Janus kinase inhibitors (JAKis) ruxolitinib, fedratinib, and pacritinib are the current standard of care in symptomatic myelofibrosis (MF). However, progressive disease and toxicities frequently lead to JAKi discontinuation. Preclinical data indicate that combining JAK and bromodomain and extraterminal (BET) domain inhibition leads to overlapping effects in MF. Pelabresib (CPI-0610), an oral, small-molecule BET1,2 inhibitor (BETi), in combination with ruxolitinib showed improvements in spleen volume reduction (SVR35) and total symptom score reduction (TSS50) from baseline in the phase 2 MANIFEST study (NCT02158858) in patients with MF. Given the absence of a head-to-head clinical comparison between JAKi monotherapy and JAKi with BETi combination therapy, we performed an unanchored matching-adjusted indirect comparison analysis to adjust for differences between studies and allow for the comparison of SVR35, TSS50, and TSS measured at several timepoints in arm 3 of MANIFEST (pelabresib with ruxolitinib in JAKi treatment-naive patients with MF), with data from the following JAKi monotherapy studies in JAKi treatment-naive patients: COMFORT-I and COMFORT-II (ruxolitinib), SIMPLIFY-1 (ruxolitinib and momelotinib), and JAKARTA (fedratinib). Response rate ratios >1 were observed for pelabresib with ruxolitinib vs all comparators for SVR35 and TSS50 at week 24. Improvements in TSS were observed as early as week 12 and were durable. These results indicate that pelabresib with ruxolitinib may have a potentially higher efficacy than JAKi monotherapy in JAKi treatment-naive MF.
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