Investigation of hERG1b Influence on hERG Channel Pharmacology at Physiological Temperature

Objective: To compare the inhibitory potencies of selected drugs (chloroquine, fluoxetine, cisapride, and ebastine [EBA]) on human Ether-a-go-go-Related Gene (hERG) potassium channel current carried by either hERG1a or co-expressed hERG 1a/1b channel isoforms. Materials and Methods: Measurements of hERG current (I-hERG) were made at 37 degrees C from HEK-293 cells expressing either the hERG1a isoform or co-expressing hERG1a and 1b isoforms. A standard "square" waveform voltage protocol was used to elicit I-hERG, and tail current measurements were used to construct concentration-response relations for each drug. Results: For fluoxetine, cisapride, and chloroquine, the observed potencies of inhibition of I-hERG were similar between hERG1 and 1a/1b expression conditions. Further experiments in which the hERG1b isoform was expressed alone also failed to show different potencies from hERG1a for these drugs. Fluoxetine was also tested at room temperature and showed similar potencies against hERG 1a and 1a/1b. EBA was more potent against hERG1a than hERG1a/1b with respective half maximal inhibitory concentration (IC50) values of 32 nM (95% confidence interval [CI] 24 nM-43 nM) and 185 nM (CI 114 nM-304 nM), a 5.8-fold difference. At ambient temperature, EBA was also more potent against hERG1a than 1a/1b, with a 2.4-fold difference in IC50. Conclusion: Comparison of these findings with prior planar patch-clamp data suggests that automated patch-clamp data on hERG1a/1b versus hERG 1a at ambient temperature cannot automatically be extrapolated to manual patch clamp at 37 degrees C. The results with EBA highlight that, during hERG screening of novel drugs, there is a case for promising candidates to incorporate some measurements on hERG1a/1b as well as hERG1a channels.