Eligible subjects will be 1:1 randomized to placebo (Control Arm A) and pembrolizumab (Experimental Arm B). Stratification factors for randomization: presence of visceral metastatic disease (lung, liver, or bone or other organs vs. lymph node only) at the time of initiation of first-line chemotherapy, and response to first-line chemotherapy (CR/PR vs. SD. Subjects who progress on placebo will be assessed to determine if they are eligible to cross over to unblinded treatment with pembrolizumab.