Abstract LBA004: Identification of GSPT1-directed molecular glue degrader (MGD) for the treatment of Myc-driven breast cancer

The Myc family of transcription factors is a well-established driver of human cancers. However, despite being amongst the most frequently mutated, translocated and overexpressed oncogenes, no therapy directly targeting the Myc family members has been developed to date. Abnormal activation of Myc results in uncontrolled cell growth that is associated with high translational output and ramp up of the protein translational machinery. This creates a dependency to protein translation and in turn represents a potential therapeutic vulnerability for Myc-driven tumors. Based on these considerations, we hypothesized that targeting the translational termination factor GSPT1, a key player of protein synthesis, may constitute a vulnerability for Myc-driven tumors. Using our proprietary Quantitative and Engineered Elimination of Neosubstrates (QuEEN^TM) platform we characterized and explored the known G-loop degron in GSPT1 that renders it amenable to cereblon-induced degradation by molecular glue degraders (MGDs). We rationally designed and subsequently screened a proprietary library of cereblon-binding small molecules, including GSPT1-directed MGDs, in human mammary epithelial cells (HMECs) expressing doxycycline-inducible c-Myc. Doxycycline treatment led to sustained c-Myc expression and as a consequence to the induction of key biomarkers of enhanced protein translation, such as phospho 4EBP1 (p4EBP1). We identified MRT-048 as a potent and highly selective GSPT1 degrader and demonstrated its ability to induce cell death in Myc-driven HMEC cells whilst sparing control cells (EC50 0.64 μM vs 30 μM respectively). This confirmed the selective vulnerability of Myc-driven cell growth to GSPT1 degradation. In follow-up studies, we confirmed the correlation between p4EBP1 as biomarker of Myc-activation and sensitivity to MRT-048 in a large panel of breast cancer cell lines. Moreover, MRT-048 treatment of animals xenografted with breast cancer cells induced tumor regression and was associated with complete GSPT1 degradation. Mechanistically, we observed that GSPT1 degradation induced by MRT-048 led to inhibition of genes regulated by Myc and ribosomal stalling at stop codons of several mRNAs. Additionally, polysome profiling of cancer cells treated with MRT-048 was associated with a global reduction of the intensities of the polysome peaks and concomitant increase in the monosome peaks as previously observed in GSPT1 knockdown experiments, suggesting that GSPT1 degradation by our MGD molecules affects both the termination and initiation stages of protein translation. We believe these data support the therapeutic potential of GSPT1-directed MGDs in Myc-driven tumors dependent on protein translation machinery. Citation Format: Gerald Gavory, Bernhard Fasching, Debora Bonenfant, Amine Sadok, Ambika Singh, Martin Schillo, Vittoria Massafra, Anne-Cecile d’Alessandro, John Castle, Mahmoud Ghandi, Agustin Chicas, Frederic Delobel, Alexander Flohr, Giorgio Ottaviani, Thomas Ryckmans, Anne-Laure Laine, Oliv Eidam, Hannah Wang, Ilona Bernett, Laura Chan, Chiara Gorrini, Theo Roumiliotis, Jyoti Choudhary, Yann-Vai LeBihan, Marc Cabry, Mark Stubbs, Rosemary Burke, Rob Van Montfort, John Caldwell, Rajesh Chopra, Ian Collins, Silvia Buonamici. Identification of GSPT1-directed molecular glue degrader (MGD) for the treatment of Myc-driven breast cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr LBA004.