S-40503 is an investigational

selective androgen receptor modulator Selective Androgen Receptor Modulators or SARMs are a class of androgen receptor ligands that maintain some of the desirable effects of androgens, such as preventing osteoporosis and muscle loss while reducing risks of developing prostate cance ...

(SARM) developed by the Japanese company Kaken Pharmaceuticals, which was developed for the treatment of osteoporosis. SARMs are a new class of drugs which produce tissue-specific

anabolic Anabolism () is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy, known also as an endergonic process. Anabolism is the building-up aspect of metabolism, whereas catabolism is the breaking ...

effects in some tissues such as muscle and bone, but without stimulating androgen receptors in other tissues such as in the

prostate gland The prostate is both an accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found only in some mammals. It differs between species anatomically, chemically, and physio ...

, thus avoiding side effects such as

benign prostatic hypertrophy Benign prostatic hyperplasia (BPH), also called prostate enlargement, is a noncancerous increase in size of the prostate gland. Symptoms may include frequent urination, trouble starting to urinate, weak stream, inability to urinate, or loss o ...

which can occur following treatment with unselective androgens like

testosterone Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristi ...

or anabolic steroids. S-40503 is a SARM that shows good

functional selectivity Functional selectivity (or “agonist trafficking”, “biased agonism”, “biased signaling”, "ligand bias" and “differential engagement”) is the ligand-dependent selectivity for certain signal transduction pathways relative to a referen ...

for bone tissue, and has relatively little effect on muscle mass and no observable effect on the prostate gland. In animal studies it was shown to increase both bone mineral density and biomechanical strength of femoral cortical bone, and at low doses showed anabolic effects only on bone tissue, while at higher doses both bone and muscle growth were affected, yet prostate gland enlargement was not seen at any dose tested. The lack of

virilizing Virilization or masculinization is the biological development of adult male characteristics in young males or females. Most of the changes of virilization are produced by androgens. Virilization is most commonly used in three medical and biology ...

effects means that S-40503 may even be suitable for use in women, which would be a substantial advantage over existing drugs as women tend to be more likely to suffer from osteoporosis, and are generally contraindicated from taking anabolic steroids due to the risk of side effects such as masculinisation and

hirsutism Hirsutism is excessive body hair on parts of the body where hair is normally absent or minimal. The word is from early 17th century: from Latin ''hirsutus'' meaning "hairy". It usually refers to a "male" pattern of hair growth in a female that ...

. Since these promising initial studies, no further data about S-40503 has been published by Kaken, and it is thought that rather than being developed for human use itself, S-40503 may be more likely to be used as a lead compound for the development of novel derivatives with similar bone anabolic effects, but longer ''in vivo'' half-life and better bioavailability. Selective androgen receptor modulators may also be used by athletes to assist in training and increase physical stamina and fitness, potentially producing effects similar to anabolic steroids but with significantly less side effects. For this reason, SARMs have already been banned by the

World Anti-Doping Agency The World Anti-Doping Agency (WADA; french: Agence mondiale antidopage, AMA) is a foundation initiated by the International Olympic Committee based in Canada to promote, coordinate, and monitor the fight against drugs in sports. The agency's key ...

since January 2008 despite no drugs from this class yet being in clinical use, and blood tests for all known SARMs are currently being developed.

References

{{Androgen receptor modulators Primary alcohols Nitro compounds Quinolines Selective androgen receptor modulators