How to determine cardiac ion channels targeted by drugs using the isolated rabbit ventricular wedge model.

Introduction: The rabbit left ventricular wedge (RLVW) has been demonstrated as a highly sensitive and specific preclinical model in assessing drug-induced QT prolongation and proarrhythmias. However, there is a need to determine drugs' cardiac ion channel profiles beyond QT measurement. In this study, we present an approach to determine cardiac ion channels targeted by drugs with analyzing a few key ECG parameters plus a contractility parameter obtained from the RLVW.
Methods: The RLVW assay was used for testing 18 drugs with well-known ion channel profiles. A transmural ECG and isometric contractility were recorded. Five parameters including QRS, QT, Tp-e/QT ratio, QT-BCL slope and the positive staircase response of contractility were analyzed.
Results: There were distinguished drug-induced ECG and contractility changes from which targeted cardiac ion channels by drugs could be determined. Inhibition of sodium channel resulted in rate-dependent QRS widening, QT and Tp-e shortening and a reduced QT-BCL slope. Although both IKr and IKs blockers prolonged QT interval, IKr blockers but not IKs increased Tp-e/QT ratio. Both potassium channel openers and calcium channel blockers markedly shortened QT and Tp-e intervals, but only calcium channel blockers could reverse the positive staircase response of contractility.
Discussion: The results in the present study are correlated closely to the drugs' well-known clinical profiles. This indicates that the RLVW assay with an adequate experimental protocol plus analysis of 5 key parameters is highly valuable in preclinical assessment of drug candidates for their detailed ion channel activities, proarrhythmic risks and other adverse effects. The limitations of the RLVW assay are also addressed.
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