A Phase I/II Study of CYT387, An Oral JAK-1/2 Inhibitor, In Myelofibrosis: Significant Response Rates In Anemia, Splenomegaly, and Constitutional Symptoms

CYT387 is a potent JAK-1/2 inhibitor that suppresses the in vitro growth of cells harboring JAK2V617F (Leukemia 2009;23:1441) and was effective in a murine model of myeloproliferative neoplasms (MPN) (Blood 2010;115:5232).

To assess the safety, tolerability, and pharmacokinetic behavior of CYT387 in a Phase I dose-escalation study in patients with high- or intermediate-risk primary myelofibrosis (PMF) and post-PV or post-essential thrombocythemia (ET) myelofibrosis. The secondary objective was evaluation of preliminary efficacy. CYT387 was administered orally once daily in 28-day cycles. Once dose-limiting toxicity (DLT) was identified, a dose-confirmation cohort initiated treatment at the maximum tolerated dose (MTD) or lower.

Thirty six subjects (median age 64 years) have been enrolled (targeted accrual 120); 18 each in the dose escalation and dose confirmation phases. Twenty-three subjects had PMF, 8 post-PV MF, and 5 post-ET MF; 81% were JAK2V617F-positive. Median palpable spleen size was 18 cm and 20 subjects (56%) were red cell transfusion-dependent at study entry. Prior treatment included JAK inhibitors (9 and 1 subjects with INCB018424 and TG101348, respectively) and pomalidomide in 9 patients. The median treatment duration to date is 15 weeks (range 4–38). Dose-linear plasma exposures were observed up to 300 mg/day, with mean elimination T_1/2 at steady state ranging from 3.9 to 5 hours across doses.

All 36 subjects were evaluable for toxicity. At 400 mg/day, 2 of 6 subjects experienced DLT (1 each with asymptomatic grade 3 hyperlipasemia and grade 3 headache that were reversible upon holding drug); consequently, the MTD was declared at 300 mg/day. In the dose-confirmation phase, subjects were started at one of 2 dose levels that were deemed clinically effective: 150 mg/day (n=15) and 300 mg/day (n=3). Thirty-five subjects are currently on active therapy: 100 mg/day (n=2), 150 mg/day (n=20), 300 mg/day (n=10), and 400 mg/day (n=3).

CYT387 was well tolerated. No grade 4 non-hematological toxicities were observed. Grade 3 non-hematologic adverse events were infrequent and included increased transaminases (n=2), increased alkaline phosphatase (n=2), headache/head pressure (n=2), increased lipase (n=1), and QTc prolongation (n=1). Thirteen (36%) subjects experienced “first-dose effect” characterized by grade 1 lightheadedness and hypotension; this phenomenon was self-limited and generally resolved within 3–4 hours with rare recurrence.

Grade 3/4 thrombocytopenia was seen in 8 (22%) subjects, and treatment-emergent grade 3 anemia was seen in 1 subject only (3%). Treatment-emergent grade 3/4 neutropenia was not observed.

Thirty two of 36 subjects who completed at least 1 cycle were eligible for response assessment:

Twenty two subjects were evaluable for anemia response (baseline Hgb <10 g/dL or red cell transfusion-dependent). Of these, 9 subjects (41%) achieved the threshold of response for “Clinical Improvement (CI)” per the International Working Group for MPN Research and Treatment (IWG-MRT) criteria, including 2 of 4 subjects who were previously treated with INCB018424. An additional 5 subjects experienced a >50% reduction in transfusion requirement, thus increasing the total anemia response rate to 63%.

Thirty of 32 evaluable subjects had splenomegaly at baseline: median 20 cm; range 10–32 cm. Twenty nine subjects (97%) had some degree of spleen size reduction (median 9 cm; range 2–18 cm): 11 (37%) patients have achieved a minimum 50% decrease in palpable spleen size, thus qualifying them for a CI, including 3 of 8 subjects (38%) who were previously treated with INCB018424.

The proportion of patients with the following symptoms at baseline, are: fatigue (97%), pruritus (22%), night sweats (38%), cough (13%), bone pain (28%), and fever (16%). At last follow up, improvement (complete resolution) in these symptoms was reported by 68% (16%), 86% (57%), 83% (75%), 75% (50%), 78% (44%), and 100% (100%), respectively.

CYT387 is first-in-class of the JAK inhibitors with a significant response rate in anemia in myelofibrosis patients. The drug also shows substantial activity in reducing spleen size and controlling constitutional symptoms. CYT387 is well tolerated, and treatment responses have been seen both at (300 mg/day) and below (150 mg/day) the MTD.

Pardanani:Cytopia Inc.: Research Funding. Off Label Use: Clinical trial data for CYT387 use in Myelofibrosis. Fida:YM Biosciences Australia: Employment, Equity Ownership. Burns:YM Biosciences Australia: Employment, Equity Ownership. Smith: YM Biosciences Australia: Employment, Equity Ownership.