Prostate cancer (PCa) is the most common male malignancy in U.S.. In contrast to the localized early-stage disease, prognosis of castration-resistant prostate cancer (CRPC), i.e., the relapsed/metastatic advanced disease phenotype, is extremely poor. Because the sustained androgen receptor (AR) signaling remains one of the central driving forces of CRPC, new-generation nonsteroid AR antagonist has been developed in recent years, exemplified by the approval of enzalutamide (Enz) in 2012 for post-chemo and in 2014 for chemotherapy-naïve CRPC. However, drug resistance inevitably develops and survival benefits of Enz in CRPC patients are short-lived. Isothiocyanates (ITCs), e.g., sulforaphane (SFN) and phenethyl isothiocyanate (PEITC), are active metabolites of dietary glucosinolate from cruciferous vegetables with broad anti-PCa activities, such as suppressing AR expression, downregulating antiapoptotic proteins, inducing cell cycle arrest and apoptosis, and inhibiting PCa stem-like cells, etc. SFN-rich extract was recently tested in recurrent PCa patients and notably prolonged PSA doubling time without severe adverse events. By modifying Enz structure, we designed and synthesized AR ligand-ITC hybrid drug, aimed at antagonizing androgen-stimulated AR transcriptional activity and meanwhile downregulating AR expression and inducing other anticancer effects in PCa cells. To increase water solubility, ITCs were further reacted with N-acetyl cysteine (NAC) to obtain NAC conjugates. A representative hybrid drug NAC conjugate 2-63 was selected for in vitro study. In PBS buffer (pH 7.4), 2-63 gradually decomposes and releases free ITC with a half-life of about 3.4h. Measured using MTT assay, 2-63 was more tolerable in noncancerous RWPE-1 cell, significant differences were observed between RWPE-1 and PCa LNCaP cells, suggesting NAC conjugate could be a water-soluble carrier of AR ligand-ITC hybrid drug. The slower release of ITC constrains the concentration of electrophilic ITC within limit that is tolerated in healthy tissue. The newly designed hybrid drug chemical scaffold retains AR antagonist activity, evidenced by the inhibition of ARE-regulated luciferase activity in kb2 cell and the suppression of DHT-stimulated proliferation of LNCaP cell. 2-63 also dose- and time-dependently inhibited growth of CRPC VCaP cell and disrupted mitochondrial membrane potential, more effectively than SFN and Enz. Analyzed using Western blot, 2-63 downregulated the expression of full-length AR (fAR) and AR splice variants 7 (AR-V7). The AR downregulating properties were also observed in LNCaP and in 22Rv1 cells carrying different AR mutants. Since AR-V expression is one of the mechanisms causing clinical resistance to Enz treatment, targeting both fAR and AR-Vs using the newly discovered AR ligand-ITC hybrids may provide a new modality to improve the prognosis in CRPC patients.

Citation Format: Siyu Ou, Liping Xu, Zhihui Qin. Synthesis and in vitro anti-prostate cancer activities of androgen receptor ligand-isothiocyanate hybrid drug [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A197.