MRX-2843 is a potent and orally active inhibitor of MERTK and FLT3(IC50s of 1.3 nM and 0.64 nM, respectively). MRX-2843, a type 1 small-molecule tyrosine kinase inhibitor that abrogates activation of both MERTK and FLT3 and their downstream effectors. MRX-2843 treatment induces apoptosis and inhibits colony formation in AML cell lines and primary patient samples expressing MERTK and/or FLT3-ITD, with a wide therapeutic window compared with that of normal human cord blood cells[1].In murine orthotopic xenograft models, once-daily oral therapy prolonged survival 2- to 3-fold over that of vehicle-treated controls. Additionally, MRX-2843 retained activity against quizartinib-resistant FLT3-ITD-mutant proteins with clinically relevant alterations at the D835 or F691 loci and prolonged survival in xenograft models of quizartinib-resistant AML. Together, these observations validate MRX-2843 as a translational agent and support its clinical development for the treatment of AML.(In Vitro):In the Kasumi-1 cell line, treatment with MRX-2843 results in dose-dependent inhibition of MERTK phosphorylation. Decreased phosphorylation is evident at concentrations as low as 10 nM, with near-complete abrogation of MERTK activation at 100 to 300 nM. Similarly, treatment of Kasumi-1 cells with MRX-2843 mediates inhibition of downstream signaling through pathways important for tumor cell survival and proliferation. MRX-2843 treatment results in a decrease in relative cell numbers, with an IC50 of 143.5±14.1 nM, indicating that MRX-2843 significantly inhibits tumor cell proliferation and/or survival. Similarly, there are 34.1%±5.6% and 67.1%±2.7% apoptotic and dead cells in NOMO-1 cultures treated with 150 nM or 300 nM MRX-2843, respectively, compare with 6.8%±0.7% in vehicle-treated cultures (P<0.001). Treatment with 50 nM and 100 nM MRX-2843 results in 62.3%±6.4% and 84.1%±7.8% inhibition of colony formation, respectively, in Kasumi-1 cultures (P<0.01). Similarly, in NOMO-1 cultures, colony formation is inhibited by 54.8%±18.1% in response to treatment with 100 nM MRX-2843 (P<0.001). In MOLM-14 cells, treatment with MRX-2843 inhibits phosphorylation of FLT3 and downstream signaling through STAT5, ERK1/2, and AKT. Activation of FLT3 and its signaling pathways is almost completely abrogated by treatment with 50 nM MRX-2843, indicating somewhat higher cellular potency against FLT3 relative to MERTK.\n(In Vivo):MRX-2843 is 78% orally bioavailable at a dose of 3 mg/kg with a Cmax of 1.3 μM and a t1/2 of 4.4 hours. In MOLM-14 parental xenografts, both quizartinib and MRX-2843 increase median survival compare with that of vehicle-treated mice (172.5 days versus 40 days and 121 days versus 36 days, respectively, P<0.001). In this model, quizartinib is more effective than MRX-2843 (P<0.005), although higher doses of MRX-2843 are not evaluated. In MOLM-14:D835Y xenografts, quizartinib prolongs survival compare with that of vehicle-treated mice, but the effect is minimal (median survival 45 days vs. 36 days, P<0.001). In MOLM-14:F691L xenografts, treatment with MRX-2843 prolongs survival by almost 2-fold in NSG and NSGS mice (median survival 87 vs. 44.5 days and 87 vs. 48 days, respectively, P<0.005). Increased survival is observed in response to treatment with MRX-2843 versus quizartinib, but the difference is only significant in NSG mice.
