This is a Phase 1/2a dose-escalation study of E6201, a dual MEK1 and FLT3 inhibitor, in subjects with advanced hematologic malignancies with documented FLT3 mutation. The Phase1 portion of the study will be a safety run-in to establish a recommended Phase 2 dose (RP2D). The Ph. 2a portion of the study will evaluate two specific patients groups: Cohort 1 will enroll up to 26 patients with relapsed or refractory AML and confirmed FLT3 mutation without prior exposure to a FLT3 inhibitor while Cohort 2 will enroll up to 12 patients with relapsed or refractory AML and confirmed FLT3 mutation with prior exposure to a FLT3 inhibitor. Each cohort of the expansion phase will incorporate a Simon 2-stage optimal design. A total of up to N=62 subjects will be enrolled in the study.