AC220, a Potent and Specific FLT3 Inhibitor, Enhances the Cytotoxic Effects of Chemotherapeutic Agents in Cell Culture and in Mouse Tumor Xenografts.
Authors: Belli Barbara A. , Dao Alan , Bhagwat Shripad , Wierenga Wendell , Armstrong Robert C 
Published: October 2019

Abstract
Abstract 2052
Poster Board II-29
AC220, a potent and selective FLT3 inhibitor, has a sub-nanomolar anti-proliferative EC50 against cells containing activated FLT3 receptor. Acute myeloid leukemia patients harboring an activated FLT3 receptor have a poorer prognosis following standard chemotherapy compared to patients with a wild-type receptor. This suggests that the combination of FLT3 inhibitor with standard chemotherapeutic regimens may improve efficacy. Other investigators have reported potential antagonistic consequences when dosing the clinical compound lestaurtinib prior to chemotherapy agents utilizing both in vitro tumor cell lines (Levis, M. et al. Blood 2004;104:1145-1150) and primary patient samples (Brown, P. et al. Leukemia 2006;20:1368-1376). We therefore explored the effect of combining AC220 with cytarabine (ara-C), cladribine, etoposide and daunorubicin against the homozygous FLT3 ITD (internal tandem duplication) cell line MV4-11 in vitro. Data were analyzed utilizing the median effect method of Chou and Talalay (Chou TC, Talalay P. Adv. Enz. Reg. 1984;22:27-55). The results indicated that delivery of AC220 concurrent with or the day following treatment with the individual chemotherapeutic agents generally provided additive to slightly synergistic effects. Unlike the results with lestaurtinib, treating the cells with AC220 prior to the addition of the chemotherapeutic agent did not result in significant antagonism. To determine whether these in vitro results translated to increased in vivo efficacy, two experiments were performed. In the first experiment, mice bearing MV4-11 solid tumors were treated with decitabine (5-aza) at 1 or 3 mg/kg QD X 5 alone or in combination with AC220 at 0.5 mg/kg delivered QD either concurrent with 5-aza or immediately following treatment. Three cycles of treatment were delivered, and, in cycle two and three, AC220 dosing was continuously maintained. AC220 alone was tumor-static regardless of treatment initiation date. 5-aza alone at 3 mg/kg reduced tumor volume by 50%, whereas 5-aza at 1 mg/kg was ineffective. When the two drugs were delivered concurrently, or when a single cycle of 5-aza preceded AC220, the combination led to an additive antitumor effect. Importantly, there was no indication of antagonism on tumor volume, body weight and WBC counts with either of the tested schedules, indicating that the combination of AC220 with 5-aza is safe and efficacious in the MV4-11 solid tumor mouse flank model. In the second experiment, mice were treated with ara-C at 30 mg/kg on a 10 day on, 14 day off, 10 day on regimen, either alone or in combination with AC220 at 1.0 mg/kg QD. AC220 dosing was initiated concurrent with the first treatment of ara-C and maintained over the 34 day period, or immediately following the first ara-C cycle and given for 14 days. AC220 alone led to tumor regression and a 20% remission rate with no cures. Ara-C alone reduced tumor burden by approximately 50% following the first cycle of treatment. Ara-C further reduced tumor burden in mice treated with AC220, and AC220 was efficacious when dosed following 10 day treatment with ara-C. Episodic dosing of ara-C concurrent with continuous AC220 leads to a significant increase in TTE (time to endpoint, 1500mm3) relative to AC220 alone (105 vs. 80 days, respectively). In this cohort, remission rate was 30% and two mice achieved cures. As with 5-aza, there was no indication of drug specific antagonism with ara-C on either of the schedules tested. Together, these data indicate that the combination of AC220 with ara-C or 5-aza, when delivered concurrently or episodically, is safe and leads to increased efficacy in the MV4-11 solid tumor mouse flank model. This result merits further investigation in future clinical trials.

Disclosures:
Belli: Ambit Biosciences: Employment. Dao:Ambit Biosciences: Employment. Bhagwat:Ambit Biosciences: Consultancy. Wierenga:Ambit Biosciences: Employment. Armstrong:Ambit Biosciences: Employment.