Development of 5N-Bicalutamide, a High-Affinity Reversible Covalent Antiandrogen

Resistance to clinical antiandrogens has plagued the evolution of effective therapeutics for advanced prostate cancer. As with the first-line therapeutic bicalutamide (Casodex), resistance to newer antiandrogens (enzalutamide, ARN-509) develops quickly in patients, despite the fact that these drugs have ∼10-fold better affinity for the androgen receptor than bicalutamide. Improving affinity alone is often not sufficient to prevent resistance, and alternative strategies are needed to improve antiandrogen efficacy. Covalent and reversible covalent drugs are being used to thwart drug resistance in other contexts, and activated aryl nitriles are among the moieties being exploited for this purpose. We capitalized on the presence of an aryl nitrile in bicalutamide, and the existence of a native cysteine residue (Cys784) in the androgen receptor ligand binding pocket, to develop 5N-bicalutamide, a cysteine-reactive antiandrogen. 5N-bicalutamide exhibits a 150-fold improvement in K_i and 20-fold improvement in IC₅₀ over the parent compound. We attribute the marked improvement in affinity and activity to the formation of a covalent adduct with Cys784, a residue that is not among the more than 160 androgen receptor point mutations associated with prostate cancer. Increasing the residence time of bound antiandrogen via formation of a covalent adduct may forestall the drug resistance seen with current clinical antiandrogens.