Mechanism of interaction between theophylline and mexiletine

Severe caffeine poisoning with mexiletine successfully treated by extracorporeal methods: A case report

A 20-year-old woman was brought to the hospital in an ambulance after ingesting 18 g of caffeine and 3500 mg of mexiletine 80 min earlier. On arrival at the emergency room, her vital signs were as follows: blood pressure, 65/37 mmHg; heart rate, 140 beats/min; and Glasgow Coma Scale, E4V4M6. Laboratory analyses revealed hypokalemia and lactic acidosis. The patient was treated with mechanical ventilation after intratracheal intubation, intravenous noradrenaline infusion, gastric lavage, and activated charcoal administration. Shortly afterwards, she developed pulseless ventricular tachycardia, and veno-arterial extracorporeal membrane oxygenation (VA-ECMO) was initiated. As the circulatory collapse continued, hemodialysis (HD) was performed with continuous intravenous infusion of noradrenaline. After the completion of HD, the noradrenaline dose was reduced. On hospital day 2, HD was performed on the second day of hospitalization. On hospital days 3 and 4, the patient was weaned off VA-ECMO and ventilator. The blood concentrations of caffeine and mexiletine at presentation were 387 μg/mL and 1.1 μg/mL respectively. During the first HD, blood concentrations of both drugs were markedly reduced. It has been reported that mexiletine may reduce the clearance of caffeine probably via inhibition of N-demethylation. In this case, the endogenous clearance of caffeine, calculated from blood concentrations, was considerably lower than estimated. If HD had not been performed, it may have taken longer to wean off the VA-ECMO because of reduced caffeine clearance in the presence of mexiletine. Notably, caffeine poisoning is more severe and prolonged when mexiletine is administered.

Differential pharmacokinetics of theophylline in elderly patients

The clinical use of theophylline as a first-line bronchodilator has declined during the last two decades. However, in many clinical settings, such as an emergency bronchial asthma attack, theophylline may have a first-line role, in combination with beta(2)-adrenoreceptor agonists and corticosteroids, for improving the asthmatic status. Furthermore, many therapeutic mechanisms of theophylline for bronchial asthma have been reported, and recent studies have suggested that theophylline therapy may have a beneficial role in the management of chronic stable asthma as well as exacerbated disease. However, theophylline has a low therapeutic index because the bronchodilation it produces has a linear relationship with logarithmic increases in serum concentration for the therapeutic range of 5-20 mg/L. Thus, the knowledge of its basic pharmacokinetics and the factors that can alter its clearance is clinically relevant for physicians. Especially when used in elderly asthmatic patients, dosage adjustment of theophylline is a requisite since the elderly have several risk factors that may increase the plasma theophylline level, such as reduced clearance, various underlying diseases and multiple coadministered drugs. After theophylline treatment has been initiated, therapeutic drug monitoring is required.

Evaluation of mexiletine clearance in a Japanese population

OBJECTIVE

To evaluate mexiletine clearance in a Japanese population and to clarify the roles of CYP2D6 and CYP1A2 in mexiletine disposition.

METHODS

Concentrations of serum and urinary mexiletine and its metabolites were determined and mexiletine clearances were estimated in 334 inpatients receiving mexiletine therapy. Concentrations of mexiletine and its metabolites in serum and urine samples were determined by HPLC.

RESULTS

Although interindividual variation of mexiletine clearance was small, the effect of age on mexiletine clearance was comparatively large. Mexiletine clearance in patients with dilated cardiomyopathy (DCM) was decreased when compared with other diagnoses (Non-DCM). The fractional contents of p-hydroxymexiletine (POH) and 2-hydroxymexiletine (OHMEX) in urine amounted to approximately 50%. Almost all of the POH was conjugated, whereas less than one-third of the OHMEX was conjugated. Although no significant differences in POH and OHMEX were observed between patients with DCM and those without, a trend toward an increase in conjugation pathway of DCM patients was observed.

CONCLUSIONS

The interindividual variation of mexiletine clearance was small, while the effect of age on the mexiletine clearance in Non-DCM was comparatively large. A significant difference in mexiletine clearance between patients with DCM and those with Non-DCM was observed. Therefore, when mexiletine is administered to patients with DCM, careful monitoring is needed.

Clinical pharmacokinetics of mexiletine

Mexiletine, a class Ib antiarrhythmic agent, is rapidly and completely absorbed following oral administration with a bioavailability of about 90%. Peak plasma concentrations following oral administration occur within 1 to 4 hours and a linear relationship between dose and plasma concentration is observed in the dose range of 100 to 600 mg. Mexiletine is weakly bound to plasma proteins (70%). Its volume of distribution is large and varies from 5 to 9 L/kg in healthy individuals. Mexiletine is eliminated slowly in humans (with an elimination half-life of 10 hours). It undergoes stereoselective disposition caused by extensive metabolism. Eleven metabolites of mexiletine are presently known, but none of these metabolites possesses any pharmacological activity. The major metabolites are hydroxymethyl-mexiletine, p-hydroxy-mexiletine, m-hydroxy-mexiletine and N-hydroxy-mexiletine. Formation of hydroxymethyl-mexiletine, p-hydroxy-mexiletine and m-hydroxy-mexiletine is genetically determined and cosegregates with polymorphic debrisoquine 4-hydroxylase [cytochrome P450 (CYP) 2D6] activity. On the other hand, CYP1A2 seems to be implicated in the N-oxidation of mexiletine. Various physiological, pathological, pharmacological and environmental factors influence the disposition of mexiletine. Myocardial infarction, opioid analgesics, atropine and antacids slow the rate of absorption, whereas metoclopramide enhances it. Rifampicin (rifampin), phenytoin and cigarette smoking significantly enhance the rate of elimination of mexiletine, whereas ciprofloxacin, propafenone and liver cirrhosis decrease it. Cimetidine, ranitidine, fluconazole and omeprazole do not modify the disposition of mexiletine. Conversely, mexiletine is known to alter the disposition of other drugs, such as caffeine and theophylline. Factors affecting the elimination of mexiletine may be clinically important and dosage adjustments are often necessary.