Jacques Turgeon

Hydrophilicity/ lipophilicity: relevance for the pharmacology and clinical effects of HMG-CoA reductase inhibitors

The recent development of specific competitive inhibitors of the hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase such as lovastatin, simvastatin, pravastatin and fluvastatin has provided an important new and effective approach to the treatment of hyperlipidaemia and atherosclerosis. These agents are designed to be hepatoselective because the primary site of cholesterol synthesis is the liver and peripheral inhibition of cholesterol synthesis would be more likely to cause adverse drug effects. In this review, Bettina Hamelin and Jacques Turgeon discuss how specific physico-chemical and pharmacological properties (first-pass effect or carrier-mediated uptake) confer hepatoselectivity to either lipophilic or hydrophilic HMG-CoA reductase inhibitors.

Block of the rapid component of the delayed rectifier potassium current by the prokinetic agent cisapride underlies drug-related lengthening of the QT interval.

BACKGROUND Lengthening of the QT interval and torsades de pointes resulting in cardiac arrests and deaths have been noticed during treatment with cisapride, a newly developed gastrointestinal prokinetic agent. The rapid (I[Kr]) and slow (I[Ks]) components of the delayed rectifier current (I[K]) are candidate ionic currents to explain cisapride-related toxicity because of their role in repolarization of cardiac ventricular myocytes. Our objectives were to (1) characterize effects of cisapride on two major time-dependent outward potassium currents involved in the repolarization of cardiac ventricular myocytes, I(Kr) and I(Ks), and (2) determine action potential-prolonging effects of cisapride on isolated hearts. METHODS AND RESULTS A first set of experiments was performed in isolated guinea pig ventricular myocytes with the whole-cell configuration of the patch-clamp technique. Cells were held at -40 mV while time-dependent outward currents were elicited by depolarizing pulses lasting either 250 ms (I[K250]) or 5000 ms (I[K5000]). Effects of cisapride on the I(Kr) component were assessed by measurement of time-dependent activating currents elicited by short pulses (250 ms; I[K250]) to low depolarizing potentials (-20, -10, and 0 mV). Time-dependent activating currents elicited by long pulses (5000 ms; I[K5000]) to positive potentials (>+30 mV) were recorded to assess effects of the drug on the I(Ks) component. A second set of experiments was conducted in isolated guinea pig hearts buffer-perfused in the Langendorff mode to assess effects of the drug on monophasic action potential duration measured at 90% repolarization (MAPD90). Hearts were exposed to cisapride 100 nmol/L at decremental pacing cycle lengths of 250, 225, 200, 175, and 150 ms to determine reverse frequency-dependent effects of the drug. Overall, 112 myocytes were exposed to seven concentrations of cisapride (10 nmol/L to 10 micromol/L). Cisapride inhibited I(Kr), the major time-dependent outward current elicited by short pulses (I[K250]) to low depolarizing potentials, in a concentration-dependent manner with an IC50 of 15 nmol/L (therapeutic levels, 50 to 200 nmol/L). Conversely, block of I(Ks) by the drug was less potent (estimated IC50 >10 micromol/L). In isolated hearts (n=9 experiments), cisapride 100 nmol/L increased MAPD90 by 23+/-3 (P<.05) at a basic cycle length of 250 ms but by only 7+/-1 ms (P<.05) at a basic cycle length of 150 ms. CONCLUSIONS Block of I(Kr) gives an explanation to lengthening of cardiac repolarization observed in isolated guinea pig hearts. Potent block of I(Kr) is also likely to underlie prolongation of the QT interval observed in patients receiving clinically recommended doses of cisapride as well as severe cardiac toxicity (torsades de pointes) observed in patients with increased plasma concentrations of the drug.

Domperidone Should Not Be Considered a No-Risk Alternative to Cisapride in the Treatment of Gastrointestinal Motility Disorders

BackgroundSeveral cases of QT prolongation and ventricular tachyarrhythmia have been reported with domperidone, a gastrokinetic and antiemetic agent available worldwide but still under investigation in the United States. Although electrolyte disturbances such as hypokalemia could account for some of these events, we hypothesized that domperidone may have unsuspected electrophysiological effects predisposing some patients to proarrhythmia. Methods and ResultsStudies were undertaken in 9 isolated guinea pig hearts, which demonstrated reverse use–dependent prolongation of cardiac repolarization by 100 nmol/L domperidone. Action potential duration increased 27% from baseline with domperidone (from 114±3 to 145±2 ms) during pacing at a cycle length of 250 ms, and a 9% increase (from 97±2 to 106±3 ms) was seen with pacing at a cycle length of 150 ms. Experiments in human ether-a-go-go–related gene (HERG)-transfected Chinese hamster ovary cells (n=32) demonstrated a concentration-dependent block of the rapid component (IKr) of the delayed rectifier potassium current. The tail current decreased by 50% at 162 nmol/L domperidone. ConclusionsDomperidone possesses cardiac electrophysiological effects similar to those of cisapride and class III antiarrhythmic drugs. These effects are observed at clinically relevant concentrations of the drug. Therefore, domperidone should not be considered a no-risk alternative to cisapride, a drug that was recently withdrawn from the US market.

Effect of a low-glycaemic index -low-fat-high protein diet on the atherogenic metabolic risk profile of abdominally obese men

It has been suggested that the current dietary recommendations (low-fat-high-carbohydrate diet) may promote the intake of sugar and highly refined starches which could have adverse effects on the metabolic risk profile. We have investigated the short-term (6-d) nutritional and metabolic effects of an ad libitum low-glycaemic index-low-fat-high-protein diet (prepared according to the Montignac method) compared with the American Heart Association (AHA) phase I diet consumed ad libitum as well as with a pair-fed session consisting of the same daily energy intake as the former but with the same macronutrient composition as the AHA phase I diet. Twelve overweight men (BMI 33.0 (sd 3.5) kg/m2) without other diseases were involved in three experimental conditions with a minimal washout period of 2 weeks separating each intervention. By protocol design, the first two conditions were administered randomly whereas the pair-fed session had to be administered last. During the ad libitum version of the AHA diet, subjects consumed 11695.0 (sd 1163.0) kJ/d and this diet induced a 28 % increase in plasma triacylglycerol levels (1.77 (sd 0.79) v. 2.27 (sd 0.92) mmol/l, P<0.05) and a 10 % reduction in plasma HDL-cholesterol concentrations (0.92 (sd 0.16) v. 0.83 (sd 0.09) mmol/l, P<0.01) which contributed to a significant increase in cholesterol:HDL-cholesterol ratio (P<0.05), this lipid index being commonly used to assess the risk of coronary heart disease. In contrast, the low-glycaemic index-low-fat-high-protein diet consumed ad libitum resulted in a spontaneous 25 % decrease (P<0.001) in total energy intake which averaged 8815.0 (sd 738.0) kJ/d. As opposed to the AHA diet, the low-glycaemic index-low-fat-high-protein diet produced a substantial decrease (-35 %) in plasma triacylglycerol levels (2.00 (sd 0.83) v. 1.31 (sd 0.38) mmol/l, P<0.0005), a significant increase (+1.6 %) in LDL peak particle diameter (251 (sd 5) v. 255 (sd 5) A, P<0.02) and marked decreases in plasma insulin levels measured either in the fasting state, over daytime and following a 75 g oral glucose load. During the pair-fed session, in which subjects were exposed to a diet with the same macronutrient composition as the AHA diet but restricted to the same energy intake as during the low-glycaemic index-low-fat-high-protein diet, there was a trend for a decrease in plasma HDL-cholesterol levels which contributed to the significant increase in cholesterol:HDL-cholesterol ratio noted with this condition. Furthermore, a marked increase in hunger (P<0.0002) and a significant decrease in satiety (P<0.007) were also noted with this energy-restricted diet. Finally, favourable changes in the metabolic risk profile noted with the ad libitum consumption of the low-glycaemic index-low-fat-high-protein diet (decreases in triacyglycerols, lack of increase in cholesterol:HDL-cholesterol ratio, increase in LDL particle size) were significantly different from the response of these variables to the AHA phase I diet. Thus, a low-glycaemic index-low-fat-high-protein content diet may have unique beneficial effects compared with the conventional AHA diet for the treatment of the atherogenic metabolic risk profile of abdominally obese patients. However, the present study was a short-term intervention and additional trials are clearly needed to document the long-term efficacy of this dietary approach with regard to compliance and effects on the metabolic risk profile.

Significant interaction between the nonprescription antihistamine diphenhydramine and the CYP2D6 substrate metoprolol in healthy men with high or low CYP2D6 activity

The prototype “classic” over‐the‐counter antihistamine diphenhydramine was shown to interact with the polymorphic P450 enzyme CYP2D6. This project was undertaken to investigate (1) whether diphenhydramine inhibits the biotransformation of the clinically relevant CYP2D6 substrate metoprolol in vitro and (2) whether this in vitro interaction results in a clinically significant pharmacokinetic and pharmacodynamic drug interaction in vivo. In vitro incubations were carried out with microsomes obtained from lymphoblastic cells transfected with CYP2D6 complementary deoxyribonucleic acid to determine the type and extent of inhibition. We then randomized 16 subjects with genetically determined high (extensive metabolizers) or low (poor metabolizers) CYP2D6 activity to receive metoprolol (100 mg) in the presence of steady‐state concentrations of diphenhydramine or placebo. In vitro, diphenhydramine was a potent competitive inhibitor of metoprolol α‐hydroxylation, exhibiting an inhibitory constant of 2 μmol/L and increasing the Michaelis‐Menten constant of metoprolol sixfold. In vivo, diphenhydramine decreased metoprolol oral and nonrenal clearances twofold and metoprolol→α‐hydroxymetoprolol partial metabolic clearance 2.5‐fold in extensive metabolizers (all P < .05) but not in poor metabolizers (P > .2). Although the hemodynamic response to metoprolol was unaltered by diphenhydramine in poor metabolizers (P > .05), metoprolol‐related effects on heart rate, systolic blood pressure, and Doppler‐derived aortic blood flow peak velocity were more pronounced and lasted significantly longer in extensive metabolizers receiving diphenhydramine compared with poor metabolizers and extensive metabolizers receiving placebo. We conclude that diphenhydramine inhibits the metabolism of metoprolol in extensive metabolizers, thereby prolonging the negative chronotropic and inotropic effects of the drug. Clinically relevant drug interactions may occur between diphenhydramine and many CYP2D6 substrates, particularly those with a narrow therapeutic index.

Droperidol lengthens cardiac repolarization due to block of the rapid component of the delayed rectifier potassium current

Droperidol Blocks Cardiac IKr. Introduction: Torsades de pointes have been observed during treatment with droperidol, a butyrophenone neuroleptic agent. Our objectives were (1) to characterize the effects of droperidol on cardiac repolarization and (2) to evaluate effects of droperidol on a major time‐dependent outward potassium current involved in cardiac repolarization (IKr).

The disposition of fluoxetine but not sertraline is altered in poor metabolizers of debrisoquin

Substrates and inhibitors of the cytochrome P450 isozyme CYP2D6 have overlapping structural characteristics. Two prototype serotonin uptake inhibitors, sertraline and fluoxetine, share these structural criteria and have been identified as potent inhibitors of CYP2D6 in vitro. The current study was undertaken to investigate whether genetically determined CYP2D6 activity alters the disposition of sertraline or fluoxetine or both.

Erythromycin Blocks the Rapid Component of the Delayed Rectifier Potassium Current and Lengthens Repolarization of Guinea Pig Ventricular Myocytes

BACKGROUND Administration of erythromycin to humans has been associated with lengthening of cardiac repolarization and even proarrhythmia. The objectives of our study were to describe effects of erythromycin on repolarization of isolated hearts and to determine effects of the drug on major K+ currents involved in cardiac repolarization. METHODS AND RESULTS A first set of experiments was conducted in isolated, buffer-perfused guinea pig hearts electrically stimulated at a basic cycle length of 250 ms. In this model, erythromycin 10(-4) mol/L increased monophasic action potential duration measured at 90% repolarization (MAPD90) by 40 +/- 7 ms. Increase in MAPD90 was reproducibly observed in seven hearts studied. To study the mechanism of these effects on cardiac repolarization, a second set of experiments was performed in isolated guinea pig ventricular myocytes using the whole cell configuration of the patch-clamp technique. In these cells, erythromycin 10(-4) mol/L decreased by about 40% (P < .05 versus baseline) the time-dependent outward K+ current elicited by short depolarizations (250 ms) to low depolarizing voltages (-20 to 0 mV). In contrast, the drug was without significant effects on the time-dependent K+ current elicited by long pulses (5000 ms) to high depolarizing voltages (+10 to +50 mV), on the time-independent background current (mostly IKl), and on the slow inward calcium current. CONCLUSIONS The outward time-dependent K+ current blocked by erythromycin in isolated guinea pig ventricular myocytes had characteristics similar to those described for IKr. Selective block of this component of IK gives an explanation for the effects of erythromycin on cardiac repolarization. These effects were observed at clinically relevant concentrations reached after intravenous administration of the drug and warn for potential interactions with other action potential-lengthening drugs.

Sildenafil (Viagra) Prolongs Cardiac Repolarization by Blocking the Rapid Component of the Delayed Rectifier Potassium Current

BACKGROUND-Several cases of unexpected death have been reported with sildenafil in patients predisposed to ischemic cardiac events. Although acute episodes of ischemia could account for some of these deaths, we hypothesized that sildenafil may have unsuspected electrophysiological effects predisposing some patients to proarrhythmia. METHODS AND RESULTS-Studies were undertaken in 10 isolated guinea pig hearts that demonstrated prolongation of cardiac repolarization in a reverse use-dependent manner by sildenafil 30 mcmol/L. Action potential duration increased 15% from baseline 117+/-3 to 134+/-2 ms with sildenafil during pacing at 250 ms cycle length, whereas a 6% increase from 99+/-2 to 105+/-2 ms was seen with pacing at 150 ms cycle length. Experiments in human ether-a-go-go-related gene (HERG)-transfected HEK293 cells (n=30) demonstrated concentration-dependent block of the rapid component (I(Kr)) of the delayed rectifier potassium current: activating current was 50% decreased at 100 mcmol/L. This effect was confirmed using HERG-transfected Chinese hamster ovary (CHO) cells, which exhibit no endogenous I(K)-like current. CONCLUSIONS-Sildenafil possesses direct cardiac electrophysiological effects similar to class III antiarrhythmic drugs. These effects are observed at concentrations that may be found in conditions of impaired drug elimination such as renal or hepatic insufficiency, during coadministration of another CYP3A substrate/inhibitor, or after drug overdose and offer a new potential explanation for sudden death during sildenafil treatment.

Involvement of CYP2D6 activity in the N-oxidation of procainamide in man.

Occurrence of a lupus-like syndrome in a significant number of patients treated with procainamide has limited the clinical use of this antiarrhythmic drug. In-vitro studies conducted in our laboratory have demonstrated that CYP2D6 is the major cytochrome P450 isozyme involved in the formation of N-hydroxyprocainamide, a metabolite potentially involved in the drug-induced lupus erythematosus syndrome observed with procainamide. In the current study, we evaluated the role of CYP2D6 activity in the in-vivo oxidation of procainamide in man. Nineteen healthy individuals, 13 with high (extensive metabolizers) and six with low (poor metabolizers) CYP2D6 activity, received a single 500 mg oral dose of procainamide hydrochloride on two occasions, once alone (period 1) and once during the concomitant administration of the selective inhibitor quinidine (50 mg four times daily; period 2). Blood and urine samples were collected over 36 h after drug administration of procainamide and analysed for procainamide and its major metabolites (N-acetylprocainamide, desethylprocainamide, N-acetyl-desethylprocainamide, p-aminobenzoic acid and its N-acetylated derivative, and nitroprocainamide). No differences were observed in the oral and renal clearances of procainamide between extensive metabolizers and poor metabolizers during either study period. However, partial metabolic clearance of procainamide to desethylprocainamide was significantly greater in extensive metabolizers than in poor metabolizers during both periods. Most importantly, the urinary excretion of nitroprocainamide during period 1 was measurable in 7/13 extensive metabolizers but in none of the poor metabolizers. During the concomitant administration of quinidine, nitroprocainamide could not be detected in the urine of any individuals tested. Therefore, our results suggest that CYP2D6 is involved in the in-vivo aliphatic amine deethylation and N-oxidation of procainamide at its arylamine function in man. Further studies are needed to demonstrate whether a low CYP2D6 activity, either genetically determined or pharmacologically modulated, could prevent drug-induced lupus erythematosus syndrome observed during chronic therapy with procainamide.