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.

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

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 (I(K)r).

METHODS AND RESULTS

Isolated, buffer-perfused guinea pig hearts (n = 32) were stimulated at different pacing cycle lengths (150 to 250 msec) and exposed to droperidol in concentrations ranging from 10 to 300 nmol/L. Droperidol increased monophasic action potential duration measured at 90% repolarization (MAPD90) in a concentration-dependent manner by 9.8+/-2.3 msec (7.3%+/-0.7%) at 10 nmol/L but by 32.7+/-3.6 msec (25.7%+/-2.2%) at 300 nmol/L (250-msec cycle length). Increase in MAPD90 also was reverse frequency dependent. As noted previously, droperidol 300 nmol/L increased MAPD90 by 32.7+/-3.6 msec (25.7%+/-2.2%) at a pacing cycle length of 250 msec but by only 14.1+/-1.3 msec (13.6%+/-2.3%) at a pacing cycle length of 150 msec. Patch clamp experiments performed in isolated guinea pig ventricular myocytes demonstrated that droperidol decreases the time-dependent outward K+ current elicited by short depolarizations (250 msec; I(K)250) in a concentration-dependent manner. Estimated IC50 for I(K)250, which mostly underlies I(K)r, was 28 nmol/L. Finally, HERG K+ current elicited in HEK293 cells expressing high levels of HERG protein was decreased 50% by droperidol 32.2 nmol/L.

CONCLUSION

Potent block of I(K)r by droperidol is likely to underlie QT prolongation observed in patients treated at therapeutic plasma concentrations (10 to 400 nmol/L) of the drug.

CYP2D6 mutations and therapeutic outcome in schizophrenic patients

STUDY OBJECTIVE

To investigate whether a relationship exists between the most common known cytochrome P450 (CYP) isozyme 2D6 mutations and schizophrenia. Because most antipsychotic and antidepressant agents interact with CYP2D6, we also investigated clinical outcomes in schizophrenic poor metabolizers (PMs) and extensive metabolizers (EMs).

DESIGN

Prospective, observational study.

SETTING

Two psychiatric hospitals and a university-affiliated nonpsychiatric hospital.

SUBJECTS

Thirty-nine consecutive schizophrenic patients (POP 1), 89 schizophrenics of French Canadian origin (POP 2), and 384 healthy French Canadians (POP 3).

INTERVENTION

All study subjects were genotyped for CYP2D6 mutant alleles. POP 1 patients were evaluated before and after 21 or more days of treatment with antipsychotic drugs metabolized at least in part by CYP2D6.

MEASUREMENTS AND MAIN RESULTS

Whole blood was collected to determine CYP2D6 alleles *1, *3, *4, *5, *6, and *7 using standard restriction fragment length polymorphisms and polymerase chain reaction techniques. In comparison, CYP2D6 genotypes were determined in POP 2 and POP 3. Twenty-three (59.0%) of 39 patients in POP 1 were genotypically EM homozygotes, 15 (38.4%) were EM heterozygotes, and 1 (2.6%) was a PM. Similar genotype distributions were determined in POP 2 and in POP 3. Genotype distributions for all three populations were in Hardy-Weinberg equilibrium (p>0.05), and there was no significant difference among them (p=0.857). In POP 1, no differences were seen among genotypes in disease symptom severity, number and severity of adverse drug effects, or attitudes toward drug treatment at baseline and at the end of the study. In fact, all patients improved significantly during their hospital stay (all p<0.05), although independent of the CYP2D6 genotype.

CONCLUSION

Common CYP2D6 mutant alleles were not associated with schizophrenia or with disease symptoms, antipsychotic-related adverse effects, or attitudes toward treatment.

Thioridazine lengthens repolarization of cardiac ventricular myocytes by blocking the delayed rectifier potassium current

Proarrhythmia has been observed with the antipsychotic agent thioridazine (THIO). The mechanisms underlying these effects are unknown. The objectives of this study were 1) to characterize the effects of THIO on cardiac repolarization and 2) to determine whether lengthening of the Q-T interval could be explained by blocking major K+-repolarizing currents. Isolated, buffer-perfused guinea pig hearts (n = 32) were stimulated at various pacing cycle lengths (150-250 ms) and exposed to THIO at concentrations ranging from 300 nM to 3 microM. THIO increased monophasic action potential duration at 90% repolarization (MAPD90) in a concentration-dependent manner from 14.9 +/- 1.8 at 300 nM to 37.1 +/- 3.2 ms at 3 microM. Increase in MAPD90 was also reverse frequency-dependent; THIO (300 nM) increased MAPD90 by 14.9 +/- 1.8 ms at a pacing cycle length of 250 ms, but by only 7.7 +/- 1.2 ms at a pacing cycle length of 150 ms. Patch-clamp experiments demonstrated that THIO decreases the time-dependent outward K+ current elicited by short depolarizations (250 ms; IK250) in a concentration-dependent manner. Estimated IC50 for IK250, which mostly underlies IKr, was 1.25 microM. Time-dependent outward K+ current elicited in tsA201 cells expressing high levels of HERG protein was also decreased approximately 50% by 1.25 microM THIO. On the other hand, THIO was less potent (IC50 of 14 microM) to decrease time-dependent K+ current elicited by long pulses (5000 ms; IK5000). Under the latter conditions, IK5000 corresponds mainly to IKs. Thus, these results demonstrate block of K+ currents and lengthening of cardiac repolarization by THIO in a concentration-dependent manner. This may provide an explanation of Q-T prolongation observed in some patients treated with THIO.

Atherogenic, hemostatic, and other potential risk markers in subjects with previous isolated myocardial infarction compared with long-standing uncomplicated stable angina

BACKGROUND

Several atherogenic, hemostatic, inflammatory, and genetic parameters and markers have been implicated as risk factors in coronary artery disease, although whether they are risk factors for acute as opposed to chronic coronary disease is unclear.

METHODS AND RESULTS

Fifty subjects with an isolated myocardial infarction >3 months previously were compared with 50 subjects with a minimum 3-year history of stable angina, documented coronary artery disease, normal electrocardiogram and normal ventricular wall motion, and no episode suggesting infarction or unstable angina. Biologic variables analyzed included apolipoprotein B (apo B), lipoprotein (a), C-reactive protein (CRP), fibrinogen, factor VII, tissue plasminogen activator (TPA) and inhibitor (PAI-1), thrombin-antithrombin (TAT), fragment 1+2 (F1+2), von Willebrand factor (vWF), activated protein C resistance, homocyst(e)ine, anticardiolipin antibodies, blood group, and the angiotensin-converting enzyme insertion/deletion (I/D) and angiotensin II receptor gene polymorphisms. There were no significant differences between the 2 groups for any of the variables studied, although fibrinogen and F 1+2 tended to be slightly higher in the angina group (P = .09 for each). These significant correlations were present: age with fibrinogen, homocyst(e)ine, and vWF; factor VII with apo B, homocyst(e)ine, and TPA; apo B with TPA and CRP; CRP with fibrinogen, TPA, PAI-1, and factor VII; fibrinogen with vWF.

CONCLUSIONS

Examination of atherogenic, hemostatic, inflammation, and genetic variables in the clinically quiescent state permitted no distinction between subjects with a previous isolated myocardial infarction in contrast to those with long-standing uncomplicated stable angina, favoring the notion that acute coronary events occur at random on a varying background of atherosclerosis. The multiple correlations found among these variables also underscore their complex interaction in the atherosclerotic process.

Differential accumulation of two glycine-rich proteins during cold-acclimation alfalfa

Two mRNAs, MsaCiA and MsaCiB, encoding for proteins harboring glycine-rich motifs, accumulate in alfalfa during cold acclimation. Fusion polypeptides containing the amino acid sequences deduced from these mRNAs were produced in Escherichia coli and used to raise antibodies. Each antibody cross-reacted specifically with soluble polypeptides, MSACIA-32 and MSACIB, respectively. These polypeptides were detectable only in crowns of cold-acclimated plants, even though MsaCiA mRNA accumulated in both crows and leaves during cold acclimation. The analysis of parietal proteins showed that several MSACIA-related proteins, with a molecular mass of 32, 41 and 68 kDa, did accumulate in leaf cell walls and one of 59 kDa crown cell walls. This diversity is most probably due to a tissue-specific maturation of MSACIA. A discrepancy was found between the time-course of accumulation of MSACIB and the one of the corresponding transcript. These results indicate that timing and localization of MSACIA and MSACIB expression are different, and suggest that this differential expression involves both transcriptional and post-transcriptional events. Comparisons made among six cultivars of contrasting freezing tolerance suggest that low tolerance could be explained by failure to accumulate proteins like MSACIA and MSACIB at a sufficient level.