Advances in the systemic treatment of metastatic melanoma

Abstract

Prior to 2011, the only commercially available agents commonly used to treat metastatic melanoma, including dacarbazine, temozolomide, fotemustine, carboplatin, paclitaxel, and interleukin-2, demonstrated limited efficacy, and no study had shown an improvement in overall survival. The standard of care for treatment of metastatic melanoma was radically changed by the subsequent approval of two agents, ipilimumab and vemurafenib, both of which improved survival in randomized phase 3 trials. Within the relatively short period that ipilimumab and vemurafenib have been commercially available, phase 2 data for the investigational agents nivolumab, MK-3475, the combination of dabrafenib and trametinib, and adoptive cell therapy, strongly suggest even further improvements in treatment outcome. Within this rich context of effective agents, the challenge for clinicians and investigators will be to develop predictive biomarkers of response, optimal sequence of therapy for individual patients, and effective combinations. An additional challenge will be to find the appropriate venue and populations to test promising new agents arising from substantial advances in our understanding of molecular alterations in melanoma cells, mechanisms of resistance to current agents, and tumor-host immune interactions.

Introduction

Before 2011, no systemic treatment for unresectable locally advanced stage III or stage IV melanoma had been consistently proven to increase median survival, and no large studies had compared existing treatments to best supportive care. In large controlled randomized trials, the median survival was consistently in the range of 8-11 months (1–3). High dose interleukin-2 was approved for treatment of metastatic melanoma based on durable tumor remissions in approximately 5%, but it can only be administered to patients with excellent performance status and normal organ function, and to date has not been compared to any other standard treatment in a randomized trial(4).

The current advances in treatment of advanced disease stem from identification of two specific driver mutations in subsets of melanoma, braf and ckit, and advances in understanding of mechanisms that control T-lymphocyte activation, proliferation, and function, specifically the immune regulatory checkpoints (Table 1). Controlled clinical trials of vemurafenib, which potently inhibits signaling from mutant braf, and of ipilimumab, which blocks the immune checkpoint CTLA-4, demonstrated meaningful improvements in median survival(5, 6). Ipilimumab was also shown to produce durable survival benefit in approximately 10% of patients. As a consequence, both vemurafenib and ipilimumab were approved in the United States in 2011. Results of clinical trials of monoclonal antibodies designed to block another immune checkpoint, PD-1 or its ligand, and of combined inhibitors of mutant braf and mek, suggest even further improvements in outcome for subsets of patients. Additional treatment gains may be achieved over the next 5-10 years by combination of active agents, introduction of new agents against novel molecular and immune targets, and improvements in technology that will increase the feasibility of adoptive cellular therapy outside of a few highly specialized treatment centers.

Table 1.

Agents/Approaches Contributing to Treatment Advances in Metastatic Melanoma

Agent	Target	Evaluation	Agent Type	Major Treatment Effect
Ipilimumab	Antagonist of CTLA-4	Phase 3	Antibody - Immune Therapy	Prolonged remissions in approximately 10%; increase in median survival
Nivolumab*	Antagonist of PD-1	Phase 1-2	Antibody-Immune Therapy	Objective response in approximately 30%; Prolonged remissions in subset of responders
MK-3475*	Antagonist of PD-1	Phase 1-2	Antibody-Immune Therapy	Objective response reported in approximately 47%; preliminary data indicate responses are prolonged in a subset
Adoptive Cell Therapy*	Melanoma antigens	Phase 2	Immune Therapy	Objective response in 50% of selected patients; prolonged complete response in 20%
Vemurafenib	Mutant braf	Phase 3	Small molecule signaling antagonist	Objective response rate approx 50%; increase in progression-free and median survival
Dabrafenib*	Mutant braf	Phase 3	Small molecule signaling antagonist	Objective response rate approx 50%; increase in progression-free survival; survival data pending
Trametinib*	mek	Phase 3	Small molecule signaling antagonist	Objective response rate approx 22%; increase in progression-free survival and overall survival
Dabrafenib + trametinib*	Mutant braf and mek	Randomized phase 2	Small molecule signaling antagonist	Objective response rate approx 75%; marked increase in progression-free survival
Imatinib* Sorafenib* Sutininib* Dasatinib* Nilotinib*	Mutant ckit	Phase 2, case reports	Small molecule signaling antagonist	Objective responses

^*Investigational agents for treatment of melanona

Immune based therapies

Ipilimumab

Ipilimumab is a human IgG1 monoclonal antibody that blocks cytotoxic T-lymphocyte antigen 4 (CTLA-4), a co-inhibitory receptor that regulates T-cell activation and the function of T-regulatory cells (Figure 1). Approval followed presentation of results from a phase III trial that compared ipilimumab 3 mg/kg every 3 weeks for 4 doses to ipilimumab in combination with a gp100 peptide vaccine or the gp100 vaccine alone in patients who had received at least one prior treatment for advanced disease (6). Although the complete and partial response rate for ipilimumab was only approximately 11%, median survival for patients receiving ipilimumab on either of the two arms was increased to 10 months compared to 6.4 months on the vaccine alone arm. Survival rates at 1 and 2 years were also improved for the ipilimumab arms, from 25% to 44-46% and 14% to 22-24%. Recent long-term follow-up from earlier phase 2 trials of ipilimumab showed that survival rates remain nearly flat from 3 to 5 years, indicating a long-term benefit for a subset of patients(7). A second phase III trial was conducted in previously untreated patients, comparing ipilimumab at a dose of 10 mg/kg administered with dacarbazine to dacarbazine alone (8). Although median survival in the ipilimumab plus dacarbazine arm was increased to 11.2 months from 9.1 months, the contribution of dacarbazine to the activity of ipilimumab remains unclear.

Figure 1.

Mechanism of Immune Checkpoint Inhibitors

There are several unique features of ipilimumab treatment described extensively in prior publications including the induction of autoimmune/inflammatory adverse events and clinical response in small brain metastases in a subset of patients(9). Several patterns of systemic tumor response have been observed, including mixed responses, disease progression followed by regression, or prolonged disease stabilization that appears to be associated with patient benefit(10). The unique patters of response led to development of new criteria for assessing clinical response to immune therapy agents. Data from the initial phase 3 study of ipilimumab also demonstrated that patients treated with ipilimumab or ipilimumab plus vaccine whose disease was progressing after achieving stable disease or tumor response at 24 weeks, could achieve a second response or prolonged stable disease with a second induction course of ipilimumab(11). Indeed, among 31 patients eligible for retreatment, objective response or stable disease of at least 24 weeks were observed in 68%. In contrast, the effect of administering maintenance ipilimumab, for example every 12 weeks, remains unclear.

Several combinations of ipilimumab with other agents may further increase activity and improve treatment outcome. Promising data including increased overall response rate, progression-free survival or complete response rate compared to prior trials were presented for ipilimumab in combination with bevacizumab, ipilimumab in combination with high dose IL-2, and tremelimumab (another anti-CTLA-4) in combination with interferon-alfa(12–14). A reliable predictive biomarker for response to ipilimumab has not yet been identified (15–18).

Inhibitors of Programmed Death 1 (PD-1) or its ligand (PDL1)

Programmed Death 1 (PD-1) is an inhibitory receptor up-regulated on activated lymphocytes. PD-1 has two known ligands, PD-L1 (also called B7-H1) and PD-L2 (B7-DC), which can be expressed on tumor and stromal cells, and are also up-regulated by cytokines produced by tumor-infiltrating lymphocytes(19–21). Several agents targeting either PD-1 or PD-L1 are being developed. In a Phase I/II study of nivolumab (BMS-936558, MDX-1106), a human IgG4 monoclonal antibody that blocks PD-1, an overall objective response rate of 31% was observed among 106 evaluable patients with previously treated advanced melanoma (22, 23). Sixteen of 23 patients with objective response followed at least 6 months from onset of treatment had an ongoing response. A similarly high objective response rate of 47% was observed among 83 advanced melanoma patients treated with MK-3475, another antagonist antibody of PD-1(24). Among the 25 patients in the latter group that had been treated with prior ipilimumab, MK-3475 produced an objective response rate of 40%. Overall, several complete responses were observed and most patients were continuing in response with a minimum follow-up of 16 weeks. In a multi-tumor phase I trial of the anti-PDL1 antibody BMS-936559, nine of 52 (17%) melanoma patients achieved a complete or partial response(25).

Toxicities associated with blockade of the PD-1 pathway have been similar in spectrum but less frequent and less severe than for ipilimumab(22). Grade 3 or 4 adverse events were observed in only 14% of patients treated with nivolumab and 9% treated with the anti-PD-L1 BMS-936559. Pneumonitis was observed in 3% and was fatal in 1% of patients treated with nivolumab, leading to implementation of early detection and management algorithms in an attempt to reduce life-threatening reactions.

In the Nivolumab phase 1 trial, a strong association was discovered between expression of PD-L1 on pre-treatment tumor samples, defined as expression on 5% or more of tumor cells, and response to therapy (22). Additional data will be required to confirm this association in melanoma. Studies conducted by Taube et al demonstrate that metastatic lesions in melanoma expressing PD-L1 are almost always associated with the presence of tumor-infiltrating lymphocytes (TIL), while those metastatic lesions without PD-L1 expression generally have no TIL(26). Increasing the activity of PD-1 blockade may require different approaches in the two subsets of tumors; for example, combining PD-1 blockade with other antagonists of lymphocyte functional suppression in PD-L1/TIL positive tumors, and combining PD-1 blockade with agents that drive lymphocyte infiltration into tumors that are PD-L1/TIL negative.

Adoptive cell therapy (ACT)

Existing immune therapies attempt to induce or expand tumor antigen-specific immune responses in vivo. An alternate approach is to isolate tumor antigen-specific T cells from the patient, either from peripheral blood or a resected tumor, and expand the cells ex vivo before re-infusing the cells back into the patient. Early studies of ACT in the late 1980’s and early 1990s produced limited activity, believed to be a result of the limited persistence of the lymphocytes after adoptive transfer (27, 28). Preclinical models demonstrated that persistence of the cells in vivo after adoptive transfer could be increased if the host was preconditioned with lymphoablating chemotherapy and/or whole body radiation(29). Subsequent studies of lymphoablation, followed by transfer of TIL in combination with systemic administration of IL-2, demonstrated high response rates in the range of 50% (30) (31, 32). In the largest study published to date, approximately 20% of patients achieved durable complete remissions. Responses were observed in patients progressing on anti-CTLA-4, and in a subsequent trial, we are aware of a patient responding after exposure to anti-PD-1, suggesting that ACT provides an anti-tumor effect that is non cross-resistant to the checkpoint inhibitors.

Currently, ACT is applicable only to a select subset of patients that have good performance status and normal organ function, resectable tumors from which cells can be isolated and expanded, ability to travel to one of a few specialized centers studying ACT, and ability to maintain their performance while waiting for cells to expand in vitro for 3-6 weeks. Various technological advances may allow export of the technology to multiple centers and increase access to more patients, for example, by reducing the generation time and cost of expanding lymphocytes ex vivo. Better selection of antigen-specific T cells from resected tumors, improved expansion techniques, identification of populations with greatest potential for in vivo activity, and improved approaches to support cell expansion and function after adoptive transfer, perhaps by concurrent administration of other cytokines and checkpoint inhibitors, may produce greater efficacy. Several trials have been conducted using peripheral blood lymphocytes genetically engineered ex vivo to express either a tumor specific T-cell receptor or chimeric antigen receptor (CAR)(33) (34) (35, 36). CARS combine the signal activating machinery of a T cell and an antigen binding site of a monoclonal antibody. By engineering peripheral blood lymphocytes to confer tumor antigen specificity, the costly and labor intensive process of harvesting cells from tumors and the delay in treatment could possibly be avoided. Moreover, the techniques may extend therapy options to a larger group of patients. Some of the attempts to administer genetically engineered T cells have been associated with unexpected toxicity and overall response rates are currently lower than reported with TIL, but advances in the technology can be expected over time(33).

Small molecule antagonists of tumor-specific mutations

Melanomas harboring braf mutations

The ras- raf mitogen activated protein kinase (MAPK) intracellular signaling cascade has been shown to be critical for malignant behavior in the majority of melanomas. It directly impacts several cellular processes including cell survival, differentiation, and proliferation. Somatic braf missense mutations present in approximately 40-60% of melanoma patients produce elevated kinase activity and activation of the MAPK pathway independent of upstream activation by ras (37). Mutations in braf are more common in cutaneous melanomas and are significantly less frequent in sun shielded areas such as mucosal or acral-lentiginous melanomas (0-9% and approximately 15-23%, respectively) (38–41). Mutations in braf are not found in uveal melanomas (42, 43). Approximately 80-90% of braf mutations are V600E and 10-20% are V600K. Other rare mutations have been noted in the literature, some of which may be less responsive to the selective mutant BRAF inhibitors (44).

Vemurafenib is a small molecule potent inhibitor of mutant BRAF(45). In vitro assays, it has little effect on melanoma cells with wild type BRAF at concentrations that markedly inhibit the growth of cells carrying a mutation in BRAF V600E or V600K. Phase 1 and 2 studies demonstrated rapid anti-tumor activity in the majority of patients carrying a tumor BRAF V600E mutation. In a phase III trial in patients with BRAF V600E mutations, objective responses were observed in 48% receiving vemurafenib and 5% treated with dacarbazine (5, 46). The median progression free survival in the vemurafenib arm was 6.9 months versus 1.6 months for dacarbazine. Vemurafenib increased median survival from 9.7 to 13.6 months despite eventual crossover from dacarbazine to vemurafenib (5, 46). Based on these remarkable data, vemurafenib was approved by FDA in 2011. The most common side effects were arthralgia, rash, fatigue, alopecia, keratoacanthoma, squamous cell carcinoma, photosensitivity, nausea, and diarrhea. Thirty eight percent of patients in the trial required a dose reduction because of adverse effects. Dabrafenib, another relatively selective inhibitor of mutated BRAF, was also compared to dacarbazine and produced an increase in the median progression free survival from 2.7 months to 5.1 months and in objective response rate from 7% to 50%, similar to vemurafenib (47). The toxicity profile of dabrafenib was also similar to that of vemurafenib, although pyrexia was noted more frequently.

Despite the impressive activity of vemurafenib and dabrafenib, current data indicate that most patients will develop progressive disease, and responses are generally not maintained when drug is stopped(48). Treatment can be continued after limited progression in some patients with probable additional benefit. Several mechanisms of resistance have been identified, some involving reactivation of signaling through downstream mek and persistent phosphorylation of erk (49, 50) (51-53). In addition, in normal cells, vemurafenib and dabrafenib can activate mek signaling through upstream activation of craf, which is the cause of the secondary squamous cell skin carcinomas observed in patients treated with these agents(54–56). The identified mechanisms of tumor resistance and development of secondary skin cancers suggested that the combined inhibition of mutant braf and mek would produce improved anti-tumor effects and may reduce the skin-related toxicities of the braf inhibitors.

Trametinib is a small molecule that binds to and potently inhibits mek1 and mek2(57). A phase III trial compared trametinib to standard chemotherapy (dacarbazine or paclitaxel) in patients whose tumors contained a braf mutation. Median progression free survival was improved from 1.5 to 4.8 months and overall survival at 6 months was increased from 67% to 81%. Crossover was allowed once patients had progressed on chemotherapy. Rash, diarrhea, and peripheral edema were the most common side effects (58). Overall activity for the mek inhibitor appeared less than for the mutant braf inhibitors in the same patient population. Trametinib was subsequently shown to have minimal activity and produced no objective responses in patients whose disease progressed on a braf inhibitor (59).

In a phase 1 trial of dabrafenib combined with trametinib, full doses of both agents could be given together safely(60). The combination was associated with a greater incidence of pyrexia, sometimes requiring concurrent administration of steroids, but a lower incidence of cutaneous toxicities including the development of skin squamous cell carcinomas. Subsequently, 162 patients were randomized to dabrafenib 150 mg orally twice daily alone versus dabrafenib in combination with either trametinib at 1 or 2 mg orally daily. Median progression free survival for the 150/2 combination group was 9.4 months versus 5.8 months in the dabrafenib alone arm. The overall objective response rate was also higher in the combination group, 76% vs. 54% (p=0.03). Long-term follow up data are not yet available to determine effects on overall survival and on duration of responses, however, the results suggest that the combination will become the treatment of choice for targeting braf mutations in patients with metastatic melanoma.

Melanoma harboring c-kit mutations

ckit is a member of the receptor tyrosine kinase family of proteins. Activation of the intracellular signaling cascade by the endogenous ligand, stem cell factor, is involved in several cellular processes including proliferation and inhibition of apoptosis (61). ckit mutations are found in approximately 20% (range 6-39%) of mucosal and acral-lentiginous melanomas, rarely in conjuctival melanomas and approximately 15% of melanomas arising from chronic sun-damaged skin (62–67). Inhibitors of ckit tyrosine kinase such as imatinib, dasatinib, sorafenib, sunitinib, and nilotinib have been studied in melanoma patients. Trials in patients overexpressing ckit by immunohistochemistry demonstrated minimal activity (68) (69) (70) (71). Subsequently, several case reports and a few limited series provided evidence for the therapeutic activity of various ckit inhibitors in patients with tumors containing an activating ckit mutation(65, 72, 73). In the largest treatment study reported in the literature, 21 patients with ckit mutations were treated with imatinib, and 6 objective responses were observed, including 2 complete responses. All responses occurred in patients with L576P or K642E mutations(66).

Sequencing of therapy, combinations, and future directions

Without question, the approvals of ipilimumab and vemurafenib marked a major advance in the treatment of locally advanced and metastatic melanoma. Within a little more than one year after the approval of these agents, compelling clinical data were presented for 2 investigational monoclonal antibodies against PD1, nivolumab and MK-3475, and for the investigational combination of braf and mek inhibitors, which respectively appear to be more effective and perhaps better tolerated than ipilimumab and vemurafenib. Moreover, high dose interleukin-2 remains a viable option for selected patients because of its ability to induce durable remissions in a small subset, and select centers are able to offer trials of adoptive cellular therapy which have shown substantial promise in phase 2 trials.

Assuming anti-PD1 antibodies and the combination of braf and mek inhibitors will become more widely available in the near future, clinicians and investigators will be faced with an array of active therapies, and with major questions regarding how to select and sequence therapies for individual patients. A major question for patients with tumor braf mutations will be which sequence of molecular targeted therapy and immunotherapy to administer in order to yield the best survival outcome with least toxicity. The relative rapidity of tumor response to molecular targeted therapy must be weighed against the current assumption that immunotherapy takes longer to produce response but may be more likely to produce longer and unmaintained remissions. However, the assumption of slower response to immune therapy may be challenged by agents such as anti-PD1 or combinations with anti-PD1 which may produce more rapid onset of tumor regression. The impact of a prior therapy on response and toxicity to a subsequent therapy, for example, a braf inhibitor followed by an immune therapy, or sequencing of two immune therapies, remains mostly unknown. Current clinical experience indicates that resistance to one immune therapy does not preclude objective response to a subsequent immune therapy, for example, anti-PD1 following ipilimumab, or ipilimumab following anti-PD1.

Combination therapies offer the possibility of synergistic anti-tumor activity but may be complicated by increased or unexpected toxicities. Inhibitors of braf have been shown to increase tumor T-cell infiltration(74), thus the rationale for combining with immune therapies, but could also enhance T-cell activation and toxicity through the paradoxical activation of craf in normal cells. Combinations of certain immune checkpoint inhibitors or checkpoint inhibitors with cytokines or immune co-stimulatory antibodies may lead to greater autoimmune adverse events. Nevertheless, combinations offer the greatest promise for further improvements in outcome, and each combination will be judged on the relative risk-benefit ratio and ability to manage induced adverse events.

With more effective therapies and more combinations to test, it will be challenging to develop new agents for certain subsets of melanoma patients. Nevertheless, there are few effective therapies for patients with metastatic ocular melanoma, and no compelling effective molecular therapy for patients progressing after immune therapy who have tumor nras mutations (approximately 15% of all melanoma patients) or tumors that do not have mutations in either braf or nras(75) (76) (77) (78). Preliminary results from a phase II trial showed some activity for a mek inhibitor in patients with nras tumor mutations (79), however, response durations were relatively short. Sequencing of the melanoma genome did not reveal additional common driver mutations amenable to rapid drug development(80). Testing novel agents in these subsets of patients, including combinations of signaling pathway antagonists, new immune therapy agents, antibody-drug conjugates and angiogenesis inhibitors, will be required to produce additional meaningful treatment advances in the near to mid-term.