Identification of ADAR1i-124: The first effective A-to-I RNA editing inhibitor with promising cancer therapeutic potential.

Two ADAR1 isoforms, p150 and p110, are involved in adenosine-to-inosine RNA editing. ADAR1p150-mediated hyper-editing of endogenous dsRNAs prevents their activation of type I interferon signaling-mediated via Melanoma Differentiation-Associated Protein 5 (MDA5), which enables cancer resistance to immune checkpoint blockade. ADAR1p150 also inhibits Z-RNA-mediated activation of Z-DNA Binding Protein 1 (ZBP1) and induction of necroptosis. ADAR1p110 suppresses the formation of telomeric repeat R-loops, which would otherwise induce apoptosis in telomerase-reactivated cancer cells. Together, ADAR1 inhibitors could serve as novel cancer therapeutics. Here, we identified, ADAR1i-124, which inhibits the catalytic activities of both ADAR1p150 and ADAR1p110. ADAR1i-124 activated MDA5 and ZBP1 pathways and dose-dependently inhibited viability across different types of cancer cell lines. Some cancer cell lines, unresponsive to ADAR1i-124 alone, became responsive when co-treated with 5-Aza-CdR. The DNA methylase inhibitor reactivated endogenous retroviruses, leading to the formation of retrovirus dsRNAs and the emergence of a new ADAR1 dependency. Our study establishes the potential of ADAR1i-124 as a future cancer therapeutic.