Mexiletine [(2',6'-dimethylphenoxy)-2-amino propane] is a class 1 antiarrhythmic agent with a similar chemical structure and electrophysiological effects to those of lidocaine. It is a chiral drug which is used clinically in the racemic form (i.e. 50:50 ratio of two enantiomers). This thesis describes the stereoselective HPLC analysis, pharmacokinetics, tissue distribution and pharmacodynamics of mexiletine enantiomers. The development of a highly sensitive and stereoselective HPLC assay for mexiletine enantiomers, using 2-anthroyl chloride as a derivatization reagent, was attempted. The synthesis and characterization of the acid chloride was successfully carried out. The 2-anthroyl derivatives of the enantiomers were resolved on a Pirkle[formula omitted] ionic (phenyl glycine) chiral column using a mobile phase of ethyl acetate/2-propanol/Hexane (4:6:90). Detection was accomplished by fluorescense (ex = 270 nm, em = 400 nm) with a lower limit of 0.5 ng/ml. However, there was an interfering peak coeluting with S(+)-mexiletine which could not be resolved. This precluded the use of the assay for the proposed pharmacokinetic and pharmacodynamic studies. A previously developed stereoselective HPLC method, with 2-naphthoyl chloride as a derivatization reagent, was subsequently used. The in vitro protein binding of mexiletine enantiomers was examined with human serum, lipoprotein deficient serum, albumin and α[formula omitted]-acid glycoprotein. The binding of the enantiomers to human serum was moderate (45 to 50%) within the therapeutic range of mexiletine. This binding was due, mainly, to albumin and α[formula omitted]-acid glycoprotein. The free fractions of the enantiomers decreased significantly (P<0.05) as pH was increased from 7.0 to 8.0. Stereoselective binding was apparent at pH 8.0 such that the free fraction of S(+)-mexiletine was significantly (p<0.05) greater than that of the R(-)-enantiomer. However, stereoselective binding was not observed at physiological pH (≈ 7.4). These results indicated that the serum binding of mexiletine enantiomers is pH-dependent. Binding was not concentration-dependent, nor was there any competitive binding interaction between the enantiomers, within the therapeutic range. Scatchard analysis of the binding data obtained with serum and albumin both showed the presence of 2 classes of binding sites. A high affinity, low capacity site and a low affinity, high capacity site. In contrast, α[formula omitted]-acid glycoprotein showed only 1 class of binding sites and this was a high affinity, low capacity site. Pharmacokinetic and tissue distribution studies in rats following the administration of racemic mexiletine (10 mg/kg) indicated extensive tissue uptake and rapid elimination of the enantiomers. R(-)-Mexiletine showed a 32% greater systemic clearance (161.8 ml/min/kg vs 122.9 ml/min/kg) than the S(+)-enantiomer. The steady state-volume of distribution was also greater for the R(-)-enantiomer (9.0 L/kg vs 7.4 L/kg), while the elimination half-lives of the enantiomers (1.4 and 1.3 h for R(-)- and S(+)-mexiletine, respectively) were not different. Maximum tissue concentrations were observed at 5 min in all the tissues studied (heart, brain, lung, kidney, liver and fat). These concentrations were not significantly different, except for the liver tissue where a 2.4-fold greater concentration of the S(+)-enantiomer was found. High tissue/serum ratios (>20) were observed for each enantiomer in the brain, lungs and kidneys. The brain accumulated 3-fold the heart concentrations of the enantiomers. Pharmacodynamic studies on the relative antiarrhythmic effects of racemic mexiletine and its enantiomers were carried out using electrical and ischaemia-induced arrhythmias in rats. Racemic mexiletine and its enantiomers significantly (P<0.05) increased VFT and ERP. However, the differences between the effects of the 3 drugs on these variables were not statistically significant. R,S-, S(+)- and R(-)-mexiletine caused significant bradycardia and PR prolongation in both pentobarbitone anaesthetized and conscious rats. These effects of the drugs were also not significantly different from each other. In the ischaemic conscious rats, the 3 drugs did not significantly reduce the incidence of VT and VF, the number of PVCs nor the "arrhythmia score" when compared to saline (control). Racemic mexiletine and its enantiomers produced comparable CNS toxicity in the conscious rats. / Pharmaceutical Sciences, Faculty of / Graduate

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<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<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<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. / Pharmaceutical Sciences, Faculty of / Graduate

Thesis (M.Sc.)--Memorial University of Newfoundland, School of Pharmacy, 1997. / Typescript. Bibliography: leaves 104-121.