AIM: To compare the pharmacokinetics of the enantiomers of trans-tramadol (trans-T) and its active metabolite, trans-O-demethyltramadol (M1), in male and female rats. METHODS: Following a single oral dose of 10 mg/kg trans-T hydrochloride to rats, (+)-trans-T, (-)-trans-T, (+)-M1, and (-)-M1 in plasma were determined by a high performance capillary electrophoresis method. RESULTS: The females showed higher plasma concentrations of (+)-trans-T, (-)-trans-T, and (+)-M1 than the males. The enantiomers of trans-T were absorbed and eliminated more slowly in the females than in the males. (+)-M1 was eliminated more slowly in the females than in the males. All pharmacokinetic parameters but Tmax of the two enantiomers of trans-T were significantly different in both sex rats. The (+)/(-)-enantiomeric ratios of the pharmacokinetic parameters for trans-T in the males were similar to those in the females. The values of Cmax, AUC0-∞ of the two enantiomers of M1 were significantly different in both sex rats. The (+)/(-)-enantiomeric ratios of Cmax, AUC0-∞ for M1 were lower than 1 in the males, larger than 1 in the females. CONCLUSIONS: Systemic exposure of (+)-trans-T, (-)-trans-T, and (+)-M1 was higher in female rats than in male rats. The stereoselectivity in pharmacokinetics of trans-T was similar, and that of M1 was different in male and female rats.
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