Pierre M. Bélanger

Resolution and Electrophysiological Effects of Mexiletine Enantiomers

Abstract— Resolution of mexiletine enantiomers from the racemic mixture has been achieved by fractional crystallization through the formation of diastereoisomeric p‐toluoyl tartrate salts. Following three crystallization steps in methanol, R‐(–)‐ and S‐(+)‐mexiletine were resolved with an optical purity > 98 % (yield ∼ 30%) and their hydrochloride salts formed. Incremental doses of mexiletine enantiomers were administered to dogs with experimentally‐induced arrhythmias to investigate the stereoselective antiarrhythmic and electrophysiological effects of these compounds. Using up to three extrastimuli, programmed electrical stimulation was performed in conscious animals 7–30 days after coronary ligation. R‐(–)‐Mexiletine prevented ventricular tachycardia in 3/6 dogs (2 after 0·5 mg kg−1, 1 after 8 mg kg−1); two animals died after 1 and 8 mg kg−1, respectively; one remained unchanged even at the highest dosage (16 mg kg−1). S‐(+)‐Mexiletine prevented ventricular tachycardia in only one dog (after 1 mg kg−1); two died after 4 and 8 mg kg−1, respectively; 2/5 remained unchanged even after the administration of 16 mg kg−1. No significant changes in any electrocardiographic intervals (PR, QRS, QTc) or refractory periods were induced by mexiletine enantiomers at any doses used (0·5–16·0 mg kg−1). These results suggest that R‐(–)‐mexiletine possesses greater antiarrhythmic properties than the opposite enantiomer.

Time-dependent variations in the organ extraction ratios of acetaminophen in rat

The pharmacokinetics of a single 40mg/kg dose of acetaminophen was investigated at 09h00 and 21h00 in Sprague-Dawley rats synchronized to a 12-h light-dark cycle. Acetaminophen was administered by the intraarterial, intravenous, intraperitoneal, and oral routes in order to determine the contribution of the gastrointestinal tract, liver, and lung to the oral extraction ratio of the drug. A mean oral extraction ratio of 0.46 was obtained at 21h00 as compared to 0.39 at 09h00. The mean extraction ratios of the gastrointestine, liver, and lung were 0.05, 0.41, and 0 at 09h00 and 0.18, 0.24, and 0.13 at 21h00, respectively. These results indicate that the extrahepatic metabolism of acetaminophen is important at 21h00, but is barely detectable at 09h00, whereas the hepatic extraction ratio is higher at 09h00 than at 21h00. Thus, there are temporal variations in the disposition of acetaminophen in the rat.

Colorimetric determination of N-hydroxylated metabolites in microsomal studies.

A colorimetric method is described which can be used for the routine determination of primary and secondary N-hydroxylamino compounds in drug metabolism studies using microsomal preparations. The assay is carried out on the supernatant obtained after protein precipitation of the incubation mixture. The method is based on the reduction of ferric ion by the hydroxylamino moiety with the resulting ferrous ion being quantitated by coupling with 2,4,6-tripyridyl-s-triazine to form a purple color with a maximum absorbance at 595 nm. The method is specific to primary and secondary hydroxylamino compounds and calibration curves can be obtained in the range of concentrations of 1 to 20 μg/ml. Hydroxamic acid, amido, amino, phenolic, nitro, nitroso, oxime, nitrone, aldehyde, and ketone compounds do not produce any color reaction when analyzed by the same method. The method is simple, rapid, and many samples can be analyzed in a short period of time.

Toward Optimal Treatment in Women: The Effect of Sex on Metoprolol‐Diphenhydramine Interaction

The objective of this study was to determine if sex influences the pharmacokinetics and hemodynamics of the CYP2D6 substrate metoprolol and its interaction with diphenhydramine (CYP2D6 inhibitor) in healthy young participants with high (extensive metabolizer [EM]) or low (poor metabolizer [PM]) CYP2D6 activities. A prespecified comparative analysis of data from 2 sequential clinical trials that included 16 EM and 4 PM women and 10 EM and 6 PM men was performed. The participants in the 2 trials were administered a single oral dose of 100 mg metoprolol in the presence of steady‐state diphenhydramine or placebo. Serial plasma and urine samples were obtained for 48 hours, and hemodynamic data was obtained for 12 hours after metoprolol. In the placebo arm, EM and PM women had 62% and 59% higher S‐metoprolol AUC0–∞ and 26% and 71% lower CL/F, respectively, compared to men with the same phenotype (all Ps < .05 women compared to men). These differences dissipated on body weight (WT) correction. Women (especially PMs) experienced greater negative chronotropic effects of metoprolol than men (P < .0001 women compared to men). Diphenhydramine coadministration increased S‐metoprolol AUC by 84% in EM women and 45% in EM men (P < .009 women compared to men). In the diphenhydramine arm, sex differences in pharmacokinetics persisted even after WT correction, resulting in greater negative chronotropic effects in women (all Ps < .05 women compared to men). Metoprolol dose should be adjusted for body weight, particularly in women. Coadministration of a CYP2D6 inhibitor such as diphenhydramine, by a patient at similar doses in the 2 sexes, could result in a greater inhibition of clearance of CYP2D6 substrates with a resulting higher risk of pronounced pharmacological and adverse effects in women compared to men.

TEMPORAL VARIATION IN THE EFFECTS OF WARFARIN ON THE VITAMIN K CYCLE

In this in vivo study, the time-dependent effect of oral sodium warfarin was studied in male rats synchronized under a 12-hr light-dark cycle (light 0600-1800). Groups of 5 animals received an oral dose of 500 micrograms/kg of warfarin or saline at 0600 or 1800 and 1 mg/kg of vitamin K 8 hr later and the rats were sacrificed 240 min after vitamin K administration. The activities of the vitamin K reductase and vitamin K epoxide reductase were measured indirectly by determining the content of vitamin K1 and vitamin K epoxide reductase in the plasma and liver. The data obtained in control rats indicated that vitamin K and vitamin K 2,3 epoxide concentrations in plasma and liver were higher (P less than 0.05) at 1800 than at 0600. Warfarin had a greater (P less than 0.05) inhibitory effect on the vitamin K and vitamin K-epoxide reductases at 0600 compared to 1800; plasma levels of S- and R-warfarin did not vary with time of administration. The findings suggest that the activity of both reductases under control conditions, and the warfarin-induced inhibition of these enzymes varied depending on the time of drug administration.

The inhibitory effect of monoamine oxidase inhibitors on hepatic microsomal enzyme systems of rat liver

Abstract The inhibitory action of five monoamine oxidase inhibitors (MAOI) on microsomal oxidative drug metabolism of rat liver was studied. The addition of iproniazid, nialamide, pargyline, phenelzine and tranylcypromine to rat liver microsomes inhibits the N-dealkylation of aminopyrine and N,N-dimethylaniline, O-demethylation of p-nitroanisole and the hydroxylation of aniline. The increasing order of inhibition was pargyline, iproniazid, nialamide, phenelzine and tranylcypromine. The MAOI tested exhibited type II difference spectra in microsomes under aerobic conditions. A correlation was found to exist between the spectral dissociation constants of the MAOI with cytochrome P-450 and their degree of inhibition of microsomal metabolism.

Circannual Variations in Carrageenan-Induced Paw Edema and in the Anti-Inflammatory Effect of Phenylbutazone in the Rat

Circannual variation in the edema produced by intraplantar injection of carrageenan (carr) was examined over a period of 31 months. The anti-inflammatory effect of a dose of 30 mg/kg of phenylbutazone (PHZ) was also determined over a period of 1 year. Carr-induced edema was significantly larger in spring while it was smaller in winter. PHZ induced a pronounced reduction of paw edema throughout the year except in spring when the anti-inflammatory effect of the drug could not be detected. Mechanisms responsible for these circannual variations are discussed.