Stereoselective HPLC analysis, pharmacokinetics, serum protein binding and metabolism of mexiletine enantiomers in healthy human subjects
Creators: name: Kwok, David W. K.
Publisher: name: University of British Columbia
Year: 2011

description: Mexiletine (MexitilR) is an orally effective antiarrhythmic drug used clinically as a racemate of the R(-)- and S(+)-enantiomers. A stereoselective high-performance liquid chromatographic assay was developed for the determination of mexiletine enantiomers in serum, saliva, red blood cells and urine. The mexiletine enantiomers were resolved as their N-anthroyl derivatives on a PirkleR phenylglycine ionic chiral HPLC column. The present study examined the serum free (unbound) and total drug kinetics for the mexiletine enantiomers in twelve healthy volunteers following oral administration of 200 mg of racemic mexiletine hydrochloride. To further characterize serum free mexiletine in the body, the concentrations of mexiletine enantiomers in saliva and in red blood cells were examined. Since mexiletine was largely eliminated by metabolic processes, p-hydroxy-mexiletine and hydroxymethyl-mexiletine metabolites were examined in the urine of four healthy subjects. Following oral drug administration, the disposition of mexiletine enantiomers was described by one or two-compartment open models. The mean peak serum total mexiletine concentration of 217 ± 68 ng/ml for R(-)-mexiletine was found to be significantly greater (p&lt;0.01) than a mean value of 196 ± 56 for S(+)-mexiletine. The mean serum total R(-)-mexiletine concentrations were also found to be significantly greater than those for S(+)-mexiletine during the first six hours. The absorption, rapid and terminal disposition kinetic parameters between the two enantiomers were not significantly different. From urinary data, the mean percentages of mexiletine enantiomers recovered from the urine over 72 hours were found to be 3.5 ± 3.4% and 3.7 ± 3.9% for R(-)- and S(+)-mexiletine, respectively. The mean terminal elimination half-lives were found to be 5.8 ± 1.5 h and 5.6 ± 1.4 h for R(-)- and S(+)-mexiletine, respectively. Both the urinary recoveries and the half-1ives for the enantiomers were not significantly different. Comparative in vitro studies on the serum protein binding of mexiletine enantiomers by ultrafiltration and by equilibrium dialysis indicated a serum pH-dependent stereoselective protein binding of mexiletine enantiomers. A serum pH range from 6.3 to 9.4 was correlated with the serum protein binding of mexiletine enantiomers from ≈30% to ≈80%. Within this pH range, the serum free drug R(-)/S(+) ratio was found to decrease from 1.0 to 0.7. At serum pH 7.4, the serum protein binding of mexiletine enantiomers was similar, and was not dependent on the therapeutic concentration range of 0.25 to 3.0 µg/ml. The in vivo serum protein binding of mexiletine enantiomers was found to be non-stereoselective. The mean serum free fractions of 0.57 ± 0.07 and 0.56 ± 0.06 for R(-)- and S(+)-mexiletine, respectively, were not significantly different. The overall mean serum free R(-)/S(+) mexiletine ratio of 1.09 was also indicative of a non-stereoselective binding of mexiletine enantiomers. Following the collection of unstimulated saliva, the overall mean saliva / serum free mexiletine area under the concentration-time curve ratios of 6.10 ± 2.82 and 7.49 + 3.48 for R(-)- and S(+)-mexiletine, respectively, were found to be significantly different (p&lt;0.01). The overall mean saliva R(-)/S(+) ratio of 0.89 ± 0.02 (mean ± S.E.) over 48 hours suggested that the disposition of mexiletine enantiomers in saliva was stereoselective. In addition, saliva mexiletine concentrations were found to correlate poorly with serum free mexiletine concentrations. In vitro studies on the distribution of mexiletine enantiomers into red blood cells indicated a distribution equilibrium of ≈40 minutes. The mean red blood cell mexiletine area under the concentration-time curve of 2.3 ±1.5 µg/ml/h and 2.8 ± 2.1 µg/ml/h for R(-)- and S(+)-mexiletine, respectively, were not significantly different. The overall mean red blood cell mexiletine R(-)/S(+) ratio of 0.91 ± 0.13 suggested a similar distribution of the enantiomers into the red blood cells. A stereoselective HPLC assay was developed for the simultaneous determination of mexiletine, p-hydroxy-mexiletine, and hydroxymethyl-mexiletine enantiomers in urine. Mexiletine and hydroxymethyl -mexiletine enantiomers were resolved on a PirkleR isoleucine covalent HPLC column as their N-anthroyl derivatives, while p-hydroxy-mexiletine enantiomers were resolved as their 0-ethyl-N-anthroyl derivatives. For mexiletine and p-hydroxy-mexiletine, chromatographic retention was found to favour the R(-)-enantiomer, thus leading to the initial elution of the S(+)-enantiomer. For hydroxymethyl-mexiletine, the order of elution was found to be reversed. The mean cumulative amounts of p-hydroxy-mexiletine enantiomers recovered from the urine over 72 hours were found to be 1.31 ± 0.33 mg and 1.27 + 0.39 mg for the R(-) and S(+)-enantiomers, respectively. The two values were not significantly different. The mean cumulative amounts of R(-)-hydroxymethyl-mexiletine (2.94 ± 1.70 mg) recovered from the urine over 72 hours were found to be significantly (p&lt;0.01) greater than a value of 1.17 ± 0.60 mg for the S(+)-enantiomer, suggesting the presence of a stereoselective metabolic pathway for hydroxymethyl-mexiletine.