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

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 contractile activity and, in selected cases, on action potentials (sharp microelectrode) and Ca2+ transients (Fura-2) and contraction under electrical pacing. The I-to-blocker inhibitor 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 1-10-100x free therapeutic plasma concentrations (FTPC). Inhibitors of I-Kr, I-Ks, I-to, antiarrhythmics (8), drugs withdrawn from market for torsades des pointes arrhythmias (TdP, 5), drugs with measurable (7) or isolated TdP incidence (13), drugs considered safe (14), 28 new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 I-Kr or I-Ks had no effect alone, but substantially prolonged relaxation time (T2) when combined at high concentration. 15/33 drugs associated with TdP and 6/14 drugs considered non-torsadogenic (cibenzoline, diltiazem, ebastine, ketoconazole, moxifloxacin, and phenytoin) induced concentration-dependent T2 prolongations (10-100x FTPC). Bepridil, desipramine, imipramine, thioridazine, and erythromycin induced irregular beating. Three NCE prolonged T2, one reduced force. Drugs inhibiting repolarization prolong relaxation in rat EHTs and cause aftercontractions involving RyR2 and NCX. Insensitivity to I-Kr inhibitors makes rat EHTs unsuitable as general proarrhythmia screen, but favors detection of effects on I-to, I-Ks + I-to or I-Ks + I-Kr. Screening a large panel of drugs suggests that effects on these currents, in addition to I-Kr, are more common than anticipated.