The potential consequences of drug-drug interaction on the strategies adopted by anti-doping laboratories to report an adverse analytical finding for morphine were investigated. We evaluated in vitro the effects of 14 drugs on the principal metabolic pathways of morphine. The selected drugs are among those most commonly used by the athletes, none of them presently included in the World Anti-Doping Agency (WADA) Prohibited List. The non-prohibited drugs included 4 antifungals (fluconazole, itraconazole, ketoconazole, and miconazole), 6 benzodiazepines (alprazolam, bromazepam, clonazepam, lorazepam, lormetazepam, and triazolam), and 4 non-steroidal anti-inflammatory drugs (diclofenac, ibuprofen, ketoprofen, and nimesulide). The in vitro assays were based on the use of either human liver microsomes or uridine 5'-diphospho-glucuronosyl-transferases. Morphine and its glucuronides were determined by developed liquid chromatography-mass spectrometry procedure after dilution with an aqueous solution containing their deuterated isotopologues as internal standards. Morphine is mainly excreted as phase II metabolites: about 70% of the parent compound is found to be biotransformed by UGT2B7 to morphine-3-glucuronide (6065%) and morphine-6-glucuronide (5-10%). A reduction of the enzymatic activity of the UGT2B7 was recorded in the presence of 9 of the 14 drugs under investigation (ketoconazole, miconazole, itraconazole, diclofenac, ibuprofen, clonazepam, lorazepam, lormetazepam, and triazolam), with a consequent significant reduction of the levels of the glucuronide metabolites. This phenomenon in vivo may affect the rate of the urinary excretion of morphine with the risk of reporting "false negative" results, especially in case of results close to the decision limit value set by WADA.
Keywords: anti-doping analysis; drug-drug interactions; masking strategy; morphine.
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