S-23 SARM capsules contain an aryl-propionamide selective androgen receptor modulator, CAS 1010396-29-8, molecular formula C18H13ClF4N2O3, molecular weight 416.75. It emerged from the University of Tennessee programme and was characterised by Jones and colleagues in 2009 in a specific and unusual research context.
Specification Table
S-23 verified chemical and handling data
| Property |
Value |
| Compound |
S-23 |
| CAS number |
1010396-29-8 |
| Molecular formula |
C18H13ClF4N2O3 |
| Molecular weight |
416.75 g/mol |
| Systematic name |
(S)-N-(4-cyano-3-trifluoromethyl-phenyl)-3-(3-fluoro,4-chlorophenoxy)-2-hydroxy-2-methyl-propanamide |
| Chemical class |
Aryl-propionamide, non-steroidal |
| Stereochemistry |
S enantiomer |
| Molecular target |
Androgen receptor (AR, NR3C4) |
| Reported binding affinity |
High, in the low nanomolar range |
| Originating programme |
University of Tennessee, Dalton and Miller group |
| Primary publication |
Jones et al., Endocrinology, 2009;150(1):385-395 |
| Reported research context |
Reversible suppression of spermatogenesis in mouse models |
| Capsule strength |
10 mg |
| Appearance |
White to off-white powder within a gelatin capsule shell |
| Purity |
Per lot-specific certificate of analysis |
| Solubility |
Soluble in DMSO and ethanol. Poorly soluble in water |
| Storage |
Room temperature, sealed, protected from light and moisture |
| Human pharmacokinetic data |
None published |
| Anti-doping status |
SARMs are prohibited by the World Anti-Doping Agency |
| Regulatory status |
No approved human or veterinary formulation in any jurisdiction |
What Did the Primary Research Actually Investigate?
This is the most interesting thing about the compound and the part most often omitted from supplier descriptions.
Jones and colleagues characterised S-23 as a candidate component of a hormonal male contraceptive regimen (Jones et al., Endocrinology, 2009). Not as an anabolic agent. The study reported that in mouse models the compound reversibly suppressed spermatogenesis, and that the suppression reversed on withdrawal.
The mechanism sits in negative feedback. An androgen receptor agonist reaching the hypothalamic-pituitary axis suppresses gonadotropin-releasing hormone, and downstream of that luteinizing hormone and follicle-stimulating hormone fall. Both are required for spermatogenesis. Suppressing them suppresses sperm production while the peripheral androgen signal is maintained by the compound itself.
The authors described this as the first demonstration that an aryl-propionamide SARM could serve as a component of an oral male contraceptive regimen. The same study also reported increased lean mass and bone mineral density with reduced fat mass in the same models, which is the tissue-selectivity profile this compound class is known for.
Any accurate description of S-23 leads with the contraception research, because that is what the primary literature is about.
What Does the Structure Tell You?
The systematic name is dense but readable, and it locates the compound precisely within its class.
The core is an aryl-propionamide, the same scaffold as andarine and the other University of Tennessee compounds. Attached to it is a 4-cyano-3-trifluoromethyl-phenyl group on the amide nitrogen, and a 3-fluoro-4-chlorophenoxy group at the propionamide 3-position.
Two halogen substitutions on that phenoxy ring, fluorine and chlorine, distinguish S-23 from its relatives. Halogen substitution on aromatic rings is a standard medicinal chemistry approach to increasing binding affinity and metabolic stability, and the reported affinity of this compound is high even by the standards of the class.
The S designation is stereochemical and matters. The molecule has a chiral centre at the propionamide 2-position, and the two enantiomers do not behave identically at the receptor. A preparation containing significant R enantiomer is not the characterised compound, and chiral chromatography is the method that establishes which is present.
How Do S-23 SARM Capsules Compare Within the Class?
Placing S-23 SARM capsules against the others in the catalogue is more useful than describing it in isolation.
Against andarine, the shared scaffold makes comparison meaningful. Both are aryl-propionamides from the same programme. Andarine has the broader preclinical literature and the visual-disturbance association. S-23 has higher reported binding affinity and a narrower, more specific published research context.
Against AC-262,536, there is no structural relationship at all, since that compound is an azabicyclo-naphthalenecarbonitrile from a separate discovery effort. AC-262,536 has the better-documented selectivity counter-screen across 47 nuclear receptors. S-23 has the more interesting in vivo characterisation.
Against RAD-140 and its benzoate ester, S-23 has the significant advantage of a genuine primary publication describing what it does. TLB-150 has none at all.
The summary is that S-23 earns its place on a specific, well-executed piece of in vivo pharmacology addressing an unusual question, rather than on breadth of evidence.
What Is Not Established?
Human pharmacokinetics are absent. No published half-life, no absorption or clearance data, no human exposure study of any kind. The compound did not progress to clinical evaluation.
Long-term safety data does not exist beyond the duration of the published rodent work. The reversibility observation in mice concerns the specific exposure period tested, and reversibility over longer periods has not been examined.
Independent replication is limited. The 2009 Endocrinology paper remains the principal source for essentially every characterisation figure quoted about this compound, and subsequent work from other laboratories is sparse.
A note on framing that matters for interpretation: the contraception context means the published exposure levels were chosen to achieve gonadotropin suppression, not to optimise any other endpoint. Reading anabolic observations from that study without noting the exposure design misrepresents what was measured.
What Does Reversibility Actually Mean in the Published Work?
The reversibility observation is the most cited feature of this compound and the one most loosely repeated, so it is worth stating what was and was not shown.
In the mouse work, spermatogenesis was suppressed during exposure and recovered after withdrawal. That is the observation. The recovery was assessed within the timeframe of the study, in the specific strain used, at the specific exposure levels tested.
What that does not establish is reversibility after longer exposure, in other species, or at other exposure levels. Suppression-recovery relationships in reproductive biology are frequently duration-dependent, and a system that recovers after weeks may behave differently after months. No published work addresses that for this compound.
The mechanism itself predicts reversibility in principle, since gonadotropin suppression through negative feedback should lift once the suppressing signal is removed and the axis is not being damaged directly. Predicting reversibility from mechanism and demonstrating it over an extended period are different levels of evidence.
For study design the implication is straightforward. If recovery is an endpoint, it needs its own measurement arm with an adequate follow-up window, rather than being assumed from the 2009 observation.
Where S-23 SARM Capsules Sit in the Regulatory Picture
No approved formulation of S-23 exists anywhere, and the compound did not progress beyond preclinical evaluation. That is the straightforward part.
The less straightforward part concerns anti-doping. Selective androgen receptor modulators as a class are prohibited by the World Anti-Doping Agency, and analytical methods for detecting them in biological matrices are an active published field. S-23 appears in that literature as a target analyte rather than as a subject of pharmacological study.
For laboratories developing or validating detection methodology, that creates a legitimate demand for reference material entirely separate from any pharmacological interest. Method development requires a characterised standard and a documented chain of custody.
Researchers working with athlete populations, or with samples that could enter anti-doping analysis, should confirm current WADA listings and any sport-governing-body requirements before beginning, since listings are revised annually.
How the Aryl-Propionamide Class Developed
Understanding where this scaffold came from explains why several compounds in this catalogue look structurally similar.
The starting point was bicalutamide, a non-steroidal antiandrogen already in clinical use at the time. Researchers at the University of Tennessee observed that structural modification of that antagonist scaffold could convert it into an agonist, which is an unusual and useful transformation.
From that observation came a series of aryl-propionamides sharing a common core: an amide linking a substituted aniline to a propionamide bearing a phenoxy group. Andarine, S-23 and several related compounds all sit on that scaffold, differing in the substitution pattern on the two aromatic rings.
Those substitution differences are what drive the pharmacology. Halogen placement, cyano and trifluoromethyl positioning all shift receptor affinity, tissue selectivity and metabolic stability, which is why compounds that look nearly identical on paper behave differently in vivo.
It also explains the shared limitations. Compounds from one scaffold tend to share solubility behaviour, metabolic routes and analytical challenges, so a method developed for one is often adaptable to another.
Analytical Confirmation and Handling
Chiral purity is the check that matters most for this compound and the one least often performed. Standard reversed-phase chromatography separates S-23 from unrelated impurities but does not resolve enantiomers, which co-elute. Establishing that a preparation is the S enantiomer requires a chiral stationary phase.
Mass spectrometry confirms molecular weight at 416.75 and the halogen content produces a distinctive isotope pattern from the chlorine, which makes identity confirmation straightforward. It cannot distinguish enantiomers, since they have identical mass.
For handling, the compound is supplied at 10 mg per capsule. Weigh contents rather than relying on nominal fill for analytical work. DMSO and ethanol are the practical stock solvents given poor aqueous solubility, and precipitation on dilution into aqueous media is the expected failure mode to watch for.
Sealed at ambient temperature and shielded from light and moisture, the dry powder holds well. Record lot, solvent, concentration and preparation date, and note whether chiral purity was confirmed, since that determines what the material actually is.
Published Literature
Traced to publisher records or primary indexes. The 2009 Endocrinology paper is the primary characterisation and the source for the contraception observations described above.
- Jones A, Chen J, Hwang DJ, Miller DD, Dalton JT. Endocrinology. 2009;150(1):385-395.
- Narayanan R, Mohler ML, Bohl CE, Miller DD, Dalton JT. Nuclear Receptor Signaling. 2008;6:e010.
- Gao W, Dalton JT. Drug Discovery Today. 2007;12(5-6):241-248.
- Christiansen AR, Lipshultz LI, Hotaling JM, Pastuszak AW. Translational Andrology and Urology. 2020;9(Suppl 2):S135-S148.
Frequently Asked Questions
What are S-23 SARM capsules?
Capsules containing S-23, an aryl-propionamide selective androgen receptor modulator with CAS 1010396-29-8, molecular formula C18H13ClF4N2O3 and molecular weight 416.75. Ten milligrams per capsule. Laboratory research use only, and not approved for human or veterinary use anywhere in the world.
What was S-23 originally studied for?
Reversible suppression of spermatogenesis, as a candidate component of a hormonal male contraceptive regimen. Jones and colleagues reported in 2009 that the compound suppressed spermatogenesis in mouse models and that suppression reversed after withdrawal. That is the primary research context, not anabolic activity.
How does the contraception mechanism work?
Through negative feedback on the hypothalamic-pituitary axis. An androgen receptor agonist reaching that axis suppresses gonadotropin-releasing hormone, which lowers both luteinizing hormone and follicle-stimulating hormone. Spermatogenesis requires both. The compound meanwhile maintains the peripheral androgen signal.
Why does stereochemistry matter for S-23?
The molecule carries a chiral centre at the propionamide 2-position, and the two enantiomers do not behave identically at the receptor. The characterised compound is the S enantiomer. A preparation containing substantial R enantiomer is simply not what the literature describes.
Can mass spectrometry confirm chiral purity?
No. Enantiomers share identical mass, so mass spectrometry confirms molecular weight and, through the chlorine isotope pattern, compound identity, but cannot distinguish handedness. Standard reversed-phase chromatography fails too, since enantiomers co-elute. A chiral stationary phase is required.
How does S-23 compare with andarine?
They share the aryl-propionamide scaffold and the same originating programme at the University of Tennessee, which makes comparison meaningful rather than superficial. Andarine carries the broader preclinical literature and the visual-disturbance association. S-23 has higher reported affinity and a narrower research context.
What do the halogen substitutions do?
The phenoxy ring carries both fluorine and chlorine. Halogen substitution on aromatic rings is a standard approach to raising binding affinity and metabolic stability, and reported affinity for S-23 is high even by the standards of its class.
Is there human data on S-23?
None at all. No published half-life, no absorption or clearance data, and no human exposure study of any kind, because the compound never progressed to clinical evaluation. The 2009 Endocrinology paper remains the principal source for essentially every figure quoted about it.
Why does the exposure design of the primary study matter?
Because exposure levels in that study were chosen to achieve gonadotropin suppression rather than to optimise anything else. The same paper reported increased lean mass and bone mineral density alongside reduced fat mass, and reading those observations without noting the design misrepresents what was measured.
How should the capsules be stored?
Ambient temperature in a sealed container, away from light and moisture, keeps the powder stable. DMSO and ethanol are the practical stock solvents given poor aqueous solubility, and precipitation on dilution into aqueous media is the expected failure mode. Record whether chiral purity was confirmed.
What did the reversibility observation actually show?
That spermatogenesis was suppressed during exposure and recovered afterwards, assessed within the study timeframe, in one mouse strain, at the exposure levels tested. It established nothing about reversibility after longer exposure, in other species, or at other levels.
Does the mechanism predict reversibility?
In principle it does, since gonadotropin suppression through negative feedback should lift once the suppressing signal is withdrawn and the axis has not been directly damaged. Predicting reversibility from mechanism is a different level of evidence from demonstrating it over an extended period.
How should recovery be studied?
With a dedicated measurement arm and an adequate follow-up window, rather than assumed from what the 2009 study reported. Suppression-recovery relationships in reproductive biology are often duration-dependent. A system that recovers after weeks may behave quite differently after months.
Compliance Statement
S-23 SARM capsules is sold exclusively for laboratory research use. It is not a drug, food, or cosmetic product, and it is not a dietary product of any kind. It is not approved by the FDA or any comparable authority for human or veterinary use, and selective androgen receptor modulators are prohibited in competitive sport under World Anti-Doping Agency rules. This product is not intended to diagnose, treat, cure, or prevent any disease. It must not be given to humans or animals. Purchase is restricted to qualified researchers and institutions operating within applicable laws. All handling is the responsibility of the purchasing laboratory.
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