Mechanism of hERG K+ channel blockade by the fluoroquinolone antibiotic moxifloxacin

1 The fluoroquinolone antibiotic moxifloxacin has been associated with the acquired long QT syndrome and is used as a positive control in the evaluation of the QT-interval prolonging potential of new drugs. In common with other QT-prolonging agents, moxifloxacin is known to inhibit the hERG potassium K+ channel, but at present there is little mechanistic information available on this action. This study was conducted in order to characterise the inhibition of hERG current (I-hERG) by moxifloxacin, and to determine the role in drug binding of the S6 aromatic amino-acid residues Tyr652 and Phe656. 2 hERG currents were studied using whole-cell patch clamp (at room temperature and at 35-37 degrees C) in an HEK293 cell line stably expressing hERG channels. 3 Moxifloxacin reversibly inhibited currents in a dose-dependent manner. We investigated the effects of different voltage commands to elicit hERG currents on moxifloxacin potency. Using a 'step-ramp' protocol, the IC50 was 65 mu M at room temperature and 29 mu M at 35 degrees C. When a ventricular action potential waveform was used to elicit currents, the IC50 was 114 mu M. 4 Block of hERG by moxifloxacin was found to be voltage-dependent, occurred rapidly and was independent of stimulation frequency. 5 Mutagenesis of the S6 helix residue Phe656 to Ala failed to eliminate or reduce the moxifloxacin-mediated block whereas mutation of Tyr652 to Ala reduced moxifloxacin block by similar to 66%. 6 Our data demonstrate that moxifloxacin blocks the hERG channel with a preference for the activated channel state. The Tyr652 but not Phe656 S6 residue is involved in moxifloxacin block of hERG, concordant with an interaction in the channel inner cavity.