RAD-140, designated Testolone, is a non-steroidal selective androgen receptor modulator (SARM) developed by Radius Health, Inc. Originally investigated as a potential therapeutic for muscle wasting conditions and breast cancer, RAD-140 has emerged as the most anabolic compound in the SARM class. Its chemical structure — a substituted benzonitrile with a dicyanopyridine core — was engineered to produce high binding affinity at the androgen receptor while maintaining oral bioavailability and tissue selectivity.

RAD-140 functions by selectively binding to androgen receptors in skeletal muscle and bone tissue, initiating the same downstream anabolic signaling cascade as testosterone — activation of AR-dependent gene transcription, increased nitrogen retention, and enhanced protein synthesis — without significant activity in prostate or sebaceous gland tissue. Preclinical data demonstrates an anabolic-to-androgenic selectivity ratio of approximately 90:1, compared to testosterone's 1:1 baseline. RAD-140 also exhibits neuroprotective properties, with in vitro research suggesting it may protect hippocampal neurons against kainate-induced excitotoxicity through AR-dependent mechanisms.

In the research literature, RAD-140 is consistently associated with significant lean muscle mass accrual, measurable strength increases, and modest improvements in endurance capacity. Studies in primates demonstrated lean mass gains of over 10% in 28 days at clinically relevant doses, with no significant changes in prostate weight — the hallmark of true tissue selectivity. Fat loss via nutrient partitioning is a commonly reported secondary effect.

RAD-140 is best suited for experienced researchers pursuing lean mass acquisition during caloric surplus protocols, or for body recomposition where simultaneous muscle gain and fat reduction are the objective. Its potency makes it a poor first compound for researchers unfamiliar with SARMs; MK-2866 or ACP-105 are more appropriate entry points. RAD-140 is frequently stacked with MK-677 for synergistic anabolic and recovery effects.

RAD-140 has an elimination half-life of approximately 60 hours, supporting once-daily oral administration. Oral bioavailability is high. Suppression of endogenous testosterone production is dose-dependent but generally moderate at standard research doses — significantly less than equivalent anabolic steroid protocols, though enough to warrant post-cycle therapy (PCT) with a SERM such as Nolvadex or Clomid for cycles exceeding 8 weeks. Liver enzyme elevation is minimal at standard doses. Typical research protocols range from 10–30mg daily for 8–12 weeks.