Electrophysiological effects of the optical isomers of disopyramide and quinidine in the dog. Dependence on stereochemistry

Stereoselective block of the hERG potassium channel by the Class Ia antiarrhythmic drug disopyramide

Potassium channels encoded by human Ether-à-go-go-Related Gene (hERG) are inhibited by diverse cardiac and non-cardiac drugs. Disopyramide is a chiral Class Ia antiarrhythmic that inhibits hERG at clinical concentrations. This study evaluated effects of disopyramide enantiomers on hERG current (IhERG) from hERG expressing HEK 293 cells at 37 °C. S(+) and R(-) disopyramide inhibited wild-type (WT) IhERG with IC50 values of 3.9 µM and 12.9 µM respectively. The attenuated-inactivation mutant N588K had little effect on the action of S(+) disopyramide but the IC50 for the R(-) enantiomer was ~ 15-fold that for S(+) disopyramide. The enhanced inactivation mutant N588E only slightly increased the potency of R(-) disopyramide. S6 mutation Y652A reduced S(+) disopyramide potency more than that of R(-) disopyramide (respective IC50 values ~ 49-fold and 11-fold their WT controls). The F656A mutation also exerted a stronger effect on S(+) than R(-) disopyramide, albeit with less IC50 elevation. A WT-Y652A tandem dimer exhibited a sensitivity to the enantiomers that was intermediate between that of WT and Y652A, suggesting Y652 groups on adjacent subunits contribute to the binding. Moving the Y (normally at site 652) one residue in the N- terminal (up) direction in N588K hERG markedly increased the blocking potency of R(-) disopyramide. Molecular dynamics simulations using a hERG pore model produced different binding modes for S(+) and R(-) disopyramide consistent with the experimental observations. In conclusion, S(+) disopyramide interacts more strongly with S6 aromatic binding residues on hERG than does R(-) disopyramide, whilst optimal binding of the latter is more reliant on intact inactivation.

Stereoselective Blockage of Quinidine and Quinine in the hERG Channel and the Effect of Their Rescue Potency on Drug-Induced hERG Trafficking Defect

Diastereoisomers of quinidine and quinine are used to treat arrhythmia and malaria, respectively. It has been reported that both drugs block the hERG (human ether-a-go-go-related gene) potassium channel which is essential for myocardium repolarization. Abnormality of repolarization increases risk of arrhythmia. The aim of our research is to study and compare the impacts of quinidine and quinine on hERG. Results show that both drugs block the hERG channel, with quinine 14-fold less potent than quinidine. In addition, they presented distinct impacts on channel dynamics. The results imply their stereospecific block effect on the hERG channel. However, F656C-hERG reversed this stereoselectivity. The mutation decreases affinity of the two drugs with hERG, and quinine was more potent than quinidine in F656C-hERG blockage. These data suggest that F656 residue contributes to the stereoselective pocket for quinidine and quinine. Further study demonstrates that both drugs do not change hERG protein levels. In rescue experiments, we found that they exert no reverse effect on pentamidine- or desipramine-induced hERG trafficking defect, although quinidine has been reported to rescue trafficking-deficient pore mutation hERG G601S based on the interaction with F656. Our research demonstrated stereoselective effects of quinidine and quinine on the hERG channel, and this is the first study to explore their reversal potency on drug-induced hERG deficiency.

Successful use of quinine in the treatment of electrical storm in a child with Brugada syndrome

A 10-year-old girl developed life-threatening recurrent polymorphic ventricular tachycardia following surgical closure of a simple secundum atrial septal defect. Post hoc analysis of a Holter recording suggested Brugada syndrome. After managing the acute phase, a dual chamber defibrillator was implanted. One week later she experienced VF storm, needing 96 appropriate shocks within a few hours. Quinidine, by virtue of its I(to) blocking property, is the only drug reported to be useful in managing VF storm in Brugada syndrome. Nonavailability of quinidine led us to try its diastereomer, intravenous quinine, which succeeded in controlling the ventricular tachycardia. Arrhythmia storm in the setting of ion channelopathy can be difficult to manage, and sometimes requires innovative therapies.

Stereoselective pharmacokinetics and pharmacodynamics of disopyramide and its metabolite in rabbits

The extent to which interactions between enantiomers of disopyramide and between disopyramide and its metabolite, mono-N-dealkylated disopyramide (MND), contribute to stereoselectivity of the anti-arrhythmic effect has been investigated in rabbits by measuring the prolongation of the QUc interval. The plasma unbound fraction of disopyramide enantiomers was constant at a concentration range of 1.44-28.9 microM. An intravenous infusion study of the disopyramide enantiomer or racemate suggested that the S-enantiomer had a pharmacological effect, determined by linear regression analysis, approximately 3.3-times more potent than that of the R-enantiomer. Furthermore, the effect caused by racemic disopyramide was the sum of that elicited by both enantiomers individually. No significant difference was observed between the slope of linear regression analysis of intravenous infusion and that of intravenous bolus injection. Single intravenous bolus injection of MND did not affect the QUc intervals. In conclusion, the S-enantiomer of disopyramide was approximately 3.3-times more potent pharmacologically than the R-enantiomer. The relationship between plasma concentration of the disopyramide enantiomers and pharmacological effect was the sum of each enantiomer individually.

Enantioselective analysis of disopyramide and mono-N-dealkyldisopyramide in plasma and urine by high-performance liquid chromatography on an amylose-derived chiral stationary phase

An enantioselective high-performance liquid chromatography method was developed for the simultaneous determination of disopyramide (DP) and mono-N-dealkyldisopyramide (MND) enantiomers in plasma and urine. The drugs were extracted from plasma samples by liquid-liquid extraction with dichloromethane after protein precipitation with trichloroacetic acid; the urine samples were processed by liquid-liquid extraction with dichloromethane. The enantiomers were resolved on a Chiralpak AD column using hexane-ethanol (91:9, v/v) plus 0.1% diethylamine as the mobile phase and monitored at 270 nm. Under these conditions the enantiomeric fractions of the drug and of its metabolite were analyzed within 20 min. The extraction procedure was efficient in removing endogenous interferents and low values for the relative standard deviations were demonstrated for both within-day and between-day assays. The method described in this paper allows the determination of DP and MND enantiomers at plasma levels as low as 12.5 ng/ml and can be used in clinical pharmacokinetic studies.

Vasoconstrictor and vasodilator effects of disopyramide on isolated rat vascular smooth muscle

We investigated the effects of disopyramide on the isometric contractions and intracellular Ca2+ concentrations ([Ca2+]i) measured by Fura-2 fluorescence in isolated rat aorta and portal veins. Disopyramide at concentrations > or = 10(-5) M increased the duration and complexity of the spontaneous contractions in rat portal veins. At > 10(-6) M, it induced a concentration-dependent contraction in the rat aorta. This effect was endothelium independent, associated with an increase in [Ca2+]i and abolished in aortic rings incubated in Ca2+-free solution or pretreated with 10(-7) M nifedipine, suggesting that disopyramide increased [Ca2+]i through the activation of L-type Ca2+ channels. In aortic rings precontracted by KCl (30 and 80 mM), 80 mM KCl in a low-concentration (26.2 mM) Na+ solution or 10(-5) M noradrenaline, disopyramide induced a concentration-dependent relaxation. The relaxant response in 80 mM KCl-precontracted arteries was associated with a parallel reduction in [Ca2+]i, an effect attributable to its Ca2+ channel blocking properties. In contrast, disopyramide had no effect on the concentration-response curves to noradrenaline in the presence of nifedipine. Disopyramide also inhibited in a concentration-dependent manner the relaxation induced by levcromakalim in aortic rings precontracted by 30 mM KCl because of its inhibitory action on K(ATP) channels, whereas it had no effect on the relaxant response to sodium nitroprusside. These effects, together with the negative inotropic effects of the drug, may account for the increase in mean arterial pressure observed in disopyramide-treated patients and the profound hypotension observed after overdosages of disopyramide.

Antiarrhythmic effects of optical isomers of disopyramide on canine ventricular arrhythmias

Disopyramide is an effective class I antiarrhythmic drug and widely used for the treatment of arrhythmias, but it has anticholinergic side effects. In vitro studies demonstrated that dextrorotatory (D-) disopyramide has a stronger anticholinergic action, whereas the levorotatory (L-) isomer has a stronger Na channel blocking action. Because the antiarrhythmic mechanism of disopyramide suppressing digitalis- and two-stage coronary ligation-induced canine ventricular arrhythmias is the drug-induced Na channel block, we examined the antiarrhythmic efficacy of D- and L-disopyramide on two arrhythmia models. On ouabain-induced ventricular tachycardia (VT), L-disopyramide 3 mg/kg decreased the arrhythmic ratio (number of ectopic beats/total heart rate), whereas the same dose of the D-isomer was ineffective and a higher dose (5 mg/kg) was needed to suppress the arrhythmia. The effective plasma concentrations (IC50) decreasing the arrhythmic ratio to 50% of the control were 5.3 and 11.3 mu g/ml for L- and D-disopyramide, respectively. We obtained similar results using 24-h two-stage coronary ligation VT. The IC50 were 8.9 and 22.2 mu g/ml for the L- and D-isomers, respectively. Our results indicate that L-disopyramide is about twice as strong an antiarrhythmic drug as the D-isomer.

Mechanism of block of a human cardiac potassium channel by terfenadine racemate and enantiomers

1. The cardiac toxicity of racemic terfenadine (marked QT prolongation and polymorphic ventricular arrhythmias) is probably due to potassium channel blockade. To test whether one of its enantiomers would be a less efficient potassium channel blocker, we compared the mechanism of action of the racemate with that of the individual enantiomers. 2. We synthesized the individual enantiomers of terfenadine and examined under whole cell voltage-clamp conditions the mechanism of action of the racemate, both enantiomers and a major metabolite on a cloned human cardiac potassium channel, hKv1.5. This delayed rectifier is sensitive to quinidine, clofilium and other 'class III' antiarrhythmic drugs at clinically relevant concentrations. 3. Upon depolarization, racemic terfenadine and its enantiomers induced a fast decline of hKv1.5 current towards a reduced steady state current level. During subsequent repolarization the tail currents deactivated more slowly than the control, resulting in a 'crossover' phenomenon. 4. The voltage-dependence of block was biphasic with a steep increase in block over the voltage range of channel opening (-30 to 0 mV), and a more shallow phase positive to 0 mV (where the channel is fully open). The latter was consistent with a binding reaction sensing 21% of the transmembrane electrical field (with reference to the cell interior). 5. The EC50 for hKv1.5 block by racemic terfenadine was 0.88 microM, while the values for R- and S-terfenadine were 1.19 microM and 1.16 microM, respectively. In contrast, the acid metabolite reduced hKv1.5 current by only 5% at a concentration of 50 microM. 6. These findings suggest that terfenadine blocks the hKvl.5 channel after it opens by entering into the internal mouth of the channel. We have previously shown that quinidine blocks hKvl.5 in a similar manner but with an apparent affinity of ~6 micro M. Thus, terfenadine and its enantiomers are approximately equipotent open state blockers of this human K+ channel and about 6 times more potent than quinidine. The similar state-, time-, and voltage-dependence of hKvl.5 block by both enantiomers also indicates that the chiral centre does not significantly constrain the orientation of critical binding determinants of terfenadine with respect to the receptor site.

Interactions of ethanol and quinidine on contractility and myocyte action potential in the rat ventricle

The combined effects of ethanol and quinidine on cardiac electromechanical coupling are unknown, but both drugs affect cardiac conduction and can cause myocardial depression. Isolated left ventricular papillary and ventricular myocytes were used to assess the combined effects of quinidine and ethanol on the electrophysiologic and mechanical properties of rat myocardium. The combination of quinidine (1-300 microM) and ethanol (120-240 mg/dL) depressed active papillary muscle tension within the clinically useful concentration range. In electrophysiologic studies of isolated ventricular myocytes, quinidine prolonged the action potential duration at 50% (APD50) and 90% (APD90) repolarization, the absolute refractory period, and the relative refractory period, but decreased the maximum rate of change of depolarization in phase 0 (Vmax). When cells were exposed to ethanol (240 mg/dL) and quinidine (1.5 microM) together, a significant decrease in the quinidine-induced prolongation of the absolute refractory and relative refractory periods was seen. Additional changes in action potential parameters from the quinidine values included slight reductions in Vmax and in APD50 and APD90, but these reductions were not consistently displayed, nor were they statistically significant.

Delayed cardiotoxicity following quinine overdose: a case report

Quinine poisoning typically results in a constellation of non-life threatening symptoms which include tinnitus, deafness, nausea, vomiting, vision changes, headache, and hypotension. Cardiac conduction defects, dysrhythmias, and cardiovascular collapse have all been reported after overdose and generally occur within 8 hours of ingestion. We report a unique case of delayed cardiotoxicity following quinine ingestion. Toxicity included marked ventricular conduction abnormalities for which serum alkalinization appeared to be therapeutically beneficial, and torsades de pointes requiring overdrive pacing for termination.

Electrophysiologic and anticholinergic effects of pirmenol enantiomers in guinea-pig myocardium

Since it has been reported that several class I drugs stereoselectively block sodium channels, potassium channels and muscarinic receptors in cardiac tissues, electrophysiologic and anticholinergic effects of enantiomers of pirmenol, a class I antiarrhythmic drug, were examined. Both (+) and (-) pirmenol depressed the maximum upstroke velocity (Vmax) of the action potential in a concentration-dependent manner in guinea-pig papillary muscles driven at 1.0 Hz, and there was no significant difference in the potency of the class I effect between the enantiomers. The onset rates of use-dependent block (UDB) of Vmax at 2.0 Hz for 10 mumol/l (+) and (-) pirmenol were 0.30 +/- 0.03 and 0.29 +/- 0.01 per action potential, and the recovery time constants from UDB for (+) and (-) pirmenol were 27.0 +/- 2.7 and 27.7 +/- 1.9 s, respectively, indicating no difference in the binding and unbinding kinetics to the sodium channel between the enantiomers. Both (+) pirmenol and (-) pirmenol prolonged action potential duration (APD) at low concentrations (1-10 mumol/l) and shortened it at high concentrations (30-100 mumol/l). Again, there was little difference with respect to the effects on APD between the enantiomers. However, in the isolated guinea-pig left atria (-) pirmenol more potently antagonized the negative inotropic effect of carbachol than (+) pirmenol, and the pA2 values for (+) and (-) pirmenol were 6.41 and 6.71, respectively. The functional study was supported by the radioligand binding experiments using [3H]N-methylscopolamine ([3H]NMS) in guinea-pig left atrial membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the enantiomers of disopyramide and its major metabolite on the electrophysiological characteristics of the guinea-pig papillary muscle

Disopyramide, a Class Ia antiarrhythmic drug, is clinically used as a racemic mixture; R(-)disopyramide and S(+)disopyramide. The major metabolite in man is desisopropyldisopyramide: R(-)desisopropyldisopyramide and S(+)desisopropyldisopyramide. The effects of the four compounds were compared on the electrophysiological characteristics of the guinea-pig papillary muscle using the standard microelectrode technique. At an external K+ concentration of 5.4 mmol/l and a stimulation frequency of 1 Hz, S(+)disopyramide (20 mumols/l) increased action potential duration (APD) by more than 18%, while it was diminished by 6% in the presence of R(-)disopyramide. Resting membrane potential amounted to -87.1 +/- 0.5 mV (n = 14) and -85.6 +/- 1.2 mV (n = 10), respectively. Also a small but significant difference in effect on the maximal rate of depolarization was observed, R(-)disopyramide being more potent, related with a slower recovery of the maximal rate of depolarization. The enantiomers of the metabolite appeared to be three times less potent than those of the parent drug in their effect on the maximal rate of depolarization. The characteristics of the enantiomers of the metabolite correlated with those of the parent drug: also the R(-)enantiomer was more potent in decreasing the maximal rate of depolarization and caused more shortening of the action potential than the S(+)enantiomer. Time constants for onset and recovery of/from rate dependent block of the maximal rate of depolarization were dependent upon the external K+ concentration, both for the enantiomers of the parent drug and those of the metabolite. Onset slowed down while recovery accelerated when external K+ was increased. Time constants were lower for the metabolite. When stimulation interval was shortened, the effect on the maximal rate of depolarisation increased. Only for the metabolite statistical significant stereoselective differences were observed at all stimulation intervals. The effects on the action potential duration were dependent upon stimulation interval; for all enantiomers the action potential duration tended to be relatively (% of control) higher at short stimulation intervals than at large stimulation intervals. The effect on the maximal rate of depolarization was also voltage dependent, but no significant differences were observed between the enantiomers, for the parent drug as well as for the metabolite.

Stereoselective interaction of tocainide and its chiral analogs with the sodium channels in human myoballs

The effects of both enantiomers of tocainide and of some of its chiral analogs on the inactivation of the sodium current in human myoballs were investigated with the whole-cell recording technique. Structure and electron densities of the applied compounds were calculated and compared to the results. Both the R(-) and the S(+) enantiomers had little effect on fast inactivation determined with short prepulses according to Hodgkin and Huxley (1952; h infinity curve). When the inactivating prepulses used in this pulse protocol were prolonged to 1024 ms, both tocainide enantiomers increased inactivation severely, suggesting that the drug binds to the channel when it is in the state of intermediate inactivation (Fakler et al. 1990). Tetrodotoxin-resistant "juvenile" sodium channels were more affected than tetrodotoxin-sensitive "adult" channels. The R form was four times as effective as the S form. The compound obtained by substitution of the methyl group on the chiral centre of tocainide with a benzyl group, although in the less potent S form, affected inactivation of the juvenile sodium channels much more than the potent (R)-tocainide. Two additional substitutions, performed on the aromatic ring of tocainide, gave a compound that was most potent in shifting the inactivation curves, but without any selectivity for juvenile or adult channels.

Enantioselective steady-state kinetics of unbound disopyramide and its dealkylated metabolite in man

Disopyramide is provided as a racemic mixture of R and S enantiomers, which have different pharmacodynamic and pharmacokinetic characteristics. Five volunteers were given racemic disopyramide 100 mg and 200 mg t.d.s. in a cross-over design. Plasma and urine concentrations of disopyramide and its active metabolite monodesisopropyl-disopyramide (MND) were determined at steady state by an enantioselective HPLC method. Unbound drug in plasma was measured after ultrafiltration. There was enantioselective clearance of unbound disopyramide (0.39 l.h-1.kg-1 for R-disopyramide and 0.58 l.h-1.kg-1 for S-disopyramide after 100 mg t.d.s.). The enantioselectivity was due to differences in the metabolism of disopyramide to MND and in further non-renal clearance, and the renal clearance of disopyramide was not enantioselective. The in vivo protein binding of disopyramide, which was saturable for both enantiomers, was also enantioselective. The difference in binding of the two enantiomers was explained by a difference in apparent binding capacity rather than in apparent binding affinity. The renal clearance of S-MND was significantly higher than R-MND (0.29 and 0.19 l.h-1.kg-1, respectively, after 100 mg t.d.s.). The renal clearance of MND also showed a tendency to saturation at higher concentrations.

Chiral aspects of drug action at ion channels: a commentary on the stereoselectivity of drug actions at voltage-gated ion channels with particular reference to verapamil actions at the Ca2+ channel

Ion channels may be considered as pharmacological receptors possessing specific drug binding sites with defined structure-activity relationships. Accordingly drug binding to ion channels is stereoselective. Interpretation of this stereoselectivity may be complex because of the existence of differences in affinity and access to different channel states. Such state-dependent interactions may give rise to quantitative and qualitative differences in stereoselectivity. The implications of such differences are reviewed for drug action at Na+, K+ and Ca2+ channels. Detailed attention is paid to the actions of verapamil enantiomers in the cardiovascular system where activities differ in vascular and cardiac tissues because of state-dependent interactions and stereoselective first-oass metabolism.

Stereoselective determination of flecainide in human plasma by high-performance liquid chromatography with fluorescence detection

Enantiomers of a drug may differ in their pharmacological activities or their disposition constants. We now describe a stereoselective analytical method for the determination of the antiarrhythmic agent flecainide in plasma. The resolution of the enantiomers is achieved by high-performance liquid chromatography (HPLC) on a normal phase silica column following derivatization with the optically active reagent (-)-methyl chloroformate. The eluting diastereoisomers are monitored by fluorescence detection at an excitation wavelength of 305 nm and an emission wavelength of 340 nm. The limit of sensitivity for the assay is as low as 2.5 ng/mL for each enantiomer using 1 mL of plasma. A new liquid-liquid extraction procedure with high recovery (greater than 95%) and high selectivity is also reported. The intra- and interassay coefficient of variation for replicated analysis of spiked plasma samples is less than 4.0% and 7.0%, respectively. The method is suitable for single and multiple dose pharmacokinetic studies in healthy volunteers or in patients.

Clinical pharmacokinetics of levorotatory and racemic disopyramide, at steady state, following oral administration in patients with ventricular arrhythmias

Electrophysiological effects, antiarrhythmic activity and kinetics of levorotatory disopyramide (R(-) DP) and racemic disopyramide (equimolar mixture of R(-) DP and S(+) DP) were compared in patients with ventricular arrhythmias. This double blind cross-over randomized trial was achieved, at steady-state, following oral administration of 200 mg three times a day. In comparison with baseline values, electrophysiological data indicated that R(-) DP and racemic DP prolonged, significantly and similarly, PR interval (+11.7% and +10%, respectively, P less than .01), and QTc interval (+9.2% and +7%, respectively, P less than .001), while QRS interval was not significantly affected. The antiarrhythmic activity, assessed by percent reduction in ventricular extrasystoles frequency, showed a similar efficiency of levorotatory and racemic DP: 80% and 74%, respectively (P = .24). Ventricular tachycardias disappeared with both treatments in the three patients concerned. During the racemic period, the mean total plasma clearance, expressed as CL/F, of S(+) DP (114.6 ml/min), was significantly lower than that of R(-) DP (157 ml/min), (P less than .001). The mean total plasma clearance of R(-) DP, during the levorotatory period (163 ml/min), did not differ from the respective value determined during the racemic period (P = .32). During the racemic period, the stereoselective difference in total plasma clearances, which is not observed when DP enantiomers are administered separately, may result from an increase in unbound fraction of R(-) DP, due to the presence of S(+) DP, which is known to be a potent displacer of R(-) DP.

Inhibitory effects of diprafenone stereoenantiomers on cardiac Na+ channels

The potency of (-)- and (+)-diprafenone to depress the Vmax of Na+-dependent action potentials and to block single cardiac Na+ channels was analyzed in microelectrode experiments with guinea pig papillary muscles and in patch clamp experiments with DPI-modified Na+ channels using neonatal cardiocytes. Within 20-30 min, both optical enantiomers caused a Vmax depression which occurred predominantly as a phasic blockade at a low dosage (10 mumol/l). (-)- and (+)-diprafenone were equally effective in evoking a tonic and phasic depression of Vmax. Exposing the cytoplasmic side of inside-out patches to 10 mumol/l of (-)- or (+)-diprafenone evoked a flicker block of DPI-modified Na+ channels within 1-2 s. Kinetic analysis of the latter revealed a KD value for the blocking action of 6.3 X 10(-5) mol for the (-) enantiomer and 7.1 X 10(-5) mol for the (+) enantiomer. Nevertheless, larger association and dissociation rate constants were obtained with (+)-diprafenone than with (-)-diprafenone. This indicates that there are stereoselective reaction kinetics in blocking open modified Na+ channels.

Immediate quantitation of antiarrhythmic drug effect by monophasic action potential recording in coronary artery disease

A contact electrode catheter, which permits clinical recording of cardiac monophasic action potentials (MAPs), was used as a means of quantifying the electrophysiologic effect of 2 antiarrhythmic drugs, procainamide and quinidine. MAP recordings were made in continuous fashion from the right ventricle in 16 patients, before and after the intravenous administration of procainamide (11 patients) or quinidine (5). Increases in the MAP duration at 90% repolarization (MAPD90) were used as indexes of drug effect and related to plasma drug level. Surface electrocardiographic (QRS duration, corrected QT interval [QTC]) and electrophysiologic (ventricular effective refractory period) measurements, in addition to MAPD90, were made at the same time as blood sampling for plasma drug level determination. Dose response curves, plotting change in MAPD90 versus plasma drug level, showed strong linear correlation for both procainamide (p less than 0.0001) and quinidine (p less than 0.0001). The variance (error of estimation) of the predictive relation, change in MAPD90 versus plasma drug level, was significantly lower than that of change in QTC (p less than 0.001), QRS duration (p less than 0.0001) or ventricular effective refractory period (p less than 0.0001) versus plasma drug level for both procainamide and quinidine. Changes in MAP duration closely correlate with plasma drug level, and as such, may serve as an immediate, quantitative indicator of myocardial drug effect during the administration of antiarrhythmic agents.

Differing electrophysiological effects of class IA, IB and IC antiarrhythmic drugs on guinea-pig sinoatrial node

Standard microelectrode techniques were used to study the effects of class IA (quinidine, disopyramide, procainamide), IB (lignocaine, mexiletine, tocainide) and IC (flecainide, encainide, lorcainide) antiarrhythmic drugs on action potentials in spontaneously beating sino-atrial node cells from guinea-pigs. The IA drugs all produced significant slowing of spontaneous rate in therapeutic concentrations. The IB agents did so only in concentrations well above therapeutic levels and the IC drugs were of intermediate potency. All nine drugs markedly slowed the repolarization rate and this was the major mechanism of sinus slowing for the IA and IC compounds. The IB drugs shared this effect but prolongation of phase 4 by reduction of the slope of diastolic depolarization was also a prominent feature of their action.

A kinetic analysis of the endplate ion channel blocking action of disopyramide and its optical isomers

The effects of the antiarrhythmic agent disopyramide was studied on responses from voltage-clamped endplates at the neuromuscular junction of the garter snake. Disopyramide reduced endplate current amplitude and decay time constant in a concentration- and voltage-dependent manner. Endplate current decays remained monophasic in the presence of the drug. These results were interpreted in terms of the drug blocking the open form of the acetylcholine receptor-ion channel complex. Disopyramide produced a greater reduction of the amplitude of endplate currents than of miniature endplate currents. The reduction in miniature endplate current amplitude was not voltage-dependent. Analysis of endplate current driving functions showed that this was due to the rapid occurrence of channel block during the rising phase of the endplate current. The residual reduction, apart from that produced by channel block, is most probably due to receptor block. Disopyramide had a voltage-dependent blocking rate constant of about 10(7) M-1 S-1 at -90 mV. The unblocking rate constant was estimated from the results of experiments using paired ionophoretically applied pulses of acetylcholine. This value was again voltage-dependent and approximately 1 s-1. The actions of the (+)- and (-)-stereoisomers of disopyramide on endplate current decay were identical, indicating that the channel binding site at the neuromuscular junction is not stereoselective.

In vivo interaction of the enantiomers of disopyramide in human subjects

Disopyramide, an antiarrhythmic agent, is marketed as a racemic mixture of two enantiomers. The racemic drug has unusual pharmacokinetic properties because of its concentration-dependent binding to plasma proteins in the therapeutic plasma concentration range. This study examined, in healthy subjects, the individual pharmacokinetic properties of both total and unbound d- and 1-disopyramide in plasma after intravenous administration of each enantiomer separately (1.5 mg/kg). Also investigated is the pharmacokinetics of total d- and 1-disopyramide in plasma after intravenous administration of a pseudoracemate. Both d- and 1-disopyramide are found to exhibit concentration-dependent binding to plasma proteins, with d-disopyramide being more avidly bound at lower concentrations. The stereoselective, concentration-dependent binding to plasma proteins resulted in distinct pharmacokinetic properties when the enantiomers were given together as the pseudoracemate. d-Disopyramide had a lower plasma clearance and renal clearance, a longer half-life, and a smaller apparent volume of distribution than 1-disopyramide. However, when the enantiomers were administered separately, there were no differences in the clearance, renal clearance, and volume of distribution between enantiomers calculated from either total or unbound drug concentrations. The results reveal an important pharmacokinetic interaction between the enantiomers of disopyramide when given as a racemic mixture, which may be dose-dependent and is not apparent upon administration of the enantiomers separately.