QT PRODACT: A Multi-site Study of In Vitro Action Potential Assays on 21 Compounds in Isolated Guinea-Pig Papillary Muscles

To construct a non-clinical database for drug-induced QT interval prolongation, the electrophysiological effects of 11 positive and 10 negative compounds on action potentials (AP) in guinea-pig papillary muscles were investigated in a multi-site study according to a standard protocol. Compounds with a selective inhibitory effect on the rapidly activated delayed rectifier potassium current (IKr) prolonged action potential duration at 90% repolarization (APD90) in a concentration-dependent manner, those showing Ca2+ current (ICa) inhibition shortened APD30, and those showing Na+ current (INa) inhibition decreased action potential amplitude (APA) and Vmax. Some of the mixed ion-channel blockers showed a bell-shaped concentration-response curve for APD90, probably due to their blockade of INa and/or ICa, sometimes leading to a false-negative result in the assay. In contrast, all positive compounds except for terfenadine and all negative compounds with IKr-blocking activity prolonged APD30-90 regardless of their INa- and/or ICa-blocking activities, suggesting that APD30-90 is a useful parameter for evaluating the IKr-blocking activity of test compounds. Furthermore, the assay is highly informative regarding the modulation of cardiac ion channels by test compounds. Therefore, when APD90 and APD30-90 are both measured, the action potential assay can be considered a useful method for assessing the risk of QT interval prolongation in humans in non-clinical safety pharmacology studies.