Phase I Study Of LY2784544, a JAK2 Selective Inhibitor, In Patients With Myelofibrosis (MF), Polycythemia Vera (PV), and Essential Thrombocythemia (ET)

Background MF, PV, and ET are neoplasms characterized by activation of JAK2 signaling, and the majority of cases are associated with the gain of function JAK2 V617F mutation. Given that wild-type JAK2 plays a pivotal role in multiple stages of hematopoiesis it is desirable to treat JAK2 V617F-positive cases by selectively inhibiting JAK2 V617F while minimizing inhibition of wild-type JAK2 in order not to impede normal marrow function. LY2784544 is a selective inhibitor of JAK2 which demonstrates dose dependent selectivity for JAK2 V617F/STAT5 signaling. LY2784544 entered clinical testing in a Phase I trial in MF, PV, and ET patients (I3X-MC-JHTA, [NCT01134120][1]). We have previously reported early interim analysis from this study and provide here an update following completion of study enrollment. Methods The study’s primary objectives were to determine the safety and tolerability of LY2784544 and define a recommended oral daily dose for further study in patients with JAK2 V617F-positive MF, ET, or PV. Secondary objectives included determination of pharmacokinetics and evaluation of response using the IWG-MRT response criteria (2006), European Leukemia Net response criteria for PV and ET (EULN 2009), and symptom burden using the Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF) (Scherber et. al. Blood 2011). Study design included a standard 3+3 dose-escalation to define the maximum tolerated dose (MTD) and was amended to allow cohort expansions at multiple doses demonstrating potential clinical merit. Patients Prior to closing to enrollment, 47 patients were consented; 9 did not meet eligibility screening. The 38 treated patients included 31 MF (22 pMF, 6 post-PV MF and 3 post-ET MF), 6 PVs, and 1 ET patient(s), and 86% had received prior systemic therapy for their MPN. Dosing and tolerability of LY2784544 : Systemic exposures were approximately dose proportional across the tested range from 30 to 300 mg. Daily doses of 200 mg or greater were associated with CTCAE Grade 3 increases in serum creatinine. In general, increases in serum creatinine occurred within the first 2 weeks of either starting treatment or after an increase in study drug dose. With the available data, the elevations in serum creatinine appear to be fully reversible after dose reduction or drug holiday in the majority of patients. The maximum tolerated daily dose of LY2784544 when given on a continuous daily basis was determined to be 120 mg per day. Serious AEs, across all grades and doses, included increased creatinine/acute renal insufficiency (4 grade 3, 5 grade 2, 1 grade 1), hyperuricemia (2 grade 4, 1 grade 1), hyperkalemia (1 grade 1), and anemia (1 grade 3). This study allowed intrapatient dose escalation, and 34 patients received 120 mg during their study participation. There were 4 patients dosed at 120 mg with SAEs but all were unrelated to study drug treatment (anemia, influenza, enterococcal bacteremia, hematuria and pneumonia). At 120 mg, the most frequently reported drug-related AEs across all grades were diarrhea (44%), nausea (29%), increased creatinine (21%), and anemia, vomiting, fatigue (9% each). Of these related AEs at 120 mg, there was 1 Grade 3 anemia, 3 Grade 3 increased creatinine, and 2 Grade 3 fatigue. There were no Grade 4 AEs. Efficacy Among a cohort of 10 MF patients who only received 120 mg, 3 achieved a clinical improvement. Analyzing across dose levels, a palpable spleen length reduction of ≥50% was seen in 15 of 27 evaluable patients (56%) at any time. The median duration of these responses was 18.3 weeks (range from 0.7 to 148.1 weeks). At 12 weeks, 56% of patients (15/27) reported a ≥50% improvement in Total Symptom Score on the MPN-SAF. Of the 6 PV patients, 3 achieved a clinicohematologic PR. Findings from a central pathology review of serial bone marrow samples will be presented at the meeting. Conclusions A recommended phase II dose for daily dosing of LY2784544 was identified as 120 mg. This dose has been well tolerated and associated with clinical improvements, supporting ongoing Phase II testing of LY2784544 in MPNs. Disclosures: Verstovsek: Eli Lilly and Co: Research Funding. Mesa: Eli Lilly and Co: Research Funding; Genetech: Research Funding; Gilead: Research Funding; Incyte: Research Funding; Sanofi: Research Funding; NS pharma: Research Funding; Celgene: Research Funding. Salama: Eli Lilly and Co: Research Funding. Giles: Eli Lilly and Co: Employment. Pitou: Eli Lilly and Co: Employment. Hayden Zimmermann: Eli Lilly and Co: Employment. Price: Eli Lilly and Co: Employment. Walgreen: Eli Lilly and Co: Employment. Prchal: Eli Lilly and Co: Research Funding. [1]: /lookup/external-ref?link_type=CLINTRIALGOV&access_num=NCT01134120&atom=%2Fbloodjournal%2F122%2F21%2F665.atom