A Study of Fedratinib With IDH Inhibition in Advanced-Phase, IDH-Mutated Ph-Negative Myeloproliferative Neoplasms

Inclusion Criteria: Must be diagnosed with advanced-Phase IDH-mutated Ph-neg MPNs (both untreated and relapsed/refractory) including any of the following: * polycythemia vera with (PV) ≥ 5% blasts * essential thrombocythemia (ET) with ≥ 5% blasts * primary myelofibrosis (PMF) with ≥ 5% blasts * Atypical CML with ≥ 5% blasts * MPN-NOS with ≥ 5% blasts * MDS/MPN Overlap Syndromes with ≥ 5% blasts including CMML * post-PV myelofibrosis with ≥ 5% blasts * post-ET myelofibrosis with ≥ 5% blasts * Patients can be on cytoreduction at time of study enrollment with hydroxyurea or steroids. * Age ≥18 years. Because no dosing or adverse event data are currently available on the use of fedratinib in combination with IDH inhibitors in patients \<18 years of age, children are excluded from this study, but will be eligible for future pediatric trials. * Eastern Cooperative Oncology Group performance status ≤2 (see Appendix A). Patients must have normal organ and marrow function as defined below: * Creatinine clearance ≥30 mL/min, determined by the Cockroft-Gault formula, OR serum creatinine ≤ 1.5 x ULN * AST and ALT ≤3 x ULN and bilirubin ≤1.5 x ULN (unless considered due to Gilbert's syndrome, leukemic involvement, or extravascular hemolysis in the spleen) Other eligibility criteria includes the following: * Patients must be at least 2 weeks from major surgery, radiation therapy, or participation in other investigational trials, and must have recovered from clinically significant tox…