The enantioselective determination of chlorpheniramine and its major metabolites in human plasma using chiral chromatography on a beta-cyclodextrin chiral stationary phase and mass spectrometric detection.

A sensitive enantioselective high-performance liquid chromatographic (HPLC) method has been developed for the simultaneous determination of plasma concentrations of (-)(R)- and (+)(S)-chlorpheniramine (CP) and their metabolites, desmethyl-chlorpheniramine (DCP), didesmethyl-chorpheniramine (DDCP) and chlorpheniramine N-oxide (CPNO). Enantioselective separations were achieved on a beta-cyclodextrin chiral stationary phase (CYCLOBOND I 2000) with a mobile phase consisting of diethylamine acetate (0.25%, pH 4.4):methanol:acetonitrile [85:7.5:7.5, (v/v/v)]and a flow-rate of 0.5 ml/min. For CP, the enantioselectivity (alpha) of the separation was 1.12 with a resolution factor (R(s)) of 1.17. The method was validated for CP by using mass spectroscopy detection (MSD). Concentrations of each enantiomer could be measured down to 125 pg/ml from a 1-ml plasma sample. Extracted calibration curves were linear from 0.13 to 50.00 ng/ml for each enantiomer. The method was applied to samples from two clinical studies.

The roles of CYP2D6 and stereoselectivity in the clinical pharmacokinetics of chlorpheniramine.

AIMS: To examine the stereoselective disposition of chlorpheniramine and to evaluate the role of CYP2D6 in chlorpheniramine pharmacokinetics in humans. METHODS: Eight healthy volunteers (six extensive metabolizers with respect to CYP2D6 and two poor metabolizers) received a single 8 mg oral dose of rac-chlorpheniramine either given alone or following administration of quinidine 50 mg every 6 h for 2 days prior to the study day and every 6 h thereafter until the end of the study. Plasma concentrations of (S)-(+)- and (R)-(-)-enantiomers of chlorpheniramine were determined using liquid chromatography/mass spectrometry. RESULTS: In extensive metabolizers, mean Cmax was greater (12.55+/-1.51 ng ml-1vs 5.38+/-0.44 ng ml-1) and CLoral was lower (0.49+/-0.08 l h-1 kg-1vs 1.07+/-0.15 l h-1 kg-1) for (S)-(+)- than for (R)-(-)-chlorpheniramine (P<0.005). For (S)-(+)-chlorpheniramine, administration of quinidine, an inhibitor of CYP2D6, resulted in an increase in Cmax to 13.94+/-1.51 (P<0.01), a reduction in CLoral to 0.22+/-0.03 l h-1 kg-1 (P<0.01), and a prolongation of elimination half-life from 18.0+/-2.0 h to 29.3+/-2.0 h (P<0.001). Administration of quinidine decreased CLoral for (R)-(-)-chlorpheniramine to 0.60+/-0.10 l h-1 kg-1 (P<0.005). In CYP2D6 poor metabolizers, systemic exposure was greater after chlorpheniramine alone than in extensive metabolizers, and administration of quinidine resulted in a slight increase in CLoral. CONCLUSIONS: Stereoselective elimination of chlorpheniramine occurs in humans, with the most pharmacologically active (S)-(+)-enantiomer cleared more slowly than the (R)-(-)-enantiomer. CYP2D6 plays a role in the metabolism of chlorpheniramine in humans.