Phase 2 study of the lysine-specific demethylase 1 inhibitor bomedemstat for essential thrombocythemia

Novel treatments that can improve outcomes of essential thrombocythemia (ET) are needed. In a phase 2 trial, participants with ET who required cytoreduction and had inadequate response to, or were intolerant of, ≥1 standard therapy received bomedemstat at a starting dose of 0.6 mg/kg per day, titrated to achieve a target platelet count (200 × 109/L to 400 × 109/L). Primary end points were safety and response, defined as a platelet count of ≤400 × 109/L without new thromboembolic events. Of 73 who received bomedemstat, at 24 weeks, 49 of 64 evaluable participants (77%) achieved a response. Durable reductions in platelet count (≤400 × 109/L for ≥12 weeks) were observed in 52 of 72 participants (72%). Durable reduction in white blood cell count (<10 × 109/L for ≥12 weeks) was observed in 61 of 72 participants (85%); of 10 participants with elevated white blood cell count at baseline, 9 had normal white blood cell count (<10 × 109/L) at week 24. Hemoglobin levels remained stable. After 24 weeks of treatment, a decrease in variant allele frequency of CALR, JAK2, or MPL was observed in 39 of 46 (85%) evaluable participants. By week 24, 2 of 73 participants (3%) had experienced ≥1 thrombotic event, and 15 of 73 (21%) experienced ≥1 hemorrhagic event. During overall treatment period, grade 3 or 4 adverse events (AEs) occurred in 34 of 73 participants (47%). AEs led to temporary treatment interruption in 29 participants (40%) and permanent discontinuation in 11 (15%). No participants died due to AEs. Bomedemstat had clinically relevant activity and manageable safety in participants with ET. This trial was registered at www.clinicaltrials.gov as #NCT04254978.