Frequency dependent effects of class I antiarrhythmic agents studied in patients with implanted pacemakers

PKPD modelling of PR and QRS intervals in conscious dogs using standard safety pharmacology data

INTRODUCTION

Pharmacokinetic-pharmacodynamic (PKPD) modelling can improve safety assessment, but few PKPD models describing drug-induced QRS and PR prolongations have been published. This investigation aims to develop and evaluate PKPD models for describing QRS and PR effects in routine safety studies.

METHODS

Exposure and telemetry data from safety pharmacology studies in conscious beagle dogs were acquired. Mixed effects baseline and PK-QRS/PR models were developed for the anti-arrhythmic compounds AZD1305, flecainide, quinidine and verapamil and the anti-muscarinic compounds AZD8683 and AZD9164. RR interval correction and circadian rhythms were investigated for predicting baseline variability. Individual PK predictions were used to drive the pharmacological effects evaluating linear and non-linear direct and effect compartment models.

RESULTS

Conduction slowing induced by the tested anti-arrhythmics was direct and proportional at low exposures, whilst time delays and non-linear effects were evident for the tested anti-muscarinics. AZD1305, flecainide and quinidine induced QRS widening with 4.2, 10 and 5.6% μM(-1) unbound drug. AZD1305 and flecainide also prolonged PR with 13.5 and 11.5% μM(-1). PR prolongations induced by the anti-muscarinics and verapamil were best described by Emax models with maximal effects ranging from 55 to 95%. RR interval correction and circadian rhythm improved PR but not QRS modelling. However, circadian rhythm had minor impact on estimated drug effects.

DISCUSSION

Baseline and drug-induced effects on QRS and PR intervals can be effectively described with PKPD models using routine data, providing quantitative safety information to support drug discovery and development.

On the relationship between block of the cardiac Na⁺ channel and drug-induced prolongation of the QRS complex

BACKGROUND AND PURPOSE

Inhibition of the human cardiac Na(+) channel (hNa(v) 1.5) can prolong the QRS complex and has been associated with increased mortality in patients with underlying cardiovascular disease. The safety implications of blocking hNa(v) 1.5 channels suggest the need to test for this activity early in drug discovery in order to design out any potential liability. However, interpretation of hNa(v) 1.5 blocking potency requires knowledge of how hNa(v) 1.5 block translates into prolongation of the QRS complex.

EXPERIMENTAL APPROACH

We tested Class I anti-arrhythmics, other known QRS prolonging drugs and drugs not reported to prolong the QRS complex. Their block of hNa(v) 1.5 channels (as IC(50) values) was measured in an automated electrophysiology-based assay. These IC(50) values were compared with published reports of the corresponding unbound (free) plasma concentrations attained during clinical use (fC(max)) to provide an IC(50) : fC(max) ratio. KEY RESULTS For 42 Class I anti-arrhythmics and other QRS prolonging drugs, 67% had IC(50) : fC(max) ratios <30. For 55 non-QRS prolonging drugs tested, 72% had ratios >100. Finally, we determined the relationship between the IC(50) value and the free drug concentration associated with prolongation of the QRS complex in humans. For 37 drugs, QRS complex prolongation was observed at free plasma concentrations that were about 15-fold lower than the corresponding IC(50) at hNa(v) 1.5 channels.

CONCLUSIONS AND IMPLICATIONS

A margin of 30- to 100-fold between hNa(v) 1.5 IC(50) and fC(max) appears to confer an acceptable degree of safety from QRS prolongation. QRS prolongation occurs on average at free plasma levels 15-fold below the IC(50) at hNa(v) 1.5 channels.

LINKED ARTICLE

This article is commented on by Gintant et al., pp. 254-259 of this issue. To view this commentary visit http://dx.doi.org/10.1111/j.1476-5381.2011.01433.x.

Aggravation of Arrhythmia by Antiarrhythmic Drugs (Proarrhythmia)

Arrhythmia aggravation by antiarrhythmic drugs (proarrhythmia) can be caused by worsening or a change of a preexisting arrhythmia, development of a new arrhythmia, or development of a bradyarrhythmia. Aggravation of arrhythmia usually occurs within several days of beginning an antiarrhythmic drug or increasing the dose of the drug. The time of occurrence is based on the particular drug and its pharmacokinetic properties. Although there are no ways to predict the patient at risk for developing arrhythmia aggravation with any specific agents, risk factors include QT interval prolongation, elevated serum levels of the drug, electrolyte abnormalities, presence of heart failure, a history of a sustained ventricular tachyarrhythmia, and underlying myocardial ischemia.