Up to 30 subjects who have active disease despite having received a minimum of 6 months of ruxolitinib therapy including last 2 months at a stable dose will be enrolled to evaluate the safety, PK, and MTD of intravenous (IV) PU-H71 administered in combination ruxolitinib. Four ascending dose levels are planned. The planned dose levels of PU-H71 are 225 mg/m2, 300 mg/m2, 400 mg/m2, and 600 mg/m2. Additional dosing cohorts may be added at the discretion of the Safety Review Committee (SRC).