MK-2866

Also known as: Ostarine, Enobosarm, GTx-024, S-22

Evidence: Clinical

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Overview

Enobosarm (ostarine), a non-steroidal selective androgen receptor modulator - not a peptide; human trials at 1-3 mg daily showed gains in lean body mass and physical function.

MK-2866 (enobosarm, ostarine, GTx-024, S-22) is a non-steroidal selective androgen receptor modulator, not a peptide. In a 12-week phase 2 trial in 120 healthy elderly men and postmenopausal women, 3 mg daily raised total lean body mass (p<0.001) and improved stair-climb physical function and insulin resistance (Dalton 2011). In a phase 2 trial in 159 patients with cancer-related weight loss, 1 mg and 3 mg daily increased lean body mass over 113 days (median +1.5 kg and +1.0 kg) while placebo did not (Dobs 2013). The phase 3 POWER programme in non-small-cell lung cancer did not deliver an approval, and enobosarm has since been tested at 9 and 18 mg daily in AR-positive advanced breast cancer (Palmieri 2024, n=136). It is not approved anywhere, and it is on the WADA prohibited list.

Benefits

Lean body mass (human phase 2 RCTs: +1.5 kg median at 1 mg daily in cancer patients, dose-dependent gain in healthy elderly), physical function (human phase 2, 3 mg vs placebo p=0.013), insulin resistance (same trial, p=0.013). Bone and recovery claims are not from human trials.

Dosing reference

Research dose: 1 to 3 mg orally once daily in the lean-mass trials; 9 or 18 mg daily tested in breast cancer
Range: 1000 to 3000 mcg, 1x daily

Oral, once daily. Not a peptide: a non-steroidal SARM, not approved by any regulator. Lean-mass trials used 1 to 3 mg daily; oncology trials have gone to 9 and 18 mg daily (Palmieri 2024). The 15 to 25 mg range common in community protocols is above every muscle-wasting dose tested and within the breast-cancer range, where it was given under oncology monitoring. Rifampin cut enobosarm exposure by 43% (AUC) and probenecid raised it by 50% (Coss 2016). It is prohibited in sport by WADA and products sold as research chemicals are frequently mislabeled or contaminated (Vasireddi 2025).

Mechanism

Enobosarm is a non-steroidal ligand that binds the androgen receptor and acts as a tissue-selective agonist, anabolic in muscle and bone while largely sparing prostate in men and hair-growth-related tissue in women (Dalton 2011). Receptor activation in skeletal muscle drives protein synthesis and nitrogen retention, which is what the lean-body-mass gains in the phase 2 trials measure. Selectivity is relative, not absolute: human trials report dose-dependent suppression of endogenous testosterone through negative feedback on the hypothalamic-pituitary axis, along with falls in HDL and SHBG. It is cleared mainly by glucuronidation with a CYP3A4 component, which is why rifampin lowers and probenecid raises exposure (Coss 2016). It is not a peptide and is included for reference.

Evidence

Clinical: at least one published human trial or an approved drug

In 120 healthy elderly men and postmenopausal women over 12 weeks, 3 mg daily increased total lean body mass (p<0.001 vs placebo) and improved physical function and insulin resistance (both p=0.013) (Dalton et al., 2011)
In 159 patients with cancer and at least 2% weight loss, 113 days of 1 mg or 3 mg daily increased lean body mass (median +1.5 kg and +1.0 kg; placebo +0.02 kg, not significant) (Dobs et al., 2013)
The phase 3 POWER programme in non-small-cell lung cancer was designed around lean body mass and stair-climb power as co-primary endpoints; no approval followed (Crawford et al., 2016)
In AR-positive, ER-positive advanced breast cancer, 9 mg (n=72) and 18 mg (n=64) daily were given in an open-label phase 2 trial, the highest doses studied in humans (Palmieri et al., 2024)
A systematic review of SARM use in athletes found no approved indication for any SARM and case reports of drug-induced liver injury, myocarditis and tendon rupture, plus frequent product contamination (Vasireddi et al., 2025)
Human-trial status: multiple published phase 2 RCTs and a phase 3 programme; not approved by FDA, EMA or any other regulator, and prohibited in sport by WADA

References

Side effects

Human trials report dose-dependent suppression of endogenous testosterone and LH and falls in HDL cholesterol. Case reports and a 2025 systematic review of athlete SARM use document drug-induced liver injury, myocarditis and tendon rupture, with frequent mislabeling or contamination of products sold as research chemicals (Vasireddi 2025). In the cancer trial the serious adverse events were disease-related, not drug-attributed (Dobs 2013). Not approved for human use and prohibited in sport by WADA.

Storage

Storage: Room temperature
Lyophilized: 24+ months

Pharmacology

Half-life: Dosed once daily in every human trial; no primary human t½ measurement located (the ~24 h figure in circulation is unsourced)
Bioavailability: Oral: active orally in all human trials; the ~80% figure has no primary source
Molecular weight: 389.33 Da