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Keywords:

  • kinase inhibitors;
  • proteasome inhibitors;
  • histone deacetylase inhibitors;
  • histone deacetylase inhibitors;
  • methyl transferase inhibitors;
  • angiogenesis

Abstract

  1. Top of page
  2. Abstract
  3. Inhibitors of the RAS-MAPK Signal Transduction Pathway
  4. Inhibitors of the PI3K/AKT Signal Transduction Pathway
  5. Reversing Resistance to Apoptosis
  6. Inhibitors of the Proteasome
  7. De-repression of Tumor-Suppressor Genes
  8. Antiangiogenic Therapy
  9. CONCLUSIONS
  10. REFERENCES

Effective therapy for melanoma remains an unmet goal, with most traditional therapies representing inadequate trade-offs among the several goals of specificity, efficacy, and toxicity. Targeted molecular therapeutics are tailored to genetic abnormalities that are associated with tumor progression. Modulation of aberrant signaling pathways in cancer cells has the potential to provide more effective and potentially nontoxic therapy for a broad range of cancers, including melanoma. Among the possible targets in melanoma are the Ras-MAPK and PI3K/AKT signal transduction pathways, the proteasome, histone deacetylases, methyltransferases, and melanoma-induced angiogenesis. Cancer 2006. © 2006 American Cancer Society.

Melanoma has become a major public health problem.1 Since the 1960s, melanoma incidence has risen by 3% to 8% per year in the Caucasian population. Despite this increase, and an overall rise in mortality due to melanoma, the case survival rate for newly diagnosed melanoma has improved substantially. This improvement is largely attributable to earlier detection and surgery. The prognosis of patients with distant metastases not amenable to surgery remains poor, and is uninfluenced by any treatment intervention yet applied in large randomized multicenter studies.2 Thus, advances in understanding melanoma cell biology have fueled hopes for a breakthrough in the therapy of disseminated melanoma.3

Genetic and epigenetic alterations play an important role in the etiology and pathogenesis of melanoma. Mammalian cells have numerous safeguards that protect against neoplastic transformation, and only when lesions occur in multiple genes does invasive cancer develop.4 Consequently, individual mutations are contributors to, rather than the sole causes of, cancer.

Transforming mutations in oncogenes may render them constitutively active, or active under conditions in which the wildtype gene is inactive. These activations may result from chromosomal translocations, amplifications, or subtle intragenic mutations affecting crucial residues that regulate the activity of the gene product. Indeed, the most common activating mutation of an oncogene has been described for melanoma affecting the BRAF gene.5 Activated BRAF kinase phosphorylates downstream targets via MEK such as ERK, thereby influencing aberrant growth. Nevertheless, mutations in other genes involved in numerous cellular pathways important to cell proliferation, apoptosis, or metastasis such as N-RAS, PTEN, and KIT or changes in expression levels of various proteins (PTEN, BCL-2, NF-κB, and CDK2) are also relevant.6

Tumor suppressor genes can also contribute to oncogenesis by epigenetic silencing, truncating mutations, deletions and insertions of various sizes, as well as missense mutations at residues essential for tumor suppressor activity. Although haploinsufficiency of tumor-suppressor genes has been suggested to exert a selective advantage for the cell, mutations in both alleles (loss of heterozygosity) are generally required to confer neoplastic transformation.7, 8

Solid tumors such as melanoma are not mere accumulations of tumor cells, but are composed of dynamic compartments of neoplastic and stromal cells.9 In addition, the host response must also be considered—although this is the realm of tumor immunology, and a topic separate from that of the present review. The impact of interactions between tumor cells and stroma on tumor biology is increasingly recognized as important. The observation that naturally occurring inhibitors of angiogenesis hinder tumor growth, and that virtually all oncogenes and tumor-suppressor gene pathways are either directly or indirectly implicated in angiogenesis, have propelled the stromal component of endothelial cells into the spotlight.10 An understanding of tumor angiogenesis, blood flow, oxygenation, and related issues involving the tumor/host relation is becoming essential to studies of cancer biology as well as to the design of more effective forms of cancer therapy.

Inhibitors of the RAS-MAPK Signal Transduction Pathway

  1. Top of page
  2. Abstract
  3. Inhibitors of the RAS-MAPK Signal Transduction Pathway
  4. Inhibitors of the PI3K/AKT Signal Transduction Pathway
  5. Reversing Resistance to Apoptosis
  6. Inhibitors of the Proteasome
  7. De-repression of Tumor-Suppressor Genes
  8. Antiangiogenic Therapy
  9. CONCLUSIONS
  10. REFERENCES

Oncogenic relevance

The importance of the RAS-MAPK signal transduction pathway for the genesis of melanoma has recently been highlighted by the discovery of activating BRAF mutations in up to 66% of cutaneous melanoma.5 Many of the processes involved in tumor growth, progression, and metastasis are mediated by signaling pathways initiated by activated receptor tyrosine kinases (RTKs) (Fig. 1). RAS functions downstream of several RTKs, and activation of RAS signaling pathways is an important mechanism through which human cancers develop. Constitutive activation of the RAS pathway occurs through mutational activation of the RAS oncogene or of downstream components.11 Thus, numerous inhibitors of the RAS-MAPK signal transduction have been developed. These include farnesyl transferase inhibitors that interfere with the translocation of RAS to the cell membrane, and direct inhibitors of RAF such as sorafenib, or MEK such as ARRY142886/AZD6244 or PD0325901 (Table 1). The most promising results so far have been observed for the combination of the multikinase inhibitor sorafenib and chemotherapy.

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Figure 1. The RAS-MAPK signal transduction pathway. RAF kinases are serine/threonine protein kinases that initiate the mitogenic kinase cascade that ultimately modulates gene expression by way of the phosphorylation of transcription factors such as Jun, Elk1, c-Ets1/2, Stat 1/3, or Myc.

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Table 1. Clinical Trials with New Agents
Class/SubstanceTrial designResultsToxicityReferences
  1. PR indicates partial response; SD, stable disease; CR, complete response; MR, mixed response; PFS, progression-free survival; OS, overall survival; CNS, central nervous system.

Signal transduction pathway inhibitors
SorafenibPhase II (n = 54)37% PRGrade 3/4 toxicity (neutropenia/thrombopenia) associated with chemotherapyK. Flaherty (presentation Berlin, September 2004)
Sorafenib + Carboplatin + Paclitaxel48% SD
CCI-779Phase II (n = 33)1/33: PR (3%)Mild toxicity: hyperlipidemia, stomitis, diarrhea, skin rash, fatigue21
CCI-779 alone0% SD
PD 0325901Phase I/II (27 melanomas)2/27 MM patients: PRIn 12% of patients Grade 3 adverse effects (rash, CHF, syncope, anemia, diarrhea, mucositis)LoRusso P, et al. ASCO 2005 Abstract 3011
5/27 MM patients: SD
Reverting resistance to apoptosis
Oblimersen SodiumPhase III (n = 771)OR 6.8 vs 12.4%The addition of oblimersen sodium to dacarbazine was not associated with serious, previously unreported adverse reactions compared with the use of dacarbazine alone, but increased the incidence of fever, neutropenia, thrombocytopenia, and catheter-related complications.22
DTIC ± Oblimersen sodiumPFS 1.6 vs 2.4 m
OS (LDH < 2x unl)
8.7 vs 10.2 m
Proteasome inhibitor
BortezomibPhase II (n = 27)0% PRGrade 3 toxicity: neuropathy, thrombopenia, constipation, fatigue, etc.28
Bortezomib alone22% SD
HDAC inhibitors
MS-275Phase I (n = 31, 6 melanoma patients)2/6: SDAnorexia, nausea, vomiting (dose-dependent)38
1 PR (long-lasting)A. Schott (Schering, Germany)
Antiangiogenetic drugs
ThalidomidePhase II (n = 14)1 mixed responseDLTs: constipation, dizziness, somnolence46
Thalidomide alone1 SDGrade 3/4: hematologic toxicity, CNS hemorrhage, thrombosis, etc. 
Phase II (n =15 evaluable for response)2 CR + 1 PR + 1 MR + 6 SD in cranial lesions47
Thalidomide + Temozolomide4 MR + 1 SD in extracranial lesions
In brain metastatic patients
Integrin receptor inhibitors
Vitaxin (MEDI-522)Randomized Phase II (n = 112)Vitaxin alone: 0% PR2 patients with lethal myocardial infarction/pulmonary embolism54
Vitaxin ± DTICVitaxin + DTIC: 13% PR
OS: longer for Vitaxin alone

Clinical experience

Currently, only preliminary results of small to medium-sized Phase II clinical trials are available for metastatic melanoma.12 In addition to blocking activated, mutated, and wildtype RAF-1 and BRAF in tumor cells, sorafenib demonstrated significant activity against several receptor tyrosine kinases involved in neovascularization and tumor progression including vascular endothelial growth factor receptor (VEGF)-2 and -3, platelet-derived growth factor receptor β (PDGF-β), Flt-3, and KIT.13 As a single agent, however, sorafenib seems to have little or no antitumor activity in advanced melanoma patients.14 Nevertheless, ongoing trials are evaluating sorafenib combination therapies. In this regard, a study of sorafenib (BAY 43-9006) combined with carboplatin and paclitaxel was first presented by Flaherty at the VIII Perspectives in Melanoma Meeting in Berlin (September 2004). Carboplatin and paclitaxel were given every 21 days followed by an oral application of sorafenib from Days 2 to 19. A maximum of 10 cycles was applied for patients with AJCC stage IV melanoma without brain metastases. Only 43% of the patients had no prior chemotherapy; 54 patients with a median age of 47 years were classified as stages M1a (13%), M1b (19%), or M1c (68%). The interim analysis revealed a partial response rate of 37% (n = 20), stable disease in another 48% (n = 26), and progressive disease only in 5 (9%) patients. At the time of this presentation, 6% (n = 3) of the patients were not evaluable. The duration of response was 6 to 22 months, and the duration of stable disease 3 to 12 months. An update on 105 patients recently presented at the ASCO annual meeting demonstrated that progression-free survival for this treatment was 8.8 months. It is notable that responses were independent of the BRAF mutational status. Thus, inhibitors more specific to BRAF will be required to elucidate the impact of targeting BRAF mutations for the treatment of melanoma.

The main toxicities observed were chemotherapy-related neutropenia and thrombocytopenia, reaching grade 4 (CTC classification system) in 43% of the patients (neutropenia) and grade 3 in 43% (thrombocytopenia), respectively. No serious additional toxicity of sorafenib added to carboplatin and paclitaxel was obvious.15 However, sorafenib as well as other kinase inhibitors have specific, important side effects. The most predominant and limiting side effect of sorafenib is the hand-foot syndrome and skin rash. The pathogenesis of these cutaneous toxicities is presently unknown. Robert et al.16 summarized the cutaneous side effects of small molecule kinase inhibitors and blocking antibodies, including the appropriate dermatologic management, in a recent overview.

Currently, a randomized Phase II study is under way to evaluate a combination of sorafenib with dacarbazine (DTIC) in advanced metastatic melanoma patients. Furthermore, the sponsor has initiated 2 large, prospective, randomized trials in stage IV disease. These include an National Cancer Institute, Cancer Therapy Evaluation Program (NCI-CTEP)-sponsored intergroup trial led by the Eastern Cooperative Oncology Group (ECOG) in the US, and an industrially sponsored trial being conducted in Europe/Australia/Canada and the US to evaluate the efficacy of carboplatin/paclitaxel plus sorafenib versus carboplatin/paclitaxel plus an oral placebo. The ECOG-led intergroup trial is directed at chemotherapy-naive patients in first-line treatment, whereas the international trial is directed at chemotherapy-resistant melanoma patients with unresectable stage III or stage IV disease. Accrual to the latter has been completed and analysis is anticipated before the end of 2006.

Inhibitors of the PI3K/AKT Signal Transduction Pathway

  1. Top of page
  2. Abstract
  3. Inhibitors of the RAS-MAPK Signal Transduction Pathway
  4. Inhibitors of the PI3K/AKT Signal Transduction Pathway
  5. Reversing Resistance to Apoptosis
  6. Inhibitors of the Proteasome
  7. De-repression of Tumor-Suppressor Genes
  8. Antiangiogenic Therapy
  9. CONCLUSIONS
  10. REFERENCES

Oncogenic relevance

The phosphoinositide 3-kinase (PI3K) pathway is responsible for the production of 3-phosphoinositide lipid molecules that serve as second messengers in the cell, the most well known of these being phosphoinositol triphosphate (PIP3).17 The pathway controls a cascade of signals that regulates many basic cellular properties, including apoptosis resistance, survival, and motility (Fig. 2). Molecular cloning efforts identified an additional family of high molecular mass kinases whose catalytic domains bear a resemblance to those of PI3Ks. Based on this sequence homology, these kinases were named PI3K-related kinases (PIKKs).18 The PIKKs include several subfamilies, such as the target of rapamycin (TOR) family, ataxia telangiectasia (the ATM) gene product, and the DNA-dependent protein kinase.

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Figure 2. The PI3K/AKT signal transduction pathway. Multiple mechanisms for activation of the PI3K-AKT pathway have been identified such as amplification and overexpression as well as loss, mutation, or epigenetic silencing of the PTEN tumor suppressor gene.

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Members of the TOR subfamily are uniquely targeted for inhibition by rapamycin. Rapamycin is a macrolide antibiotic with antifungal and immunosuppressive properties that has been approved as an immunosuppressive drug for organ transplantation.19

Clinical experience

Results of clinical studies had been reported using the three water-soluble rapamycin analogs: RAD001 (Novartis, Hanover, NJ), AP23573 (ARIAD Pharmaceuticals, Cambridge, MA), and CCI-779 (Wyeth, Philadelphia, PA).18 CCI-779 has demonstrated activity against melanoma in preclinical models and has shown clinical benefits in patients with breast and renal cell cancer, where CCI-779 significantly increases the overall survival of first-line, poor-risk advanced renal cell cancer patients compared with interferon (IFN).20 Recently, in a Phase II trial metastatic melanoma patients were treated with weekly doses of 250 mg CCI-779 administered intravenously after diphenhydramine premedication.21 Patients with measurable disease, no more than 1 previous chemotherapy for metastatic melanoma, and normal organ functions were eligible. In total, 33 patients were treated, of which 21 had received previous chemo- and/or biological therapy for advanced metastatic melanoma disease. One patient showed a partial response lasting 2 months. The median time to disease progression and overall survival were 10 weeks and 5 months, respectively. Toxicity was mild, and predominantly comprised of stomatitis, diarrhea, and a skin rash. Hyperlipidemia was cumulative and was able to be managed with lipid-lowering statins. The authors concluded that CCI-779 was not sufficiently active to warrant further testing as a single agent in melanoma.21

Reversing Resistance to Apoptosis

  1. Top of page
  2. Abstract
  3. Inhibitors of the RAS-MAPK Signal Transduction Pathway
  4. Inhibitors of the PI3K/AKT Signal Transduction Pathway
  5. Reversing Resistance to Apoptosis
  6. Inhibitors of the Proteasome
  7. De-repression of Tumor-Suppressor Genes
  8. Antiangiogenic Therapy
  9. CONCLUSIONS
  10. REFERENCES

Oncogenic relevance

One of the major consequences of the constitutive activation of the MAPK and PI3K/AKT signal transduction pathway in tumor cells is the induction of tumor cell resistance to apoptosis. Indeed, this resistance is believed to be an element of the resistance of melanoma to most of the classical approaches to treatment, i.e., chemotherapy and irradiation. Thus, to target and reverse this antiapoptotic mechanism is attractive as a component of multiple therapies.

Clinical experience

Members of the BcL2 gene family as well as the “inhibitor of apoptosis protein” (IAP) families have been successfully targeted to render tumor cells more susceptible to apoptosis. To this end, BcL2 protein expression can be directly down-regulated by antisense oligonucleotides, i.e., oblimersen sodium (Genasense, Genta, Berkeley Heights, NJ). Long-term follow-up results from a Phase 3 trial of dacarbazine alone or in combination with oblimersen sodium in patients with advanced malignant melanoma did not demonstrate significance (P = .077) with respect to the primary endpoint of this trial, i.e., overall survival. However, a significant increase in “durable” responses (i.e., responses longer than 6 months duration) (P = .02)—which, among progression-free survival, antitumor response, and safety, was a secondary endpoint of the trial—in the arm treated with oblimersen sodium could be observed.22 Post-hoc evaluation of the serum lactate dehydrogenase (LDH) level at study entry suggested that the benefit from oblimersen sodium treatment was limited to patients with a normal serum LDH.

An alternative approach to overcome tumor cell resistance to apoptosis is to inhibit the function of IAPs. One of these proteins is survivin, which has been found to be highly expressed in most cancers. YM155 is a small molecule (Astellas Pharma, Japan) that has been demonstrated in preclinical models to suppress the function of survivin, and that has now entered a Phase II trial in several solid tumors including melanoma (ClinicalTrials.gov Identifier [ct.gI] NCT00281541 at www.clinicaltrials.gov).

Inhibitors of the Proteasome

  1. Top of page
  2. Abstract
  3. Inhibitors of the RAS-MAPK Signal Transduction Pathway
  4. Inhibitors of the PI3K/AKT Signal Transduction Pathway
  5. Reversing Resistance to Apoptosis
  6. Inhibitors of the Proteasome
  7. De-repression of Tumor-Suppressor Genes
  8. Antiangiogenic Therapy
  9. CONCLUSIONS
  10. REFERENCES

Oncogenic relevance

Protein homeostasis is critical to biological processes and fundamental to cancer cell survival. The proteasome is a multienzyme complex that serves as a major protein degradation pathway.23 The orderly degradation of cellular proteins is critical for regulation of signal transduction, transcriptional regulation, response to stress, and the control of receptor function. As such, the proteasome controls the levels of proteins that are important for cell-cycle progression and apoptosis in normal and malignant cells including cyclins, caspases, BcL2, and NFκB (Fig. 3). In cancer, deregulation of the ubiquitin-proteasome pathway may contribute to tumor progression, drug resistance, and altered immune surveillance.24

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Figure 3. Target structures for proteasome inhibition. The first step for protein degradation via the proteasome pathway involves polyubiquitination. Ubiquitinated proteins are targeted to the proteasome complex, which results in their subsequent degradation in the central portion of the proteasome complex. Proteasome substrates include p53 and IκB.

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Boronic acid-derived compounds inhibit the proteasome pathway in a highly specific manner.25, 26 Most of these boronated proteasome inhibitors were active across a large panel of tumor cell lines and the potency of proteasome inhibition correlated with growth-inhibitory effects. On the basis of its potency and cytotoxicity, bortezomib was identified as a lead candidate for further clinical testing.27

Clinical experience

Amiri et al.28 published work on the proteasome inhibitor bortezomib, which when combined with temozolomide significantly reduced tumor growth in an animal model. Complete remissions of palpable tumors were observed after 30 days and based on this and other studies bortezomib was pursued in patients with metastatic melanoma. A Phase II study of bortezomib in 27 patients (originally intended to recruit 45 patients), was terminated due to lack of clinical responses. Six (22%) of the treated patients achieved stable disease. Of these 6 patients, 4 were stable after 4 cycles of treatment, but were removed from the study due to significant toxicity. The median time to disease progression was 1.5 months, but curiously the median overall survival was 14.5 months. Although no grade 4 treatment-related toxicity was observed, 11 (42%) patients demonstrated grade 3 toxicity including sensory neuropathy, thrombocytopenia, constipation, fatigue, ileus/abdominal pain, and infection without neutropenia. The authors concluded that single-agent bortezomib administered twice weekly every 2 or 3 weeks at a dose of 1.5 mg/m2 was not efficacious in metastatic melanoma.25 Nevertheless, because in the animal model the combination of chemotherapy and bortezomib was promising, a Phase II trial is currently evaluating the effects of this agent in combination with carboplatin and paclitaxel as a first-line treatment in patients with metastatic cutaneous and ocular melanomas (ct.gI NCT00288041).

De-repression of Tumor-Suppressor Genes

  1. Top of page
  2. Abstract
  3. Inhibitors of the RAS-MAPK Signal Transduction Pathway
  4. Inhibitors of the PI3K/AKT Signal Transduction Pathway
  5. Reversing Resistance to Apoptosis
  6. Inhibitors of the Proteasome
  7. De-repression of Tumor-Suppressor Genes
  8. Antiangiogenic Therapy
  9. CONCLUSIONS
  10. REFERENCES

Oncogenic relevance

Cancer is associated with epigenetic as much as genetic alterations30 and the fate of an individual cell may rely on a delicate balance between gene expression and repression. Activation of transcriptional machinery is driven by the presence of transcription factors in the nucleus and their accessibility to the DNA. Access to DNA is determined in part by chromatin-modifying enzymes, which have recently become possible to target therapeutically.

Epigenetic gene silencing occurs mainly through 2 mechanisms, histone deacetylation by deacetylases (HDAC) and DNA hypermethylation.30–32 Both mechanisms are frequently altered in human tumors. The archetypal gene silenced by histone modification in human cancer is the cyclin-dependent kinase inhibitor CDKN1A.25 DNA hypermethylation has been the subject of substantial study, and many tumor-suppressor genes, including CDKN2A, are known to undergo methylation-associated silencing in human neoplasms.34

Although histone deacetylation has a fundamental role in regulating gene expression, HDAC inhibitors directly affect transcription of only a relatively small number of genes, ranging between 2% and 10%. The majority of these genes are directly or indirectly involved in the regulation of cell growth and survival, providing a mechanistic explanation of the anticancer properties of HDAC inhibitors. Inhibitors of HDAC induce cell cycle arrest, differentiation, and promote the apoptosis of melanoma cells in vitro, and many have potent antitumor activities in vivo. Short-chain fatty acids, such as the antiepileptic drug valproic acid, inhibit HDAC activity and affect the expression of numerous genes with disparate cellular functions.36, 37 Hydroxamic acids such as suberoylanilide hydroxamic acid (SAHA) has potent biological effects at micromolar and nanomolar concentrations, with longer in vivo half-life and bioavailability than short-chain fatty acids. Fungal metabolites like depsipeptide (FK228) are tetrapeptides that exert potent HDAC inhibitory activity.

Nucleoside DNA methyltransferase inhibitors, such as 2′-deoxy-5-azacytidine (decitabine), reverse epigenetic silencing of aberrantly methylated genes and have been shown to exhibit antitumor activity.37 Indeed, demethylating agents demonstrated reactivation of tumor suppressor genes aberrantly methylated in tumor cells, leading to inhibition of tumor growth. An important consequence of this is that, unlike conventional cytotoxic agents, it may be best to use such drugs at concentrations lower than the maximum tolerated dose and in a manner that optimizes their demethylating activity.38

Clinical experience

MS-275 is an orally active synthetic pyridyl carbamate HDAC inhibitor. Recently, a Phase I pharmacokinetic study of MS-275 in patients with advanced and refractory solid tumors and lymphomas was published.39 Dose-limiting toxicities were predominantly nausea, vomiting, anorexia, and fatigue. No complete or partial responses were observed in this clinical trial. However, 15 cases of disease stabilization lasting from 62 to 309 days were noted. Among these, there were 2 melanoma patients with stable disease of 4 and 5 months. Based on this and another Phase I trial in which a long-lasting, near-complete remission of a patient with advanced metastatic melanoma was observed (A. Schott, Schering, Berlin, Germany), a Phase II trial on 28 patients in Germany has been performed and the results are pending.

Antiangiogenic Therapy

  1. Top of page
  2. Abstract
  3. Inhibitors of the RAS-MAPK Signal Transduction Pathway
  4. Inhibitors of the PI3K/AKT Signal Transduction Pathway
  5. Reversing Resistance to Apoptosis
  6. Inhibitors of the Proteasome
  7. De-repression of Tumor-Suppressor Genes
  8. Antiangiogenic Therapy
  9. CONCLUSIONS
  10. REFERENCES

Oncogenic relevance

Tumor vasculature is essential for tumor growth and progression, and the broad requirement of vascularization has made antiangiogenic therapies attractive for many solid tumors.40 Both antiangiogenic and antivascular approaches are being developed. Targeting tumor-angiogenesis has several potential advantages over standard chemotherapy for cancer. These include easy access to targets within the vasculature, independence of tumor cell resistance mechanisms, broad applicability to tumors of different histologic types, and the potential to develop very specific therapies with minimal toxicities, because angiogenesis is not a requirement of normal adult tissues (Fig. 4).

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Figure 4. Tumor angiogenesis and its inhibition. Angiogenesis depends on the expression of specific mediators such as VEGF (vascular endothelial growth factor), FGF (fibroblast growth factor), interleukin-8, and angiopoietins. Newly formed tumor-related blood vessels sprout into the extracellular matrix (ECM), a process dependent on the ability of proliferating endothelial cells to interact with diverse glycoprotein components of this ECM. This interaction is mediated by endothelial transmembrane receptors or integrins αvβ3 and αvβ5.

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Clinical experience: endostatin

Several Phase I clinical trials of recombinant human endostatin were recently initiated to evaluate chronic administration of the protein to patients with advanced, refractory solid tumors.41, 42 These studies demonstrated that no clinically significant toxicities were associated with the administration of endostatin.41 For example, among a group of 15 patients who received a total of 50 monthly cycles of daily 20-minute intravenous injections with doses ranging from 15 to 240 mg/m2 per day experience only transient skin rashes at the highest doses. However, the best clinical responses to treatment in the referred trial consisted of a minor tumor reduction in 1 patient and disease stabilization in 2 others.

Clinical experience: bevacizumab

Because VEGF has a well-established role in stimulating tumor angiogenesis required for tumor growth in many tumors, numerous compounds have been developed to counteract its angiogenic effect. Bevacizumab, a humanized anti-VEGF monoclonal antibody, received FDA approval for colorectal cancer therapy in the US in 2005.40 Results obtained from initial trials of this agent showed that it was generally well tolerated, and trials in combination with chemotherapy have been completed in a number of solid tumors. The most striking results were observed for patients with advanced colorectal cancer, where the combination of bevacizumab, 5-fluorouracil, leucovorin, and irinotecan demonstrated significant improvement in response rates and overall survival compared with chemotherapy alone.43 Bevacizumab is currently being tested in four different Phase II trials in metastatic melanoma and other advanced solid cancers. It is combined with low- and high-dose interferon α (ct.gI NCT00026221), a chemotherapy regimen comprised of carboplatin and paclitaxel (ct.gI NCT00255762), imatinib mesylate (ct.gI NCT00074308), and lastly with erlotinib.6

Increased risk of grade 3/4 hypertension was an infrequently observed toxicity associated with bevacizumab.44 However, in these studies 3 bleeding-related deaths in the bevacizumab-containing arms were documented.

Thalidomide has been evaluated as a cancer therapy after original use to treat pregnancy-associated morning sickness, when it was banned in the 1960s due to serious congenital birth defects (phocomelia) that have been attributed to its antiangiogenic effects on the limb buds.45 The precise mechanism of its antiangiogenic activity remains unknown, however. Some of its effects may result from blocking angiogenic factors such as VEGF.46 Thalidomide has also been suggested to have immunomodulating effects, which have been suggested to be greater for a new group of agents in this family termed IMiDs (CC-5013 and CC-4047).46

Thalidomide has been studied as a single agent or in combination with temozolomide in patients with metastatic melanoma. A Phase II clinical trial by Reiriz et al.47 reported on 14 patients treated orally with a daily dose of 200 mg, which was escalated to a maximum of 800 mg per day. One mixed response, 1 stable disease, but no objective responses were observed in this clinical trial. Dose-limiting toxicities were constipation, dizziness, and somnolence. Because thalidomide alone showed poor activity, various combinations have been used in metastatic melanoma. An initially interesting result was published by Hwu et al.,48 who examined this combination in chemotherapy-naive patients with brain metastases. The regimen consisted of temozolomide at a dose level of 75 mg/m2 per day for 6 weeks with a 2-week break between cycles plus thalidomide (200 mg/day with escalating doses). Among the 26 patients included, 16 were symptomatic with brain metastases and 25 had extracranial metastases. Fifteen patients were evaluable for response, and 3 had complete or partial responses, whereas 7 had minor responses or stable disease in the brain. Unfortunately, 5 of these 10 patients had progressive metastases at extracranial sites. The median survival was 5 months for all 26 patients and 6 months for the 15 evaluable patients. After this encouraging experience, more recent data from the same institution has demonstrated no objective responses, and an elevated frequency of deep venous thrombosis or pulmonary embolism serve as a caveat against further pursuit of this regimen.

Clinical testing of other IMiDs has been conducted over the past 6 years, although the results of trials have been largely negative.46 CC-5013 and CC-4047 are the lead substances under clinical development. These 2 compounds vary in the extent of their immune-modulating activity, although they seem to have similar antiangiogenic activity. They also differ with respect to their pharmacokinetics and stability in plasma. Clinical development of IMiDs has been initiated with the use of CC-5013 (lenalimolide) in multiple myeloma. In Phase I clinical trials it was demonstrated that it was safe to use and well tolerated. CC-5013 turned out to be a safe drug at dose levels between 5 mg and 50 mg per day in a dose-escalating study approach. The predominating toxicities were paresthesia, altered taste, skin rashes, fatigue, nausea, poor appetite, and vomiting. There was no evidence that somnolence is a problem for patients treated with CC-5013. Furthermore, apart from mild paresthesia no evidence for any neurologic defects became apparent. Out of 20 evaluable patients there was 1 partial remission in a patient with metastatic melanoma. Some other patients with melanoma demonstrated stable disease during treatment.49 This trial led to the use of CC-5013 in a larger Phase III trial to study safety and efficacy for previously treated stage IV melanoma patients. The study subjects were either randomized to lenalimolide alone or to an oral placebo. However, the trial was closed by its Data and Safety Monitoring Committee due to failure to meet the required interim therapeutic endpoint.

As mentioned previously, quiescent cells rarely express integrins and proliferating endothelial cells are the primary source of increased αvβ3 expression in various human epithelial tumors.50 Indeed, increased expression of endothelial αvβ3 has been found to have major prognostic impact in patients with breast cancer. Hence, antiangiogenic treatment by blocking endothelial integrin activity seems to be an attractive approach, and monoclonal antibodies directed against αvβ3 as well as low-molecular-mass peptides blocking αv or vitronectin receptors have been developed.

Cilengitide (EMD 121974, Cyclo-l-Arg-Gly-l-Asp-d-Phe-N (Me) l-Val) is an example of such a low-molecular-mass peptide (MW 588.7 g/mol).51 It has an IC50 (inhibitory concentration 50%) for the inhibition of αvβ3 receptor binding to vitronectin of 1 nM, whereas the IC50 for inhibiting the αvβ5 receptor binding to vitronectin is 140 nM. Notably, cilengitide does not block the αIIβ3 fibrinogen receptor, which is the receptor for inhibitors of platelet aggregation; it exerts no antiaggregatory activity in vitro in concentrations up to 10 μM. Results from the first Phase I trial demonstrated a good safety profile, with no drug-related hematologic toxicity and only mild drug-related nonhematologic toxicity, mainly consisting of gastrointestinal and constitutional complaints.51 There was no indication of increasing toxicity with either increasing dose or prolonged exposure. Cilengitide is currently being tested in a randomized Phase II study at 2 different doses in unresectable stage III or stage IV melanoma patients (ct.gI NCT00082875).

Vitaxin (MEDI-522) is a humanized monoclonal antibody targeted against the αvβ3 integrin expressed by endothelial and tumor cells. It has been tested in a pilot trial in patients with metastatic cancer of different histologies.52 Posey et al.53 published 2001 results that demonstrate that the infusion of vitaxin is not associated with significant toxicity at 3 different dose levels from 10 to 200 mg. No tumor responses were observed in this pilot trial. The study suggested that a schedule of vitaxin at doses of 200 mg every 3 weeks maintains circulating levels of antibodies with little or no toxicity. In May 2005, the results of a randomized, open-label Phase II international study performed in metastatic melanoma patients were presented.54 The study explored the antitumor activity and safety of vitaxin (8 mg/kg/week) with or without DTIC (1000 mg/m2 once every 3 weeks) in patients with stage IV metastatic melanoma. Patients with ECOG performance status 0 or 1 with no prior therapy except for adjuvant immunotherapy were enrolled; 112 melanoma patients were randomized. Deaths associated with myocardial infarction and pulmonary embolism occurred in 2 patients, 1 in each arm. Grade 3 and 4 adverse events were chiefly neutropenia and thrombocytopenia. Vitaxin alone showed no objective responses, whereas the combination with DTIC demonstrated a 13% response rate. However, the median survival for the trial as a whole was prolonged, and was longer in the vitaxin-alone group at 13.7 months. The preliminary overall survival data suggest clinical activity of vitaxin in metastatic melanoma, and are now in Phase III trial compared with DTIC.

CNTO-95 is a fully human monoclonal antibody against integrins (αVβ3 and αVβ5) that has shown antitumor and antiangiogenic activity in animal models in vivo.55, 56 A Phase I study of CNTO95 in patients with different solid tumors assessed the safety and pharmacokinetics of this drug in advanced refractory malignancies. Among the treated tumors, 1 patient with a cutaneous angiosarcoma demonstrated a partial response at a dose level of 10 mg/kg. This integrin receptor antibody was well tolerated in general, with only 1 grade 3 toxicity (infusion-related fever).57 CNTO-95 has also been tested in a multicenter Phase I/II study in advanced metastatic melanoma patients. The results are not yet available. The company is currently preparing for a Phase III trial in previously untreated melanoma patients (stage IV).

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. Inhibitors of the RAS-MAPK Signal Transduction Pathway
  4. Inhibitors of the PI3K/AKT Signal Transduction Pathway
  5. Reversing Resistance to Apoptosis
  6. Inhibitors of the Proteasome
  7. De-repression of Tumor-Suppressor Genes
  8. Antiangiogenic Therapy
  9. CONCLUSIONS
  10. REFERENCES

The last decade has been characterized by fundamental advances in the understanding of the tumor biology and particularly the molecular markers of progression in human melanoma. The identification and validation of surrogate markers that have prognostic and predictive value remains a challenge.58 In other tumors such as breast, colorectal, or pancreatic cancer, as well as hematologic malignancies, targeted therapies with antibodies have been approved by the regulatory authorities. Melanoma belongs to a subset of human solid tumors that have been especially difficult to treat, and in which no prospective randomized clinical trials have ever shown reproducible superiority of 1 drug or regimen compared with another. Because the standards of care for advanced metastatic melanoma are not well defined, and because for many cases tumor is often accessible in the skin and/or lymph nodes, this tumor is 1 for which clinical research ought to be able to make more rapid inroads, using the tools of modern molecular biology.

The careful development of promising new agents in clinical trials that include translational laboratory assessment of the putative molecular targets in tumor tissue is likely to yield more rapid progress.59 Ideally, the identification of more critical targets and the more precise application of new therapies to subgroups of patients who manifest the particular molecular derangement will increase the likelihood of benefit from the specific targeted therapy. Current clinical trials can be considered as a beginning for more rational and less empiric trial designs, bringing the hope for urgently needed breakthroughs. Ultimately, the results observed in pilot Phase I and II trials will still require rigorous Phase III clinical trials in larger numbers of patients, but even in this setting a molecular understanding of the impact of therapy may allow greater efficiency in our efforts to achieve survival prolongation without toxicity.

REFERENCES

  1. Top of page
  2. Abstract
  3. Inhibitors of the RAS-MAPK Signal Transduction Pathway
  4. Inhibitors of the PI3K/AKT Signal Transduction Pathway
  5. Reversing Resistance to Apoptosis
  6. Inhibitors of the Proteasome
  7. De-repression of Tumor-Suppressor Genes
  8. Antiangiogenic Therapy
  9. CONCLUSIONS
  10. REFERENCES