Enclomiphene: Scientific Profile

Publicado el 21 de mayo de 2026 · Actualizado el 28 de agosto de 2026

Selective estrogen receptor modulator (SERM) for research. It acts as an antagonist in the hypothalamus, stimulating the release of endogenous gonadotropins.

Chemical identity. Enclomiphene is the E (trans) isomer of clomiphene and belongs to the triphenylethylene family, the selective estrogen receptor modulators (SERM). Its molecular formula is C26H28ClNO, with a molecular weight of 405.9 g/mol and CAS number 15690-57-0. In the literature it usually appears as enclomiphene citrate, which is the salt used in the published clinical trials, and as the clinical development that Repros Therapeutics carried out with this compound. It is described here exclusively as a reference material for research.

Pharmacology in the hypothalamic-pituitary-gonadal axis. Enclomiphene behaves as an antagonist of estrogen receptors at the hypothalamic and pituitary level. By blocking the negative feedback that estradiol exerts on that axis, it increases the pulsatile secretion of gonadotropin-releasing hormone (GnRH) and, with it, the pituitary release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In gonadal models, that signal stimulates the Leydig and Sertoli cells. What makes the compound interesting as a research tool is that the stimulus is endogenous: whereas exogenous administration of testosterone suppresses LH and FSH, in the published trials with enclomiphene the gonadotropins were maintained or rose above baseline.

Enclomiphene versus clomiphene: a difference of isomers, not of class. The clomiphene citrate marketed as a drug is not a single molecule, but a mixture of two geometric isomers: approximately 62% enclomiphene (E isomer, trans) and 38% zuclomiphene (Z isomer, cis). The two behave differently. Enclomiphene is the antiestrogenic fraction and has a short plasma half-life, on the order of 8 to 10 hours. Zuclomiphene has estrogenic agonist activity and a half-life greater than 40 hours; in disposition studies, plasma concentrations have been measured more than a month after exposure, which produces accumulation with repeated administration. Hence the rationale for the isolated isomer: it allows the study of the blockade of estrogenic feedback without the persistent estrogenic component contributed by zuclomiphene.

Enclomiphene versus tamoxifen. Both are triphenylethylene SERMs and both raise LH and FSH by antagonizing the estrogen receptor in the hypothalamus, but their tissue profile and pharmacokinetics differ. Tamoxifen is characterized by mixed behavior—antagonist in breast tissue, partial agonist in bone and endometrium—and acts largely through long-half-life active metabolites, such as 4-hydroxytamoxifen and endoxifen. Enclomiphene has no comparable accumulating metabolite and its antagonism is more limited in time. In laboratory practice this means they are not interchangeable as pharmacological probes: tamoxifen is used above all as a breast-tissue antiestrogen, and enclomiphene as a probe of the hypothalamic-pituitary-gonadal axis.

Regulatory status: what it means that this development was never approved. This is the point that is most frequently misinterpreted. Enclomiphene is not an approved medication. Repros Therapeutics submitted this compound to the FDA for secondary hypogonadism and received Complete Response Letters in 2009, 2012, and December 2015. In the last one, the agency considered that the design of the phase 3 studies was no longer sufficient to demonstrate clinical benefit and requested at least one additional trial, in addition to noting objections regarding the inclusion criteria, the titration, and the validation of the bioanalytical method. That additional trial was never completed. Enclomiphene does not have marketing authorization in the United States, the European Medicines Agency never received an application, Health Canada does not have it in its product database, and the British MHRA has not licensed it. In Mexico, the active ingredient with sanitary registration before COFEPRIS is clomiphene—the mixture of the two isomers, indicated for ovulation induction—not isolated enclomiphene. The practical reading for a laboratory is direct: any enclomiphene material is a research reagent and not a pharmaceutical specialty, and must be handled and documented to that standard.

Anti-doping control. Clomiphene is expressly listed in class S4 (hormone and metabolic modulators) of the World Anti-Doping Agency Prohibited List of Substances and Methods, prohibited at all times, in and out of competition. Enclomiphene, being one of the constituent isomers of clomiphene and given the open wording of that class, is included in the same prohibition. This is a pertinent datum for any research protocol with sporting implications.

What has been studied and with what design. The published evidence base is limited and comes mainly from the Repros clinical program. The pharmacokinetic and pharmacodynamic study by Wiehle and colleagues (BJU International, 2013) compared arms of 6.25, 12.5, and 25 mg daily of enclomiphene citrate over six weeks against a testosterone gel: the enclomiphene arms raised total testosterone while maintaining LH and FSH, whereas the gel suppressed them. Subsequent phase 3 reports further describe an increase in sperm concentration in the enclomiphene arms versus a decrease in the testosterone-gel arms. Later reviews (Rodriguez and colleagues, 2016; Saffati and colleagues, 2024; Hohl and colleagues, 2025) systematize those findings and the comparisons with clomiphene. These figures describe the design of published trials and are cited as bibliographic reference; they do not constitute a usage guideline.

Laboratory handling. Enclomiphene is presented as a solid and is stored between 15 and 30 °C, in a dry place protected from light. As with any nuclear receptor modulator without a pharmacopeial reference standard, it is advisable to document lot, identity and purity by HPLC before incorporating it into a protocol, and to record the date the container was opened.

FAQ

Are Enclomiphene and clomiphene the same? No. Clomiphene is a mixture of two isomers, approximately 62% enclomiphene and 38% zuclomiphene. Enclomiphene is only the isolated E isomer, without the long-half-life estrogenic fraction contributed by zuclomiphene.

What is the difference between enclomiphene and tamoxifen? Both are SERMs, but tamoxifen has a mixed tissue profile and acts through long-half-life active metabolites, whereas enclomiphene is studied specifically as an estrogen antagonist of the hypothalamic-pituitary-gonadal axis, with a competitive, short-duration action.

Is enclomiphene approved in Mexico? No. In Mexico the sanitary registration corresponds to clomiphene, not to isolated enclomiphene. The enclomiphene development by Repros Therapeutics was never approved by the FDA nor did it obtain marketing authorization in the European Union, Canada or the United Kingdom.

Is enclomiphene a SARM? No. A SARM is a selective androgen receptor modulator; enclomiphene is a SERM and acts on the estrogen receptor. They appear grouped together in some catalogs, but the molecular target is different.

Why does the research separate the E isomer? Because zuclomiphene accumulates: its half-life exceeds 40 hours and it has been detected in plasma more than a month after exposure. Isolating the E isomer eliminates that confounding variable from the experimental design.

What is investigated with enclomiphene? The blockade of hypothalamic-pituitary estrogenic feedback, the increase of endogenous LH and FSH and the recovery of the HPG axis after periods of suppression, including the effect on spermatogenesis in the models evaluated.

This content is a reference technical data sheet on a compound intended exclusively for research. It is not medical information or a recommendation for use.