Use- and state-dependent Nav1.5 blockers on QPatch X and in vivo and in vitro assays

In ion channel safety assessment and drug discovery it is often important to determine the mode of action of a drug candidate, which requires high-quality recordings. We successfully use the QPatch X in multi-hole mode in a screening scenario on Na_v1.5. This correlates with data from Langendorff perfused rabbit hearts, and an in vivo ECG assay using anesthetized rabbit, and effectively discriminates different modes of action of compounds on NaV1.5.
The protocol for QPatch X both tests the decay of the sodium current (30 Hz pulsetrain), and the recovery of the current. The QPatch assay allows combination of these elements into a single protocol, thereby shortening experiment time and costs.
The results show that: 1) Flecainide is a use-dependent blocker of open channels, as demonstrated by a slow decay of I_Na (30 Hz pulsetrain), and delayed recovery from inactivation in the QPatch experiments. Flecainide produces ECG changes (marked PR and QRS prolongation, and slight QTc prolongation), and lethal arrhythmia (VT ~Vf) in both Langendorff hearts and anesthetized rabbits at low doses. 2) Lidocaine is a state-dependent blocker of inactivated channels, inducing fast decay of I_Na (30 Hz pulsetrain) and fast recovery from inactivation. Lidocaine produces slight ECG changes (PR and QRS prolongation), but no arrhythmia in Langendorff-hearts or anesthetized rabbits. 3) Quinidine demonstrates a slow decay of I_Na (30 Hz pulsetrain), and delayed recovery from inactivation. Quinidine shows moderate PR and QRS prolongation, and severe morphological changes in ECGs.
The present results suggest that: 1) The QPatch X offers a time- and cost-effective screening scenario to select safe compounds with Na_v1.5 blocking activity and no proarrhythmic activity, and 2) the pro-arrhythmic activity of the Na_v1.5 blockers flecainide and quinidine might be attributable to their marked delay of recovery from inactivation.