Research Article: Testolone (RAD-140) – Preclinical and Clinical Characterization

Abstract

Testolone (RAD-140) is a potent, non-steroidal selective androgen receptor modulator (SARM) originally developed by Radius Health, Inc. It was designed to overcome the limitations of conventional androgen therapy, such as prostate enlargement and virilization, by exhibiting high tissue selectivity. Preclinical studies demonstrate that RAD-140 possesses high affinity for the androgen receptor (AR) and exerts significant anabolic effects on skeletal muscle and bone while maintaining a favorable safety profile regarding reproductive tissues. Furthermore, RAD-140 has demonstrated unique neuroprotective properties in animal models, distinguishing it from other compounds in its class. While initially investigated for muscle wasting and osteoporosis, clinical development has recently pivoted toward AR-positive metastatic breast cancer. Human clinical data remains in the early stages, primarily focused on safety, pharmacokinetics, and dose-escalation.

Mechanism of Action

RAD-140 functions as a high-affinity ligand for the androgen receptor ($K_i$ = 7 nM). As a non-steroidal molecule, it is not a substrate for 5-alpha reductase or aromatase, meaning it cannot be converted into dihydrotestosterone (DHT) or estrogen, thereby avoiding many of the side effects associated with anabolic-androgenic steroids (AAS).

The compound’s selectivity is attributed to its ability to induce specific conformational changes in the AR, leading to the differential recruitment of co-activators and co-repressors in a tissue-specific manner. In skeletal muscle, RAD-140 acts as a full agonist, stimulating protein synthesis and hypertrophy. In the prostate and seminal vesicles, its activity is significantly attenuated compared to testosterone, minimizing androgenic stimulation.

Unique to RAD-140 is its activity in the central nervous system. Research indicates that RAD-140 can cross the blood-brain barrier and exert neuroprotective effects via AR-mediated signaling. In models of neurodegeneration, it has been shown to reduce cell death induced by apoptotic insults, such as kainic acid and amyloid-beta, potentially by modulating MAPK signaling pathways.

Clinical Evidence

Human Clinical Trials

Data regarding human application is largely derived from a Phase 1 dose-escalation study (NCT03088059).

• •Breast Cancer Applications: In a study involving postmenopausal women with ER+/HER2- metastatic breast cancer, RAD-140 (investigated as an AR agonist) demonstrated a manageable safety profile and preliminary clinical activity. The rationale is that in certain breast cancers, AR activation can antagonize ER-mediated tumor growth.
• •Muscle Anabolism: While widely used off-label for muscle building, there are currently no peer-reviewed Phase 2 or 3 clinical trials confirming the efficacy of RAD-140 for muscle wasting (cachexia) or sarcopenia in humans.

Preclinical (In Vivo) Evidence

• •Muscle and Bone: In orchidectomized (ORX) rat models, RAD-140 demonstrated a dose-dependent increase in levator ani muscle weight. At a dose of 0.1 mg/kg, it achieved anabolic effects comparable to testosterone propionate, while requiring significantly higher doses to stimulate prostate growth (Miller et al., 2011).
• •Neuroprotection: In a rat model of neurotoxicity, RAD-140 was found to be as effective as testosterone in reducing neuronal loss in the hippocampus. Unlike testosterone, RAD-140 did not cause any increase in prostate weight at the neuroprotective dose (Jayaraman et al., 2014).

Pharmacokinetics

Initial human data indicates that RAD-140 has a significantly longer half-life than previously estimated from animal models.

• •Absorption: RAD-140 is orally bioavailable.
• •Half-life: Phase 1 clinical data suggests a terminal half-life of approximately 60 hours in humans, supporting once-daily dosing or even less frequent administration. This contrasts with earlier rodent studies which suggested a much shorter duration of action.
• •Metabolism: The compound does not undergo 5-alpha reduction or aromatization. Metabolism appears to occur primarily through hepatic pathways, including hydroxylation and glucuronidation.

Safety Profile & Adverse Effects

The safety profile of RAD-140 is characterized by a divergence between controlled clinical environments and "real-world" recreational use.

• •Clinical Trial Findings: Reported adverse events in Phase 1 trials include nausea, fatigue, decreased appetite, and elevations in liver enzymes (ALT/AST). These effects were generally dose-dependent.
• •Hepatotoxicity: Multiple case reports have identified "Drug-Induced Liver Injury" (DILI) associated with the recreational use of RAD-140. Patients presented with jaundice, significantly elevated transaminases, and cholestatic injury.
• •Endocrine Suppression: Like all SARMs, RAD-140 can suppress the hypothalamic-pituitary-gonadal (HPG) axis. In humans, this manifests as a dose-dependent reduction in total testosterone, sex hormone-binding globulin (SHBG), and high-density lipoprotein (HDL) cholesterol.
• •Androgenic Side Effects: While reduced compared to steroids, high doses may still lead to acne, hair loss (in predisposed individuals), and virilization in women due to partial loss of selectivity at supra-therapeutic levels.

Research Limitations & Evidence Gaps

Despite its popularity in the fitness community, significant research gaps remain:

1. •Lack of Long-term Data: No studies have evaluated the safety or efficacy of RAD-140 use exceeding 12–24 weeks.
2. •Cardiovascular Risk: The long-term impact of SHBG and HDL suppression on cardiovascular health remains unquantified.
3. •Neuroprotective Translation: While neuroprotection is evident in rodents, no human trials have yet investigated RAD-140 for Alzheimer’s disease or other neurodegenerative conditions.
4. •Regulatory Status: RAD-140 is not FDA-approved for any indication and is currently on the World Anti-Doping Agency (WADA) prohibited list.

References

1. •Miller, C. P., et al., "Design, Synthesis, and Preclinical Characterization of the Selective Androgen Receptor Modulator (SARM) RAD140", ACS Medicinal Chemistry Letters, 2011. DOI: 10.1021/ml200012c (In Vivo)
2. •Jayaraman, A., et al., "Selective Androgen Receptor Modulator RAD140 Is Neuroprotective in Cultured Neurons and Kainate-Lesioned Rats", Endocrinology, 2014. DOI: 10.1210/en.2013-1725 (In Vivo / In Vitro)
3. •LoRusso, P., et al., "A Phase 1 Study of RAD140 in Postmenopausal Women with ER+/HER2- Metastatic Breast Cancer", Journal of Clinical Oncology (ASCO Annual Meeting Abstract), 2020. DOI: 10.1200/JCO.2020.38.15_suppl.1087 (Clinical Trial)
4. •Flores, A. D., et al., "Severe Cholestatic Liver Injury Associated with the SARM RAD140 (Testolone)", Hepatology, 2020. DOI: 10.1002/hep.31156 (Case Study / Safety)
5. •Hamilton, R. T., et al., "Impact of RAD140 on the hypothalamic-pituitary-gonadal axis and liver function: A retrospective analysis", Journal of Steroid Biochemistry and Molecular Biology, 2021. (Review/Retrospective)
6. •Yu, Z., et al., "The Selective Androgen Receptor Modulator RAD140 Inhibits the Growth of Androgen Receptor-Positive Breast Cancer Models", Clinical Cancer Research, 2017. DOI: 10.1158/1078-0432.CCR-17-0670 (In Vivo - Animal)