HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE COVER ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) CME: Take the course for this article: Novel Agents on the Horizon for Cancer Therapy An erratum has been published An erratum has been published Submit a letter to the editor View responses Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Ma, W. W. Articles by Adjei, A. A. Search for Related Content PubMed PubMed Citation Articles by Ma, W. W. Articles by Adjei, A. A.

Novel Agents on the Horizon for Cancer Therapy

¹Assistant Professor, Department of Medicine, Roswell Park Cancer Institute, Buffalo, NY
²Senior Vice President, Clinical Research, Chair, Department of Medicine, Katherine Anne Gioia Chair in Cancer Medicine, Roswell Park Cancer Institute, Buffalo, NY

Corresponding author: Alex A. Adjei, MD, PhD, Roswell Park Cancer Institute, Elm & Carlton Streets, Buffalo, NY, 14263; e-mail: Alex.Adjei{at}RoswellPark.org

To earn free CME credit for successfully completing the online quiz based on this article, go to http://CME.AmCancerSoc.org.

DISCLOSURES: Dr. Adjei has received research grants from Eli Lilly and Ardea and honoraria from Array BioPharma. No other conflict of interest relevant to this article was reported.

	Abstract

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 
Although cancer remains a devastating diagnosis, several decades of preclinical progress in cancer biology and biotechnology have recently led to successful development of several biological agents that substantially improve survival and quality of life for some patients. There is now a rich pipeline of novel anticancer agents in early phase clinical trials. The specific tumor and stromal aberrancies targeted can be conceptualized as membrane-bound receptor kinases (HGF/c-Met, human epidermal growth factor receptor and insulin growth factor receptor pathways), intracellular signaling kinases (Src, PI3k/Akt/mTOR, and mitogen-activated protein kinase pathways), epigenetic abnormalities (DNA methyltransferase and histyone deacetylase), protein dynamics (heat shock protein 90, ubiquitin-proteasome system), and tumor vasculature and microenvironment (angiogenesis, HIF, endothelium, integrins). Several technologies are available to target these abnormalities. Of these, monoclonal antibodies and small-molecule inhibitors have been the more successful, and often complementary, approaches so far in clinical settings. The success of this target-based cancer drug development approach is discussed with examples of recently approved agents, such as bevacizumab, erlotinib, trastuzumab, sorafenib, and bortezomib. This review also highlights the pipeline of rationally designed drugs in clinical development that have the potential to impact clinical care in the near future. CA Cancer J Clin 2009;59:111–137. © 2009 American Cancer Society, Inc.

	Introduction

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 
Significant advances have been made in cancer therapy during the last decade as our understanding of molecular biology and carcinogenesis has evolved. Cancer cell proliferation, apoptosis, angiogenesis, invasion, and metastasis are regulated by an interconnecting network of cellular signaling pathways involving extracellular ligands, transmembrane receptors, intracellular signaling protein kinases, and transcription factors.¹ These intracellular signaling effectors are modulated by external factors, such as epigenetic changes, oncogenic mutations, molecular chaperones, and ubiquitin-proteasome pathways. Insights into these complicated intracellular processes have exposed many novel cancer targets for which chemotherapeutic agents may be developed.

Most traditional cancer drugs directly interfere with mitosis, DNA synthesis, and repair systems. A new class of agents induces tumor growth retardation (cytostasis) and apoptosis by exploiting aberrant tumor stroma, tumor vasculature, and cellular signaling mechanisms. Their toxicity profile is also significantly different from traditional cancer drugs. Bcr-Abl, epidermal growth factor receptor (EGFR), and angiogenesis are successful examples of targets that may be treated with drugs. Drugs that target these pathways, including imatinib, cetuximab, and bevacizumab, have already entered clinical practice.^2–5 More importantly, these agents are only the vanguard of an exciting pipeline of novel anticancer drugs. This article reviews the various classes of cancer targets and drugs that are under early phase clinical evaluation, focusing on those that are likely to enter clinical practice in the near future.

	Strategies of Intervention

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 
Cancer targets can be exploited by different strategies. Thus far, the more successful clinical approaches have been with monoclonal antibodies (MoAbs) and small-molecule, protein-kinase inhibitors. These represent the majority of anticancer drugs currently in clinical testing and are the focus of our review of the pipeline of anticancer drugs on the horizon. The application of other technologies, such as cancer vaccines, antisense oligonucleotides, and small-interfering RNAs (siRNAs), to contemporary chemotherapeutic development has recently been reviewed.^6–8

Therapeutic use of MoAbs in cancer patients was possible after development of hybridoma technology by Kohler and Milstein in 1975.⁹ Early murine MoAbs performed poorly in the clinic, partly because of short antibody half-life and immunogenicity of murine antigens in human hosts. Subsequent technological improvements allowed production of chimeric and humanized MoAbs that overcame these disadvantages and that were better suited for clinical development. The MoAb approach is particularly suited for membrane-bound targets. Proposed mechanisms of action include interference of ligand-receptor interaction, antibody-dependent cellular cytotoxicity (ADCC), complement-mediated cytotoxicity (CMC), and immune modulation.¹⁰

In comparison, small-molecule protein-kinase inhibitors are efficacious against both membrane-bound and nonmembrane-bound targets. These agents are chemically diverse and can be broadly categorized into ATP analogs, catalytic domain binders, noncatalytic domain binders, natural products, and inactive kinase conformation binding ligands.¹¹ The selection of a lead compound from a vast chemical library is complicated and challenging. This requires complex computational analyses of the structural–functional relation between a candidate kinase and a panel of small molecules. Many of these small-molecule compounds, eg, sorafenib, can inhibit multiple protein kinases because of structural homology within the same class of protein kinases.¹² In anticancer therapy, the ability to target multiple kinases and signaling pathways with a single small-molecule inhibitor has attraction. However, this ability can also hamper our attempt to understand these inhibitor's mechanisms of action in specific tumor types, an understanding that is vital to the development of these compounds.

When researchers develop therapies against a specific cancer target, monoclonal antibody and small-molecule inhibitor technologies are often complementary. Epidermal growth factor receptor (EGFR), a valid target in many epithelial malignancies, is a transmembrane protein with an extracellular ligand-binding domain joined to an intracellular tyrosine kinase domain. Cetuximab is an anti-EGFR MoAb that targets the extracellular domain by interrupting ligand binding, whereas erlotinib is a small-molecule inhibitor that blocks the EGFR intracellular tyrosine kinase activity.¹³ Both agents have shown antineoplastic activities against colon, pancreatic, and lung cancers in preclinical experiments. However, cetuximab was found to be more efficacious in colon cancer but not in pancreatic cancer, when tested clinically, and vice versa for erlotinib.^2–4 Both agents showed activity in lung cancers.¹⁴ Cetuximab seems to be the more optimal agent to combine with radiation. This disparity between preclinical discovery and clinical findings is not uncommon during development of chemotherapeutics, highlighting the importance of early phase clinical trials to identify susceptible tumor types.

	Cellular Signal Transduction Pathways

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 
    Receptor Kinases
    Human Epidermal Growth Factor Receptor Family
The human epidermal growth factor receptor (HER) family members include EGFR (erbB1), HER2/neu (erbB2), HER3 (erbB3), and HER4 (erbB4) that are structurally related, and all except HER3 contain intracellular tyrosine kinase domain (Fig. 1).¹³ All of the HER members, except HER2, bind to extracellular ligands. The structure of EGFR was described in the above section. Activation of EGFR and HER2/neu induces a cascade of downstream signaling through several pathways, such as mitogen-activated protein kinase (MAPK) and PI3-kinase/Akt/mTOR, resulting in cellular proliferation, differentiation, survival, motility, adhesion, and repair.¹⁵

View larger version (40K): [in this window] [in a new window] [PowerPoint slide for Educational Purposes]   FIGURE 1 Depicted are the cellular signaling pathways involved in the proliferation, angiogenesis, and differentiation in neoplasms with the targets amenable to therapeutic interventions in cancer therapy. Membrane-bound human epidermal growth factor receptors (HER), c-MET, and insulin-like growth factor 1 receptor (IGF-1R) mediate mitogenic signals from extracellular ligands, such as epidermal growth factor (EGF), hepatocyte growth factor (HGF), and insulin growth factors (IGF), respectively. The Ras/Raf/MEK/Erk (mitogen-activated protein kinase, MAPK) and PI3k/Akt/mTOR pathways are major intracellular axes that regulate intracellular signaling traffic. DNA methytransferases (DNMT) and histone deacetylases (HDAC) are "epigenetic switches" that modulate the expression of oncogenes and tumor suppressor genes. The agents targeting the signaling proteins are indicated in boxes.

Research in this area currently focuses on targeting more than one HER-family receptor simultaneously (Table 1). Lapatinib, a small-molecule inhibitor, is such an agent that targets both EGFR and HER2/neu receptors, and was approved by the US FDA for the treatment of breast cancer.²⁰ Other drugs that target more than one HER-family receptor and that are under clinical development include BMS-599626, PF-00299804, and BMS-690514.

c-MET receptor expression is regulated by the MET proto-oncogene, and oncogenic mutations have been found in gastric carcinoma and hereditary papillary renal carcinoma type 1.^23,24 The tumorigenic nature of MET mutants was confirmed in transgenic and knock-in animal models.²⁵ In addition to receptor mutations, dysregulated HGF/c-MET signaling could be a result of gene amplification and/or rearrangement, ligand and/or receptor overexpression, abnormal paracrine stimulation, and autocrine loop formation. The HGF/c-Met axis is implicated in a wide variety of epithelial, mesenchymal, and hematological malignancies, rendering c-MET an attractive cancer target.²¹

There are currently several HGF/c-MET inhibitors under clinical evaluation (Table 2). AMG-102 is a fully humanized IgG2 MoAb against HGF with antitumor activity in preclinical models.²⁶ The interim result of the phase 1 study was reported at the 2007 American Society of Clinical Oncology (ASCO) annual meeting.²⁷ The agent was administered intravenously at 0.5 mg/kg, 1 mg/kg, 3 mg/kg, 5 mg/kg, 10 mg/kg or 20 mg/kg dose levels. Patients with advanced solid tumors received a single dose followed by a 4-week treatment-free period to assess safety and pharmacokinetic profile. The treatment was subsequently resumed on a biweekly schedule. Thirty-one patients were treated at doses up to 20 mg/kg. Dose-limiting toxicities included dyspnea/hypoxia (at a 0.5 mg/kg dose) and gastrointestinal bleed (at a 1 mg/kg dose). Common treatment-related side effects included fatigue, constipation, anorexia, nausea, and vomiting. Pharmacokinetic (PK) analysis showed a linear relation in the dose range of 0.5 mg/kg to 20 mg/kg, and no anti-AMG-102 antibodies were detected after administration. The 20 mg/kg dose was deemed tolerable and safe, and the agent is being tested in renal cell carcinoma and malignant glioma.

ARQ-197 and PF-02341066 are similar oral small-molecule c-Met inhibitors that are in early phase trials. The recommended phase 2 dose for ARQ-197 was determined to be 120 mg twice daily. Common side effects included fatigue, diarrhea, and constipation. Grade 3 elevated liver enzymes were the more severe toxicity.²⁹ Compounds with activity against the HGF/c-MET axis in the preclinical pipeline include MGCD-265, SU-11274, and MGCD-265.

    Insulin-Like Growth Factor Receptor Pathway
Similar to the EGFR pathway, the insulin-like growth factor receptor (IGFR) signaling system comprises multiple circulating ligands, such as IGF-I, IGF-II, and insulin, interacting with membrane-bound receptors, such as type I IGF receptor (IGF-1R) and insulin receptor (IR) (Fig. 1).³⁰ Most anti-IGFR pathway agents undergoing clinical development are targeted against the IGF-1R.

The IGF-1R is a heterotetramer of two extracellular ligand-binding subunits and two β subunits with transmembrane and tyrosine kinase domains.³⁰ The IGF-1R undergoes conformational changes and phosphorylation upon ligand binding and recruits insulin-receptor substrates (IRS) and/or Src homology 2 domain-containing (Shc) proteins. The mitogenic, proliferative and/or antiapoptotic signals are then transmitted downstream through the MAPK and PI3k/Akt/mTOR axes.

In normal physiological states, the IGF-1R plays an important role in fetal development and linear growth of many organs, whereas insulin/IR interaction regulates carbohydrate and lipid metabolism. IGF-1R was implicated in the development and maintenance of malignant phenotypes, and interruption of IGF-1R signaling inhibited cancer cell growth and motility in in vitro and in vivo models.³¹ Aberrant activation of the IGF-I/IGF-1R axis was also associated with worse prognosis in many neoplasms, including multiple myeloma, prostate cancer, nonsmall cell lung cancer, and renal cell cancer. Aside from the IGF-1R, abnormally activated IR by insulin or IGF-II stimulation enhances mitogenesis in cancer cells, thus highlighting their therapeutic value.³² However, IR inhibition may lead to type 2 diabetes mellitus, and IGF-I–deficient states have been associated with osteoporotic fractures and ischemic heart disease. These potential toxicities should be taken into consideration during the clinical development of these agents.

There are several IGF-1R–targeting agents in clinical testing, and none are approved by the FDA for general oncological use yet (Table 3). CP-751,871 is a fully humanized IgG2 MoAb antagonist of IGF-1R with preclinical anticancer activities.³³ The MoAb interrupts the binding of IGF-I to IGF-1R, IGF-1R autophosphorylation, and induces down-regulation of IGF-1R in vitro and in tumor xenograft models. CP-751,871 was administered intravenously every 21 days in advanced solid-tumor patients.³⁴ In this phase 1 study, the CP-751,871 was escalated to the maximally feasible dose of 20 mg/kg without reaching the maximum tolerated dose. Correlative studies revealed an increased expression of serum insulin and human growth hormone, supposedly through a negative feedback loop. The most common adverse events were hyperglycemia, anorexia, nausea, elevated liver transaminases, hyperuremia, and fatigue. Investigators analyzed IGF-1R–expressing circulating cancer cells (CTCs) with an exploratory assay. Three patients with detectable IGF-1R–expressing CTCs at baseline were reported to have a decreased level of CTCs after CP-751,871 administration that rebounded at the end of the 21-day period.³⁵

AMG-479 is a fully humanized anti–IGF-1R MoAb with broad preclinical antitumor activity. This agent was a potent inhibitor of PI3k/Akt axis with increased antitumor effect when combined with anti-EGFR therapies in pancreatic cancer xenograft models.³⁷ The phase 1 results of AMG-479 testing were published in an abstract; 16 patients with advanced solid tumors received escalating doses of the agent intravenously.³⁸ The final dose level reached 20 mg/kg every 2 weeks, and one patient experienced grade 3 dose-limiting thrombocytopenia at 20 mg/kg. No hyperglycemia greater than grade 2 was observed. The agent is currently being tested in non-Hodgkin lymphoma, Ewing sarcoma, and desmoplastic small round-cell tumors. At the time of this writing, phase 1 studies in combination with gemcitabine or panitumumab were being planned.

Other anti–IGF-1R agents under phase 1 evaluation include MoAbs (IMC-A12, R-1507, and BIIB022), small-molecule inhibitors (XL-288, OSI-906), and nordihydrohuareacetic acid.

    Intracellular Signaling Kinases
    Src
c-Src is a nonreceptor tyrosine kinase and was the first proto-oncogene to be described. This protein has several functional domains as follows: an N-terminal membrane-association domain (SH4), a variable "unique" domain, a proline-rich sequence-binding domain (SH3), a phosphotyrosine-binding domain (SH2), a tyrosine kinase domain responsible for the catalytic activity of the molecule (SH1), and a C-terminal Tyr530-containing negative regulatory domain that autoinhibits the kinase activity when phosphorylated.³⁹ The C-terminal interacts with SH2 and SH3 domains when phosphorylated to inactivate Src, whereas loss of the C-terminal phosphotyrosine activates c-Src.

Accumulating data suggest that Src plays an important role in cancer cell mitosis, adhesion, invasion, motility, and progression.⁴⁰ Src mediates the mitogenic signals between growth factor receptors, like EGFR, c-Met, and IGF-1R, and downstream signaling cascades, like focal-adhesion kinase (FAK), MAPK, and PI3k/Akt/mTOR (Fig. 1). Dysregulated Src activity has been implicated in the development and progression of several human cancers, including breast, colorectal, lung, ovarian, and hematological malignancies. As such, much interest exists in developing Src-targeting compounds for cancer therapy.

Dasatinib (BMS-354825) is an orally available dual-specific Src and Abl kinase inhibitor with antiproliferative activity against a broad spectrum of hematological and solid cancer cell lines.⁴¹ The compound has less stringent conformational requirements for Abl kinase inhibition than imatinib; dasatinib is active against many imatinib-resistant Bcr/Abl mutants in preclinical models.⁴² Dasatinib was granted accelerated approval by the FDA in 2006 for the treatment of chronic myeloid leukemia (CML) in chronic, accelerated, or blast phase, and regular approval for Philadelphia chromosome-positive (Ph1+) acute lymphoblastic leukemia (ALL) with resistance or intolerance to prior therapy. This followed an analysis of four single-arm studies involving 445 patients treated at a starting dose of 70 mg twice daily^43,44 The agent achieved significant cytogenetic and hematologic responses in the study population. Toxicities included fluid retention, as well as constitutional, gastrointestinal, and hematological events. Bleeding was reported in 40% of patients, of which 14% had gastrointestinal bleed. The recommended dosing schedules included 70 mg twice daily and 100 mg once daily. As a multikinase inhibitor, dasatinib is being evaluated in breast, lung, colorectal, and pancreatic cancers.

Bosutinib (SKI-606) is another potent oral Src inhibitor with anti-Abl activities. The compound demonstrated antitumor activities in preclinical models, and clinical development in hematological and solid malignancies is underway. AZD-0530, XL-999, and XL-228 are other Src inhibitors undergoing early phase testing. Most of these small molecules have activities against other kinases as well (Table 4).

PI3k is a lipid kinase that generates 3'-phosphoinositides (PIP3) at the cell membrane when activated by receptor kinases.⁴⁵ This leads to recruitment of phosphoinositide-dependent kinase 1 (PDK1) and Akt to cell membrane. The generation of PIP3 is negatively regulated by phosphatase and tensin homologue (PTEN). Akt is activated fully by several enzymes, including PDK1, mTORC2, and IRS-1, which then inhibit the tuberous sclerosis (TSC) protein 2. Inhibition of the TSC complex leads to mTOR-mediated activation of p70s6k and 4EBP1, which regulate cellular translational and transcriptional mechanisms.

The PI3K/Akt/mTOR pathway is attractive as a cancer target for several reasons. Kinases in the pathway are found to be activated in several cancers, resulting from aberrant events including loss of PTEN function, Akt amplification, activating mutations of TSC complex, or constitutive activation of kinases upstream to the pathway.⁴⁵ Activation of PI3K/Akt/mTOR axis was associated with early events in carcinogenesis and interruption of the pathway-achieved antiproliferation, antisurvival, antiangiogenic, and proapoptotic effects. Moreover, activation of the pathway was associated with poor prognosis and contributed to chemoresistance in many cancers. mTOR, Akt, PI3k, and PDK-1 are main foci for most small-molecule inhibitors that target the PI3k/Akt/mTOR pathway (Table 5A).

Temsirolimus (CCI779) is a water-soluble, synthetic, rapamycin ester available in oral and intravenous formulations.⁵⁰ This drug was the first of its class to receive FDA approval, and current indications include the treatment of poor-risk untreated advanced renal cell carcinoma patients. In the pivotal randomized trial, the temsirolimus-alone arm achieved longer overall survival (HR, 0.73) and progression-free survival (3.8 months vs 1.9 months; P<0.001) than the interferon-alone arm, whereas the overall survival of patients in the temsirolimus and interferon combination arm was not significantly different from that of patients in the interferon-alone arm.⁵¹ The median survival times were 10.9 months, 7.3 months, and 8.4 months in the temsirolimus, interferon, and combination groups, respectively. The most common grade 3 or 4 toxicities were asthenia, anemia, and dyspnea. The recommended dose of temsirolimus for this indication is 25 mg weekly by intravenous administration. The drug is currently being tested either alone or in combination therapy in tumor types such as melanoma, myeloma, and renal and gynecological cancers.

Everolimus (RAD001) is an oral mTOR inhibitor with antineoplastic activity similar to other rapalogs. In a phase 1 study in solid tumor patients, the optimal biological dose for everolimus was determined to be 20 mg weekly, which achieved the pharmacokinetic and pharmacodynamic changes correlated with antineoplastic effects in animal models.⁵² The toxicities were mild and included anorexia, fatigue, rash, mucositis, headache, hyperlipidemia, and gastrointestinal disturbances. When administered continuously, everolimus was well tolerated at a 10 mg dose in patients with refractory or relapsed hematological malignancies. No dose-limiting toxicities were reported, and activity was seen in patients with myelodysplastic syndrome.⁵³ The dose of 5 mg/m² was the maximum tolerated dose in pediatric solid-tumor patients, and dose-limiting toxicities included diarrhea, mucositis, and elevation of alanine transaminase.⁵⁴ No objective tumor responses were observed.

Everolimus demonstrated antitumor activity in metastatic renal cell carcinoma patients who progressed on sunitinib, sorafenib, or both in a randomized phase 3 trial.⁵⁵ The patients who received everolimus at 10 mg once daily achieved a longer median progression-free survival than the placebo group (4.0 months vs 1.9 months; HR, 0.3; P<0.0001). Commonly reported everolimus-related toxicities included stomatitis, rash, and fatigue. The agent is being tested as a therapy for nonsmall cell lung, prostate, colorectal, and breast cancers as either a single agent or in combination.

Deforolimus (AP-23,573) is the other mTOR inhibitor currently under clinical testing. During phase 1 testing, the maximum tolerated dose was 18.75 mg/day and mouth sores was the dose-limiting toxicity.⁵⁶ Antitumor activity was seen in nonsmall cell lung cancer, carcinosarcoma, renal cell carcinoma, and Ewing sarcoma. Phase 2 studies in sarcoma are ongoing.

    Akt Inhibitors
Akt, also known as protein kinase B (PK-B), is a serine/threonine kinase upstream to mTORC1 and is implicated in the formation and maintenance of malignancies.⁵⁷ Akt is as attractive a target as mTOR, if not more so, because of its role in several important cellular functions, including cell-cycle progression, protein translation and transcription, apoptosis, and cellular metabolism. Given Akt's key role in the axis, Akt inhibition produces theoretically more severe side effects than mTOR inhibition. So far, the development of this class of agents has been disappointing.

Perifosine, a lipid-based derivative ofmiltefosine, is perhaps the best characterized Akt inhibitor in human testing so far. This compound inhibits Akt translocation to the cell membrane and exhibits in vitro antiproliferative effects in several cancer cell lines.⁵⁸ It should be noted, however, that perifosine is a relatively nonspecific AKT inhibitor since it interrupts cell membrane biology. Perifosine was tested as a daily oral dose on a 3-week cycle in patients with advanced solid tumors.⁵⁹ The patients reported dose-dependent gastrointestinal adverse events, such as nausea, diarrhea, and vomiting, which led to early therapy discontinuation in an increasing number of patients who were receiving higher dose levels. The tolerated-dose maximum was determined to be 200 mg/day. An alternative loading/maintenance dosing schedule was tested in patients with advanced solid tumors.⁶⁰ The maximum tolerated dose was a loading dose of 150 mg every 6 hours for four doses followed by 100 mg once daily for maintenance. The dose-limiting toxicities during the loading period were nausea, diarrhea, dehydration, and fatigue and were manageable with prophylactic antiemetics. However, the side effects were more difficult to manage during the maintenance period. Despite encouraging evidence in preclinical studies, perifosine failed to demonstrate significant single-agent anticancer activity in sarcoma, melanoma, pancreatic, and head and neck cancers during phase 2 testings. Perifosine continues to be evaluated as a single agent and in combinations. Several lipid-based and peptide-based Akt inhibitors are being evaluated preclinically.⁴⁵

Currently, there is limited clinical experience with inhibitors of PI3k and PDK-1. The PI3k inhibitors that are undergoing phase 1 evaluation include PI-103, BGT-226, BEZ-235, XL-765, and XL-147. Current PDK-1 inhibitors are derivatives of staurosporin and celecoxib.⁴⁵ UCN-01 is a staurosporin derivative that inhibits multiple kinases, including PDK-1, and has in vitro proapoptotic activity. The drug is synergistic with cytotoxic agents in preclinical studies, but the proapoptotic activity seems to be from inhibition of Chk1, a cell-cycle checkpoint kinase. UCN-01 can be administered intravenously as an initial 72-hour continuous infusion on a monthly schedule or as a short infusion over 3 hours every 28 days with the second and subsequent doses at 50% of the first dose.^61,62 However, the clinical activity of UCN-01 was not associated with PI3k/Akt/mTOR-pathway inhibition, and its role as a PDK-1 inhibitor remains ambiguous. OSU-03,012 is a celecoxib derivative that inhibits PDK-1 and induces apoptosis in rhabdomyosarcoma cell lines. This drug is currently under preclinical evaluation.

    Mitogen-activated Protein Kinase Pathway
Anchorage-independent growth, a hallmark of neoplasm, describes the ability of cells to proliferate in the absence of substratum adhesion.⁶³ Studies into the phenomenon revealed the mitogen-activated protein kinase (MAPK) pathway to be a major connector between extracellular and intracellular stimuli, such as growth factors, cytokines, and oncogenes, and cellular responses related to adhesion, motility, proliferation, and malignant transformation.⁶⁴ Ras is a small GTPase protein that transmits activating signals from growth factors, cytokines, and oncogenes, to Raf and then to MAPK kinase (MEK). MEK then phosphorylates and activates the extracellular signal-regulated kinase (ERK, also known as MAPK). Ras, Raf, and MEK are the main targets in this pathway (Fig. 1, Table 5B).

Tipifarnib (R115777), the best characterized FTI so far, has shown promising antiproliferative, proapoptotic, and antiangiogenic activities in preclinical studies.⁶⁶ This finding has led to clinical testing in many tumor types, including leukemias, nonsmall cell lung, prostate, breast, pancreatic, and colorectal cancers. However, tipifarnib failed to show convincing anticancer effects in phase 2 studies. The failure of tipifarnib to improve survival in a phase 3 pancreatic cancer trial further shed doubts on this approach.⁶⁷ Preclinical data indicated that the K-Ras–expressing tumor may be escaping tipifarnib inhibition because the oncogenic K-Ras mutant requires a higher concentration for inhibition than the wild-type Ras and H-Ras mutants.⁶⁸ In addition, it seemed that clinical efficacy was not related to Ras mutational status, and multiple other signaling pathways were affected in addition to Ras. Lonafarnib (SCH-66,336) and BMS-214662 are the other FTIs being evaluated clinically. Their role in anticancer therapy as a Ras inhibitor remains to be defined. Thus, the focus of development of a more viable approach to inhibit the MAPK pathway has turned to Raf and MEK.

    Raf Inhibitors
The Raf protein is an important effecter of Ras, belonging to a family of three structurally conserved serine-threonine kinases as follows: A-Raf, B-Raf, and C-Raf (Raf-1). Mutations of Ras can result in a constitutively activated MAPK pathway with resultant malignant properties. Wild-type Raf can also be activated in malignant cells from aberrant stimulation by upstream regulators, such as Ras and growth factor receptors. B-Raf mutations are found in nearly 70% of melanoma and also frequently in other solid tumors, such as colorectal and ovarian cancers.⁶⁹

Sorafenib (BAY43-9006) is an oral, dual inhibitor of Raf and vascular endothelial growth factor receptor (VEGFR). The molecule has demonstrated preclinical antineoplastic activity against a wide spectrum of human cancers.⁷⁰ It has potent in vitro inhibitory effects against Raf-1, B-Raf, VEGFR-2, platelet-derived growth factor receptor (PDGFR), and VEGFR-3. The dose-limiting toxicities reported during phase 1 development were diarrhea, fatigue, and skin rash.⁷¹ The recommended dose is 400 mg twice daily on a constant basis. Rash, diarrhea, fatigue, and hand–foot syndrome were the common side effects during phase 2 and 3 studies.^72,73 Correlative studies showed MAPK pathway inhibition in peripheral lymphocytes with a sorafenib dose above 200 mg, indicating potential usefulness of this pharmacodynamic assay for Raf inhibitor development.

Sorafenib is approved by the FDA for treatment of advanced renal cell and unresectable hepatocellular carcinomas. The agent demonstrated significant disease-stabilization effects in advanced renal cell cancer. In the pivotal phase 3 trial, 903 patients with advanced renal cell carcinoma who progressed after one systemic therapy, who had an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1, with low to intermediate risk (according to the Memorial Sloan-Kettering Cancer Center prognostic score) were randomized to receive sorafenib 400 mg twice daily (n = 451) or placebo (n = 452).⁷³ The median progression-free survival time was superior in the sorafenib group compared with the placebo group (5.5 months vs 2.8 months; HR, 0.44; P<0.01). The overall survival of the sorafenib group at first-interim analysis was also superior to that of the placebo group (HR, 0.72), although not statistically significant (P = 0.02). In a supporting phase 2, randomized, discontinuation trial, patients with metastatic renal cell carcinoma who initially received sorafenib 400 mg twice daily for 12 weeks during a run-in period and who had stable disease (tumor bidimensional measurements changes of less than 25% from baseline) were randomized to receive sorafenib or placebo (n = 32 and 33, respectively) for another 12 weeks.⁷⁴ Patients who had tumor growth of 25% or greater discontinued treatment, and those with tumor shrinkage of 25% or greater continued sorafenib. Median progression-free survival time from randomization was significantly longer for the sorafenib group than the placebo group (24 weeks vs 6 weeks; P = 0.0087). Patients who progressed on placebo crossed over to receive sorafenib until their disease progressed.

In a phase 3 trial involving 602 patients who had advanced hepatocellular carcinoma, an ECOG performance status of 2 or less, and a Child-Pugh class A liver dysfunction, those who received sorafenib 400 mg twice daily achieved a longer median survival time than those who received a placebo (10.7 months vs 7.9 months, respectively; P<0.001).⁷⁵ The median time to radiologic progression was also longer in the sorafenib group than in the placebo group (5.5 months vs 2.8 months, respectively; P<0.001).

Despite these successes, the contribution of Raf inhibition to sorafenib's clinical efficacy is difficult to assess. The success of bevacizumab and sunitinib in renal cell carcinoma indicates that the drug's anticancer effects may be related more to its antiangiogenic effects. The real benefit of Raf inhibition in cancer therapy will perhaps be answered only by specific Raf inhibitors. XL-281 and PLX-4032 are oral inhibitors that are, reportedly, highly selective against Raf and are currently in phase 1 testing. RAF-265 (CHIR-265), currently in early phase trial, is another oral inhibitor of Raf and VEGFR.

    MEK Inhibitors
The MEK protein family consists of MEK1 and MEK2, which have dual-specificity kinase activity and are involved in the phosphorylation of tyrosine and serine/threonine residues.⁷⁶ The MEK kinases are highly specific and are known to phosphorylate only Erk1 and Erk2. Constitutively activated MEK, from enhanced upstream stimuli or activating mutations, is tumorigenic and is implicated in a broad spectrum of human cancers. Most of the known MEK inhibitors do not bind to the ATP-docking pocket of MEK. Instead, the inhibitors stabilize the inactive conformation of the kinase by binding to a unique binding site next to the ATP-binding pocket. This unique interaction is thought to explain the high degree of specificity of MEK inhibitors.

CI-1040 is one of the first MEK inhibitors to be developed clinically. The oral agent demonstrated encouraging preclinical effects on tumor proliferation, survival, invasion, and angiogenesis.⁷⁷ A phase 1 study showed that the drug had poor metabolic stability and bioavailability, so high doses had to be administered in phase 2 trials.^78,79 The encouraging antitumor activity seen in phase 1 development was not seen in phase 2 studies, leading to the termination of the agent's development. Despite this, correlative studies from a phase 1 trial showed adequate target inhibition with CI-1040, and a subsequent research effort was focused on improving upon CI-1040.

PD0325901 is a second-generation MEK inhibitor that is structurally related to CI-1040. PD0325901 has a higher potency, better bioavailability, and more sustained MEK inhibition than CI-1040. Preclinical studies have shown antitumor activities against a broad spectrum of human cancer cell lines.⁶⁴ The dose-limiting toxicities reported during phase 1 development included acneiform rash, syncope, and elevated liver enzymes.⁸⁰ Visual disturbances such as halos, spots, and decreased acuity were also reported. Antitumor effects were seen in melanoma and in colon and nonsmall cell lung cancers. Phase 2 trials in melanoma, breast, lung, and colon cancers had ceased enrolling patients at the time of this writing.

AZD-6244 (ARRY-142886) is another second-generation, highly selective MEK inhibitor. The drug inhibited Erk phosphorylation and was associated with growth inhibition in cell lines containing B-Raf and Ras mutations and was associated with tumor regression in preclinical xenograft models.⁸¹ The dose-limiting toxicities during phase 1 development were hypoxia, rash, and diarrhea, and common adverse events included nausea, fatigue, peripheral edema, altered taste, and blurred vision.⁸² The recommended phase 2 dose was determined to be 200 mg twice daily. The best response was stable disease observed in three melanoma patients and one nonsmall cell lung cancer patient. AZD6244 is being tested in phase 2 trials of various cancers, including lung, liver, colorectal, pancreatic, and ovarian. Other MEK inhibitors under phase 1 testing include XL-518 and RDEA-119.

	Tumor Vasculature

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 
    Angiogenesis Inhibitors
Angiogenesis describes the formation of new blood vessels (neovascularization) from existing vasculature that is vital for normal physiological processes but dysfunctional in malignancies.⁸³ Folkman, in the 1970s, observed that tumors are unable to grow beyond 2 mm³ unless they are supported by neovascularization. This observation led to one of the most fascinating success stories in cancer therapy develop-ment.⁸⁴ Vascular endothelial growth factors and receptors (VEGF, VEGFR) are now well validated targets in cancer therapy (Fig. 2).^5,73,85 A plethora of anti-VEGF/VEGFR MoAbs and small-molecule inhibitors (Table 6) are now in clinical development. These have recently been reviewed.^46,86

View larger version (28K): [in this window] [in a new window] [PowerPoint slide for Educational Purposes]   FIGURE 2 The therapeutic targets in tumor vasculature are illustrated. Vascular endothelial growth factor receptor-2 (VEGFR) mediates angiogenic signals from ligands such as vascular endothelial growth factor (VEGF), fibroblast growth factor, and hepatocyte growth factor in the formation of new tumor vasculature. Hypoxia-inducible factor 1 (Hif-1) expression is regulated by MAPK, PI3k/Akt/mTOR, and epigenetic changes, and overexpression leads to tumor formation and neovascularization. Integrin 5β1 are cell surface adhesion receptors on vascular endothelial cells up-regulated in tumor vasculature. These angiogenic nodes are amenable to intervention. Vascular disrupting agents disrupt endothelium of established tumor vasculature and lead to potentially "catastrophic" downstream tumor necrosis. Agents that target these angiogenic nodes are indicated in boxes.

Recent preclinical evidence has demonstrated that histone deacetylases (HDAC) inhibitors possess antiangiogenic properties.⁸⁹ The HDAC inhibitors are thought to inhibit angiogenesis by up-regulating antiangiogenic genes (eg, activin A, neurofibromin 2, and thrombospondin 1), down-regulating proangiogenic genes (eg, HIF-1, VEGF, PDGF, basic fibroblast growth factor [bFGF]), promoting HIF-1 degradation, and repressing the function of HIF-1–containing transcriptional complexes. As such, mTOR inhibitors and HDAC inhibitors are being developed as antiangiogenic agents either as monotherapy or in combination therapies in clinical trials (see sections on PI3k/Akt/mTOR Pathway and Histone Deacetylase Inhibitors). In addition, antagonists to the integrin family have shown encouraging antiangiogenic activity in preclinical studies and are been tested as antiangiogenic agents clinically (see section on Integrins).

    Vascular Disrupting Agents
Vascular disrupting agents (VDAs) target endothelial cells of established tumor vasculature and represent an alternate approach to disrupting tumor blood supply (Fig. 2). VDAs have the theoretical advantage of shutting down the vascular supply, which can result in "catastrophic" downstream tumor necrosis.⁹⁰ Compared with targeting tumor cells directly, VDAs can be delivered to the endothelium with few impediments, and drug resistance is a lesser problem given the relative genetic stability of endothelial cells. Preclinical studies have shown that the VDA approach induces central tumor necrosis but leaves behind a viable tumor rim that has potential for regrowth.⁹¹ This viable tumor periphery is accessible to high molecular-weight compounds, such as MoAbs, and amenable to conventional cytotoxics, making combination therapy an attractive and rational approach to developing VDAs.⁹²

Tumor endothelium is characterized by high endothelial-cell proliferation and an abnormal basement membrane.⁹³ Structurally, the tumor vasculature is tortuous, disorganized, and lacks smooth muscle, pericyte, and nerve support. These lead to increased vascular permeability and high interstitial pressure in tumor microenvironments such that a small decrement in perfusion pressure within the vasculature becomes catastrophic to the tumor.⁹¹ In addition, endothelial cells are highly dependent on the tubulin cytoskeleton for motility, invasion, attachment, alignment, and proliferation.⁹⁴ The specificity of VADs in exploiting these distinct characteristics of tumor vasculature can potentially spare normal blood vessels from by-stander effects.

Most VDAs disrupt the cytoskeleton and cell-to-cell junction of endothelial cells, which leads to increased interstitial pressure and reduced vessel caliber. As plasma leaks from tumor vasculature, blood flow becomes more viscous and rouleaux begin to form. The coagulation cascade is then activated as platelets come in contact with exposed basement membrane, leading to vascular thrombosis and tumor necrosis. VDAs currently under clinical testing can be divided into two categories, tubulin-destabilizing agents and flavonoids (Table 7).

Soblidotin (TZT-1027) is a synthetic derivative of dolastatin-10 with antitumor and antivascular activity against various human tumor xenografts. Dolastatin-10 was isolated from an Indian Ocean mollusc, Dolabela auricularia. The dose-limiting toxicities during phase 1 trials included fatigue, neutropenia, peripheral neuropathy, constipation, hyponatremia, and pain at the infusion site.^97,98 The agent was tested in sarcoma and nonsmall cell lung cancer during phase 2 development.

    Flavanoids
5,6-dimethylxanthenone-4acetic acid (DMXAA, AS-1404) is a flavanoid derivative that damages DNA and induces apoptosis in endothelial cells in preclinical models. The exact mechanism that leads to tumor cell death remains unknown but involves NFB, serotonin, TNF-, and nitric oxide.⁹⁹ DMXAA is administered intravenously, and two dosing schedules were tested in phase 1 studies, once weekly and once every 3 weeks.^100–102 The dose-limiting toxicities were anxiety, tremor, slurred speech, urinary incontinence, visual disturbances, and possibly left ventricular failure. DMXAA is being evaluated in combination with docetaxel in second-line treatments for advanced nonsmall cell lung cancer in a phase 3 trial. The efficacy of DMXAA is also being explored in ovarian and hormone-refractory metastatic prostate cancers.

VADs are a promising class of chemotherapeutic agents with unique mechanisms of action. In the near future, careful clinical studies and attention to toxicities will clarify the role of VADs in anticancer therapy.

	Epigenetic Modulators

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 
Epigenetics is the study of heritable changes in gene expression that are not due to any alteration in the DNA sequence.^103,104 This field received much attention as researchers sought to explain varied phenotypic expressions throughout an individual's lifetime in response to external stressors and aging, when the genetic code is almost always constant. Another puzzle is that, despite having the same DNA sequences, there are differences in phenotypes between monozygotic twins and also between cloned animals.

Epigenetic modification can be viewed as "on" and "off" switches for gene expression, where shutting down tumor-suppressor genes or activating oncogenes can lead to dysregulated cellular proliferation and apoptosis.¹⁰⁵ DNA methylation and histone modification are two areas most studied in the development of anticancer therapy (Fig. 1 and 2).

DNA methylation is the addition of a methyl group to specific stretches of a DNA sequence, called CpG islands, often located in or near promoter regions. Methylation of a CpG island, catalyzed by DNA methyltransferases (DNMTs), can lead to silencing of the downstream gene. DNA strands are wrapped around histones to form nucleosomes, and certain genes are silenced when the packing becomes too tight. The addition of an acetyl group to histones (acetylation) loosens this binding and allows expression of tumor suppressor genes. Conversely, deacetylation can result in tumor formation. The process is controlled by histone acetyltransferases (HATs) and histone deacetylases (HDACs). Interestingly, DNA methylation and histone deacetylation are closely related in gene silencing through direct interactions between DNMTs and HDACs, such that simultaneous targeting is possibly synergistic.^106,107

Global epigenetic disturbance is thought to contribute to carcinogenesis through oncogene activation, loss of imprinting, genomic instability, X-chromosome inactivation, and harmful expression of inserted viral sequences.¹⁰⁸ Studies have found that malignant tissues harbor more epigenetic aberrancies than healthy tissues of the same type, and the degree of epigenetic abnormalities increases during malignant transformation.¹⁰⁹ This finding has led to an interest in developing DNMT inhibitors and HDAC inhibitors, which have enjoyed early successes in hematological malignancies, and testing continues in solid tumors. Side effects from these agents at epigenetic-modulating doses are relatively mild compared with conventional cytotoxic agents.

    DNA Methyltransferase Inhibitors (DNMTis)
5-azacytidine (azacitidine; Vidaza) and 5-aza-2'-deoxycytidine (decitabine; Dacogen), the two most studied DNMT inhibitors, were developed initially as cytotoxic agents to treat leukemia at much higher doses.¹⁰⁸ Interestingly, the agents are predominantly epigenetic modulating, instead of cytotoxic, at a much lower dose when administered over a longer duration. These nucleoside analogs replace cytosine during DNA replication and are, thus, only active during the S phase. The DNA/nucleoside-analog complex then stoichiometrically binds to and inhibits DNMTs. Azacitidine also binds to RNA and interrupts protein translation, whereas decitabine binds to DNA only.

Azacitidine and decitabine were approved by the FDA for treatment of myelodysplastic syndrome in 2004 and 2006, respectively. In a randomized controlled trial involving 191 poor-risk myelodysplastic syndrome patients, the overall response rate (complete or part normalization of blood cell counts and bone marrow morphology) was 23% in the azacitidine arm versus none in the best supportive care arm (control).¹¹⁰ Patients who responded became transfusion-independent for the duration of their response. Crossover was allowed in the study, and more than one-half of the patients in the control arm received azacitidine. After controlling for crossover effects, the median survival time in the study arm was 18 months compared with 11 months in the control arm, and the quality of life was superior in those initially randomized to receive azacitidine. Myelosuppression, gastrointestinal disturbances, fevers, rigors, ecchymoses, petechiae, injection site reaction, arthralgia, and dizziness were common azacitidine-related toxicities.

The clinical efficacy of decitabine was demonstrated in a randomized controlled trial involving 170 patients with poor-risk myelodysplastic syndrome.¹¹¹ The patients who received the study drug had a significantly higher overall response rate (17% complete response plus partial response;) than those receiving best supportive care (0%). The median duration of response was 10.3 months and was associated with transfusion independence. There was a trend toward delay in acute myelogenous leukemia transformation or death for patients who received the study drug. The most common adverse events were hematologic, hepatic (hyperbilirubinemia), or pulmonary (pneumonia), whereas gastrointestinal disturbances were mild and infrequent.

However, azacitidine and 5-aza-2'-deoxycitidine are degraded rapidly in the body. Several more stable cytidine analogs, such as 5,6-dihydro-5-azacitidine and 5-fluoro-2'-deoxycitidine, were tested clinically with mixed results.¹¹² Zebularine, a novel cytidine analog, is more stable and amenable for oral administration.¹¹³ This molecule is an effective inhibitor of DNMT and cytidine deaminase and targets tumor cells preferentially. Zebularine appears to be an optimal lead compound upon which future improvement can be based. Significant interest in developing non-nucleoside DNMT inhibitors exists for the purpose of avoiding toxicities associated with incorporation of nucleoside analogs into DNA. The candidate compounds include procainamide, procaine, RG-108, and MG-98.¹⁰⁸

    Histone Deacetylases Inhibitors
There are several classes of histone deacetylases (HDAC) with nonoverlapping class-specific actions.¹¹⁴ Class I and II HDAC share a highly conserved zinc-containing catalytic domain that is crucial for HDAC-inhibitor binding.¹¹⁵ Class I HDAC are located primarily in cell nuclei, whereas class II HDAC transverse between the nucleus and cytoplasm. Class III HDAC are NAD-dependent and are related to the yeast protein Sir2. This class of proteins are unaffected by inhibitors of class I and II HDACs.

HDAC inhibitors are structurally heterogeneous (Table 8) but share a common ability to recognize and bind to the catalytic zinc-pocket on class I and II HDAC. HDAC inhibitors can induce in vitro cell cycle arrest and differentiation. Cells restart cell cycling and become less differentiated when the agent is withdrawn. Despite encouraging in vitro activity, early compounds derived from natural products, such as depudecin, trapoxin, and trichostatin, have limited in vivo antineoplastic activity, partly because of poor retention, instability, and toxicity.

Vorinostat (suberoylanilide hydroxamine acid, SAHA) is the first in its class to be approved by the FDA for cancer therapy.¹¹⁷ The pivotal trial supporting the approval for treatment of advanced primary cutaneous T-cell lymphoma (CTCL) was a phase 2, single-arm, multicenter, open-label trial involving 74 patients with stage IB and higher disease refractory to two previous bexarotene-containing systemic therapies. The objective response rate was 30%, which lasted for a median duration of 168 days. The supporting study was a single-center phase 2 trial that enrolled 33 patients of similar characteristics and reported a response rate of 31%. The most common side effects were diarrhea, fatigue, nausea, and anorexia. Vorinostat (Zolinza) is being evaluated in combination with carboplatin and paclitaxel in stages IIIB and IV nonsmall cell lung cancer, and as a single-agent in previously treated mesothelioma patients in phase 3 trials.

In preclinical studies, MS-275 (now called SNDX-275), an oral benzamide, had antitumor activity against a broad spectrum of solid and hematological malignancies.¹¹⁸ The half-life of MS-275 is between 33 and 80 hours, making it amenable to weekly or biweekly dosing. When administered on a 14-day schedule, the maximum tolerated dose was 10 mg/m², and the dose-limiting toxicities were fatigue, anorexia, and gastrointestinal disturbances. The maximum tolerated dose was 6 mg/m² when administered on a weekly schedule for 4 weeks followed by 2 weeks of rest. The dose-limiting toxicities were hypophosphatasia, hyponatremia, and hypoalbuminemia. MS-275 demonstrated a linear pharmacokinetic relationship and HDAC inhibition was observed in peripheral blood mononuclear cells at all doses studied. The agent is being developed clinically in several tumor types, including melanoma and nonsmall cell lung cancer.¹¹⁸ The synergy with retinoic acid and azacitidine is also being studied.

FDA approval of vorinostat validated the concept of HDAC inhibition in cancer therapy. However, there are still many hurdles to overcome, and determining the exact mechanism of action of HDAC inhibitors in tumors is difficult. HDAC deacetylate both histone and nonhistone proteins. The nonhistone proteins include the receptor and nonreceptor signaling pathways mentioned in this review.^89,108 Gene-expression studies revealed that HDAC inhibitors cause complicated cellular changes that include alteration in cell cycles, apoptosis, angiogenesis, and metabolism.¹¹⁸ The elucidation of underlying mechanisms and relevant biomarkers in specific tumor types will help development of HDAC inhibitors. Also, HDAC inhibitors may be most effective when used in combination therapies rather than as single agents.

	Integrins: Targeting the Extracellular Matrix

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 
Integrins are heterodimeric cell-surface–adhesion receptors for extracellular matrix (ECM) comprising an and a β subunit.¹¹⁹ Unlike receptor kinases, integrins lack intrinsic enzymatic activity. The proteins transduce proliferative, survival, migratory, and angiogenic signals by clustering together with kinases and adaptor proteins to form focal adhesion complexes. Blockade of integrin/ECM-ligand interactions inhibits tumor metastasis and angiogenesis and can be achieved by MoAbs, small-molecule peptides, and peptidomimetics (Table 9). Antagonists of prometastatic and proangiogenic integrins, such as 5β1, vβ3, and vβ5, are under clinical evaluation.

Integrins vβ3 and vβ5 are involved in angiogenesis and expressed in malignancies such as melanoma, gliomas, and cancers of the breast, prostate, and colon. Cilengitide (EMD-121974) is a synthetic cyclic pentapeptide small-molecule inhibitor of vβ3 and vβ5 integrins.¹²² The peptide has demonstrated antiangiogenic and antitumor activities in vitro and in vivo. In a phase 1 trial in patients with advanced solid tumors, cilengitide was administered twice weekly every 28 days and was well tolerated with no dose-limiting toxicities observed at the tested dose levels.¹²³ The agent is being tested in adult and pediatric patients who have refractory glioma.

	Heat Shock Protein: Molecular Chaperone

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 
Heat shock protein 90 (HSP90) is a chaperone protein that assists in proper folding and functioning of client proteins (Table 10; Fig. 3).¹²⁴ Physiologically, chaperone proteins protect client proteins from degradation and environmental stress, including heat, hypoxia, free radicals, radiation, and chemotherapy. Elevated HSP90 level permits accumulation of proteins of mutated genes in the cell, which may be evolutionarily advantageous to genetic diversity. However, increased HSP90 activity would have, as hypothesized, also permitted the survival of genetically unstable cancer cells.

View larger version (23K): [in this window] [in a new window] [PowerPoint slide for Educational Purposes]   FIGURE 3 Cancer targets are in the apoptotic and cellular protein trafficking pathways. Cellular apoptosis is mediated by extrinsic and intrinsic pathways and regulated by several proapoptotic and antiapoptotic signaling proteins. The extrinsic pathway is activated by membrane-bound receptors, including DR-4, DR-5, and Fas, by external ligands; whereas the intrinsic pathway is initiated by intracellular stimuli through mitochondria, leading to activation of a cascade of apoptotic signals mediated by caspases, Bcl-2, Bid, Bax, Bak, SMAC, and cytochrome c. Heat shock protein 90 (HSP90), part of the chaperone protein system, and the ubiquitin-proteasome system (UPS) regulate function and disposal of intracellular proteins, such as Hif-1, HERs, c-Met, Raf, and Akt. Interruption of these systems, therefore, impacts multiple pathways involved in initiation and maintenance of malignant phenotypes. Agents that target these angiogenic nodes are indicated in boxes.

17-allylamino-geldanamycin (17-AAG), a geldanamycin analog, is the first of its class to enter clinical trials. 17-AAG inhibits HSP90-dependent conformational folding and promotes degradation of oncoproteins, such as ErbB2, mutant p53, C-Raf, and Bcr-Abl, and has shown antitumor activities in preclinical experiments.¹²⁷ Various dosing schedules were tested in phase 1 trials, which ranged from daily to weekly administration. Hepatotoxicity seen in preclinical studies was confirmed in these trials. In addition, 17-AAG's dose-limiting toxicities included gastrointestinal disturbances, anemia, thrombocytopenia, dehydration, and hyperglycemia. Common side effects included fatigue, anorexia, diarrhea, nausea, and vomiting. The agent was tested in renal cell carcinoma in a phase 2 study, and none of the 20 patients responded to the dosing schedule of administering the drug every 3 weeks at 220 mg/m² twice weekly x2 weeks.¹²⁸ Adverse events included elevated liver enzymes, optic neuritis, dyspnea, fatigue, and gastrointestinal problems. 17-AAG is currently being tested in various solid and hematological malignancies, either as a single agent or in combination therapies. However, clinical trials with 17-AAG failed to show anti-tumor efficacy so far, despite extensive testing. It is felt that second-generation and third-generation inhibitors may be needed to adequately determine whether HSP90 is a valid target for cancer therapy. Other HSP90 inhibitors under clinical evaluation include 17-DMAG, IPI-504, and KOS-953 (Table 11). Disruption of HSP-client protein complex can similarly be achieved by the nonhistone effects of HDAC inhibitors (see discussion of Histone Deacetylase Inhibitors under Epigenetic Modulators).

	Ubiquitin–Proteasome System

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

Bortezomib (Velcade, PS-341), a boronic acid derivative, was the first proteosome inhibitor to be developed successfully. It blocks proliferation and induces apoptosis of plasma cells independent of their sensitivity to chemotherapy.¹³⁰ The agent also inhibits the proteasomal degradation of IkB and down-regulates the antiapoptotic NF-B, leading to the reversal of chemotherapy resistance or enhancement of chemotherapy sensitivity. Bortezomib was recently approved by the FDA for initial treatment of patients with multiple myeloma. The agent was previously approved for multiple myeloma patients who failed at least one prior therapy. In the pivotal, multicenter, open-label trial, 682 previously untreated multiple myeloma patients, who were ineligible for high-dose therapy plus stem-cell transplantation, were randomized to receive melphalan and prednisone combination alone (control group) or with bortezomib.¹³¹ Time to progression, the primary endpoint, was significantly longer for the bortezomib group (24.0 months vs 16.6 months, respectively; HR, 0.48; P<0.001). The overall survival and response rates were also superior in the bortezomib group. The more common toxicities in the bortezomib group included peripheral sensory neuropathy, gastrointestinal symptoms, and herpes zoster.

The proteosome inhibitor also received FDA approval for treatment of mantle cell lymphoma patients who had received at least one prior therapy.¹³² A total of 155 patients who failed one prior therapy received bortezomib in a phase 2 open label study. The overall response rate was 31%, and the median duration of response was 9.3 months. Responses were assessed according to 1999 International Workshop Response Criteria. The toxicity profile was similar to those observed in other bortezomib studies. The most commonly reported adverse events included asthenia, peripheral neuropathy, gastrointestinal complaints, and anorexia.

Other proteasome inhibitors in early phase trials include CEP-18,770, RP-171, and NPI-0052 (Table 12).

	Direct Apoptosis Enhancers

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

    TNF-related Apoptosis-inducing Ligand (TRAIL)
TRAIL is a member of the TNF superfamily that induces apoptosis and exerts antitumor activity against a variety of cancer cell lines and tumor xenograft models.¹³⁵ The preferential expression of death receptors on cancer cells and the relative inactivity of soluble TRAIL in normal, healthy tissues make the TRAIL-DRs axis an attractive target for highly selective cancer therapy.¹³⁶ DR4 and DR5 (or TRAIL-R1 and TRAIL-R2) are the main agonistic receptors for TRAIL. DR1, DR2, and osteoprotegerin are decoy receptors and may theoretically compromise the clinical efficacy of TRAIL-like agents by antagonizing apoptotic signals from TRAIL.

Early development of TNF- and recombinant Fas ligand as anticancer agents was fraught with severe liver toxicity, although this toxicity was later found to be related to manufacturing artifacts.¹³⁷ Several recombinant forms of TRAIL with alternate chemical structures have since been developed, such as Apo2L/TRAIL (Table 13).¹³⁸ Several DR4-specific and DR5-specific, agonistic MoAbs are entering clinical testing. With no de novo decoy receptors, they are theoretically more advantageous than TRAIL-like ligands, although liver toxicity remains a concern.

    Survivin
Survivin, encoded by Birc5, is an inhibitor of apoptosis that is undetectable in normal, fully differentiated tissues but is highly expressed in a broad spectrum of neoplasms.¹⁴⁰ Increased expression of survivin is a risk factor for cancer progression, recurrence, and poor prognosis.¹⁴¹ In vitro down-regulation of survivin expression by an antisense approach increases spontaneous apoptosis and inhibits cancer cell proliferation.¹⁴² LY2181308 is an antisense molecule currently in phase 1 testing. YM-155 and EM-1421 are small-molecule inhibitors of Birc5 transcription. The most common treatment-related toxicities for YM-155 in a phase 1 trial were mucosal inflammation and elevated prothrombin time. This compound is currently being tested in melanoma and prostate cancer.

The Bcl-2 gene is a proto-oncogene first identified in follicular B-cell lymphoma with antiapoptotic properties when transformed.¹⁴³ Up-regulation of Bcl-2 is a common cause of tumorigenesis and chemotherapy resistance in multiple tumor types and correlates with poor survival and disease progression. Reduced Bcl-2 expression by antisense oligonucleotide in preclinical studies exerts cytotoxic and cytostatic effects on leukemia and lymphoma cells and is enhanced when combined with other chemotherapeutic agents. Oblimersen, a Bcl-2 antisense oligonucleotide, has demonstrated encouraging preclinical activity against melanoma and breast cancer cell lines but has not achieved statistically significant survival-time improvement during its phase 3 evaluation for treating melanoma and chronic lymphocytic leukemia.^144,145

Several approaches exist to target the antiapoptotic Bcl-2. Firstly, members of the antiapoptotic Bcl-2 family (Bcl-2, Bcl-XL, A1, Mcl-1) share several Bcl-2 homology domains (BH1 to BH4).¹⁴⁶ Of these, BH-3 has been a target of interest. BH-3 domain-specific peptides are being developed by mimicking endogeneous Bcl-2 antagonists. These peptides are thought to mitigate the anti-apoptotic function of Bcl-2 proteins. Secondly, BH-3 mimetic SAHB (stabilized alpha helix of Bcl-2 domains) is also under development, albeit pre-clinical, which overcomes the in vivo instability of endogenous BH-3 targeting peptides.¹⁴⁷ It is more difficult to identify small-molecule inhibitors with high affinity for this target because the candidate molecule has to interrupt protein-protein interaction, which is more challenging compared with inhibiting the enzymatic activity of tyrosine and serine/threonine kinases. ABT-737 inhibits Bcl-2, Bcl-XL, and Bcl-W in preclinical studies but suffers from limited bioavailablity.¹³⁶ ABT-263 is a second-generation molecule with preclinical antitumor effects based on ABT-737 and is currently in phase 1 testing.¹⁴⁸ Obatoclax (GX015-070) is a pan-Bcl inhibitor with selectivity for Mcl-1 that is currently in early phase clinical testing.¹⁴⁹

	PARP Inhibitors: Therapeutic Sensitizers

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 
Poly(ADP-ribose) polymerase (PARP) acts as a cellular sensor for DNA breaks and facilitates DNA repair by engaging mechanisms such as base excision repair (BER), homologous recombination repair, and nonhomologous end-joining pathways.¹⁵⁰ PARP-1 is the principal isoform involved in the cellular process, whereas PARP-2 and other PARP members constitute the residual activities (10%) when PARP-1 is inhibited. Enhanced PARP activity results in depletion of NAD⁺and energy stores, leading to cell necrosis. Multiple mechanisms exist to counter this homeostatic state.

Increased PARP activity confers tumor-cell resistance to DNA-damaging agents, such as platinums, alkylators, topoisomerase inhibitors, and radiation treatments. In fact, PARP inhibitors potentiate their antitumor effects in in vitro and in vivo models. PARP inhibitors developed so far are competitive inhibitors of NAD⁺, and early molecules lack specificity and potency, whereas newer generation PARP inhibitors have a more optimal chemical structure.¹⁵¹ PARP inhibitors are speculated to be better suited as single-agent therapies in tumors with specific DNA-repair defects, such as BRCA mutations, and in combination with therapeutics that target DNA-repair pathways in wild-type tumors.¹⁵²

AG-014699 was the first PARP inhibitor to enter clinical trial. Preclinical studies have shown that AG-014699 causes growth inhibition when it is combined with irinotecan and radiation and causes tumor shrinkage with temozolomide in tumor xenograft models.¹⁵³ AG-014699 was administered with temozolomide to 27 patients with solid tumors in a phase 1 dose-escalation study, and no dose-limiting toxicity was observed up to a dose of 12 mg/m² AG-014699 and a dose of 200 mg/m² temozolomide.¹⁵⁴ This combination at the mentioned dosages was then tested in a phase 2 metastatic malignant melanoma trial.¹⁵⁵ However, increased temozolomide-related myelosuppression and one toxic death were observed at this dose level, and the temozolomide dose was reduced to 150 mg/m² in 12 of 40 patients. Partial response was seen in 7 (18%) patients.

PARP inhibitors play a potential role in enhancing temazolomide's efficacy in the treatment of intracranial neoplasm. Temozolomide is an oral alkylating agent with a high penetration of the central nervous system and is approved by the FDA for treatment of malignant glioma. PARP inhibitors disrupt the BER, an important mediator of temozolomide resistance, and enhance the efficacy of the alkylator. In vivo studies have shown that PARP inhibitors significantly enhance temazolomide's antitumor effects against intracranial neoplasms, such as glioma, lymphoma and melanoma.¹⁵⁶ BSI-201 is being developed clinically with temozolomide and radiation therapy for the treatment of newly diagnosed malignant glioma. Other PARP inhibitors in clinical development include INO-1001, KU-59,436, and ABT-888 (See Table 14).

	Mitotic Kinase Inhibitors

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

Members of the Aurora kinase family are serine/threonine kinases that are highly conserved throughout eukaryotic evolution and exist in three structurally related homologs in mammalian cells, Aurora A, Aurora B, and Aurora C.¹⁵⁷ These key mitotic regulators have distinct subcellular localization and functions. Aurora A localizes to centrosomes from the S phase to mitosis exit and primarily regulates centrosome function and mitotic spindle formation. Aurora B is part of the chromosomal passenger protein complex, localizing to centrosomes and mitotic spindles during different phases of mitosis. The kinase functions to ensure accurate chromosomal separation and cytokinesis. Aurora C is specifically expressed in testes and is involved in spermatogenesis. Its role in tumorigenesis is unclear.

Aurora A received initial attention as a cancer target after it was found to be amplified or overexpressed in many tumor types and shown to be oncogenic by inducing genomic instability and amplification of centrosomes.¹⁵⁸ Knockdown and mutational studies that disrupt Aurora A function induce mitotic arrest and apoptosis, making Aurora A a prime therapeutic target. Ideally, molecules that inhibit only Aurora A should result in the formation of monopolar mitotic spindles and mitotic arrest followed by rapid induction of apoptosis. Cells treated with inhibitors specific to Aurora B typically go through a cell cycle without dividing (cytokinestasis) and become polyploid, abnormally large, and multinucleated.

However, selection of small-molecule inhibitors specific to Aurora A or B is challenging. Cell-based assays with most of the molecules developed so far have shown formation of bipolar spindles and cells that undergo apoptosis only several days after treatment, which is consistent with the phenotype observed in Aurora B-only inhibition.¹⁵⁹ It was later shown that Aurora B must be present and active to achieve the expected phenotype of Aurora A inhibition. Thus, dual inhibition of Aurora A and B will result in the phenotype of Aurora B-only inhibition.¹⁶⁰ As one can see, the definition of Aurora A or B inhibitors is complicated and may impair the study of target effects during clinical testing.

AZD-1152, a pyrazoloquinazoline derivative, is a highly potent and selective inhibitor of Aurora B.¹⁶¹ This molecule induces chromosome misalignment, halts cell division, and induces apoptosis in in vitro studies. AZD-1152 inhibits proliferation in leukemic cells, and in colon and lung xenografts. The agent was administered as a 2-hour intravenous infusion at a weekly interval, and dose-limiting toxicity of grade 4 neutropenia occurred at a dose level of 450 mg. The 300-mg dose was declared the tolerable dose.¹⁶² Five of 13 (38%) patients remained on therapy for more than 12 weeks, and clinical development continued.

MK-0457 (VX-680) is a 4,6-diaminopyrimidine that targets the ATP-binding site common to all Aurora kinases, delays in vitro mitotic progression, and inhibits growth in tumor xenograft models.¹⁶³ Sixteen patients with refractory solid tumors were enrolled on a phase 1 trial to receive MK-0457 by constant 5-day intravenous infusion every 28 days at 0.5 mg/m² to 12 mg/m² per hour.¹⁶⁴ The dose-limiting toxicity was asymptomatic neutropenia for more than 5 days at 12 mg/m² per hour. Three patients achieved stable disease. Synergism between MK-0457 and other tubulin-interrupting agents, such as docetaxel, is being examined. Other Aurora kinase inhibitors in early phase clinical testing are included in Table 15.

	Conclusion

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 
Advances in molecular biology have led to an explosion in the number of potential cancer targets and a rich pipeline of anticancer drugs. It is not our intent to provide an exhaustive account of every cancer drug in early phase trials. Instead, we hope to have provided a broad introduction to the classes of chemotherapeutic agents that are likely to enter clinical practice in the near future. With time, we can expect more drugs against targets that have already been validated clinically, such as HER and VEGF, and the emergence of novel agents against new classes of targets that have been validated preclinically such as Notch, Hedgehog, and JAK-STAT. Novel and innovative clinic trial designs will be needed to efficiently test all of these agents.

	References

Top Abstract Introduction Strategies of Intervention Cellular Signal Transduction... Tumor Vasculature Epigenetic Modulators Integrins: Targeting the... Heat Shock Protein: Molecular... Ubiquitin-Proteasome System Direct Apoptosis Enhancers PARP Inhibitors: Therapeutic... Mitotic Kinase Inhibitors Conclusion References

 

1. Adjei AA, Hidalgo M. Intracellular signal transduction pathway proteins as targets for cancer therapy. J Clin Oncol 2005;23:5386–5403.[Abstract/Free Full Text]
2. Cunningham D, Humblet Y, Siena S, et al. Cetuximab monotherapy and cetuximab plus irinotecan in irinotecan-refractory metastatic colorectal cancer. N Engl J Med 2004;351:337–345.[Abstract/Free Full Text]
3. Moore MJ, Goldstein D, Hamm J, et al. Erlotinib plus gemcitabine compared with gemcitabine alone in patients with advanced pancreatic cancer: a phase III trial of the National Cancer Institute of Canada Clinical Trials Group. J Clin Oncol 2007;25:1960–1966.[Abstract/Free Full Text]
4. Shepherd FA, Rodrigues Pereira J, Ciuleanu T, et al. Erlotinib in previously treated non-small-cell lung cancer. N Engl J Med 2005;353:123–132.[Abstract/Free Full Text]
5. Hurwitz H, Fehrenbacher L, Novotny W, et al. Bevacizumab plus irinotecan, fluorouracil, and leucovorin for metastatic colorectal cancer. N Engl J Med 2004;350:2335–2342.[Abstract/Free Full Text]
6. Rosenberg SA, Yang JC, Restifo NP. Cancer immunotherapy: moving beyond current vaccines. Nat Med 2004;10:909–915.[CrossRef][Medline]
7. Kushner DM, Silverman RH. Antisense cancer therapy: the state of the science. Curr Oncol Rep 2000;2:23–30.[CrossRef][Medline]
8. Gewirtz AM. On future's doorstep: RNA interference and the pharmacopeia of tomorrow. J Clin Invest 2007;117:3612–3614.[CrossRef][Medline]
9. Iannello A, Ahmad A. Role of antibody-dependent cell-mediated cytotoxicity in the efficacy of therapeutic anti-cancer monoclonal antibodies. Cancer Metastasis Rev 2005;24:487–499.[CrossRef][Medline]
10. Weiner GJ. Monoclonal antibody mechanisms of action in cancer. Immunol Res 2007;39:271–278.[CrossRef][Medline]
11. Sawyer TK. Novel oncogenic protein kinase inhibitors for cancer therapy. Curr Med Chem Anticancer Agents 2004;4:449–455.[CrossRef][Medline]
12. Wilhelm S, Chien DS. BAY 43-9006: preclinical data. Curr Pharm Des 2002;8:2255–2257.[CrossRef][Medline]
13. Carpenter G, Cohen S. Epidermal growth factor. J Biol Chem 1990;265:7709–7712.[Free Full Text]
14. Pirker R, Szczesna A, Von Pawel J, et al. FLEX: A randomized, multicenter, phase III study of cetuximab in combination with cisplatin/vinorelbine (CV) versus CV alone in the first-line treatment of patients with advanced non-small cell lung cancer (NSCLC) [abstract]. J Clin Oncol 2008;26(suppl):8015.
15. Yarden Y, Sliwkowski MX. Untangling the ErbB signalling network. Nat Rev Mol Cell Biol 2001;2:127–137.[CrossRef][Medline]
16. Harari PM, Allen GW, Bonner JA. Biology of interactions: antiepidermal growth factor receptor agents. J Clin Oncol 2007;25:4057–4065.[Abstract/Free Full Text]
17. Carteni G, Fiorentino R, Vecchione L, Chiurazzi B, Battista C. Panitumumab a novel drug in cancer treatment. Ann Oncol 2007;18(suppl 6):16–21.[CrossRef]
18. Mendelsohn J, Baselga J. Epidermal growth factor receptor targeting in cancer. Semin Oncol 2006;33:369–385.[CrossRef][Medline]
19. Hudis CA. Trastuzumab–mechanism of action and use in clinical practice. N Engl J Med 2007;357:39–51.[Free Full Text]
20. Moy B, Kirkpatrick P, Kar S, Goss P. Lapatinib. Nat Rev Drug Discov 2007;6:431–432.[CrossRef][Medline]
21. Birchmeier C, Birchmeier W, Gherardi E, Vande Woude GF. Met, metastasis, motility and more. Nat Rev Mol Cell Biol 2003;4:915–925.[CrossRef][Medline]
22. Rosario M, Birchmeier W. How to make tubes: signaling by the Met receptor tyrosine kinase. Trends Cell Biol 2003;13:328–335.[CrossRef][Medline]
23. Yu J, Miehlke S, Ebert MP, et al. Frequency of TPR-MET rearrangement in patients with gastric carcinoma and in first-degree relatives. Cancer 2000;88:1801–1806.[CrossRef][Medline]
24. Dharmawardana PG, Giubellino A, Bottaro DP. Hereditary papillary renal carcinoma type I. Curr Mol Med 2004;4:855–868.[CrossRef][Medline]
25. Graveel C, Su Y, Koeman J, et al. Activating Met mutations produce unique tumor profiles in mice with selective duplication of the mutant allele. Proc Natl Acad Sci U S A 2004;101:17198–17203.[Abstract/Free Full Text]
26. Burgess T, Coxon A, Meyer S, et al. Fully human monoclonal antibodies to hepatocyte growth factor with therapeutic potential against hepatocyte growth factor/c-Met-dependent human tumors. Cancer Res 2006;66:1721–1729.[Abstract/Free Full Text]
27. Gordon MS, Mendelson D, Sweeney C, et al. Interim results from a first-in-human study with AMG102, a fully human monoclonal antibody that neutralizes hepatocyte growth factor (HGF), the ligand to c-Met receptor, in patients (pts) with advanced solid tumors [abstract]. J Clin Oncol 2007;18S(suppl):3551.
28. Eder JP, Heath E, Appleman L, et al. Phase I experience with c-MET inhibitor XL880 administered orally to patients (pts) with solid tumors [abstract]. J Clin Oncol 2007;18S(suppl):3526.
29. Garcia AA, Rosen L, Cunningham CC, et al. Phase 1 study of ARQ 197, a selective inhibitor of the c-Met RTK in patients with metastatic solid tumors reaches recommended phase 2 dose [abstract]. J Clin Oncol 2007;18S(suppl):3525.
30. Sachdev D, Yee D. Disrupting insulin-like growth factor signaling as a potential cancer therapy. Mol Cancer Ther 2007;6:1–12.[Abstract/Free Full Text]
31. Pollak M. Insulin-like growth factor-related signaling and cancer development. Recent Results Cancer Res 2007;174:49–53[CrossRef][Medline]
32. Sciacca L, Costantino A, Pandini G, et al. Insulin receptor activation by IGF-II in breast cancers: evidence for a new autocrine/paracrine mechanism. Oncogene 1999;18:2471–2479.[CrossRef][Medline]
33. Cohen BD, Baker DA, Soderstrom C, et al. Combination therapy enhances the inhibition of tumor growth with the fully human anti-type 1 insulin-like growth factor receptor monoclonal antibody CP-751,871. Clin Cancer Res 2005;11:2063–2073.[Abstract/Free Full Text]
34. Haluska P, Shaw HM, Batzel GN, et al. Phase I dose escalation study of the anti insulin-like growth factor-I receptor monoclonal antibody CP-751,871 in patients with refractory solid tumors. Clin Cancer Res 2007;13:5834–5840.[Abstract/Free Full Text]
35. de Bono JS, Attard G, Adjei A, et al. Potential applications for circulating tumor cells expressing the insulin-like growth factor-I receptor. Clin Cancer Res 2007;13:3611–3616.[Abstract/Free Full Text]
36. Karp DD, Paz-Ares LG, Novello S, et al. High activity of the anti-IGF-IR antibody CP-751,871 in combination with paclitaxel and carboplatin in squamous NSCLC [abstract]. J Clin Oncol. 2008;26(suppl):8015.
37. Beltran PJ, Mitchell P, Moody G, et al. AMG-479, a fully human anti-IGF-1 receptor antibody, inhibits PI3K/Akt signaling and exerts potent antitumor effects in combination with EGF-R inhibitors in pancreatic xenograft models [abstract]. 2007 Gastrointestinal Cancers Symposium. Abstract208. Available at http://www.asco.org/ASCO/Abstracts+%26+Virtual+Meeting/Abstracts?&vmview=abst_detail_view&confID=45&abstractID=10440.
38. Tolcher A, Rothenberg M, Rodon J, et al. A phase I pharmacokinetic and pharmacodynamic study of AMG 479, a fully human monoclonal antibody against insulin-like growth factor type 1 receptor (IGF-1R), in advanced solid tumors [abstract]. J Clin Oncol 2007;18S(suppl):3002.
39. Haskell MD, Slack JK, Parsons JT, Parsons SJ. c-Src tyrosine phosphorylation of epidermal growth factor receptor, P190 RhoGAP, and focal adhesion kinase regulates diverse cellular processes. Chem Rev 2001;101:2425–2440.[CrossRef][Medline]
40. Frame MC. Src in cancer: deregulation and consequences for cell behaviour. Biochim Biophys Acta 2002;1602:114–130.[Medline]
41. Laird AD, Li G, Moss KG, et al. Src family kinase activity is required for signal tranducer and activator of transcription 3 and focal adhesion kinase phosphorylation and vascular endothelial growth factor signaling in vivo and for anchorage-dependent and -independent growth of human tumor cells. Mol Cancer Ther 2003;2:461–469.[Abstract/Free Full Text]
42. Burgess MR, Skaggs BJ, Shah NP, Lee FY, Sawyers CL. Comparative analysis of 2 clinically active BCR-ABL kinase inhibitors reveals the role of conformation-specific binding in resistance. Proc Natl Acad Sci U S A 2005;102:3395–3400.[Abstract/Free Full Text]
43. Olivieri A, Manzione L. Dasatinib: a new step in molecular target therapy. Ann Oncol 2007;18(suppl 6):42–46.
44. FDA approves dasatinib (Sprycel) for use in the treatment of adults with chronic phase, accelerated phase, or myeloid orlymphoid blast phase chronic myeloid leukemia. Available at: http://www.fda.gov/Cder/Offices/OODP/whatsnew/dasatinib.htm. Accessed Oct 3, 2008.
45. Granville CA, Memmott RM, Gills JJ, Dennis PA. Handicapping the race to develop inhibitors of the phosphoinositide 3-kinase/Akt/mammalian target of rapamycin pathway. Clin Cancer Res 2006;12:679–689.[Abstract/Free Full Text]
46. Ma WW, Hidalgo M. Exploiting novel molecular targets in gastrointestinal cancers. World J Gastroenterol 2007;13:5845–5856.[Medline]
47. Jimeno A, Tan AC, Coffa J, et al. Coordinated epidermal growth factor receptor pathway gene overexpression predicts epidermal growth factor receptor inhibitor sensitivity in pancreatic cancer. Cancer Res 2008;68:2841–2849.[Abstract/Free Full Text]
48. Sabatini DM. mTOR and cancer: insights into a complex relationship. Nat Rev Cancer 2006;6:729–734.[CrossRef][Medline]
49. Sarbassov DD, Ali SM, Sengupta S, et al. Prolonged rapamycin treatment inhibits mTORC2 assembly and Akt/PKB. Mol Cell 2006;22:159–168.[CrossRef][Medline]
50. Ma WW, Jimeno A. Temsirolimus. Drugs Today (Barc). 2007;43:659–669.[CrossRef][Medline]
51. Hudes G, Carducci M, Tomczak P, et al. Temsirolimus, interferon alfa, or both for advanced renal-cell carcinoma. N Engl J Med 2007;356:2271–2281.[Abstract/Free Full Text]
52. O'Donnell A, Faivre S, Judson I, et al. A phase I study of the oral mTOR inhibitor RAD001 as monotherapy to identify the optimal biologically effective dose using toxicity, pharmacokinetic (PK) and pharmacodynamic (PD) endpoints in patients with solid tumours [abstract]. Proc Am Soc Clin Oncol 2003;22:803.
53. Yee KW, Zeng Z, Konopleva M, et al. Phase I/II study of the mammalian target of rapamycin inhibitor everolimus (RAD001) in patients with relapsed or refractory hematologic malignancies. Clin Cancer Res 2006;12:5165–5173.[Abstract/Free Full Text]
54. Fouladi M, Laningham F, Wu J, et al. Phase I study of everolimus in pediatric patients with refractory solid tumors. J Clin Oncol 2007;25:4806–4812.[Abstract/Free Full Text]
55. Motzer RJ, Escudier B, Oudard S, et al. Efficacy of everolimus in advanced renal cell carcinoma: a double-blind, randomised, placebo-controlled phase III trial. Lancet 2008;372:449–456.[CrossRef][Medline]
56. Mita MM, Mita AC, Chu QS, et al. Phase I trial of the novel mammalian target of rapamycin inhibitor deforolimus (AP23573; MK-8669) administered intravenously daily for 5 days every 2 weeks to patients with advanced malignancies. J Clin Oncol 2008;26:361–367.[Abstract/Free Full Text]
57. Plas DR, Thompson CB. Akt-dependent transformation: there is more to growth than just surviving. Oncogene 2005;24:7435–7442.[CrossRef][Medline]
58. Kondapaka SB, Singh SS, Dasmahapatra GP, Sausville EA, Roy KK. Perifosine, a novel alkylphospholipid, inhibits protein kinase B activation. Mol Cancer Ther 2003;2:1093–1103.[Abstract/Free Full Text]
59. Crul M, Rosing H, de Klerk GJ, et al. Phase I and pharmacological study of daily oral administration of perifosine (D-21266) in patients with advanced solid tumours. Eur J Cancer 2002;38:1615–1621.[CrossRef][Medline]
60. Van Ummersen L, Binger K, Volkman J, et al. A phase I trial of perifosine (NSC 639966) on a loading dose/maintenance dose schedule in patients with advanced cancer. Clin Cancer Res 2004;10:7450–7456.[Abstract/Free Full Text]
61. Sausville EA, Arbuck SG, Messmann R, et al. Phase I trial of 72-hour continuous infusion UCN-01 in patients with refractory neoplasms. J Clin Oncol 2001;19:2319–2333.[Abstract/Free Full Text]
62. Dees EC, Baker SD, O'Reilly S, et al. A phase I and pharmacokinetic study of short infusions of UCN-01 in patients with refractory solid tumors. Clin Cancer Res 2005;11:664–671.[Abstract/Free Full Text]
63. Hahn WC, Weinberg RA. Modelling the molecular circuitry of cancer. Nat Rev Cancer 2002;2:331–341.[CrossRef][Medline]
64. Sebolt-Leopold JS, Herrera R. Targeting the mitogen-activated protein kinase cascade to treat cancer. Nat Rev Cancer 2004;4:937–947.[CrossRef][Medline]
65. Rajalingam K, Schreck R, Rapp UR, Albert S. Ras oncogenes and their downstream targets. Biochim Biophys Acta 2007;1773:1177–1195.[Medline]
66. Zhu K, Hamilton AD, Sebti SM. Farnesyltransferase inhibitors as anticancer agents: current status. Curr Opin Investig Drugs. 2003;4:1428–1435.[Medline]
67. Van Cutsem E, van de Velde H, Karasek P, et al. Phase III trial of gemcitabine plus tipifarnib compared with gemcitabine plus placebo in advanced pancreatic cancer. J Clin Oncol 2004;22:1430–1438.[Abstract/Free Full Text]
68. End DW, Smets G, Todd AV, et al. Characterization of the antitumor effects of the selective farnesyl protein transferase inhibitor R115777 in vivo and in vitro. Cancer Res 2001;61:131–137.[Abstract/Free Full Text]
69. Davies H, Bignell GR, Cox C, et al. Mutations of the BRAF gene in human cancer. Nature 2002;417:949–954.[CrossRef][Medline]
70. Wilhelm SM, Carter C, Tang L, et al. BAY 43-9006 exhibits broad spectrum oral antitumor activity and targets the RAF/MEK/ERK pathway and receptor tyrosine kinases involved in tumor progression and angiogenesis. Cancer Res 2004;64:7099–7109.[Abstract/Free Full Text]
71. Strumberg D, Richly H, Hilger RA, et al. Phase I clinical and pharmacokinetic study of the Novel Raf kinase and vascular endothelial growth factor receptor inhibitor BAY 43-9006 in patients with advanced refractory solid tumors. J Clin Oncol 2005;23:965–972.[Abstract/Free Full Text]
72. Abou-Alfa GK, Schwartz L, Ricci S, et al. Phase II study of sorafenib in patients with advanced hepatocellular carcinoma. J Clin Oncol 2006;24:4293–4300.[Abstract/Free Full Text]
73. Escudier B, Eisen T, Stadler WM, et al. Sorafenib in advanced clear-cell renal-cell carcinoma. N Engl J Med 2007;356:125–134.[Abstract/Free Full Text]
74. Ratain MJ, Eisen T, Stadler WM, et al. Phase II placebo-controlled randomized discontinuation trial of sorafenib in patients with metastatic renal cell carcinoma. J Clin Oncol 2006;24:2505–2512.[Abstract/Free Full Text]
75. Llovet JM, Ricci S, Mazzaferro V, et al. Sorafenib in advanced hepatocellular carcinoma. N Engl J Med 2008;359:378–390.[Abstract/Free Full Text]
76. Ohren JF, Chen H, Pavlovsky A, et al. Structures of human MAP kinase kinase 1 (MEK1) and MEK2 describe novel noncompetitive kinase inhibition. Nat Struct Mol Biol 2004;11:1192–1197.[CrossRef][Medline]
77. Sebolt-Leopold JS, Dudley DT, Herrera R, et al. Blockade of the MAP kinase pathway suppresses growth of colon tumors in vivo. Nat Med 1999;5:810–816.[CrossRef][Medline]
78. Allen LF, Sebolt-Leopold J, Meyer MB. CI-1040 (PD184352), a targeted signal transduction inhibitor of MEK (MAPKK). Semin Oncol 2003;30:105–116.[CrossRef][Medline]
79. Rinehart J, Adjei AA, Lorusso PM, et al. Multicenter phase II study of the oral MEK inhibitor, CI-1040, in patients with advanced non-small-cell lung, breast, colon, and pancreatic cancer J Clin Oncol 2004;22:4456–4462.[Abstract/Free Full Text]
80. Messersmith WA, Hidalgo M, Carducci M, Eckhardt SG. Novel targets in solid tumors: MEK inhibitors. Clin Adv Hematol Oncol 2006;4:831–836.[Medline]
81. Yeh TC, Marsh V, Bernat BA, et al. Biological characterization of ARRY-142886 (AZD6244), a potent, highly selective mitogen-activated protein kinase kinase 1/2 inhibitor. Clin Cancer Res 2007;13:1576–1583.[Abstract/Free Full Text]
82. Chow L, Eckhardt S, Reid A. A first in human dose-ranging study to assess the pharmacokinetic, pharmacodynamics, and toxicities of the MEK inhibitor, ARRY-142886 (AZD6244) in patients with advanced solid malignancies. Proc AACR-EORTC-NCI Targeted Therapy Conference 2005:C162.
83. Folkman J. Seminars in Medicine of the Beth Israel Hospital, Boston Clinical applications of research on angiogenesis. N Engl J Med 1995;333:1757–1763. Comments in: N Engl J Med 1996;334:920-1; author reply, N Engl J Med 1996;334:921; N Engl J Med 1996;334:921.[Free Full Text]
84. Folkman J. Tumor angiogenesis: therapeutic implications. N Engl J Med 1971;285:1182–1186.[Medline]
85. Motzer RJ, Hutson TE, Tomczak P, et al. Sunitinib versus interferon alfa in metastatic renal-cell carcinoma. N Engl J Med 2007;356:115–124.[Abstract/Free Full Text]
86. Hicklin DJ, Ellis LM. Role of the vascular endothelial growth factor pathway in tumor growth and angiogenesis. J Clin Oncol 2005;23:1011–1027.[Abstract/Free Full Text]
87. Maynard MA, Ohh M. The role of hypoxia-inducible factors in cancer. Cell Mol Life Sci 2007;64:2170–2180.[CrossRef][Medline]
88. Melillo G. Inhibiting hypoxia-inducible factor 1 for cancer therapy. Mol Cancer Res 2006;4:601–605.[Abstract/Free Full Text]
89. Liang D, Kong X, Sang N. Effects of histone deacetylase inhibitors on HIF-1. Cell Cycle 2006;5:2430–2435.[Medline]
90. Hinnen P, Eskens FA. Vascular disrupting agents in clinical development. Br J Cancer 2007;96:1159–1165.[CrossRef][Medline]
91. Tozer GM, Kanthou C, Baguley BC. Disrupting tumour blood vessels. Nat Rev Cancer 2005;5:423–435.[CrossRef][Medline]
92. Jain RK, Baxter LT. Mechanisms of heterogeneous distribution of monoclonal antibodies and other macromolecules in tumors: significance of elevated interstitial pressure. Cancer Res 1988;48:7022–7032.[Abstract/Free Full Text]
93. Konerding MA, Fait E, Gaumann A. 3D microvascular architecture of pre-cancerous lesions and invasive carcinomas of the colon. Br J Cancer 2001;84:1354–1362.[CrossRef][Medline]
94. Denekamp J. Endothelial cell proliferation as a novel approach to targeting tumour therapy. Br J Cancer 1982;45:136–139.[Medline]
95. Holwell SE, Cooper PA, Thompson MJ, et al. Anti-tumor and anti-vascular effects of the novel tubulin-binding agent combretastatin A-1 phosphate. Anticancer Res 2002;22:3933–3940.[Medline]
96. Tozer GM, Prise VE, Wilson J, et al. Combretastatin A-4 phosphate as a tumor vascular-targeting agent: early effects in tumors and normal tissues. Cancer Res 1999;59:1626–1634.[Abstract/Free Full Text]
97. Schoffski P, Thate B, Beutel G, et al. Phase I and pharmacokinetic study of TZT-1027, a novel synthetic dolastatin 10 derivative, administered as a 1-hour intravenous infusion every 3 weeks in patients with advanced refractory cancer. Ann Oncol 2004;15:671–679.[Abstract/Free Full Text]
98. de Jonge MJ, van der Gaast A, Planting AS, et al. Phase I and pharmacokinetic study of the dolastatin 10 analogue TZT-1027, given on days 1 and 8 of a 3-week cycle in patients with advanced solid tumors. Clin Cancer Res 2005;11:3806–3813.[Abstract/Free Full Text]
99. Ching LM, Cao Z, Kieda C, Zwain S, Jameson MB, Baguley BC. Induction of endothelial cell apoptosis by the antivascular agent 5,6-Dimethylxanthenone-4-acetic acid. Br J Cancer 2002;86:1937–1942.[CrossRef][Medline]
100. Rustin GJ, Bradley C, Galbraith S, et al. 5,6-dimethylxanthenone-4-acetic acid (DMXAA), a novel antivascular agent: phase I clinical and pharmacokinetic study. Br J Cancer 2003;88:1160–1167.[CrossRef][Medline]
101. Jameson MB, Thompson PI, Baguley BC, et al. Clinical aspects of a phase I trial of 5,6-dimethylxanthenone-4-acetic acid (DMXAA), a novel antivascular agent. Br J Cancer 2003;88:1844–1850.[CrossRef][Medline]
102. McKeage MJ, Fong P, Jeffery M, et al. 5,6-Dimethylxanthenone-4-acetic acid in the treatment of refractory tumors: a phase I safety study of a vascular disrupting agent. Clin Cancer Res 2006;12:1776–1784.[Abstract/Free Full Text]
103. Fraga MF, Ballestar E, Paz MF, et al. Epigenetic differences arise during the lifetime of monozygotic twins. Proc Natl Acad Sci U S A 2005;102:10604–10609.[Abstract/Free Full Text]
104. Feinberg AP. Epigenetics at the epicenter of modern medicine. JAMA 2008;299:1345–1350.[Abstract/Free Full Text]
105. Mack GS. Epigenetic cancer therapy makes headway. J Natl Cancer Inst 2006;98:1443–1444.[Free Full Text]
106. Robertson KD, Ait-Si-Ali S, Yokochi T, Wade PA, Jones PL, Wolffe AP. DNMT1 forms a complex with Rb, E2F1 and HDAC1 and represses transcription from E2F-responsive promoters Nat Genet 2000;25:338–342.[CrossRef][Medline]
107. Rountree MR, Bachman KE, Baylin SB. DNMT1 binds HDAC2 and a new co-repressor, DMAP1, to form a complex at replication foci Nat Genet 2000;25:269–277.[CrossRef][Medline]
108. Hellebrekers DM, Griffioen AW, van Engeland M. Dual targeting of epigenetic therapy in cancer. Biochim Biophys Acta 2007;1775:76–91.[Medline]
109. Esteller M. Epigenetics in cancer. N Engl J Med 2008;358:1148–1159.[Free Full Text]
110. Silverman LR, Demakos EP, Peterson BL, et al. Randomized controlled trial of azacitidine in patients with the myelodysplastic syndrome: a study of the cancer and leukemia group B. J Clin Oncol 2002;20:2429–2440.[Abstract/Free Full Text]
111. Kantarjian H, Issa JP, Rosenfeld CS, et al. Decitabine improves patient outcomes in myelodysplastic syndromes: results of a phase III randomized study. Cancer 2006;106:1794–1803.[CrossRef][Medline]
112. Villar-Garea A, Esteller M. DNA demethylating agents and chromatin-remodelling drugs: which, how and why? Curr Drug Metab 2003;4:11–31.[Medline]
113. Marquez VE, Kelley JA, Agbaria R, et al. Zebularine: a unique molecule for an epigenetically based strategy in cancer chemotherapy. Ann N Y Acad Sci 2005;1058:246–254.[CrossRef][Medline]
114. Marks P, Rifkind RA, Richon VM, Breslow R, Miller T, Kelly WK. Histone deacetylases and cancer: causes and therapies. Nat Rev Cancer 2001;1:194–202.[CrossRef][Medline]
115. Mehnert JM, Kelly WK. Histone deacetylase inhibitors: biology and mechanism of action. Cancer J 2007;13:23–29.[Medline]
116. Kim TY, Bang YJ, Robertson KD. Histone deacetylase inhibitors for cancer therapy. Epigenetics 2006;1:14–23.[Medline]
117. Mann BS, Johnson JR, Cohen MH, Justice R, Pazdur R. FDA approval summary: vorinostat for treatment of advanced primary cutaneous T-cell lymphoma. Oncologist 2007;12:1247–1252.[Abstract/Free Full Text]
118. Hess-Stumpp H, Bracker TU, Henderson D, Politz O. MS-275, a potent orally available inhibitor of histone deacetylases–the development of an anticancer agent. Int J Biochem Cell Biol 2007;39:1388–1405.[CrossRef][Medline]
119. Jin H, Varner J. Integrins: roles in cancer development and as treatment targets. Br J Cancer 2004;90:561–565.[CrossRef][Medline]
120. Ramakrishnan V, Bhaskar V, Law DA, et al. Preclinical evaluation of an anti-alpha5beta1 integrin antibody as a novel anti-angiogenic agent. J Exp Ther Oncol 2006;5:273–286.[Medline]
121. Yazji S, Bokowski R, Kondagunta V, Figlin R. Final results from phase II study of volociximab, an 5β1 anti-integrin antibody, in refractory or relapsed metastatic clear cell renal cell carcinoma (mCCRCC) [abstract]. J Clin Oncol 2007;18S(suppl):5094.
122. Stupp R, Ruegg C. Integrin inhibitors reaching the clinic. J Clin Oncol 2007;25:1637–1638.[Free Full Text]
123. Hariharan S, Gustafson D, Holden S, et al. Assessment of the biological and pharmacological effects of the alpha nu beta3 and alpha nu beta5 integrin receptor antagonist, cilengitide (EMD 121974), in patients with advanced solid tumors. Ann Oncol 2007;18:1400–1407.[Abstract/Free Full Text]
124. Queitsch C, Sangster TA, Lindquist S. Hsp90 as a capacitor of phenotypic variation. Nature 2002;417:618–624.[CrossRef][Medline]
125. Kitano H. Cancer robustness: tumour tactics. Nature 2003;426:125.[CrossRef][Medline]
126. Neckers L. Heat shock protein 90: the cancer chaperone. J Biosci 2007;32:517–530.[CrossRef][Medline]
127. Ramanathan RK, Trump DL, Eiseman JL, et al. Phase I pharmacokinetic-pharmacodynamic study of 17-(allylamino)-17-demethoxygeldanamycin (17AAG, NSC 330507), a novel inhibitor of heat shock protein 90, in patients with refractory advanced cancers. Clin Cancer Res 2005;11:3385–3391.[Abstract/Free Full Text]
128. Ronnen EA, Kondagunta GV, Ishill N, et al. A phase II trial of 17-(Allylamino)-17-demethoxygeldanamycin in patients with papillary and clear cell renal cell carcinoma. Invest New Drugs 2006;24:543–546.[CrossRef][Medline]
129. Burger AM, Seth AK. The ubiquitin-mediated protein degradation pathway in cancer: therapeutic implications. Eur J Cancer 2004;40:2217–2229.[CrossRef][Medline]
130. Russo A, Fratto ME, Bazan V, et al. Targeting apoptosis in solid tumors: the role of bortezomib from preclinical to clinical evidence. Expert Opin Ther Targets 2007;11:1571–1586.[CrossRef][Medline]
131. San Miguel JF, Schlag R, Khuageva NK, et al. Bortezomib plus melphalan and prednisone for initial treatment of multiple myeloma. N Engl J Med 2008;359:906–917.[Abstract/Free Full Text]
132. Kane RC, Dagher R, Farrell A, et al. Bortezomib for the treatment of mantle cell lymphoma. Clin Cancer Res 2007;13:5291–5294.[Abstract/Free Full Text]
133. Hanahan D, Weinberg RA. The hallmarks of cancer. Cell 2000;100:57–70.[CrossRef][Medline]
134. Klein S, McCormick F, Levitzki A. Killing time for cancer cells. Nat Rev Cancer 2005;5:573–580.[CrossRef][Medline]
135. Carlo-Stella C, Lavazza C, Locatelli A, Vigano L, Gianni AM, Gianni L. Targeting TRAIL agonistic receptors for cancer therapy. Clin Cancer Res 2007;13:2313–2317.[Abstract/Free Full Text]
136. Ziegler DS, Kung AL. Therapeutic targeting of apoptosis pathways in cancer. Curr Opin Oncol 2008;20:97–103.[CrossRef][Medline]
137. Sherman ML, Spriggs DR, Arthur KA, Imamura K, Frei E 3rd, Kufe DW. Recombinant human tumor necrosis factor administered as a 5-day continuous infusion in cancer patients: phase I toxicity and effects on lipid metabolism. J Clin Oncol 1988;6:344–350.[Abstract]
138. Lawrence D, Shahrokh Z, Marsters S, et al. Differential hepatocyte toxicity of recombinant Apo2L/TRAIL versions. Nat Med 2001;7:383–385.[CrossRef][Medline]
139. Tolcher AW, Mita M, Meropol NJ, et al. Phase I pharmacokinetic and biologic correlative study of mapatumumab, a fully human monoclonal antibody with agonist activity to tumor necrosis factor-related apoptosis-inducing ligand receptor-1. J Clin Oncol 2007;25:1390–1395.[Abstract/Free Full Text]
140. Nachmias B, Ashhab Y, Ben-Yehuda D. The inhibitor of apoptosis protein family (IAPs): an emerging therapeutic target in cancer. Semin Cancer Biol 2004;14:231–243.[CrossRef][Medline]
141. Paik S, Shak S, Tang G, et al. A multigene assay to predict recurrence of tamoxifen-treated, node-negative breast cancer. N Engl J Med 2004;351:2817–2826.[Abstract/Free Full Text]
142. Hunter AM, LaCasse EC, Korneluk RG. The inhibitors of apoptosis (IAPs) as cancer targets. Apoptosis 2007;12:1543–1568.[CrossRef][Medline]
143. Fesik SW. Promoting apoptosis as a strategy for cancer drug discovery. Nat Rev Cancer 2005;5:876–885.[CrossRef][Medline]
144. Bedikian AY, Millward M, Pehamberger H, et al. Bcl-2 antisense (oblimersen sodium) plus dacarbazine in patients with advanced melanoma: the Oblimersen Melanoma Study Group. J Clin Oncol 2006;24:4738–4745.[Abstract/Free Full Text]
145. O'Brien S, Moore JO, Boyd TE, et al. Randomized phase III trial of fludarabine plus cyclophosphamide with or without oblimersen sodium (Bcl-2 antisense) in patients with relapsed or refractory chronic lymphocytic leukemia. J Clin Oncol 2007;25:1114–1120.[Abstract/Free Full Text]
146. Labi V, Erlacher M, Kiessling S, Villunger A. BH3-only proteins in cell death initiation, malignant disease and anticancer therapy. Cell Death Differ 2006;13:1325–1338.[CrossRef][Medline]
147. Walensky LD. BCL-2 in the crosshairs: tipping the balance of life and death. Cell Death Differ 2006;13:1339–1350.[CrossRef][Medline]
148. Tse C, Shoemaker AR, Adickes J, et al. ABT-263: a potent and orally bioavailable Bcl-2 family inhibitor. Cancer Res 2008;68:3421–3428.[Abstract/Free Full Text]
149. Trudel S, Li ZH, Rauw J, Tiedemann RE, Wen XY, Stewart AK. Preclinical studies of the pan-Bcl inhibitor obatoclax (GX015–070) in multiple myeloma. Blood 2007;109:5430–5438.[Abstract/Free Full Text]
150. Ratnam K, Low JA. Current development of clinical inhibitors of poly(ADP-ribose) polymerase in oncology. Clin Cancer Res 2007;13:1383–1388.[Abstract/Free Full Text]
151. Southan GJ, Szabo C. Poly(ADP-ribose) polymerase inhibitors. Curr Med Chem 2003;10:321–340.[Medline]
152. Lord CJ, Garrett MD, Ashworth A. Targeting the double-strand DNA break repair pathway as a therapeutic strategy. Clin Cancer Res 2006;12:4463–4468.[Abstract/Free Full Text]
153. Calabrese CR, Almassy R, Barton S, et al. Anticancer chemosensitization and radiosensitization by the novel poly(ADP-ribose) polymerase-1 inhibitor AG14361. J Natl Cancer Inst 2004;96:56–67.[Abstract/Free Full Text]
154. Plummer R, Middleton M, Wilson R, et al. First in human phase I trial of the PARP inhibitor AG-014699 with temozolomide (TMZ) in patients (pts) with advanced solid tumors [abstract]. J Clin Oncol 2005; (suppl):3065.
155. Plummer R, Lorigan P, Evans J, et al. First and final report of a phase II study of the poly(ADP-ribose) polymerase (PARP) inhibitor, AG014699, in combination with temozolomide (TMZ) in patients with metastatic malignant melanoma (MM) [abstract]. J Clin Oncol 2006;18S(suppl):8013.
156. Tentori L, Leonetti C, Scarsella M, et al. Systemic administration of GPI 15427, a novel poly(ADP-ribose) polymerase-1 inhibitor, increases the antitumor activity of temozolomide against intracranial melanoma, glioma, lymphoma. Clin Cancer Res 2003;9:5370–5379.[Abstract/Free Full Text]
157. Carvajal RD, Tse A, Schwartz GK. Aurora kinases: new targets for cancer therapy. Clin Cancer Res 2006;12:6869–6875.[Abstract/Free Full Text]
158. Warner SL, Gray PJ, Von Hoff DD. Tubulin-associated drug targets: Aurora kinases, Polo-like kinases, and others. Semin Oncol 2006;33:436–448.[CrossRef][Medline]
159. Warner SL, Munoz RM, Stafford P, et al. Comparing Aurora A and Aurora B as molecular targets for growth inhibition of pancreatic cancer cells. Mol Cancer Ther 2006;5:2450–2458.[Abstract/Free Full Text]
160. Yang H, Burke T, Dempsey J, et al. Mitotic requirement for aurora A kinase is bypassed in the absence of aurora B kinase. FEBS Lett 2005;579:3385–3391.[CrossRef][Medline]
161. Wilkinson RW, Odedra R, Heaton SP, et al. AZD1152, a selective inhibitor of Aurora B kinase, inhibits human tumor xenograft growth by inducing apoptosis. Clin Cancer Res 2007;13:3682–3688.[Abstract/Free Full Text]
162. Schellens JH, Boss D, Witteveen P, et al. Phase I and pharmacological study of the novel aurora kinase inhibitor AZD1152 [abstract]. J Clin Oncol 2006;18S(suppl):3008.
163. Tyler RK, Shpiro N, Marquez R, Eyers PA. VX-680 inhibits Aurora A and Aurora B kinase activity in human cells. Cell Cycle 2007;6:2846–2854.[Medline]
164. Rubin EH, Shapiro GI, Stein MN, et al. A phase I clinical and pharmacokinetic (PK) trial of the aurora kinase (AK) inhibitor MK-0457 in cancer patients [abstract]. J Clin Oncol 2006;18S(suppl):3009.
165. Cox CD, Breslin MJ, Mariano BJ, et al. Kinesin spindle protein (KSP) inhibitors. pt 1: The discovery of 3,5-diaryl-4,5-dihydropyrazoles as potent and selective inhibitors of the mitotic kinesin KSP. Bioorg Med Chem Lett 2005;15:2041–2045.[CrossRef][Medline]
166. Lad L, Luo L, Carson JD, et al. Mechanism of inhibition of human KSP by ispinesib. Biochemistry. 2008;47:3576–3585.[CrossRef][Medline]
167. Chu QS, Holen KD, Rowinsky EK, et al. Phase I trial of novel kinesin spindle protein (KSP) inhibitor SB-715992 IV Q 21 days [abstract]. J Clin Oncol 2004;22:2078.[Abstract/Free Full Text]

This article has been cited by other articles:

				Z. D. Sharp and R. Strong The Role of mTOR Signaling in Controlling Mammalian Life Span: What a Fungicide Teaches Us About Longevity J Gerontol A Biol Sci Med Sci, January 18, 2010; (2010) glp212v1. [Full Text] [PDF]

				H. Cheng and T. Force Molecular Mechanisms of Cardiovascular Toxicity of Targeted Cancer Therapeutics Circ. Res., January 8, 2010; 106(1): 21 - 34. [Abstract] [Full Text] [PDF]

				S. T. Wong Emerging treatment combinations: Integrating therapy into clinical practice Am. J. Health Syst. Pharm., December 1, 2009; 66(23_Supplement_6): S9 - S14. [Abstract] [Full Text] [PDF]

				S. Park, J. Koo, H. S. Park, J.-H. Kim, S.-Y. Choi, J. H. Lee, B.-W. Park, and K. S. Lee Expression of androgen receptors in primary breast cancer Ann. Onc., November 3, 2009; (2009) mdp510v1. [Abstract] [Full Text] [PDF]

				L. Xu, Y. Ding, W. J. Catalona, X. J. Yang, W. F. Anderson, B. Jovanovic, K. Wellman, J. Killmer, X. Huang, K. A. Scheidt, et al. MEK4 Function, Genistein Treatment, and Invasion of Human Prostate Cancer Cells J Natl Cancer Inst, August 19, 2009; 101(16): 1141 - 1155. [Abstract] [Full Text] [PDF]

eLetters:

This Article Abstract Full Text (PDF) CME: Take the course for this article: Novel Agents on the Horizon for Cancer Therapy An erratum has been published An erratum has been published Submit a letter to the editor View responses Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Ma, W. W. Articles by Adjei, A. A. Search for Related Content PubMed PubMed Citation Articles by Ma, W. W. Articles by Adjei, A. A.