Observations on conducting whole‐cell patch clamping of the hERG cardiac K⁺channel in pure human serum

Inhibition of the human ether‐a‐go‐go‐related gene (hERG) K + channel by drugs leads to QT prolongation on the electrocardiogram and can result in serious cardiac arrhythmia. For this reason, screening of drugs on hERG is mandatory during the drug development process. Patch clamp electrophysiology in a defined physiological saline solution (PSS) represents the standard method for assaying drug effects on the channel. To make the assay more translatable to clinical studies, we have conducted whole‐cell patch clamping of hERG using pure human serum as the extracellular medium. Pure human serum had little effect on the hERG channel waveform or the current–voltage relationship when compared to PSS. hERG current recordings were highly stable in serum at room temperature, but prolonged recordings at the physiological temperature required prior heat inactivation of the serum. Compared to PSS, the IC 50 values, conducted at room temperature, of the classic hERG blocking drugs cisapride, moxifloxacin, and terfenadine were shifted to the right by an extent predicted by their known plasma protein binding, but we did not detect any differences in IC 50 s between male and female serum. Total plasma levels of these drugs associated with clinical QT prolongation corresponded to small (<15%) inhibition of hERG current in pure serum suggesting that minor inhibition of the channel leads to observable pharmacodynamic effects. Conducting whole‐cell patch clamping of hERG in human serum has the potential to make the assay more translatable to clinical studies and improve its predictive value for safety testing.