|Other names||GTx-024; MK-2866; Ostarine; S-22|
|Elimination half-life||24 hours|
|CompTox Dashboard (EPA)|
|Chemical and physical data|
|Molar mass||389.334 g·mol−1|
|3D model (JSmol)|
|Melting point||132 to 136 °C (270 to 277 °F)|
Enobosarm, also known as ostarine or MK-2866, is an investigational selective androgen receptor modulator (SARM) developed by GTx, Inc. for the treatment of conditions such as muscle wasting and osteoporosis, formerly under development by Merck & Company.
According to a 2009 paper authored by GTx, "Readers are cautioned to note that the name ostarine is often mistakenly linked to the chemical structure of [S-4], which is also known as andarine. The chemical structure of ostarine has not been publicly disclosed." A 2009 review stated "Recently, GTx disclosed that compound 5 had advanced into clinical trials. The patent application described detailed data in an initial proof-of-concept Phase IIa clinical trial. It is not explicitly stated that compound 5 is Ostarine (MK-2866).
As of 2012, the mechanism of action of Enobosarm is still being debated and requires further investigation.
GTx Incorporated was founded in Memphis in 1997 and licensed rights to enobosarm from the University of Tennessee Research Foundation; the SARM compounds were invented by James T. Dalton, Duane D. Miller, Karen A. Veverka and their research teams at Ohio State University, the University of Tennessee and GTx, respectively.
In August 2011, there was a double-blind, placebo controlled phase II trial that focused on elderly men and postmenopausal women concluded that Enobosarm showed statistically significant improvements in total lean body mass and physical function without the negative side effects that are normally present with steroids.
In August 2013, GTx announced that enobosarm had failed in two Phase III clinical trials to treat wasting in people with lung cancer. The company had invested around $35 million in the development of the drug. The company said at that time that is planned to pursue approval of enobosarm in Europe; the company was also still developing GTx-758 for castration-resistant prostate cancer.
The FDA have warned that SARMs can have serious side effects ranging from risk of heart attack to stroke and liver damage.
Society and culture
SARMs including Enobosarm may be and have been used by athletes to assist in training and increase physical stamina and fitness, potentially producing effects similar to anabolic steroids. For this reason, SARMs were banned by the World Anti-Doping Agency in January 2008, despite no drugs from this class yet being in clinical use, and blood tests for all known SARMs have been developed. There are a variety of known cases of doping in sports with enobosarm by professional athletes.
In October 2018, UFC fighter Sean O'Malley tested positive for Enobosarm and was suspended by the Nevada State Athletic Commission and USADA for six months. O'Malley tested positive again on May 25, 2019 and was suspended for nine months by the same agencies. USADA determined that none of O'Malley's positive tests were consistent with intentional use and he was allowed to compete at UFC 248 as long as he kept his levels below the threshold of 100 ng/ml.
On January 7 2019, the College National Football Championship was played between University of Alabama and Clemson University. Prior to the College Football National Championship game, three Clemson players who were suspended, Dexter Lawrence, Braden Galloway and Zach Giella, all tested positive for a substance known as Enobosarm(ostarine). On June 23, 2019 Clemson will not release ostarine investigation findings, citing privacy law. 
On 23rd October 2020,The Union Cycliste Internationale (UCI) announced that the Italian rider Matteo Spreafico has been notified of two Adverse Analytical Findings (AAFs) for Enobosarm in two samples collected during the Giro d’Italia on 15 and 16 October 2020.
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- Bieler D (25 July 2019). "Failed PED test has a highly paid offensive lineman sharing polygraph results". Washington Post. Retrieved 25 July 2019.
One of the NFL’s highest-paid offensive linemen claimed Wednesday that he did not knowingly take a banned substance he says got him a four-game suspension — and he took a polygraph test in an attempt to prove it.
- "UCI statement on Matteo Spreafico". UCI. Retrieved 2020-10-23.