FLX925 Is a Rationally Designed FLT3, CDK4/6 Inhibitor with a Desirable Resistance Profile
Authors: Marubayashi Sachie , Park Adam , Noubade Rajkumar , Phan Huyen , Cutler Gene , Kassner Paul , Fridman Jordan S 
Published: October 2019

Abstract
Acquired secondary resistance mutations in FLT3 have been shown to limit the therapeutic benefit of FLT3 inhibitors in FLT3-ITD mutated AML. Multiple strategies have been pursued to address such resistance, including the development of kinase inhibitors that use alternate binding modes or by simultaneously targeting additional pathways. This latter approach may be focused on suppression of parallel oncogenic pathways to treat the acquired resistance, or on strategies to reduce or delay the acquisition of resistance. Here, we describe a rationally conceived next generation FLT3 inhibitor, FLX925, that was prospectively designed to address or avoid common resistance mechanisms with a unique binding mode and potent activity against CDK4/CDK6. We contextualize our findings by comparing FLX925 to other FLT3 inhibitors (quizartinib and gilteritinib) currently in late-stage clinical development and demonstrate that FLX925 has a superior resistance profile.
FLX925 is a potent and selective type-1 inhibitor of FLT3 that retains its cellular potency against clinically relevant secondary resistance mutations in FLT3. This was evaluated in multiple experimental systems including isogenic Ba/F3 cells engineered to express FLT3-ITD with, or without, various known secondary FLT3 mutations. In addition, these data were extended to the human setting using both the MOLM13 and MOLM14 FLT3-ITD mutated AML cell lines and subclones of these lines harboring well-characterized resistance mutations described elsewhere. Data have been published previously demonstrating a superior resistance profile for FLX925 when compared to quizartinib and sorafenib. When compared to gilteritinib, FLX925 had a favorable profile of relative potencies against a range of FLT3-ITD resistance mutations.
In addition to the isogenic murine and human models of FLT3 inhibitor resistance, with engineered known genetic alterations, we explored the activity of FLX925 and other FLT3 inhibitors in a model of in vitro acquired resistance. Using MOLM13 cells and standard protocols for the generation of drug resistance, we demonstrate that the magnitude of resistance to quizartinib and gilteritinib greatly exceeds that observed with FLX925 during the same timeframe. Pools of resistant cells from each compound treatment are being analyzed by next-generation sequencing in an effort to better understand the mechanism of resistance associated with each compound. We hypothesize that the CDK4/6 inhibitory activity, which is unique to FLX925, contributes to its superior resistance profile. Whether this is solely linked to an impact on the cell cycle or more recent findings of CDK6-driven transcription of FLT3 and PIM1 is an active area of investigation. Nonetheless, the CDK4/6 activity of FLX925 potentially broadens the utility of this compound to FLT3 wild-type AML. Here, we show FLX925, in contrast to other FLT3 inhibitors, potently suppresses the proliferation of a panel of AML cell lines. These data have been extended to FLT3 wild-type AML patient samples in which FLX925, but not gilteritnib, induced a desirable pharmacodynamic effects. The totality of the preclinical data suggest FLX925 may be a best-in-class inhibitor for the treatment of AML with, or without, FLT3-ITD mutations. FLX925 is currently being investigated in a Ph1/b dose-escalation study in subject with relapsed or refractory AML (NCT02335814).

Disclosures
Marubayashi: FLX Bio: Employment, Equity Ownership. Park:FLX Bio: Employment, Equity Ownership. Noubade:FLX Bio: Employment, Equity Ownership. Phan:FLX Bio: Employment. Cutler:FLX Bio: Employment, Equity Ownership; Amgen, Inc: Equity Ownership. Kassner:FLX Bio: Employment, Equity Ownership. Fridman:FLX Bio: Employment, Equity Ownership.