THE HUMAN ETHER-A'-GO-GO RELATED GENE (hERG) K + CHANNEL BLOCKADE BY THE INVESTIGATIVE SELECTIVE-SEROTONIN REUPTAKE INHIBITOR CONA-437: LIMITED DEPENDENCE ON S6 AROMATIC RESIDUES

channel current (IhERG) by a novel investigative serotonin-selective reuptake inhibitor (SSRI), CONA-437, was investigated. Whole-cell patch-clamp measurements of IhERG were made from human embryonic kidney (HEK 293) cells expressing wild- type (WT) or mutant forms of the hERG channel. With a step-ramp voltage-command, peak IhERG was inhibited with an IC50 of 1.34 µM at 35 ±1°C; the IC50 with the same protocol was not significantly different at room temperature. Voltage- command waveform selection had only a modest effect on the potency of IhERG block: the IC50 with a ventricular action potential command was 0.72 µM. IhERG blockade developed rapidly with time following membrane depolarisation and showed a weak dependence on voltage, accompanied by a shift of ~ -5 mV in voltage-dependence of activation. There was no significant effect of CONA-437 on voltage-dependence of IhERG inactivation, though at some voltages an apparent acceleration of the time-course of inactivation was observed. Significantly, mutation of the S6 aromatic amino acid residues Y652 and F656 had only a modest effect on IhERG blockade by CONA-437 (a 3-4 fold shift in affinity). CONA-437 at up to 30 µM had no significant effect on either Nav1.5 sodium channels or L-type calcium channels. In conclusion, the novel SSRI CONA-437 is particularly notable as a gating-dependent hERG channel inhibitor for which neither S6 aromatic amino-acid constituent of the canonical drug binding site on the hERG channel appears obligatory for IhERG inhibition to occur.