A Phase I Study of Ruxolitinib Plus Nilotinib in Chronic Phase CML Patients with Molecular Evidence of Disease

Background: BCR-ABL tyrosine kinase inhibitors (TKI) are the standard treatment for CP-CML. A subset of patients have profound molecular responses with BCR-ABL transcripts no longer detectable using RT-PCR (MR^4.5). The ENESTnd trial compared nilotinib versus imatinib as frontline therapy in CML, and reported an increase in the cumulative incidence of MR^4.5 of approximately 11% per year for the first five years in nilotinib treated patients. Discontinuation of TKIs is successful in 40-50% of patients who have a durable MR^4.5. The phosphorylation of STAT3-Y705 via the JAK-STAT signaling pathway provides a protective microenvironment for the leukemic stem cells (LSC) and is a well described mechanism of resistance to TKIs. The residual LSCs likely contribute to relapse after TKI discontinuation. Data suggests that by simultaneously blocking JAK2 and TYK2, pSTAT3 is inhibited, thereby eliminating the protective environment in the bone marrow, and sensitizing the LSCs to TKIs. Ruxolitinib is a JAK2 and TYK2 inhibitor. Here we used ruxolitinib in combination with nilotinib in CP-CML patients to establish the maximal tolerated dose (MTD) of ruxolitinib, and obtain preliminary data about the impact of this combination on BCR-ABL transcript levels.

Methods: This phase I, dose-escalation study used ruxolitinib plus nilotinib in CP-CML. All subjects were taking nilotinib prior to enrollment. Eligible subjects had a complete cytogenetic response (CCyR), yet had detectable BCR-ABL transcripts by RT-PCR at enrollment. We used a 3+3 design with 3 cohorts. The nilotinib dose remained unchanged, and the three doses of ruxolitinib were 5mg BID, 10mg BID and 15mg BID. Two additional subjects were treated at the MTD. Subjects remained on combination therapy for six months, at which point ruxolitinib was discontinued. RT-PCR was used to measure BCR-ABL transcript levels in the peripheral blood and/or bone marrow at baseline and every 3 months. The primary endpoint was the MTD of ruxolitinib. Secondary endpoints included toxicity assessment, incidence of MR^4.5 at six months, change in fatigue severity scores and impact of ruxolitinib on pSTAT3/5 inhibition assessed with a plasma inhibitory assay (PIA) Descriptive statistics were used for baseline demographics, toxicity, MR^4.5 and pSTAT3 levels. Subjects completed the fatigue severity index (FSI) questionnaire at baseline and every 3 months. A paired samples t-test was used to measure the difference in fatigue severity over time.

Results: A total of 11 patients were enrolled between April 2013 and March 2016. Median age was 41 (25-63). 73% (n=8) were male. 36% (n=4) had received one TKI prior to nilotinib. The nilotinib dose was 300mg (n=8) or 400mg BID (n=3). Median time from diagnosis to enrollment was 11 months (6-135). Each cohort enrolled 3 subjects, and two additional subjects were treated at the MTD. There were no dose limiting toxicities; therefore the MTD/RP2D of ruxolitinib was 15mg PO BID. There were no grade 3/4 adverse events in any cohort, and no clinically significant cytopenias. Grade 1/2 transaminitis occurred in 1 subject in cohorts 1 and 2. No dose reductions were needed.

At data cutoff, 9 subjects have completed six months on trial, and 2 remain active. Of those nine, 3 (33%) had ≥1-log reduction in BCR-ABL transcripts from baseline and 4 (44%) achieved MR^4.5. One subject in cohort 1 progressed after three months and a kinase domain mutation analysis found a T315I mutation.

FSI data available on seven subjects showed a non-significant decline in average fatigue severity from baseline (mean 2.78, SD 1.79) to follow-up (mean 1.86, SD 1.21), p=0.29.

Results from the plasma inhibitory assay and updated results of all 11 subjects will be presented at the meeting after all subjects will have completed the trial.

Conclusion: Our data suggest that ruxolitinib is safe and tolerable at 15mg PO BID when combined with nilotinib in CP-CML, and with no grade 3/4 adverse events reported, this should be considered the RP2D for future studies. The incidence of MR^4.5 after six months was 44% which surpasses that of historical controls, although the sample size is small and a larger study is needed to confirm these results. The combination leads to an improvement in fatigue severity that did not reach statistical significance. This data serves as justification for future studies using ruxolitinib in combination with TKIs to determine the true impact on eradication of MRD in CP-CML.