Blast phase myeloproliferative neoplasm: contemporary review and 2024 treatment algorithm

Leukemic transformation in myeloproliferative neoplasms (MPN), also referred to as "blast-phase MPN", is the most feared disease complication, with incidence estimates of 1-4% for essential thrombocythemia, 3-7% for polycythemia vera, and 9-13% for primary myelofibrosis. Diagnosis of MPN-BP requires the presence of ≥20% circulating or bone marrow blasts; a lower level of excess blasts (10-19%) constitutes "accelerated phase" disease (MPN-AP). Neither "intensive" nor "less intensive" chemotherapy, by itself, secures long-term survival in MPN-BP. Large-scale retrospective series have consistently shown a dismal prognosis in MPN-BP, with 1- and 3-year survival estimates of <20% and <5%, respectively. Allogeneic hematopoietic stem cell transplant (AHSCT) offers the possibility of a >30% 3-year survival rate and should be pursued, ideally, while the patient is still in chronic phase disease. The value of pre-transplant bridging chemotherapy is uncertain in MPN-AP while it is advised in MPN-BP; in this regard, we currently favor combination chemotherapy with venetoclax (Ven) and hypomethylating agent (HMA); response is more likely in the absence of complex/monosomal karyotype and presence of TET2 mutation. Furthermore, in the presence of an IDH mutation, the use of IDH inhibitors, either alone or in combination with Ven-HMA, can be considered. Pre-transplant clearance of excess blasts is desired but not mandated; in this regard, additional salvage chemotherapy is more likely to compromise transplant eligibility rather than improve post-transplant survival. Controlled studies are needed to determine the optimal pre- and post-transplant measures that target transplant-associated morbidity and post-transplant relapse.