Formestane is a steroidal aromatase inhibitor, prohibited at all times under the IFHA and ARCI rules. In 2023, a postrace urine sample collected from a female Thoroughbred racehorse returned positive for formestane, 4-hydroxytestosterone, and 6α-hydroxyandrostenedione, from the Official Laboratory (RASL). The initial investigation identified a commercial supplement containing dehydroepiandrosterone (DHEA) and pregnenolone, administered at twice the daily dose recommended by the manufacturer. Subsequently, the responsible person submitted a report from their private veterinary advisor claiming endogenous production, requesting threshold implementation and Isotope Ratio Mass Spectrometry (IRMS) analysis, as recommended by WADA in human anti-doping.
The investigation extended over several months so both hypotheses could be evaluated. This included compositional/metabolic assessment of the supplement, review of pharmacokinetic pathways from DHEA and pregnenolone, comparison of carnivorous versus herbivorous models, and literature review on endogenous formestane production. Importantly, the Official Laboratory provided continuous cooperation, hosting technical meetings to review data, while other national regulatory entities agreed to participate in interviews.
The theory of an endogenous source was unlikely due to the gender of the horse and the estimated concentration of formestane detected in the sample. Additionally, the comparison between herbivore equine models and human omnivore/carnivore models was not applicable, as dietary sources markedly influence carbon isotope ratios (δ13C), making IRMS interpretations unreliable. In contrast, the metabolite profile was quite plausible, since DHEA is converted to androstenedione, which can yield 6α-hydroxyandrostenedione, while androstenedione may also be hydroxylated to formestane (4-hydroxyandrostenedione), which is subsequently metabolized to 4-hydroxytestosterone. Unfortunately, as the supplement administered was no longer available for analysis, the possibility of the presence of an undeclared ingredient could not be ruled out.
This case illustrates the challenges in equine anti-doping regulation. Case resolution depended on laboratory findings, international consultation and interdisciplinary collaboration. Subsequent international discussions underscored the importance of global scientific dialogue in equine anti-doping.