Abstract B071: Evaluating the synergy between enzalutamide and SRC kinase inhibitors against castration-resistant prostate cancer
Authors: White Ralph E. , Tan Victor , Bannister Maxwell , Nguyen Hai Dang , Drake Justin M. 
Published: June 2023

Abstract
Prostate Cancer (PCa) remains the most diagnosed non-skin cancer amongst the American male population. Treatment for localized prostate cancer consists of Androgen Deprivation Therapies (ADTs), which inhibit the full-length androgen receptor (AR-FL). Though initially effective, a subset of patients will develop resistance to ADTs and the tumors will transition to Castration-Resistant Prostate Cancer (CRPC). Second-generation hormonal therapies such as abiraterone acetate and enzalutamide (Enza) are typically given to men with CRPC. However, these treatments are not curative and typically prolong survival by a few months, prompting investigators to understand the mechanisms of resistance corresponding to CRPC. Several resistance mechanisms contribute to this lack of efficacy of these therapies such as the emergence of AR mutations, AR amplification, AR splice variants (AR-Vs) and in some cases increased kinase signaling. In this study, we evaluated whether inhibition of a hyperactive tyrosine kinase in CRPC, SRC, synergizes with enzalutamide or chemotherapy in several prostate cancer cell line models expressing variable AR isoforms. Five cell lines were chosen: DU145 (AR-negative), AD-1 (AR-FL positive), R1-D567 (AR-V positive), LNCaP95 and 22Rv1 (AR-FL/AR-V positive), all of which express SRC protein. We then administered enzalutamide, the SRC family kinase inhibitor saracatinib (Sara), or docetaxel (DTX) at varying doses to each cell line to generate dose response curves and IC50 values. Each drug’s IC50 was used for combination therapy studies. To determine synergy, we implemented the Bliss Independence equation and the Combination Index Equation via Chou-Talalay method. To define the mechanism of synergy, we used western blotting and immunofluorescence to measure molecular changes (protein and mRNA) in our cell lines. In our CRPC cell lines, LNCaP95 and 22rv1, we observed robust synergy for Enza plus Sara. Interestingly, we found a lack of synergy for DTX plus Sara, in 22rv1 cells. In this cell line, we also found that Enza and Sara individually induced a greater amount of DNA damage when compared to DTX. We observed higher activation of Caspase 3/7 and cleaved PARP expression when Enza plus Sara was administered. We also observed a significant decrease in AR Y534phosphorylation, a key SRC substrate residue, on both AR-FL and AR-V with a concomitant decrease in AR-V protein expression after saracatinib administration. In addition, we observed a decrease in mRNA expression of AR target genes, TMPRSS2 and KLK3. Overall, our results suggest that SRC inhibition is a potential therapeutic strategy to be used in combination with potent AR targeting agents in the treatment of CRPC, due to its effects on AR/AR-Vs at the protein and mRNA level. By elucidating this synergy, we provide pre-clinical data that suggests combining SRC kinase inhibitors with enzalutamide in certain patients with AR-Vs.
Citation Format: Ralph E. White, Victor Tan, Maxwell Bannister, Hai Dang Nguyen, Justin M. Drake. Evaluating the synergy between enzalutamide and SRC kinase inhibitors against castration-resistant prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr B071.