Xiao-Liang Chen

Characterization of HERG potassium channel inhibition using CoMSiA 3D QSAR and homology modeling approaches

A data set consisting of twenty-two sertindole analogues and ten structurally diverse inhibitors, spanning a wide range in potency, was analyzed using CoMSiA. A homology model of HERG was constructed from the crystal structure of the open MthK potassium channel. A complementary relationship between our CoMSiA and homology models is apparent when the long inhibitor axis is oriented parallel to the longitudinal axis of the pore, with the tail region pointed toward the selectivity filter. The key elements of the pharmacophore, the CoMSiA and the homology model are: (1) The hydrophobic feature optimally consists of an aromatic group that is capable of engaging in pi-stacking with a Phe656 side chain. Optionally, a second aromatic or hydrophobic group present in some inhibitors may contact an additional Phe656 side chain. (2) The basic nitrogen appears to undergo a pi-cation interaction with Tyr652. (3) The pore diameter (12A+), and depth of the selectivity loop relative to the intracellular opening, act as constraints on the conformation-dependent inhibitor dimensions.

A comparison of the receptor binding and HERG channel affinities for a series of antipsychotic drugs

Many antipsychotic drugs produce QT interval prolongation on the electrocardiogram (ECG). Blockade of the human cardiac K(+) channel known as human ether-a-go-go-related gene (HERG) often underlies such clinical findings. In fact, HERG channel inhibition is now commonly used as a screen to predict the ability of a drug to prolong QT interval. However, the exact relationship between HERG channel blockade, target receptor binding affinity and clinical QT prolongation is not known. Using patch-clamp electrophysiology, we examined a series of seven antipsychotic drugs for their ability to block HERG, and determined their IC(50) values. We then compared these results to their binding affinities (K(i) values) for the dopamine D(2) receptor, the 5-HT(2A) receptor and, where available, to clinical QT prolongation data. We found that sertindole, pimozide and thioridazine displayed little (<10-fold) or no selectivity for dopamine D(2) or 5-HT(2A) receptors relative to their HERG channel affinities. This lack of selectivity likely underlies the significant QT interval prolongation observed with administration of these drugs. Of the other drugs tested (ziprasidone, quetiapine, risperidone and olanzapine), olanzapine displayed the greatest selectivity for dopamine D(2) and 5-HT(2A) receptor binding (100-1000-fold) compared to its HERG channel IC(50). We also compared these HERG channel IC(50) values to QT interval prolongation and plasma drug levels obtained in a recent clinical study. We found that the ratio of total plasma drug concentration to HERG IC(50) value was indicative of the degree of QT prolongation observed. Target receptor affinity and expected clinical plasma levels are important parameters to consider for the interpretation of HERG channel data.

Mechanisms underlying the QT interval-prolonging effects of sevoflurane and its interactions with other QT-prolonging drugs.

Background:Sevoflurane prolongs ventricular repolarization in patients, but the mechanisms are not fully characterized. The effects of sevoflurane on many cloned human cardiac ion channels have not been studied, and the interactions between sevoflurane and other drugs that prolong cardiac repolarization have not been detailed. Methods:The effects of sevoflurane on action potentials and L-type Ca2+ channels in guinea pig myocytes were examined. Sevofluranes effects on cloned human cardiac K+ channels and the cloned human cardiac Na+ channel were studied. The consequences of combining sevoflurane and the class III antiarrhythmic drugs sotalol or dofetilide on action potential duration were also examined. Results:Sevoflurane produced an increase in action potential duration at concentrations of 0.3–1 mm. Contrary to most drugs that delay ventricular repolarization, sevoflurane was without effect on the human ether-a-go-go–related gene cardiac potassium channel but instead produced a reduction in KvLQT1/minK K+ channel currents and inhibited the Kv4.3 K+ channel by speeding its apparent rate of inactivation. Sevoflurane had little effect on Na+ and Ca2+ channel currents at concentrations of 1 mm or less. When the authors coadministered sevoflurane with sotalol or dofetilide, synergistic effects on repolarization were observed, resulting in large increases in action potential duration (up to 66%). Conclusion:Prolonged ventricular repolarization observed with administration of sevoflurane results from inhibition of KvLQT1/minK and Kv4.3 cardiac K+ channels. Combining sevoflurane with class III antiarrhythmic drugs results in supra-additive effects on action potential duration. The results indicate that sevoflurane, when administered with this class of drug, could result in excessive delays in ventricular repolarization. The results suggest the need for further clinical studies.

Ca++ channel activators reveal differential L-type Ca++ channel pharmacology between native and stem cell-derived cardiomyocytes

Human stem cell-derived cardiomyocytes provide new models for studying the ion channel pharmacology of human cardiac cells for both drug discovery and safety pharmacology purposes. However, detailed pharmacological characterization of ion channels in stem cell-derived cardiomyocytes is lacking. Therefore, we used patch-clamp electrophysiology to perform a pharmacological survey of the L-type Ca2+ channel in induced pluripotent and embryonic stem cell-derived cardiomyocytes and compared the results with native guinea pig ventricular cells. Six structurally distinct antagonists [nifedipine, verapamil, diltiazem, lidoflazine, bepridil, and 2-[(cis-2-phenylcyclopentyl)imino]-azacyclotridecane hydrochloride (MDL 12330)] and two structurally distinct activators [methyl 2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-1,4-dihydropyridine-3-carboxylate (Bay K8644) and 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester (FPL 64176)] were used. The IC50 values for the six antagonists showed little variability between the three cell types. However, whereas Bay K8644 produced robust increases in Ca2+ channel current in guinea pig myocytes, it failed to enhance current in the two stem cell lines. Furthermore, Ca2+ channel current kinetics after addition of Bay K8644 differed in the stem cell-derived cardiomyocytes compared with native cells. FPL 64176 produced consistently large increases in Ca2+ channel current in guinea pig myocytes but had a variable effect on current amplitude in the stem cell-derived myocytes. The effects of FPL 64176 on current kinetics were similar in all three cell types. We conclude that, in the stem cell-derived myocytes tested, L-type Ca2+ channel antagonist pharmacology is preserved, but the pharmacology of activators is altered. The results highlight the need for extensive pharmacological characterization of ion channels in stem cell-derived cardiomyocytes because these complex proteins contain multiple sites of drug action.

Interactions of the narcotic l-α-acetylmethadol with human cardiac K+ channels

Abstract l-α-acetylmethadol is a long-acting narcotic analgesic that is used in the treatment of opiate addiction. However, the drug has been associated with cases of QT interval prolongation and ventricular arrhythmia. To understand the mechanism underlying these clinical findings, we examined the effects of l-α-acetylmethadol on the cloned human cardiac K+ channels HERG (human ether-a-go-go-related gene), KvLQT1/minK and Kv4.3. Using patch clamp electrophysiology, we found that l-α-acetylmethadol inhibited HERG channel currents in a voltage-dependent manner displaying an IC50 value of 3 μM. The major active metabolite of l-α-acetylmethadol, noracetylmethadol, inhibited HERG with an estimated IC50 values of 12 μM. l-α-acetylmethadol had little or no effect on Kv4.3 or KvLQT1/minK K+ channel currents at concentration up to 10 μM. We conclude that the proarrhythmic effects of l-α-acetylmethadol are due to specific blockade of the HERG cardiac K+ channel and that its active metabolite noracetylmethadol may provide a safer alternative in the treatment of opiate addiction.

FUNCTIONAL INTERACTION BETWEEN DPI 201-106, A DRUG THAT MIMICS CONGENITAL LONG QT SYNDROME, AND SEVOFLURANE ON THE GUINEA-PIG CARDIAC ACTION POTENTIAL

1 Sevoflurane produces QT prolongation on the electrocardiogram, predominantly via inhibition of the slow delayed rectifier K+ current. DPI 201‐106 is an experimental drug that produces QT prolongation by reducing Na+ channel inactivation, thereby mimicking congenital long QT syndrome type 3 (LQT3). The present study explores the electrophysiological consequences of administration of sevoflurane in the presence of impaired Na+ channel activity. 2 We examined the effects of sevoflurane and DPI 201‐106, alone and in combination, on the cardiac action potential of guinea‐pig ventricular myocytes using standard microelectrode techniques. 3 Both sevoflurane and DPI‐201‐106 prolonged action potential duration, with the combination of the two drugs producing greater than additive effects. Similarly, instability and triangulation of the action potential waveform, measures of pro‐arrhythmia, were more pronounced when both drugs were combined. 4 Sevoflurane treatment significantly alters cardiac action potential waveforms when administered in the presence of impaired Na+ channel inactivation. These results indicate the potential for ventricular arrhythmia when sevoflurane is administered to LQT3 patients and suggests caution when using sevoflurane in this population.

Manual Whole-Cell Patch-Clamping of the HERG Cardiac K + Channel

Delayed ventricular repolarization, as measured by a prolongation of the QT interval on the electrocardiogram, is a major safety issue in the drug development process. It is now recognized that most cases of drug-induced QT prolongation arise from direct pharmacological inhibition of the human ether-a-go-go-related gene (HERG) cardiac K+ channel. It is standard practice to test a drugs ability to interact with the HERG channel prior to entry into clinical trials. This testing is used, as part of a larger battery of tests, to help predict the cardiac safety profile of a drug. Manual whole-cell patch-clamping provides the most sensitive and accurate way to examine the biophysical and pharmacological properties of the HERG cardiac K+ channel.