REDUCTIONS IN JAK2V617F ALLELE BURDEN WITH RUXOLITINIB TREATMENT IN COMFORT-II, A PHASE III STUDY COMPARING THE SAFETY AND EFFICACY OF RUXOLITINIB TO BEST AVAILABLE THERAPY (BAT)

6514^ Background: Ruxolitinib is a potent and selective JAK1/2 inhibitor approved for the treatment of myelofibrosis (MF) based on results of the phase 3 COMFORT studies. Ruxolitinib demonstrated rapid and durable reductions in splenomegaly and improved MF-related symptoms and quality of life of patients (pts) with MF. Since one measure of efficacy is molecular response, this analysis correlates changes in mutant allele burden (%V617F) with spleen size reduction in COMFORT-II. Methods: COMFORT-II is a randomized, open-label, phase 3 study comparing ruxolitinib 15 or 20 mg twice daily (BID) with BAT. The primary endpoint was a ≥ 35% reduction in spleen volume from baseline (BL) at week 48. Change in %V617F was measured by allele specific qPCR. Pts were stratified by reduction in %V617F ( 20%) and results were correlated with achievement of the primary endpoint. Results: More pts in the ruxolitinib arm had ≥ 10% V617F reductions compared with BAT (41% vs 5%; P = .01; Table). The majority of ...