What is Ostarine (MK-2866)?

Ostarine (MK-2866, Enobosarm, GTx-024) is a non-steroidal selective androgen receptor modulator (SARM) developed by GTx, Inc. It is the most extensively studied SARM with the largest body of published clinical data — including Phase II and Phase III trials in muscle wasting and cancer cachexia. Ostarine binds the androgen receptor with high affinity and selectivity, producing anabolic effects in muscle and bone with reduced androgenic effects on prostate and other tissues compared to testosterone. For Research Use Only.

Did Ostarine complete Phase III clinical trials?

Ostarine (Enobosarm) reached Phase III development — further than any other SARM. GTx conducted Phase III trials (POWER 1 and POWER 2) in non-small cell lung cancer (NSCLC) patients experiencing cancer cachexia. While Phase II data was positive (significant lean mass retention and functional improvement vs placebo), Phase III results were mixed — one of two pivotal trials met its primary endpoint, the other did not. This was insufficient for FDA approval. GTx subsequently partnered with Merck and explored other indications. No SARM has received FDA approval to date.

How does Ostarine compare to RAD-140 and LGD-4033?

Among the most-researched SARMs: Ostarine has the most clinical data (Phase II/III trials) but shows less potent lean mass effects per dose than LGD-4033 or RAD-140 in preclinical comparisons. LGD-4033 showed significant lean mass effects in Phase II at just 1 mg/day, with higher AR binding affinity than Ostarine. RAD-140 has stronger preclinical data for neuroprotection and a higher AR binding affinity. For muscle wasting models where translational human data is important, Ostarine's extensive clinical dataset is the primary advantage.

Ostarine (MK-2866) Research Guide | The Most Studied SARM — Phase II/III Data

What Is Ostarine?

Ostarine (MK-2866, Enobosarm, GTx-024) is a non-steroidal selective androgen receptor modulator developed by GTx, Inc. (now Oncternal Therapeutics). It is the SARM with the largest body of published clinical research and the furthest clinical development — Phase III trials — of any compound in the SARM class.

Ostarine binds the androgen receptor with high affinity and acts as a full agonist in skeletal muscle and bone, with partial or no agonism in androgen-sensitive tissues such as prostate and scalp. This tissue selectivity is the defining feature and research rationale.

Ostarine is prohibited in sport under WADA S1 (Anabolic Agents). It is not approved by any regulatory authority. All supply is For Research Use Only.

AR Selectivity Mechanism

Ostarine's selectivity for muscle over prostate is quantified in preclinical anabolic:androgenic assays. In orchiectomized rat models:

Ostarine produces approximately 3:1 anabolic:androgenic activity ratio — lower selectivity than LGD-4033 (~500:1) or RAD-140 (~89:1) but still significantly more selective than testosterone (~1:1)
Non-aromatizable and non-5α-reducible: no estradiol or DHT conversion
Oral bioavailability: ~95% in rodents — very high for AR agonist research use
Half-life: ~24 hours in humans, enabling once-daily dosing protocols

Phase II Clinical Data

Key Phase II studies for Ostarine/Enobosarm:

Papanicolaou et al. (2013): Phase II in cancer patients (mixed solid tumors). Enobosarm 1mg or 3mg vs placebo for 16 weeks. Results: significant lean body mass gains at both doses vs placebo; functional improvement (stair climb power); no significant androgenic side effects. This study prompted Phase III advancement.
Healthy subject dose-finding (Dalton et al., 2011): 120 healthy elderly men and postmenopausal women; doses 0.1–3 mg/day. Dose-dependent lean mass gains; bone-specific alkaline phosphatase increases at 3 mg/day in women.
Stress urinary incontinence (SUI) trial: GTx explored Ostarine for SUI — an androgen-responsive condition — in a Phase II trial showing significant improvement in urinary leakage frequency.

Phase III Context: POWER 1 & 2

GTx's Phase III program (POWER 1: NCT01355497, POWER 2: NCT01355484) enrolled NSCLC patients experiencing cachexia:

POWER 1: Met primary endpoint — statistically significant lean mass retention and total lean mass benefit vs placebo.
POWER 2: Did not meet primary endpoint (primary endpoint was 10% or greater improvement in stair climb power from baseline — not met).
FDA outcome: Both pivotal trials required to meet primary endpoints for approval. With mixed results, FDA did not approve the NDA.

The POWER trials remain the most advanced clinical dataset for any SARM in a cancer cachexia indication, providing important translational context for researchers designing muscle wasting models.

Cancer Cachexia Research

Cancer cachexia — progressive skeletal muscle loss driven by inflammatory cytokines and tumor-derived factors — is a major unmet medical need. SARMs are of research interest because:

Testosterone, the classic anabolic agent, causes prostate-related side effects that are unacceptable in many cancer patients
SARMs can potentially provide the anabolic benefit without androgenic complications
Ostarine's Phase II/III dataset is the primary evidence base for SARM-based cachexia research

The research framework: in vitro (C2C12 or primary myocyte cultures + inflammatory stimuli) → rodent cachexia models (LLC tumor, ApcMin mouse) → translational clinical endpoints. Ostarine's established clinical data provides reference values for translational research design.

Ostarine vs RAD-140 vs LGD-4033

Parameter	Ostarine (MK-2866)	RAD-140	LGD-4033
Developer	GTx / Oncternal	Radius Health	Ligand / Viking Therapeutics
AR binding (Ki)	~3.8 nM	~7 nM	~1 nM
A:A ratio	~3:1	~89:1	~500:1
Max clinical stage	Phase III (NDA submitted)	Phase I	Phase II
Human lean mass data	Yes (+1–2 kg in Phase II)	No Phase II	Yes (+1.2 kg/1mg/21d)
Primary strength	Most clinical data	Neuroprotection data	Potency, Phase II LBM data
WADA status	S1 Prohibited	S1 Prohibited	S1 Prohibited

Cart

Ostarine (MK-2866) Research Guide