MK-2866
Selective Androgen Receptor Modulator | Muscle Wasting Research
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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.
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
Research Indications
MK-2866 was originally developed for cancer-related muscle wasting. Phase II trials in cancer patients demonstrated statistically significant increases in lean body mass at doses of 1 mg and 3 mg daily over 16 weeks, regardless of cancer type, chemotherapy regimen, or baseline nutritional status.
Age-related loss of muscle mass and strength. MK-2866 has shown improvements in lean body mass and physical function in elderly subjects, though Phase III trial endpoints for regulatory approval were not fully met.
Potential application for preserving lean mass during prolonged immobilization or recovery from surgery, based on its anti-catabolic properties demonstrated in clinical populations.
At research doses of 12.5-25 mg daily, users report gains of 2-4 kg of lean body mass over 6-8 week cycles. Clinical trials at lower doses (1-3 mg) still showed statistically significant lean mass increases, suggesting a dose-dependent anabolic response.
MK-2866 is commonly used for simultaneous muscle preservation and fat loss during caloric restriction. Its ability to maintain nitrogen balance in a deficit makes it suitable for recomposition protocols.
While not a direct fat-burning agent, MK-2866 preserves metabolically active lean tissue during a cut, indirectly supporting a higher basal metabolic rate and improved body composition outcomes.
Preclinical and early clinical data suggest MK-2866 increases bone mineral density through direct osteoblast stimulation. This effect is consistent with its selective AR agonism in bone tissue and may benefit populations at risk for osteoporosis.
Users report modest but noticeable improvements in strength, particularly in compound lifts, during MK-2866 cycles. Clinical trials measured improved stair-climbing power as a functional endpoint.
Accelerated recovery between training sessions is commonly reported, likely mediated by improved nitrogen retention and reduced exercise-induced muscle damage.
Dosing Protocols
MK-2866 is administered exclusively via the oral route, either as a liquid solution or in capsule form. Its high oral bioavailability and long half-life of approximately 24 hours make once-daily dosing practical and effective. In clinical trials, MK-2866 was administered as oral capsules at doses of 0.1 mg to 3 mg. Research and off-label use typically involves higher doses in the 10-25 mg range.
| Goal | Dose | Frequency | Route |
|---|---|---|---|
| Muscle Preservation (Cut / Deficit) | 10-15 mg/day | Once daily | Oral (liquid or capsule) |
| Lean Mass Gain (Recomposition) | 12.5-20 mg/day | Once daily | Oral (liquid or capsule) |
| Lean Mass Gain (Bulk) | 20-25 mg/day | Once daily | Oral (liquid or capsule) |
| Female Protocol | 5-10 mg/day | Once daily | Oral (liquid or capsule) |
Interactions
What to Expect
Side Effects & Safety
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
Stop Signs - Discontinue if:
- Persistent or severe liver pain (right upper quadrant tenderness)
- Yellowing of skin or eyes (jaundice)
- Severe or persistent headaches not relieved by standard measures
- Chest pain, heart palpitations, or shortness of breath
- Signs of significant hormonal disruption: severe mood changes, persistent erectile dysfunction, or testicular pain
- Unusual swelling in extremities
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
Quality Checklist
Good Signs
- Third-party lab testing (HPLC/MS) confirming compound identity and purity above 98%
- Clear labeling with compound name, concentration, batch number, and expiration date
- Certificate of analysis (CoA) available with matching batch number
- Liquid solution is clear with no visible particulates, cloudiness, or discoloration
- Capsules are uniformly filled with consistent weight across the batch
- Vendor has an established reputation with verified independent lab testing
Warning Signs
- No third-party lab testing or certificate of analysis available
- Unusually low price compared to market average (potential underdosing or substitution)
- Vague or incomplete labeling missing concentration, batch number, or expiration
- Vendor makes explicit therapeutic or curative claims
- Product marketed as a dietary supplement rather than a research chemical
Bad Signs
- Lab testing reveals wrong compound, contamination, or significant underdosing
- Product contains undisclosed active ingredients (prohormones, methylated steroids)
- Liquid is discolored, cloudy, or contains visible particles or precipitate
- No labeling or labeling does not match contents
- Vendor provides no verifiable contact information or has a history of failed third-party testing
- Product seized or flagged by regulatory authorities for adulteration
References
- The Safety, Pharmacokinetics, and Effects of LGD-4033, a Novel Nonsteroidal Oral, Selective Androgen Receptor Modulator, in Healthy Young Men (Dalton et al. - Ostarine Phase II)Dalton, J.T., Barnette, K.G., Bohl, C.E., et al.Journal of Cachexia, Sarcopenia and Muscle (2011)
Phase II randomized, double-blind, placebo-controlled trial of enobosarm (GTx-024/MK-2866) in 120 healthy elderly men and postmenopausal women. Doses of 1 mg and 3 mg daily for 12 weeks produced statistically significant, dose-dependent increases in total lean body mass and improvements in physical function (stair-climbing power). The compound was well tolerated with no serious adverse events.
- Effects of Enobosarm on Muscle Wasting and Physical Function in Patients with Cancer: A Double-Blind, Randomised Controlled Phase 2 TrialDobs, A.S., Boccia, R.V., Croot, C.C., et al.The Lancet Oncology (2013)
Phase II trial of enobosarm in 159 cancer patients with muscle wasting. Both 1 mg and 3 mg doses produced significant increases in lean body mass compared to placebo at 16 weeks, regardless of cancer type or concurrent chemotherapy. Median lean body mass increased by 1.5 kg (3 mg group) versus a loss of 0.02 kg in placebo. The drug was well tolerated with a safety profile comparable to placebo.
- Enobosarm (GTx-024) Phase III Clinical Trials for Cancer Cachexia: POWER 1 and POWER 2Crawford, J., Prado, C.M., Johnston, M.A., et al.Annals of Oncology (2016)
Two pivotal Phase III trials (POWER 1 and POWER 2) evaluating enobosarm 3 mg daily in patients with non-small cell lung cancer cachexia. Both trials met the lean body mass co-primary endpoint, demonstrating significantly greater muscle mass versus placebo. The stair-climbing power co-primary endpoint showed a positive trend but did not reach statistical significance in the responder analysis, contributing to the FDA's decision not to approve the drug for this indication.
- Selective Androgen Receptor Modulators (SARMs) as Function Promoting TherapiesNarayanan, R., Coss, C.C., Dalton, J.T.Current Opinion in Clinical Nutrition & Metabolic Care (2018)
Comprehensive review of SARM development with focus on enobosarm as the most clinically advanced SARM. Describes the tissue-selective mechanism of action, clinical trial results, and potential therapeutic applications for muscle wasting, sarcopenia, breast cancer, and stress urinary incontinence.
- Pharmacology of Selective Androgen Receptor ModulatorsMohler, M.L., Bohl, C.E., Jones, A., et al.Bioorganic & Medicinal Chemistry (2009)
Detailed pharmacological characterization of aryl propionamide SARMs including MK-2866. Describes structure-activity relationships, androgen receptor binding kinetics, and the molecular basis for tissue selectivity. Provides the foundational medicinal chemistry context for understanding how MK-2866 achieves selective anabolic activity without significant androgenic effects in non-target tissues.
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This information is for educational and research purposes only. Consult a healthcare professional before use.