Hydroxymethylmexiletine Research Articles

Influence of experimental diabetes and insulin treatment on the enantioselective pharmacokinetics of mexiletine and its metabolites

This study evaluates the influence of streptozotocin-induced diabetes on the kinetic disposition and metabolism of mexiletine (MEX) enantiomers in rats. Animals in the control (n = 6 for each blood collection time), diabetic (single intravenous dosage of 45 mg·(kg body mass)(-1) of streptozotocin), and insulin-treated groups (diabetic rats treated daily with 2 IU insulin) received by gavage a single dose of 10 mg·(kg body mass)(-1) racemic MEX. MEX enantiomers and the metabolites hydroxymethylmexiletine (HMM) and p-hydroxymexiletine PHM) were analyzed by LC-MS/MS. Statistical analysis was based on a serial sacrifice design, and parameter estimation was performed using a Bayesian modeling procedure. Area under the curve (AUC) for the (-)-(R) enantiomers of MEX, HMM, and PHM did not differ between the control and diabetic groups. However, AUC for (+)-(S)-MEX and (+)-(S)-HMM were lower in the diabetic than in the control group. Insulin treatment recovered glucose levels to normal and the (+)-(S)-MEX AUC and (+)-(S)-HMM AUC became similar to the AUCs observed in the nondiabetic animals.

Enantioselective determination of mexiletine and its metabolites p‐hydroxymexiletine and hydroxymethylmexiletine in rat plasma by normal‐phase liquid chromatography‐tandem mass spectrometry: Application to pharmacokinetics

Mexiletine (MEX), hydroxymethylmexiletine (HMM) and p-hydroxymexiletine (PHM) were analyzed in rat plasma by LC-MS/MS. The plasma samples were prepared by liquid-liquid extraction using methyl-tert-butyl ether as extracting solvent. MEX, HMM, and PHM enantiomers were resolved on a Chiralpak(R) AD column. Validation of the method showed a relative standard deviation (precision) and relative errors (accuracy) of less than 15% for all analytes studied. Quantification limits were 0.5 ng ml(-1) for the MEX and 0.2 ng ml(-1) for the HMM and PHM enantiomers. The validated method was successfully applied to quantify the enantiomers of MEX and its metabolites in plasma samples of rats (n = 6) treated with a single oral dose of racemic MEX.

Synthesis of ( R)-, ( S)-, and ( RS)-hydroxymethylmexiletine, one of the major metabolites of mexiletine

Hydroxymethylmexiletine (HMM), one of the main metabolites of mexiletine, has been synthesized in both racemic and optically active forms following two alternative routes. The ee values for both HMM enantiomers were 98%, as assessed by 1H NMR analysis in the presence of (−)-( R)-(2-naphthyloxy)phenylacetic acid as a chiral solvating agent and electrophoretic analysis using β-cyclodextrine sulfated sodium salt as a chiral auxiliary.

Evaluation of the pharmacological activity of the major mexiletine metabolites on skeletal muscle sodium currents

Mexiletine (Mex), an orally effective antiarrhythmic agent used to treat ventricular arrhythmias, has also been found to be effective for myotonia and neuropathic pain. It is extensively metabolized in humans but little information exists about the pharmacodynamic properties of its metabolites. To determine their contribution to the clinical activity of Mex, p-hydroxy-mexiletine (PHM), hydroxy-methyl-mexiletine (HMM), N-hydroxy-mexiletine (NHM) (phase I reaction products) and N-carbonyloxy beta-D-glucuronide (NMG) (phase II reaction product) were tested on sodium currents (I(Na)) of frog skeletal muscle fibres. Sodium currents were elicited with depolarizing pulses from different holding potentials (HP=-140, -100, -70 mV) and stimulation frequencies (0.25, 0.5, 1, 2, 5, 10 Hz) using the vaseline-gap voltage-clamp method. All the hydroxylated derivatives blocked the sodium channel in a voltage- and use-dependent manner. The PHM, HMM and NHM metabolites were up to 10-fold less effective than the parent compound. However, HMM showed a greater use-dependent behaviour (10 Hz), compared to Mex and the other metabolites. Similar to Mex, these products behaved as inactivating channel blockers. Conjugation with glucuronic acid (NMG) resulted in almost complete abolition of the pharmacological activity of the parent compound. Thus, although less potent, the phase I metabolites tested demonstrated similar pharmacological behaviour to Mex and might contribute to its clinical profile.

Influence of the CYP2D6*10 allele on the metabolism of mexiletine by human liver microsomes.

To study the influence of CYP2D6*10 on the formation of p-hydroxymexiletine (PHM) and hydroxymethylmexiletine (HMM) using microsomes from human liver of known genotypes. Microsomes from human livers of genotype CYP2D6*1/*1 (n = 5), *1/*10 (n = 6) and *10/*10 (n = 6) were used in this study. The formation of PHM and HMM was determined by high-performance liquid chromatography. The formation rates of PHM and HMM were decreased by more than 50% and 85% in CYP2D6*1/*10 and *10/*10 microsomes, respectively, compared with *1/*1 microsomes. The metabolism of mexiletine to form PHM and HMM appears to be impaired to a significant extent in human liver microsomes from hetero- and homozygotes of CYP2D6*10.

Inhibition Kinetics of Theophylline Metabolism by Mexiletine and Its Metabolites.

To further characterize the mode of drug interaction between theophylline (TP) and mexiletine (ME), in vitro kinetic studies were carried out using rat liver microsomes and 9000 x g supernatant. The kinetic study revealed that the Km value and Vmax/Km ratio for the metabolic conversion of TP to 1,3-dimethyluric acid (1,3-DMU) were the second lowest and the highest, respectively, of four metabolic pathways. Thus, the rank of efficiency of the oxidative metabolism by microsomal cytochrome P-450 (P-450) isozymes was TP to 1,3-DMU > TP to 1-methylxanthine (1-MX) > TP to 3-MX > 1,3-DMU to 1-methyluric acid, suggesting that the isozyme metabolizing TP would have a higher affinity for the oxidation at the 8-position in TP molecules than at the 1- and 3-positions. Lineweaver-Burk plots showed that the conversion of TP to 3-MX and to 1,3-DMU was inhibited competitively by ME and its metabolites, and that the pathway of TP to 1-MX was inhibited noncompetitively. In consideration of the Ki values calculated, it seems probable that deamino-p-hydroxy ME (DApHME) might be the most potent inhibitor of the metabolic pathways of TP, and that the rank order of inhibition is approximately DApHME > p-hydroxy ME > deamino-hydroxymethyl ME > ME > hydroxymethyl ME, with some exceptions. The mechanism of the interaction between TP and ME is probably due to the metabolic antagonism in the liver, and TP, ME and their metabolites share some of the same metabolic pathways, mediated by P-450 isozymes.

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.

The metabolism of mexiletine in relation to the debrisoquine/sparteine‐ type polymorphism of drug oxidation.

1. The relationship between the metabolism of the antiarrhythmic drug mexiletine and the debrisoquine/sparteine-type polymorphism was studied in vitro, using microsomes from six human livers, and in vivo, in nine healthy drug-free volunteers with wide variation in their ability to hydroxylate debrisoquine. 2. There was a strong and similar correlation between the formation rate of both major mexiletine metabolites, p-hydroxymexiletine (PHM) and hydroxymethylmexiletine (HMM), and the high affinity component of dextromethorphan O-demethylase activity in human liver microsomes (rs = 0.94; P less than 0.01). 3. There were marked interindividual differences in the amounts of PHM and HMM excreted in the urine over 48 h after a single 200 mg oral dose of mexiletine hydrochloride. Recoveries of both metabolites were correlated inversely with the debrisoquine/4-hydroxydebrisoquine (D/HD) urinary metabolic ratio (rs = -0.83; P = 0.006 and rs = -0.85; P = 0.004, respectively) and were lower in poor metabolisers of debrisoquine (PM) than in extensive metabolisers (EM). Moreover, PM had the highest values of mexiletine/PHM and mexiletine/HMM urinary ratios. In addition, there was a strong correlation between these two indices of mexiletine hydroxylation and the D/HD metabolic ratios (rs = 0.92; P = 0.001 and rs = 0.90; P = 0.001, respectively). 4. After mexiletine pretreatment, the values for D/HD ratio were significantly increased in EM while corresponding values in PM were similar. 5. These findings are in accordance with previous in vitro data suggesting that PHM and HMM formation is predominantly catalyzed by the genetically variable human liver cytochrome P450IID6 isoenzyme responsible for the debrisoquine/sparteine-type polymorphism of drug oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)