Halotestin

Fluoxymesterone is a synthetic anabolic-androgenic steroid (AAS) belonging to the 17α-alkylated testosterone derivative class. Its chemical structure is based on testosterone with three key modifications: a 9α-fluoro group, an 11β-hydroxyl group, and a 17α-methyl group, giving it the systematic name 9-fluoro-11β,17β-dihydroxy-17α-methylandrost-4-en-3-one.

The 17α-methyl substitution is the structural feature primarily responsible for oral bioavailability. Unmodified testosterone undergoes extensive first-pass hepatic metabolism when taken orally, rendering it largely inactive. The methyl group at the 17α position sterically hinders this rapid hepatic breakdown, allowing the compound to reach systemic circulation. This same modification is, however, associated with hepatotoxic potential, a property shared with other 17α-alkylated androgens.

Human study: In a clinical study of infants and children, administration of fluoxymesterone produced an approximately two-fold decrease in plasma sex steroid-binding protein (SBP), an effect that was absent in subjects with androgen insensitivity syndrome, suggesting the response is androgen-receptor-mediated. The mechanism by which androgens lower SBP in vivo was acknowledged to be not yet understood. Note: Oral fluoxymesterone and injectable depot-testosterone compared in separate groups (PMID 2847619)

Human study: In a randomized trial of 108 female patients with metastatic breast carcinoma, the combination of tamoxifen and fluoxymesterone achieved a complete and partial remission rate of 38% compared with 15% for tamoxifen alone, and prolonged overall time to treatment failure. Side effects were not meaningfully different across tamoxifen dose levels but androgen-related effects were noted in the combination arm. Note: Oral fluoxymesterone combined with tamoxifen in a randomized trial (PMID 6824247)

Human study: Human excretion studies identified 6β-hydroxylation as the major metabolic pathway for fluoxymesterone, in contrast to testosterone and 17α-methyltestosterone for which this pathway is negligible. The 6β-hydroxy metabolite was synthesized, isolated, and characterized by multiple analytical techniques to support doping detection. Note: Doping detection study; human excretion after single administered dose (PMID 8539789)

Human study: Liquid chromatography quadrupole time-of-flight mass spectrometry analysis of human urine identified twelve metabolites of fluoxymesterone including three previously unreported structures, one of which (a 6,16-dihydroxylated metabolite) had previously been detected only in equine urine. The findings expand the known urinary metabolite profile relevant to doping control. Note: Doping detection study; analysis of positive control urine samples (PMID 22521423)

Human study: LC-MS/MS and GC-MS analysis of urine from a positive fluoxymesterone doping case detected nine analytes including the parent compound, with five previously unreported metabolites identified. A neutral-loss scan for loss of hydrogen fluoride was proposed as a selective detection strategy for fluoxymesterone metabolites. Note: Doping detection study; analysis of positive doping control urine samples (PMID 18035854)

Human study: In a randomized trial of 279 elderly postmenopausal patients with metastatic breast cancer, the addition of fluoxymesterone to tamoxifen did not significantly improve overall response rate, time to treatment failure, or survival compared with tamoxifen alone. Toxicity was more pronounced in the combined treatment groups, with most toxicity attributable to fluoxymesterone. Note: Oral fluoxymesterone used in combination with tamoxifen (PMID 10942095)

Human study: In a study of 184 patients with previously treated metastatic breast cancer, two doxorubicin-based regimens each containing fluoxymesterone (DAVH and TAVH) produced overall response rates of 32–38%, with similar duration of response and survival between arms. Seven treatment-related deaths occurred across the two groups, and the regimens were assessed as reasonable second-line options. Note: Fluoxymesterone used as one component of two multi-drug chemotherapy regimens (PMID 2505919)

Human study: A clinical review noted that fluoxymesterone, as an orally active testosterone derivative, has been used as androgen replacement in males with hypogonadism, but its use in idiopathic male infertility and erectile dysfunction in elderly men was shown to be less effective. The review also noted high-dose use of synthetic androgens by athletes for performance enhancement and as an approach to male hormonal contraception. Note: Review covers oral androgens including fluoxymesterone; injectable testosterone esters described separately (PMID 8164356)

Animal study: In a neonatal hamster study, fluoxymesterone failed to masculinize or defeminize sexual behavior in either castrated males or females despite producing external genital virilization comparable to testosterone. The authors concluded that fluoxymesterone has little or no central neural androgenic effect on sexual behavior in this species. Note: Animal behavioral study in golden hamsters (PMID 7250522)

Human study: In a prospective randomized trial of 62 evaluable patients with metastatic breast cancer, the tamoxifen plus fluoxymesterone combination produced greater toxicity than tamoxifen plus danazol, notably increased masculinization, with equivalent anti-tumor response rates of approximately 11–12%. The investigators concluded tamoxifen plus danazol was preferable due to the higher toxicity burden of fluoxymesterone. Note: Used in combination with tamoxifen vs tamoxifen plus danazol (PMID 3058238)

Human study: A case report documented pulmonary peliosis, peliosis hepatis, and splenic peliosis in a 40-year-old man who received long-term therapy with fluoxymesterone and methyltestosterone for Fanconi's anemia. The authors noted this represented only the second reported case of pulmonary peliosis in the English literature, highlighting the rarity but severity of this vascular complication. Note: Used alongside methyltestosterone in a patient with Fanconi's anemia (PMID 3839663)

Human study: In a randomized clinical trial of patients with cancer anorexia/cachexia, fluoxymesterone resulted in significantly less appetite enhancement than megestrol acetate or dexamethasone and did not demonstrate a favorable toxicity profile. The investigators concluded fluoxymesterone was clearly an inferior choice for treating cancer-related anorexia/cachexia. Note: Oral fluoxymesterone compared with megestrol acetate and dexamethasone (PMID 10506633)

Laboratory study: In vitro testing in cultured human lymphocytes and in vivo testing in mouse bone marrow demonstrated that fluoxymesterone induced chromosome aberrations, sister chromatid exchanges, and increased micronuclei frequencies, indicating clastogenic potential. Fluoxymesterone did not cause significant mutagenic activity in the Ames Salmonella assay with or without metabolic activation. (PMID 7715612)

Laboratory study: In primary rat hepatocyte cultures, significant increases in lactate dehydrogenase release and decreases in cell viability were observed with other 17α-alkylated steroids (methyltestosterone, stanozolol, oxymetholone) but not with fluoxymesterone at the doses and timepoints evaluated. The authors identified a structural-activity relationship in which 17α-alkylation generally predicted direct hepatocyte toxicity, though fluoxymesterone did not fit this pattern in this in vitro model. Note: In vitro hepatocyte culture study; fluoxymesterone tested but did not show significant toxicity at doses tested (PMID 8527826)

Open questions: Several important questions remain unresolved. The precise molecular mechanism by which fluoxymesterone and other androgens suppress sex steroid-binding protein (SHBG) in vivo is not understood, despite in vitro evidence of stimulatory effects on hepatoma cells. It is unclear whether the genotoxic (clastogenic) effects observed in cultured human lymphocytes and mouse bone marrow translate to clinically meaningful cancer risk in humans. The relative contribution of fluoxymesterone versus co-administered agents to hepatic and other toxicities in older clinical trials is difficult to disentangle, since it was almost always used in combination regimens.

Its apparent inability to masculinize central neural circuits in neonatal hamsters raises unanswered questions about whether the 9α-fluoro and 11β-hydroxyl groups alter androgenic activity in specific tissues in humans. Finally, long-term safety data from controlled trials are lacking, as most clinical studies were conducted decades ago in heavily pre-treated oncology populations.