ORIGINAL CONTRIBUTION Effects of proarrhythmic drugs on relaxation time and beating pattern in rat engineered heart tissue

Abstract The assessment of proarrhythmic risks of drugs remains challenging. To evaluate the suitability of rat engineered heart tissue (EHT) for detecting proarrhythmic effects. We monitored drug effects on spontaneous con-tractile activity and, in selected cases, on action potentials (sharp microelectrode) and Ca2? transients (Fura-2) and contraction under electrical pacing. The Ito-blocker inhib-itor 4-aminopyridine increased action potential duration and T2 and caused aftercontractions, which were abolished by inhibitors of ryanodine receptors (RyR2; JTV-519) or sodium calcium exchanger (NCX; SEA0400). 77 Drugs were then tested at 1-10-1009 free therapeutic plasma concentrations (FTPC): Inhibitors of IKr, IKs, Ito, antiar-rhythmics (8), drugs withdrawn from market for torsades des pointes arrhythmias (TdP, 5), drugs with measurable (7) or isolated TdP incidence (13), drugs considered safe (14), 28 new chemical entities (NCE). Inhibitors of IKr or IKs had no effect alone, but substantially prolonged relax-ation time (T2) when combined at high concentration. 15/33 drugs associated with TdP and 6/14 drugs considered non-torsadogenic (cibenzoline, diltiazem, ebastine, keto-conazole, moxifloxacin, and phenytoin) induced concen-tration-dependent T2 prolongations (10-1009 FTPC). Bepridil, desipramine, imipramine, thioridazine, and erythromycin induced irregular beating. Three NCE pro-longed T2, one reduced force. Drugs inhibiting repolari-zation prolong relaxation in rat EHTs and cause aftercontractions involving RyR2 and NCX. Insensitivity to IKr inhibitors makes rat EHTs unsuitable as general proarrhythmia screen, but favors detection of effects on Ito, IKs? Ito or IKs? IKr. Screening a large panel of drugs suggests that effects on these currents, in addition to IKr, are more common than anticipated