Clinical pharmacokinetics of the newer antiarrhythmic agents

Effectiveness and Safety of Different Oral Anticoagulants with P-glycoprotein/ CYP3A4 Inhibitors: A Network Meta-analysis

BACKGROUND

Metabolism of oral anticoagulants (OAC) is affected by P-glycoprotein (P-gp)/ CYP3A4 enzyme. However, the P-gp/CYP3A4 inhibitors are unavoidably used with OACs.

METHODS

Medline, Cochrane, and Embase were systematically searched for randomized controlled trials and cohort studies from inception till 23rd November, 2022 to assess the safety and effectiveness of OACs when concomitantly used with P-gp/CYP3A4 inhibitors. The primary outcomes were major bleeding and gastrointestinal (GI) bleeding. Secondary outcomes were stroke/systemic embolism (SE), all-cause mortality, any bleeding as well as intracranial hemorrhage (ICH). We estimated summary odds ratios (OR) with 95% credible intervals (CI) using pairwise and network meta-analysis with random effects.

RESULTS

A total of 11 studies involving 37,973 patients were included. When concomitantly used with P-pg/ CYP3A4 inhibitors, network meta-analysis indicated that dabigatran, apixaban, and edoxaban were associated with significantly lower risk of major bleeding compared to rivaroxaban, with ORs of 0.56, 0.51 and 0.48, respectively. Rivaroxaban and dabigatran were associated with a significantly increased risk of GI bleeding than warfarin, apixaban and edoxaban. Dabigatran and apixaban were linked with significantly lower risk of any bleeding compared with warfarin (ORs were 0.75 and 0.68, respectively) or rivaroxaban (ORs were 0.67 and 0.60, respectively). Apixaban (OR 0.32) and edoxaban (OR 0.35) were associated with a lower risk of ICH compared with warfarin. There was no difference between any OACs in terms of stroke/SE or all-cause mortality.

CONCLUSION

When concomitantly used with P-gp/CYP3A4 inhibitors, apixaban and edoxaban were associated with a lower risk of bleeding, though no significant difference in effectiveness was observed among all OACs.

Gut microbiota and cardiac arrhythmia: a pharmacokinetic scope

BACKGROUND

Dealing with cardiac arrhythmia is a difficult challenge. Choosing between different anti-arrhythmic drugs (AADs) while being cautious about the pro-arrhythmic characteristics of some of these drugs and their diverse interaction with other drugs is a real obstacle.

MAIN BODY

Gut microbiota (GM), in our bodies, are now being considered as a hidden organ which can regulate our immune system, digest complex food, and secrete bioactive compounds. Yet, GM are encountered in the pathophysiology of arrhythmia and can affect the pharmacokinetics of AADs, as well as some anti-thrombotics, resulting in altering their bioavailability, therapeutic function and may predispose to some of their unpleasant adverse effects.

CONCLUSIONS

Knowledge of the exact role of GM in the pharmacokinetics of these drugs is now essential for better understanding of the art of arrhythmia management. Also, it will help deciding when to consider probiotics as an adjunctive therapy while treating arrhythmia. This should be discovered in the near future.

Hemodynamic and antiarrhythmic effects of lidocaine or amiodarone in dogs anesthetized with halothane

The effects of continuous rate infusion of lidocaine or amiodarone on hemodynamic and arrhythmias induced by epinephrine in dogs anesthetized with halothane were evaluated. Thirty dogs were distributed into three groups: amiodarone group (AG), lidocaine group (LG), or control group (CG). Anesthesia was induced with etomidate and maintained with halothane. Thirty minutes later a bolus and continuous rate infusion (CRI) of amiodarone in AG, lidocaine in LG and NaCl at 0.9% in CG was administered. After 10 minutes, arrhythmias were induced by epinephrine CRI at 0.0001mg/kg/minute, which was increased in 0.0001mg/kg/minute every ten minutes, until 0.0003mg/kg/minute. The measurements were performed 30 minutes after the induction of anesthesia (T0), 10 minutes after beginning the drug CRIs (T1), 10 minutes after beginning epinephrine administration (T2) and 10 minutes after increasing epinephrine CRI (T3 and T4). In CG, at T3 heart rate (HR) was greater than in LG, while at T4, HR in GC was higher than in LG and AG. In LG and CG, from T2, central venous pressure (CVP), cardiac index (CI), systolic arterial pressure (SAP), diastolic arterial pressure (DAP), mean arterial pressure (MAP), mean pulmonary arterial pressure (mPAP) increased. In AG, SAP, DAP and MAP it decreased from T1 to T3. The ventricular ectopic beats (VEB) were lower in AG. Amiodarone has better antiarrhythmogenic effects, although it was also associated with hypotension.

Amiodarone and the risk of cancer: a nationwide population-based study

BACKGROUND

In postmarketing surveillance, the US Food and Drug Administration has reported the development of lung masses, thyroid cancer, and skin cancer after amiodarone therapy.

METHODS

Using the Taiwan National Health Insurance Research database, the authors conducted a population-based cohort study. Patients who were treated with amiodarone between 1997 and 2008 were enrolled. Those with antecedent cancer were excluded. Standardized incidence ratios (SIRs) of cancers were calculated to compare the cancer incidence of the study cohort with that of the general population. A multivariate Cox regression model was used to evaluate the association between cumulative defined daily doses (cDDDs) of amiodarone and cancer occurrence.

RESULTS

The study included 6418 subjects, with a median follow-up of 2.57 years. A total of 280 patients developed cancer. The risk of cancer increased with borderline significance (SIR, 1.12; 95% confidence interval [95% CI], 0.99-1.26 [P = .067]). Male patients had a higher risk (SIR, 1.18; 95% CI, 1.02-1.36 [P = .022]). The total cohort of patients and the male patients with > 180 cDDDs within the first year were found to have SIRs of 1.28 (95% CI, 1.00-1.61; P = .046) and 1.46 (95% CI, 1.11-1.89; P = .008), respectively. After adjustment for age, sex, and comorbidities, the hazards ratio was 1.98 (95% CI, 1.22-3.22; P = .006) for the high tertile of cDDDs compared with the low tertile.

CONCLUSIONS

The results of the current study indicate that amiodarone may be associated with an increased risk of incident cancer, especially in males, with a dose-dependent effect.

Digoxin is not a substrate for organic anion-transporting polypeptide transporters OATP1A2, OATP1B1, OATP1B3, and OATP2B1 but is a substrate for a sodium-dependent transporter expressed in HEK293 cells

Digoxin, an orally administered cardiac glycoside cardiovascular drug, has a narrow therapeutic window. Circulating digoxin levels (maximal concentration of ∼1.5 ng/ml) require careful monitoring, and the potential for drug-drug interactions (DDI) is a concern. Increases in digoxin plasma exposure caused by inhibition of P-glycoprotein (P-gp) have been reported. Digoxin has also been described as a substrate of various organic anion-transporting polypeptide (OATP) transporters, posing a risk that inhibition of OATPs may result in a clinically relevant DDI similar to what has been observed for P-gp. Although studies in rats have shown that Oatps contribute to the disposition of digoxin, the role of OATPs in the disposition of digoxin in humans has not been clearly defined. Using two methods, Boehringer Ingelheim, GlaxoSmithKline, Pfizer, and Solvo observed that digoxin is not a substrate of OATP1A2, OATP1B1, OATP1B3, and OATP2B1. However, digoxin inhibited the uptake of probe substrates of OATP1B1 (IC(50) of 47 μM), OATP1B3 (IC(50) > 8.1 μM), and OATP2B1 (IC(50) > 300 μM), but not OATP1A2 in transfected cell lines. It is interesting to note that digoxin is a substrate of a sodium-dependent transporter endogenously expressed in HEK293 cells because uptake of digoxin was significantly greater in cells incubated with sodium-fortified media compared with incubations conducted in media in which sodium was absent. Thus, although digoxin is not a substrate for the human OATP transporters evaluated in this study, in addition to P-gp-mediated efflux, its uptake and pharmacokinetic disposition may be partially facilitated by a sodium-dependent transporter.

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.

Early recurrence of arrhythmia in patients taking amiodarone or class 1C agents for treatment of atrial fibrillation or atrial flutter

Amiodarone use for the prevention of recurrent atrial fibrillation or atrial flutter requires drug loading over a period of several days to weeks, whereas class 1C agents do not. Three hundred thirty-nine patients were evaluated during drug loading with amiodarone or class 1C agents, and it was found that recurrent arrhythmia was common, usually not persistent, and frequently asymptomatic. Recurrent arrhythmia was not more common with amiodarone.

Development of enzyme-linked immunosorbent assay for therapeutic drug monitoring of mexiletine

This paper reports a sensitive and specific enzyme-linked immunosorbent assay for determination of the antiarrhythmic drug mexiletine in human serum. Anti-mexiletine antibody was obtained by immunizing rabbits with an antigen conjugated with mercaptosuccinyl bovine serum albumin using N-(epsilon-maleimidocaproyloxy)succinimide as a heterobifunctional coupling agent. Enzyme labeling of mexiletine with beta-D-galactosidase was performed using glutaraldehyde. In this assay, the mexiletine to be quantified is chemically modified by acetic anhydride allowed to compete with a mexiletine-beta-D-galactosidase conjugate for binding to a limited amount of an anti-mexiletine antibody which was used to coat the wells of a microtiter plate. Mexiletine concentrations lower than 80 ng/ml were measurable reproducibly by the enzyme-linked immunosorbent assay. This assay was specific for mexiletine and showed very slight cross-reactivity with its major metabolite, 2-hydroxymethylmexiletine (1.5%), but none with p-hydroxymexiletine. The values of serum mexiletine levels from 15 patients by this enzyme-linked immunosorbent assay were comparable with those measured by HPLC. There was a good correlation between the values determined by the two methods. The enzyme-linked immunosorbent assay should be a valuable tool in therapeutic drug monitoring and pharmacokinetic studies of mexiletine.

Effects of amiodarone on cardiac function and mitochondrial oxidative phosphorylation during ischemia and reperfusion

This study was carried out in order to determine if the efficiency of amiodarone, a class III antiarrhythmic agent, is associated with changes in mitochondrial oxidative phosphorylation. A population of 30 rats were treated with amiodarone (100 mg/kg/day) for 5 days. A second population receiving only vehicle was used as control. The hearts were perfused according to the working mode. After 15 min of normoxic perfusion, the left main coronary artery was ligated and the ligation was maintained for 20 min. The ligation was removed and reperfusion continued for a further 30 min. The electrocardiogram was monitored continuously. At the end of perfusion, the ischemic and non ischemic areas were visually separated and mitochondria were harvested from each area. Their oxidative and energy metabolism were assessed with palmitoylcarnitine as substrate in 2 respiration media differing in their free calcium concentration (0 or 0.34 microm). In normoxic conditions, amiodarone treatment increased the cardiac metabolic efficiency (mechanical work to oxygen consumption ratio). The local ischemia decreased the aortic and coronary flows without modifying the cardiac metabolic efficiency. Amiodarone treatment maintained the aortic flow at a significantly higher value; the duration of severe arrhythmias was significantly decreased by the drug. The reperfusion of the ischemic area allowed the partial recovery of fluid dynamics. The coronary flow was restored to 89% of the pre ischemic value. Conversely, the aortic flow never exceeded that measured at the end of ischemia, partly due to the important development of severe arrhythmias. The recovery of aortic flow and metabolic efficiency during reperfusion was improved by amiodarone treatment; ventricular tachycardia and fibrillation duration were reduced. In the mitochondria issued from the normoxic area, the energy metabolism was not altered by the amiodarone treatment, but the presence of calcium in the respiration medium modified the oxidative phosphorylation. The divalent cation slightly decreased the state III respiration rate and increased noticeably the state IV respiration rate. This was associated with an important mitochondrial AMP production and maintenance of ADP in the respiration medium. This energy wasting was reported to decrease the mitochondrial metabolic efficiency. After an ischemia-reperfusion sequence, mitochondrial oxidation phosphorylation was reduced and amiodarone treatment amplified this decrease. This was presumably due to an increased mitochondrial calcium accumulation. Thus, the beneficial properties of amiodarone during reperfusion are supposed to be due to a protection against the deleterious effect of excess matrix calcium on mitochondrial energy metabolism.

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.

Pharmacokinetics and metabolism of dofetilide in mouse, rat, dog and man

1. Pharmacokinetics of dofetilide were studied in man, dog, rat and mouse after single i.v. and oral doses of dofetilide or 14C-dofetilide. 2. Dofetilide was absorbed completely in all species. Low metabolic clearance in man resulted in complete bioavailability following oral administration. Higher metabolic clearance in rodents, and to a lesser extent dogs, resulted in decreased bioavailability because of first-pass metabolism. 3. Following i.v. administration, the volume of distribution showed only moderate variation in all species (2.8-6.3 l/kg). High plasma clearance in rodents resulted in short half-life values (mouse 0.32, male rat 0.5 and female rat 1.2 h), whilst lower clearance in dog and man gave longer terminal elimination half-lives (4.6 and 7.6 h respectively). 4. After single i.v. doses of 14C-dofetilide, unchanged drug was the major component excreted in urine of all species with several metabolites also present. 5. Metabolites identified in urine from all species were formed by N-oxidation or N-dealkylation of the tertiary nitrogen atom of dofetilide. 6. After oral and i.v. administration of 14C-dofetilide to man, parent compound was the only detectable component present in plasma and represented 75% of plasma radioactivity. No single metabolite accounted for greater than 5% of plasma radioactivity.

A dose-ranging study of UK-68,798, a novel class III anti-arrhythmic agent, in normal volunteers

1. UK-68,798, a novel class III anti-arrhythmic agent was administered intravenously to twelve healthy volunteers in a placebo controlled, double-blind, dose-escalating study. 2. Doses of 5 and 10 micrograms kg-1 of UK-68,798 selectively and significantly prolonged the QT interval, with mean maximum changes of 35 and 107 ms respectively, without affecting other ECG intervals. 3. There were dose-related increases in AUC but clearance (23 l h-1), terminal elimination half-life (8 h) and volume of distribution (245 l) were found to be independent of dose with low levels of intra- and inter-patient variability. 4. UK-68,798 has electrophysiological effects indicative of selective class III anti-arrhythmic activity and merits further assessment in clinical studies.

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)

Pharmacokinetics of newer drugs in patients with renal impairment (Part II)

Cardiovascular diseases occur frequently in patients with renal failure. Any pharmacokinetic impairment in these diseases should be considered when individualizing drug therapy. The pharmacokinetics of new cardiovascular drugs in uraemic patients are reviewed: alpha- and beta-blocking agents, ACE inhibitors, centrally acting antihypertensive agents, calcium antagonists, antiarrhythmic agents and inotropic agents. Guidelines are proposed for adjustment of dosage regimens as a function of renal impairment. Renal or extrarenal elimination of drugs and their metabolites, and the activity of the latter, are taken into account. The disposition of new drugs such as flestolol, alacepril, delapril, propafenone, milrinone or enoximone, is not well documented in patients with renal failure. Further characterizations of the elimination of these compounds are needed and the potential therapeutic or toxic effects of the metabolites require evaluation to determine whether the dosage needs to be adjusted. Until such investigations are performed, those drugs should not be used in uraemic patients; if no therapeutic alternative is available, clinical controls are necessary at regular intervals. Relationships between pharmacological or therapeutic effects and drug plasma concentrations should be evaluated for such long term use drugs. The knowledge of a plasma concentration therapeutic window is important to provide information which will be useful in determining appropriate drug dosage in renal failure.

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.

Altered flecainide disposition in healthy volunteers taking quinine

The pharmacokinetics of flecainide and its two sequential metabolites, both free and conjugated, its pharmacodynamics, and the influence of simultaneously administered quinine, have been studied in 10 healthy subjects. The study comprised two, 48-h open phases at an interval of 1 week. Flecainide acetate 150 mg was given as a 30-min i.v. infusion and quinine sulphate orally 500 mg x 3 over 24 h. Quinine administration did not change the apparent volume of distribution or the renal clearance of flecainide, but it significantly reduced its systemic clearance (9.1 vs 7.6 ml.kg-1.min-1), thus increasing the elimination half-life (9.6 vs 11.5 h). The amount of flecainide transformed to its first, meta-O-dealkylated metabolite (MODF) fell with no change in the renal excretion of the latter, either in its free or conjugated forms. This finding, in association with a fall in amount of the second, meta-O-dealkylated lactam metabolite (MODLF) recovered in its conjugated forms in the urine, suggests that quinine inhibits both the first and the second steps of the sequential metabolism of flecainide. When the subjects received quinine in addition to flecainide, the PR interval in the ECG was slightly more prolonged than with flecainide alone. Due to the study design, an effect of quinine per se and the consequence of increased serum flecainide levels could not be distinguished.

Resolution of enantiomers of the antiarrhythmic drug encainide and its major metabolites by chiral derivatization and high-performance liquid chromatography

Commercially available chiral columns were unable to provide adequate resolution of enantiomers of the antiarrhythmic drug encainide or its major metabolites. The homochiral derivatizing agent, (-)-menthyl chloroformate, was found to react at the tertiary piperidine nitrogen of racemic encainide providing two menthyl carbamate diastereomers. The individual diastereomers could be separated with baseline resolution on normal-phase high-performance liquid chromatography on a silica column. Structures of the derivatives were confirmed by electron impact mass spectrometry and 1H NMR spectroscopy. The method was adapted for the chiral analysis of the major metabolites of encainide. The limit of sensitivity for racemic encainide was 10 ng on column and it was possible to detect a mixture containing (+)- and (-)-encainide in a ratio of 1:99. Preliminary studies indicated that (-)-encainide was O-demethylated to a greater extent than the (+)-enantiomer by rat liver microsomes.

Pharmacokinetic interaction between diltiazem and amiodarone in the dog

The pharmacokinetic interaction between diltiazem and amiodarone was investigated in dogs. In the presence of amiodarone, diltiazem's AUC values were significantly increased and its total body clearance and volume of distribution at steady-state significantly decreased. This study indicates that cardiac patients on combined diltiazem-amiodarone therapy may indeed be in a high risk situation in regards to the unexpectedly high blood levels of diltiazem induced: with the ultimate introduction of such side-effects as (1) the lowering of blood pressure, (2) A/V block, and (3) sinus node depression. Such cases would require immediate dosage adjustment. Assuming that the data obtained in this study can be extrapolated to humans, a patient's physiological parameters should be monitored at periodic intervals and, more importantly, the patient should report the first sign of any untoward effect.

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.

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.

Pharmacokinetic implications of lorcainide therapy in patients with normal and depressed cardiac function

The influence of cardiac function as measured by the left ventricular ejection fraction on the pharmacokinetic variables of a new antiarrhythmic drug, lorcainide, was investigated in 20 cardiac patients. Patients were divided into two groups: those with normal (ejection fraction greater than .40) or depressed (ejection fraction less than .40) left ventricular function. The elimination half-life, plasma clearance rates, or volume of distribution of lorcainide were not significantly different in patients with either normal or depressed cardiac function. A decrease in arrhythmia frequency could be correlated to plasma lorcainide concentration in the majority of patients, and it was noted that at least 0.1 mg/L of lorcainide was required for the presence of an antiarrhythmic effect. Three unusual cases are presented to illustrate the importance of measuring plasma drug concentrations and calculating the drug pharmacokinetics and to correlate these to the antiarrhythmic response in order to minimize the risk of plasma drug accumulation and side effects. A review of published data shows a three- to sixfold interpatient variation in the elimination half-life of lorcainide with practical implications in its use as an antiarrhythmic drug.

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.

Propafenone for the treatment of refractory complex ventricular ectopic activity

The results of therapy with propafenone were evaluated in 45 patients with complex ventricular ectopic activity that had been refractory to a mean of 3.8 antiarrhythmic drugs. The cardiac diagnoses were ischemic heart disease (in 16 patients), cardiomyopathy (in 7), mitral valve prolapse (in 7), mitral valve prolapse (in 7), idiopathic ventricular ectopic beats (in 6), valvular heart disease (in 5), and hypertension (in 4). The frequency of ventricular ectopic beats was established after therapy with antiarrhythmic agents had been discontinued. Patients then received propafenone during a dose-ranging protocol. An effective response was defined as a reduction in total ventricular ectopic beats of 80% or more. During dose ranging, therapy failed in four patients because of side effects, in eight because of a reduction in ventricular ectopic beats of less than 80%, and in three because of an aggravation of the arrhythmia. Thirty patients had a reduction in total ventricular ectopic beats of 80% or more. During a mean follow-up of 12.4 months, therapy failed in 1 patient because of sustained ventricular tachycardia and in 7 because of intolerable side effects; 22 patients continued to receive propafenone. PR and QRS intervals were significantly prolonged (P = 0.001), but the corrected QT interval and the heart rate were unchanged. The mean trough plasma level of propafenone associated with an effective response was 756 ng/ml, and that associated with intolerable side effects was 920 ng/ml. Thus, in patients with refractory complex ventricular ectopic beats, propafenone was effective and well tolerated initially in 67% of patients and during long-term administration in 49%, and toxicity was minor in most patients.

A seizure induced by concurrent lidocaine-tocainide therapy--is it just a case of additive toxicity?

A 79-year-old white male was admitted to the hospital for treatment of a right-lower-lobe pneumonia. His past medical history included: mild congestive heart failure, asymptomatic ventricular tachycardia, and ethanol abuse. He was initially treated with furosemide for his heart failure, lidocaine for his arrhythmias, and Bactrim for his pneumonia. On day 13 of hospitalization he experienced a tonic-clonic seizure during the time he was being converted from lidocaine to tocainide. At the time of the seizure both tocainide and lidocaine were well within their respective therapeutic ranges. Since the seizure, the patient has tolerated treatment with each drug separately, and at serum concentrations similar to those preceding the seizure, without neurological complications, indicating the possibility of a tocainide-lidocaine induced seizure.