S23 is an orally available, selective androgen receptor modulator (SARM). This agent was originally developed by GTX Inc. as a male form of hormonal contraception. It has a high affinity for the androgen receptor, with a Ki value of 1.7 nM. A study published by Jones et al. compared the effectiveness of S23 in both castrated and intact male rats.

Effects of S23 on Fertility

In castrated male rats, the effective dose of S23 was 0.43 mg/day in the prostate and 0.079 mg/day in the levator ani muscle. In intact male rats, a dose higher than 0.1 mg/day resulted in a suppression of luteinizing hormone (LH) by more than 50%. This dose was consistent with a reduction in prostate size and an increase in the size of the levator ani muscle.

In a study with intact male subjects treated with both S23 and estradiol benzoate (to maintain sexual behavior), no sperm was found in the testicles in four of six subjects after 10 weeks. In addition, no pregnancy was reported in any of the six laboratory animals. After treatment, the infertility was completely reversible; all subsequent mating trials resulted in pregnancy.

Effects of S23 on Muscle Mass and Body Fat

S23 shows effects on muscle mass and body fat similar to those of anabolic steroids. Studies in male rats showed that treatment with S23 led to a decrease in total body weight and fat mass. The rats in this study were also administered estradiol benzoate to maintain sexual behavior. Despite the known muscle loss due to estrogens, the combination with S23 neutralized this effect and resulted in an increase in muscle mass in the male rats.

In addition, another study showed that S23 may be effective against muscle loss caused by long-term use of glucocorticoids. This study also looked at the effects of S23 on muscle atrophy caused by castration. Muscle loss was noted in both scenarios, but the administration of S23 and testosterone blocked the dexamethasone-induced dephosphorylation of Akt. The results suggest that S23 may be an effective treatment against glucocorticoid-induced muscle loss.

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