For laboratory research use only. Not for human consumption. All compounds referenced in this article are sold by Explicit SARMs solely as reference materials for in-vitro and animal research by qualified professionals. Nothing here is medical advice, and no statement has been evaluated by the FDA. These materials are not intended to diagnose, treat, cure or prevent any disease.
Ostarine, also known by its development codes MK-2866 and GTx-024 and by the international nonproprietary name enobosarm, is the most extensively studied selective androgen receptor modulator (SARM). Unlike most compounds in the class, it progressed through multiple controlled clinical trials, so the literature on Ostarine includes animal pharmacology, phase 2 data and two phase 3 trials. That makes it the reference point many researchers use when comparing newer SARMs.
This article reviews the mechanism, research history and published findings on Ostarine, along with practical points on documentation and purity. Explicit SARMs supplies Ostarine MK-2866 Capsules and MK-2866 Liquid as laboratory reference material only.
What Is Ostarine?
Ostarine is a nonsteroidal aryl-propionamide ligand of the androgen receptor (AR). It belongs to the same chemical family as S4 (Andarine) and S-23, all of which trace back to aryl-propionamide scaffolds first explored in academic and GTx, Inc. programs in the late 1990s and 2000s. [3]
- Common names: Ostarine, MK-2866, GTx-024, enobosarm
- Class: nonsteroidal selective androgen receptor modulator (aryl-propionamide)
- Primary target: androgen receptor
- Regulatory status: not approved by the FDA or any other regulator for any indication
Mechanism of Action
Ostarine binds the ligand-binding domain of the androgen receptor and acts as an agonist in tissues such as skeletal muscle and bone, with comparatively weaker activity in androgen-dependent reproductive tissues in rodent assays. The proposed basis for this selectivity has three parts. First, Ostarine is not a substrate for 5-alpha reductase, so it is not amplified into a more potent androgen in tissues like the prostate. Second, it is not aromatized to an estrogen. Third, the receptor conformation it induces appears to recruit a different set of coactivators and corepressors than DHT does, and coregulator availability differs among tissues. [3]
As with every SARM, selectivity is relative. At higher exposures, androgenic tissues respond and endocrine feedback suppresses gonadotropins.
Research History
Ostarine was developed by GTx, Inc. as a candidate for muscle wasting associated with cancer (cancer cachexia) and other conditions involving loss of lean tissue. Development followed a conventional path: rodent pharmacology, phase 1 pharmacokinetic studies, a phase 2 study in healthy older men and postmenopausal women, a phase 2 study in patients with cancer, and finally the phase 3 POWER program in non-small cell lung cancer.
The compound was later studied in oncology settings, including AR-positive breast cancer, and its development history passed to other sponsors. As of this writing it has not received marketing approval anywhere.
What Published Studies Report
Animal research
Ostarine has been widely used in rat models of hormone deficiency. In a 2020 study in Frontiers in Endocrinology, Roch and colleagues compared Ostarine and Ligandrol in ovariectomized rats and reported changes in muscle tissue characteristics, including fiber size and capillary density, alongside effects on other tissues that varied by exposure. [5]
Phase 2 clinical research
A double-blind, placebo-controlled phase 2 trial published in 2011 enrolled healthy elderly men and postmenopausal women. The investigators reported dose-dependent increases in lean body mass measured by DXA and improvements on a stair-climb test compared with placebo, with reductions in HDL cholesterol noted among the laboratory findings. [1]
A second phase 2 trial, published in Lancet Oncology in 2013, studied patients with cancer and muscle wasting. The investigators reported increases in lean body mass relative to baseline in the Ostarine groups. [2]
Phase 3 POWER trials
Two phase 3 trials, POWER 1 (NCT01355484) and POWER 2 (NCT01355497), studied Ostarine in patients with non-small cell lung cancer receiving chemotherapy. Both trials were completed. In 2013 the sponsor reported that neither trial met the overall criteria for the co-primary responder endpoints (lean body mass and stair climb power) agreed with the FDA, although continuous-variable analyses of lean body mass favored Ostarine over placebo in both trials. [8] The program did not result in approval. This is an important part of the record: favorable phase 2 lean-mass data did not translate into a regulatory win.
Safety findings in the literature
Published trials report reductions in HDL cholesterol and suppression of sex-hormone binding globulin and gonadotropins as recurring laboratory findings. [1][3] Ostarine is also among the most frequently identified substances in anti-doping adverse analytical findings, which has generated a large analytical literature on its detection. [6]
Research Considerations
Points laboratories commonly build into study designs with Ostarine:
- Lipid endpoints. Because HDL reduction is a consistent clinical finding, lipid panels are a sensible secondary endpoint in animal work.
- Hormonal axis. Measure LH, FSH and endogenous androgens where the HPG axis is relevant, and include vehicle controls.
- Comparator choice. Because Ostarine has the deepest dataset in the class, it is often used as the comparator when characterizing newer SARMs such as LGD-4033 or RAD-140.
- Model selection. Ovariectomized and orchidectomized rat models are the most common published settings, which helps with cross-study comparison.
- Analytical controls. Run an authenticated reference standard when developing HPLC or LC-MS methods.
Analytical Characterization
Because Ostarine is so widely studied, its analytical chemistry is well developed. Anti-doping laboratories have published LC-MS/MS methods for the parent compound and its main metabolites, including hydroxylated and glucuronidated forms. [6] Those methods are a practical starting point for laboratories that need to confirm exposure in animal studies.
Useful checks when qualifying a new lot include:
- HPLC purity with a defined detection wavelength and a reported impurity profile
- Mass spectrometric identity confirmation against the expected molecular ion
- Consistency of retention time against an authenticated reference standard
- For the liquid format, a concentration check, since solvent loss over time changes the effective concentration
Documenting these checks in the methods section makes results easier to compare across laboratories and across the large body of published Ostarine work.
WADA Status
Enobosarm (ostarine) is named explicitly in WADA section S1.2 (Other Anabolic Agents) and is prohibited at all times. [7]
Storage, Purity and COA
Ostarine is a stable small molecule, but good practice still matters:
- Store sealed, protected from light, heat and humidity.
- Keep the liquid format tightly capped and upright to avoid concentration drift from solvent loss.
- Label secondary containers with compound name, lot number and date.
- Keep the Certificate of Analysis with your records. A credible COA reports HPLC purity, confirms identity by mass spectrometry, and lists the lot number and analysis date. Material without lot-level documentation is difficult to defend in a methods section.
For lot documentation, use the contact page.
Related Research Compounds
- S4 Andarine Capsules, an earlier aryl-propionamide SARM from the same chemical family
- LGD-4033 Ligandrol Capsules, compared directly with Ostarine in ovariectomized rats [5]
- Frequently Asked Questions About the Best Ostarine Source
- Class overview: SARMs vs Traditional Research Compounds and the SARMs collection
Frequently Asked Questions
Are Ostarine, MK-2866 and enobosarm the same compound?
Yes. MK-2866 and GTx-024 are development codes, enobosarm is the nonproprietary name, and Ostarine is the common trade-style name. All refer to one molecule.
Is Ostarine approved by the FDA?
No. Despite phase 3 trials, Ostarine has never been approved by the FDA or any other regulator.
Why did the phase 3 trials not lead to approval?
The POWER trials did not meet the overall responder criteria for their co-primary endpoints agreed with the FDA, so the program was not approved for cancer-related muscle wasting. [8]
What safety signals appear in published trials?
Reductions in HDL cholesterol and changes in hormone levels are the most consistently reported laboratory findings. [1][3]
Is Ostarine prohibited in sport?
Yes. It is listed by name in WADA section S1.2 and is prohibited at all times. [7]
Capsules or liquid for laboratory work?
Both contain the same compound. Capsules provide pre-measured solid material, while the liquid suits protocols built around solution preparation.
References
- Dalton JT, et al. The selective androgen receptor modulator GTx-024 (enobosarm) improves lean body mass and physical function in healthy elderly men and postmenopausal women: results of a double-blind, placebo-controlled phase II trial. J Cachexia Sarcopenia Muscle. 2011. PubMed 22031847
- Dobs AS, et al. Effects of enobosarm on muscle wasting and physical function in patients with cancer: a double-blind, randomised controlled phase 2 trial. Lancet Oncol. 2013. PubMed 23499390
- Narayanan R, et al. Development of selective androgen receptor modulators (SARMs). Mol Cell Endocrinol. 2018. PubMed 28624515
- ClinicalTrials.gov records for the POWER phase 3 trials: NCT01355484 and NCT01355497
- Roch PJ, et al. Ostarine and Ligandrol Improve Muscle Tissue in an Ovariectomized Rat Model. Front Endocrinol (Lausanne). 2020. PubMed 33042018
- Thevis M, Schanzer W. Detection of SARMs in doping control analysis. Mol Cell Endocrinol. 2018. PubMed 28137616
- World Anti-Doping Agency. 2026 Prohibited List. wada-ama.org
- GTx, Inc. press release on POWER trial results, August 19, 2013 (SEC filing exhibit). sec.gov
For laboratory research use only. Not for human consumption. All compounds referenced in this article are sold by Explicit SARMs solely as reference materials for in-vitro and animal research by qualified professionals. Nothing here is medical advice, and no statement has been evaluated by the FDA. These materials are not intended to diagnose, treat, cure or prevent any disease.
For the compound itself, Explicit SARMs lists ostarine MK-2866 capsules, and for comparison work outside the androgen receptor, GW-501516 cardarine capsules.