Enobosarm

What is Enobosarm?#

Enobosarm (Ostarine, GTx-024, MK-2866) is an oral nonsteroidal selective androgen receptor modulator (SARM) that promotes lean body mass accretion and physical function preservation through selective activation of androgen receptors in muscle and bone tissue. Unlike testosterone and other anabolic steroids, enobosarm was designed to minimize androgenic effects in reproductive tissues (prostate, seminal vesicles, sebaceous glands), offering tissue-selective anabolic activity.

Originally developed by GTx Inc. for cancer-associated muscle wasting, enobosarm is now being developed by Veru Inc. for a compelling new indication: preserving muscle mass during GLP-1 receptor agonist-induced weight loss. This repositioning addresses one of the central limitations of current obesity pharmacotherapy -- the loss of lean mass that typically accounts for 25-40% of total weight loss during GLP-1 treatment.

The Phase 2b QUALITY trial demonstrated that adding enobosarm 3 mg to semaglutide therapy eliminated lean mass loss entirely (shifting weight loss composition to 0% lean and 100% fat) while preserving physical function. The FDA has confirmed 3 mg as an acceptable dosage and incremental weight loss as a valid primary endpoint for regulatory approval.

Mechanism of Action#

Enobosarm's mechanism of action is based on selective androgen receptor modulation:

• Androgen receptor binding: Enobosarm binds to the androgen receptor (AR) with high affinity and selectivity, functioning as a partial agonist with tissue-selective activity
• Muscle anabolism: In skeletal muscle, AR activation stimulates muscle protein synthesis through the Akt/mTOR signaling pathway and inhibits muscle protein breakdown via FoxO transcription factor suppression
• Bone preservation: AR activation in osteoblasts promotes bone formation and may help preserve bone mineral density during weight loss
• Tissue selectivity: Unlike testosterone, enobosarm has reduced activity in prostate and other reproductive tissues due to differential cofactor recruitment -- the AR-SARM complex recruits coactivators differently than the AR-testosterone complex, leading to tissue-specific gene expression patterns
• Fat metabolism: AR activation in adipose tissue may enhance lipolysis and contribute to preferential fat loss over lean mass loss
• Satellite cell effects: Research suggests enobosarm may modulate adult skeletal muscle mass independently of the androgen receptor in the satellite cell lineage

The tissue selectivity of SARMs is their defining advantage over traditional androgens. Enobosarm achieves anabolic effects in muscle and bone while avoiding or minimizing virilizing effects (hair growth, voice deepening), prostate stimulation, and hepatotoxicity associated with oral anabolic steroids.

Clinical Development#

Enobosarm has one of the longest clinical development histories of any SARM:

• Phase 1 (GTx): Initial safety and pharmacokinetic studies establishing the dose range
• Phase 2 in healthy elderly (2011): 120 participants; dose-dependent lean mass gains and improved stair climb at 1 mg and 3 mg (PMID: 22031847)
• Phase 2 in cancer cachexia (2013): Significant lean mass gain of 1.0-1.5 kg in cancer patients (PMID: 23499390; Lancet Oncology)
• Phase 3 POWER trials (2013-2014): Two Phase 3 trials in NSCLC muscle wasting. Did not meet co-primary endpoints of lean mass and physical function
• AR+ breast cancer (2024): Phase 2 trial by Veru evaluating enobosarm in AR-positive, ER-positive, HER2-negative breast cancer
• Phase 2b QUALITY (2025): Positive results -- 71% reduction in lean mass loss with semaglutide; 0% lean/100% fat weight loss composition at 3 mg
• QUALITY Maintenance Extension (2025): Enobosarm reduced weight regain by 46% and completely prevented fat regain after semaglutide discontinuation
• FDA Regulatory Meeting (September 2025): FDA confirmed 3 mg as acceptable dosage and incremental weight loss as valid approval endpoint
• Phase 2b PLATEAU (planned 2026): 180 patients evaluating enobosarm with tirzepatide over 72 weeks

Important Considerations#

Enobosarm is an investigational drug that has not been approved by the FDA or any regulatory authority for any indication. Key considerations include:

• Enobosarm sold online as "Ostarine" or "MK-2866" is not pharmaceutical-grade and may contain contaminants, incorrect doses, or other undisclosed substances. The FDA has issued warning letters to companies selling SARMs
• Ostarine/SARMs are banned by WADA and all major sporting organizations as prohibited anabolic agents
• Liver enzyme elevations (ALT) have been observed in clinical trials (up to 20.8% at 3 mg in cancer patients), though generally transient and reversible
• Suppression of sex hormones (testosterone, SHBG, LH, FSH) has been observed in clinical studies
• The Phase 3 POWER trials in cancer cachexia did not meet co-primary endpoints, though this may reflect the difficulty of the cancer cachexia indication rather than drug failure
• The obesity/muscle preservation indication represents a fundamentally different clinical context

Key Research Findings#

QUALITY Phase 2b Trial (Veru press release, January 2025):

• 71% reduction in lean mass loss with enobosarm + semaglutide vs placebo + semaglutide at 16 weeks
• Weight loss composition: 0% lean mass and 100% fat mass in enobosarm 3 mg group
• 44.8% of semaglutide-only patients had clinically significant stair climb decline at 16 weeks; enobosarm preserved physical function

QUALITY Maintenance Extension (Veru press release, June 2025):

• During 12-week maintenance after semaglutide discontinuation, placebo group regained 43% of lost weight
• Enobosarm 3 mg reduced weight regain by 46% and completely prevented fat regain

Dalton et al., J Cachexia Sarcopenia Muscle 2011 (PMID: 22031847):

• Dose-dependent lean mass increase in 120 healthy elderly participants over 12 weeks
• Clinically meaningful improvement in stair climb test performance

Related Reading#

• Enobosarm research studies and evidence
• Enobosarm dosing protocols
• Enobosarm side effects profile
• Bimagrumab research guide
• Apitegromab research guide
• Trevogrumab research guide