MK-2866 — Ostarine, developmental name enobosarm — is the SARM with the most substantial human
trial record, which makes it the reference point for the class. This monograph summarizes that
record and where it stops.
Chemical identity & structure.
MK-2866 is a nonsteroidal selective androgen receptor modulator (SARM) developed by GTx Inc.
It is a small molecule (not a steroid), designed to activate the androgen receptor with tissue
selectivity favouring muscle and bone.
Mechanism of action.
Ostarine binds the androgen receptor and produces anabolic signalling in muscle and bone, with the
SARM design intent of sparing androgenic activity in tissues like the prostate. In trials this
translated to lean-mass gains without the full androgenic profile of testosterone — though
suppression of the body's own hormone axis is still reported.
Key research findings.
Enobosarm was studied in multiple human clinical trials, including Phase II work showing
increases in lean body mass and physical function in older adults and in cancer patients, and a
Phase III program (POWER trials) in non-small-cell-lung-cancer muscle wasting, where it improved
lean mass but did not consistently meet co-primary physical-function endpoints. It has also been
studied in stress urinary incontinence. This is far more human data than any other SARM has.
The research / citation base.
Despite the trial program, enobosarm is not an approved drug for these indications. It is on
the WADA Prohibited List and is one of the most common causes of "tainted supplement"
anti-doping positives, because it has been found illegally added to consumer products. The
human-data advantage is real but should not be read as regulatory approval.
Human clinical trial data
In the randomised, double-blind, placebo-controlled phase 2 trial by Dobs et al. (2013) in patients with cancer and at least 2% weight loss, participants received enobosarm at one of two dose levels or placebo for up to 113 days (PubMed). In that trial, 159 patients were analysed for safety, and both enobosarm groups showed significant increases in total lean body mass by day 113 or end of study compared with baseline.
Quality & sourcing notes.
Given how frequently Ostarine appears as an undeclared adulterant and as a mislabelled research
product, a batch-specific COA confirming identity (mass spectrometry) and purity (HPLC ≥98%) is
essential. Underdosed and substituted product is common.
*Research-use note: This monograph is an educational summary of the published research literature.
MK-2866 is not an approved drug and is prohibited in sport; it is described here for research
context only. Nothing here is medical advice or a usage recommendation.*