Ostarine

Muscle Growth & Body Composition

Also known as: MK-2866, Enobosarm, GTx-024, S-22

Selective Androgen Receptor ModulatorsResearch phase: Phase 3Regulatory: Not FDA-approved. WADA prohibited. Investigational drug. Widely available as research chemical.

Mechanism

Ostarine is the most extensively studied SARM (selective androgen receptor modulator), designed to selectively stimulate androgen receptors in muscle and bone without the broad hormonal side effects of traditional anabolic steroids. It promotes lean muscle gain, bone density, and fat loss. It has completed Phase 3 clinical trials for muscle wasting and stress urinary incontinence.

Technical detail

Ostarine (enobosarm/MK-2866) is a non-steroidal selective androgen receptor modulator that binds AR with high affinity (Ki ~3.8 nM) and exhibits tissue-selective agonism in muscle and bone with minimal androgenic effects on prostate and sebaceous glands. Its selectivity arises from tissue-specific cofactor recruitment and differential AR conformational changes. Phase 3 trials (POWER 1 & 2) demonstrated significant improvements in lean body mass and physical function in cancer-associated cachexia, though FDA approval was not granted due to inconsistent functional endpoints.

Evidence

Activity and safety of enobosarm, a novel, oral, selective androgen receptor modulator, in androgen receptor-positive, oestrogen receptor-positive, and HER2-negative advanced breast cancer (Study G200802): a randomised, open-label, multicentre, multinational, parallel design, phase 2 trial
Palmieri et al. (2024) — Lancet Oncology — PMID: 38342115
In postmenopausal women with AR-positive, ER-positive, HER2-negative advanced breast cancer, 9 mg and 18 mg oral enobosarm were evaluated in a multicenter randomized phase 2 study and showed measurable clinical activity with a reported 24-week clinical benefit endpoint in the AR-positive evaluable population.
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Effects of enobosarm on muscle wasting and physical function in patients with cancer: a double-blind, randomised controlled phase 2 trial
Dobs et al. (2013) — Lancet Oncology — PMID: 23499390
In patients with cancer-related muscle wasting, oral enobosarm 1 mg and 3 mg produced significant increases in lean body mass by day 113 versus baseline, while placebo did not; common serious adverse events were largely related to underlying cancer progression rather than study drug.
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