Cutaneous malignant melanoma represents an aggressive form of skin cancer, with 132,000 new cases of melanoma and 50,000 melanoma-related deaths diagnosed worldwide each year.1 In recent decades, major progress has been made in the treatment of melanoma, especially metastatic and advanced-stage disease. Approval of new treatments, such as immunotherapy with anti–PD-1 (pembrolizumab and nivolumab) and anti–CTLA-4 (ipilimumab) antibodies, has revolutionized therapeutic strategies (Figure 1). Molecularly, melanoma has the highest mutational burden among solid tumors. Approximately 40% of melanomas harbor the BRAF V600 mutation, leading to constitutive activation of the mitogen-activated protein kinase (MAPK) signaling pathway.2 The other described genomic subtypes are mutated RAS (accounting for approximately 28% of cases), mutated NF1 (approximately 14% of cases), and triple wild type, though these other subtypes have not been as successfully targeted with therapy to date.3 Dual inhibition of this pathway using combination therapy with BRAF and MEK inhibitors confers high response rates and survival benefit, though efficacy in metastatic patients often is limited by development of resistance. The US Food and Drug Administration (FDA) has approved 3 combinations of targeted therapy in unresectable tumors: dabrafenib and trametinib, vemurafenib and cobimetinib, and encorafenib and binimetinib. The oncolytic herpesvirus talimogene laherparepvec also has received FDA approval for local treatment of unresectable cutaneous, subcutaneous, and nodal lesions in patients with recurrent melanoma after initial surgery.2
FIGURE 1. Schematic representation of various therapeutic strategies for the treatment of melanoma.
In this review, we explore new therapeutic agents and novel combinations that are being tested in early-phase clinical trials (Table). We discuss newer promising tools such as nanotechnology to develop nanosystems that act as drug carriers and/or light absorbents to potentially improve therapy outcomes. Finally, we highlight challenges such as management after resistance and intervention with novel immunotherapies and the lack of predictive biomarkers to stratify patients to targeted treatments after primary treatment failure.
Targeted Therapies
Vemurafenib was approved by the FDA in 2011 and was the first BRAF-targeted therapy approved for the treatment of melanoma based on a 48% response rate and a 63% reduction in the risk for death vs dacarbazine chemotherapy.4 Despite a rapid and clinically significant initial response, progression-free survival (PFS) was only 5.3 months, which is indicative of the rapid development of resistance with monotherapy through MAPK reactivation. As a result, combined BRAF and MEK inhibition was introduced and is now the standard of care for targeted therapy in melanoma. Treatment with dabrafenib and trametinib, vemurafenib and cobimetinib, or encorafenib and binimetinib is associated with prolonged PFS and overall survival (OS) compared to BRAF inhibitor monotherapy, with response rates exceeding 60% and a complete response rate of 10% to 18%.5 Recently, combining atezolizumab with vemurafenib and cobimetinib was shown to improve PFS compared to combined targeted therapy.6 Targeted therapy usually is given as first-line treatment to symptomatic patients with a high tumor burden because the response may be more rapid than the response to immunotherapy. Ultimately, most patients with advanced BRAF-mutated melanoma receive both targeted therapy and immunotherapy.
Mutations of KIT (encoding proto-oncogene receptor tyrosine kinase) activate intracellular MAPK and phosphatidylinositol 3-kinase (PI3K) pathways (Figure 2).7 KIT mutations are found in mucosal and acral melanomas as well as chronically sun-damaged skin, with frequencies of 39%, 36%, and 28%, respectively. Imatinib was associated with a 53% response rate and PFS of 3.9 months among patients with KIT-mutated melanoma but failed to cause regression in melanomas with KIT amplification.8
FIGURE 2. Binding of ligands to receptors with tyrosine kinase activity (eg, c-KIT) promotes the activation of downstream signaling pathways, including RAS, CRAF, MEK, ERK (extracellular signal-regulated kinase), PI3K (phosphoinositide 3-kinase), and AKT. Inhibition by imatinib or by different BRAF and MEK inhibitors represents clinically relevant strategies. mTOR indicates mammalian target of rapamycin; PTEN, phosphotase and tensin homolog deleted on chromosome 10; RTK, receptor tyrosine kinase.
Anti–CTLA-4 Immune Checkpoint Inhibition
CTLA-4 is a protein found on T cells that binds with another protein, B7, preventing T cells from killing cancer cells. Hence, blockade of CTLA-4 antibody avoids the immunosuppressive state of lymphocytes, strengthening their antitumor action.9 Ipilimumab, an anti–CTLA-4 antibody, demonstrated improvement in median OS for management of unresectable or metastatic stage IV melanoma, resulting in its FDA approval.8 A combination of ipilimumab with dacarbazine in stage IV melanoma showed notable improvement of OS.10 Similarly, tremelimumab showed evidence of tumor regression in a phase 1 trial but with more severe immune-related side effects compared with ipilimumab.11 A second study on patients with stage IV melanoma treated with tremelimumab as first-line therapy in comparison with dacarbazine demonstrated differences in OS that were not statistically significant, though there was a longer duration of an objective response in patients treated with tremelimumab (35.8 months) compared with patients responding to dacarbazine (13.7 months).12
Anti–PD-1 Immune Checkpoint Inhibition
PD-1 is a transmembrane protein with immunoreceptor tyrosine-based inhibitory signaling, identified as an apoptosis-associated molecule.13 Upon activation, it is expressed on the cell surface of CD4, CD8, B lymphocytes, natural killer cells, monocytes, and dendritic cells.14 PD-L1, the ligand of PD-1, is constitutively expressed on different hematopoietic cells, as well as on fibroblasts, endothelial cells, mesenchymal cells, neurons, and keratinocytes.15,16 Reactivation of effector T lymphocytes by PD-1:PD-L1 pathway inhibition has shown clinically significant therapeutic relevance.17 The PD-1:PD-L1 interaction is active only in the presence of T- or B-cell antigen receptor cross-link. This interaction prevents PI3K/AKT signaling and MAPK/extracellular signal-regulated kinase pathway activation with the net result of lymphocytic functional exhaustion.18,19 PD-L1 blockade is shown to have better clinical benefit and minor toxicity compared to anti–CTLA-4 therapy. Treatment with anti-PD1 nivolumab in a phase 1b clinical trial (N=107) demonstrated highly specific action, durable tumor remission, and long-term safety in 32% of patients with advanced melanoma.20 These promising results led to the FDA approval of nivolumab for the treatment of patients with advanced and unresponsive melanoma. A recent clinical trial combining ipilimumab and nivolumab resulted in an impressive increase of PFS compared with ipilimumab monotherapy (11.5 months vs 2.9 months).21 Similarly, treatment with pembrolizumab in advanced melanoma demonstrated improvement in PFS and OS compared with anti–CTLA-4 therapy,22,23 which resulted in FDA approval of pembrolizumab for the treatment of advanced melanoma in patients previously treated with ipilimumab or BRAF inhibitors in BRAF V600 mutation–positive patients.24
Lymphocyte-Activated Gene 3–Targeted Therapies
Lymphocyte-activated gene 3 (LAG-3)(also known as CD223 or FDC protein) is a type of immune checkpoint receptor transmembrane protein that is located on chromosome 12.25 It is present on the surface of effector T cells and regulatory T cells that regulate the adaptive immune response.26 Lymphocyte-activated gene 3 is reported to be highly expressed on the surface of tumor-infiltrating lymphocytes, thus the level of LAG-3 expression was found to corelate with the prognosis of tumors. In some tumors involving the kidneys, lungs, and bladder, a high level of LAG-3 was associated with a worse prognosis; in gastric carcinoma and melanoma, a high level of LAG-3 indicates better prognosis.27 Similar to PD-1, LAG-3 also is found to be an inhibitory checkpoint that contributes to decreased T cells. Therefore, antibodies targeting LAG-3 have been gaining interest as modalities in cancer immunotherapy. The initial clinical trials employing only LAG-3 antibody on solid tumors found an objective response rate and disease control rate of 6% and 17%, respectively.25,26,28 Given the unsatisfactory results, the idea that combination therapy with an anti–PD-L1 drug and LAG-3 antibody started gaining attention. A randomized, double-blind clinical trial, RELATIVITY-047, studying the effects of a combination of relatlimab (a first-in-class LAG-3 antibody) and nivolumab (an anti–PD-L1 antibody) on melanoma found longer PFS (10.1 months vs 4.6 months) and a 25% lower risk for disease progression or death with the combination of relatlimab and nivolumab vs nivolumab alone.28 The FDA approved the combination of relatlimab and nivolumab for individuals aged 12 years or older with previously untreated melanoma that is surgically unresectable or has metastasized.29 Zhao et al30 demonstrated that LAG-3/PD-1 and CTLA-4/PD-1 inhibition showed similar PFS, and LAG-3/PD-1 inhibition showed earlier survival benefit and fewer treatment-related adverse effects, with grade 3 or 4 treatment-related adverse effects occurring in 18.9% of patients on anti–LAG-3 and anti–PD-1 combination (relatlimab plus nivolumab) compared with 55.0% in patients treated with anti–CTL-4 and anti–PD-1 combination (ipilimumab plus nivolumab)(N=1344). Further studies are warranted to understand the exact mechanism of LAG-3 signaling pathways, effects of its inhibition and efficacy, and adverse events associated with its combined use with anti–PD-1 drugs.