Optimization of Cav1.2 screening with an automated planar patch clamp platform

Introduction: Ca_v1.2 channels play an important role in shaping the cardiac action potential. Screening pharmaceutical compounds for Ca_v1.2 block is very important in developing drugs without cardiac liability. Ca_v1.2 screening has been traditionally done using fluorescence assays, but these assays have some limitations. Patch clamping is considered the gold standard for ion channel studies, but is very labor intensive. The purpose of this study was to develop a robust medium throughput Ca_v1.2 screening assay in PatchXpress^® 7000A by optimizing cell isolation conditions, recording solutions and experimental parameters. Under the conditions established, structurally different standard Ca_v1.2 antagonists and an agonist were tested. Methods: HEK-293 cells stably transfected with hCa_v1.2 L-type Ca channel were used. For experiments, cells were isolated using 0.05% Trypsin. Currents were recorded in the presence of 30 mM extracellular Ba²⁺ and low magnesium intracellular recording solution to minimize rundown. Ca_v1.2 currents were elicited from a holding potential of ? 60 mV at 0.05 Hz to increase pharmacological sensitivity and minimize rundown. Test compounds were applied at increasing concentrations for 5 min followed by a brief washout. Results: Averaged peak Ca_v1.2 current amplitudes were increased from 10 pA/pF to 15 pA/pF by shortening cell incubation and trypsin exposure time from 2.5 min at 37 °C to 1 min at room temperature and adding 0.2 mM cAMP to the intracellular solution. Rundown was minimized from 2%/min to 0.5%/min by reducing the intracellular free Mg²⁺ from 2.7 mM to 0.2 mM and adding 100 nM Ca²⁺. Under the established conditions, we tested 8 structurally different antagonists and an agonist. The IC₅₀ values obtained ranked well against published values and results obtained using traditional clamp experiments performed in parallel using the expressed cell line and native myocytes. Discussion: This assay can be used as a reliable pharmacological screening tool for Ca_v1.2 block to assess compounds for cardiac liability during lead optimization.