Immunotherapy Combined or Sequenced With Targeted Therapy in the Treatment of Solid Tumors: Current Perspectives

Abstract

The advent of newer immunotherapeutic and molecularly targeted agents has provided a number of effective options for cancer treatment but has also added much complexity in selecting the best initial treatment or treatment plan for each patient. Molecularly targeted agents offer selectivity and are the cornerstone for “precision medicine.” While targeted agents are associated with high tumor response rates, patients inevitably develop resistance to these drugs. Immunotherapies exploit the endogenous immune system to eradicate cancer and can produce durable disease control that results in long-term, treatment-free survival in some patients. Guidelines for treatment selection in patients with specific tumor types and clinical features are routinely being reconsidered in order to accommodate the increasingly complex treatment landscapes. Here, we review current perspectives on the use of immunotherapeutic agents, particularly immune checkpoint inhibitors (nivolumab, pembrolizumab, and ipilimumab), in combination or in sequence with molecularly targeted agents in patients with advanced melanoma as well as other tumor types. We further discuss remaining unmet needs for patient selection and treatment with approved therapies.

It is now well established that endogenous immune responses play a key role in the recognition and elimination of cancer cells (“immunosurveillance”), and the ability of tumor cells to avoid immune destruction is now a recognized hallmark of cancer ( 1 ). This feature of cancer has served as a foundation for the emerging field of immuno-oncology, a scientific discipline that reflects the intersection between tumor immunology, cancer biology, and cancer immunotherapy.

All components of the innate and adaptive immune systems play a role in tumor identification and elimination (or control), including natural killer cells, effector T-cells, and antibodies produced by B-cells ( 2 ). These immune components recognize specific antigens expressed by tumor cells, which are typically the result of nonspecific mutations in the tumor that alter the structure of normal proteins, making them appear foreign to the immune system (so-called “neoantigens”). As these antigens are typically not expressed by normal cells, the immune response selectively targets the tumor. However, changes in the tumor itself, eg, modification of antigen expression, and/or within the tumor microenvironment, such as the release of immunosuppressive factors or expression of inhibitory checkpoints, allow the tumor cells to avoid immunosurveillance and ultimately escape immune destruction ( 2 ). The goal of immunotherapy, therefore, is to enhance the patient’s own immune response in order to shift the balance toward antitumor immunity, allowing for durable and adaptable control of tumor growth ( 2 ).

Along with major advances in the understanding of tumor immunology in the past 30 years, genomic studies have identified oncogenic mutations that drive tumorigenesis in several cancer types. The identification of such mutations has ushered in the era of personalized or precision medicine, in which patients can be treated with a specific molecularly targeted agent based on the mutation status of their tumor. In non–small cell lung cancer, for example, the tyrosine kinase inhibitors gefitinib, erlotinib, and afatinib are used to treat patients with epidermal growth factor receptor (EGFR) mutations, and crizotinib is used to treat patients with anaplastic lymphoma kinase rearrangements ( 3 ). The BRAF serine-threonine kinase in the mitogen-activated protein kinase (MAPK) pathway (RAS-RAF-MEK-ERK) regulates cell proliferation, differentiation, and survival and is mutated in 60% of thyroid cancers, 50% of melanomas, 10% of colorectal carcinomas, and 6% of lung cancers ( 4 ). With that knowledge, the BRAF inhibitors vemurafenib and dabrafenib and the MEK inhibitor trametinib (either alone or in combination with dabrafenib) have been clinically developed, tested, and approved in recent years for the treatment of patients with BRAF V600 mutation–positive melanoma. However, several mechanisms underlying resistance to BRAF inhibitors have been identified and can occur through oncogene reactivation or activation of a bypass pathway ( 5 ).

With the availability of new immunotherapeutic approaches and targeted agents, the challenge for clinicians is to understand how to use these agents together in order to provide greater clinical benefit for their patients. Potential regimens may include a “sequenced” approach, where one treatment is used first and then followed by a switch to a second treatment only after progression occurs or a defined number of weeks later or where two or more treatments are used concurrently or in “combination” in a patient. Here, we provide a current perspective on this topic, using advanced melanoma as an example of the potential for using these agents in sequence or in combination in different tumor types.

Search Strategy and Selection Criteria

We conducted a systematic literature review as part of the development of this review article. Databases searched included Medline and PubMed, using search terms “vemurafenib AND melanoma,” “dabrafenib AND melanoma,” “trametinib AND melanoma,” “BRAF inhibitor,” “MEK inhibitor,” “BRAF inhibitor AND resistance,” “immunotherapy AND melanoma,” “ipilimumab AND melanoma,” “nivolumab AND melanoma,” “pembrolizumab AND melanoma,” “anti-PD-1 antibody,” “anti-PD-1 antibody AND renal cell carcinoma,” “immunotherapy and renal cell carcinoma,” “tyrosine kinase inhibitors AND renal cell carcinoma,” “tyrosine kinase inhibitors AND lung cancer,” “immunotherapy AND lung cancer,” and “ipilimumab AND lung cancer”. We reviewed all evidence for immunotherapies and molecular targeted therapies currently being evaluated for the treatment of advanced cancers, with an emphasis on reports related to the combination or sequencing of agents. Only articles published in English in the past seven years, with a focus on advanced cancers (melanoma, renal cell carcinoma, and lung cancer), were considered. There was no restriction on stage of clinical development, but preclinical studies were excluded. We further searched clinical trial databases for completed and ongoing studies evaluating combinations of immunotherapies, particularly immune checkpoint inhibitors, combinations of targeted agents, combinations of immunotherapies or immune checkpoint inhibitors with targeted agents, and immunotherapies or immune checkpoint inhibitors prior to or following targeted agents. Our search was limited to advanced melanoma, metastatic renal cell carcinoma, and stage IV lung cancer. All dosing and treatment schedules were reviewed, and all phases of clinical development were considered.

Immunotherapy of Melanoma

In 1998, high-dose interleukin-2 (HDIL-2) was approved by the Food and Drug Administration (FDA) in the United States for the treatment of patients with advanced melanoma. In a 2013 consensus statement on tumor immunotherapy by the Society for Immunotherapy of Cancer (SITC), HDIL-2 remains a reasonable treatment option in any line of therapy for appropriate patients with advanced melanoma ( 6 ). HDIL-2 has been shown to induce complete, durable tumor regression in a small subset of patients (~6%), but the potential for serious toxicities (eg, hypotension, decreased renal function, respiratory distress) limits its use to the healthiest patients treated at centers experienced with inpatient administration and management of the side effects ( 7 ).

The identification of key regulatory components in immune cell pathways has led to new targets for cancer immunotherapy. In the 1990s, the results of several pivotal studies were reported demonstrating that cytotoxic T-lymphocyte antigen-4 (CTLA-4) plays a key role in downregulating the activity of T-cells in order to maintain immune tolerance and homeostasis ( 8 ). It was hypothesized that blocking the inhibitory function of the CTLA-4 immune checkpoint molecule could sustain a T-cell response against tumor cells ( 9 ). This led to the development of ipilimumab, a fully human monoclonal antibody that binds to CTLA-4 to augment antitumor immune responses ( 9 ). In two phase III trials, ipilimumab demonstrated an improvement in overall survival (OS) in patients with advanced melanoma, and in 2011 ipilimumab at 3mg/kg was approved by the FDA and the European Union for the treatment of patients with advanced melanoma ( 10 , 11 ). Studies have demonstrated that ipilimumab can be safely used in patients with treated or untreated brain metastases ( 10 , 12 , 13 ), and pooled OS data from several clinical studies have shown that a proportion of patients treated with ipilimumab experience long-term survival ( 14 ). In 1861 patients, a plateau in the OS curve was observed beginning around three years, with three-year OS rates that were 22% (95% confidence interval [CI] = 20% to 24%) for the entire population, 26% (95% CI = 21% to 30%) for treatment-naive patients, and 20% (95% CI = 18% to 23%) for previously treated patients. This pooled OS analysis included some patients that were followed for up to 10 years ( 14 ).

Another immune checkpoint molecule, programmed death-1 (PD-1), inhibits effector T-cell responses within tissues, and its ligand PD-L1 can be manipulated by tumor cells to avoid immune-mediated destruction ( 15 ). Two anti-PD-1 monoclonal antibodies, pembrolizumab (humanized IgG4) and nivolumab (fully human IgG4), were approved by the FDA in 2014 for the treatment of patients with unresectable or metastatic melanoma whose disease had progressed following ipilimumab and, if BRAF V600 mutation–positive, a BRAF inhibitor ( 16 , 17 ). These approvals were based on a phase II study for pembrolizumab (KEYNOTE-002) and a phase III study for nivolumab (CheckMate 037) that compared each agent with investigator’s choice of chemotherapy ( 18 , 19 ). A phase III, double-blind, randomized controlled trial involving treatment-naive patients with BRAF wild-type melanoma (CheckMate 066) was stopped early in 2014 by an independent data monitoring committee because of a statistically significant OS benefit observed in the nivolumab arm compared with the dacarbazine control arm ( 20 ). In 2015, both nivolumab and pembrolizumab were approved in the EU for treatment-naive and previously treated advanced melanoma, regardless of BRAF mutational status.

National Comprehensive Cancer Network (NCCN) guidelines now recommend nivolumab or pembrolizumab for the first-line treatment of advanced melanoma, regardless of BRAF mutational status ( 21 ). In contrast to ipilimumab, these agents are recommended even in patients who have anticipated clinical deterioration within 12 weeks ( 21 ). Phase III trials of pembrolizumab (KEYNOTE-006) and nivolumab (CheckMate 067) have recently demonstrated improved progression-free survival (PFS), with less high-grade toxicity compared with ipilimumab alone ( 22 , 23 ). Thus, anti-PD-1 therapy is likely to be preferred over ipilimumab as a first-line treatment option for patients with advanced melanoma.

Several ongoing studies seek to answer the question of how to sequence these new anti-PD-1 agents or potentially use them in combination with approved agents in the current treatment landscape ( Table 1 ). Immune checkpoint inhibitors, such as ipilimumab and anti-PD-1 antibodies, modulate the immune system through different mechanisms and may exert synergistic activity when used in combination. Therefore, the first question that needs to be answered pertains to the optimal use of immune checkpoint inhibitors with one another. Will these agents produce better outcomes when used in combination, in sequence, or in some regimen involving both? Evidence from a phase I clinical study initially suggested that ipilimumab plus nivolumab may produce more frequent tumor responses and longer OS than with either agent alone in patients with advanced melanoma ( 24 , 25 ). Greater toxicities with the combination were also reported, but were similar in nature to those observed in ipilimumab clinical trials and appeared to be manageable ( 24 , 25 ). More recently, data from a randomized phase II trial (Checkmate 069) demonstrated that the combination of ipilimumab plus nivolumab statistically significantly improved objective response rate (ORR) and PFS compared with ipilimumab monotherapy ( 26 ). In 109 patients with previously untreated BRAF wild-type metastatic melanoma, the ORR was 61% (95% CI = 49% to 72%) with combination therapy vs 11% (95% CI = 3% to 25%) with ipilimumab monotherapy. Treatment-related grade 3 or 4 adverse events were reported in 54% of patients in the combination therapy group and 24% of patients receiving ipilimumab monotherapy ( 26 ).

Table 1.

Key ongoing studies in melanoma evaluating immunotherapy in combination or in sequence with molecularly targeted agents*

Study identifier	Title	Agents	Study design
Combination studies
NCT01767454	Phase 1 Study of the BRAF Inhibitor Dabrafenib +/- MEK Inhibitor Trametinib in Combination With Ipilimumab for V600E/K Mutation Positive Metastatic or Unresectable Melanoma	Ipilimumab,  dabrafenib, trametinib	(Doublet arm) Dabrafenib 150mg BID for 2 weeks, then  dabrafenib 150mg BID + ipilimumab 3mg/kg q3w x 4 doses  dabrafenib 100mg BID + ipilimumab 3mg/kg q3w x 4 doses (Triplet arm) Dabrafenib dose above + trametinib, then  dabrafenib 100mg BID + trametinib 1mg QID + ipilimumab 3mg/kg q3w x 4 doses  dabrafenib 150mg BID + trametinib 1mg QID + ipilimumab 3mg/kg q3w x 4 doses Targeted therapy continued postipilimumab until PD or unacceptable toxicity
NCT02130466	A Study of the Safety and Efficacy of Pembrolizumab (MK-3475) in Combination With Trametinib and Dabrafenib in Participants With Advanced Melanoma (MK-3475-022/KEYNOTE-022)	Pembrolizumab, dabrafenib, trametinib	(Arm A) Pembrolizumab + trametinib + dabrafenib (Arm B) Placebo + trametinib + dabrafenib (Arm C) Pembrolizumab + trametinib (Arm D) Pembrolizumab + dabrafenib
NCT02357732	Study of the Anti-PD-1 Antibody Nivolumab in Combination With Dabrafenib and/or Trametinib	Nivolumab, dabrafenib, trametinib	(Doublet) Nivolumab + dabrafenib (Doublet) Nivolumab + trametinib (Triplet) Nivolumab + dabrafenib + trametinib
NCT02200562	A Phase I/II Study of Concurrent Ipilimumab and Dabrafenib in Unresectable Stage III or Stage IV Melanoma	Ipilimumab, dabrafenib	Safety of ipilimumab and dabrafenib in combination
NCT01656642	A Phase Ib, Open-Label Study of the Safety and Pharmacology of MPDL3280A Administered in Combination With Vemurafenib or Vemurafenib Plus Cobimetinib in Patients With Previously Untreated BRAFV600- Mutation Positive Metastatic Melanoma	MPDL3280A (anti-PD-L1), vemurafenib,  cobimetinib	(Arm 1) MPDL3280A + vemurafenib (Arm 2) MPDL3280A + vemurafenib (Arm 3) MPDL3280A + vemurafenib (Arm 4) MPDL3280A + vemurafenib + cobimetinib (Arm 5) MPDL3280A + vemurafenib + cobimetinib
NCT02400385	A Phase II Trial of Sunitinib and Nivolumab for KIT-mutated Advanced Melanoma	Nivolumab,  sunitinib	Sunitinib 50mg/day, 4 weeks on and 2 weeks off, and concurrent nivolumab 3mg/kg q2w, both for three years if tolerated
Sequencing studies
NCT01940809	A Sequential Safety and Biomarker Study of BRAF-MEK Inhibition on the Immune Response in the Context of CTLA-4 Blockade for BRAF Mutant Melanoma	Ipilimumab,  dabrafenib, trametinib	(Arm A) Dabrafenib BID and trametinib QID for 25 days, followed by ipilimumab q3w x 4 doses (Arm B) Trametinib QID for 25 days, followed by ipilimumab q3w x 4 doses (Arm C) Dabrafenib BID for 25 days, followed by ipilimumab q3w x 4 doses (Arm D) Ipilimumab q3w x 4 doses All treatments continue until PD or unacceptable toxicity
NCT01673854	A Single Arm Open-Label Phase 2 Study of Vemurafenib Followed by Ipilimumab in Subjects With Previously Untreated V600 BRAF Mutated Advanced Melanoma	Ipilimumab, vemurafenib	Vemurafenib 960mg BID for 6 weeks, followed by ipilimumab 10mg/kg q3w for 4 doses, then q12w starting week 24, until PD or unacceptable toxicity Vemurafenib will be restarted at time of PD or unacceptable toxicity on ipilimumab
NCT01783938	An Open-Label, Randomized, Phase 2 Study of Nivolumab Given Sequentially With Ipilimumab in Subjects With Advanced or Metastatic Melanoma	Ipilimumab, nivolumab	Cohort A: Nivolumab followed by ipilimumab Cohort B: Ipilimumab followed by nivolumab
NCT02224781	A Randomized Phase III Trial of Dabrafenib + Trametinib Followed by Ipilimumab + Nivolumab at Progression vs. Ipilimumab + Nivolumab Followed by Dabrafenib + Trametinib at Progression in Patients With Advanced BRAFV600 Mutant Melanoma	Ipilimumab, nivolumab dabrafenib, trametinib	(Arm A) Ipilimumab + nivolumab induction, nivolumab maintenance; crossover to Arm C upon PD (Arm B) Dabrafenib + trametinib; crossover to Arm D upon PD (Arm C) Dabrafenib + trametinib (Arm D) Ipilimumab + nivolumab induction, nivolumab maintenance

* BID = twice a day; CTLA-4 = cytotoxic T-lymphocyte antigen-4; PD = progressive disease; PD-1 = programmed death-1; QID = four times a day; q2w = every 2 weeks; q3w = every 3 weeks.

A randomized phase III trial (Checkmate 067) has now reported efficacy and safety of the combination of nivolumab and ipilimumab or nivolumab monotherapy compared with ipilimumab monotherapy in 945 treatment-naive patients with unresectable stage III or IV melanoma ( 23 ). Median PFS (a coprimary endpoint of the study) was 11.5 months in the combination group, 6.9 months for nivolumab alone, and 2.9 months for ipilimumab alone. The rates of investigator-assessed objective response were 58% (95% CI = 52% to 63%) in the combination group, 44% (95% CI = 38% to 49%) in the nivolumab group, and 19% (95% CI = 15% to 24%) in the ipilimumab group. Treatment-related grade 3 or 4 adverse events occurred in 55% of those in the combination group, 16% of the patients in the nivolumab group, and 27% of those in the ipilimumab group. Patients continue to be followed for OS, the other coprimary endpoint of the study.

Considering the higher rates of severe adverse events and more discontinuations as a result of drug toxicity with the combination of nivolumab and ipilimumab ( 23 ), monotherapy may be preferred in some patients. This may be especially true if patients who progress on anti-PD-1 monotherapy can subsequently achieve a benefit to combination therapy or ipilimumab monotherapy. In CheckMate 067, similar PFS was observed in the nivolumab alone and combination groups among patients with PD-L1-positive tumors, suggesting that tumor cell PD-L1 expression could identify patients who will achieve similar benefit from nivolumab monotherapy and thus avoid the enhanced toxicity of the combination. However, it should be noted that only approximately 25% of patients in the trial had PD-L1-positive tumors and PFS is not the preferred endpoint for measuring the effectiveness of an immunotherapy, making these results hypothesis-generating rather than definitive. It should also be noted that 41% of patients whose tumors were PD-L1-negative responded to nivolumab monotherapy, indicating that many such patients may also do as well with monotherapy as with the combination. In addition, because the assay for PD-L1 expression is different in the pembrolizumab trials (where as many as 80% of patients tumors expressed PD-L1) ( 22 ), the results of the Checkmate 067 study cannot be generalized to decision-making for pembrolizumab treatment. Tumor PD-L1 expression cannot, therefore, be recommended as a biomarker to select patients for initial treatment at this time. Clearly, more precise and standardized biomarkers will be necessary to better define the patient population that will benefit maximally from anti-PD-1 monotherapy.

While OS data in the CheckMate 067 study remain immature and biomarker work is ongoing, it is recommended that choices of therapy be based on clinical grounds. Combination therapy, if available, should be given to patients who need a rapid response and are perceived to be able to tolerate the side effects while monotherapy with either pembrolizumab or nivolumab is acceptable, regardless of PD-L1 status, for patients who are either frailer or wish to avoid the toxicity risk.

A key unanswered question is how OS with anti-PD-1 monotherapy followed by combination therapy (or sequential ipilimumab monotherapy) will compare with OS with initial combination therapy. Conceivably, such a trial could include state-of-the-art biomarker analyses examining, in addition to PD-L1 expression by various assays, the CD8 infiltration at the leading edge of the tumor, T-cell receptor clonality, tumor mutational frequency, and interferon-driven gene expression signature, all of which have been proposed to aid in the selection of patients poised to respond maximally to single-agent anti-PD1 therapy ( 27–29 ). Though sequencing of combination therapy and monotherapy still requires exploration, studies have been designed to help address the issue of sequencing individual immune checkpoint inhibitors. The phase II CheckMate 064 study (NCT01783938) will evaluate nivolumab followed ipilimumab vs ipilimumab followed by nivolumab in patients with advanced melanoma, in which switch to the other agent occurs at a prespecified timepoint.

Once these questions regarding combination or sequenced immunotherapy have been answered, the next important question is how to use immune checkpoint inhibitors in treatment regimens involving BRAF and/or MEK inhibitors. Safety and tolerability will inevitably guide optimization of these treatment regimens, based on current evidence as discussed below.

Targeted Therapy in Melanoma

Beginning with the FDA approval of vemurafenib in 2011 ( 30 ), several targeted therapy options have also become available for patients with melanoma within the past few years. Most recently, the combination of dabrafenib and trametinib was approved in the United States in 2014 ( 31 , 32 ) and in September 2015 in the EU ( 33 ). Targeted therapy (vemurafenib, dabrafenib, trametinib, or the combination of dabrafenib and trametinib) is recommended for patients with BRAF mutation–positive tumors and symptomatic disease or in patients who have progressed despite immunotherapy ( 21 ). In the EU, ESMO Clinical Practice Guidelines recommend a BRAF inhibitor for patients with BRAF mutation–positive disease and symptomatic, bulky metastases ( 34 ). Like ipilimumab, BRAF inhibitors have been safely used in patients with treated or untreated brain metastases ( 35–37 ).

The combination of BRAF inhibitor and MEK inhibitor may be considered the targeted therapy standard of care for patients with BRAF mutation–positive melanoma. In a phase III study, the combination of dabrafenib and trametinib improved PFS compared with dabrafenib alone in treatment-naive patients with BRAF V600 mutation–positive melanoma ( 38 ). Continued follow-up of the patients in this study demonstrated a statistically significant improvement in OS with the combination compared with dabrafenib alone (25.1 months vs 18.7 months, respectively) ( 39 ). Dabrafenib combined with trametinib also improved OS compared with vemurafenib as first-line therapy in a phase III trial of patients with mutated BRAF V600E or V600K melanoma ( 40 ).

An investigational MEK inhibitor, cobimetinib, has also demonstrated promising antitumor activity ( 41 ). In a phase III trial, the combination of vemurafenib and cobimetinib demonstrated statistically significant improvement in PFS compared with vemurafenib alone ( 42 ). BRAF inhibitors have been associated with immunologic effects that may be favorable for antitumor immunity ( 43 ), and evidence suggests that these agents can promote T-cell infiltration into human melanoma tumors ( 44 ). Additionally, inhibition of the MAPK pathway may enhance dendritic cell function and, therefore, tumor-specific T-cell responses ( 45 ). In mouse models, the immune-stimulating effects of BRAF inhibitors have been shown to enhance the antitumor activity of adoptive immunotherapy ( 46 ). The precise mechanisms by which targeted therapies stimulate the immune system remain unclear at present. However, one hypothesis is that by leading to tumor lysis and antigen release, they prime the T-cell responses that can be enhanced via immunotherapy ( Figure 1 ).

Figure 1.

Hypothesis for the antitumor effects of a BRAF/MEK inhibitor in sequence with checkpoint inhibitors. APC = antigen-presenting cell; MHC = major histocompatibility complex class I or class II molecules; TCR = T-cell receptor.

Due to the lack of clinical trial data at present, limited information on the potential for immunotherapies combined or sequenced with targeted therapies is available in current guidelines. The increasingly complex treatment landscape poses a challenge for clinicians, but several ongoing trials may soon provide greater clarity regarding patient selection and treatment decisions ( Table 1 ).

Combination of Targeted Agents and Immunotherapy

The rationale for combination regimens is to provide the durable clinical benefit of immunotherapy along with the rapid and high response rates produced by molecularly targeted therapy. This approach potentially offers both short- and long-term benefit to patients. A further rationale for combination strategies is based on the distinct but complementary mechanisms of action for such agents. To this end, there is great interest in understanding how immune checkpoint inhibitors can be used with molecularly targeted agents to improve clinical outcomes for patients with advanced cancers.

Several important questions relate to the value of combining targeted therapy and immunotherapy. It is unknown whether infiltrating immune cells associated with targeted therapy are functional and can recognize the tumor, whether the combinations would be tolerable, and whether combinations would result in more durable response vs sequencing of agents. In a phase I trial of patients with BRAF V600 mutation–positive melanoma evaluating concurrent vemurafenib and ipilimumab treatment, dose-limiting hepatotoxicity was observed and further patient accrual to the study was stopped ( 47 ). It is unclear whether this effect was because of the combined immune-stimulating properties of vemurafenib and ipilimumab, as BRAF inhibition paradoxically activates the MAPK pathway in T lymphocytes ( 43 ), or the enhancement by ipilimumab of what otherwise would have been subclinical liver toxicity from vemurafenib. While the underlying causes of hepatotoxicity with concurrent vemurafenib and ipilimumab remain unclear, the results of this study highlight the importance of rigorous clinical trial evaluation and optimization of combination regimens involving immunotherapy and molecular targeted therapy.

Future studies involving the optimization of doses, timing, and other factors may result in lower, more manageable toxicity. Several different approaches could be considered for evaluation ( Figure 2 ). For example, initial results are available from a study evaluating the combination of dabrafenib and ipilimumab (with or without trametinib) in patients with BRAF-mutant melanoma. While the combination of standard doses of dabrafenib and ipilimumab was tolerated and showed clinical activity, triplet therapy with dabrafenib, trametinib, and ipilimumab resulted in dose-limiting toxicities and is not being pursued ( 48 ). Studies involving combinations of anti-PD-1 or anti-PD-L1 antibodies in combination with BRAF +/- MEK inhibitors have been initiated, and some initial data have been reported. The results of a phase I study showed evidence of clinical activity and a manageable safety profile for an anti-PD-L1 antibody in combination with dabrafenib and trametinib in BRAF mutation–positive melanoma, in combination with trametinib in BRAF wild-type melanoma, and following trametinib in BRAF wild-type melanoma ( 49 ). While these data are encouraging, longer follow-up including duration of response data is necessary to assess whether the clinical activity can be distinguished from that expected with BRAF/MEK inhibitors or MEK inhibitor therapy alone.

Figure 2.

Potential combination or sequencing regimens for immunotherapy and molecularly targeted agents in melanoma. OR = objective response; PD-1 = programmed death-1.

Sequencing of Immunotherapy and Targeted Therapy

Sequencing of immunotherapy and targeted therapy may potentially have benefits similar to or superior to combination therapy while not exposing patients to the toxicity and cost of simultaneous approaches for long periods of time. Available evidence suggests that targeted therapies may be effective when administered after immunotherapy, but little prospective clinical trial evidence exists regarding the optimal sequencing of such agents. Current evidence is largely anecdotal and based on retrospective analyses. One phase II trial (CA184-240/NCT01673854) was designed to introduce ipilimumab following vemurafenib, thus avoiding liver toxicity observed when the agents are used concurrently ( 47 ). In addition, ipilimumab was initiated after six weeks of vemurafenib therapy, before the time when resistance to BRAF inhibitors normally occurs. This study found that the safety profile of the sequence was tolerable, with a median PFS of 4.4 months and an ORR of 30% after initial vemurafenib and ipilimumab treatment ( 50 ).

Within the limitations of the existing data, ipilimumab appears to be more effective when given prior to (rather than after) a BRAF inhibitor for most patients with BRAF mutation–positive melanoma. In a retrospective analysis of patients who had received a BRAF inhibitor (with or without trametinib) before or after immunotherapy, PFS and response rates were roughly equivalent irrespective of the timing of BRAF inhibitor therapy ( 51 ). However, longer OS was observed when ipilimumab was given prior to a BRAF inhibitor compared with a BRAF inhibitor first followed by ipilimumab, or with either agent alone, supporting the use of immunotherapy as the first-line treatment whenever possible ( 52 ). A caveat of this retrospective analysis is that different criteria were used for the selection of patients for initial BRAF inhibitor vs ipilimumab therapy. Furthermore, these results may not apply to anti-PD-1-based therapy. As noted above, BRAF inhibitor treatment increases T-cell infiltration into the tumor and may enhance expression of PD-L1. This may result in a tumor microenvironment more predisposed to respond to subsequent anti-PD-1 therapy ( Figure 2 ) ( 43 , 53 ). In an open-label, randomized phase III study, patients who progressed on both ipilimumab and a BRAF inhibitor still had a higher response rate with nivolumab compared with chemotherapy ( 19 ). Similar results were reported in a randomized phase II trial of pembrolizumab vs chemotherapy (KEYNOTE- 002) ( 18 ).

As nivolumab plus ipilimumab has been shown to produce greater response rates than either agent alone, the combination may have even greater benefit as the initial treatment for patients with rapidly progressive disease. Thus, although the combination is not yet approved, it has the potential to be as effective as combined targeted therapy (BRAF + MEK inhibition) in patients with BRAF mutation–positive melanoma. In the CheckMate 067 trial, median PFS was 11.7 months in patients with BRAF mutation–positive melanoma who received nivolumab plus ipilimumab ( 23 ) and was 11.0 months with the combination of dabrafenib and trametinib in a phase III trial of BRAF V600 mutation–positive melanoma ( 39 ). The optimal sequence of the anti-CTLA-4 and anti-PD-1 antibody combination with the BRAF and MEK inhibitor combination in patients with BRAF mutation–positive melanoma is being prospectively explored in intergroup clinical trial EA6134 (NCT02224781), in which dabrafenib plus trametinib is being evaluated in sequence with ipilimumab plus nivolumab ( Table 1 ).

Another potential approach to overcome the problem of resistance to BRAF inhibitor therapy in patients with rapidly progressive BRAF mutation–positive melanoma is to introduce the immunotherapy (or other therapy) before resistance to the BRAF inhibitor is expected to occur or is documented clinically ( Figure 2 ). It remains to be determined whether this approach is feasible and more effective than either giving the treatment regimens in sequence following onset of resistance or in combination. As a biomarker for resistance has not been identified, the timing of when to initiate or switch to immunotherapy relative to the start of a BRAF and/or MEK inhibitor is currently empirical. Future studies are therefore needed to address when resistance occurs, what is the mechanism of resistance, and what is the possibility of adding a drug to block or delay the development of resistance. The identification of a biomarker of resistance could emerge as an important component of the on-treatment decision process. For example, blood tests examining circulating tumor cells or more sensitive study of mutations in pretreatment specimens may allow ascertainment of the predominant mechanism of resistance and guide the choice of agents to prevent the development of resistance. The safety profile of various sequenced regimens also must be evaluated in clinical trials in order to assess the risk-benefit ratio for patients.

Application to Other Tumors

The lessons learned from melanoma may be applicable to other tumor types. Both nivolumab and pembrolizumab are being evaluated in other tumor types, in first-line and later therapy settings. For example, pembrolizumab has been evaluated in patients with non–small cell lung cancer, showing activity in a phase I trial ( 54 ). Of note, nivolumab has been approved by the FDA for the treatment of patients with metastatic squamous non–small cell lung cancer during progression on or after platinum-based chemotherapy ( 17 ).

Sunitinib, a tyrosine kinase inhibitor used to treat patients with metastatic renal cell carcinoma, has been shown to enhance the proliferation of antigen-specific T-cells when given prior to (but not when co-administered with) a vaccine in mouse models ( 55 , 56 ). A phase I trial evaluating nivolumab in combination with sunitinib or pazopanib in patients with metastatic renal cell carcinoma showed encouraging antitumor activity. However, liver and other toxicities were an issue, particularly for patients treated with pazopanib in combination with nivolumab ( 57 ). This preliminary evidence suggests that such a combination may not be feasible. Therefore, additional studies may be aimed at evaluating the optimal sequencing of such agents (ie, anti-PD-1 antibody followed by anti-VEGF therapy or vice versa) in order to optimize the overall risk-benefit ratio or more selective VEGF pathway inhibitors (ie, bevacizumab or axitinib in combination with anti-PD-1 agents) in order to optimize the benefit from both classes of drugs.

As VEGF is believed to suppress the immune system, combination with bevacizumab and ipilimumab has already been tested ( 58 ), and further investigation with checkpoint inhibitors is warranted. Similarly, the potential immune-potentiating effects of radiation therapy suggest benefits of concurrent radiation and immune checkpoint inhibition ( 59 , 60 ), though safety needs to be evaluated in clinical trials. In lung cancer, the results of a phase II trial showed that a phased schedule of chemotherapy and ipilimumab produced better outcomes than a concurrent schedule ( 61 ). This result emphasizes the need for optimal evaluation of various agents used in conjunction with immunotherapies, regardless of treatment or tumor type.

Conclusions

Beginning in 2011, six new systemic therapies have been approved for the treatment of patients with unresectable or metastatic melanoma both in the United States and the EU. These new agents have filled great unmet needs for advanced melanoma as the treatment options available prior to 2011 had limited effectiveness. Ipilimumab has demonstrated durable tumor responses and long-term survival in a proportion of patients with advanced melanoma. Newer immune checkpoint inhibitors, nivolumab and pembrolizumab, have better safety profiles and efficacy outcomes than ipilimumab and offer the possibility of combination or sequential treatment strategies that may extend long-term survival beyond that achieved with ipilimumab alone. The challenge for melanoma practitioners is how to combine or sequence immune checkpoint inhibitors with each other and, in patients with BRAF mutation–positive melanoma with BRAF/MEK inhibitors, to ensure the best possible outcomes for patients. At the top of any treatment decision algorithm will be the selection of patients for initial therapy. We will need to build upon existing data with new clinical trial results and real-world experience in order to inform melanoma treatment guidelines, which will undoubtedly require constant updating in the next several years. In addition, although not ready for clinical use at present, ongoing research offers the potential to identify biomarkers to guide the treatment selection process. Much of the data regarding the combination or sequencing of immunotherapy with molecularly targeted agents is derived from the melanoma experience, but it is anticipated that these key learnings will apply to other tumor types as well.

Funding

Professional medical writing and editing assistance were provided by StemScientific, an Ashfield Company, funded by Bristol-Myers Squibb.

Professional medical writing assistance was provided by Ward A. Pedersen and Zenab Amin at StemScientific, an Ashfield Company, and was funded by Bristol-Myers Squibb. Bristol-Myers Squibb did not influence the content of the manuscript, nor did the authors receive any financial compensation for their work on the manuscript.

JL and MBA both contributed to the literature search, conception of review, data interpretation, and writing of the manuscript.

Dr Atkins reports acting as a consultant for BMS, Caladrius, Genentech, GSK, Merck, Nektar, Novartis, and Pfizer, outside the submitted work. Dr Larkin reports acting as a non-remunerated consultant for BMS, Eisai, MSD, Novartis, Pfizer, and Roche/Genentech. Dr Larkin is supported by the NIHR Royal Marsden Hospital/Institute of Cancer Research Biomedical Research Centre for Cancer.