Drug-drug interaction and doping, part 2: an in vitro study on the effect of non-prohibited drugs on the phase I metabolic profile of stanozolol.

The present study was designed to provide preliminary information on the potential impact of metabolic drug-drug interaction on the effectiveness of doping control strategies currently followed by the anti-doping laboratories to detect the intake of prohibited agents. In vitro assays based on the use of human liver microsomes and recombinant cytochrome P450 isoforms were developed and applied to characterize the phase I metabolic profile of the prohibited agent stanozolol, both in the absence and in the presence of substances (ketoconazole, itraconazole, miconazole, cimetidine, ranitidine, and nefazodone) not included in the World Anti-Doping Agency (WADA) list of prohibited substances and methods and frequently administered to athletes. The results show that the in vitro model utilized in this study is adequate to simulate the in vivo metabolism of stanozolol. Furthermore, our data showed that ketoconazole, itraconazole, miconazole, and nefazodone caused a marked modification in the production of the metabolic products (3'-hydroxy-stanozolol, 4β-hydroxy-stanozolol and 16β-hydroxy-stanozolol) normally selected by the anti-doping laboratories as target analytes to detect stanozolol intake. On the contrary, moderate variations were registered in the presence of cimetidine and no significant modifications were measured in the presence of ranitidine. This evidence confirms that the potential effect of drug-drug interactions is duly taken into account also in anti-doping analysis.