Choice of cardiac tissue plays an important role in the evaluation of drug-induced prolongation of the QT interval in vitro in rabbit.
BACKGROUND
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 x 10(-8) M), sertindole (1 x 10(-6) M), erythromycin (3 x 10(-4) M) and sparfloxacin (1 x 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.


CONCLUSIONS
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.
Center of Excellence for Cardiovascular Safety Research, Johnson and Johnson Pharmaceutical Research and Development, B-2340 Beerse, Belgium. hlu@prdbe.jnj.com
Hua Rong
HR
Lu
Eddy
E
Vlaminckx
Ard
A
Teisman
David J
DJ
Gallacher
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002317:Cardiovascular Agents / Q000009:adverse effects* / Q000145:classification; D004353:Drug Evaluation, Preclinical / Q000379:methods*; D064420:Drug-Related Side Effects and Adverse Reactions; D004594:Electrophysiology; D004917:Erythromycin / Q000494:pharmacology; D005260:Female; D024841:Fluoroquinolones / Q000494:pharmacology; D006321:Heart / Q000187:drug effects* / Q000503:physiopathology; D007093:Imidazoles / Q000494:pharmacology; D007211:Indoles / Q000494:pharmacology; D008133:Long QT Syndrome / Q000139:chemically induced* / Q000503:physiopathology; D009206:Myocardium; D009924:Organ Culture Techniques; D010477:Perfusion; D004364:Pharmaceutical Preparations / Q000145:classification; D010627:Phenethylamines / Q000494:pharmacology; D011817:Rabbits; D012680:Sensitivity and Specificity; D013449:Sulfonamides / Q000494:pharmacology; D017180:Tachycardia, Ventricular / Q000139:chemically induced; D016171:Torsades de Pointes / Q000139:chemically induced
D002317:Cardiovascular Agents; D024841:Fluoroquinolones; D007093:Imidazoles; D007211:Indoles; D004364:Pharmaceutical Preparations; D010627:Phenethylamines; D013449:Sulfonamides; D004917:Erythromycin; C066304:sertindole; C061363:sparfloxacin; C063533:dofetilide