Choice of cardiac tissue plays an important role in the evaluation of drug-induced prolongation of the QT interval in vitro in rabbit

Abstract

Introduction Regulatory guidelines (CPMP/986/96, ICHS7B) recommend the use of isolated cardiac tissues, including Purkinje fibers, papillary muscles and ventricular trabeculae, for detecting potential drug-induced long QT. However, the differential sensitivity of these tissues in experimental drug-induced long QT is relatively unknown. We investigated the electrophysiological characteristics of these tissue types in vitro together with their different responses to drugs that are known to induce prolongation of the QT interval in man. Methods Electrophysiological parameters were measured in vitro using a micro-electrode technique. The isolated rabbit Purkinje fibers, papillary muscles or ventricular trabeculae were superperfused with Tyrode's solution and stimulated according to different stimulation protocols. The effects of dofetilide (1 × 10 − 8 M), sertindole (1 × 10 − 6 M), erythromycin (3 × 10 − 4 M) and sparfloxacin (1 × 10 − 4 M) were evaluated relative to solvent ( n = 8 to 12 for each group). Results In isolated Purkinje fibers, action potential duration at 90% repolarization (APD 90 at 1 Hz) was markedly prolonged by 55% (erythromycin), 103% (dofetilide), 118% (sertindole) and 88% (sparfloxacin) from baseline. The prolongation of APD 90 caused by these 4 compounds was associated with a 28% to 78% incidence of early afterdepolarizations (EADs) at 0.2 Hz only in the Purkinje fiber. In contrast, APD 90 was altered by erythromycin, dofetilide, sertindole and sparfloxacin only by + 15%, + 6%, − 7% or + 15%, respectively, in isolated papillary muscles, and by 33%, + 28%, + 4% or + 16%, respectively, in ventricular trabeculae. EADs were not induced by these four compounds in papillary muscles or in trabeculae. Reducing the stimulation rate to 0.2 Hz resulted in a 33% prolongation of APD 90 in Purkinje fibers, while APD 90 was shortened by 10% in papillary muscles and by 20% in ventricular trabeculae. Conclusion The present study demonstrates that the differential sensitivity of tissue types play an important role in the detection of drug-induced long APD and EADs. Indeed the Purkinje fiber was the only tissue type to display the well known phenomenon associated with I kr channel blockade (inverse-use dependence), when the stimulation rate was decreased below 1 Hz. Rabbit Purkinje fibers constitute the most sensitive tissue type for detecting drug-induced long action potential duration and EADs. As such the selection of tissue type needs to be taken into careful consideration in cardiac safety assessments when exploring drug induced long QT.