Enobosarm (Ostarine / MK-2866): A Comprehensive Research Review

Class: SARM (Selective Androgen Receptor Modulator)
Category: SARMs
Tagline: Non-steroidal SARM with tissue-selective anabolic activity

Abstract

Enobosarm (also known as Ostarine, MK-2866, or GTx-024) is a non-steroidal selective androgen receptor modulator (SARM) developed to treat conditions involving muscle wasting and bone loss, such as cancer cachexia and sarcopenia. By selectively targeting the androgen receptor (AR), enobosarm aims to provide the anabolic benefits of traditional steroids—such as increased lean body mass (LBM) and bone mineral density (BMD)—without the deleterious androgenic side effects on the prostate, skin, and hair. While multiple Phase II clinical trials demonstrated significant efficacy in increasing LBM, Phase III trials encountered challenges in meeting primary functional endpoints. This article reviews the mechanism, clinical data, and safety profile of enobosarm based on peer-reviewed literature.

Mechanism of Action

Enobosarm (MK-2866) is a non-steroidal ligand that binds to the androgen receptor with high affinity ($K_i$ of approximately 3.8–10 nM). Unlike testosterone and synthetic anabolic-androgenic steroids (AAS), enobosarm is not a substrate for 5-alpha reductase or aromatase; consequently, it does not convert into dihydrotestosterone (DHT) or estrogen.

Its tissue selectivity is driven by the recruitment of tissue-specific coregulators. In skeletal muscle and bone, enobosarm acts as a full agonist, stimulating protein synthesis and osteoblast activity. Conversely, in androgenic tissues like the prostate and sebaceous glands, it functions as a partial agonist or antagonist, significantly reducing the potential for prostate hypertrophy or acne. This selectivity allows for the decoupling of anabolic and androgenic effects, which is the hallmark of SARM pharmacology.

Clinical Evidence

Muscle Wasting and Cancer Cachexia

The most significant clinical data for enobosarm stems from its evaluation in cancer-induced muscle wasting. In a pivotal Phase II randomized, double-blind, placebo-controlled trial, 120 healthy elderly men and postmenopausal women received enobosarm (0.1 mg to 3 mg) for 12 weeks. The study found a dose-dependent increase in LBM, with the 3 mg group gaining an average of 1.3 kg ($p < 0.001$) compared to placebo.

Following this, the POWER 1 and POWER 2 Phase III trials evaluated enobosarm in patients with advanced non-small cell lung cancer (NSCLC). While the trials confirmed that enobosarm significantly increased LBM compared to placebo, they failed to meet the co-primary endpoint of improved physical function (measured by the Stair Climb Power Test) as required by the FDA.

Sarcopenia and Physical Function

In a trial involving 120 healthy elderly subjects (Dalton et al., 2011), enobosarm treatment resulted in significant improvements in physical function. Subjects in the 3 mg dose group demonstrated a statistically significant improvement in the time required to climb 12 steps and a significant increase in leg press strength.

Breast Cancer (AR+ Metastatic)

Recent research has pivoted toward oncology, specifically androgen receptor-positive (AR+) breast cancer. Because enobosarm stimulates the AR without the virilizing effects of testosterone, it has been investigated as a targeted therapy. A Phase II trial (Palmieri et al., 2021) demonstrated clinical benefit rates in heavily pre-treated postmenopausal women with AR+ metastatic breast cancer, suggesting a potential role beyond muscle preservation.

Pharmacokinetics

Enobosarm is characterized by high oral bioavailability and a favorable pharmacokinetic profile for once-daily dosing.

• •Absorption: Rapidly absorbed following oral administration.
• •Half-life: Approximately 24 hours in humans.
• •Metabolism: Primarily hepatic, though it does not undergo significant 5-alpha reduction or aromatization.
• •Excretion: Eliminated largely unchanged via urine and feces.

Safety Profile & Adverse Effects

Clinical trials have generally reported that enobosarm is well-tolerated at doses up to 3 mg, but several side effects have been consistently identified:

• •Hepatotoxicity: Transient, dose-dependent elevations in serum liver enzymes (ALT/AST) have been observed. These levels typically return to baseline without intervention upon cessation.
• •Lipid Profile: A consistent reduction in high-density lipoprotein (HDL) cholesterol levels and a decrease in serum triglycerides have been noted across multiple studies.
• •Endocrine Suppression: Enobosarm causes a dose-dependent suppression of Sex Hormone-Binding Globulin (SHBG) and total testosterone in men. However, significant suppression of Luteinizing Hormone (LH) or Follicle-Stimulating Hormone (FSH) is generally not observed at lower clinical doses.
• •Absence of Androgenic Effects: Clinical data shows no significant increase in PSA (Prostate Specific Antigen), no change in sebum production, and no signs of virilization in women.

Research Limitations & Evidence Gaps

Despite promising Phase II results, the "failure" of the Phase III POWER trials to demonstrate functional improvement (despite gains in muscle mass) highlights a significant gap in SARM research: the disconnect between muscle quantity and muscle quality/function.

Furthermore:

1. •Long-term Safety: Most human trials have not exceeded 4–6 months. The long-term effects of AR modulation on cardiovascular health and bone density remain unknown.
2. •Regulatory Status: Enobosarm is not FDA-approved for any indication and is currently listed on the WADA Prohibited List, complicating its use in athletic populations.
3. •Dosing Standardization: While 3 mg is the most studied dose for cachexia, optimal dosing for other indications (e.g., osteoporosis or SUI) has not been finalized.

References

1. •Dalton et al., "The selective androgen receptor modulator GTx-024 (enobosarm) improves lean body mass and physical function in healthy elderly men and postmenopausal women: results of a double-blind, placebo-controlled phase II trial", Journal of Cachexia, Sarcopenia and Muscle, 2011. DOI: 10.1007/s13539-011-0034-6 (Clinical Trial)
2. •Dobs et al., "Effects of enobosarm on muscle wasting and physical function in patients with cancer: a double-blind, randomised-controlled phase 2 trial", The Lancet Oncology, 2013. DOI: 10.1016/S1470-2045(13)70055-X (Clinical Trial)
3. •Crawford et al., "A Phase III extension study of enobosarm, a selective androgen receptor modulator, in patients with non-small cell lung cancer", Journal of Clinical Oncology, 2016. DOI: 10.1200/JCO.2016.34.15_suppl.9063 (Phase III Clinical Trial Data)
4. •Palmieri et al., "Enobosarm, a Selective Androgen Receptor Modulator, for the Treatment of Advanced Breast Cancer", The Lancet Oncology, 2021. DOI: 10.1016/S1470-2045(21)00150-1 (Clinical Trial)
5. •Mohler et al., "Nonsteroidal selective androgen receptor modulators (SARMs): dissociating the anabolic and androgenic activities of the androgen receptor", Journal of Medicinal Chemistry, 2009. DOI: 10.1021/jm900280m (Review/In Vitro)
6. •Peters et al., "Enobosarm for the treatment of stress urinary incontinence in women", International Urogynecology Journal, 2021. DOI: 10.1007/s00192-020-04538-x (Clinical Trial)