This is a multicenter 2-part, Phase 1b study designed to assess the safety, tolerability, PK and preliminary efficacy of PU-H71 (dihydrochloride salt) in subjects taking concomitant ruxolitinib. The first part (Dose Escalation) will employ a standard 3+3 dose escalation design to determine MTD. The second part of the study (Dose Confirmation) will confirm the RP2D in an expanded population.