Study Comparing Investigational Drug HBI-8000 + Nivolumab vs. Placebo + Nivolumab in Patients With Advanced Melanoma

This is a multicenter, randomized, double-blind, placebo-controlled Phase 3 study of HBI-8000 or Placebo combined with nivolumab. Randomization of eligible patients will be stratified by PD-L1 expression (positive, ≥1% expression level versus negative, \<1% expression level) and LDH (normal versus elevated) in the main study. Adults with new, progressive brain metastasis, or adolescents with or without new progressive brain metastasis will be enrolled in a separate, non-randomized, open-label cohort to receive the combination of HBI-8000 and nivolumab. In the main study, eligible patients will be randomized within the appropriate stratum at a 1:1 ratio to the Test arm or the Control arm. Study treatment will be initiated within 3 days of randomization. A treatment cycle consists of 28 days. Patients will be treated with one of the following: Test arm: HBI-8000 30 mg oral BIW + nivolumab IV at specific doses on specific days Control arm: Placebo oral BIW + nivolumab IV at specific doses on specific days The Study Treatment (HBI-8000 or Placebo) is administered approximately 30 minutes after a full meal. The Study Treatment (HBI-8000 or Placebo) will be administered twice a week on the following days of every 28-day cycle: * CxW1: Days 1, 4 * CxW2: Days 8, 11 * CxW3: Days 15, 18 * CxW4: Days 22, 25 Study treatment must commence within 3 days after randomization and continue up to 2 years or until disease progression (confirmed), unacceptable toxicity or patient withdrawal of consent. In addition to Study Treatment, nivolumab is administered at specific doses on specific days as an intravenous infusion over approximately 30 minutes. Nivolumab will be administered on Day 1 of each cycle. For non-randomized cohort for special population, eligible subjects will receive HBI-8000 30 mg oral BIW and nivolumab IV at specific doses on specific days, under the same schedule as described above. For adolescents weighing \< 40 kg, nivolumab will be dosed at specific doses every 4 weeks. Nivolumab will be administered on Day 1 of each cycle.