Abstract LB516A: The irreversible FLT3 inhibitor FF-10101 is active against a diversity of FLT3 inhibitor resistance mechanisms
Authors: Ferng Timothy T. , Terada Daisuke , Ando Makoto , Tarver Theodore C. , Chaudhary Fihr , Lin Kimberly C. , Logan Aaron C. , Smith Catherine C. 
Published: June 2022

Abstract
Background: FLT3 tyrosine kinase inhibitors (TKIs) such as midostaurin and gilteritinib have improved clinical outcomes for patients with FLT3-mutant acute myeloid leukemia (AML), but resistance remains a problem. FF-10101 is the first irreversible, covalent inhibitor of FLT3 and has shown activity against FLT3 F691L and D835 mutations. We evaluated FF-10101 against an expanded panel of FLT3 resistance mutations and activating mechanisms of MAPK signaling associated with FLT3 TKI resistance in AML cell lines and patient samples.
Results: FF-10101 was uniformly active against a series of previously unevaluated FLT3 TKI resistant D835 mutations (sensitivity = IC50 &lt;10X of FLT3-ITD alone in Ba/F3 cells). FF-10101 was more active against F691L than gilteritinib (8-fold vs 14-fold increase in IC50 vs. FLT3-ITD, respectively). K429E and N676K mutations associated with clinical resistance to crenolanib and midostaurin, respectively, were also both FF-10101-sensitive. G697S and Y693N mutations, previously identified by random saturation mutagenesis screening to confer resistance to gilteritinib, also conferred moderate resistance to FF-10101 while Y693C conferred a high degree of resistance to both (IC50 &gt;50X higher than FLT3-ITD). D698N, however, was FF-10101-sensitive and gilteritinib-resistant. Using a cell-based model, we confirmed C695 as the lone residue covalently bound by FF-10101. Moreover, a mutagenesis screen for FF-10101 resistance identified only C695Y/R mutations. Structural modeling showed an aromatic π-π interaction between FF-10101 and Y693 that orients an electrophilic acrylamide portion for covalent bonding of FLT3 at C695, likely explaining resistance conferred by Y693C. Confirmatory mutagenesis experiments showed that Y693F, by recapitulating the native aromatic interaction, retained FF-10101 sensitivity while the non-aromatic Y693S mutation conferred a high degree of resistance. Finally, FF-10101 exerted generally greater cytotoxicity than gilteritinib in FLT3-ITD+ AML cell lines (MOLM-14 and MV4-11) and three FLT3-mutant AML patient samples cultured in HS5 stromal cell conditioned media that models bone marrow microenvironment (BME) induced TKI resistance. In MOLM-14s, FF-10101 more effectively suppressed rebound ERK re-activation after 24h drug exposure than gilteritinib, potentially explaining this enhanced activity. Nevertheless, MOLM-14 and MV4-11s with NRAS G12C and Q61K co-mutations exhibited significant resistance to FF-10101 and gilteritinib.
Conclusions: FF-10101 is active against nearly all mutations associated with FLT3 TKI resistance except for C695 mutations (site of FF-10101 binding) and the gilteritinib/crenolanib-resistant Y693C mutation (which has yet to be identified clinically). FF-10101 potentially overcomes resistance associated with the BME but likely remains vulnerable to RAS activating mutants. In report of phase 1 study of FF-10101 in relapsed/refractory AML (NCT03194685), 8/30 evaluable patients exhibited response (27%) with five responding patients previously receiving a FLT3 TKI, including one CRi after progression on gilteritinib. These results and our findings show that FF-10101 has potential to benefit AML patients that acquire certain FLT3 TKD resistance mutations or exhibit progression with other FLT3 TKIs, including gilteritinib.
Citation Format: Timothy T. Ferng, Daisuke Terada, Makoto Ando, Theodore C. Tarver, Fihr Chaudhary, Kimberly C. Lin, Aaron C. Logan, Catherine C. Smith. The irreversible FLT3 inhibitor FF-10101 is active against a diversity of FLT3 inhibitor resistance mechanisms [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB516A.