Ostarine (MK-2866/Enobosarm) for Muscle Preservation & Body Recomposition — SARMs
Most extensively studied SARM with Phase III data. GTx Inc (Enobosarm). FDA rejected NDA in 2018. NOT FDA-approved.
Overview
Ostarine (MK-2866, Enobosarm) is a non-steroidal selective androgen receptor modulator developed by GTx Inc., representing the most extensively studied SARM in human clinical trials. It has progressed through Phase I, Phase II, and two pivotal Phase III trials. The Phase II trial in 120 elderly men and postmenopausal women (Dalton et al., 2011; PMID: 21154150) demonstrated significant lean body mass increases (+1.3kg at 3mg/day over 12 weeks) with concurrent stair-climb improvement. GTx conducted two Phase III trials (POWER 1 and POWER 2) in non-small cell lung cancer patients with cachexia, evaluating Enobosarm at 1mg and 3mg daily. While the trials met one co-primary endpoint (lean body mass increase ≥10% responder rate), the FDA rejected the NDA in 2018 because the physical function endpoint (stair-climb power) failed to reach statistical significance in both trials simultaneously. Ostarine has moderate AR binding affinity with an oral half-life of approximately 24 hours. It demonstrates the mildest suppressive profile among popular SARMs, with only modest testosterone reduction at 3mg/day and relatively rapid recovery. However, at doses commonly used recreationally (10-25mg/day), significant HPG axis suppression occurs. Ostarine does not aromatize and has minimal androgenic effects on skin and prostate tissue. Multiple cases of liver injury have been reported with gray-market products, though attribution is complicated by product contamination. It remains NOT FDA-approved despite being the closest SARM to regulatory approval, and is prohibited by WADA.
Indications
- Phase III investigation for cancer cachexia in NSCLC patients (POWER trials)
- Phase II demonstrated lean mass gains in elderly and postmenopausal populations
- Research into stress urinary incontinence treatment (GTx pipeline)
- Investigation of muscle preservation during caloric deficit
- Potential treatment for age-related muscle loss and sarcopenia
- Research into body recomposition (simultaneous fat loss and lean mass gain)
- Preclinical investigation for breast cancer treatment
Mechanism of Action
MK-2866 is rapidly absorbed orally with Tmax ~1 hour. High oral bioavailability ensures predictable plasma levels. 24-hour half-life supports once-daily dosing with steady-state in 7-10 days
Dosing
| Compound | Dose | Frequency | Notes |
|---|---|---|---|
| Ostarine (MK-2866) | 10 mg/day | Once daily | Common starting research dose; mild-moderate suppression expected |
| Ostarine (MK-2866) | 20-25 mg/day | Once daily | Higher research dose; significant suppression, greater recomposition effect |
| Ostarine (MK-2866) | 3 mg/day | Once daily | Phase III clinical trial dose; demonstrated lean mass preservation in cachexia |
Evidence Grade
GRADE D
Safety & Contraindications
- NOT FDA-approved — NDA rejected 2018 due to failed co-primary endpoint
- HPG axis suppression at recreational doses (10-25mg) — PCT may be needed
- Mild suppression at clinical doses (1-3mg) with recovery within 4-6 weeks
- Drug-induced liver injury cases reported (PMID: 31607478) — contamination suspected in some cases
- HDL cholesterol suppression observed in clinical trials
- WADA prohibited substance since 2008
- Dose-dependent insulin resistance observed in some studies
- Long-term cardiovascular safety unknown beyond 12-week trial data
- Gray-market products frequently adulterated with prohormones or other compounds
- No established safe dose for long-term use
- Potential teratogenic effects — absolute contraindication in pregnancy