AC220, a Potent, Selective, Second Generation FLT3 Receptor Tyrosine Kinase (RTK) Inhibitor, in a First-in-Human (FIH) Phase 1 AML Study.

Abstract 636

Activating mutations in the FLT3 RTK are present in ∼30% of AML patients (pts), who have a significantly worse prognosis than pts with wild type (WT) FLT3. AC220 is a novel 2^nd generation RTK inhibitor with potent in vitro and in vivo activity in FLT3- and KIT-dependent tumor cell lines. It is highly selective for both WT and mutant FLT3 with significant activity against KIT. A first-in-human Phase 1 study investigating AC220 in predominantly relapsed or refractory AML pts, unselected for FLT3 mutations, was recently completed, using a standard 3+3 dose escalation with 50% dose increments. AC220 was administered once daily as an oral solution initially with an intermittent dosing (ID) regimen: 14 days on and 14 days off (1 cycle). Starting dose was escalated from 12 to 450 mg/day ID. Additional cohorts were investigated on a continuous dosing (CD) regimen: 200 and 300 mg/day for 28 days (1 cycle). A total of 76 pts (46 male, 30 female) were dosed with AC220. Median age was 60 yrs (23-86), median number of prior therapies was 4 (0-9), 12 pts had prior allogeneic transplant and 3 elderly pts (≥72 yrs) unfit for induction chemotherapy were previously untreated. Eighteen pts (24%) had FLT3 ITD mutations, 47 (62%) were WT, and 11 (14%) were undetermined. Pts were also evaluated for PK, phosphorylated (p) FLT3, pKIT, pSTAT5, and ex vivo plasma inhibitory activity. AC220 plasma exposure was sustained between dose intervals and continued to increase in a dose-proportional manner from 12 to 450 mg with a half-life of ∼1.5 days. An active metabolite, AC886, was detected that has similar potency and activity to AC220. Patient plasma at '12 mg potently inhibited pFLT3 in ex vivo FLT3-ITD cell lines and complete inhibition of pFLT3 in WT cell lines was observed at higher doses. Target inhibition was also observed, with suppression of pFLT3, pSTAT5 and pKIT in peripheral blasts. The most commonly reported possibly drug-related AEs were GI events, peripheral edema, and dysguesia, which were Grade ≤2. DLT was observed at 300 mg CD and 200 mg CD was declared as the MTD. Two pts at 300 mg CD had possibly study drug-related DLTs with grade 3 QTc prolongation, but had confounding factors including concomitant medications known to prolong QTc. Responses (IWG criteria) were observed in 23 (30%) pts. PR and CR were observed as low as the 18 and 40 mg cohorts, respectively. Most responses occurred within cycle 1. Overall, 9 (12%) pts had a complete response (CR) with 2 CR, 5 CRi, and 2 CRp. One of these pts also had complete resolution of leukemia cutis. In addition, 14 (18%) pts achieved PR. Overall median duration of response (MDOR) was 14 (4-62+) weeks and overall median survival (MS) was 14 (1-68+) weeks. In FLT3-ITD pts the MDOR was 12 (4-27+) weeks and the MS was 18 (3-42) weeks. In FLT3-WT pts the MDOR was 32 (8-62+) weeks and the MS was 11 (1-68+) weeks. 10 (56%) of 18 FLT3-ITD pts were responders (1 CR, 3 CRi, 6 PR), 9 (19%) of 47 FLT3-WT pts (1 CRi, 2 CRp, 6 PR), and 4 (36%) of 11 undetermined pts (1 CR, 1 CRi, 2 PR). At 200 mg CD (MTD expansion), 4 of 6 FLT3-ITD pts responded (1 CR, 1 CRp, 1 CRi, 1 PR). Of the 4 responders, 2 failed prior treatment with sorafenib and 2 previously refractory pts went onto transplant. The 2 FLT3-ITD non-responders had 6 and 8 prior lines of therapy, respectively. These encouraging efficacy results and an acceptable safety profile warrant continued evaluation of AC220 as monotherapy and in combination therapy for the treatment of AML. Phase 2 studies in FLT3-ITD positive and WT pts are in progress.