Abstract 249: MOMA-313, an inhibitor of the DNA polymerase theta helicase domain, prevents BRCA reversions in preclinical models.

Abstract MOMA-313 is a small molecule inhibitor of the DNA polymerase theta (Polθ) helicase domain in development for the treatment of homologous recombination (HR)-deficient tumors. Polθ repairs DNA double-strand breaks (DSBs) by annealing flanking regions of microhomology, a process called theta-mediated end-joining (TMEJ). In preclinical models, MOMA-313 synergizes with PARP inhibitors to drive deeper responses both in vitro and in vivo, demonstrating the potential for combination benefit in the clinic. Clinical resistance to PARP inhibitors is caused by BRCA reversions – secondary mutations that restore BRCA function – that often have the hallmarks of Polθ-mediated DNA repair. It has been hypothesized that, in addition to providing mechanistic synergy, inhibiting Polθ may extend the duration of PARP inhibitor responses by preventing or delaying the emergence of BRCA reversions. However, due to the challenge of creating reversion mutations via a clinically relevant mechanism, this hypothesis has not been experimentally addressed. Here, we use genetic tools to create targeted DNA single-strand breaks (SSBs) – the most common form of endogenous DNA damage – which can be converted to DSBs during DNA replication and are efficiently repaired by HR. As expected, SSBs in HR-proficient cells show no evidence of mutagenic DNA repair. In contrast, we find that in the context of HR deficiency, SSBs are repaired as genomic deletions flanked by regions of microhomology, suggesting a specific role for Polθ in the DNA repair process. Using MOMA-313 to inhibit Polθ activity, we demonstrate a broad role for Polθ in DNA repair initiated by SSBs in HR-deficient models, with MOMA-313 preventing the vast majority of DNA repair products. The extent of Polθ-mediated repair was unexpectedly broad, extending beyond canonical TMEJ products to include a large proportion of repair products with only one base pair of DNA microhomology. Importantly, the induction of SSBs adjacent to the pathogenic BRCA2 mutation in Capan-1 cells produced genomic deletions that restored the BRCA2 reading frame and caused cellular resistance to PARP inhibitor treatment. MOMA-313 dramatically reduced the frequency of these BRCA reversion mutations, preventing the emergence of PARP inhibitor resistance. These data provide experimental evidence that Polθ activity is a key contributor to PARP inhibitor resistance and provide a strong rationale for early and sustained Polθ inhibition in BRCA-mutant tumors to drive deeper and more durable clinical responses. Citation Format: Giulia Bottoni, Anthony Tubbs, Kelly McGlynn, Vinny Motwani, Michael Reutershan, Timothy Guzi, Erica Evans, Peter Hammerman, Jordan A. Krall. MOMA-313, an inhibitor of the DNA polymerase theta helicase domain, prevents BRCA reversions in preclinical models abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 249.