Reliable identification of cardiac conduction abnormalities in drug discovery using automated patch clamp II: Best practices for Nav1.5 peak current in a high throughput screening environment.

INTRODUCTION: For reliable identification of cardiac safety risk, compounds should be screened for activity on cardiac ion channels in addition to hERG, including NaV1.5 and CaV1.2. We identified different parameters that might affect IC50s of compounds on NaV1.5 peak and late currents recorded using automated patch clamp (APC) and suggest outlines for best practices.
METHODS: APC instruments SyncroPatch 384 and Patchliner were used to record NaV1.5 peak and late current. Up to 24 CiPA compounds were used to investigate effects of voltage protocol, holding potential (-80 mV or - 95 mV) and temperature (23 ± 1 °C or 36 ± 1 °C) on IC50 values on hNaV1.5 overexpressed in HEK or CHO cells either as frozen cells or running cultures.
RESULTS: The IC50s of 18 compounds on the NaV1.5 peak current recorded on the SyncroPatch 384 using the CiPA step-ramp protocol correlated well with the literature. The use of frozen or cultured cells did not affect IC50s but voltage protocol and holding potential did cause differences in IC50 values. Temperature can affect Vhalf of inactivation and also compound potency. A compound incubation time of 5-6 min was sufficient for most compounds, however slow acting compounds such as terfenadine required longer to reach maximum effect. DISCUSSION: We conclude that holding potential, voltage protocol and temperature can affect IC50 values and recommend the use of the CiPA step-ramp protocol at physiological temperature to record NaV1.5 peak and late currents for cardiac safety. Further recommendations include: a minimum compound incubation time of 5 min, a replicate number of 4 and the use of positive and negative controls for reliable IC50s.