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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 2nd 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. Disclosures: Cortes: Ambit Biosciences: Research Funding. Foran:Ambit Biosciences: Research Funding. Ghirdaladze:Ambit Biosciences: Research Funding. DeVetten:Ambit Biosciences: Research Funding. Zodelava:Ambit Biosciences: Research Funding. Holman:Ambit Biosciences: Research Funding. Levis:Ambit Biosciences: Consultancy, Research Funding. James:Ambit Biosciences: Employment. Zarringkar:Ambit Biosciences: Employment. Gunawardane:Ambit Biosciences: Employment. Armstrong:Ambit Biosciences: Employment. Padre:Ambit Biosciences: Employment. Wierenga:Ambit Biosciences: Employment. Corringham:Ambit Biosciences: Employment. Trikha:Ambit Biosciences: Employment.

Activating mutations in the receptor tyrosine kinase FLT3 are present in approximately 30% of patients (pts) with acute myeloid leukemia (AML), and they have a significantly worse prognosis than pts with wild type (WT) FLT3, suggesting that the activated kinase is a driver of the disease and a potential target for kinase inhibitor therapy. AC220 is a novel 2nd generation class III receptor tyrosine kinase (RTK) inhibitor with potent in vitro and in vivo activity in FLT3-dependent tumors. It is highly selective for WT and mutant FLT3 and several other class III RTKs, including KIT, CSF1R, RET and PDGFR. AC220 is in a first-in-human phase 1 study for relapsed or refractory AML pts, unselected for FLT3 mutations. The study has a standard 3+3 dose escalation design with 50% dose increments. AC220 is administered once daily as an oral solution for 14 days followed by a 14 day rest period (1 cycle) with a starting dose of 12 mg. Concurrently, pts are being dosed on a continuous dosing regimen starting at 200 mg/day for 28 days (1 cycle). Pts with clinical benefit may continue to receive further cycles. Currently, 52 pts have been dosed with AC220 up to 450 mg/day (10 dose cohorts). Median age was 60 yrs (range, 23 to 86 yrs), median number of prior therapies was 3 (range, 0 to 8) and 2 pts had prior allogeneic hematopoietic stem cell transplant (HSCT). Two elderly patients (age ≥ 78 yrs) unfit for induction chemotherapy were previously untreated. Fifteen patients have FLT3 mutations (12 ITD and 3 TKD), 25 are WT, and 12 are undetermined. Pts are also evaluated for PK, pFLT3, pSTAT5, FLT3 genotyping and ex vivo plasma inhibitory activity. AC220 is well tolerated and MTD has not yet been observed with either schedule. One pt had a possibly drug-related DLT in the 18 mg cohort (grade 3 CHF, although pt had a pre-existing heart condition) leading to cohort expansion, but no other cases of drug-related CHF or other DLT have been seen. Other possibly drug-related AEs (most frequently gastrointestinal events) were mild (grade ≤ 2). Response data based on investigator’s assessment are available on the first 45 pts. Responses were observed in 11 (24%) pts. Four pts achieved a complete response (CR) – 2 with incomplete platelet recovery (CRp) and 2 with incomplete platelet and neutrophil recovery (CRi), one of these pts also had complete resolution of leukemia cutis. In addition, 7 pts had partial responses (PR, defined as a decrease of ≥ 50% blasts to levels of 5%–25% in the bone marrow). Most responses (8/11, 73%) occurred after cycle 1 and one was observed after cycle 3. Median duration of response is 18 weeks (range, 4 to 26+ weeks). Three responders are FLT3 mutants (2 ITD and 1TKD), 5 are WT, 3 are undetermined. Six of the 9 non-responding pts with ITD mutations had initial rapid clearing of peripheral blasts with intermittent AC220 dosing, but subsequently progressed or had disease-related mortality. All these pts had aggressive disease and received a median of 6 prior treatment regimens (range, 3 to 8). AC220 plasma exposure is sustained between dose intervals and continues to increase in a dose-proportional manner from 12 mg to 300 mg. FLT3 phosphorylation is strongly suppressed when plasma obtained from study pts is tested ex vivo in FLT3-ITD and WT cell lines at 12 mg and 60 mg doses, respectively. Assessments of pFLT3 and pSTAT5 from treated pts’ peripheral blood are ongoing and will be presented. Encouraging preliminary efficacy results and an acceptable safety profile warrants continued evaluation of AC220 as a single agent and in combination with other therapeutics for the treatment of AML.