Jiyun Chen

Structural Basis for Accommodation of Nonsteroidal Ligands in the Androgen Receptor

The mechanism by which the androgen receptor (AR) distinguishes between agonist and antagonist ligands is poorly understood. AR antagonists are currently used to treat prostate cancer. However, mutations commonly develop in patients that convert these compounds to agonists. Recently, our laboratory discovered selective androgen receptor modulators, which structurally resemble the nonsteroidal AR antagonists bicalutamide and hydroxyflutamide but act as agonists for the androgen receptor in a tissue-selective manner. To investigate why subtle structural changes to both the ligand and the receptor (i.e. mutations) result in drastic changes in activity, we studied structure-activity relationships for nonsteroidal AR ligands through crystallography and site-directed mutagenesis, comparing bound conformations of R-bicalutamide, hydroxyflutamide, and two previously reported nonsteroidal androgens, S-1 and R-3. These studies provide the first crystallographic evidence of the mechanism by which nonsteroidal ligands interact with the wild type AR. We have shown that changes induced to the positions of Trp-741, Thr-877, and Met-895 allow for ligand accommodation within the AR binding pocket and that a water-mediated hydrogen bond to the backbone oxygen of Leu-873 and the ketone of hydroxyflutamide is present when bound to the T877A AR variant. Additionally, we demonstrated that R-bicalutamide stimulates transcriptional activation in AR harboring the M895T point mutation. As a whole, these studies provide critical new insight for receptor-based drug design of nonsteroidal AR agonists and antagonists.

A Selective Androgen Receptor Modulator for Hormonal Male Contraception

The recent discovery of nonsteroidal selective androgen receptor modulators (SARMs) provides a promising alternative for testosterone replacement therapies, including hormonal male contraception. The identification of an orally bioavailable SARM with the ability to mimic the central and peripheral androgenic and anabolic effects of testosterone would represent an important step toward the “male pill”. We characterized the in vitro and in vivo pharmacologic activity of (S)-3-(4-chloro-3-fluorophenoxy)-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethylphenyl)propionamide (C-6), a novel SARM developed in our laboratories. C-6 was identified as an androgen receptor (AR) agonist with high AR binding affinity (Ki = 4.9 nM). C-6 showed tissue-selective pharmacologic activity with higher anabolic activity than androgenic activity in male rats. The doses required to maintain the weight of the prostate, seminal vesicles, and levator ani muscle to half the size of the maximum effects (i.e., ED50) were 0.78 ± 0.06, 0.88 ± 0.1, and 0.17 ± 0.04 mg/day, respectively. As opposed to other SARMs, gonadotropin levels in C-6-treated groups were significantly lower than control values. C-6 also significantly decreased serum testosterone concentration in intact rats after 2 weeks of treatment. Marked suppression of spermatogenesis was observed after 10 weeks of treatment with C-6 in intact male rats. Pharmacokinetic studies of C-6 in male rats revealed that C-6 was well absorbed after oral administration (bioavailability 76%), with a long (6.3 h) half-life at a dose of 10 mg/kg. These studies show that C-6 mimicked the in vivo pharmacologic and endocrine effects of testosterone while maintaining the oral bioavailability and tissue-selective actions of nonsteroidal SARMs.

Preclinical Characterization of a (S)-N-(4-Cyano-3-Trifluoromethyl-Phenyl)-3-(3-Fluoro, 4-Chlorophenoxy)-2-Hydroxy-2-Methyl-Propanamide: A Selective Androgen Receptor Modulator for Hormonal Male Contraception

The pharmacologic effects of (S)-N-(4-cyano-3-trifluoromethyl-phenyl)-3-(3-fluoro, 4-chlorophenoxy)-2-hydroxy-2-methyl-propanamide (S-23) were characterized in male rats as an animal model of hormonal male contraception. S-23 showed high binding affinity (inhibitory constant = 1.7 +/- 0.2 nm) and was identified as a full agonist in vitro. In castrated male rats, the ED50 of S-23 in the prostate and levator ani muscle was 0.43 and 0.079 mg/d, respectively. In intact male rats treated for 14 d, S-23 alone suppressed LH levels by greater than 50% at doses greater than 0.1 mg/d, with corresponding decreases in the size of the prostate but increases in the size of levator ani muscle. In intact male rats treated for up to 10 wk with S-23 and estradiol benzoate (EB; necessary to maintain sexual behavior in rats), S-23 showed biphasic effects on androgenic tissues and spermatogenesis by suppressing serum concentrations of LH and FSH. EB alone showed no effect on spermatogenesis. In the EB + S-23 (0.1 mg/d) group, four of six animals showed no sperm in the testis and zero pregnancies (none of six) in mating trials. After termination of treatment, infertility was fully reversible, with a 100% pregnancy rate observed after 100 d of recovery. S-23 increased bone mineral density and lean mass but reduced fat mass in a dose-dependent manner. This is the first study to show that a selective androgen receptor modulator combined with EB is an effective and reversible regimen for hormonal male contraception in rats. The beneficial effects of S-23 on the muscle, tissue selectivity, and favorable pharmacokinetic properties make it a strong candidate for use in oral male contraception.

Effect of B-ring substitution pattern on binding mode of propionamide selective androgen receptor modulators

Selective androgen receptor modulators (SARMs) are essentially prostate sparing androgens, which provide therapeutic potential in osteoporosis, male hormone replacement, and muscle wasting. Herein we report crystal structures of the androgen receptor (AR) ligand-binding domain (LBD) complexed to a series of potent synthetic nonsteroidal SARMs with a substituted pendant arene referred to as the B-ring. We found that hydrophilic B-ring para-substituted analogs exhibit an additional region of hydrogen bonding not seen with steroidal compounds and that multiple halogen substitutions affect the B-ring conformation and aromatic interactions with Trp741. This information elucidates interactions important for high AR binding affinity and provides new insight for structure-based drug design.