This is a multicenter, multi-arm, international, phase Ib, 3 x 3 dose-escalation study with an initial phase I followed by an expansion phase. The primary objective of the phase Ib is to determine the maximum tolerated dose (MTD), recommended phase 2 dose (RP2D), and dose limiting toxicity (DLT) of various combinations of pembrolizumab, pralatrexate and decitabine.