Thrombocytopenia is common in patients with myelofibrosis and is a well-established adverse prognostic factor. Both of the approved Janus kinase (JAK) inhibitors, ruxolitinib and fedratinib, can worsen thrombocytopenia and have not been evaluated in patients with severe thrombocytopenia (<50 × 109/L). Pacritinib, a novel JAK2/interleukin-1 receptor-associated kinase 1 inhibitor, has been studied in two Phase 3 trials (PERSIST-1 and PERSIST-2), both of which enrolled patients with myelofibrosis and severe thrombocytopenia. In order to better characterize treatment outcomes for this population with advanced disease, we present a retrospective analysis of efficacy and safety data in the 189 patients with severe thrombocytopenia treated in the PERSIST studies. The proportion of patients in the pacritinib group meeting efficacy endpoints was greater than in the BAT group for n35% spleen volume reduction (23% vs 2%, P = 0.0007), =50% modified Total Symptom Score reduction (25% vs 8%, P = 0.044), and self-reported symptom benefit (“much” or “very much” improved; 25% vs 8%, P = 0.016) at the primary analysis timepoint (week 24). The adverse event profile of pacritinib was manageable, and dose modification was rarely required. There was no excess in bleeding or death in pacritinib-treated patients. These results indicate that pacritinib is a promising treatment for patients with myelofibrosis who lack safe and effective therapeutic options due to severe thrombocytopenia.
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