Therapeutic Targeting of FLT3 Gate Keeper Mutation with E2082-0047 in Traditional and a Novel Immunocompetent Murine Adoptive Transfer Model of AML

Signaling mutations such as Fms-like tyrosine kinase 3 (FLT3) internal tandem duplication (FLT3-ITD) occur in approximately 30% of AML patients which provide AML cells with a constitutively active aberrant growth advantage, resulting in inferior overall survival. Multiple phase 3 studies with different FLT3 inhibitors validated FLT3-ITD as an important therapeutic target in AML. However, resistance to FLT3-ITD inhibitors occurs via mutations in the tyrosine kinase domain (FLT3-TKD), including the gatekeeper mutation F691L for which all approved FLT3 inhibitors are ineffective. Other mechanisms of secondary FLT3 inhibitor resistance include induction of IRAK4 activity and acquisition of RAS/PTPN11/MAPK activating mutations. New FLT3 inhibitors targeting both intrinsic (FLT3 acquired mutations) and extrinsic mechanisms of FLT3 resistance (e.g. IRAK4 induction) are urgently needed.

We demonstrated previously the selectivity and potency of a novel FLT3 inhibitor, E2082-0047, against FLT3-ITD and TKD mutations in vitro using a Ba/F3 cell line harboring different TKD mutations. Guided by KinomeScan data, we demonstrated simultaneous IRAK4 inhibition and verified superior in vivo efficacy compared to gilteritinib using the FLT3-ITD MOLM-13 cell line derived xenograft (CDX) murine model (Elgamal OA et al., ASH 2022). Recognizing the exceptional safety profile of E2082-0047 in pre-clinical IND enabling toxicology studies, we moved forward to test the potency of E2082-0047 versus gilteritinib in cells harboring the FLT3-ITD-F691L gatekeeper mutation using two in vivo models, MOLM-14-F691L CDX model and a novel immunocompetent murine adoptive transfer model that does not require conditioning or irradiation. This model is generated by the crossing of the CD45.2 specific Npm1^cA cre inducible model (Vassiliou et al., 2011) with the Flt3^ITD (Lee et al., 2007) model which was previously reported to harbor a F692L mutation, analogous to the human F691L mutation (Dovey et al., 2016) thus, generating the Npm1^cA;Flt3^ITD-F692L AML murine model.

In the MOLM-14-F691L CDX model study, we found a consistent reduction in human CD45 cells in bone marrow and increased survival benefit in mice receiving E2082-0047 compared to gilteritinib. Further, mice in the gilteritinib group with progressing disease (> 50% human CD45+ cells in bone marrow) were switched to E2082-0047 with consequent > 5-fold reduction in bone marrow tumor burden within 8 days. These data suggest that E2082-0047 will be effective in gilteritinib resistant disease. Strikingly, in the AML adoptive transfer model, E2082-0047 resulted in a compelling long-term survival benefit. In our first study (donor #1), vehicle and gilteritinib cohorts had 29- and 32-days median survival, respectively with evident circulating CD45.2 AML. In the E2082-0047 cohort, mice remained on study with an average of <2% circulating CD45.2 cells therefore, we stopped treatment and euthanized three mice for tissue collection, leaving eight mice for disease relapse monitoring for an additional two weeks where we lost 3/8 mice due to early removal criteria (ERC) and 5/8 were euthanized at end of study day 100, with an average of <10% circulating CD45.2 cells. This initial study showed the capacity for E2082-0047 to induce long-term survival in a model with a clinically relevant TKD gatekeeper mutation. We repeated our study with a more aggressive donor (donor #2) where vehicle and gilteritinib arms had a median survival of 22 days. While mice receiving E2082-0047 remain on study (week 8 of an ongoing study), with an average of <5% peripheral blood CD45.2 level (Figure 1).

Collectively, our previous and follow-up studies herein demonstrate the superiority of E2082-0047 over gilteritinib against FLT3-ITD and with TKD mutations including the gatekeeper F691L variant. We demonstrate significantly improved survival after E2082-0047 treatment in an immune deficient in vivo model of FLT3-ITD/TKD-F691L MOLM-14 AML and a durable long-term survival in a large subset of mice using two donors in an immune competent adoptive transfer model of NPM1+FLT3-ITD/TKD-F692L AML. Given the curative potential of E2082-0047 in the immune competent model, ongoing efforts are focused on investigating the host immune response to AML. Given the safety and efficacy studies performed with E2082-0047 to date, a healthy volunteer study to best estimate dose for trials in AML is ongoing.