Mexiletine: a new type I antiarrhythmic agent

The efficacy and safety of first-line therapies for preventing chronic post-surgical pain: a network meta-analysis

Background

Due to conflicting evidence regarding first-line therapies for chronic post-surgical pain (CPSP), here we comparatively evaluated the efficacy and safety of first-line therapies for the prevention of CPSP.

Materials and Methods

MEDLINE, EMBASE, and Cochrane CENTRAL databases were searched for randomized, controlled trials (RCTs) of systemic drugs measuring pain three months or more post-surgery. Pairwise meta-analyses (a frequentist technique directly comparing each intervention against placebo) and network meta-analyses (a Bayesian technique simultaneously comparing several interventions via an evidence network) compared the mean differences for primary efficacy (reduction in all pain), secondary efficacy (reduction in moderate or severe pain), and primary safety (drop-out rate from treatment-related adverse effects). Ranking probabilities from the network meta-analysis were transformed using surface under the cumulative ranking analysis (SUCRA). Sensitivity analyses evaluated the impact of age, gender, surgery type, and outlier studies.

Results

Twenty-four RCTs were included. Mexiletine and ketamine ranked highest in primary efficacy, while ketamine and nefopam ranked highest in secondary efficacy. Simultaneous SUCRA-based rankings of the interventions according to both efficacy and safety revealed that nefopam and mexiletine ranked highest in preventing CPSP. Through the sensitivity analyses, gabapentin and ketamine remained the most-highly-ranked in terms of efficacy, while nefopam and ketamine remained the most-highly-ranked in terms of safety.

Conclusions

Nefopam and mexiletine may be considered as first-line therapies for the prevention of CPSP. On account of the paucity of evidence available on nefopam and mexiletine, gabapentin and ketamine may also be considered. Venlafaxine is not recommended for the prevention of CPSP.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Mexiletine in the symptomatic treatment of diabetic peripheral neuropathy

OBJECTIVE

To evaluate the efficacy and safety of mexiletine in the symptomatic treatment of diabetic peripheral neuropathy (DPN).

METHODS

In this prospective, double-blind study, 29 patients were randomized to receive mexiletine 600 mg/d or matching placebo for 3 weeks. A four-item symptom score (FIS), which rated pain, dysesthesias, paresthesias, and nightly exacerbations of symptoms, and a 100-mm visual analog scale (VAS), which rated pain intensity, were completed by patients before and after treatment. At the end of treatment independent patient and investigator global assessments were made. Patients reported adverse effects after 1 and 3 weeks of treatment.

RESULTS

Treatment groups were similar at baseline. The difference between the median changes in FIS scores (mexiletine = 5.5, placebo = 2) was not statistically significant. Overall symptom response was similar in both treatment groups as demonstrated by both global assessments (p = 0.19). The mean change in VAS score from baseline to posttreatment was determined for both groups and the difference between these mean scores was 16.5 mm (95% CI, -7.1 to 40.2 mm) (p = 0.16). Inadequate statistical power (1-beta = 0.40) may have resulted from small sample size, small magnitude of effect, or variability in the measured effect. Adverse effects were more common in the mexiletine group, though not statistically significant. One patient receiving mexiletine was hospitalized for palpitations.

CONCLUSIONS

Because of conflicting reports of mexiletine's efficacy in the treatment of symptomatic DPN, this drug should be reserved for patients unresponsive or intolerant to standard therapy, without evidence of heart disease, and with sensations of burning heat, formication, or stabbing pain.

High-performance liquid chromatographic method for resolving the enantiomers of mexiletine and two major metabolites isolated from microbial fermentation media

(+/-)-Mexiletine is a class Ib antiarrhythmic drug useful in the treatment of premature ventricular contractions. It is predominantly metabolized by the liver with less than 15% being excreted in urine as unchanged drug. p-Hydroxymexiletine (PHM) and hydroxymethylmexiletine (HMM) are the two major mammalian metabolites. The purpose of our study was to develop a stereospecific high-performance liquid chromatographic (HPLC) method to determine whether the fungus, Cunninghamella echinulata (UAMH 4145), was able to biosynthesize these same two metabolites from the substrate (+/-)-mexiletine. Furthermore, it was desirable to ascertain whether metabolism of mexiletine was stereoselective. The method requires pre-column derivatization of the drug and metabolites with S-(+)-1-(1-naphthyl)ethyl isocyanate (NEIC) followed by normal-phase HPLC. Mexiletine, PHM, HMM and (+/-)-1-(4-hydroxyphenoxy)-3-isopropylaminopropan-2-ol (internal standard) were extracted from microbial broth using two volumes of diethyl ether after basifying with sodium carbonate. The combined ether extracts were evaporated to dryness, using a gentle stream of nitrogen, and reconstituted in 0.3 ml of chloroform to which was added 0.075 ml of NEIC (0.1%, v/v, in chloroform). This solution was immediately evaporated to dryness under a nitrogen stream. The residue was reconstituted with 0.220 ml of chloroform and 0.030 ml of n-butylamine (0.33%, v/v, in chloroform) and injected into the HPLC system.

Advances in oral anti-arrhythmic therapy: implications for the anaesthetist

Surgical patients often are receiving antiarrhythmic therapy. Thus, because anaesthetic agents can affect cardiac function and may interact with concurrent antiarrhythmic medications, the anaesthetist should be aware of the electrophysiology associated with dysrhythmias and their management. Tocainide, flecainide, mexiletine, encainide and amiodarone have been introduced recently and each has an unique pattern of bioavailability, metabolism and toxicity. Patients treated with these drugs need special concern as they have abnormal cardiovascular systems and may be at increased risk for perioperative morbidity. In addition, unexpected untoward reactions and toxicity can result from interactions of anaesthetic agents and these drugs. This review discusses normal cardiac electrophysiology, common dysrhythmias and the electrophysiological effects of the newer oral antiarrhythmic drugs.

Poisoning due to class 1B antiarrhythmic drugs. Lignocaine, mexiletine and tocainide

Since most of the toxicity associated with class 1B antiarrhythmic drugs is dose-related, this review examines adverse effects seen in both therapeutic practice and accidental or premeditated overdose. Toxicity is very common with these agents and can be life-threatening. A high percentage of patients must discontinue therapy because of adverse effects. Mexiletine and tocainide are structural analogues of lignocaine (lidocaine) and toxicity is similar with all 3 drugs. With gradual intoxication (the most common form) central nervous system effects such as lightheadedness, dizziness, drowsiness and confusion are seen first. Seizures and respiratory arrest can occur. Cardiovascular toxicity is manifested by progressive heart block, reduced cardiac contraction, hypotension and asystole. Both mexiletine and tocainide may have proarrhythmic effects. Gastrointestinal toxicity is also common. Shock, hypotension, cardiac failure and beta-blocker therapy reduce lignocaine clearance and enhance the risk of intoxication during routine therapy. Both lignocaine and mexiletine elimination is impaired in severe liver disease while tocainide clearance is reduced in renal failure. Management of toxicity is largely supportive and symptomatic. Lignocaine infusion must be discontinued and decontamination of the gut in the case of oral preparations is recommended. Serious intoxication requires intensive care unit admission. Haemodialysis or haemoperfusion may be helpful in serious lignocaine and tocainide poisoning. In institutions where extracorporeal circulatory assistance is available, massive lignocaine poisoning has been successfully treated with this intervention. In the therapeutic setting serious toxicity can be prevented by close clinical surveillance and appropriate dose reduction in patients with reduced drug clearance. Because of the large interindividual variation in lignocaine pharmacokinetic parameters, therapeutic drug monitoring is recommended if results can be reported quickly. Mexiletine and tocainide have stereoselective metabolism and assays do not distinguish the more active isomers. Therapeutic drug monitoring is less useful in this situation.

Prolongation of ventricular depolarization. ECG manifestation of mexiletine toxicity

Mexiletine is a type 1B antiarrhythmic drug similar to lidocaine. Prolongation of ventricular depolarization has not been previously reported with the usual oral dosage of mexiletine. We describe a patient with renal failure and heart failure on low-dose oral therapy who developed mexiletine toxicity, which was manifested by ECG prolongation of ventricular depolarization. This was confirmed by elevated plasma concentration of mexiletine. This case illustrates that contrary to the usual belief, mexiletine pharmacokinetics are altered by renal failure. It is important to monitor mexiletine therapy by plasma levels in patients with impaired renal function to avoid mexiletine toxicity.

New antiarrhythmic drugs: tocainide, mexiletine, flecainide, encainide, and amiodarone

Tocainide, mexiletine, flecainide, encainide, and amiodarone are antiarrhythmic agents that have recently been approved by the Food and Drug Administration for general use in the treatment of ventricular arrhythmias. All five agents are effective in the treatment of patients with ventricular arrhythmias, whereas encainide, flecainide, and amiodarone are also useful in patients with supraventricular arrhythmias and the Wolff-Parkinson-White syndrome (although not yet approved for these indications). Tocainide and mexiletine are similar to lidocaine and are as effective as quinidine in patients with ventricular arrhythmias. Encainide and flecainide are superior to quinidine for the control of ventricular ectopic beats and as effective as quinidine for patients with ventricular tachycardia. Amiodarone is the most effective agent available for treating patients with ventricular tachycardia, but it is also the most toxic antiarrhythmic agent and should be used only when other antiarrhythmic drugs have not been effective or tolerated.