Abstract 6898: Identification of a novel first-in-class inhibitor targeting the ADAR1 p150 isoform demonstrates robust anti-tumor efficacy in a syngeneic mouse model

Abstract Background: Adenosine deaminase acting on RNA (ADAR1) catalyzes the conversion of adenosine (A) to inosine (I) in double-stranded RNAs (dsRNAs). It exists in two isoforms: nuclear p110 and p150, which shuttles between the nucleus and cytoplasm and is regulated by interferon (IFN) signaling. The p150 isoform contains a Z-DNA/Z-RNA binding domain and suppresses immunogenic dsRNAs and Z-RNAs arising from endogenous retroviral elements. Loss of ADAR1 leads to Z-RNA accumulation, activation of the sensor ZBP1, and RIPK3-driven necroptosis. ADAR1 promotes tumor cell survival and resistance to immune responses, making p150 inhibition a promising strategy to boost anti-tumor immunity. Herein, we report the discovery of a first-in-class Zα-targeted ADAR1 p150 inhibitor for cancer immunotherapy. Methods: A class of ADAR1 p150 inhibitors (AVA-ADR) were identified through a high-throughput ADAR1p150 binding assay. The ability of these inhibitors to induce interferons was confirmed in various cell-lines. The on-target mechanisms were confirmed by downstream PKR activation and MDA5 mediated upregulation of ISGs. Pharmacokinetic studies demonstrate the oral availability of the AVA-ADR compounds. The anti-tumor efficacy of our ADAR1 p150 inhibitors was evaluated in B16F10 syngeneic melanoma mice model as monotherapy and in combination with anti-PD-1.Results: We have identified a potential first-in-class small molecule inhibitor specifically targeting the ADAR1 enzyme via the Zα domain. Our compounds exhibit potent in vitro IFN response via an MDA5-dependent mechanism. Binding studies revealed a series of AVA-ADR compounds with strong sub-micromolar affinity for the Zα-domain. ADME and pharmacokinetic analyses demonstrate favorable PK profiles and oral bioavailability. Our ADAR1 inhibitors demonstrated promising anti-tumor activity in preclinical melanoma models, with notable tumor growth inhibition observed both as monotherapy and in combination with immune checkpoint inhibitors. The combination therapy led to enhanced efficacy compared to either treatment alone. Additionally, markers associated with immune activation were significantly upregulated in tumor samples from the combination group. Target engagement was supported by the development of a PROTAC derivative, which selectively degraded the target protein.Conclusions: To our knowledge, our AVA-ADR series of compounds are the first report of a selective small molecule inhibitor of ADAR1 p150. The 2nd generation AVA-ADR compounds described here are stable/orally bioavailable first-in-class ADAR1 inhibitors that demonstrated significant IFN induction in-vitro and in-vivo, demonstrating significant tumor growth inhibition as monotherapy and synergistically in combination with Anti -PD1. Citation Format: Avijit Goswami, Sandeep Goyal, Kawaljit Singh, Princy Khurana, Aditya Kulkarni. Identification of a novel first-in-class inhibitor targeting the ADAR1 p150 isoform demonstrates robust anti-tumor efficacy in a syngeneic mouse model [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 6898.