Abstract 3303: Paralogue selective p300 degraders induce synthetic lethality in pre-clinical models of CBP-deficient and p300-dependent malignancies

Abstract EP300 (or p300) acts as histone acetyltransferase (HAT) and transcriptional adapter or co-activator regulating transcription via chromatin remodeling. Both histone and non-histone proteins are acetylated by p300. In addition to HAT function, p300 has crotonyl-transferase activities, and that p300-catalyzed histone crotonylation directly stimulates transcription to a greater degree. p300 functions by scaffolding or as co-activator and enhancer of different transcription factors like HIF1a, BRCA-1, p53, NFκB, c-Myc, estrogen receptor (ER) and androgen receptor (AR) and other proteins such as PD-L1 and FOXP3. Selective targeting of p300 is expected to lead to therapeutic efficacy in CBP-mutant and p300-dependent malignancies with high degree of tolerability because of sparing of the other paralog CBP in normal cells. CBP mutant cancers comprise of several solid and hematological malignancies including 10% to 15% of non-small cell and small cell lung cancers harboring loss-of-function (LOF) aberrations. p300-dependent malignancies include prostate cancer, in which p300 plays a major role for androgen-dependent and -independent transactivation of the AR, MYCN-amplified neuroblastoma and ER+ breast cancers. Since the conventional CBP/p300 inhibitors do not discriminate between CBP and p300 proteins due to high sequence homology, we adopted a degrader approach, which has the potential to lead to paralog selectivity due to differentiated ternary complex formation. Herein we report best-in-class novel, potent and paralog selective p300 degraders with excellent selectivity over CBP and and other bromodomain containing proteins such as BRD4 by implementing molecular modeling, iterative medicinal chemistry and SAR based approaches. Synthetic lethality has been demonstrated while selectively degrading p300 in the cell lines having LOF mutation of another paralogue, CBP. Identified selective p300 degraders displayed excellent selectivity towards p300 degradation across tested cell lines and are orally bioavailable. Selective antiproliferative activity was observed in a panel of cell lines with CBP mutations or p300 dependency including AR+/ER+ cell lines along with the downregulation of target genes. In summary, we have identified highly selective degraders of p300 with desirable profile. Efforts are underway to advance them further towards nominating a development candidate. Citation Format: Saravanan Thiyagarajan, Chandrasekhar Abbineni, Krishna Chaitanya T, Achala Apte, Iram Khan Iqbal, Naveen Kumar R, Aravind A B, Avinash Kumar, Dabbeeru Madhu Babu, Shwetha Shwetha, Ankita Manna, Monalisha Mandal, Rituparna Panigrahi, Sivapriya Marappan, Suraj T Gore, Mohamad Fairus, Lim Yang Jin, Subhendu Mukherjee, Girish Daginakatte, Shekar Chelur, Kavitha Nellore, Murali Ramachandra, Susanta Samajdar. Paralogue selective p300 degraders induce synthetic lethality in pre-clinical models of CBP-deficient and p300-dependent malignancies 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 3303.