AC220, a FLT3 Inhibitor, Increases Survival in Two Genotypically Distinct FLT3-ITD Models of Acute Myeloid Leukemia and Provides Sustained Protection Following Chronic Administration.
Authors: Brigham Daniel , Belli Barbara A. , Breider Michael , Bhagwat Shripad , Wierenga Wendell , Armstrong Robert C 
Published: October 2019

Abstract
Abstract 2053
Poster Board II-30
Activating mutations in the receptor tyrosine kinase FLT3 are present in up to 30% of acute myeloid leukemia (AML) patients, implicating FLT3 as a potential target for kinase inhibitor therapy. AC220, a potent and selective FLT3 inhibitor, is currently in Phase II clinical trials in both FLT3-ITD positive and WT patients. Similar to imatinib therapy for CML, it is possible that targeted therapies will be delivered chronically. Therefore, we examined the efficacy of AC220 in a limited versus chronic dosing regimen in the homozygous FLT3-ITD-dependent MV4-11 disseminated mouse disease model. As AML is a heterogeneous disease, we also examined the in vivo efficacy of AC220 in the MOLM-14 disease model. This cell line is heterozygous for a FLT-3 ITD mutation in addition to carrying the MLL-AF9 fusion. In the MV4-11 model, a 30 day dosing regimen was compared to chronic dosing. In the vehicle control group, median survival time following inoculation was 47 days, with mortality preceded by clinical signs of disease and detection of MV4-11 cells in the blood. No specific clinical signs or body weight loss were attributed to the study drug. AC220 demonstrated dose-dependent efficacy from 0.1 mg/kg to 10 mg/kg orally once per day for 30 days. The 0.1 mg/kg group had a marginal (10%) increase in life span (ILS) relative to vehicle control, while a significant increase of survival was observed at the 1.0 mg/kg dose (55% ILS). The 10 mg/kg dose led to 80% survival at study termination, day 172 (>250% ILS). Prolonged survival with AC220 correlated with delayed disease onset as measured by clinical signs and detection of circulating MV4-11 cells. Chronic dosing in the 1.0 mg/kg group further delayed disease onset and mortality with an ILS of 155% relative to vehicle, and 63% relative to 30-day dosing. Similar to the 30-day dosing group, chronic administration of AC220 at 10 mg/kg led to 80% survival at day 172. A separate study was conducted to examine the relationship between bone marrow engraftment, tumor burden in peripheral blood and disease onset. At day 20, engraftment was detected only in the bone marrow. At day 35, when clinical signs of disease are typically apparent, levels of tumor cells as high as 80% and 35% were detected in the bone marrow and peripheral blood, respectively. AC220 given for 28 days at 1.0mg/kg delayed median onset of disease by 24 days (63%), with tumor burden undetectable in the absence of clinical signs of disease. The 28 day dosing at 10.0mg/kg completely inhibited disease onset, with no detectable tumor burden in either blood or bone marrow through the end of the study (terminated on day 130, >200% ILS). In the MOLM-14 model, median survival time of untreated or vehicle treated animals was 22 days. In a 21 day dosing regimen, AC220 demonstrated dose dependent efficacy, providing 9%, 64% and 127% ILS at 0.1, 1.0 and 10 mg/kg respectively. Prolonged survival correlated with a delay in both disease onset and detection of circulating MOLM-14 cells. Similar to the MV4-11 model, temporal analysis confirmed bone marrow engraftment prior to detectable levels in the circulation. At 13 days post-transplant, tumor cells are only detectable in the bone marrow, while by day 19, when clinical signs are apparent, tumor burden in the bone marrow and periphery of vehicle treated animals are as high as 35% and 6%, respectively. Unlike the MV4-11 model, dosing at 10 mg/kg for 21 days did not prevent disease onset after cessation of dosing, indicating that tumor cells were not completely eliminated, although detectable levels are not present in bone marrow or circulation until days 47 and 52, respectively. These data suggest that AC220 is efficacious against both FLT3-ITD homozygous and heterozygous genotypes, and that a chronic dosing regimen may provide greater disease protection than a limited course of therapy. This study is consistent with observed efficacy in AML patients treated continuously with AC220.

Disclosures:
Brigham: Ambit Biosciences: Employment. Belli:Ambit Biosciences: Employment. Breider:Ambit Biosciences: Employment. Bhagwat:Ambit Biosciences: Consultancy. Wierenga:Ambit Biosciences: Employment. Armstrong:Ambit Biosciences: Employment.