S-23 Overview
S-23, also known as Mastorin, is a synthetic nonsteroidal selective androgen receptor modulator (SARM) of the arylpropionamide chemical class. It was originally developed by GTx, Inc. (the same developer as MK-2866 / Ostarine) and is referenced in the primary literature primarily for its investigation as a male hormonal contraceptive candidate, with documented dose-dependent suppression of spermatogenesis in animal models and reversible recovery upon cessation. S-23 has not reached human clinical trials but has been extensively characterized in preclinical androgen receptor pharmacology and analytical chemistry literature.
S-23 is widely cited as a reference SARM in androgen receptor binding studies, structure-activity relationship analysis of the arylpropionamide class, and anti-doping analytical method development. WADA adverse analytical findings for S-23 have increased year-over-year, reflecting growing relevance as an analytical target.
S-23 is not approved for human or veterinary use and is supplied strictly for laboratory and analytical research purposes.
Mechanism of Action
S-23 binds the androgen receptor ligand-binding domain with high affinity (reported K_i ≈ 1.7 nM) and acts as a tissue-differential AR ligand. Published in vitro and preclinical characterizations describe differential transcriptional output across androgen-responsive cell types, with documented effects on bone mineral density, body composition, and reproductive endocrine endpoints in rodent models. The compound has been characterized as one of the higher-affinity arylpropionamide-class AR ligands.
Reported characterizations include:
- AR binding K_i ≈ 1.7 nM
- AR-mediated transcriptional activation at low-nanomolar concentrations in CV-1 cell reporter assays
- Documented suppression of spermatogenesis in preclinical male contraceptive research, with spontaneous recovery upon cessation
- Investigated in glucocorticoid-induced muscle loss prevention research
- Documented urinary metabolite profile relevant to anti-doping analytical chemistry (characterized in Ameline et al. and related forensic toxicology literature)
Chemical Structure & Identification
- Chemical Name: (2S)-3-(4-Chloro-3-fluorophenoxy)-N-[4-cyano-3-(trifluoromethyl)phenyl]-2-hydroxy-2-methylpropanamide
- Synonyms: S23; S 23; Mastorin
- Chemical Class: Nonsteroidal arylpropionamide SARM
- CAS Number: 1010396-29-8
- Molecular Formula: C₁₈H₁₃ClF₄N₂O₃
- Molecular Weight (average): 416.75 g/mol
- PubChem CID: 44256407
Structural Description: S-23 is a small-molecule, nonsteroidal androgen receptor ligand of the arylpropionamide chemical class — structurally related to MK-2866 (Ostarine) and andarine (S-4). The molecule consists of a 4-chloro-3-fluorophenoxy ether, a chiral (S)-configured 2-hydroxy-2-methylpropanamide central group, and an aniline-derived arm bearing both a 4-cyano substituent and a 3-trifluoromethyl group. The 4-chloro-3-fluoro substitution pattern on the phenoxy ring (versus the 4-cyano substitution in MK-2866) is a key structural feature distinguishing S-23 from other arylpropionamide-class SARMs and contributing to its higher reported AR binding affinity. Structural identification and molecular data are referenced against PubChem.
Physical and Chemical Properties
- Appearance: White to off-white solid powder
- Solubility: Soluble in DMSO and ethanol; poorly soluble in water
- Stability: Stable under dry, cool, light-protected storage conditions
- Recommended storage: -20 °C for long-term stability
Research Focus Areas
S-23 is commonly referenced in laboratory research involving:
- Androgen receptor binding, competition, and transactivation assays
- SARM structure-activity relationship (SAR) analysis, particularly within the arylpropionamide chemical class
- Reproductive endocrinology research (S-23’s male contraceptive research history makes it a reference compound for AR-mediated spermatogenesis suppression studies)
- Bone mineral density and body composition signaling research in preclinical models
- Analytical and reference-standard work, including LC-MS/MS method development referenced in anti-doping analytical literature
- Urinary metabolite identification and forensic toxicology research
- Comparative SARM evaluation alongside other arylpropionamide-class compounds (MK-2866, andarine / S-4)
Research Background & Regulatory Context
S-23 is included on the World Anti-Doping Agency (WADA) Prohibited List under the anabolic agents category. Adverse analytical findings for S-23 in WADA-administered testing programs have increased year-over-year, reflecting its growing prevalence as an analyte of interest in anti-doping research. The U.S. Food and Drug Administration has issued multiple public consumer advisories naming SARMs broadly, including arylpropionamide-class compounds, in the context of unapproved bodybuilding and dietary supplement products. This published regulatory and analytical record is part of the scientific literature on the compound and reinforces that S-23 is supplied strictly as a research reagent for laboratory and analytical use.
Research Use Disclaimer
S-23 is intended strictly for laboratory research use only (RUO). Not for human consumption, clinical use, veterinary use, therapeutic application, dietary supplement use, or diagnostic purposes. All handling and use must comply with applicable federal, state, and local laws and institutional laboratory safety standards.
Summary
S-23 (Mastorin) is a structurally defined nonsteroidal arylpropionamide SARM developed by GTx, Inc. as a candidate male hormonal contraceptive and characterized extensively in preclinical androgen receptor pharmacology, reproductive endocrinology, and analytical chemistry research. Its high AR binding affinity (K_i ≈ 1.7 nM), distinctive 4-chloro-3-fluorophenoxy substitution pattern, and documented urinary metabolite profile make it a widely used reference compound in controlled laboratory studies of androgen receptor signaling and anti-doping analytical method development.
| CAS Number | 1010396-29-8 |
| Other Names | S23, S 23, CCTH-methylpropionamide, UNII-XDK89456WM, SCHEMBL2816704, SSFVOEAXHZGTRJ-UHFFFAOYSA-N, BCP30652 |
| IUPAC Name | 3-(4-chloro-3-fluorophenoxy)-N-[4-cyano-3-(trifluoromethyl)phenyl]-2-hydroxy-2-methylpropanamide |
| Molecular Formula | C₁₈H₁₃ClF₄N₂O₃ |
| Molecular Weight | 416.8 |
| Dry-Fill Capsule Concentration | 15mg |
| Liquid Concentration And Solution | 30mg/ml PEG400/DMSO |
| Aliquot Concentration And Solution | 50mg/ml (PEG400, Propylene Glycol, Benzyl Benzoate, Benzyl Alcohol) |
