MK-2866 (Ostarine)

Ostarine — also known as MK-2866 or Enobosarm — is the first SARM to have undergone Phase II and Phase III human clinical trials, generating the most comprehensive published human pharmacokinetic and efficacy dataset of any SARM. Developed by GTx Inc., Ostarine was investigated primarily for muscle-wasting conditions associated with cancer.

Development History

Ostarine was synthesised as part of GTx Inc.'s SARM programme under Professor James T. Dalton at the University of Tennessee — the same research group credited with discovering the non-steroidal SARM scaffold. Following promising preclinical data, GTx advanced Ostarine through Phase I, Phase II, and ultimately Phase III clinical trials in cancer-related muscle wasting (cachexia).

The POWER 1 and POWER 2 Phase III trials enrolled patients with non-small cell lung cancer experiencing involuntary weight loss. These trials represent the most advanced clinical development of any SARM to date, though both failed to meet their primary endpoints on a population-wide basis, leading to discontinuation of the FDA new drug application in 2013. Significant lean mass benefit was nonetheless observed in subgroup analyses.

Key Research Findings

Across the GTx Phase I and II trials in healthy postmenopausal women and elderly men:

• Dose-dependent lean mass gains of ~0.7–1.4 kg at doses of 1–3 mg/day over 12 weeks
• Statistically significant improvements in stair-climbing power and physical function scores
• Mild testosterone suppression at doses of 1 mg/day; more pronounced at 3 mg/day in healthy men
• No significant changes in PSA, haematocrit, or liver enzyme levels at therapeutic doses
• Excellent oral bioavailability and half-life (~24 hours)

The preservation of lean mass without meaningful androgenic side effects in both male and female subjects is a defining characteristic of Ostarine's research profile — consistent with the tissue-selective SARM mechanism.

Compound Profile

• Classification: Non-steroidal SARM (partial AR agonist)
• Developer: GTx Inc. / University of Tennessee
• Half-life: ~24 hours
• Typical research dose range: 10–25 mg/day
• Typical research duration: 8–12 weeks
• Aromatisation: None
• Testosterone suppression: Mild to moderate, dose-dependent

Ostarine vs. Other SARMs

Ostarine is widely considered the mildest SARM in terms of both anabolic potency and testosterone suppression. This makes it a common starting point in comparative SARM research protocols. LGD-4033 typically produces greater lean mass effects at equivalent doses but with more pronounced suppression (per the Basaria 2013 trial). RAD-140 has a higher preclinical anabolic selectivity ratio. Ostarine's extensive published safety dataset, however, makes it the most reliably characterised compound for informed research purposes.

Frequently Asked Questions

Is Ostarine the weakest SARM?

In terms of anabolic potency per milligram, Ostarine is generally considered less potent than LGD-4033, RAD-140, or YK-11. However, "weakest" is relative — Ostarine has demonstrated statistically significant lean mass gains and functional improvements in Phase II controlled human trials at 1–3 mg/day, which is meaningful by any clinical standard.

Does Ostarine require post-cycle support?

Ostarine suppresses testosterone in a dose-dependent manner. At lower doses (1–3 mg/day), suppression in the GTx trials was mild and recovery occurred rapidly. At doses commonly used in research (10–25 mg/day), more significant suppression should be anticipated. Post-cycle support is standard practice in research protocols involving Ostarine.

Why did the POWER trials fail?

The POWER trials failed to meet their primary endpoint (improvement in physical function across the entire intent-to-treat population), partly due to significant heterogeneity in the patient population (varying degrees of cachexia severity). Subgroup analyses showed significant lean mass benefit in more severely wasted patients. The FDA was not satisfied with the dataset for NDA approval.