Programmed cell death-1 (PD-1) is an immune-inhibitory receptor that belongs to the CD28/CTLA4 receptor family [
44‐
47]. PD-1 binds to two known ligands PD-L1 (B7-H1) [
44‐
48] and PD-L2 (B7-DC) which are widely expressed in a variety of tissues [
49,
50]. Once PD-1 binds to PD-L1, it negatively regulates T cell functions [
45‐
48].
PD-L1 is expressed in many tumors, including melanoma [
51,
52]. PD-1/PD-L1 interactions have been studied in animal models, as well as in vitro, and they have been shown to inhibit the effector functions of tumor-specific CD8+ T cells, thereby contributing to tumor-induced immunosuppression leading to tumor resistance to cytotoxic T cell responses [
51‐
53].
High expression of PD-L1 on tumor cells has been found to correlate with poor prognosis and survival in various cancer types, including renal cell carcinoma (RCC), ovarian carcinoma, and melanoma [
54‐
56]. However, more recently, studies have shown that expression of PD-L1 metastatic melanoma correlates with the presence of tumor-infiltrating lymphocytes (TILs) in human melanocytic lesions, such that 98% of PD-L1(+) tumors were associated with TILs compared with only 28% of PD-L1(−) tumors. PD-L1(+) melanocytes were almost always localized immediately adjacent to TILs [
57]. Interestingly, IFN-γ, a primary inducer of PD-L1 expression, was detected at the interface of PD-L1(+) tumors and TILs suggesting that TILs trigger their own inhibition by secreting cytokines that drive tumor PD-L1 expression. Consistent with this hypothesis, overall survival of patients with PD-L1(+) metastatic melanoma was significantly longer than patients with PD-L1(−) metastatic melanoma [
57]. Multiple anti-PD-1 monoclonal antibodies are currently in use and have shown promising activity in the management of advanced melanoma.
Pembrolizumab
Pembrolizumab is a humanized monoclonal antibody (IgG4/kappa isotype) that blocks the interaction between PD-1 and its ligands, PD-L1 and PD-L2. It was evaluated in an open-label phase I trial (KEYNOTE-001), which initially evaluated three different doses: 1, 3, and 10 mg/kg administered every 2 weeks. All three doses were tolerated, and given that pembrolizumab has a half-life of 21 days, the protocol changed the dosing frequency to every 3 weeks. Patients with advanced melanoma who were ipilimumab naïve (
n = 179) and ipilimumab treated (
n = 115) were enrolled and given pembrolizumab at 10 mg/kg (
n = 183) or 2 mg/kg (
n = 111) [
58]. The overall response rate was 34%: 44% in the treatment-naïve, 40% in the ipilimumab-naïve, and 28% in the ipilimumab-treated patients. These responses were durable, and the median duration of response was not reached (6–76+ weeks). Median progression-free survival (PFS) was 5.5 months, and OS was 69% at 1 year [
59]. Of note, response rates and PFS were significantly higher in patients who had high PD-L1 tumor expression [
60]. A 3-year OS update was presented at the 2016 ASCO Annual Meeting and included 655 patients who were enrolled and treated on this trial. There was a 40% 3-year OS rate in all patients including 45% OS rate in treatment-naïve patients [
61]. FDA approval of pembrolizumab in September 2014 at the dose of 2 mg/kg every 3 weeks was granted based on initial data analysis from a cohort of the phase I trial in which 173 patients received pembrolizumab 2 mg/kg (
n = 89) or 10 mg/kg (
n = 84) every 3 weeks and covered pretreated patients. The label was later expanded to include treatment-naïve patients as later data became available [
62].
The phase II clinical trial, KEYNOTE-002, evaluated two doses of pembrolizumab (2 or 10 mg/kg) compared with the investigator’s choice of chemotherapy in patients with advanced melanoma [
63]. Both the 2 and 10 mg/kg doses of pembrolizumab had improved PFS over chemotherapy (HR 0.57; 95% CI 0.45–0.73;
p < 0.0001 and HR 0.50; 95% CI 0.39–0.64;
p < 0.0001, respectively) across all subgroups, as well as a higher overall response rate compared to chemotherapy (21 and 25 vs. 4%).
Pembrolizumab was also evaluated in a multicenter, randomized phase III trial (KEYNOTE-006), which compared two different dosing schedules (10 mg/kg every 2 weeks or every 3 weeks) with ipilimumab [
64]. The overall response rate (ORR) was 33% (pembrolizumab) vs. 12% (ipilimumab). The PFS after 6 months of treatment was 45% for the pembrolizumab arms and 26% for the ipilimumab arm, with an OS of 87 versus 75%. At 12 months, OS rates were 74% (every 2 weeks) and 68% (every 3 weeks) for the two pembrolizumab arms and 58% for the ipilimumab arm. Finally, pembrolizumab was superior to ipilimumab in this study in all subset analyses of pre-specified groups, including PD-L1(+) and PD-L1(−) groups. Final OS analysis was presented at the 2016 ASCO Annual Meeting. ORR was 36–37% in the pembrolizumab groups (12–13% CR) versus 13% (5% CR) in the ipilimumab groups. At a median follow-up of 23 months, median OS was not reached for pembrolizumab. At 24 months, 55% of pembrolizumab-treated patients overall were alive, including approximately 30% who were alive and progression free [
65].
Nivolumab
Nivolumab is a fully human anti-PD-1 monoclonal antibody (IgG4). In studies evaluating efficacy and safety, nivolumab was given at various doses ranging from 0.1 to 10 mg/kg. It was tolerated at up to 10 mg/kg, which is the highest dose tested, and no maximum tolerated dose was identified [
66]. The 10 mg/kg dose of nivolumab had more high grade 3/4 drug-related adverse events (AEs) than the other doses, although the spectrum, frequency, and severity of AEs was generally similar across all doses. The incidence of immune-related AEs (irAEs) was approximately 20% and included pruritus, rash, and diarrhea. Other irAEs include increase of TSH, increase of ALT/AST, pneumonitis, infusion reaction, and vitiligo.
In a phase I trial of nivolumab in ipilimumab-naïve patients with advanced melanoma, the median OS was 17.3 months (all doses) and 20.3 months at the 3 mg/kg dose. Survival rates were 63% at 1 year, 48% at 2 years, and 41% at 3 years. Median PFS was 3.7 months across doses and 9.7 months at 3 mg/kg [
67]. Based on safety data and further studies (including CheckMate 037), nivolumab is given at a dose of 3 mg/kg every 2 weeks in subsequent trials and became the second monoclonal antibody against PD-1 receptor to be approved by the FDA for the treatment of patients with unresectable or metastatic melanoma and disease progression following ipilimumab and a BRAF inhibitor (if
BRAF V600 mutation positive).
CheckMate 037 was a phase III trial on patients with metastatic melanoma who progressed on or after anti-CTLA-4 therapy and a BRAF inhibitor (if
BRAF V600 mutation positive) which demonstrated the efficacy of nivolumab compared to the investigator’s choice of chemotherapy, with an overall response rate (ORR) of 32 vs. 11% [
68]. Nivolumab also demonstrated significant efficacy in ipilimumab-naïve patients with advanced melanoma [
69]. Long-term follow-up in the phase I study of nivolumab determined 2-year and 3-year overall survival rates of 48 and 41%, respectively, with nivolumab when given to treatment-naïve patients [
70].
The combination of ipilimumab and nivolumab given concurrently or sequentially was evaluated in a phase I study, and depending on the dose, the combination resulted in response rates of approximately 50% with many durable responses [
71]. Updated data from this trial demonstrated that concurrent treatment with nivolumab and ipilimumab resulted in a 2-year survival rate of 79% [
72]. However, there was a 62% rate of grade 3/4 irAEs at the optimal doses.
CheckMate 069 was a randomized phase II double-blind trial with 142 patients with metastatic melanoma who are treatment-naïve patients [
73]. Patients were assigned in a 2:1 fashion to ipilimumab (3 mg/kg) combined with either nivolumab (1 mg/kg) or placebo every 3 weeks for four doses, followed by nivolumab (3 mg/kg) or placebo every 2 weeks until disease progression or toxic side effects. Patients with BRAF wild-type tumors had an objective response rate of 61% in the combination group versus 11% in the ipilimumab monotherapy group (
p < 0.001). Furthermore, there were complete responses in 22% of the patients in the combination group and none in the ipilimumab monotherapy group. Median PFS was not reached in the combination therapy group and was 4.4 months in the ipilimumab group (HR 0.40; 95% CI 0.23 to 0.68;
p < 0.001). Similar results were also seen in patients with BRAF mutation-positive tumors. In a later update with a median follow-up of 24.5 months, the 2-year overall survival rate in the combination arm was 63.8% (95% CI 53.3–72.6) and 53.6% (95% CI 38.1–66.8) for those with ipilimumab alone [
74].
CheckMate 067 was a phase III double-blind study comparing nivolumab plus ipilimumab to nivolumab alone and to ipilimumab alone in treatment-naïve patients (
n = 945) with advanced melanoma. The ORR with nivolumab alone was 43.7%, in combination with ipilimumab was 57.6%, and ipilimumab monotherapy was 19% [
75]. Treatment-related AEs were more frequently seen in the combination group (grade 3/4, 55%) than with nivolumab (grade 3/4, 16%) or with ipilimumab alone (grade 3/4, 27%).