Abstract 4033: Chemical trapping of inactive GRB2 dimer unveils a novel cancer vulnerability by coupling replication stress to anti-tumor immunity

Abstract Growth Factor Receptor-Bound protein 2 (GRB2), a critical signal transduction adaptor, has been an elusive therapeutic target due to its ubiquitous SH2/SH3 domains, despite prior clinical evidence (e.g. a striking 75% complete remission in AML patients using liposomal antisense GRB2 RNA) supporting the high value of its inhibition. We overcame this long-standing "undruggable" challenge by leveraging our discovery that the GRB2 dimer is its auto-inhibited form with a unique druggable interface. We will report the structure-guided discovery and optimization of small molecule GRB2 inhibitors (GRB2i) that act by stabilizing this inactive dimer. Combining experimental structural biochemistry and AI-Machine Learning (AI-ML) refinement across chemical space, we have developed potent clinical leads. Our data reveal GRB2i primarily targets cancer-critical DNA Damage Response (DDR) pathways, rather than canonical GRB2 roles in cell growth and proliferation. Specifically, our GRB2i chemically induces catastrophic Homologous Recombination (HDR) deficiency and replication fork instability. This vulnerability renders cancer cells exquisitely sensitive to PARP inhibitors and other DNA damaging agents. Importantly, we show that GRB2i activates the cGAS/STING pathway, triggering the release of pro-inflammatory cytokines and creating a significant immune destruction liability for cancer cells. These collective results unveil a novel structure-guided strategy for small molecule GRB2 inhibition, demonstrating its potential as a single agent or strategic combination therapy to enhance anti-tumor immunity and exploit a central cancer vulnerability. Citation Format: Zamal Ahmed, Darin E. Jones, John A. Tainer. Chemical trapping of inactive GRB2 dimer unveils a novel cancer vulnerability by coupling replication stress to anti-tumor immunity 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 4033.