Effects of Drug Metabolism, Metabolites, and Stereoselectivity on Antiarrhythmic Drug Action

Anti-tussive activity of benproperine enantiomers on citric-acid-induced cough in conscious guinea-pigs

The anti-tussive effect of the R-(+)- and S-(-)-enantiomers of benproperine was evaluated and compared with that of the racemate on cough induced by 7.5% citric acid in conscious guineapigs. All the three compounds, intraperitoneally administered 1.5 h before the test, significantly inhibited citric-acid-induced cough. The ID50 values (effective doses for 50% inhibition) (with 95% confidence intervals) were 16.1 (9.1–28.4), 23.3 (11.2–48.6), 25.4 (11.7–55.1) mg kg−1 for the number of coughs in the 3 min of challenge, and 11.9 (5.3–26.6), 13.5 (5.6–32.4), 19.2 (12.8–28.9) mg kg−1 for the number of coughs in the 5 min immediately after the challenge, for (+)-benproperine, R-(+)-benproperine and S-(-)-benproperine, respectively. These findings suggest that the use of either enantiomer does not show any advantage over the racemate with regard to their anti-tussive effect.

CYP2D6 mutations and therapeutic outcome in schizophrenic patients

STUDY OBJECTIVE

To investigate whether a relationship exists between the most common known cytochrome P450 (CYP) isozyme 2D6 mutations and schizophrenia. Because most antipsychotic and antidepressant agents interact with CYP2D6, we also investigated clinical outcomes in schizophrenic poor metabolizers (PMs) and extensive metabolizers (EMs).

DESIGN

Prospective, observational study.

SETTING

Two psychiatric hospitals and a university-affiliated nonpsychiatric hospital.

SUBJECTS

Thirty-nine consecutive schizophrenic patients (POP 1), 89 schizophrenics of French Canadian origin (POP 2), and 384 healthy French Canadians (POP 3).

INTERVENTION

All study subjects were genotyped for CYP2D6 mutant alleles. POP 1 patients were evaluated before and after 21 or more days of treatment with antipsychotic drugs metabolized at least in part by CYP2D6.

MEASUREMENTS AND MAIN RESULTS

Whole blood was collected to determine CYP2D6 alleles *1, *3, *4, *5, *6, and *7 using standard restriction fragment length polymorphisms and polymerase chain reaction techniques. In comparison, CYP2D6 genotypes were determined in POP 2 and POP 3. Twenty-three (59.0%) of 39 patients in POP 1 were genotypically EM homozygotes, 15 (38.4%) were EM heterozygotes, and 1 (2.6%) was a PM. Similar genotype distributions were determined in POP 2 and in POP 3. Genotype distributions for all three populations were in Hardy-Weinberg equilibrium (p>0.05), and there was no significant difference among them (p=0.857). In POP 1, no differences were seen among genotypes in disease symptom severity, number and severity of adverse drug effects, or attitudes toward drug treatment at baseline and at the end of the study. In fact, all patients improved significantly during their hospital stay (all p<0.05), although independent of the CYP2D6 genotype.

CONCLUSION

Common CYP2D6 mutant alleles were not associated with schizophrenia or with disease symptoms, antipsychotic-related adverse effects, or attitudes toward treatment.

An enantiomer-enantiomer interaction of (S)- and (R)-propafenone modifies the effect of racemic drug therapy

BACKGROUND

Therapy with racemic compounds produces effects that can be attributed to both (S)- and (R)-enantiomers. Here we have tested the hypothesis that an enantiomer-enantiomer interaction would modulate the effects of treatment with a racemate, the antiarrhythmic propafenone. Previous studies have shown that while the enantiomers of propafenone exert similar sodium channel-blocking (QRS widening) effects, it is the (S)-enantiomer that produces beta-blockade; moreover, we have demonstrated recently that (R)-propafenone inhibits the metabolism of (S)-propafenone in vitro.

METHODS AND RESULTS

This single-blind, randomized study compared the effects of (R/S)-, (S)-, (R)-propafenone (150 mg q 6 hours for 4 days) and placebo on QRS duration (delta QRS) and on maximum exercise heart rate (delta HRmax), an index of beta-blockade. The clearance of (S)-propafenone was significantly lower (-55 +/- 24%, P < .001) during treatment with (R/S)-propafenone than with the (S)-enantiomer alone, and delta HRmax was significantly altered during (R/S)-propafenone (-8.8 +/- 6.6 beats per minute; P < .01) and during (S)-propafenone (-4.3 +/- 4.8 beats per minute; P < .01) but not during (R)-propafenone (-1.8 +/- 6.4 beats per minute) or placebo (0.3 +/- 7.1 beats per minute). In contrast, (R/S)-, (S)-, and (R)-propafenone all prolonged QRS compared with placebo.

CONCLUSIONS

These data indicate that (R)-propafenone impairs the disposition of (S)-propafenone in humans. As a result, the beta-blocking effects of 150 mg of racemic propafenone (75 mg of the [S]-enantiomer) were more pronounced than those of 150 mg of (S)-propafenone alone. Thus, the effects of racemic drug therapy are not necessarily those predicted by summation of the effects of the individual enantiomers.

Improved high-performance liquid chromatographic assay for the stereoselective determination of mexiletine in plasma

A simple and sensitive high-performance liquid chromatographic procedure for resolution of mexiletine enantiomers has been developed. Proteins from plasma samples containing RS-mexiletine were precipitated with a mixture of barium hydroxide and zinc sulphate before extraction under alkaline conditions with diethyl ether. Organic extracts were evaporated to dryness, and the residues reconstituted with 0.03 M hydrochloric acid (20 microliters). Derivatization with o-phthalaldehyde N-acetyl-L-cysteine reagent was performed after alkalinization with 0.1 M sodium borate. An aliquot of the resulting solution was injected onto a reversed-phase C18 column and resolution of mexiletine diastereoisomeric derivatives was achieved with a mobile phase consisting of methanol-50 mM sodium acetate (65:35), at a flow-rate of 1 ml/min. The retention times of S-(+)- and R-(-)-mexiletine diastereoisomeric peaks were 14 and 15 min, respectively. Product elution was monitored by fluorescence detection using excitation and emission wavelengths fixed at 350 and 445 nm, respectively. Calibration curves were linear over the concentration range 2.5-500 ng/ml for each enantiomer (r greater than 0.99). The assay is shown to be suitable for pharmacokinetic studies after administration of a single oral dose of 200 mg of RS-mexiletine hydrochloride to healthy volunteers.

On the mechanism of action of antiarrhythmic agents

Cardiac arrhythmias arise from disturbances in the functioning of the specific ion channels that normally control excitation or from the functional expression of previously latent channels. Antiarrhythmic agents act by blocking the membrane sodium, potassium, and calcium channels, but no agent has exclusive action on a given type of channel. Arrhythmias resulting from reentry form the largest group of clinically significant arrhythmias. Most arrhythmias result from depressed sodium channel function. The local anesthetic class of sodium channel blockers (class I agents) acts by slowing conduction and converting regions of unidirectional block to bidirectional block. Class III agents act by prolonging the action potential duration. Because potassium currents are normally responsible for repolarization of the cardiac action potentials, these agents are generally assumed to be potassium channel blockers. Class IV antiarrhythmics--calcium channel blockers--are used when a group of reentrant arrhythmias arises in regions in which conduction is primarily sustained by increases in permeability to calcium ions. The mechanisms of action of antiarrhythmic agents are discussed with respect to the basic cellular mechanisms of cardiac arrhythmias.

Resolution and electrophysiological effects of mexiletine enantiomers

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 greater than 98% (yield approximately 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.