Bisphenol A binds to the local anesthetic receptor site to block the human cardiac sodium channel

Bisphenol A (BPA) has attracted considerable public attention as it leaches from plastic used in food containers, is detectable in human fluids and recent epidemiologic studies link BPA exposure with diseases including cardiovascular disorders. As heart-toxicity may derive from modified cardiac electrophysiology, we investigated the interaction between BPA and hNav1.5, the predominant voltage-gated sodium channel subtype expressed in the human heart. Electrophysiology studies of heterologously-expressed hNav1.5 determined that BPA blocks the channel with a K_d of 25.4±1.3 µM. By comparing the effects of BPA and the local anesthetic mexiletine on wild type hNav1.5 and the F1760A mutant, we demonstrate that both compounds share an overlapping binding site. With a key binding determinant thus identified, an homology model of hNav1.5 was generated based on the recently-reported crystal structure of the bacterial voltage-gated sodium channel NavAb. Docking predictions position both ligands in a cavity delimited by F1760 and contiguous with the DIII–IV pore fenestration. Steered molecular dynamics simulations used to assess routes of ligand ingress indicate that the DIII–IV pore fenestration is a viable access pathway. Therefore BPA block of the human heart sodium channel involves the local anesthetic receptor and both BPA and mexiletine may enter the closed-state pore via membrane-located side fenestrations.

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

BACKGROUND AND PURPOSE

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.

EXPERIMENTAL APPROACH

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.

KEY RESULTS

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.

CONCLUSIONS AND IMPLICATIONS

Thus, although less potent, the phase I metabolites tested demonstrated similar pharmacological behaviour to Mex and might contribute to its clinical profile.

Catalytic roles of CYP2D6.10 and CYP2D6.36 enzymes in mexiletine metabolism: In vitro functional analysis of recombinant proteins expressed in Saccharomyces cerevisiae

Cytochrome P450 2D6 (CYP2D6) metabolizes approximately one-third of the medicines in current clinical use and exhibits genetic polymorphism with interindividual differences in metabolic activity. To precisely investigate the effect of CYP2D6*10B and CYP2D6*36 frequently found in Oriental populations on mexiletine metabolism in vitro, CYP2D6 proteins of wild-type (CYP2D6.1) and variants (CYP2D6.10 and CYP2D6.36) were heterologously expressed in yeast cells and their mexiletine p- and 2-methyl hydroxylation activities were determined. Both variant CYP2D6 enzymes showed a drastic reduction of CYP2D6 holo- and apoproteins compared with those of CYP2D6.1. Mexiletine p- and 2-methyl hydroxylation activities on the basis of the microsomal protein level at the single substrate concentration (100 microM) of variant CYP2D6s were less than 6% for CYP2D6.10 and 1% for CYP2D6.36 of those of CYP2D6.1. Kinetic analysis for mexiletine hydroxylation revealed that the affinity toward mexiletine of CYP2D6.10 and CYP2D6.36 was reduced by amino acid substitutions. The Vmax and Vmax/Km values of CYP2D6.10 on the basis of the microsomal protein level were reduced to less than 10% of those of CYP2D6.1, whereas the values on the basis of functional CYP2D6 level were comparable to those of CYP2D6.1. Although it was impossible to estimate the kinetic parameters for the mexiletine hydroxylation of CYP2D6.36, the metabolic ability toward mexiletine was considered to be poorer not only than that of CYP2D6.1 but also than that of CYP2D6.10. The same tendency was also observed in kinetic analysis for bufuralol 1''-hydroxylation as a representative CYP2D6 probe. These findings suggest that CYP2D6*36 has a more drastic impact on mexiletine metabolism than CYP2D6*10.

Inhibitory effects of psychotropic drugs on mexiletine metabolism in human liver microsomes: prediction of in vivo drug interactions

Mexiletine, an anti-arrhythmic agent, is used for the control of ventricular arrhythmias and for neuropathic pain from cancer or diabetes mellitus. It is sometimes used together with psychotropic drugs in patients with depression, schizophrenia or sleep disorder. It is metabolized mainly by cytochrome P450 (CYP) 2 D 6 and, to a minor extent, by CYP1A2. To predict possible drug interactions between mexiletine and psychotropic drugs, the inhibitory effects of 14 psychotropic drugs (phenytoin, carbamazepine, fluvoxamine, paroxetine, fluoxetine, citalopram, sertraline, imipramine, desipramine, haloperidol, thioridazine, olanzapine, etizolam, and quazepam) on mexiletine metabolism in human liver microsomes were determined. Fluoxetine (Ki=0.6+/- 0.1 microM), sertraline (Ki=7.6+/- 0.8 microM) and desipramine (Ki=3.2+/- 0.5 microM) competitively inhibited the mexiletine p-hydroxylation in human liver microsomes. Thioridazine (Kis=0.5+/- 0.2 microM; Kii =3.6+/-1.6 microM) and paroxetine (Kis=1.7+/- 0.7 microM; Kii=3.6+/- 0.9 microM) exhibited a mixed-type inhibition (competitive and non-competitive) toward mexiletine p-hydroxylation in human liver microsomes. The changes of the in vivo clearance of mexiletine by the psychotropic drugs were predicted by 1+(I/Ki) using the in vitro Ki and unbound inhibitor concentrations in liver. The values were calculated as 2.4 for paroxetine, 5.5 for fluoxetine, 1.1 for sertraline, 2.8 for desipramine and 2.2 for thioridazine. In addition, paroxetine exhibited a mechanism-based inactivation with Ki=0.7 microM and Kinact=0.15 min(-1). The present study predicted the possibility of drug interactions between mexiletine and paroxetine, fluoxetine, desipramine, and thioridazine in clinical use.

A Multifunctional Aromatic Residue in the External Pore Vestibule of Na+ Channels Contributes to the Local Anesthetic Receptor

Voltage-gated Na(+) (Na(v)) channels are responsible for initiating action potentials in excitable cells and are the targets of local anesthetics (LA). The LA receptor is localized to the cytoplasmic pore mouth formed by the S6 segments from all four domains (DI-DIV) but several outer pore-lining residues have also been shown to influence LA block (albeit somewhat modestly). Many of the reported amino acid substitutions, however, also disrupt the inactivated conformations that favor LA binding, complicating the interpretation of their specific effects on drug block. In this article, we report that an externally accessible aromatic residue in the Na(v) channel pore, DIV-Trp1531, when substituted with cysteine, completely abolished LA block (e.g., 300 microM mexiletine induced a use-dependent block with 65.0 +/- 2.9% remaining current and -11.0 +/- 0.6 mV of steady-state inactivation shift of wild-type (WT) channels versus 97.4 +/- 0.7% and -2.4 +/- 2.1 mV of W1531C, respectively; p < 0.05) without destabilizing fast inactivation (complete inactivation at 20 ms at -20 mV; V(1/2) = -70.0 +/- 1.6 mV versus -48.6 +/- 0.5 mV of WT). W1531C also abolished internal QX-222 block (200 microM; 98.4 +/- 3.4% versus 54.0 +/- 3.2% of WT) without altering drug access. It is interesting that W1531Y restored WT blocking behavior, whereas W1531A channels exhibited an intermediate phenotype. Together, our results provide novel insights into the mechanism of drug action, and the structural relationship between the LA receptor and the outer pore vestibule.

Mexiletine block of wild-type and inactivation-deficient human skeletal muscle hNav1.4 Na+ channels

Mexiletine is a class 1b antiarrhythmic drug used for ventricular arrhythmias but is also found to be effective for paramyotonia congenita, potassium-aggravated myotonia, long QT-3 syndrome, and neuropathic pain. This drug elicits tonic block of Na(+) channels when cells are stimulated infrequently and produces additional use-dependent block during repetitive pulses. We examined the state-dependent block by mexiletine in human skeletal muscle hNav1.4 wild-type and inactivation-deficient mutant Na(+) channels (hNav1.4-L443C/A444W) expressed in HEK293t cells with a beta1 subunit. The 50% inhibitory concentrations (IC(50)) for the inactivated-state block and the resting-state block of wild-type Na(+) channels by mexiletine were measured as 67.8 +/- 7.0 microm and 431.2 +/- 9.4 microm, respectively (n= 5). In contrast, the IC(50) for the block of open inactivation-deficient mutant channels at +30 mV by mexiletine was 3.3 +/- 0.1 microm (n= 5), which was within the therapeutic plasma concentration range (2.8-11 microm). Estimated on- and off-rates for the open-state block by mexiletine at +30 mV were 10.4 microm(-1) s(-1) and 54.4 s(-1), respectively. Use-dependent block by mexiletine was greater in inactivation-deficient mutant channels than in wild-type channels during repetitive pulses. Furthermore, the IC(50) values for the block of persistent late hNav1.4 currents in chloramine-T-pretreated cells by mexiletine was 7.5 +/- 0.8 microm (n= 5) at +30 mV. Our results together support the hypothesis that the in vivo efficacy of mexiletine is primarily due to the open-channel block of persistent late Na(+) currents, which may arise during various pathological conditions.

Mexiletine carbonyloxy beta-D-glucuronide: a novel metabolite in human urine

1. The study was performed to isolate and characterize a glucuronic acid conjugate of mexiletine that releases mexiletine on acid hydrolysis from urine samples obtained from healthy volunteers following a single oral dose of mexiletine. 2. The [M-H]- ion of the isolated metabolite was observed at m/z 398 in the negative electrospray ionization mass spectrum. This mass number was 44 higher than that of the product generated when mexiletine is subjected to direct glucuronidation. In positive-ion mode, collision-induced dissociation of the quasimolecular ion [M+NH4]+, m/z 417, gave product ions at m/z 224, 180 and 58. These mass spectral data indicated that the metabolite contained a carbonyloxy moiety in its structure in addition to mexiletine and a glucuronic acid moiety. 3. The presence of this carbonyloxy moiety was further supported by the following chemical reactions. When the metabolite was hydrolysed with an aqueous solution of 1 M sodium hydroxide at room temperature, mexiletine was released, whereas the N-methoxycarbonyl derivative of mexiletine was obtained after treatment of the metabolite with methanolic sodium hydroxide solution. 4. The results indicated that the structure of the isolated metabolite was the N-carbonyloxy beta-D-glucuronic acid conjugate of mexiletine.

Role of specific cytochrome P450 enzymes in the N-oxidation of the antiarrhythmic agent mexiletine

1. Mexiletine is extensively metabolized in man by C- and N-oxidation and the aim of the present study was to characterize major cytochrome P450 enzyme(s) involved in the formation of N-hydroxymexiletine. 2. Incubations with genetically engineered microsomes indicated that the formation rate of N-hydroxymexiletine was highest in the presence of microsomes expressing high levels of either CYP1A2 or CYP2E1 and the formation of N-hydroxymexiletine by human liver microsomes was inhibited about 40% by antibodies directed against CYP1A1/1A2 or CYP2E1. Additional incubations demonstrated that formation of N-hydroxymexiletine was decreased 47 and 51% by furafylline, 40 microm and 120 microm, respectively, and decreased 55 and 67% by alpha-naphthoflavone, 1 microm and 3 microm, respectively (all p < 0.05 versus control). 3. The formation rate of N-hydroxymexiletine in human liver microsomes was highly correlated with CYP2B6 (RS-mexiletine, r = 0.7827; R-(-)-enantiomer, r = 0.7034; S-(+)-enantiomer, r = 0.7495), CYP2E1 (S-(+)-enantiomer, r = 0.7057) and CYP1A2 (RS-mexiletine, r = 0.5334; S-(+)-enantiomer, r = 0.6035). 4. In conclusion, we have demonstrated that CYP1A2 is a major human cytochrome P450 enzyme involved in the formation of N-hydroxymexiletine. However, other cytochrome P450 enzymes (CYP2E1 and CYP2B6) also appear to play a role in the N-oxidation of this drug.

Mexiletine block of disease-associated mutations in S6 segments of the human skeletal muscle Na(+) channel

1. Over twenty different missense mutations in the alpha-subunit of the adult skeletal muscle Na(+) channel (hSkM1) have been identified as a cause of myotonia or periodic paralysis. We examined state-dependent mexiletine block for mutations involving the putative binding site in S6 segments (V445M, S804F, V1293I, V1589M and M1592V). Whole-cell Na(+) currents were measured from wild-type (WT) and mutant channels transiently expressed in HEK cells. 2. Use-dependent block (10 ms pulses to -10 mV, at 20 Hz) in 100 microM mexiletine was reduced modestly by mutations in IVS6 (V1589M, M1592V) and enhanced by the mutation in IS6 (V445M). For mutations in IIS6 (S804F) and IIIS6 (V1293I) use-dependent block was not statistically different from that of wild-type channels. 3. Resting-state block (10 ms pulses to -10 mV from -150 mV, at 0.1 Hz) of S6 mutants was comparable to that of WT (dissociation constant for resting channels, K(R) = 650 +/- 40 microM, n = 9). The S6 mutant with the greatest change in K(R) was V445M (K(R) = 794 +/- 45 microM, n = 5), but this difference was only marginally significant (P = 0.047). 4. A modified technique for estimating local anaesthetic affinity of inactivated channels was developed to reduce errors due to slow inactivation and to failure of drug binding to reach equilibrium. Mexiletine affinity for inactivated channels was reduced by mutations in IVS6 (V1589M: dissociation constant for the inactivated state (K(I)) = 44.7 microM; M1592V: K(I) = 40.0 microM) and increased by the mutation in IS6 (V445M: K(I) = 15.0 microM), compared to wild-type channels (K(I) = 28.3 microM). 5. We conclude that the disease-associated S6 mutations in domains I-IV cause at most a 2-fold change in inactivated state affinity and have even less of an effect on resting block. Model simulations show that the reduced use-dependent block of IVS6 mutants derives primarily from an increased off-rate at hyperpolarized potentials, whereas the enhanced use-dependent block of the IS6 mutant was due to a higher affinity for inactivated V445M channels.

A pyrroline derivative of mexiletine offers marked protection against ischemia/reperfusion-induced myocardial contractile dysfunction

The efficacy and mechanism of protection of a new 2,2,5, 5-tetramethylpyrroline derivative of mexiletine, MEX-NH, against ischemia/reperfusion-induced cardiac dysfunction are reported. The MEX-NH and its nitroxide metabolite are membrane-permeable antioxidants. Studies were performed in an isolated rat heart model to measure the efficacy of MEX-NH in preventing postischemic injury. Serial measurements of contractile function and coronary flow were performed on hearts subjected to 30 min of global 37 degrees C ischemia followed by 45 min of reperfusion. Hearts were either untreated or treated with 25 microM MEX-NH or MEX for 1 min before ischemia. The hearts treated with MEX-NH showed marked recovery of left ventricular developed pressure (96.3 +/- 2.7% of preischemic value) compared with untreated (13.7 +/- 1.0%) or MEX-treated (19.9 +/- 2.7%) hearts. The cardiac sarcolemmal Na(+),K(+)-ATPase activity showed that the enzyme activity was fully restored in hearts treated with MEX-NH compared with 65 +/- 5.3% inhibition in the untreated hearts. Competitive inhibition of [(3)H]ouabain binding revealed that the MEX-NH binds at the K(+)-binding site of the enzyme. The present study establishes that the compound MEX-NH provides marked protection against ischemia/reperfusion-induced contractile dysfunction in isolated hearts. A combination of reversible inhibition of Na(+)/K(+)-ATPase activity during ischemia and site-targeted antioxidative effect upon reperfusion seems to contribute to this cardioprotection.

Theoretical study of mexiletine and its interaction with cationic and anionic receptor sites

Theoretical methods are applied to study the antiarrhythmic (AA) mexiletine (1-(2,6-dimethylphenoxy)-2-aminopropane). The AM1 method is used to construct a three-centre binding model for this drug. This model consists of an amine nitrogen atom that is protonated to a higher degree at physiological pH, flat hydrophobic regions of aromatic rings and additional functional groups with lone electron pairs of oxygen. Based on these ideas, a model for the binding of mexiletine at the transmembrane protein was constructed. An ab initio SCF method was used to study the two-component mexiletine-receptor binding site composed of acetate (Glu-, Asp-) and protonated methylamine (Lys+, Arg+). The binding of mexiletine to the receptor may be understood by considering a two-step process of recognition and binding of AA to its receptor. Within this model the mexiletine cation is recognised in the first step and bonded to the negatively-charged part of the receptor. In a subsequent step, the interaction between the amide oxygen and cationic amine group of the membrane protein may follow.

Involvement of CYP1A2 in mexiletine metabolism

AIMS

Mexiletine has been reported to be hydroxylated by cytochrome P450 2D6 (CYP2D6) in humans. However, the involvement of CYP1A2 in the metabolism of mexiletine has been proposed based on the interaction with theophylline which is mainly metabolized by CYP1A2. The aim of this study was to clarify the role of human CYP1A2 in mexiletine metabolism.

METHODS

Human CYP isoforms involved in mexiletine metabolism were investigated using microsomes from human liver and B-lymphoblastoid cells expressing human CYPs. The contributions of CYP1A2 and CYP2D6 to mexiletine metabolism were estimated by the relative activity factor (RAF).

RESULTS

Mexiletine p- and 2-hydroxylase activities in human liver microsomes were inhibited by ethoxyresorufin and furafylline as well as quinidine. Mexiletine p- and 2-hydroxylase activities in microsomes from nine human livers correlated significantly with bufuralol 1'-hydroxylase activity (r = 0.907, P < 0.001 and r = 0.886, P < 0.01, respectively). Microsomes of B-lymphoblastoid cells expressing human CYP1A2 exhibited lower mexiletine p- and 2-hydroxylase activities than those expressing human CYP2D6. It was estimated by RAF that the major isoform involved in mexiletine metabolism was CYP2D6, and the contribution of CYPIA2 to both mexiletine p- and 2-hydroxylase activities was 7-30% in human liver microsomes. However, the Km values of the expressed CYP1A2 (approximately 15 microM) were almost identical with those of the expressed CYP2D6 (approximately 22 microM) and human liver microsomes.

CONCLUSIONS

Mexiletine is a substrate of CYP1A2. The data obtained in this study suggest that the interaction of mexiletine with theophylline might be due to competitive inhibition of CYP1A2.

Stereoselective metabolism of rac-mexiletine by the fungus Cunninghamella echinulata yields the major human metabolites hydroxymethylmexiletine and p-hydroxymexiletine

rac-Mexiletine is an orally effective class 1b antiarrhythmic agent used to treat ventricular arrhythmias. In vivo experiments have demonstrated. It is predominantly metabolized by the liver with < 10% excreted as unchanged drug. The major mammalian metabolites have been identified as p-hydroxymexiletine (PHM) and hydroxymethylmexiletine (HMM). The purpose of our study was to determine whether the fungus Cunninghamella echinulata, which possesses a cytochrome P450 system analogous to that found in humans, could be used as a suitable in vitro model for studying the oxidative metabolism of rac-mexiletine. To accomplish this, a high performance liquid chromatographic assay was used that was capable of simultaneously quantifying the enantiomers of mexiletine, HMM, and PHM. Utilizing this procedure, it was demonstrated that C. echinulata stereoselectively converted rac-mexiletine into HMM (4% of added drug) and PHM (32% of added drug) after an incubation period of 50 hr. In addition, metabolite biosynthesis could be optimized by altering fermentation media components. Seven media values and seven pH values were evaluated. It was determined that a medium at pH 7.0 containing yeast extract and sucrose yielded optimal amounts of metabolites.

Direct analysis of the enantiomers of mexiletine and its metabolites in plasma and urine using an HPLC-CSP

A high-performance liquid chromatographic assay has been developed for the quantification of the enantiomers of mexiletine and its four major metabolites, in plasma and in urine. Mexiletine and all metabolites were determined, after derivatization of mexiletine and its hydroxymetabolites, p-hydroxymexiletine and hydroxymethylmexiletine, using a Chiralpak AD chiral stationary phase, based on a carbamoyl derivative of amylose. o-phthalaldehyde was chosen as derivatization reagent to increase the sensitivity of detection, to achieve separation of all compounds in one chromatographic system, and to avoid interferences.

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.

Stereoselective hydroxylation of mexiletine in human liver microsomes: implication of P450IID6--a preliminary report

Mexiletine, a class Ib antiarrhythmic drug, is used clinically as a racemic mixture of two enantiomers. Aromatic and aliphatic hydroxylation are the two major metabolic steps. In the liver, this metabolism is catalyzed by the cytochrome P450IID6, an isoenzyme of cytochrome P450 due to genetic polymorphism in humans. In the present study, the metabolism of the two stereoisomers was compared in vitro in human liver microsomes. Parahydroxylation (aromatic hydroxylation) is favored for S(+)-mexiletine with a mean intrinsic clearance higher than for R(-)-mexiletine. The R(-) enantiomer exhibits a threefold higher mean Vmax value for aliphatic hydroxylation than S(+)-mexiletine. We showed that (i) the high-affinity component of dextrometorphan O-demethylation was competitively inhibited by R(-)- and S(+)-mexiletine and that (ii) hydroxylations of the two enantiomers were very strongly competitively inhibited by quinidine. Hydroxylation reactions of mexiletine exhibit stereoselectivity in vitro in human liver microsomes and are catalyzed by P450IID6.

Isolation and structural characterization by spectroscopic methods of two glucuronide metabolites of mexiletine after N-oxidation and deamination

Urine samples from control and mexiletine-treated human subjects or rabbits (test group) were collected and passed through an ion exchange resin to isolate polar compounds. Methanolic eluates from control and test urines were analyzed by TLC. Exposure to p-dimethylaminocinnamaldehyde gave an additional intense pink band at Rt 0.40-0.45 in TLC analysis of test urine eluate when compared to control urine eluate. Non-exposed silica at this Rt was scraped and metabolites were extracted with methanol. Hydrolysis of this methanolic extract at 100 degrees C with hydrochloric acid released mexiletine. GC/MS and fast atom bombardment mass spectrometry analyses of nonhydrolyzed methanolic extracts evidenced the presence of two conjugated metabolites of mexiletine, namely, N-hydroxymexiletine glucuronide and mexiletine alcohol glucuronide. Synthetic compounds corresponding to these metabolites were obtained and spectra compared with those of isolated metabolites from urine. Definite structure assignment of N-hydroxymexiletine glucuronide was obtained from NMR spectrometry which confirmed the structure to be a hydoxylamine glucuronide (N-O-C link) and showed that the glycoside moiety was in the beta configuration. Thus, it is proposed that N-hydroxymexiletine glucuronide corresponds to mexiletine acid-labile conjugate and represents a major metabolic pathway in the disposition of mexiletine.

Simultaneous determination of mexiletine and four hydroxylated metabolites in human serum by high-performance liquid chromatography and its application to pharmacokinetic studies

A high-performance liquid chromatographic method has been developed for the simultaneous determination of mexiletine and its four hydroxylated metabolites in human serum. The method involves a single-step extraction of mexiletine, hydroxymethylmexiletine, p-hydroxymexiletine and their corresponding alcohols with diisopropyl ether-dichloromethane-propan-2-ol (2.5:1.5:0.5, v/v). Separation of the compounds on a deactivated Supelcosil LC8-DB column is accomplished by high-performance liquid chromatography with ultraviolet detection at 203 nm. Overall the recovery of each compound is reproducible and greater than 75%. The lower limit of detection is 2 ng/ml for mexiletine and its metabolites. The application of the method is shown by measuring the concentrations in serum of mexiletine and its metabolites over 24 h in a healthy volunteer after a single intravenous injection of the drug and by monitoring serum concentrations in patients receiving long-term treatment by mouth of the drug.

A model analysis for competitive binding of mexiletine and aprindine to the cardiac sodium channel

A simulation model was developed to predict complex interaction between antiarrhythmic drugs and cardiac sodium channels. This model has four assumptions: (1) Vmax of the action potential is a linear indicator of available sodium channel conductance; (2) antiarrhythmic drugs block the channel by binding to a single common receptor site associated with the channel; (3) binding and dissociation rate constants differ for the three channel states: activated, inactivated and resting, and (4) both drug-free and drug-bound channels change states far more rapidly than binding and dissociation processes. Binding and dissociation rate constants for the three channel states were calculated from single cell experiments using guinea pig hearts. Vmax changes reflecting tonic and use-dependent sodium channel block in the presence of mexiletine and aprindine were simulated and compared with those obtained in the single cell experiments. The model predicted that 'tonic' Vmax inhibition would be enhanced, whereas 'use-dependent' ones would be attenuated after admixture of mexiletine with aprindine. The mechanisms would involve competitive interaction at the common receptor site. Single-cell experiments supported this prediction. We conclude that our simple two-drug binding model provides a useful tool to predict pharmacological interaction between class I antiarrhythmic drugs given in combination.

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)

Meta-hydroxymexiletine, a new metabolite of mexiletine. Isolation, characterization, and species differences in its formation

Meta-hydroxymexiletine [1-(3-hydroxy,2,6-dimethyl)phenoxy-2-amino-propane], a novel metabolite of the antiarrhythmic drug mexiletine, was isolated from urine of rats given mexiletine. The structure of the metabolite was elucidated by 1H-NMR and mass spectrometry and by IR spectrophotometry. The metabolite is produced in vitro by hepatic microsomes of various laboratory animals including rat, guinea-pig, hamster, rabbit, and mouse. In humans, meta-hydroxymexiletine accounts for approximately 2% of the administered dose of mexiletine.

Pharmacokinetics of mexiletine and its metabolites, hydroxymethylmexiletine and p-hydroxymexiletine, after single oral administration in healthy subjects

Pharmacokinetics of mexiletine and its major metabolites, p-hydroxymexiletine and hydroxymethylmexiletine, were studied in 10 healthy subjects after administration of a single oral 400 mg dose. Mexiletine was extensively metabolized to hydroxymethylmexiletine and less to p-hydroxymexiletine. Mean metabolic ratio (Rm) was 0.54 and 0.18 respectively. Peak serum concentrations (Cmax) for mexiletine (0.686 +/- 0.110 micrograms/ml), hydroxymethylmexiletine (0.507 +/- 0.087 micrograms/ml), and p-hydroxymexiletine (0.096 +/- 0.046 micrograms/ml) occurred after 3.0, 4.0 and 4.2 +/- 0.6 h. Disposition parameters for mexiletine were as follows: volume of distribution (V lambda), 5.44 +/- 1.44 l/kg; serum clearance (CL) 8.08 +/- 1.23 ml/min/kg. There were no significant differences in elimination half-life (t1/2) and mean residence time (MRT) for mexiletine and its metabolites. Double-peak phenomenon was observed for all the subjects on elimination phase of mexiletine, hydroxymethylmexiletine and p-hydroxymexiletine.

Inhibitory studies of mexiletine and dextromethorphan oxidation in human liver microsomes

The cytochrome P-450dbl isozyme (P-450bdl) is responsible for the genetic sparteine-debrisoquine type polymorphism of drug oxidation in humans. To investigate the relationship between mexiletine oxidation and the activity of this isozyme, cross-inhibition studies were performed in human liver microsomes with mexiletine and dextromethorphan, a prototype substrate for P-450dbl. The formation of hydroxymethylmexiletine and p-hydroxymexiletine, two major mexiletine metabolites, was competitively inhibited by dextromethorphan. Mexiletine competitively inhibited the high affinity component of dextromethorphan O-demethylation. In addition, there was a good agreement between the apparent Km values for the formation of both mexiletine metabolites and the high affinity component of dextromethorphan O-demethylation and their respective apparent Ki values. Several drugs were tested for their ability to inhibit mexiletine oxidation. Quinidine, quinine, propafenone, oxprenolol, propranolol, ajmaline, desipramine, imipramine, chlorpromazine and amitryptiline were competitive inhibitors for the formation of hydroxymethylmexiletine and p-hydroxymexiletine as for prototype reactions of the sparteine-debrisoquine type polymorphism. Amobarbital, valproic acid, ethosuximide, caffeine, theophylline, disopyramide and phenytoin, known to be non-inhibitors of P-450dbl activity, were found not to inhibit the formation of these mexiletine metabolites. Moreover, the formation of both metabolites was strongly inhibited by an antiserum containing anti-liver/kidney microsomes antibodies type I (anti-LKMI) directed against P-450dbl. These data suggest that the formation of two major metabolites of mexiletine is predominantly catalysed by the genetically variable human liver P-450dbl.

High-performance liquid chromatographic assay for mexiletine hydroxylation in microsomes of human liver

A simple high-performance liquid chromatographic assay, using fluorescence detection, is described for determining simultaneously the production of the two major hydroxylated metabolites of mexiletine in human liver microsomes. The detection limits of hydroxymethylmexiletine and p-hydroxymexiletine are 0.35 and 0.08 nmol/ml, respectively. The assay is specific, reproducible and allows the simultaneous kinetic characterization of the reactions in small amounts of liver tissue. The assay may be used to acquire a better knowledge of the kinetic behaviour of mexiletine and of its metabolites, and to investigate if the large inter-individual variations of the mexiletine pharmacokinetics are of metabolic origin, due to variations of its hydroxylation processes.

2,6-Dimethylphenol: a new metabolite of mexiletine

A new metabolite, 2,6-dimethylphenol obtained by O-dealkylation of mexiletine by rabbit liver was identified. A g.c.-f.i.d. method was developed for its analysis in hepatic homogenates and some of the characteristics of the metabolic reaction were studied. Formation of 2,6-dimethylphenol is microsomal and is optimal under aerobic conditions. Both carbon monoxide and n-octylamine significantly depress this O-dealkylation reaction.

Mexiletine use in pregnancy and lactation

Treatment of arrhythmias during pregnancy is complicated by insufficient information on the effects of the drug on the fetus or possible alterations of the drug's pharmacodynamics in the mother. The use of mexiletine, a newly introduced antiarrhythmic agent, during the entire course of pregnancy and subsequent lactation is presented.

Stereoselective serum protein binding of mexiletine enantiomers in man

The binding of mexiletine enantiomers to human serum proteins was studied in vitro using serum samples collected from five healthy male subjects. Racemic mexiletine was added to an aliquot of each subject's serum to cover the concentration range 0.2 to 2.0 micrograms/mL. Following ultrafiltration of the serum samples containing racemic mexiletine, the individual enantiomers were determined using a stereoselective high-performance liquid chromatographic method developed in our laboratory. The assay data thus obtained for the free levels of the enantiomers showed that the free fraction of S(+) mexiletine was 28.32 +/- 1.45% and that of the R(-) enantiomer was 19.80 +/- 1.49%. The binding was shown to be significantly greater (p less than 0.001) for R(-) mexiletine than its antipode. There was no evidence of concentration dependence in binding over the concentration range studied, which covered the normal therapeutic range. However, significant inter-individual variability in the free fractions was observed. The total binding of the enantiomers was 76%.

Stereoselective disposition of mexiletine in man

The pharmacokinetics of S-(+)- and R-(-)-mexiletine and of the corresponding conjugates were investigated in six healthy young volunteers after administration of a single 200 mg oral dose of racemic mexiletine hydrochloride. The values for the distribution rate constants as well as for the elimination half-lives of the two enantiomers were similar but the AUC of the S-(+)-enantiomer was always significantly higher (P less than 0.01) than that of the opposite enantiomer. The mean R/S ratios for unchanged mexiletine in serum and in urine were 0.78 +/- 0.12 (s.d.) and 0.80 +/- 0.21, respectively. Urinary excretion of mexiletine conjugates consisted mainly of the R-(-)-enantiomer; the mean R/S enantiomeric ratio over 48 h was 9.65 +/- 3.10. Serum concentrations of the conjugates were measured in three subjects. The mean R/S AUC ratio was 2.94 +/- 0.48 and the renal clearance of the R-(-)-enantiomer was significantly higher (P less than 0.02) than that of the S-(+)-enantiomer.

Short-term myocardial uptake of lidocaine and mexiletine in patients with ischemic heart disease

Determination of short-term myocardial drug uptake and subsequent redistribution was performed in 27 patients with ischemic heart disease for the antiarrhythmic agents lidocaine and mexiletine, using frequent simultaneous measurements of drug concentration in aortic and coronary sinus blood, combined with measurement of coronary sinus blood flow after intravenous bolus injection of the drug. Maximal myocardial drug content per unit resting coronary sinus blood flow (MDC:F) was significantly greater in patients in whom coronary sinus pacing at 100 beat/min was performed during the initial period of drug uptake. Maximal myocardial drug content occurred after 2.4 +/- 0.2 (SEM) for lidocaine and after 5.5 +/- 0.6 min for mexiletine (p less than .001), and pacing did not affect time to maximum myocardial drug content. In nonpaced, but not paced, patients maximal MDC:F was greater in the lidocaine group than that in the mexiletine group. The subsequent efflux of lidocaine from the myocardium was more rapid that that of mexiletine in both paced and nonpaced groups.

Mexiletine: a new type I antiarrhythmic agent

Mexiletine is a type I antiarrhythmic drug that is structurally similar to lidocaine. Mexiletine has considerable potential for causing neurologic, cardiac, or gastrointestinal side effects. However, mexiletine does not undergo clinically significant first-pass metabolism and, thus, has good oral bioavailability. Mexiletine has a large and variable volume of distribution and an elimination half-life ranging from 6 to 12 hours. Mexiletine disposition is probably altered in patients with heart failure, liver disease, and severe renal dysfunction. Efficacy and toxicity are not well correlated with mexiletine serum concentrations. Mexiletine is as effective as traditional antiarrhythmics in the treatment of premature ventricular contractions. However, in patients with drug-refractory inducible ventricular tachycardia, mexiletine is usually ineffective when used alone. When mexiletine is combined with other antiarrhythmic agents, a significantly higher percentage of patients with this difficult arrhythmia have a good response. Mexiletine is a potentially important addition to the existing antiarrhythmic drugs currently available, but its place in the clinical setting and in therapeutic drug monitoring is not well defined at this time.

Pharmacology and clinical use of mexiletine

Mexiletine is an antiarrhythmic agent with structural and electrophysiologic properties similar to those of lidocaine. Mexiletine decreases ventricular automaticity while shortening both action potential duration and effective refractory period. The drug may be administered orally or intravenously. Hepatic metabolism is the major route of elimination. The elimination half-life is approximately 10 hours, but longer in patients with acute myocardial infarction, chronic congestive heart failure or hepatic insufficiency. Mexiletine suppresses ventricular ectopy in the acute phase of myocardial infarction. The drug is effective for some patients in whom lidocaine has failed. It suppresses chronic ventricular ectopy and is well tolerated in approximately two-thirds of stable outpatients treated with this agent. In that population, mexiletine is comparable in efficacy to quinidine, procainamide and disopyramide. It is effective in 30-50% of patients with ventricular arrhythmias refractory to other antiarrhythmic drugs. In patients with refractory arrhythmias, the efficacy of mexiletine may be enhanced by combination with propranolol, quinidine or amiodarone. Adverse reactions limit use of mexiletine in approximately 20% of patients. Gastrointestinal and central nervous system side effects are the most common. Mexiletine does not depress myocardial function. Aggravation of arrhythmias is uncommonly observed. The usual intravenous dose of mexiletine is 150-250 mg over at least 10 minutes. Long-term oral dosages are usually 200-300 mg 3 or 4 times daily.

Cirrhosis of the liver markedly impairs the elimination of mexiletine

To study the effects of cirrhosis of the liver on the pharmacokinetics of mexiletine a single i.v. dose of 200 mg was administered to six cirrhotic patients and to six healthy controls. The distribution of mexiletine in both study groups was similar, as indicated by similar values of V1 and Vss, but it tended to occur more slowly in the cirrhotics. The plasma protein binding of mexiletine was unchanged in the patients with cirrhosis. The elimination of mexiletine was markedly retarded in the cirrhotics, as indicated by its lower total clearance (2.31 vs. 8.27 ml/kg/h,) lower total elimination rate constant (0.059 vs 0.353 h-1), and longer elimination half-life (28.7 vs 9.9 h). The antipyrine half-life was 38.3 h in the patients and 14.7 h in the controls. One healthy volunteer had a Morgagni-Stokes-Adams type of syncopal attack 5 min after administration of mexiletine due to disturbance of AV conduction induced by the drug. Thus, on a pharmacokinetic basis the loading dose of mexiletine need not be modified in cirrhotic patients, whereas the maintenance dosage should be reduced to one fourth - one third of the usual dose.

Effect of cigarette smoking on mexiletine kinetics

The effect of cigarette smoking on the kinetics of a single, 200 mg, oral dose of the antiarrhythmic drug mexiletine was investigated in healthy subjects. Cigarette smoking had no effect on absorption or distribution of the drug, but it significantly reduced the elimination t1/2 from 11.1 +/- 3.4 to 7.2 +/- 1.8 hours; the effect on clearance was less significant. Determination of the urinary concentrations of the three major metabolites of mexiletine (mexiletine glucuronide conjugate, hydroxymethylmexiletine, and p-hydroxymexiletine) indicated that cigarette smoking selectively induced conjugation of mexiletine with glucuronic acid as well as aliphatic hydroxylation to yield hydroxymethylmexiletine, but that it had no effect on the formation of p-hydroxymexiletine.

Mexiletine kinetics in healthy subjects taking cimetidine

Cimetidine, a commonly used H2-receptor antagonist, was found to interact adversely with many drugs, including class I antiarrhythmics such as lidocaine and quinidine. To test the effect of cimetidine on the kinetics of mexiletine, a class I antiarrhythmic similar to lidocaine, the absorption and disposition of mexiletine were followed in six healthy subjects before and after 1 week of cimetidine, 300 mg by mouth four times a day. Cimetidine did not alter the distribution and elimination of mexiletine, as shown by similar mean kinetics including total body clearance, AUC, and the elimination t1/2 before and after cimetidine treatment. Cimetidine did have a significant effect on mexiletine absorption, as demonstrated by a longer mean absorption t1/2 (from 0.20 +/- 0.14 to 0.61 +/- 0.35 hours), a longer mean time to peak mexiletine concentration (from 1.13 +/- 0.31 to 1.88 +/- 0.83 hours), and decreased mexiletine plasma concentration (from 0.74 +/- 0.19 to 0.59 +/- 0.15 mg/ml). We conclude that cimetidine does not alter the disposition of oral mexiletine in normal subjects.

Pharmacokinetics and nondialyzability of mexiletine in renal failure

Mexiletine is an investigational antiarrhythmic drug eliminated primarily by hepatic metabolism. To evaluate its pharmacokinetics in patients with renal failure, we gave 14 subjects (creatinine clearance from 0 to 68.9 ml/min, including five subjects who required maintenance dialysis) a single, 200 mg dose of mexiletine. Serial blood samples were drawn and analyzed for mexiletine concentration by gas chromatography. The elimination t1/2 was 18.9 +/- 7.4 hours and oral clearance was 378 +/- 109 ml/min (means +/- SD). There was no correlation between these parameters and creatinine clearance. In subjects receiving dialysis, the study was also repeated during dialysis 1 week later. There was no significant difference between the AUCs either while receiving dialysis or when calculated on a day when the subject was not receiving dialysis. Thus dosing adjustments for mexiletine should not be necessary in patients with creatinine clearance values as low as 10 ml/min or in patients receiving dialysis. Furthermore, supplemental doses of mexiletine are not likely to be needed after dialysis. Evaluation of the kinetics at steady state are necessary to extrapolate further our observations after a single oral dose.

Antiarrhythmic drug therapy. Recent advances and current status

A number of conventional and newer antiarrhythmic agents are available for the treatment and prophylaxis of ventricular tachycardia and sudden death. Using a multifaceted approach of programmed electrical stimulation studies, drug level determinations, exercise tolerance testing, and 24-hour ambulatory electrocardiographic monitoring, the physician can identify those patients who require therapy and then predict the likelihood of efficacy with each antiarrhythmic agent. This approach affords evaluation of both aspects of the sudden death equation-ectopy frequency (triggering mechanism) and vulnerability to development of sustained ventricular tachycardia (substrate). After institution of therapy, careful follow-up is necessary to document sustained drug efficacy and detect side effects. Serious adverse reactions necessitate a change in antiarrhythmic therapy, as opposed to lowering drug dosage to an ineffective level. The unacceptably high incidence of sudden death due to electrical instability can be reversed only by a rigorous and dedicated long-term approach to the management of serious ventricular arrhythmias.

Do gastric contents modify antidotal efficacy of oral activated charcoal?

The effect of food on the antidotal efficacy of activated charcoal was studied in six healthy volunteers, who ingested aspirin 1000 mg, mexiletine 200 mg and tolfenamic acid 400 mg in a randomized cross-over study. Activated charcoal 25 g, suspended in water, was administered 5 min or 60 min after the drugs were taken on an empty stomach or after a standard meal. The serum concentrations and the cumulative excretion into urine of the drugs were followed for 48 h. When the drugs were taken on an empty stomach, activated charcoal given 5 min or 60 min afterwards reduced the bioavailability of the drugs by 75-98% or 10-60%, respectively. Food moderately weakened the effect of activated charcoal administered 5 min after the drugs, but when the charcoal was given 1 h later the effect was still practically the same, the reduction of absorption varying in both cases in the range of 45-85%. Thus the efficacy of charcoal given 60 min after the drugs was better after a standard meal than on an empty stomach. Presence of food in the stomach of patients with drug overdosage modifies the efficacy of activated charcoal and gives it more time to adsorb drugs in the gastrointestinal canal, possibly by slowing gastric emptying rate.

Clinical pharmacokinetics of the newer antiarrhythmic agents

This article reviews clinical pharmacokinetic data on 8 new antiarrhythmic agents. Some of these drugs have been studied extensively while others are relatively new, with incomplete data due to limited evaluation. Amiodarone is a class III antiarrhythmic drug which is effective in treating many atrial and ventricular arrhythmias that are refractory to other drugs. Amiodarone accumulates extensively in tissues and its disposition characteristics are best described by models with 3 and 4 compartments. Its apparent volume of distribution is very large (1300 to 11,000L) and its elimination half-life very long (53 days). A delay of up to 28 days from of treatment to onset of antiarrhythmic effect may be observed, and the antiarrhythmic effect may persist for weeks to months following cessation of therapy. Clinically significant drug interactions have been observed with amiodarone and warfarin, digoxin, quinidine and procainamide. Encainide is a class Ic antiarrhythmic drug. Although it has a short elimination half-life (1 to 3h), 2 major metabolites with antiarrhythmic effects accumulate in the plasma of patients during long term therapy. Plasma concentrations of O-demethyl encainide appear to correlate with the antiarrhythmic effect. Flecainide, another class Ic antiarrhythmic agent, has an elimination half-life of 14 hours which makes it suitable for twice daily dosing. Flecainide elimination is prolonged in patients with low output heart failure. Significant drug interactions with digoxin and cimetidine have been reported. Lorcainide is also a class Ic antiarrhythmic drug, the bioavailability of which is nonlinear. Clearance of the drug is reduced during long term therapy. A major active metabolite, norlorcainide, accumulates in the plasma of patients during long term therapy and its concentration exceeds that of lorcainide by a factor of 2. The elimination half-lives of lorcainide (9h) and norlorcainide (28h) allow for once or twice daily dosing. Mexiletine, a class Ib antiarrhythmic drug, is structurally similar to lignocaine (lidocaine). A sustained release formulation provides effective plasma concentrations when administered twice daily. The apparent volume of distribution of mexiletine is 5.0 to 6.6 L/kg, and the elimination half-life varies from 6 to 12 hours in normal subjects and from 11 to 17 hours in cardiac patients. Mexilitine is extensively metabolised but the metabolites are not pharmacologically active. Renal elimination of mexiletine is pH dependent. Drugs which induce hepatic metabolism significantly alter the pharmacokinetics of mexiletine.(ABSTRACT TRUNCATED AT 400 WORDS)

The interaction of mexiletine with other cardiovascular drugs

Drug-drug interactions can be adverse or beneficial and can be classified as pharmacokinetic or pharmacodynamic. Several adverse pharmacokinetic drug interactions have been described for mexiletine. Because it is a weak base, mexiletine undergoes several pH-dependent drug interactions in the gastrointestinal tract and kidney. Since mexiletine is metabolized by hepatic mixed-function oxidases, its metabolic rate can be altered by drugs that induce or inhibit this drug metabolizing system. Phenytoin and rifampin have been shown to increase mexiletine clearance and decrease its plasma concentration. Striking examples of beneficial pharmacodynamic interactions occur with mexiletine. Combining mexiletine with either beta-adrenergic blocking drugs or with quinidine markedly increases antiarrhythmic efficacy and substantially decreases the incidence of adverse effects. These beneficial interactions will have a major impact on the clinical use of mexiletine.

Pharmacology, electrophysiology, and pharmacokinetics of mexiletine

Mexiletine is a class I antiarrhythmic agent that is active after both oral and intravenous administration and similar in structure and activity to lidocaine. It decreases phase O maximal rate of depolarization (Vmax) by fast sodium channel blockade. The marked rate dependence of Vmax depression may explain mexiletine's lack of effect on normal conduction and its efficacy against ventricular tachyarrhythmias. Mexiletine significantly decreases the relative refractory period in His-Purkinje fibers without changing the sinus rate or atrioventricular and His-Purkinje conduction times. Action potential duration is usually shortened. Mexiletine may aggravate preexisting impairment of impulse generation and conduction. Uptake and distribution of mexiletine are rapid, systemic bioavailability is about 90%, and tissue distribution is extensive. Mexiletine is primarily metabolized in the liver; 10% to 15% is excreted unchanged in the urine. Elimination half-life is 9 to 11 hours after intravenous or oral administration. Microsomal enzyme induction shortens mexiletine's elimination half-life, whereas hepatic disease and acute myocardial infarction prolong it. Renal disease has little effect, although hemodialysis increases mexiletine clearance. Plasma concentrations from 0.75 to 2.0 mg/L are usually associated with a desirable therapeutic response.

Practical considerations in the treatment of ventricular arrhythmias with mexiletine

Mexiletine can be administered by intravenous, intramuscular, or oral route. However, the intramuscular route is seldom used because it offers no advantage over the other routes. Loading doses of about 400 mg may result in unacceptable side effects and are unnecessary for management of chronic ventricular arrhythmias. Therapeutic efficacy as well as side effects increase in proportion to increasing blood levels. At plasma levels of 2.0 mg/L, side effects are encountered in a significant number of patients. Because of its lack of serious toxicity even in patients with various systemic illnesses and its long elimination half-life, mexiletine should be considered as a first-line drug for treatment of ventricular arrhythmias.

Pharmacokinetics of intravenous mexiletine in patients with acute myocardial infarction

Acute myocardial infarction (AMI) is known to alter the pharmacokinetics of several antiarrhythmic agents. To study the effects of AMI on the kinetics of mexiletine (MEX), a single intravenous dose of 200 mg MEX HCl was infused over 30 min in 11 patients with AMI. The study was performed within 24 h of the onset of pain (study I) and repeated about 2 weeks later in seven patients at discharge (study II). MEX was quantitated in plasma and urine samples by a gas-liquid chromatographic method. The decline of MEX in plasma was three-exponential, with a terminal half-life of 14.7 +/- 3.4 (mean +/- SE) h in study I and 11.3 +/- 2.4 h (p less than 0.05) in study II, in the seven patients studied in both phases. The steady-state volume of distribution averaged 578 +/- 97 L in study I and 415 +/- 33 L in study II (p less than 0.05). The total plasma clearance, renal clearance, and recovery of MEX in urine were similar in the two studies, as was the plasma protein binding of MEX (64 +/- 2 vs. 57 +/- 3%, NS). Thus, an increase in the volume of distribution with consequent prolongation of the elimination half-life of MEX occurs in the acute phase of AMI, whereas the rate of elimination remains unchanged.