MK-2866
Also known as: Ostarine, Enobosarm, GTx-024
Selective Androgen Receptor Modulator | Muscle Wasting Research
Overview
MK-2866 (Ostarine, Enobosarm) is a non-steroidal selective androgen receptor modulator (SARM) originally developed by GTx, Inc. (now Oncternal Therapeutics) for the prevention and treatment of muscle wasting and sarcopenia. It is the most extensively studied SARM in clinical trials, having progressed through Phase I, II, and III trials for cancer-related cachexia, stress urinary incontinence, and age-related muscle loss. MK-2866 selectively binds to androgen receptors in muscle and bone tissue with high affinity while exhibiting minimal activity in prostate and sebaceous glands, which differentiates it from traditional anabolic-androgenic steroids. Despite promising clinical data demonstrating significant lean body mass gains in cancer patients and elderly subjects, the FDA has not granted approval, and GTx's New Drug Application for stress urinary incontinence was not approved in 2018. MK-2866 remains classified as an investigational compound and is prohibited by WADA in competitive sports.
Key benefits
- Increases lean body mass in a dose-dependent manner with clinical trial support
- Preserves muscle mass during caloric deficit or catabolic conditions
- Selective tissue activity reduces androgenic side effects compared to anabolic steroids
- Oral bioavailability eliminates the need for injections
- Does not aromatize to estrogen, avoiding gynecomastia and water retention
- Improves physical function and stair-climbing power in clinical populations
- Long 24-hour half-life allows convenient once-daily dosing
- Mild side effect profile at commonly studied doses
Mechanism of action
MK-2866 binds to the androgen receptor (AR) with high affinity and selectivity, functioning as a partial agonist in muscle and bone tissue. Upon binding, the MK-2866-AR complex undergoes a conformational change that promotes nuclear translocation and interaction with androgen response elements (AREs) on DNA, activating transcription of genes involved in protein synthesis, nitrogen retention, and myogenic differentiation. The tissue selectivity of MK-2866 arises from its non-steroidal structure, which produces a distinct AR conformational change compared to testosterone or DHT. This results in differential coactivator and corepressor recruitment across tissues. In skeletal muscle, MK-2866 robustly activates anabolic signaling pathways including PI3K/Akt/mTOR, increasing muscle protein synthesis and reducing protein degradation via suppression of ubiquitin-proteasome and myostatin pathways. In bone, it stimulates osteoblast differentiation and mineralization. Critically, MK-2866 demonstrates reduced androgenic activity in prostate tissue and does not undergo 5-alpha reduction to a more potent metabolite (unlike testosterone), nor does it aromatize to estrogen, which accounts for its lower incidence of androgenic and estrogenic side effects.
Molecular data
- Type
- Non-steroidal selective androgen receptor modulator (C19H14F3N3O3)
- Half-life
- ~24 hours
Indications
What the research community uses this compound for, with self-reported effectiveness.
Muscle Wasting / Cachexia
Body Composition
Bone Health
Physical Performance
Dosing protocols
Common protocols by delivery method. Adjust the curve below to model accumulation in your own cycle.
Delivery method
| Protocol | Dose | Frequency | Duration |
|---|---|---|---|
| — | 10-15 mg/day | Once daily | — |
| — | 12.5-20 mg/day | Once daily | — |
| — | 20-25 mg/day | Once daily | — |
| — | 5-10 mg/day | Once daily | — |
Safety
Reported adverse effects, contraindications, and what to monitor on cycle.
Common side effects
- Mild testosterone suppression (dose-dependent, typically 10-30% reduction at 25 mg)
- HDL cholesterol reduction (10-20% suppression observed in clinical trials)
- Headaches, particularly during the first 1-2 weeks
- Mild back pain or muscle cramps
- Transient fatigue toward the end of longer cycles
- Slight reduction in libido at higher doses or extended cycle lengths
Rare side effects
- Joint dryness or discomfort at higher doses (due to reduced estrogenic signaling)
- Nausea or gastrointestinal discomfort
- Elevated liver enzymes (AST/ALT), typically mild and reversible
- Hair shedding in individuals predisposed to androgenic alopecia
- Skin dryness or mild acne
- Significant HPTA suppression requiring full PCT (more common with prolonged or high-dose use)
Contraindications
- Active liver disease or significantly elevated liver enzymes
- Hormone-sensitive cancers (breast, prostate) without oncologist clearance
- Pregnancy or breastfeeding (potential endocrine disruption to fetus/infant)
- Individuals under 21 years of age (risk of premature HPTA disruption during development)
- Concurrent use of hepatotoxic medications without liver function monitoring
- Known hypersensitivity to MK-2866 or any formulation excipients
- Competitive athletes subject to WADA or USADA anti-doping testing
Monitoring
- Total testosterone, free testosterone, LH, FSH: baseline and 4 weeks into cycle
- Lipid panel (total cholesterol, LDL, HDL, triglycerides): baseline and end of cycle
- Liver function tests (AST, ALT, bilirubin): baseline and every 4-6 weeks
- Complete blood count: baseline and end of cycle
- Post-cycle bloodwork at 4 weeks after discontinuation to confirm hormonal recovery
References
Primary literature and clinical-trial data informing this entry.