Noncanonical pharmacological inhibition of the hERG K+ channel by a synthetic cannabinoid

Synthetic cannabinoid receptor agonists (SCRAs) are illicit 'street' drugs (colloquially known as "K2", "Spice" or "Mamba"), the use of which has been linked to QT interval prolongation and changes in T wave morphology. Machine learning models have predicted that SCRAs have some propensity to interact with hERG potassium channels. This study was conducted to evaluate effects of the SCRA 5F-AKB48 on hERG channel electrophysiology. Whole-cell patch-clamp experiments were performed on hERG-expressing HEK293 cells. Using a standard +20 mV rectangular protocol 5F-AKB48 was found to inhibit wild-type (WT) hERG current (IhERG) with an IC50 of 2.16 µM. Inhibition showed voltage-dependence over the steep range of voltage-dependent activation of IhERG and activation was left-ward shifted by the drug. 5F-AKB48 produced a significant slowing of IhERG activation. Neither the voltage dependence nor time-dependence of inactivation were significantly altered by the drug. When 5F-AKB48 was co-applied with the antipsychotic olanzapine, additive inhibition of IhERG was recorded. The N588K but not S620T attenuated inactivation mutant reduced IhERG inhibition by 5F-AKB48 compared to that for WT channels. However, neither of the Y652A or F656V mutations reduced IhERG inhibition by the drug. In contrast, the F557L and M651A mutations significantly attenuated the inhibitory action of 5F-AKB48. In silicoAI-based modelling, docking, and molecular dynamics simulations provided further evidence for distinct binding modes involving M651 and F557. We conclude that 5F-AKB48 is an effective inhibitor of the hERG potassium channel that interacts with a site which is distinct from the canonical pore site involving S6 aromatic residues.