MEK inhibition overcomes stromal-mediated resistance to a MERTK targeted therapy in AML co-cultures and vascularized mesenchymal organoids

MERTK receptor tyrosine kinase mediates pro-survival signaling and therapeutic resistance and is a potential therapeutic target in acute myeloid leukemia (AML); however, like many therapies, AML interactions with the bone marrow stroma confer resistance to MERTK inhibition, limiting therapeutic efficacy. Rational combination strategies that suppress compensatory signaling in physiologically relevant model systems are needed to overcome microenvironment-driven drug resistance in AML. Here, we demonstrate stromal-mediated ERK activation as a mechanism of resistance to the MERTK inhibitor, MRX-2843. Co-culture with stromal cell lines (Hs27 or Hs5 fibroblasts) or mesenchymal stem cells (MSCs) protected AML cell lines (NOMO-1, OCI-AML5, KASUMI-1) from induction of cell death in response to treatment with MRX-2843 (e.g. no co-culture vs. co-culture: Kasumi-1 + Hs27: 67.6% vs. 31.6% dead, OCI-AML5 + Hs5: 79.4% vs. 36.6%, NOMO-1 + MSC: 77.2% vs. 34.5%). Induction of gammaH2AX, an indicator of cell death, was also decreased in stromal co-cultures treated with MRX-2843 compared to AML mono-cultures. Mechanistically, MERTK expression was increased in AML cells cultured with stromal cells. ERK phosphorylation was also induced in stromal cell co-cultures and was refractory to inhibition by MRX-2843. Moreover, treatment with the MEK inhibitors PD0329501 or pimasertib in combination with MRX-2843 abrogated ERK phosphorylation and restored induction of gammaH2AX and anti-leukemia activity in the presence of stromal cells, confirming dependence on MEK/ERK signaling in AML cells for stromal-mediated survival. To better model the impact of the bone marrow microenvironment on anti-leukemia activity, we utilized a novel three-dimensional vascular mesenchymal organoid system that has greater stromal cell complexity and recapitulates many features of the bone marrow observed in human AML (e.g. architecture, cell-cell interactions, cytokine/chemokine production). Combined treatment with MRX-2843 and pimasertib significantly enhanced therapeutic efficacy in organoids established from NOMO-1 and KASUMI-1 AML cell lines, as evidenced by increased gammaH2AX expression compared to MRX-2843 or pimasertib monotherapies. Notably, these findings were recapitulated in organoids established from a MERTK-expressing patient-derived AML xenograft, reinforcing the translational relevance of the combination therapy. In preliminary dose-finding studies, concurrent treatment with MRX-2843 and pimasertib was well-tolerated in mice. Collectively, these data identify MEK/ERK signaling as a mechanism of stromal-mediated resistance to MERTK inhibition and establish combined treatment with MRX-2843 and a MEK inhibitor as a promising strategy for effective treatment of AML.