Mexiletine

Electrophysiological actions of mexiletine on rabbit sinoatrial node cells

The effects of mexiletine (1-100 microM) were examined on membrane potential and current of rabbit sinoatrial node cells by means of conventional microelectrode and double microelectrode voltage clamp techniques. Mexiletine decreased, in a dose-dependent manner, the maximum rate of depolarization of the action potential and the action potential amplitude, and increased the spontaneous cycle length. The slope of the diastolic depolarization (phase 4) was also reduced. In the voltage clamp experiment, mexiletine (40-100 microM) reduced the slow inward current (Isi), the potassium outward current (IK) and the hyperpolarization-activated current (Ih). The kinetic variable of IK was not altered by the drug. These results suggest that mexiletine does not have a specific effect on a single-current system, but that relatively high concentrations of mexiletine exert an inhibitory effect on the electrical activity of the sinoatrial node cells.

Effects of mexiletine on transmembrane action potentials as affected by external potassium concentration and by rate of stimulation in guinea-pig papillary muscles

1. The effects of mexiletine and quinidine were compared on transmembrane potentials in guinea-pig papillary muscles, using conventional microelectrode techniques. 2. Mexiletine (23.1 mumol/l) decreased the maximum rate of rise of the action potential (Vmax) and increased the ratio of the effective refractory period to the action potential duration at 90% repolarization level (ERP/APD90); these effects were prominent with elevation of the external potassium concentration ([K]o) from 2.7 to 5.4, 8.1 and 10.0 mmol/l. 3. The percentage decrease in Vmax induced by 5 and 10 mumol/l of quinidine was approximately constant in 2.7, 5.4 and 10.0 mmol/l [K]o solutions. 4. The decrease in Vmax produced by mexiletine was progressively increased as the driving rate was raised from 0.25 to 5Hz. This rate-dependent change was pronounced when the concentration was raised from 23.1 to 46.2 and 92.4 mumol/l. 5. Mexiletine in concentrations of 23.1 and 92.4 mumol/l delayed the recovery of Vmax in a premature action potential to the level of Vmax in the conditioning action potentials at the driving rate of 0.25 Hz. 6. It appears that mexiletine exerts its anti-arrhythmic action by a selective depressant effect on depolarized cells (high [K]0) and cells with high frequency discharges, as is the case with lignocaine.

[Electrophysiological classification of newer antiarrhythmic drugs (author's transl)]

Newer antiarrhythmic drugs are tentatively classified on the basis of their influence on the transmembrane action potential, the fast sodium inward current and its kinetics, and the slow inward current which is primarily carried by calcium ions. Disopyramide is probably a quinidine-like antiarrhythmic drug, while the effects of aprindine, lorcainide, and tocainide resemble those of lidocaine. Mexiletine and propafenon cannot be classified definitely on the basis of the hitherto available electrophysiological data. Besides their effects on the fast inward current these substances may have additional calcium antagonistic properties.

Long-term oral antiarrhythmic therapy with mexiletine

Forty-eight patients with ischaemic heart disease received oral mexiletine for the control or prevention of ventricular arrhythmias. The most frequently used doses were 200, 250, and 300 mg 8-hourly. The treatment period varied from 2 days to more than 1 year (median 3 months). No ventricular arrhythmias were detected in more than one-half of the patients. Severe side effects occurred in 15 (31%) of the 48 patients. Major ventricular arrhythmias were observed in 14 (29%) of the 48 patients at a time when the majority had plasma concentrations within the therapeutic range. The value of mexiletine in the management of patients at risk of sudden death is likely to be limited.

New electron-capture gas-liquid chromatographic method for the determination of mexiletine plasma levels in man

A method for the determination of mexiletine in human plasma by gas-liquid chromatography with electron-capture detection is described. Plasma samples are extracted at pH 12 with dichloromethane after addition of the internal standard, the 2,4-methyl analogue of mexiletine. A derivative is obtained using heptafluorobutyric anhydride; according to gas chromatography-mass spectrometry it is a monoheptafluorobutyryl compound. The minimum detectable amount of mexiletine is 5 pg. Accurate determinations of human plasma levels were performed after oral or intravenous treatment.

Long-term treatment of ventricular arrhythmias with oral mexiletine

Twenty-four patients with ventricular arrhythmias were treated with oral mexiletine for periods of from one to 16 months (total 10.4 patient-years). In 19 patients arrhythmias were satisfactorily controlled and plasma levels previously shown to be within the therapeutic range were maintained on an 8 hourly regimen. The drug was well tolerated in most patients and regular hematological and biochemical screening revealed no problems of toxicity.