A Phase I Dose Escalation Study of KW-2449, An Oral Multi-Kinase Inhibitor against FLT3, Abl, FGFR1 and Aurora in Patients with Relapsed/Refractory AML, ALL and MDS or Resistant/Intolerant CML

Background: Activating mutations of FMS-like tyrosine kinase 3 (FLT3) occur in 30% of patients with de novo AML and confer a poor prognosis. KW-2449 is an oral multi-kinase inhibitor which is highly potent against mutant FLT3 (IC₅₀=0 1–7 nmol/L) and other tyrosine kinases including FGFR1, TrkA, Abl (including T315I) and Aurora A serine threonine kinase. Based on the activity of KW-2449 and its metabolite (M1) in both in vitro and in vivo preclinical leukemia models, KW-2449 was evaluated in patients with leukemia and MDS in this first-in-man study. M1 is formed via monoamine oxidase-B and aldehyde oxidase mediated oxidation of KW-2449.

Methods: The objectives were to assess the safety, tolerability, pharmacokinetic (PK) and pharmacodynamic effects of KW-2449 in patients with refractory/relapsed AML, ALL and MDS, or resistant/intolerant CML. A range of daily doses of KW-2449 (12.5–250 mg twice daily, i.e. 25–500 mg/day) on 2 treatment schedules (14 or 28 days) with a recovery period of 7–28 days between cycles were evaluated. Dose limiting toxicity (DLT) and maximum tolerated dose (MTD) were assessed for the 1^st cycle. The 28-day schedule was later eliminated. The plasma concentration of KW-2449 and M1 were analyzed by LC-MS/MS method. A plasma inhibitory activity (PIA) assay [Blood 108(10) 3477–83] for P-FLT3 and P-STAT5 was used to measure FLT3 inhibition.

Results: 37 patients aged 26–88 years (16 male) were treated at 7 dose levels: 25, 50, 100, 200, 300, 400, and 500 mg daily. Thirty-one patients had AML, 5 CML and 1 ALL. The mean duration of therapy was 2 cycles in AML patients and 4 cycles in CML patients. KW-2449 was rapidly absorbed and metabolized to M1. The half-lives for KW-2449 and M1 were not dose-related and ranged from 2.4 to 4.9 hours and 2.6 to 6.6 hours, respectively. Administration of KW-2449 in a BID regimen led to minor accumulation and was consistent with the short half-life. The PIA assay demonstrated the near complete down-regulation of P-FLT3 and P-STAT5 2 hours post-dose at a dosing level of 400 mg daily. The extent of inhibition was lower at 8 hours and generally absent at 12 hours post-dose. The most frequently reported adverse events (AEs; any grade, regardless of causality) were nausea (70.3%), vomiting (48.6%), fatigue (45.9%), diarrhea (32.4%), dyspnea (29.7%), febrile neutropenia (29.7%), pain in extremity (29.7%), and arthralgia (27.0%). Febrile neutropenia (24.3%), pneumonia (10.8%), and thrombocytopenia (10.8%) were the most frequently reported Grade 3/4 AEs. DLTs occurred in 2 patients: Grade 3 atrial fibrillation (100 mg daily) and Grade 3 nausea and vomiting (500 mg daily). A total of 70 SAEs were reported in 27 patients including 11 on-study deaths; only atrial fibrillation and pleural effusion were considered possibly related to KW-2449. Eight of 31 patients with AML (26%) (FLT3 mutation: 5 positive and 3 negative) and 1 of 5 patients with CML (20%) exhibited a ³ 50% reduction in peripheral blasts and/or bone marrow blasts from baseline to the end of Cycle 1. One patient (500 mg daily) with AML exhibited a > 50% decrease in peripheral blasts, increased platelets, and ANC, and decreased WBC count. A patient with CML (Bcr-Abl T315I +) lost the mutant clone while on KW-2449 treatment.

Conclusions: KW-2449 was safe and well tolerated at the dose levels evaluated. There were no complete or partial responses, but transient decreases in peripheral blood and bone marrow blasts were observed, justifying continued investigation of this agent. Sustained inhibition of P-STAT5 and P-FLT3 was not achieved at trough at the highest BID dose evaluated. TID and QID dosing schedules should be evaluated to accommodate the short t_1/2 and to achieve sufficient target inhibition.