Effects of norquetiapine, the active metabolite of quetiapine, on cloned hERG potassium channels

Quetiapine is an atypical antipsychotic drug that is widely used for the treatment of schizophrenia. It is mainly metabolized by a cytochrome P450 system in the liver. Norquetiapine is a major active metabolite in humans with a pharmacological profile that differs distinctly from that of quetiapine. We used the whole-cell patch-clamp technique to investigate the effects of norquetiapine on hERG channels that are stably expressed in HEK cells. Quetiapine and norquetiapine inhibited the hERG tail currents at −50 mV in a concentration-dependent manner with IC₅₀ values of 8.3 and 10.8 μM, respectively, which suggested equal potency. The block of hERG currents by norquetiapine was voltage-dependent with a steep increase over a range of voltages for channel activation. However, at more depolarized potentials where the channels were fully activated, the block by norquetiapine was voltage-independent. The steady-state inactivation curve of the hERG currents was shifted to the hyperpolarizing direction in the presence of norquetiapine. Norquetiapine did not produce a use-dependent block. A fast application of norquetiapine inhibited the hERG current elicited by a 5 s depolarizing pulse to +60 mV, which fully inactivated the hERG currents, suggesting an inactivated-state block. During a repolarizing pulse wherein the hERG current was slowly deactivated, albeit remaining in an open state, a fast application of norquetiapine rapidly and reversibly inhibited the open state of the hERG current. Our results indicated that quetiapine and norquetiapine had equal potency in inhibiting hERG tail currents. Norquetiapine inhibited the hERG current by preferentially interacting with the open and/or inactivated states of the channels.