Abstract 6062: Potent and orally bioavailable BCL6 protein degraders demonstrate efficacy in pre-clinical models of diffuse large B-cell lymphoma (DLBCL)

Abstract The B-cell lymphoma 6 (BCL6) transcription repressor protein is a protooncogene especially in diffuse large B-cell lymphoma (DLBCL), conferring survival, protection, and maintenance of lymphoma cells. BCL6 expression in normal B cells contributes to initiation and maintenance of the germinal centers (GC). Somatic mutations of the BCL6 gene including chromosome translocations have been shown to result in the deregulation of BCL6 expression and contributes to a subgroup of poor prognostic double- and triple-hit lymphomas. To solve this unmet clinical need, we have developed highly specific, potent, and orally bioavailable BCL6 protein degrader HSK43608 that demonstrates potent in vitro and in vivo efficacy in multiple pre-clinical DLBCL models. HSK43608 achieves 90% BCL6 degradation at 10 nM and DC50 of 1 nM in the OCI-Ly1 model following 24 hr treatment. Time-course studies showed 90% BCL6 degradation within 1 hr which could be maintained at 24 hrs. Importantly, medicinal chemistry efforts have resulted in the successful development of new-generation IMIDs to improve the oral bioavailability of BCL6 degrader for in vivo treatment. As expected, concentration of HSK43608 in tumor tissue was much higher than that in plasma in the DLBCL xenograft models. This tumor tissue enrichment of HSK43608 achieved 95% BCL6 degradation 48 hr after last dosing and convincing tumor growth inhibition. It is worth noting that BCL6 knockout mice suffer from myocarditis with eosinophils infiltration in heart muscle. The tissue distribution study demonstrated less HSK43608 in heart than in plasma, partially explained why there was no sign of myocarditis with eosinophils infiltration in the heart of the HSK43608 treated mice. Taking advantage of the new IMIDs structure, HSK43608 barely induce the degradation of the first generation IMIDs targets including GSPT1, SALL4, CK1α, Aiolos and Ikaros, indicating its safety. In summary, these results indicate HSK43608 as a potent and orally bioavailable BCL6 degrader and support further development. Citation Format: Jun Xu, Yangguang Li, Ju Wang, Pingming Tang, Yuting Liao, Chen Zhang, Pangke Yan. Potent and orally bioavailable BCL6 protein degraders demonstrate efficacy in pre-clinical models of diffuse large B-cell lymphoma (DLBCL) abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 6062.