Equine metabolic investigation of the phosphodiesterase-4 inhibitor ibudilast as a potential performance enhancer

Abstract

Rationale: Phosphodiesterase 4 (PDE4) inhibitors are a newer class of drugs that induce bronchodilation and have anti-inflammatory effects, making them susceptible to misuse as performance enhancers in competitive sports. Methods: This study explores the metabolic conversion of PDE4 inhibitor ibudilast in thoroughbred horses after oral administration and in vitro using equine liver microsomes and Cunninghamella elegans. A liquid chromatography-high resolution mass spectrometry method was used to postulate the plausible structures of the detected metabolites. Results: A total of 20 in vivo metabolites were identified under experimental conditions, including 12 Phase I and 8 Phase II conjugated metabolites. Phase I metabolites were predominantly formed through hydroxylation (mono-, di-, and tri-hydroxylation). Demethylated metabolites were also identified during this investigation. Additionally, the research detected Phase II metabolites conjugated with glucuronic and sulfonic acids. Conclusions: The data presented here can assist in detecting the PDE4 inhibitor ibudilast and uncover its illicit use in competitive sports.