The potent Janus kinase (JAK) 1/JAK2 inhibitor ruxolitinib appears to provide clinically meaningful reductions in spleen size and symptoms in patients with myelofibrosis (MF), including those patients with low platelet counts, according to the largest study of ruxolitinib in patients with MF to date. A phase 3b, expanded-access study evaluating the safety and efficacy of ruxolitinib in 2233 patients found no new safety concerns and supports the notion that this treatment is appropriate for patients with MF who have platelet counts of 50-100×109/L.

The latest analysis of the JUMP Study, published in the British Journal of Haematology, included patients with MF who have not been extensively studied — those with low platelet counts (<100×109/L) and those without splenomegaly. The primary endpoint was assessment of ruxolitinib safety and tolerability by the frequency, duration, and severity of adverse events (AEs). The patients (mean age 67 years) were treated at 279 clinical sites in 26 countries with 138 patients having baseline platelet counts of <100×109/L ( identifier: NCT01493414).

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Study findings showed that the most common AEs were anemia and thrombocytopenia, and these AEs only led to discontinuation of treatment in 5.4% of the patients and in 12.3% in the low-platelet cohort. The rates of anemia were similar in patients with low-platelets but as expected the rates of worsening thrombocytopenia (all grades) were higher in the low-platelet cohort (73.2% vs 53.5% overall). The researchers found that nonhematologic AEs (including infections) mainly were only grade 1/2.

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The study showed that the use of this agent resulted in meaningful reductions in spleen length and symptom improvements in patients without splenomegaly. However, the authors write that these findings must be interpreted with caution due to the limitations associated with a single-arm study and the small number of patients with low platelet counts who were included in the study. 


Al-Ali HK, Griesshammer M, Foltz L, et al. Primary analysis of JUMP, a phase 3b, expanded-access study evaluating the safety and efficacy of ruxolitinib in patients with myelofibrosis, including those with low platelet counts [published online February 4, 2020]. Br J Haematol. doi: 10.1111/bjh.16462