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

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.

hERG toxicity assessment: Useful guidelines for drug design

All along the drug development process, one of the most frequent adverse side effects, leading to the failure of drugs, is the cardiac arrhythmias. Such failure is mostly related to the capacity of the drug to inhibit the human ether-à-go-go-related gene (hERG) cardiac potassium channel. The early identification of hERG inhibition properties of biological active compounds has focused most of attention over the years. In order to prevent the cardiac side effects, a great number of in silico, in vitro and in vivo assays have been performed. The main goal of these studies is to understand the reasons of these effects, and then to give information or instructions to scientists involved in drug development to avoid the cardiac side effects. To evaluate anticipated cardiovascular effects, early evaluation of hERG toxicity has been strongly recommended for instance by the regulatory agencies such as U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA). Thus, following an initial screening of a collection of compounds to find hits, a great number of pharmacomodulation studies on the novel identified chemical series need to be performed including activity evaluation towards hERG. We provide in this concise review clear guidelines, based on described examples, illustrating successful optimization process to avoid hERG interactions as cases studies and to spur scientists to develop safe drugs.

Stereoselective Inhibition of the hERG1 Potassium Channel

A growing number of drugs have been shown to prolong cardiac repolarization, predisposing individuals to life-threatening ventricular arrhythmias known as Torsades de Pointes. Most of these drugs are known to interfere with the human ether à-gogo related gene 1 (hERG1) channel, whose current is one of the main determinants of action potential duration. Prolonged repolarization is reflected by lengthening of the QT interval of the electrocardiogram, as seen in the suitably named drug-induced long QT syndrome. Chirality (presence of an asymmetric atom) is a common feature of marketed drugs, which can therefore exist in at least two enantiomers with distinct three-dimensional structures and possibly distinct biological fates. Both the pharmacokinetic and pharmacodynamic properties can differ between enantiomers, as well as also between individuals who take the drug due to metabolic polymorphisms. Despite the large number of reports about drugs reducing the hERG1 current, potential stereoselective contributions have only been scarcely investigated. In this review, we present a non-exhaustive list of clinically important molecules which display chiral toxicity that may be related to hERG1-blocking properties. We particularly focus on methadone cardiotoxicity, which illustrates the importance of the stereoselective effect of drug chirality as well as individual variations resulting from pharmacogenetics. Furthermore, it seems likely that, during drug development, consideration of chirality in lead optimization and systematic assessment of the hERG1 current block with all enantiomers could contribute to the reduction of the risk of drug-induced LQTS.

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.

Pharmacodynamic analysis of the electrocardiographic interaction between disopyramide and erythromycin in rats

Disopyramide (DP) is known to induce QT prolongation and Torsades de Pointes (TdP) when administered concomitantly with erythromycin (EM). To define and evaluate quantitatively the arrhythmogenic risk of the concomitant administration of DP and EM, we investigated the influence of EM on the pharmacokinetics and pharmacodynamics of DP in rats. The time profiles of change in QT interval and plasma concentration of each drug were evaluated during and after constant intravenous infusion of DP (6.0 or 15.0 mg/kg/h), EM (4.0 or 8.0 mg/kg/h), and coadministration of DP and EM (DP 6.0 mg/kg/h plus EM 4.0 mg/kg/h). Each agent induced QT prolongation at plasma concentrations within the therapeutic range in humans. DP-induced QT prolongation was proportional to its plasma concentration. In the case of EM, the Emax model with an "effect compartment" could explain the relationship between plasma EM concentrations and changes in QT interval. Although coadministration of EM with DP gave enhanced QT prolongation compared to dosing with DP alone, EM did not affect the pharmacokinetics of DP. In conclusion, it was shown that a pharmacodynamic interaction contributes to the electrocardiographic adverse reaction (i.e., QT prolongation) induced by coadministration of DP and EM in rats.

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.

Propafenone and disopyramide enhance post-ischemic contractile and metabolic recovery of perfused hearts

The effects of sodium channel blockers, propafenone and disopyramide, on post-ischemic contractile dysfunction of perfused rat hearts were examined. Isolated hearts were subjected to 35 min ischemia, followed by 60 min reperfusion with and without administration of either drug during 3 min of pre-ischemia. Ischemia/reperfusion induced complete cardiac dysfunction, rise in left ventricular end-diastolic pressure, increase in perfusion pressure, accumulation of Na+ and Ca2+ and loss of K+ and Mg2+, and release of creatine kinase and purine nucleosides and bases from the heart. These observations suggest that ischemia/reperfusion in the current study induces cardiac cell necrosis or an increase in cell membrane permeability to ions, substrates and macromolecules. Treatment of perfused hearts with either propafenone at concentrations ranging from 5 to 70 microM, or disopyramide at concentrations of 100 microM or higher resulted in a pronounced contractile recovery of the heart, associated with suppression of reperfusion-induced tissue ion alteration and inhibition of reperfusion-induced release of creatine kinase and purine nucleosides and bases. Ischemic insult itself caused tissue Na+ accumulation and K+ loss without any change in tissue Ca2+ and Mg2+. The alterations in the electrolytes were attenuated by treatment with either agent. The results suggest that prevention of ischemia- and reperfusion-induced ionic disturbance of cardiac cells by propafenone and disopyramide plays a role in the improvement of post-ischemic contractile dysfunction.

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)