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