Abstract LB157: Discovery of HP537, a potent and selective p300/CBP inhibitor for the treatment of hematologic malignancies

Abstract E1A-binding protein P300 (p300) and its paralog CREB binding protein (CBP) are key acetyl transferases (HAT) and transcriptional co-factors. p300/CBP bromodomain (BRD) inhibition reduces acetylation of H3K27 at enhancers, leading to suppression of enhancer-driven gene expression, such as MYC and IRF4 that are critical for the initiation, progression, and chemoresistance of hematological malignancies. Thus, p300/CBP inhibition is a highly potential therapeutic strategy for the treatment of hematological malignancies, including multiple myeloma (MM), non-Hodgkin lymphoma (NHL) and leukemia. We have discovered HP537 that competitively inhibits an acetylated peptide substrate to bind BRD of p300/CBP with IC50 5 nM. HP537 does not inhibit the enzymatic activity of HAT domain of p300/CBP and HAT1, nor has binding affinity for other BRD containing proteins including BRD4 and BRD9. HP537 inhibits proliferation of a wide range of cell lines derived from MM, leukemia and lymphoma with IC50 values ranged from 0. 01 μM to 0. 4 μM. Protein expression levels of acetylated H3K27, c-Myc and IRF4 are significantly decreased by HP537 in MM, NHL and acute myelogenous leukemia (AML) cell lines in a dose- and time-dependent manner. HP537 robustly and dose-dependently inhibits tumor growth in OPM-2 (MM), MOLM-16 (AML), and KARPAS-422 (NHL) cell line derived tumor xenograft (CDX) mouse models. Down-regulation of c-Myc and IRF4 are also observed in the tumor samples. In conclusion, HP537 is a novel, potent and selective inhibitor of p300/CBP BRD. The pre-clinical data supports the clinical development of HP537 for the treatment of hematological malignancies. A FIH study of HP537 in cancer patients is planned to start in 2024. HP537 structure will not be disclosed. Citation Format: Jing Li, Zhilin Tu, Jian Peng, Lei Fan, Hua Yu, Fei Wang, Xinghai Li. Discovery of HP537, a potent and selective p300/CBP inhibitor for the treatment of hematologic malignancies abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (7Suppl): Abstract nr LB157.