The anti-malarial drug halofantrine and its metabolite N-desbutylhalofantrine block HERG potassium channels

Objective: The antimalarial drug halofantrine has been associated with QT interval prolongation and with fatal and nonfatal arrhythinias in patients without known underlying cardiac abnormalities. A common target for QT interval-prolonging drugs is the human ether a-go-go gene (HERG) which encodes the pore forming subunit of the rapidly activating delayed rectifier K+ current (I-Kr). Methods: We studied the effects of halofantrine (0.1-1000 nM) and its major metabolite N-desbutylhalofantrine (3-1000 nM) on wild type HERG K+ channels stably expressed in HEK 293 cells, using the whole cell patch-clamp recording technique. Results: Halofantrine and N-desbutylhalofantrine blocked HERG K+ channels in a concentration-dependent manner with a half-maximal inhibitory concentration of 21.6 nM (n = 311 cells) and 71.7 nM (n = 18 cells), respectively. The development of drug block for both halofantrine and N-desbutylhalofantrine required channel activation indicative of open and/or inactivated state block. Drug washout or cell hyperpolarization resulted in minimal current recovery consistent with virtually irreversible binding. Using a ventricular action potential voltage clamp protocol, halofantrine and N-desbutylhalofantrine block of HERG current was greatest during phases 2 and 3 of the action potential waveform. Conclusion: We conclude that both halofantrine and N-desbutylhalofantrine cause high affinity block of HERG K+ channels. Although N-desbutylhalofantrine has been suggested to be a safer antimalarial agent compared to halofantrine, our results suggest that the gain in the safety margin for QT interval prolongation-related cardiotoxicity is minimal. (C) 2002 Elsevier Science B.V. All rights reserved.