Quinidine

Long QT syndrome: Genetic implications and drug influence

<zakljucak> Produzeni QT interval predstavlja problem u klinickoj praksi kome se posvecuje sve veca paznja i koji je oblast intenzivnog istrazivanja. Genetske studije kod pojedinaca koji imaju klinicki izrazene simptome produzenog QT intervala i/ili pozitivnu porodicnu anamnezu za iznenadnu srcanu smrt, mogu za pocetak da budu ogranicene na KCNQ1, HERG i SCN5A gene sa verovatnocom otkrivanja mutacije od 65%. Predstojece studije tek treba da objasne znacaj polimorfizma odredjenih gena za pojavu produzenja QT intervala.

Potassium channel blockers quinidine and caesium halt cell proliferation in C6 glioma cells via a polyamine-dependent mechanism

Potassium channels are ubiquitous in cells and serve essential functions in physiology and pathophysiology. Potassium channel blockers have been shown to block tumour growth by arresting cells at the G(0)/G(1) checkpoint of the cell cycle. We investigated the effect of quinidine and caesium (Cs(+)) on cell proliferation, LDH (lactate dehydrogenase) release, free internal calcium, membrane potential, polyamine concentration, ODC (ornithine decarboxylase) activity and polyamine uptake in C6 glioma cells. The EC(50) for reducing cell proliferation was 112 microM for quinidine, whereas Cs(+) was less effective with an EC(50) of 4.75 mM. KCl or sucrose did not affect proliferation. LDH release was augmented by quinidine. Quinidine caused a transient increase in free internal calcium but decreased calcium after a 48 h incubation period. Further 300 microM quinidine depolarized the cell membrane in a similar range as did 30 mM KCl. Quinidine decreased cellular putrescine beyond detection levels while spermidine and spermine remained unaffected. ODC activity was reduced. Addition of putrescine could not override the antiproliferative effect owing to a reduced activity of the polyamine transporter. Our study indicates that the antiproliferative effect of quinidine is not due to a simple membrane depolarization but is caused by a block of ODC activity.

Dextromethorphan as a Potential Neuroprotective Agent With Unique Mechanisms of Action

BACKGROUND

Dextromethorphan (DM) is a widely-used antitussive. DM's complex central nervous system (CNS) pharmacology became of interest when it was discovered to be neuroprotective due to its low-affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonism.

REVIEW SUMMARY

Mounting preclinical evidence has proven that DM has important neuroprotective properties in various CNS injury models, including focal and global ischemia, seizure, and traumatic brain injury paradigms. Many of these protective actions seem functionally related to its inhibitory effects on glutamate-induced neurotoxicity via NMDA receptor antagonist, sigma-1 receptor agonist, and voltage-gated calcium channel antagonist actions. DM's protection of dopamine neurons in parkinsonian models may be due to inhibition of neurodegenerative inflammatory responses. Clinical findings are limited, with preliminary evidence indicating that DM protects against neuronal damage. Negative findings seem to relate to attainment of inadequate DM brain concentrations. Small studies have shown some promise for treatment of perioperative brain injury, amyotrophic lateral sclerosis, and symptoms of methotrexate neurotoxicity. DM safety/tolerability trials in stroke, neurosurgery, and amyotrophic lateral sclerosis patients demonstrated a favorable safety profile. DM's limited clinical benefit is proposed to be associated with its rapid metabolism to dextrorphan, which restricts its central bioavailability and therapeutic utility. Systemic concentrations of DM can be increased via coadministration of low-dose quinidine (Q), which reversibly inhibits its first-pass elimination. Potential drug interactions with DM/Q are discussed.

CONCLUSIONS

Given the compelling preclinical evidence for neuroprotective properties of DM, initial clinical neuroprotective findings, and clinical demonstrations that the DM/Q combination is well tolerated, this strategy may hold promise for the treatment of various acute and degenerative neurologic disorders.

Cardiotoxicity of antimalarial drugs

There are consistent differences in cardiovascular state between acute illness in malaria and recovery that prolong the electrocardiographic QT interval and have been misinterpreted as resulting from antimalarial cardiotoxicity. Of the different classes of antimalarial drugs, only the quinolines, and structurally related antimalarial drugs, have clinically significant cardiovascular effects. Drugs in this class can exacerbate malaria-associated orthostatic hypotension and several have been shown to delay ventricular depolarisation slightly (class 1c effect), resulting in widening of the QRS complex, but only quinidine and halofantrine have clinically significant effects on ventricular repolarisation (class 3 effect). Both drugs cause potentially dangerous QT prolongation, and halofantrine has been associated with sudden death. The parenteral quinoline formulations (chloroquine, quinine, and quinidine) are predictably hypotensive when injected rapidly, and cardiovascular collapse can occur with self-poisoning. Transiently hypotensive plasma concentrations of chloroquine can occur when doses of 5 mg base/kg or more are given by intramuscular or subcutaneous injection. At currently recommended doses, other antimalarial drugs do not have clinically significant cardiac effects. More information on amodiaquine, primaquine, and the newer structurally related compounds is needed.

Ionic basis of pharmacological therapy in Brugada syndrome

An implantable cardioverter-defibrillator is considered the only effective therapy to terminate ventricular arrhythmias in symptomatic patients with Brugada syndrome. However, it does not prevent future arrhythmic episodes. Only antiarrhythmic drug therapy can prevent them. There have been several reports of a beneficial effect of oral quinidine in both asymptomatic and symptomatic patients. Other possible beneficial oral agents could be I(to) blockers. Intravenous isoproterenol has been reported to be especially useful in abolishing arrhythmic storms in emergency situations. Also, isolated case reports on the usefulness of cilostazol, sotalol, and mexiletine have been described. The present article reviews the mechanisms by which these drugs may act and their possible role in the pharmacotherapy of this disease.

Dextromethorphan and quinidine in adult patients with uncontrolled painful diabetic peripheral neuropathy: a 29-day, multicenter, open-label, dose-escalation study

BACKGROUND

Pain associated with diabetic peripheral neuropathy (DPN) has a substantial negative impact on patients' quality of life.

OBJECTIVES

The primary objective of this study was to evaluate the tolerability of capsules containing dextromethorphan (DM) and quinidine (Q) in patients with painful DPN. A secondary objective was to perform a preliminary assessment of the efficacy of DM/Q in this patient population.

METHODS

This was a multicenter, open-label, dose-escalation study. Eligible patients were aged between 18 and 80 years, had a confirmed diagnosis of diabetes with acceptable glycemic control, had been receiving established diabetic therapy for at least 3 months, and had a clinical diagnosis of distal symmetric sensory neuropathy with daily DPN-associated pain for the previous 3 months. On study entry, patient-rated diabetic pain had to be moderate or greater. Patients who met the inclusion criteria underwent a 2-week washout period during which all analgesics were discontinued, followed by 29 days of treatment with capsules containing DM 30 mg and Q 30 mg (DM30/Q30), beginning with 1 capsule/d and escalating at approximately 1-week intervals, as tolerated, to a maximum dose of 4 capsules/d (DM120/Q120). Tolerability was assessed based on adverse events and changes in clinical and laboratory parameters and nerve conduction velocity. Preliminary efficacy assessments included changes from baseline in scores on the pain intensity rating scale (PIRS), pain relief rating scale (PRRS), peripheral neuropathy quality-of-life instrument, and patients' diary assessments of sleep, present pain intensity, pain, and activity.

RESULTS

The study included 36 men and women (mean age, 58 years; mean body mass index, 32.8 kg/m(2)). Of the 33 subjects who completed the study, 23 (69.7%) did so at the highest permitted dose (DM120/Q120). The most commonly reported adverse events (occurring in > or =5% of subjects) were nausea (27.8%), dizziness (25.0%), and headache (25.0%). Three patients experienced 5 serious adverse events, only 1 of which was considered possibly related to study drug. The most commonly occurring laboratory abnormalities (involving glycosylated hemoglobin, serum glucose, triglycerides, and cholesterol) were considered typical of a population with diabetes. Improvements from baseline in scores on the PIRS, PRRS, and other exploratory efficacy measures were noted (P < 0.001).

CONCLUSIONS

The results of this open-label study indicated that the combination of DMIQ (dose range, DM30/Q30-DM120/Q120) was well tolerated in patients with pain associated with DPN. Based on the preliminary efficacy results, a randomized, controlled, double-blind trial is warranted to assess the tolerability and efficacy of this combination in patients with DPN.

Dextromethorphan/quinidine: a novel dextromethorphan product for the treatment of emotional lability

Dextromethorphan (DM) is among the most widely used, non-narcotic antitussives, with a predictable safety profile. In 1981, a non-opioid, high-affinity brain recognition site for DM was discovered, and since then a unique neuropharmacological profile has emerged for this 'old' drug , suggesting novel applications. However, an extensive body of research for DM alone in treating various neurological conditions has been inconsistent. This may be largely due to its rapid first-pass metabolism. DM is currently being reintroduced as the active ingredient in a novel combination product in which low-dose quinidine is added to inhibit its breakdown, elevating blood levels of DM and increasing its likelihood of reaching neuronal targets . This has opened new possibilities for therapeutic use; the best evidence at present being for neurological disorders affecting emotional control.

A rabbit Langendorff heart proarrhythmia model: predictive value for clinical identification of Torsades de Pointes

BACKGROUND AND PURPOSE

The rabbit isolated Langendorff heart model (SCREENIT) was used to investigate the proarrhythmic potential of a range of marketed drugs or drugs intended for market. These data were used to validate the SCREENIT model against clinical outcomes.

EXPERIMENTAL APPROACH

Fifty-five drugs, 3 replicates and 2 controls were tested in a blinded manner. Proarrhythmia variables included a 10% change in MAPD(60), triangulation, instability and reverse frequency-dependence of the MAP. Early after-depolarisations, ventricular tachycardia, TdP and ventricular fibrillation were noted. Data are reported at nominal concentrations relative to EFTPC(max). Proarrhythmic scores were assigned to each drug and each drug category.

KEY RESULTS

Category 1 and 2 drugs have the highest number of proarrhythmia variables and overt proarrhythmia while Category 5 drugs have the lowest, at every margin. At 30-fold the EFTPC(max), the mean proarrhythmic scores are: Category 1, 101+/-24; Category 2, 101+/-14; Category 3, 72+/-20; Category 4, 59+/-16 and Category 5, 22+/-9 points. Only drugs in Category 5 have mean proarrhythmic scores, below 30-fold, that remain within the Safety Zone.

CONCLUSIONS AND IMPLICATIONS

A 30-fold margin between effects and EFTPC(max) is sufficiently stringent to provide confidence to proceed with a new chemical entity, without incurring the risk of eliminating potentially beneficial drugs. The model is particularly useful where compounds have small margins between the hERG IC(50) and predicted EFTPC(max). These data suggest this is a robust and reliable assay that can add value to an integrated QT/TdP risk assessment.

Voltage-gated K+ channels support proliferation of colonic carcinoma cells

Plasma membrane potassium (K+) channels are required for cell proliferation. Evidence is growing that K+ channels play a central role in the development and growth of human cancer. Here we examine the contribution and the mechanism by which K+ channels control proliferation of T84 human colonic carcinoma cells. Numerous K+ channels are expressed in T84 cells, but only voltage-gated K+ (Kv) channels influenced proliferation. A number of Kv channel inhibitors reduced DNA synthesis and cell number, without exerting apoptotic or toxic effects. Expression of several Kv channels, such as EagI, Kv 3.4 and Kv 1.5, was detected in patch clamp experiments and in fluorescence-based assays using a voltage sensitive dye. The contribution of EagI channels to proliferation was confirmed by siRNA, which abolished EagI activity and inhibited cell growth. Inhibition of Kv channels did not interfere with the ability of T84 cells to regulate their cell vol, but it restricted intracellular pH regulation. In addition, inhibitors of Kv channels, as well as siRNA for EagI, attenuated intracellular Ca2+ signaling. The data suggest that Kv channels control proliferation of colonic cancer cells by affecting intracellular pH and Ca2+ signaling.

Pharmacokinetic analysis of transcellular transport of quinidine across monolayers of human intestinal epithelial Caco-2 cells

To investigate the mechanism responsible for the intestinal absorption of a lipophilic organic cation, quinidine, we performed a pharmacokinetic analysis of transcellular transport across Caco-2 cell monolayers grown on a porous membrane. Basolateral-to-apical transport of the drug was almost constant in the concentration range of 100 nM-100 microM. Transcellular transport was greater in the apical-to-basolateral direction than in the opposite direction. Apical-to-basolateral transport was greater at a concentration of 100 microM than 100 nM. The calculated influx clearance value of the apical membrane was much greater than the other influx/efflux clearance values of cell membranes, and was 5.6-fold the influx clearance value of the basolateral membrane at the drug concentration of 100 microM. We also investigated the uptake of quinidine at the apical membrane of Caco-2 cells grown on plastic dishes. The uptake was markedly increased by alkalization of the apical medium at 37 degrees C, and was decreased at low temperature (4 degrees C). In addition, it was inhibited by diphenhydramine and levofloxacin, but not by carvedilol, rifamycin SV, or L-carnitine. These findings indicated that the influx at the apical membrane was the direction-determining step in the transcellular transport of quinidine across Caco-2 cell monolayers, and that some specific transport system was involved in this influx.

Atrial Fibrillation: Modern Concepts and Management

Atrial fibrillation (AF) is a common cardiac arrhythmia responsible for significant morbidity and mortality. In recent years, progress has been made in determining the genetic abnormalities that may lead to AF. New trials have shown that rate control and anticoagulation are acceptable as a primary treatment strategy in many patients who have a high risk of recurrence. Newer and safer antiarrhythmics are now available. Pacemaker and implantable cardiac defibrillator technology is rapidly evolving and may play a significant role in future treatment and prevention of AF. Direct thrombin inhibitors are likely to add a user-friendly option to the current standard therapy for stroke prevention.

Pharmacokinetics of dextromethorphan after single or multiple dosing in combination with quinidine in extensive and poor metabolizers

Dextromethorphan (DM) pharmacological properties predict that the widely used cough suppressant could be used to treat several neuronal disorders, but it is rapidly metabolized after oral dosing. To find out whether quinidine (Q), a CYP2D6 inhibitor, could elevate and prolong DM plasma profiles, 2 multiple-dose studies identified the lowest oral dose of Q that could be used in a fixed combination with 3 doses of DM. A multiple-dose study in healthy subjects with an extensive or a poor enzyme metabolizer phenotype evaluated the safety and pharmacokinetic profile of a selected fixed-dose combination (AVP-923). Study 1 randomized 46 healthy subjects, who were extensive CYP2D6 metabolizers, to receive 0, 2.5, 10, 25, 50, or 75 mg Q twice daily in combination with 30 mg DM for 7 days. Plasma and urine samples were collected after the first and last doses for the assay of DM, dextrorphan (DX), and Q. Study 2 randomized 65 healthy extensive CYP2D6 metabolizers to 8 groups given twice-daily 45- or 60-mg DM doses combined with 0, 30, 45, or 60 mg Q for 7 days. The effects of increasing Q were not different with doses greater than 25 mg, whereas lower doses showed a dose-related increase in plasma DM concentrations. Urinary ratios of DM/DX showed a Q dose- and time-related increase in the number of subjects converted to the poor metabolizer phenotype that reached 100% on day 3 of dosing with 25 mg Q. Results from both studies indicated that 25 to 30 mg Q is adequate to maximally suppress O-demethylation of DM. Study 3 evaluated 7 extensive metabolizers and 2 poor metabolizers given an oral capsule every 12 hours containing 30 mg Q combined with 30 mg DM. DM plasma AUC values increased in both groups of subjects during the 8-day study. The mean urinary metabolic ratio (DM/DX) increased at least 27-fold in extensive metabolizers by day 8. There was no effect of Q on urinary metabolic ratios in poor metabolizers. Safety evaluations, including electrocardiograms, indicated that the combination was well tolerated, with no difference between extensive and poor metabolizer phenotypes.

Enantiodivergent Formal Synthesis of (+)- and (−)-Cyclophellitol from d-Xylose Based on the Latent Symmetry Concept

Formal synthesis of (+)- and (-)-cyclophellitol from d-xylose has been accomplished through utilization of the latent plane of chirality present in the starting carbohydrate. The synthetic pathway is suitable for preparation and biological evaluation of cyclophellitol analogues in both enantiomeric series.

Antiarrhythmic drug therapy of atrial fibrillation

In the post-AFFIRM era, treatment of AF has become the treatment of symptoms. In some patients, this will be simple rate control, but there remain a significant cohort of patients in whom rate control alone does not give acceptable symptom relief. In this group, antiarrhythmic therapy still has a role, and the AFFIRM trial indicates that this therapeutic strategy is without significant deleterious effect on mortality. The choice of antiarrhythmic agent must be individualized according to underlying cardiac pathologies and comorbidities, however. Most recently, the introduction of dofetilide has widened the therapeutic options in patients with severe heart disease, and the Canadian Trial of Atrial Fibrillation indicated the superior efficacy of amiodarone at low doses. The release/ development of newer Class III antiarrhythmic agents may offer hope for the benefits of amiodarone without the serious adverse effects with long-term therapy.

Voltage-dependent profile of human ether-a-go-go-related gene channel block is influenced by a single residue in the S6 transmembrane domain

Many common medications block delayed rectifier K(+) channels and prolong the duration of cardiac action potentials. Here we investigate the molecular mechanisms of voltage-dependent block of human ether-a-go-go-related gene (HERG) delayed rectifier K(+) channels expressed in Xenopus laevis oocytes by quinidine, an antiarrhythmic drug, and vesnarinone, a cardiotonic drug. The IC(50) values determined with voltage-clamp pulses to 0 mV were 4.6 microM and 57 microM for quinidine and quinine, respectively. Block of HERG by quinidine (and its isomer quinine) was enhanced by progressive membrane depolarization and accompanied by a negative shift in the voltage dependence of channel activation. As reported previously for other HERG blockers (e.g., MK-499, cisapride, terfenadine, chloroquine), the potency of quinidine was reduced >100-fold by the mutation of key aromatic residues (Y652, F656) located in the S6 domain. Mutations of Y652 eliminated (Y652F) or reversed (Y652A) the voltage dependence of HERG channel block by quinidine and quinine. These quinolines contain a charged N atom that might bond with Y652 by a cation-pi interaction. However, similar changes in the voltage-dependent profile for block of Y652F or Y652A HERG channels were observed with vesnarinone, a cardiotonic drug that is uncharged at physiological pH. Together, these results suggest that voltage-dependent block of HERG results from gating-dependent changes in the orientation of Y652, a critical component of the drug binding site, and not from a transmembrane field effect on a charged drug molecule.

Antiarrhythmic drugs: new agents and evolving concepts

Properties of several new antiarrhythmic drugs are summarised in this review article. Recent concepts concerning their safety and efficacy of antiarrhythmics are discussed. A brief perspective on possible future strategies for pharmacotherapy of arrhythmias is provided.

Psychotropic drugs, cardiac arrhythmia, and sudden death

A variety of drugs targeted towards the central nervous system are associated with cardiac side effects, some of which are linked with reports of arrhythmia and sudden death. Some psychotropic drugs, particularly tricyclic antidepressants (TCAs) and antipsychotic agents, are correlated with iatrogenic prolongation of the QT interval of the electrocardiogram (ECG). In turn, this is associated with the arrhythmia (TdP). This review discusses the association between psychotropic agents, arrhythmia and sudden death and, focusing on TCAs and antipsychotics, considers their range of cellular actions on the heart; potentially pro-arrhythmic interactions between psychotropic and other medications are also considered. At the cellular level TCAs, such as imipramine and amitriptyline, and antipsychotics, such as thioridazine, are associated with inhibition of potassium channels encoded by In many cases this cellular action correlates with ECG changes and a risk of TdP. However, not all psychotropic agents that inhibit HERG at the cellular level are associated equally with QT prolongation in patients, and the potential for QT prolongation is not always equally correlated with TdP. Differences in risk between classes of psychotropic drugs, and between individual drugs within a class, may result from additional cellular effects of particular agents, which may influence the consequent effects of inhibition of repolarizing potassium current.

KV2.1 K+ channels underlie major voltage-gated K+ outward current in H9c2 myoblasts

The H9c2 clonal cell line derived from embryonic rat ventricle is an in vitro model system for cardiac and skeletal myocytes. We used the whole-cell patch clamp technique to characterize the electrophysiological and pharmacological properties of an outward K+ current (IK(V)) and determined its molecular correlate in H9c2 myoblasts. IK(V) was activated by threshold depolarization to -30 mV, and its current amplitude and rate of activation increased with further depolarizations. IK(V) inactivated slowly with a time constant of 1-2 s, and the V(0.5) for steady-state inactivation was -37.9 +/- 4.6 mV (n = 10). Tetraethylammonium and quinidine suppressed IK(V) with IC(50)'s of 3.7 mM and 11.6 microM, respectively. Using RT-PCR analysis we found that the K(V )2.1 gene is the most abundantly expressed among genes for K(V)1.2, 1.4, 1.5, 2.1, 4.2, and 4.3, and by Western blotting we confirmed the synthesis of the K(V)2.1 alpha-subunit protein. We conclude that IK(V), the predominant voltage-gated outward current in H9c2 myoblasts, flows through the channels comprised of the K(V)2.1-subunit gene product.

Mode-dependent inhibition by quinidine of Na+-Ca2+ exchanger current from guinea-pig isolated ventricular myocytes

1. The aim of the present study was to determine whether or not the class Ia anti-arrhythmic agent quinidine modulates ionic current (INaCa) generated by the sarcolemmal Na+-Ca2+ exchanger of adult ventricular cardiac myocytes. 2. Selective whole-cell voltage-clamp recordings of INaCa were made from guinea-pig ventricular myocytes, with major interfering currents blocked. The INaCa was measured as the ionic current sensitive to 10 mmol/L external Ni2+ during a descending voltage ramp protocol. 3. The effects of quinidine concentrations in the range 10-100 micromol/L were studied. Quinidine produced a concentration-dependent partial blockade of outward INaCa, generated by reverse-mode exchange. At +60 mV, 100 micromol/L quinidine blocked INaCa by 33.0 +/- 4.1% (mean+/-SEM; n = 4). This was the maximal concentration that we were able to test, because concentrations of quinidine higher than 100 micromol/L were found to be toxic to cells under our conditions. The drug did not produce any significant inhibition of inward INaCa (generated by forward-mode exchange). 4. The Ni2+-insensitive residual current was not significantly altered by quinidine at any membrane potential, confirming that the inhibitory effects of quinidine we observed could be attributed to an action on the Na+-Ca2+ exchanger. 5. For the purpose of comparison, quinidine was tested against L-type Ca current (ICa,L). It blocked peak ICa,L at 0 mV, with an IC50 of 14.9 +/- 1.5 micromol/L. Thus, quinidine was less potent against the exchanger than against ICa,L. 6. Our data suggest that quinidine preferentially inhibits the Naout/Cain mode of exchanger function. We conclude that this drug is a weak inhibitor of ventricular INaCa and that the inhibitory effect is mode dependent.

Serum quinidine concentrations and effect on QT dispersion and interval

OBJECTIVE

To establish a relationship between serum quinidine concentrations (SQCs) and QT interval dispersion, compared with corresponding QT intervals, in order to identify a reason why many reports describe torsade de pointes as occurring at subtherapeutic concentrations.

DESIGN

Retrospective study.

SETTING

University teaching hospital.

PARTICIPANTS

Eleven patients with atrial arrhythmias managed with quinidine therapy.

MAIN OUTCOME MEASURES

Patients with subtherapeutic (<2 microg/mL) and therapeutic (2-5 microg/mL) SQCs with corresponding 12-lead electrocardiograms (ECGs) (25 mm/sec) and baseline ECG were evaluated for QT interval dispersion, calculated as the maximum minus the minimum QT interval on the 12-lead ECG.

RESULTS

Mean +/- SD subtherapeutic and therapeutic SQCs were 1.48 +/- 0.39 microg/mL and 3.78 +/- 0.88 microg/mL (p < 0.001). Baseline values for QT/QTc intervals were 376.4 +/- 59.2/429.5 +/- 57.3 msec. At subtherapeutic and therapeutic SQCs, mean QT/QTc intervals were 403.6 +/- 59.9/450.5 +/- 38.5 msec and 439.1 +/- 48.9/472.4 +/- 44.6 msec, respectively. Mean QT dispersion was 47 +/- 16.2 msec at baseline, 98.2 +/- 27.5 msec at subtherapeutic SQC, and 70.9 +/- 33.9 msec at therapeutic SQCs (p = 0.001 for overall analysis; p < 0.001 for baseline vs. subtherapeutic concentrations; p = NS for therapeutic vs. subtherapeutic in post hoc comparison).

CONCLUSIONS

Despite QT interval lengthening with increasing SQCs, QT dispersion was numerically greatest at subtherapeutic SQCs. Further study is required to determine the value of QT dispersion as a tool for identifying proarrhythmic risk with drugs that prolong the QT interval.

Influence of the ORM1 phenotypes on serum unbound concentration and protein binding of quinidine

BACKGROUND

Only unbound or free drug in plasma can be transported to its site of action. The fraction of unbound drug in plasma varies widely for highly bound drugs among individuals. The genetic polymorphism of orosomucoid (ORM) could be related to the interindividual variability in plasma binding of basic drugs, as ORM is the transport protein for these drugs in plasma. The ORM is a major binding protein in plasma for various basic drugs and is coded by two loci, ORM1 and ORM2, which are closely linked on chromosome 9q31-->34.1. ORM1 locus is highly polymorphic and the ORM2 locus is monomorphic in most population.

METHODS

Twenty-eight healthy volunteers were selected with three ORM1 phenotypes, containing homozygotes ORM1 F1 (n=10) and ORM1 S (n=8), and heterozygote ORM1 F1S (n=10), identified by isoelectric focusing on polyacrylamide gels followed immunoblotting after desialylation of sera. After a single oral dose of quinidine 200 mg, serum total (HPLC) and unbound concentrations in ultrafiltrate (ultrafiltration/HPLC) were determined, and the pharmacokinetic parameters and protein binding rate were calculated.

RESULTS

Serum concentrations of ORM (553.8-573.2 mg/l) and albumin proteins (57.5-58.4 mg/l) were similar in the three groups (P>0.05). Unbound quinidine concentration in ORM1 F1 phenotype subjects was higher than that in ORM1 S and ORM1 F1S phenotype; the free drug percentage for the subjects with ORM1 F1 phenotype (19.79%) was twice as high as that with ORM1 S phenotype (10.96%) (P<0.01) at 24 h after administration of oral quinidine when the state of disposition equilibrium occurred. The elimination t(1/2) values and the other pharmacokinetic parameters of quinidine were not affected by the different ORM1 phenotypes.

CONCLUSIONS

Different ORM1 phenotypes may affect the disposition of quinidine, a basic drug, rather than its hepatic metabolism and elimination. The functional heterogeneity of ORM1 could be responsible for the differences in plasma binding of quinidine. Therefore, monitoring of the unbound quinidine concentration would be important for the patients with different ORM1 phenotypes who are treated with quinidine.

Ionic basis of the resting membrane potential and action potential in the pharyngeal muscle of Caenorhabditis elegans

The pharynx of C. elegans is a rhythmically active muscle that pumps bacteria into the gut of the nematode. This activity is maintained by action potentials, which qualitatively bear a resemblance to vertebrate cardiac action potentials. Here, the ionic basis of the resting membrane potential and pharyngeal action potential has been characterized using intracellular recording techniques. The resting membrane potential is largely determined by a K(+) permeability, and a ouabain-sensitive, electrogenic pump. As previously suggested, the action potential is at least partly dependent on voltage-gated Ca(2+) channels, as the amplitude was increased as extracellular Ca(2+) was increased, and decreased by L-type Ca(2+) channel blockers verapamil and nifedipine. Barium caused a marked prolongation of action potential duration, suggesting that a calcium-activated K(+) current may contribute to repolarization. Most notably, however, we found that action potentials were abolished in the absence of external Na(+). This may be due, at least in part, to a Na(+)-dependent pacemaker potential. In addition, the persistence of action potentials in nominally free Ca(2+), the inhibition by Na(+) channel blockers procaine and quinidine, and the increase in action potential frequency caused by veratridine, a toxin that alters activation of voltage-gated Na(+) channels, point to the involvement of a voltage-gated Na(+) current. Voltage-clamp analysis is required for detailed characterization of this current, and this is in progress. Nonetheless, these observations are quite surprising in view of the lack of any obvious candidate genes for voltage-gated Na(+) channels in the C. elegans genome. It would therefore be informative to re-evaluate the data from these homology searches, with the aim of identifying the gene(s) conferring this Na(+), quinidine, and veratridine sensitivity to the pharynx.

Quantitative relationship between myocardial concentration of tacrolimus and QT prolongation in guinea pigs: pharmacokinetic/pharmacodynamic model incorporating a site of adverse effect

Clinical cases have been reported of tacrolimus (FK506)-induced QT prolongation. We have previously demonstrated sustained QT prolongation by FK506 in guinea pigs. Herein, we aimed to conduct a pharmacokinetic/pharmacodynamic (PK/PD) analysis of FK506, using a model involving the myocardial compartment. The pharmacokinetics of FK506 and its effects on QTc intervals were investigated in guinea pigs. In the pharmacokinetic study, whole blood and ventricular FK506 concentrations were analyzed, using a 4-compartment model during and after intravenous infusion of FK506 (0.01 or 0.1 mg/hr/kg). Subsequently, the concentration-response relationship between ventricular FK506 concentration and change in QTc interval was analyzed, using the maximal effect (Emax) model. Pharmacokinetic profiles of FK506 showed a delayed distribution of FK506 into the ventricle. Furthermore, the observed QT prolongation paralleled the ventricular FK506 concentrations, with no lag-time between the two. The Emax model successfully described the relationship between changes in QTc interval and ventricular FK506 concentrations. In conclusion, the PK/PD model where the myocardial drug concentration of FK506 was linked with its adverse effect could describe, for the first time, the anti-clockwise hysteresis observed in the relationship between blood FK506 concentration and QTprolongation. Such a hysteresis pattern for QTprolongation might be caused, therefore, mainly by the delayed disposition of FK506 to ventricular myocytes.

An amino acid residue whose change by mutation affects drug binding to the HERG channel

We did the experiments to search for amino acids that affect quinidine binding to the HERG channel, and have identified an amino acid whose change by mutation affects the binding of various drugs. The residue is located at position 647 in the S6 and is not involved in the recently identified methanesulfonanilide binding pocket. The homology model of the HERG channel indicated that the residue faces toward the outside of the channel pore. We conclude that the residue at position 647 does not interact directly with drug molecules but plays an important role in keeping the binding site's high affinity for drugs.

Ascorbic acid-induced modulation of venous tone in humans

Ascorbic acid appears to have vasodilatory properties, but the underlying mechanisms are not well understood. The aims of this study were to define the acute effects of locally infused ascorbic acid in human veins and to explore underlying mechanisms by using pharmacological tools in vivo. Ascorbic acid was infused in dorsal hand veins submaximally preconstricted with the alpha(1)-adrenoceptor agonist phenylephrine or with prostaglandin F(2alpha) in 23 healthy male nonsmokers, and the venodilator response was measured. Ascorbic acid produced dose-dependent dilation with maximum reversal of constriction of 38+/-4% in phenylephrine-preconstricted veins and of 51+/-13% in prostaglandin F(2alpha)-preconstricted veins. Oral pretreatment with the cyclooxygenase inhibitor acetylsalicylic acid or local coinfusion of ascorbic acid and the nitric oxide synthase inhibitor N:(G)-monomethyl-L-arginine had no effect, but coinfusion of ascorbic acid and methylene blue (to inhibit cGMP generation) abolished venodilation. Coinfusion of ascorbic acid and the nonselective potassium channel blocker quinidine abolished venodilation, whereas the inhibitor of ATP-dependent potassium channels glibenclamide had no effect. In cultured bovine endothelial cells, ascorbic acid did not affect intracellular calcium concentration but blunted the response to ATP or digitonin exposure. Ascorbic acid, in millimolar concentrations, dilates human hand veins, presumably by activation of vascular smooth muscle potassium channels through cGMP. This activation is independent of eNOS-mediated nitric oxide synthesis and cyclooxygenase products and does not involve ATP-dependent potassium channels.

Innovative pharmacologic approaches to cardiopulmonary resuscitation

The survival rate of patients undergoing cardiopulmonary resuscitation is 5 to 15%. New cardiopulmonary resuscitation treatment approaches under investigation include the use of vasopressin as a vasopressor, amiodarone for the treatment of ventricular tachyarrhythmias, and adenosine antagonists (i.e., theophylline) for bradyasystolic rhythms. More innovative approaches include the use of thyroid hormone and endothelin.

Control of paroxysmal atrial fibrillation recurrence using combined administration of propafenone and quinidine

The frequent recurrence of paroxysmal atrial fibrillation (PAF) despite the use of standard antiarrhythmic agents prompted the use of new therapeutic approaches. There are few data on systematic assessment of PAF control with stepwise dose escalation and the use of a drug combination. Low-dose quinidine may promote the efficacy of propafenone by inhibiting its degradation through the cytochrome P450 pathway (CYP2D6). We prescribed propafenone 300 to 450 mg/day to 60 patients with PAF for 8 weeks, and 62% were symptomatically controlled. The 19 refractory patients were randomized in a double-blinded fashion to receive either a higher dose of propafenone (450 to 675 mg/day) or the standard dose of propafenone with low-dose quinidine 150 mg/day, each for an 8-week study period, and subsequently crossed over to the alternative treatment. The resulting serum propafenone concentrations were 259 +/- 208 and 336 +/- 237 mg/day (p >0.5), respectively. Both treatment arms prolonged the time to the first symptomatic atrial fibrillation (AF) recurrence and the interval between attacks, and AF was controlled in 37% of patients. However, the higher dose of propafenone was associated with gastrointestinal side effects not present with the low-dose quinidine combination. Of the 10 refractory patients, 7 were further controlled with a standard dose of propafenone plus quinidine (600 mg/day). Overall, control of PAF was achieved in 85% of patients at the end of 8 months; adverse effects necessitating withdrawal were observed in 6%, and uncontrolled AF in 5% of patients. There was no difference in the mean AF rate during recurrences in all phases, and ventricular proarrhythmia was not seen. This study documents the role of stepwise antiarrhythmic treatment of PAF. The use of a standard dose of propafenone, followed by low-dose quinidine combination to reduce propafenone degradation, and the combined standard dose of propafenone and quinidine may be used to maximize efficacy and tolerability.

Antiarrhythmic agents: drug interactions of clinical significance

The management of cardiac arrhythmias has grown more complex in recent years. Despite the recent focus on nonpharmacological therapy, most clinical arrhythmias are treated with existing antiarrhythmics. Because of the narrow therapeutic index of antiarrhythmic agents, potential drug interactions with other medications are of major clinical importance. As most antiarrhythmics are metabolised via the cytochrome P450 enzyme system, pharmacokinetic interactions constitute the majority of clinically significant interactions seen with these agents. Antiarrhythmics may be substrates, inducers or inhibitors of cytochrome P450 enzymes, and many of these metabolic interactions have been characterised. However, many potential interactions have not, and knowledge of how antiarrhythmic agents are metabolised by the cytochrome P450 enzyme system may allow clinicians to predict potential interactions. Drug interactions with Vaughn-Williams Class II (beta-blockers) and Class IV (calcium antagonists) agents have previously been reviewed and are not discussed here. Class I agents, which primarily block fast sodium channels and slow conduction velocity, include quinidine, procainamide, disopyramide, lidocaine (lignocaine), mexiletine, flecainide and propafenone. All of these agents except procainamide are metabolised via the cytochrome P450 system and are involved in a number of drug-drug interactions, including over 20 different interactions with quinidine. Quinidine has been observed to inhibit the metabolism of digoxin, tricyclic antidepressants and codeine. Furthermore, cimetidine, azole antifungals and calcium antagonists can significantly inhibit the metabolism of quinidine. Procainamide is excreted via active tubular secretion, which may be inhibited by cimetidine and trimethoprim. Other Class I agents may affect the disposition of warfarin, theophylline and tricyclic antidepressants. Many of these interactions can significantly affect efficacy and/or toxicity. Of the Class III antiarrhythmics, amiodarone is involved in a significant number of interactions since it is a potent inhibitor of several cytochrome P450 enzymes. It can significantly impair the metabolism of digoxin, theophylline and warfarin. Dosages of digoxin and warfarin should empirically be decreased by one-half when amiodarone therapy is added. In addition to pharmacokinetic interactions, many reports describe the use of antiarrhythmic drug combinations for the treatment of arrhythmias. By combining antiarrhythmic drugs and utilising additive electrophysiological/pharmacodynamic effects, antiarrhythmic efficacy may be improved and toxicity reduced. As medication regimens grow more complex with the aging population, knowledge of existing and potential drug-drug interactions becomes vital for clinicians to optimise drug therapy for every patient.

Cost effectiveness of therapies for atrial fibrillation. A review

Atrial fibrillation is the most common supraventricular tachyarrhythmia encountered in clinical practice, affecting over 5% of persons over the age of 65 years. A common pathophysiological mechanism for arrhythmia development is atrial distention and fibrosis induced by hypertension, coronary artery disease or ventricular dysfunction. Less frequently, atrial fibrillation is caused by mitral stenosis or other provocative factors such as thyrotoxicosis, pericarditis or alcohol intoxication. Depending on the extent of associated cardiovascular disease, atrial fibrillation may produce haemodynamic compromise, or symptoms such as palpitations, fatigue, chest pain or dyspnoea. Arrhythmia-induced atrial stasis can precipitate clot formation and the potential for subsequent thromboembolism. Comprehensive management of atrial fibrillation requires a multifaceted approach directed at controlling symptoms, protecting the patient from ischaemic stroke or peripheral embolism and possible conversion to or maintenance of sinus rhythm. Numerous randomised trials have demonstrated the efficacy of warfarin--and less so aspirin (acetylsalicylic acid)--in reducing the risk of embolic events. Furthermore, therapeutic strategies exist that can favourably modify symptoms by restoring and maintaining sinus rhythm with cardioversion and antiarrhythmic prophylaxis. However, the risks and benefits of various treatments is highly dependent on patient-specific features, emphasising the need for an individualised approach. This article reviews the findings of cost-effectiveness studies published over the past decade that have evaluated different components of treatment strategies for atrial fibrillation. These studies demonstrate the economic attractiveness of acute management options, long term warfarin prophylaxis, telemetry-guided initiation of antiarrhythmic therapy, approaches to restore and maintain sinus rhythm, and the potential role of transoesophageal echocardiographic screening for atrial thrombus prior to pharmacological or electrical cardioversion. Further, we discuss the merits and limitations of the cost-effectiveness analyses in the context of overall treatment strategies. Finally, we identify areas that will require additional research to achieve the goal of effective and economically efficient management of atrial fibrillation.

Familial and acquired long qt syndrome and the cardiac rapid delayed rectifier potassium current

1. Long QT syndrome (LQTS) is a cardiac disorder characterized by syncope, seizures and sudden death; it can be congenital, idiopathic, or iatrogenic. 2. Long QT syndrome is so-named because of the connection observed between the distinctive polymorphic ventricular tachycardia torsade de pointes and prolongation of the QT interval of the electrocardiogram, reflecting abnormally slowed ventricular action potential (AP) repolarization. Acquired LQTS has many similar clinical features to congenital LQTS, but typically affects older individuals and is often associated with specific pharmacological agents. 3. A growing number of drugs associated with QT prolongation and its concomitant risks of arrhythmia and sudden death have been shown to block the 'rapid' cardiac delayed rectifier potassium current (IKr) or cloned channels encoded by the human ether-a-go-go-related gene (HERG; the gene believed to encode native IKr). Because IKr plays an important role in ventricular AP repolarization, its inhibition would be expected to result in prolongation of both the AP and QT interval of the electrocardiogram. 4. The drugs that produce acquired LQTS are structurally heterogeneous, including anti-arrhythmics, such as quinidine, non-sedating antihistamines, such as terfenadine, and psychiatric drugs, such as haloperidol. In addition to heterogeneity in their structure, the electrophysiological characteristics of HERG/IKr inhibition differ between agents. 5. Here, clinical observations are associated with cellular data to correlate acquired LQTS with the IKr/HERG potassium (K+) channel. One strategy for developing improved compounds in those drug classes that are currently associated with LQTS could be to design drug structures that preserve clinical efficacy but are modified to avoid pharmacological interactions with IKr. Until such time, awareness of the QT-prolongation risk of particular agents is important for the clinician.

Review of common adverse effects of selected antiarrhythmic drugs

The management of cardiac arrhythmias has changed dramatically over the past several years. New drugs and devices are now available to treat various arrhythmias. Many new agents have been developed that rely on different electrophysiologic mechanisms to elicit their effect on the heart rhythm. Though often effective, these drugs also pose a risk because all of them have a variety of potential adverse effects associated with their use. Many of these adverse reactions are common to all antiarrhythmic drugs, whereas others are unique to particular agents. This review discusses the notable adverse effects of selected antiarrhythmic drugs.

Cardiovascular pharmacology: new drugs and new indications

This review presents a brief overview of some of the many exciting developments that are taking place in the field of cardiovascular pharmacology. Research continues to progress at a rapid rate, and we can expect many drugs to enter the clinical arena within the next few years. It must be borne in mind, however, that the pharmaceutical industry and hospital budgetary restrictions sometimes limit drug development and occasionally interrupt clinical trials, even before their results have been obtained.

Efficacy and safety of sotalol versus quinidine for the maintenance of sinus rhythm after conversion of atrial fibrillation. SOCESP Investigators. The Cardiology Society of São Paulo

To compare the efficacy and safety of sotalol and quinidine after conversion of atrial fibrillation (AF) of <6 months, a prospective multicenter trial enrolled 121 patients who were randomized to receive dl-sotalol (160 to 320 mg/day, 58 patients) or quinidine sulfate (600 to 800 mg/day, 63 patients). Patients with left ventricular ejection fraction of <0.40 or left atrial diameter >5.2 cm were excluded. After 6 months of follow-up, using the Kaplan-Meier method, the probabilities of success were comparable between sotalol (74%) and quinidine (68%), but recurrences occurred later with sotalol than with quinidine (69 vs 10 days, p <0.05). Four patients developed proarrhythmic events, 3 (5%) with sotalol and 1 (2%) with quinidine, which were all associated with diuretic therapy. In patients converted from recent-onset AF (< or = 72 hours), sotalol was more effective than quinidine (93% vs 64%, p = 0.01), whereas in chronic AF (> 72 hours), quinidine was more effective than sotalol (68% vs 33%, p <0.05). During recurrences, the ventricular rate was significantly reduced in patients taking sotalol (98 to 82 beats/min, p <0.05). Independent predictors of therapeutic success were recent-onset AF in the sotalol group (p <0.001) and absence of hypertension in the quinidine group (p <0.05). In conclusion, sotalol and quinidine have comparable efficacy and safety for the maintenance of sinus rhythm in the overall group. In recent-onset AF, sotalol was more effective, whereas in chronic AF, quinidine had a better result. Recurrences occurred later with sotalol when compared with quinidine. Because of proarrhythmia, these drugs should be used judiciously in patients on diuretic therapy.

Inhibition of the current of heterologously expressed HERG potassium channels by imipramine and amitriptyline

1 Tricyclic antidepressants (TCAs) are associated with cardiovascular side effects including prolongation of the QT interval of the ECG. In this report we studied the effects of two TCAs (imipramine and amitriptyline) on ionic current mediated by cloned HERG potassium channels. 2 Voltage clamp measurements of HERG currents were made from CHO cells transiently transfected with HERG cDNA. HERG-encoded potassium channels were inhibited in a reversible manner by both imipramine and amitriptyline. HERG tail currents (IHERG) following test pulses to +20 mV were inhibited by imipramine with an IC50 of 3.4+/-0.4 microM (mean+/-s.e.mean) and a Hill coefficient of 1.17+/-0.03 (n = 5). 3 microM amitriptyline inhibited IHERG by 34+/-6% (n = 3). The inhibition showed only weak voltage dependence. 3 Using an 'envelope of tails' comprised of pulses to +20 mV of varying durations, the tau of activation was found to be 155+/-30 ms for control and 132+/-26 ms for 3 microM imipramine (n = 5). Once maximal channel activation was achieved after 320 ms (as demonstrated by maximal tail currents), further prolongation of depolarization did not increase imipramine-mediated HERG channel inhibition. 4 Taking current measurements every second during a 10 s depolarizing pulse from -80 mV to 0 mV, block was observed during the first pulse in the presence of imipramine and the level of IHERG block was similar throughout the pulse (n=5). 5 A three pulse protocol (two depolarizing pulses to +20 mV separated by 20 ms at -80 mV) revealed that imipramine did not significantly alter the kinetics of IHERG inactivation. The tau of inactivation was 8+/-2 ms and 5.6+/-0.4 ms (n = 5) in the absence and presence of 3 microM imipramine, respectively, and currents inactivated to a similar extent. 6 Our data are consistent with TCAs causing components of block of the HERG channel in both the closed and open states. Any component of open channel block occurs rapidly upon depolarization. Inhibition of IHERG by the prototype TCAs imipramine and amitriptyline may suggest a mechanism for QT prolongation associated with risks of arrhythmia and sudden death that accompany high concentrations of TCAs following overdose.

The effects of quinine on cochlear nerve fibre activity in the guinea pig

The effect of quinine on single cochlear nerve fibre activity (n = 38) was measured in four pigmented guinea pigs, which were given 10-30 mg/kg of quinine intravenously. The frequency tuning curves of these fibres exhibited significant increases in the thresholds of both 'tip' and 'tails' regions of the frequency tuning curve, but these changes did not appear to be accompanied by significant changes in tuning, as measured by the Q'10'dB. In comparison with control fibres (n = 178) from 13 untreated animals, significant changes in the proportion of low:high spontaneous rates (SR) were also seen. Using a boundary criterion of 25 sp/s, this rate changed from 26:74% to 47:53% in control and quinine-poisoned fibres, respectively. Independent of changes in the spontaneous rate, significant increases in the mean absolute refractory period from 0.85 to 1 ms were measured following quinine administration. The absence of a significant effect on fibre tuning whilst threshold was elevated indicates that quinine does not affect the integrity of the cochlear amplifier, though appears to affect cochlear sensitivity.

Evaluation of outpatient initiation of antiarrhythmic drug therapy in patients reverting to sinus rhythm after an episode of atrial fibrillation

The safety of a method of outpatient antiarrhythmic drug loading utilizing a continuous loop event recorder was evaluated. The findings suggest that the standard 2-day hospital admission for drug loading is not necessary in all patients and a method of outpatient loading may be equally safe.

Quinidine interactions with human atrial potassium channels: developmental aspects

Clinical studies have suggested that quinidine is less effective when used for the treatment of atrial arrhythmias in pediatric patients compared with its clinical effectiveness in the adult patient population. Age-related changes in the cardiac actions of quinidine on action potential duration and interaction with potassium channels in several mammalian species also have been reported. We investigated the effects of postnatal development on quinidine's interaction with major repolarizing currents (Ito, IKur, Ins, and IK1) in human atrial myocytes, using the whole-cell configuration of the voltage-clamp technique. Our results indicate that there are age-related changes in both the IC50 for quinidine blockade of Ito, as well as the mechanism of quinidine unblocking. In contrast, quinidine was found to inhibit both adult and pediatric IK1 and IKur in an age-independent manner, whereas the nonselective cation current (Ins), which contributes to the sustained outward current (Isus), was insensitive to quinidine. The results from this study help to clarify the electrophysiological mechanism by which quinidine elicits its antiarrhythmic effect in the pediatric and adult human population.

Unexpected instant death following successful coronary artery bypass graft surgery (and other clinical settings): atrial fibrillation, quinidine, procainamide, et cetera, and instant death

Primum non nocere. Atrial fibrillation (AF) occurs commonly following coronary artery bypass graft surgery, although new onset atrial fibrillation in this setting is usually transient. When AF reverts or is converted to sinus rhythm it is unlikely to recur, whether or not the patient takes preventive medication. As no benefit (and sometimes increased risk) associated with reduced mortality or morbidity in this setting has been reported for antiarrhythmic agents, standard treatment should consist of observation or control of ventricular response with an appropriate agent until AF relapses to sinus rhythm. If an antiarrhythmic agent, especially a class I agent, is used because of persistent or recurrent AF in the early postoperative period, heart rhythm should be monitored as long as the class I agent is administered and treatment initiated if an undersirable rhythm develops. Atrial fibrillation in other clinical settings in patients with structural heart disease presents a more difficult management problem. Class I agents are reported to be associated with an increased risk of death, despite an efficacious effect of maintaining sinus rhythm. Amiodarone is reported to be well tolerated with respect to the cardiovascular system, but unacceptable noncardiac effects are reported. A safe amiodarone-like agent is greatly needed. Atrial fibrillation in patients with no structural heart disease is not discussed in this presentation.

Combination of sotalol and quinidine in a canine model of torsades de pointes: no increase in the QT-related proarrhythmic action of sotalol

INTRODUCTION

Clinical treatment with a combination of Class IA and III antiarrhythmic drugs is not recommended, as they both favor bradycardia-dependent proarrhythmic events such as torsades de pointes (TdP). However, this theoretical additive effect on ventricular repolarization has never been demonstrated and could be questioned as other Class I drugs, such as mexiletine, a Class IB drug, limit the number of sotalol-induced TdP in dogs with AV block, suggesting the possibility of an antagonistic action of Class I properties against Class III effects.

METHODS AND RESULTS

We compared the electrophysiologic and proarrhythmic effects of sotalol (Class III) alone and combined with quinidine (Class IA) in a canine model of acquired long QT syndrome. Seven hypokalemic (K+: 3 +/- 0.1 mEq/L) dogs with chronic AV block had a demand pacemaker implanted and set at a rate of 25 beats/min. They were submitted to two (sotalol-alone and sotalol-plus-quinidine) experiments 48 hours apart using a randomized cross-over protocol. They were pretreated with quinidine (10 mg/kg + 1.8 mg/kg per hour) or saline infused throughout the experiment, and given sotalol (4.5 mg/kg + 1.5 mg/kg per hour) for 2 hours, 30 minutes after the beginning of the pretreatment infusion during both experiments. Ventricular and atrial cycle lengths were similarly increased by sotalol after quinidine or saline. The sotalol-induced prolongation of the QT interval was significantly shorter in quinidine-pretreated dogs (24 +/- 7 msec after quinidine vs 40 +/- 8 msec after saline). Fewer dogs developed TdP: significantly during the first hour of infusion (1/7 sotalol-plus-quinidine vs 6/7 sotalol-alone dogs, P < 0.05) but nonsignificantly during the second hour (3/7 vs 6/7).

CONCLUSION

In this model, the sotalol-plus-quinidine combination is at least no more arrhythmogenic than either of the drugs given alone.