An automated electrophysiological assay for differentiating Ca(v)2.2 inhibitors based on state dependence and kinetics

Ca(V)2.2 (N-type) calcium channels are key regulators of neurotransmission. Evidence from knockout animals and localization studies suggest that Ca(V)2.2 channels play a critical role in nociceptive transmission. Additionally, ziconotide, a selective peptide inhibitor of Ca(V)2.2 channels, is clinically used to treat refractory pain. However, the use of ziconotide is limited by its low therapeutic index, which is believed, at least in part, to be a consequence of ziconotide inhibiting Ca(V)2.2 channels regardless of the channel state. Subsequent efforts have focused on the discovery of state-dependent inhibitors that preferentially bind to the inactivated state of Ca(V)2.2 channels in order to achieve an improved safety profile relative to ziconotide. Much less attention has been paid to understanding the binding kinetics of these state-dependent inhibitors. Here, we describe a novel electrophysiology-based assay on an automated patch platform designed to differentiate Ca(V)2.2 inhibitors based on their combined state dependence and kinetics. More specifically, this assay assesses inactivated state block, closed state block, and monitors the kinetics of recovery from block when channels move between states. Additionally, a use-dependent assay is described that uses a train of depolarizing pulses to drive channels to a similar level of inactivation for comparison. This use-dependent protocol also provides information on the kinetics of block development. Data are provided to show how these assays can be utilized to screen for kinetic diversity within and across chemical classes.