Abstract A052: Discovery and characterization of a p300-selective degrader demonstrates potent anti-tumor activity in preclinical models of prostate cancer
Authors: Russell Mike , Lowenstein Cassandra L , Zhang Xuqing , Roach Jeremy , Song Jianing , Nagilla Rakesh , Orth Peter , Tudor Matt , Deng Qiaolin , Sui Zhihua , Strickland Corey , Jolivette Larry J , Priestley E Scott , Mohammad Helai P 
Published: December 2023

Abstract
Strategies targeting androgen receptor (AR) in the treatment of prostate cancer (PrCa) have shown clear clinical benefit. However, a variety of AR-signaling bypass mechanisms ultimately result in progression to metastatic castrate resistant prostate cancer (mCRPC), and therapies for these patients remain an urgent unmet need. The histone acetyltransferase paralogs p300 and CBP, key coactivators of AR, are currently under investigation as attractive therapeutic targets to abrogate aberrant AR signaling but face a challenging clinical toxicity profile in hematopoietic cells. We postulate that p300 serves as a key regulator of AR signaling, and by sparing CBP we should retain potency against AR-driven PrCa while providing a larger safety margin. We discovered potent heterobifunctional degraders with demonstrated selectivity for p300 over CBP. Our tool compound exhibits degradation of p300 within 2 hours (DC50 = 7.5nM, Dmax = 79%) with minimal impact on CBP at 48 h (DC50 &gt;1uM, Dmax = 5%). The AR/ARv7 driven VCaP PrCa cell line demonstrated exceptional sensitivity to this p300 degrader in proliferation assays (gIC50 = 0.8 nM) comparable to the dual p300/CBP inhibitor, CCS-1477, or the AR degrader, ARV-110 and undergoes a cytotoxic response. Profiled against known AR target genes, our degrader was able to suppress KLK2, KLK3 and FKBP5 transcript levels akin to CCS-1477 in both LNCaP and VCaP PrCa cell lines, supporting the hypothesis that p300, not CBP, acts as the primary coactivator of AR signaling. In addition, our compound led to greater downregulation of full-length AR, ARv7 and cMyc compared to CCS-1477. Subsequent assessment in an in vivo model (VCaP CDX) demonstrated tumor growth inhibition at the highest dose tested (88%; p=2.6e-4). Lastly, we profiled our p300 degrader in human bone marrow derived hematopoietic progenitor colony-forming assays where we observed a significant reduction in toxicity (IC50 = 3.9uM) compared to either CCS-1477 (IC50 = 121nM) or a dual degrader tool compound (IC50 = 16nM), supporting the hypothesis that a p300-specific mechanism will offer an improved therapeutic index. Treatment with our p300-specific degrader resulted in a potent anti-tumor effect on PrCa cells both in vitro and in vivo, comparing favorably to both dual p300/CBP inhibitor or AR degrader strategies. Furthermore, selective degradation of p300 has the potential to minimize toxicity to hematopoietic cells commonly observed with dual targeting of CBP and p300. Taken together, these results support the role p300 may play in driving AR-mediated signaling in PrCa and demonstrate that a p300-specific degrader may serve as an effective therapeutic modality in this disease setting.
Citation Format: Mike Russell, Cassandra L Lowenstein, Xuqing Zhang, Jeremy Roach, Jianing Song, Rakesh Nagilla, Peter Orth, Matt Tudor, Qiaolin Deng, Zhihua Sui, Corey Strickland, Larry J Jolivette, E Scott Priestley, Helai P Mohammad. Discovery and characterization of a p300-selective degrader demonstrates potent anti-tumor activity in preclinical models of prostate cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr A052.