Abstract 6381: SAMHD1high macrophages drive resistance to cytarabine in acute myeloid leukemia (AML) by pyrimidine metabolite deoxycytidine (dC)

Abstract Increasing evidence highlights macrophages as critical players in tumorigenesis and therapeutic resistance in both solid and liquid tumors. While macrophages in solid cancers are largely pro-tumor and associated with poor prognosis, their contribution to acute myeloid leukemia (AML) pathogenesis—a deadly cancer of myeloid origin—remains controversial. To elucidate precise role of macrophages in AML, we analyzed AML patient dataset (BeatAML) and revealed that high macrophage gene signatures correlated with poor survival outcomes (n=57). We demonstrated that macrophages promote chemoresistance to standard-of-care chemotherapy drug cytarabine (AraC) in AML, as evidenced by the increased sensitivity to AraC following macrophage depletion in AML patient-derived xenograft (PDX) models and mouse syngeneic leukemia models. To identify direct contribution of macrophages in mediating chemoresistance, we conducted in vitro coculture experiments and drug screen (n=31 drugs) with bone-marrow derived macrophage conditioned media (BMDM-CM). In vitro BMDM-CM fractionation experiments revealed that the resistance is primarily mediated by soluble factors (<3kDa) secreted by macrophages. Functional genetic screening using genome-wide CRISPR Knock-out studies identified deoxycytidine kinase (DCK) as a critical target of soluble factors. Metabolite profiling pinpointed deoxycytidine (dC), a pyrimidine metabolite, as the primary soluble factor secreted by macrophages that inhibited DCK activity in AML cells to drive AraC resistance. Intriguingly, single-cell sequencing of healthy and AML samples, and cancer-wide Dependency Map (DepMap) analysis identified that SAM and HD domain-containing protein 1 (SAMHD1) causes dC accumulation. Inhibition of SAMHD1 and dihydroorotate dehydrogenase (DHODH) blocked dC biogenesis and reversed AraC resistance mediated by macrophages in vitro. Further, single cell RNA sequencing analysis of paired AML patient samples revealed an enrichment of SAMHD1high macrophages/monocytes in patient bone marrow after AraC-based chemotherapies. Since SAMHD1 is an interferon-stimulated gene, we hypothesize that the inflammatory state in the bone marrow microenvironment could modulate SAMHD1 expression. Consistent with this hypothesis, the inflammatory level is indeed correlated with SAMHD1 expression in primary AML samples according to BeatAML patient database, implying that inflammation might give rise to AraC resistance by modulating SAMHD1 in immune microenvironment. Abstract Collectively, our findings uncover a novel mechanism of AraC resistance in AML mediated by macrophage-derived dC. We propose that targeting dC metabolism in macrophages could be a potential strategy to overcome chemoresistance and improve outcomes for AML patients. Citation Format: Chuqi Wang, Yuhan Wang, Camillo Benetti, Karanpreet Bhatia, Xiao Xian Lin, Petra Hyroššová, Edward Ayoub, Jakub Rohlena, Katerina Rohlenova, Michael Andreeff, Shruti Bhatt. SAMHD1high macrophages drive resistance to cytarabine in acute myeloid leukemia (AML) by pyrimidine metabolite deoxycytidine (dC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6381.