Elucidating the challenges with categorizing drugs that block cardiac sodium channels − The impact of temperature on drug-cardiac sodium channel interactions

Class I antiarrhythmic drugs (AADs) are categorized into Class IA, IB and IC subgroups based on their distinct electrophysiological consequences on the heart that are thought to arise from distinct interaction characteristics with cardiac sodium channels (Na_V). The original categorization centered on assessing electrophysiological features including drug effects on the maximal upstroke velocity of action potential (AP), AP duration, and effective refractory period using ventricular preparations from animals performed at physiological temperature (PT). Subsequently, categorization assessing drug binding and unbinding kinetics in patch clamp assays in Na_V overexpressing cells at room temperature (RT) was proposed. Temperature is known to impact Na_V gating, hence potentially affecting use- and state-dependence of drug interactions. Whether RT patch clamp data adequately capture drug-Na_V interaction characteristics that allows for the same subgroup categorization as in the original categorization using native tissues is unclear. Similarly, a systematic assessment of the effects on other cardiac ion channels following best practices is also lacking. This study characterized drug-Na_V (Na_V1.5) interaction characteristics of quinidine (IA), mexiletine (IB) and flecainide (IC) at near PT and RT using manual patch clamp. Additionally, drug's potencies on inhibiting the late Na_V1.5, hERG, and Ca_V1.2 currents were conducted at near PT following ICH S7B Q&A 2.1 best practices. At RT, use-dependent block of Na_V1.5 currents and unbinding kinetics were fastest for mexiletine, followed by quinidine, and flecainide. At near PT, use-dependent block and unbinding kinetics of quinidine remained faster than flecainide. Surprisingly, mexiletine showed no use-dependent block and no apparent unbinding at near PT. Preliminary assessment revealed that the three drugs have different effects on other cardiac ionic currents. The three Class I AADs showed distinct interaction characteristics with Na_V1.5 currents. Importantly, temperature-dependent binding and unbinding kinetics for mexiletine and different effects on other cardiac ionic currents for all three drugs were observed. These data provide mechanistic insights to drug-induced changes in myocyte APs that led to Class I AADs categorization, and lay a foundation for future research to identify the commonalities and differences of Class I AADs interacting with Na_V1.5 channels.