The effects of plasma proteins on delayed repolarization in vitro with cisapride, risperidone, and D, L-sotalol.

INTRODUCTION: Drug-induced long QT syndrome (LQTS) has been linked to arrhythmias (including Torsades de Pointes and sudden cardiac death), and has led to an increased awareness of the potential risk of delayed repolarization in vitro and in vivo. However, in vitro assessments of delayed repolarization have not been fully predictive of in vivo effects. METHODS: To define the extent to which plasma protein binding (ppb) contributes to such disparities in repolarization studies, we compared drug-induced prolongation of the canine Purkinje fiber action potential duration (APD(90)) in vitro during superfusion with 100% Tyrode's solution (Tyrodes), canine plasma [50% plasma/50% Tyrodes] and a 5% solution of recombinant human serum albumin in Tyrodes (HSA). Drugs evaluated included cisapride (>98% ppb), risperidone (90% ppb), and d, l-sotalol (negligible ppb). Effects on APD were monitored using standard microelectrode techniques under physiologic conditions and temperature ([K(+)]=4 mM, 37 degrees C) during slow stimulation (2 s basic cycle length). RESULTS: The effects of cisapride and risperidone on Purkinje fiber APD(90) were significantly attenuated in the presence of plasma proteins. However, with cisapride, the extent of reduction with plasma proteins was significantly less than predicted based on calculated free drug levels. DISCUSSION: We conclude that while plasma protein binding does reduce APD prolongation seen with bound drugs, this effect is not well correlated with the calculated plasma protein binding or expected clinical free fraction. Because of the complex drug interactions that occur in plasma, the electrophysiological effects seen with bound drugs are not well correlated with the calculated free fraction and thus caution should be exercised when assigning a predictive safety window. Thus, the canine Purkinje fiber assay is useful for defining the modulation of delayed repolarization due to plasma protein binding of novel therapeutic agents.

The utility of hERG and repolarization assays in evaluating delayed cardiac repolarization: influence of multi-channel block.

Drug-induced delayed cardiac repolarization is a recognized risk factor for proarrhythmia and is associated with block of IKr (the potassium current encoded by the human ether-a- go-go-related gene [hERG]). To evaluate the utility of 2 in vitro assays widely used to assess delayed repolarization, we compared the effects of haloperidol and 9 structurally diverse drugs in a hERG and repolarization (canine Purkinje fiber action potential duration [APD]) assay over wide concentrations. Despite potent hERG current block (IC50 = 0.174 microM), haloperidol elicited a bell-shaped concentration-response relationship for APD prolongation, with lesser prolongation (and reduced plateau height) observed with concentrations eliciting maximal hERG block, consistent with multi-channel block at higher concentrations. Consistent with this hypothesis, APD prolongation with the specific IKr blocker dofetilide was a) reduced by concomitant administration of nifedipine (calcium current block) and b) reversed by lidocaine (late sodium current block). Additional studies demonstrated prominent (>50%) hERG inhibition with most (9/10) drugs despite wide APD changes (158% prolongation - 16% shortening), consistent with multi-channel block. The poor correlation between hERG and repolarization assays suggests that the hERG assay oversimplifies drug effects on the complex repolarization process for drugs demonstrating multi-channel block and that neither assay alone adequately predicts proarrhythmic risk.