Integrating drug effects on individual cardiac ionic currents and cardiac action potentials to understand nonclinical translation to clinical ECG changes

Concomitant inhibition of the late Na⁺ current (I_NaL) and/or the L-type Ca²⁺ current (I_CaL) has been hypothesized to mitigate hERG block-mediated QT_C prolongation. This hypothesis was tested in a clinical trial using drugs selected based on available patch clamp data at the time. The results showed that hERG block-mediated QT_C prolongation with dofetilide was shortened by co-administration of lidocaine or mexiletine-drugs that inhibit I_NaL. However, diltiazem, selected as the preferential I_CaL inhibitor, did not shorten hERG block-mediated QT_C prolongation by moxifloxacin. Patch clamp results can be sensitive to experimental differences across laboratories. Therefore, this study reexamined the effects of all drugs on I_NaL, I_CaL, and hERG current using overexpression cell lines and physiologically relevant experimental protocols aimed at producing drug-channel interaction characteristics in humans. Drug effects on ventricular action potentials (APs) from adult human trabeculae were also tested to better understand the nonclinical and clinical findings. Mexiletine and lidocaine showed similar potencies on inhibiting I_NaL and I_CaL in the prior and present patch clamp studies. Both drugs reduced dofetilide-induced AP duration (APD) prolongation, consistent with the clinical data. For diltiazem, the I_CaL potency and the separation between I_CaL and hERG potencies (I_CaL: 1.3 µM; hERG: 8.9 µM; hERG-to-I_CaL ratio = 7) is much reduced comparing to the prior results (I_CaL: 112.1 nM; hERG: 6.6 µM; ratio = 59). These new findings are consistent with diltiazem-induced APD shortening and AP triangulation caused by greater reductions in the early rather than late repolarization-a signature of multi-ion channel block. Consistent with this interpretation, nifedipine, which preferentially inhibits I_CaL over hERG (I_CaL: 13.2 nM; hERG: 35 μM; ratio = 2,651) caused APD shortening without AP triangulation. Results from this study thus support the following: 1) diltiazem failed to reduce moxifloxacin-induced QT_C prolongation due to its concomitant hERG block at clinical exposure levels; and 2) the importance of using physiologically relevant protocols to generate ion channel pharmacology and obtaining functional recordings from myocytes to provide a better understanding of nonclinical data translation to clinical ECG signals. Data used in this manuscript, including the original electrophysiology records, may be found at: https://osf.io/69ght/.