Ostarine (Enobosarm, MK-2866)

Overview

Ostarine (development codes MK-2866, GTx-024, S-22; INN enobosarm) is an orally active, nonsteroidal selective androgen receptor modulator (SARM) of the arylpropionamide chemical class. It is NOT a peptide — it is a small synthetic molecule derived from structural modification of the antiandrogen bicalutamide. It appears on this reference because it is heavily discussed and stacked alongside peptides in the fitness and "research chemical" community, not because it is one.

Enobosarm was developed by GTx, Inc. (Memphis; licensed from the University of Tennessee Research Foundation), briefly partnered with Merck & Co. (2007–2010), and after a 2019 reverse merger into Oncternal Therapeutics the compound was licensed to Veru Inc. (December 2020), which now runs its oncology program. Across two decades it has been studied for muscle wasting, osteoporosis, stress urinary incontinence, and androgen-receptor-positive breast cancer.

Chemistry and structure

Enobosarm is a small-molecule arylpropionamide, not an amino-acid chain.

Mechanism of action

• Androgen receptor (AR) modulation. Enobosarm binds the androgen receptor and acts as a tissue-selective agonist — designed to drive anabolic effects in muscle and bone while producing weaker androgenic activity in prostate and other reproductive tissues than testosterone. This tissue selectivity is the entire premise of the "SARM" class. [In vitro] / [Animal]
• Anabolic effect on lean mass. Activated AR signaling in skeletal muscle increases lean body mass; in trials this was measured by DXA (lean body mass) and functional tests (e.g., stair-climb power). [Human]
• Nonsteroidal, non-aromatizing. As an arylpropionamide it is not a steroid and is not a substrate for aromatase; its downstream endocrine effects (e.g., suppression of endogenous testosterone) arise from AR-axis feedback rather than from steroid conversion. [Human] / [Hypothesis]
• Not tissue-perfectly-selective in practice. Human data show endocrine changes (e.g., reductions in SHBG and total/free testosterone), so "selective" does not mean "no hormonal effect." [Human]

Research and evidence

Human evidence in context. Enobosarm is one of the most clinically advanced SARMs — it has real, peer-reviewed Phase 2 data and reached Phase 3. But the honest summary is that its muscle-wasting program did not succeed (POWER1/POWER2 missed co-primary endpoints; ASTRID failed), and its most promising current path is oncology (breast cancer), not physique or athletic performance. No trial supports its grey-market use for bodybuilding, and it is not approved for anything. Precise human pharmacokinetics (half-life, bioavailability) were not verified against a primary PK paper here — the commonly cited ~24 h half-life comes from secondary sources.

Status and regulation

• Regulatory approval: None. Enobosarm is not approved by the FDA, EMA, or any other regulator for any indication. It is investigational (Phase 3 in oncology).
• Development history: GTx, Inc. → partnership with Merck (2007–2010, ended) → reverse merger into Oncternal Therapeutics (2019) → licensed to Veru Inc. (Dec 2020), which runs the current breast-cancer program.
• Anti-doping: Prohibited by WADA at all times (in- and out-of-competition) as an Anabolic Agent — S1.2, Other Anabolic Agents, where it is named. WADA sport-eligibility is a separate axis from legality.
• Market reality: Sold as a "research chemical." The FDA has warned against SARMs in body-building products, citing risks including liver injury. Grey-market products are of unknown identity, purity, and potency (see Safety).

Safety

Beyond DILI and endocrine suppression, long-term human safety is not established: the completed trials were of limited duration and the compound is unapproved. Grey-market products add contamination, mislabeling, and dosing errors as separate, compounding hazards.

Legal status

How it compares

Enobosarm is often grouped with other SARMs and "research chemicals," but the classes are distinct:

• Cardarine (GW-501516) — a PPARδ agonist, not a SARM and not a peptide, abandoned over a rodent cancer signal. Frequently stacked with SARMs, which is the source of the confusion.
• Compared with other true SARMs, enobosarm is the most clinically advanced (real Phase 2/3 data), yet still unapproved, and its muscle-wasting program did not meet its primary goals.

The decisive framing: enobosarm is an investigational drug with genuine oncology trials, not an approved physique or performance product — and its grey-market form is the leading source of SARM contamination. See the Muscle growth & hormone overview.

Common misconceptions

• "Ostarine is a peptide." No. It is a nonsteroidal small molecule (arylpropionamide, C19H14F3N3O3, 389.33 g/mol). It is covered here only because it is discussed and stacked with peptides.
• "It's a mild, safe, legal alternative to steroids." No. It is unapproved, causes documented liver injury and testosterone suppression, and is WADA-prohibited. Framing it as a safe steroid substitute is not supported.
• "It's approved / almost approved for muscle wasting." No. POWER1/POWER2 missed their co-primary endpoints and ASTRID failed — the muscle-wasting/incontinence programs did not succeed. Its live Phase 3 is in breast cancer.
• "A bottle labeled ostarine contains ostarine." Not reliably. In the JAMA 2017 analysis only 52% of products sold as SARMs contained the labeled compound.

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This entry is educational and summarizes published research and public regulatory information. It is not medical advice, a recommendation, or a guide to obtaining or using any substance. Not legal advice — verify your own jurisdiction.