Malignant melanoma is the most serious form of primary skin cancer and one of the only malignancies in which the incidence rate has been rising. It is estimated that in 2018 there were 91,270 newly diagnosed cases and 9320 deaths from advanced melanoma in the United States. Melanoma is the fifth most common cancer type in males and the sixth most common in females. Despite rising incidence rates, improvement in the treatment of advanced melanoma has resulted in declining death rates over the past decade.1 Although most melanoma is diagnosed at an early stage and can be cured with surgical excision, the prognosis for metastatic melanoma had been historically poor prior to recent advancements in treatment. Conventional chemotherapy treatment with dacarbazine or temozolomide resulted in response rates ranging from 7.5% to 12.1%, but without much impact on median overall survival (OS), with reported OS ranging from 6.4 to 7.8 months. Combination approaches with interferon alfa-2B and low-dose interleukin-2 resulted in improved response rates compared with traditional chemotherapy, but again without survival benefit.2
Immunotherapy in the form of high-dose interleukin-2 emerged as the first therapy to alter the natural history of advanced melanoma, with both improved response rates (objective response rate [ORR], 16%) and median OS (2 months), with some patients achieving durable responses lasting more than 30 months. However, significant systemic toxicity limited its application to carefully selected patients.3 The past decade has brought rapid advancements in treatment with immune checkpoint inhibitors and molecularly targeted agents, which have significantly improved ORRs, progression-free survival (PFS), and OS for patients with metastatic melanoma.4-8
This review is the first of 2 articles focusing on the treatment and sequencing of therapies in advanced melanoma. Here, we review the selection of first-line therapy for metastatic melanoma. Current evidence for immune checkpoint blockade and molecularly targeted agents in the treatment of metastatic melanoma after progression on first-line therapy is discussed in a separate article.
The incidence of melanoma is strongly associated with ultraviolet light–mediated DNA damage related to sun exposure. Specifically, melanoma is associated to a greater degree with intense intermittent sun exposure and sunburn, but not associated with higher occupational exposure.9 Ultraviolet radiation can induce DNA damage by a number of mechanisms, and deficient DNA repair leads to somatic mutations that drive the progression from normal melanocyte to melanoma.10
The most commonly identified genetic mutations in cutaneous melanomas are alterations in the mitogen-activated protein kinase (MAPK) pathway. Typically, an extracellular growth factor causes dimerization of the growth factor receptor, which activates the intracellular RAS GTPase protein. Subsequently BRAF is phosphorylated within the kinase domain, which leads to downstream activation of the MEK and ERK kinases through phosphorylation. Activated ERK leads to phosphorylation of various cytoplasmic and nuclear targets, and the downstream effects of these changes promote cellular proliferation. While activation of this pathway usually requires phosphorylation of BRAF by RAS, mutations placing an acidic amino acid near the kinase domain mimics phosphorylation and leads to constitutive activation of the BRAF serine/threonine kinase in the absence of upstream signaling from extracellular growth factors mediated through RAS.11 One study of tumor samples of 71 patients with cutaneous melanoma detected NRAS mutations in 30% and BRAF mutations in 59% of all tumors tested. Of the BRAF mutation–positive tumors, 88% harbored the Val599Glu mutation, now commonly referred to as the BRAF V600E mutation. The same study demonstrated that the vast majority of BRAF mutations were seen in the primary tumor and were preserved when metastases were analyzed. Additionally, both NRAS and BRAF mutations were detected in the radial growth phase of the melanoma tumor. These findings indicate that alterations in the MAPK pathway occur early in the pathogenesis of advanced melanoma.11 Another group demonstrated that 66% of malignant melanoma tumor samples harbored BRAF mutations, of which 80% were specifically the V600E mutation. In vitro assays showed that the BRAF V600E–mutated kinase had greater than 10-fold kinase activity compared to wild-type BRAF, and that this kinase enhanced cellular proliferation even when upstream NRAS signaling was inhibited.12
The Cancer Genome Atlas Network performed a large analysis of tumor samples from 331 different melanoma patients and studied variations at the DNA, RNA, and protein levels. The study established a framework of 4 notable genomic subtypes, including mutant BRAF (52%), mutant RAS (28%), mutant NF1 (14%), and triple wild-type (6%). Additionally, mRNA transcriptomic analysis of overexpressed genes identified 3 different subclasses, which were labeled as “immune,” “keratin,” and “MITF-low.” The immune subclass was characterized by increased expression of proteins found in immune cells, immune signaling molecules, immune checkpoint proteins, cytokines, and chemokines, and correlated with increased lymphocyte invasion within the tumor. Interestingly, in the post-accession survival analysis, the “immune” transcriptomic subclass was statistically correlated with an improved prognosis.13 Having an understanding of the molecular pathogenesis of advanced melanoma helps to create a framework for understanding the mechanisms of current standard of care therapies for the disease.
A 62-year-old Caucasian man with a history of well-controlled type 2 diabetes mellitus and hypertension is being followed by his dermatologist for surveillance of melanocytic nevi. On follow-up he is noted to have an asymmetrical melanocytic lesion over the right scalp with irregular borders and variegated color. He is asymptomatic and the remainder of physical examination is unremarkable, as he has no other concerning skin lesions and no cervical, axillary, or inguinal lymphadenopathy.