JAK2V617F Molecular Response to Ruxolitinib in Patients with PV and ET Is Associated with Lower Risk of Progression to Secondary Myelofibrosis

Introduction. Ruxolitinib (RUX) is approved for patients (pts) with Polycythemia Vera (PV) who are refractory or intolerant to hydroxycarbamide, based on composite endpoint of spleen volume reduction and Hct control (RESPONSE-1 trial) and Hct control (RESPONSE-2). Conversely, studies in pts with Essential Thrombocythemia (ET) were inconclusive, and the drug is not approved in that indication. Aim. The aim of current study was to analyze changes of JAK2V617F variant allele fraction (VAF) in PV and ET pts who were long-term treated with RUX, and the clinical correlates and outcome of molecular responses. Patients and Methods. The study included 65 PV and 12 ET pts; the latter had been enrolled in phase-2 studies and were still on RUX owing to continuing clinical benefits. Median time from diagnosis to RUX was 5.9y (0.6-24y), age 58y (25-87y), 50.6% were male, history of thrombosis in 21 pts (27%). A JAK2V617F mutation was found in 75 pts (97.4%), JAK2ex12 and MPLW515L in 1 pt each. At circa-annual intervals, JAK2V617F VAF was measured in granulocytes by digital PCR or highly sensitive (threshold <0.01%) RTQ-PCR. MPLW515L was measured by digital PCR. IWG-MRT criteria were used for clinical and molecular response assignment. The latter included partial (PMR: a >50% VAF reduction) and complete (CMR; JAK2V617F undetectable) molecular response, confirmed at >1y. We also considered a deep molecular response (DMR) as a confirmed VAF<2%. The nonparametric Wilcoxon rank-sum test, Kaplan-Meier estimate of survival and log-rank test were used as appropriate. Results. Median FU on RUX was 8.8y (1.3-14.1). The median RUX dose was 20 mg die (5-50mg). RUX was discontinued in 23 pts for AE (n=7, 9.1%), disease progression (n= 12, 15.6%), refractoriness/loss of response (n=3, 3.8%) or pt decision (n=1, 1.3%), after a median of 5.0y (0.8-13y). While on RUX, there were 3 thrombosis and 4 bleedings. Progression to secondary MF (sMF) occurred in 24 pts (31.1%) after a median of 6.0y (2-11.5); of these, 22 (92%) were PV, accounting for 34% of all PV compared to 16.6% of ET. Three pts (3.9%) transformed to AML after 9.2y (1.3-13.7y) and died, and 2 pts (2.6%) died after evolution to sMF while on RUX. A IWG-MRT response was reached in 92% for Hct, 59% for platelet count, 73% for leukocyte count, 77% for spleen volume (<50% palpatory). The median JAK2V617F VAF was 47% (21-98%) and 68% (15-99%) at diagnosis and RUX start, respectively. The overall mean VAF decrease was 64.6%+27.5%. A PMR was observed in 33 pts (44.2%), after a median time of 3.3y (1-7.6y), 51% and 50% of PV and ET; the baseline JAK2V617F VAF was 61.3±27.7% for PV and 32.3±10.4% for ET. A CMR (confirmed) was declared in 5 JAK2V617F mutated pts (6.5%; median baseline VAF 44.7±36.8%) of which 3 were PV, and in the MPLW515L mutated ET pt (baseline VAF 33%). A DMR was confirmed in 9 pts (11.7%; median baseline VAF 30.5±16.0), of which 6 were PV. Median time to CMR and DMR was 4.6y (1.1-7.6y) and 5.0y (2.1-12.1y), respectively. Mean duration of CM + DMR was 8y (7-12y). There was no correlation between any category of molecular response and response of Hct, platelets and spleen length, while normalization of leukocytes occurred more frequently in pts with PMR (93.8% vs 62.5%, P=0.02). Of the pts who progressed to sMF, 21 of 24 pts (87.5%) had not attained an at least PMR (P<0.0001). The HR of sMF evolution was 19.9 (95%CI, 2.4-166.7) for those who did not obtain any molecular response and 3.56 (95%CI, 0.37-34.6) for those who obtained PMR, using CMR+DMR as reference (P<0.0001) (Figure 1). By ROC analysis, a cutoff level of <60% JAK2V617F VAF at baseline (involving 35 pts, 45.5% of the entire population) was associated with a significantly greater likelihood to obtain CMR+DMR (37.1% vs 2.4%; P<0.0001) as well as PMR (60% vs 38.2%, P=0.01). All 3 pts who transformed to AML had not obtained any molecular response. Discussion. In this study, 18% of PV and ET pts receiving RUX developed sustained JAK2V617F complete plus deep molecular response (JAK2V617<2%) over long term treatment, that were associated with lower risk of progression to secondary MF; also pts attaining a PMR had reduced probability to develop sMF compared to those who did not show any changes in JAK2V617 VAF, overall reinforcing that attainment of molecular response may be a surrogate of disease modification. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal