Ostarine (MK-2866)

Ostarine (MK-2866, enobosarm) is a nonsteroidal, oral SARM developed by GTx Inc. (with Merck) originally targeting muscle wasting (cachexia) and osteoporosis — one of the most clinically studied SARMs, with real Phase 2 human trial data.

Phase 2 trial, cancer patients (n=159 for safety, 100 evaluable for efficacy, up to 113 days, 1mg or 3mg daily): both doses produced significant lean body mass gains vs. placebo — 1.5kg at 1mg (P=0.0012), 1.0kg at 3mg (P=0.046) — vs. essentially no change on placebo. Dobs et al., Lancet Oncol, 2013 Serious adverse events (disease progression, pneumonia, febrile neutropenia) occurred at similar rates across all groups and were not judged related to the study drug.

Important context on development status: despite multiple positive Phase 2 trials, subsequent Phase 3 trials failed to meet co-primary functional endpoints, and the cachexia program was ultimately discontinued without ever reaching FDA approval for any indication.

The cancer-cachexia trial tested 1mg and 3mg daily, oral, for up to 113 days — notably, the lower 1mg dose produced a larger median lean-mass gain than the 3mg dose in this trial.

In the 159-patient safety population, serious adverse events were comparable across placebo and both treatment doses, and none were attributed to the study drug. This trial was conducted in cancer patients specifically — a population with its own baseline health complexity — worth keeping in mind when interpreting the safety data for a healthy-user context.