Intestinal absorption and metabolism of chlorpheniramine enantiomers in rat

Chlorpheniramine (CPAM) is a chiral antihistaminic drug commercialized as a racemic mixture. The intestinal absorption and metabolism of CPAM have been investigated in rat using in vivo (oral and IV administration), in situ (intestinal loop model), and in vitro (everted sac model) experiments. Oral and IV administrations of 20 mg/kg of the racemic mixture show that the pharmacokinetics of CPAM are stereoselective, with higher AUCs for the (+)-S-enantiomer compared to its antipode. The monodesmethyl metabolite (DCPM) was quantifiable in blood and its pharmacokinetics are stereoselective after oral but not after IV administration. Experiments using intestinal loops and everted sacs showed that the absorption is not stereoselective and that in vivo stereoselective formation of DCPM is presumably due to stereoselective hepatic metabolism. Moreover, the in vitro and in situ absorption of CPAM are not modified by modulators of P-glycoprotein and cytochromes P450 (cyclosporin A, ketoconazole).