The incidence of cutaneous melanoma has increased over the past 2 decades, with SEER estimates indicating that the number of new cases of melanoma diagnosed annually rose from 38,300 in 1996 to 76,000 in 2016.1 Among persons younger than 50 years, the incidence is higher in females, and younger women (aged 15–39 years) are especially vulnerable.2 Among persons older than 50, melanoma incidence in men is nearly twice that of women, in whom melanomas are often thicker and often associated with worse outcomes.1,2 Approximately 85% of melanomas are diagnosed at early stages when surgery is curative, but the lifetime probability of developing invasive disease is 3% in men and 2% in women.
Prior to the advent of effective immunotherapies and targeted therapies, melanoma was often managed with chemotherapy, which had dismal response rates and commensurately poor outcomes. Advances in the understanding of the molecular etiopathogenesis and immune escape responses of cutaneous metastatic melanoma have transformed therapeutic approaches. Specifically, improved understanding of the genetic mutations driving melanoma tumorigenesis coupled with insights into mechanisms of tumor-mediated immune evasion resulted in development of inhibitors of mitogen-activated protein kinases (MAPK; BRAF and MEK) along with inhibitors of negative regulatory immune checkpoints (cytotoxic T lymphocyte–associated antigen 4 [CTLA-4] and programmed cell death-1 [PD-1]). In this review, we discuss the role of immune therapy, targeted therapy, and combinations of these in the treatment of metastatic cutaneous melanoma. We limit the immuno-therapy discussion to approved CTLA-4/PD-1 inhibitors and the targeted therapy discussion to approved BRAF/NRAS/MEK inhibitors and do not discuss non-checkpoint immunotherapies including cytokines (HD IL-2), vaccines, or adoptive T-cell approaches. Interested readers are directed to other excellent works covering these important topics.26–29
DEVELOPMENT OF TARGETED AND NOVEL IMMUNE THERAPIES
For many years the degree of ultraviolet (UV) light exposure was considered the sole major risk factor for melanoma oncogenesis, even though its mechanism was largely unknown.3 However, clinical observations regarding the occurrence of melanoma on less exposed areas (trunk and limbs) in individuals with intermittent sun exposure led to the proposition that melanomas that arose in younger patients with intermittent sun exposure were distinct from melanomas that arose in older patients in association with markers of chronic sun exposure—the “divergent pathway” hypothesis.3 Critical to this understanding were whole-exome sequencing data from multiple groups, including The Cancer Genome Atlas, that identified patterns of mutations in oncogenic drivers that were distinct in patients with and without chronically sun-damaged (CSD) skin.4–7 It is now clear that based on its association with CSD skin, melanoma can be subclassified into CSD or non-CSD melanoma. CSD and non-CSD melanoma have distinct clinico-pathological characteristics and are associated with different driver mutations. CSD melanomas typically arise in older patients on sun-exposed areas (head/neck, dorsal surfaces of distal extremities) and are associated with particular driver mutations (BRAF non-V600E, NRAS, NF1, or KIT) and genetic signatures of UV-induced DNA damage (G > T [UVA] or C > T [UVB]) transitions. Conversely, non-CSD melanomas typically arise in younger (< 55 years) patients on intermittently sun-exposed areas (trunk, proximal extremities) and are associated with BRAF V600E/K driver mutations and often lack genetic signatures of UV mutagenesis.
Identification of driver mutations in components of the MAPK pathway, including BRAF and NRAS, facilitated the development of targeted inhibitors. The BRAF inhibitors vemurafenib and dabrafenib have been shown in pivotal phase 3 studies to significantly improve overall and progression-free survival in patients with metastatic melanoma compared with chemotherapy and garnered regulatory approval (vemurafenib, BRIM-3;8,9 dabrafenib, BREAK-310). Concomitant MEK and BRAF inhibition extends the duration of benefit by preventing downstream kinase activation in the MAPK pathway. Notably, concomitant MEK inhibition alters the side-effect profile of BRAF inhibitors, with reduced incidence of keratoacanthomas and cutaneous squamous cell carcinomas that are attributable to on-target, off-tumor effects of BRAF inhibitors. Combined BRAF and MEK inhibition (vemurafenib/cobimetinib and dabrafenib/trametinib) further improved overall and progression-free survival compared to single-agent BRAF inhibition in phase 3 studies (COMBI-d,11 COMBI-v,12 and coBRIM13). Although often deep, the responses seen with the use of targeted kinase inhibitors are not often durable, with the vast majority of patients progressing after 12 to 15 months of therapy.In parallel, work primarily done in murine models of chronic viral infection uncovered the role played by co-inhibitory or co-excitatory immune checkpoints in mediating T-cell immune responses. These efforts clarified that tumor-mediated immune suppression primarily occurs through enhancement of inhibitory signals via the negative T-cell immune checkpoints CTLA-4 or PD-1.14,15 Blockade of negative T-cell immune checkpoints resulted in activation of the adaptive immune system, resulting in durable anti-tumor responses as demonstrated in studies of the CTLA-4 inhibitor ipilimumab (CA184-02016 and CA184-02417) and the PD-1 inhibitors nivolumab (CA209-003,18 CheckMate 037,19 and CheckMate 06620) and pembrolizumab (KEYNOTE-00121 and KEYNOTE-00622). Compared to the deep but short-lived responses seen with targeted kinase inhibitors, patients treated with CTLA-4 or PD-1 immune checkpoint blockade often developed durable responses that persisted even after completion of therapy. Combined CTLA-4 and PD-1 blockade results in greater magnitude of response with proportionately increased toxicity.23–25