In Silico Assessment of Class I Antiarrhythmic Drug Effects on Pitx2-Induced Atrial Fibrillation: Insights from Populations of Electrophysiological Models of Human Atrial Cells and Tissues.

Electrical remodelling as a result of homeodomain transcription factor 2 (Pitx2)-dependent gene regulation was linked to atrial fibrillation (AF) and AF patients with single nucleotide polymorphisms at chromosome 4q25 responded favorably to class I antiarrhythmic drugs (AADs). The possible reasons behind this remain elusive. The purpose of this study was to assess the efficacy of the AADs disopyramide, quinidine, and propafenone on human atrial arrhythmias mediated by Pitx2-induced remodelling, from a single cell to the tissue level, using drug binding models with multi-channel pharmacology. Experimentally calibrated populations of human atrial action po-tential (AP) models in both sinus rhythm (SR) and Pitx2-induced AF conditions were constructed by using two distinct models to represent morphological subtypes of AP. Multi-channel pharmaco-logical effects of disopyramide, quinidine, and propafenone on ionic currents were considered. Simulated results showed that Pitx2-induced remodelling increased maximum upstroke velocity (dVdtmax), and decreased AP duration (APD), conduction velocity (CV), and wavelength (WL). At the concentrations tested in this study, these AADs decreased dVdtmax and CV and prolonged APD in the setting of Pitx2-induced AF. Our findings of alterations in WL indicated that disopyramide may be more effective against Pitx2-induced AF than propafenone and quinidine by prolonging WL.

Cross - site comparison of excitation-contraction coupling using impedance and field potential recordings in hiPSC cardiomyocytes.

INTRODUCTION: Since 2005 the S7B and E14 guidances from ICH and FDA have been in place to assess a potential drug candidate's ability to cause long QT syndrome. To refine these guidelines, the FDA proposed the Comprehensive in vitro Proarrhythmia Assay (CiPA) initiative, where the assessment of drug effects on cardiac repolarization was one subject of investigation. Within the myocyte validation study, effects of pharmaceutical compounds on human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were assessed and this article will focus on the evaluation of the proarrhythmic potential of 23 blinded drugs in four hiPSC-CM cell lines. METHODS: Experiments were performed on the CardioExcyte 96 at different sites. A combined readout of contractility (via impedance) and electrophysiology endpoints (field potentials) was performed. RESULTS: Our data demonstrates that hERG blockers such as dofetilide and further high risk categorized compounds prolong the field potential duration. Arrhythmia were detected in both impedance as well as field potential recordings. Intermediate risk compounds induced arrhythmia in almost all cases at the highest dose. In the case of low risk compounds, either a decrease in FPDmax was observed, or not a significant change from pre-addition control values. DISCUSSION: With exceptions, hiPSC-CMs are sensitive and exhibit at least 10% delayed or shortened repolarization from pre-addition values and arrhythmia after drug application and thus can provide predictive cardiac electrophysiology data. The baseline electrophysiological parameters vary between iPS cells from different sources, therefore positive and negative control recordings are recommended.

Modelling the effects of quinidine, disopyramide, and E-4031 on short QT syndrome variant 3 in the human ventricles.

OBJECTIVE: Short QT syndrome (SQTS) is an inherited cardiac channelopathy, but at present little information is available on its pharmacological treatment. SQT3 variant (linked to the inward rectifier potassium current I K1) of SQTS, results from a gain-of-function mutation (Kir2.1 D172N) in the KCNJ2-encoded channels, which is associated with ventricular fibrillation (VF). Using biophysically-detailed human ventricular computer models, this study investigated the potential effects of quinidine, disopyramide, and E-4031 on SQT3. APPROACH: The ten Tusscher et al model of human ventricular myocyte action potential (AP) was modified to recapitulate the changes in I K1 due to heterozygous and homozygous forms of the D172N mutation. Wild-type (WT) and mutant WT-D172N and D172N formulations were incorporated into one-dimensional (1D) and 2D tissue models with transmural heterogeneities. Effects of drugs on channel-blocking activity were modelled using half-maximal inhibitory concentration (IC50) and Hill coefficient (nH) values. Effects of drugs on AP duration (APD), effective refractory period (ERP) and QT interval of pseudo-ECGs were quantified, and both temporal and spatial vulnerability to re-entry was measured. Re-entry was simulated in the 2D ventricular tissue. MAIN RESULTS: At the single cell level, the drugs quinidine, disopyramide, and E-4031 prolonged APD at 90% repolarization (APD90), and decreased maximal transmural voltage heterogeneity (δV); this caused the decreased transmural dispersion of APD90. Quinidine prolonged the QT interval and decreased the T-wave amplitude. Furthermore, quinidine increased ERP and reduced temporal vulnerability and increased spatial vulnerability, resulting in a reduced susceptibility to arrhythmogenesis in SQT3. In the 2D tissue, quinidine was effective in terminating and preventing re-entry associated with the heterozygous D172N condition. Quinidine exhibited significantly better therapeutic effects on SQT3 than disopyramide and E-4031. SIGNIFICANCE: This study substantiates a causal link between quinidine and QT interval prolongation in SQT3 Kir2.1 mutations and highlights possible pharmacological agent quinidine for treating SQT3 patients.

In silico assessment of the effects of quinidine, disopyramide and E-4031 on short QT syndrome variant 1 in the human ventricles.

AIMS: Short QT syndrome (SQTS) is an inherited disorder associated with abnormally abbreviated QT intervals and an increased incidence of atrial and ventricular arrhythmias. SQT1 variant (linked to the rapid delayed rectifier potassium channel current, IKr) of SQTS, results from an inactivation-attenuated, gain-of-function mutation (N588K) in the KCNH2-encoded potassium channels. Pro-arrhythmogenic effects of SQT1 have been well characterized, but less is known about the possible pharmacological antiarrhythmic treatment of SQT1. Therefore, this study aimed to assess the potential effects of E-4031, disopyramide and quinidine on SQT1 using a mathematical model of human ventricular electrophysiology. METHODS: The ten Tusscher et al. biophysically detailed model of the human ventricular action potential (AP) was modified to incorporate IKr Markov chain (MC) formulations based on experimental data of the kinetics of the N588K mutation of the KCNH2-encoded subunit of the IKr channels. The modified ventricular cell model was then integrated into one-dimensional (1D) strand, 2D regular and realistic tissues with transmural heterogeneities. The channel-blocking effect of the drugs on ion currents in healthy and SQT1 cells was modeled using half-maximal inhibitory concentration (IC50) and Hill coefficient (nH) values from literatures. Effects of drugs on cell AP duration (APD), effective refractory period (ERP) and pseudo-ECG traces were calculated. Effects of drugs on the ventricular temporal and spatial vulnerability to re-entrant excitation waves were measured. Re-entry was simulated in both 2D regular and realistic ventricular tissue. RESULTS: At the single cell level, the drugs E-4031 and disopyramide had hardly noticeable effects on the ventricular cell APD at 90% repolarization (APD90), whereas quinidine caused a significant prolongation of APD90. Quinidine prolonged and decreased the maximal transmural AP heterogeneity (δV); this led to the decreased transmural heterogeneity of APD across the 1D strand. Quinidine caused QT prolongation and a decrease in the T-wave amplitude, and increased ERP and decreased temporal susceptibility of the tissue to the initiation of re-entry and increased the minimum substrate size necessary to prevent re-entry in the 2D regular model, and further terminated re-entrant waves in the 2D realistic model. Quinidine exhibited significantly better therapeutic effects on SQT1 than E-4031 and disopyramide. CONCLUSIONS: The simulated pharmacological actions of quinidine exhibited antiarrhythmic effects on SQT1. This study substantiates a causal link between quinidine and QT interval prolongation in SQT1 and suggests that quinidine may be a potential pharmacological agent for treating SQT1 patients.

The human ether-a-go-go-related gene (hERG) current inhibition selectively prolongs action potential of midmyocardial cells to augment transmural dispersion.

The majority of drug induced arrhythmias are related to the prolongation of action potential duration following inhibition of rapidly activating delayed rectifier potassium current (I(Kr)) mediated by the hERG channel. However, for arrhythmias to develop and be sustained, not only the prolongation of action potential duration but also its transmural dispersion are required. Herein, we evaluated the effect of hERG inhibition on transmural dispersion of action potential duration using the action potential clamp technique that combined an in silico myocyte model with the actual I(Kr) measurement. Whole cell I(Kr) current was measured in Chinese hamster ovary cells stably expressing the hERG channel. The measured current was coupled with models of ventricular endocardial, M-, and epicardial cells to calculate the action potentials. Action potentials were evaluated under control condition and in the presence of 1, 10, or 100 µM disopyramide, an hERG inhibitor. Disopyramide dose-dependently increased the action potential durations of the three cell types. However, action potential duration of M-cells increased disproportionately at higher doses, and was significantly different from that of epicardial and endocardial cells (dispersion of repolarization). By contrast, the effects of disopyramide on peak I(Kr) and instantaneous current-voltage relation were similar in all cell types. Simulation study suggested that the reduced repolarization reserve of M-cell with smaller amount of slowly activating delayed rectifier potassium current levels off at longer action potential duration to make such differences. The action potential clamp technique is useful for studying the mechanism of arrhythmogenesis by hERG inhibition through the transmural dispersion of repolarization.

Impact of probe compound in MRP2 vesicular transport assays.

MRP2 is an efflux transporter that is expressed mainly in the canalicular membrane of hepatocytes, where it expels polar and ionic compounds into the bile. MRP2 is also present in the apical membrane of enterocytes and epithelial cells of proximal tubules of the kidney. Inhibition of MRP2 transport can lead to the accumulation of metabolites and other MRP2 substrates up to toxic levels in these cells. The transport properties of MRP2 are frequently studied with the vesicular transport assay. The assay identifies compounds that interact with MRP2 by measuring the effect of a compound on the transport of a radioactively labeled or fluorescent probe. We have compared the effect of eight selected test compounds (quercetin, disopyramide, paracetamol, indomethacin, diclofenac, estrone-3-sulfate, budesonide, and thioridazine) on the MRP2-mediated transport of three commonly used probes: 5(6)-carboxy-2,7-dichlorofluorescein, leukotriene C4 and estradiol-17-ß-d-glucuronide (E217ßG). Five of the test compounds had different probe-dependent effects on the MRP2-mediated transport, suggesting differences in the transport mechanism of the probes. Our results underline the complexity of substrate recognition by these efflux transporters and the difficulties in directly comparing results obtained with different assays, especially when different probes are used.

Molecular determinants of hERG potassium channel inhibition by disopyramide.

The Class Ia antiarrhythmic drug disopyramide (DISO) causes QT interval prolongation that is potentially dangerous in acquired Long QT Syndrome but beneficial in short QT syndrome, through inhibition of the hERG-encoded channels responsible for rapid delayed rectifier K(+) current (I(Kr)). In this study, alanine mutants of hERG S6 and pore helix residues and MthK-based homology modelling and ligand docking were used to investigate molecular determinants of DISO binding to hERG. Whole-cell hERG current (I(hERG)) recordings were made at 37°C from HEK-293 cells expressing WT or mutant hERG channels. WT outward I(hERG) tails were inhibited with an IC(50) of 7.3µM, whilst inward I(hERG) tails in a high [K(+)](e) of 94mM were blocked with an IC(50) of 25.7µM. The IC(50) for the Y652A mutation was ~55-fold that of WT I(hERG); this mutation also abolished a leftward shift in voltage-dependent I(hERG) activation present for WT hERG. The IC(50) for F656A I(hERG) was ~51 fold its corresponding WT control. In contrast to previously studied methanesulphonanilide hERG inhibitors, neither the G648A S6 nor the T623A and S624A pore helical mutations modified DISO IC(50). Computational docking with the hERG model showed that DISO did not exhibit a single unique binding pose; instead several low energy binding poses at the lower end of the pore cavity favoured interactions with Y652 and F656. In the WT hERG model DISO did not interact directly with residues at the base of the pore helix, consistent with the minimal effect of mutation of these residues on drug block.

Prospective comparative study of intravenous cibenzoline and disopyramide therapy in the treatment of paroxysmal atrial fibrillation after cardiovascular surgery.

BACKGROUND: It has been reported that approximately one-third of patients undergoing cardiovascular surgery experience paroxysmal atrial fibrillation (AF) during the postoperative period. There is, however, little information on the selection of anti-arrhythmic drugs for terminating postoperative paroxysmal AF. METHODS AND RESULTS: Between April 2007 and March 2009, 118 patients (76 men, 42 women, mean age 68+/-10 years) who had postoperative paroxysmal AF lasting > or =30 min were randomly assigned to receive either iv cibenzoline (70 mg, n=60) or disopyramide (50 mg, n=58) for terminating postoperative paroxysmal AF. The success rate of iv cibenzoline therapy (47%) was significantly greater than that of iv disopyramide therapy (24%; P<0.05). To identify clinical factors to increase the termination efficacy of iv cibenzoline, multivariate logistic regression was used to adjust for several covariates and to generate adjusted odds ratios (OR). The significant variables for the termination of paroxysmal AF after iv cibenzoline therapy were pretreatment with oral beta-adrenergic blockers (OR =8.224, P=0.030) and smaller left atrial dimensions (OR =0.879, P=0.039). CONCLUSIONS: The efficacy of iv cibenzoline for the termination of postoperative paroxysmal AF was significantly better than that of disopyramide, especially in patients with pre-administration of oral beta-adrenergic blockers and those with smaller left atrium.

Kv1.5 open channel block by the antiarrhythmic drug disopyramide: molecular determinants of block.

Kv1.5 is considered to be a potential molecular target for treatment of atrial fibrillation or flutter. Disopyramide is widely used in the treatment of atrial flutter and/or atrial fibrillation. The present study was undertaken to characterize the effects of disopyramide on currents mediated by Kv1.5 channels and to determine the putative binding site involved in the inhibitory effects of disopyramide. Experiments were carried out on wild-type and site directed mutated hKv1.5 channels expressed on HEK 293 cells using the patch-clamp technique. Disopyramide acting from the cytoplasmic side of the membrane produced blocking effects on Kv1.5 that exhibited several features typical of an open channel blocker. Ala-scanning mutagenesis of the Kv1.5 pore domain combined with macroscopic current analysis suggested that disopyramide interacted only with the Val512 residue that faces to the central cavity of the channel. Mutation of this key residue to Ala caused marked change in the IC(50) of disopyramide (22-fold). The single interaction between disopyramide and Val512 in the PVP region is able to change the mechanism of channel closure, reproducing the "foot-in-the-door" phenomenon.

1H NMR and UPLC-MS(E) statistical heterospectroscopy: characterization of drug metabolites (xenometabolome) in epidemiological studies.

Statistical HeterospectroscopY (SHY) is a statistical strategy for the coanalysis of multiple spectroscopic data sets acquired in parallel on the same samples. This method operates through the analysis of the intrinsic covariance between signal intensities in the same and related molecular fingerprints measured by multiple spectroscopic techniques across cohorts of samples. Here, the method is applied to 600-MHz (1)H NMR and UPLC-TOF-MS (E) data obtained from human urine samples ( n = 86) from a subset of an epidemiological population unselected for any relevant phenotype or disease factor. We show that direct cross-correlation of spectral parameters, viz. chemical shifts from NMR and m/ z data from MS, together with fragment analysis from MS (E) scans, leads not only to the detection of numerous endogenous urinary metabolites but also the identification of drug metabolites that are part of the latent use of drugs by the population. We show previously unreported positive mode ions of ibuprofen metabolites with their NMR correlates and suggest the detection of new metabolites of disopyramide in the population samples. This approach is of great potential value in the description of population xenometabolomes and in population pharmacology studies, and indeed for drug metabolism studies in general.

Disopyramide is an effective inhibitor of mutant HERG K+ channels involved in variant 1 short QT syndrome.

The recently identified idiopathic short QT syndrome (SQTS) is associated with an increased risk of arrhythmia and sudden death. The use of implantable cardioverter defibrillators helps to protect SQTS patients from ventricular fibrillation; however, pharmacological treatments to normalise the QT interval are limited: thus far only quinidine has been found to be effective in a subset of patients, with the SQT1 variant. SQT1 is associated with an amino acid substitution (N588K) in the KCNH2-encoded HERG K(+) channel that reduces HERG current (I(HERG)) inactivation and sensitivity to drug block. We demonstrate here that the N588K-HERG mutation only slightly attenuates I(HERG) blockade by the Class Ia antiarrhythmic drug disopyramide (1.5-fold elevation of IC(50)), compared to quinidine (3.5-fold elevation of IC(50)) and the Class III antiarrhythmic drug E-4031 (11.5-fold elevation of IC(50)). Thus, of the drugs studied to date, disopyramide is the one least affected by the SQT1 HERG mutation. Disopyramide is associated with QT prolongation in normal use and our findings provide a rational basis for its evaluation as a treatment for SQT1.

Repolarization dynamics in patients with idiopathic ventricular fibrillation: pharmacological therapy with bepridil and disopyramide.

The electrocardiographic parameters relating occurrence of ventricular fibrillation (VF) episodes in patients with idiopathic VF (IVF) are still unknown. The aim of this study was to clarify efficacy of pharmacological therapy in patients with IVF with respect to repolarization dynamics. The study group consisted of 8 men (age 43.6 +/- 9.1 years) with IVF (Brugada type 5 patients, prominent J wave in the inferior leads 3 patients) who had documented spontaneous episodes of VF, 7 of whom had implantable cardioverter defibrillators. The relation between QT and RR interval was analyzed from 24-hour Holter ECG using an automatic analyzing system before and after pharmacological therapy (bepridil 5 and disopyramide 3). From QT-RR linear regression lines, QT intervals were determined at RR intervals of 0.6 second [QT(0.6)], 1.0 second [QT(1.0)], and 1.2 seconds [QT(1.2)]. Pharmacological therapy increased the slope of QT-RR regression line from 0.105 +/- 0.020 to 0.144 +/- 0.037 (P < 0.05). Accordingly, QT(1.0) and QT(1.2) became longer after drug therapy [QT(1.0), 0.382 +/- 0.016 seconds vs 0.414 +/- 0.016 seconds (P < 0.01); QT(1.2), 0.403 +/- 0.017 seconds vs 0.442 +/- 0.021 seconds (P < 0.01)]. However, QT(0.6) did not change after drug administration. Before drug therapy the average episodes of VF were 5.5 +/- 5.8 (range 1 to 17) during the observation period of 19.3 +/- 17.6 months (range 6 to 60 months). After drug therapy, 6 patients had no episode of VF for 24 to 120 months (66.0 +/- 38.5 months). Two patients had a single episode of VF for 12- and 96-month follow-ups. Pharmacological therapy decreased the frequency of VF episodes in association with prolongation of QT intervals at slower heart rates. Not only J wave and ST elevation but also shorter QT intervals at slower heart rates may represent an electrophysiological substrate for development of VF episodes in these specific IVF patients.

Human organic cation transporter 3 mediates the transport of antiarrhythmic drugs.

Antiarrhythmic drugs have been considered to be transported by the organic cation transport system. The purpose of this study was to elucidate the molecular mechanism underlying the transport of antiarrhythmic drugs using cells from the second segment of the proximal tubule (S2) cells of mice expressing human-organic cation transporter 3 (S2 human-OCT3). The antiarrhythmic drugs tested were cibenzoline, disopyramide, lidocaine, mexiletine, phenytoin, pilsicanide, procainamide and quinidine. Human-OCT3 mediated a time- and dose-dependent uptake of quinidine and lidocaine, with Km values of 216 and 139 microM, respectively. Human-OCT3 also mediated the uptake of disopyramide and procainamide but not that of phenytoin. All antiarrhythmic drugs tested inhibited histamine uptake mediated by human-OCT3 in a dose-dependent manner. The IC50 values of antiarrhythmic drugs for human-OCT3 ranged between 0.75 and 656 microM. Kinetic analysis revealed that disopyramide, lidocaine, procainamide and quinidine inhibited histamine uptake mediated by human-OCT3 in a competitive manner. In conclusion, these results suggest that human-OCT3 mediates the transport of antiarrhythmic drugs, which may be the mechanism underlying the distribution and the elimination of these drugs.

Ropivacaine combined with various anti-arrhythmic drugs results in mild alterations in myocardial contractility in pigs.

PURPOSE: The present study was undertaken following the observation of a marked decrease in myocardial contractility after ropivacaine in a patient on amiodarone, in order to investigate the cardiovascular effects of combining ropivacaine with anti-arrhythmic drugs (AARD). METHODS: Anesthetized domestic pigs were treated with disopyramide, flecainide, atenolol, amiodarone, diltiazem or nicardipine at a dose leading to blood levels obtained in treated patients, then received 1 mg*kg(-1) ropivacaine. Blood pressure (BP), left venticular (LV) dP/dt max, sinus heart rate, and intraventricular conduction time were measured before and following the administration of AARD, and following ropivacaine at different time points. RESULTS: All tested AARD induced the expected hemodynamic and electrophysiologic effects. Following ropivacaine, a 20 to 35% decrease in LV dP/dt max of prolonged duration was observed with amiodarone only. A brief 10 to 20% decrease in mean BP was observed in all animals, except those treated with nicardipine who sustained an important and prolonged decrease in BP. All other variables were not significantly affected. DISCUSSION: The combination of ropivacaine with AARD was always associated with a slight drop in LV dP/dt max. The effect on mean BP was slight, except with nicardipine. Clinicians should be aware of the interactions of ropivacaine with AARD, especially amiodarone and nicardipine.

[Anti-arrhythmic therapy: diagnostic possibilities of signal-averaged electrocardiography and heart rate variability]. / Antiaritmicheskaia terapiia: diagnosticheskie vozmozhnosti signal-usrednennoi EKG i variabel'nosti ritma serdtsa.

Effects of monotherapy with class IC, II and III antiarrhythmic drugs on parameters of signal averaged (SA) ECG and heart rate variability were studied in 88 patients (mean age 45.6+/-7.8 years). Class IC drugs (ethacizine, disopyramide) caused worsening of qualitative parameters of SA ECG and appearance of ventricular late potentials. Therapy with beta-adrenoblockers, amiodarone and sotalol in patients with ventricular arrhythmias was associated with improvement of parameters of SA ECG, lowering of sympathetic and augmentation of parasympathetic activity without sings of arrhrythmogenic and negative inotropic effects. Combination of noninvasive diagnostic methods including SA ECG, temporal and spectral analysis of heart rate variability, Holter ECG monitoring can facilitate selection of appropriate antiarrhythmic therapy and control of its efficacy.

Inhibitory effect of erythromycin on potassium currents in rat ventricular myocytes in comparison with disopyramide.

Disopyramide, a class Ia antiarrhythmic agent, has been reported to induce torsades de pointes (TdP) associated with excessive QT prolongation in electrocardiogram (ECG), especially when concomitantly administered with erythromycin, a macrolide antibiotic agent. In this study, we have evaluated the effects of erythromycin on action potential duration (APD) and potassium currents in rat ventricular myocytes in comparison with disopyramide. We have evaluated the relationship between in-vitro potassium current inhibition and in-vivo QT prolongation observed in a previous study. Action potentials and membrane potassium currents, including delayed rectifier current (I(K)) and transient outward current (I(to)), were recorded using a whole-cell patch clamp method in enzymatically-dissociated ventricular cells. Erythromycin and disopyramide prolonged APD in a concentration-dependent manner. Disopyramide (10-100 microM) and erythromycin (100 microM) led to increases in the APD at 90% repolarization level. Disopyramide reduced I(K) (IC50 = 37.2 +/- 0.17 microM) and I(to) (IC50 = 20.9 +/- 0.13 microM) while erythromycin reduced I(K) (IC50 = 60.1 +/- 0.29 microM) but not I(to). The observed prolongation of APD might be ascribed to the inhibition of potassium currents. Erythromycin produced the prolongation of APD and the inhibition of potassium currents with a lag time after addition of the drugs, which suggested that erythromycin might not reach potassium channels from outside the ventricular cells. The potency of disopyramide was almost equivalent under in-vitro and in-vivo conditions. However, potency of erythromycin in-vitro was far weaker than that in-vivo reported in a previous study, presumably due to a difference in the uptake of erythromycin into ventricular myocytes between in-vivo and in-vitro conditions. Therefore, when drug-induced risks of QT prolongation are to be evaluated, the difference of potencies between in-vitro and in-vivo should be taken into consideration.

Characterisation of recombinant HERG K+ channel blockade by the Class Ia antiarrhythmic drug procainamide.

Class Ia antiarrhythmic drugs, including procainamide (PROC), are associated with cardiac sodium channel blockade, delayed ventricular repolarisation and with a risk of ventricular pro-arrhythmia. The HERG K(+) channel is frequently linked to drug-induced pro-arrhythmia. Therefore, in this study, interactions between PROC and HERG K(+) channels were investigated, with particular reference to potency and mechanism of drug action. Whole-cell patch-clamp recordings of HERG current (I(HERG)) were made at 37 degrees C from human embryonic kidney (HEK 293) cells stably expressing the HERG channel. Following activating pulses to +20 mV, I(HERG) tails were inhibited by PROC with an IC(50) value of approximately 139 microM. I(HERG) blockade was found to be both time- and voltage-dependent, demonstrating contingency upon HERG channel gating. However, I(HERG) inhibition by PROC was relieved by depolarisation to a highly positive membrane potential (+80 mV) that favoured HERG channel inactivation. These data suggest that PROC inhibits the HERG K(+) channel by a primarily 'open' or 'activated' channel state blocking mechanism and that avidity of drug-binding is decreased by extensive I(HERG) inactivation. The potency of I(HERG) blockade by PROC is much lower than for other Class Ia agents that have been studied previously under analogous conditions (quinidine and disopyramide), although the blocking mechanism appears similar. Thus, differences between the chemical structure of PROC and other Class Ia antiarrhythmic drugs may help provide insight into chemical determinants of blocking potency for agents that bind to open/activated HERG channels.

The effect of commonly used drugs on angiogenesis.

BACKGROUND: Within the last decade, there has been much interest in the area of tumor angiogenesis, with the advent of many new anti-angiogenic drugs undergoing testing in cancer clinical Phases II and III. Many of the cancer patients also take multiple medications for a variety of chronic illnesses. Because of possible drug-drug interactions, it is important to investigate the effect that commonly prescribed medications may have on angiogenesis. MATERIALS AND METHODS: In this pilot study, we assessed the effect of the following drugs on in vitro angiogenesis: atenolol, diltiazem, enalapril, disopyramide, mexiletine, coumadin, cimetidine and omeprazole. RESULTS & CONCLUSION: We observed that, although some of these drugs at massive doses inhibited endothelial proliferation, they did not affect in vitro angiogenesis at human therapeutic ranges.

Comparison of the effect of class IA antiarrhythmic drugs on transmembrane potassium currents in rabbit ventricular myocytes.

BACKGROUND: The blockade of cardiac transmembrane potassium channels, which is commonly seen with various antiarrhythmic drugs, plays an important role in their mechanism of action. We studied and compared the less-explored effects of three Class IA antiarrhythmics on the transient outward current (I(to)) and on the inward rectifier (I(kl)), ATP sensitive (I(KATP)), and delayed rectifier (I(K)) potassium currents in rabbit ventricular myocytes. METHODS AND RESULTS: Transmembrane currents were measured by applying the whole-cell configuration of the patch-clamp technique at 37 degrees C in myocytes enzymatically isolated from rabbit ventricular preparations. Quinidine (10 microM), disopyramide (10 microM), and procainamide (50 microM) were studied at concentrations close to or exceeding the therapeutic plasma level. All studied drugs significantly decreased the amplitude of I(KATP) (activated by 50 microM pinacidil) and I(K) currents. None of them influenced significantly I(kl). The amplitude of I(to) was decreased by quinidine and disopyramide but was not considerably altered by procainamide. The fast inactivation of I(to) was not changed by procainamide and was significantly accelerated by quinidine and disopyramide. CONCLUSION: Although quinidine, disopyramide, and procainamide are all classified as Class IA antiarrhythmics, these drugs had different effects on various potassium currents, which may partially explain their distinct effect on repolarization in various cardiac tissues and on cardiac arrhythmias in clinical settings.