Prolongation of the QT interval observed in a Japanese patient with vivax malaria following treatment with halofantrine

Trends in malaria cases in Japan

Just after World War II, more than 10,000 malaria cases per year were reported in Japan, including indigenous, imported and induced malaria. Malaria has been successfully eradicated since 1961 in Japan and now only imported malaria cases are encountered. However, as the number of Japanese people who are going abroad and also the number of foreigners who are visiting Japan increases (about 16 and 5 millions, respectively, in 2001), so does the chance for Japanese doctors to see imported malaria or transfusion-transmitted malaria cases. In fact, the total number of the patients with acute malaria in Japan has been around 100–150 annually for the last 10 years. Of those, about 75% are Japanese and 25% are foreigners, and about 75% are male and 25% are female. The peak age is in the 20 s. Recently, about 45% of patients are Plasmodium falciparum and another 45% Plasmodium vivax infections. The former species is likely to be seen in travelers coming back from African countries and the latter is mainly from Asian countries. The important issue is that patients in Japan have not been diagnosed promptly nor treated properly because doctors in Japan are no longer familiar with tropical medicine. Therefore, some patients are dying from severe malaria as a consequence. As it is, most of the effective medicines for drug-resistant malaria or severe malaria have not been registered in Japan. There is now a need for medical practitioners to focus on travel medicine in Japan.

Inhibition of HERG potassium channels by the antimalarial agent halofantrine

Halofantrine is a widely used antimalarial agent which has been associated with prolongation of the 'QT interval' of the electrocardiogram (ECG), torsades de pointes and sudden death. Whilst QT prolongation is consistent with halofantrine-induced increases in cardiac ventricular action potential duration, the cellular mechanism for these observations has not been previously reported. The delayed rectifier potassium channel, I(Kr), is a primary site of action of drugs causing QT prolongation and is encoded by the human-ether-a-go-go-related gene (HERG). We examined the effects of halofantrine on HERG potassium channels stably expressed in Chinese hamster ovary (CHO-K1) cells. Halofantrine blocked HERG tail currents elicited on repolarization to -60 mV from +30 mV with an IC(50) of 196.9 nM. The therapeutic plasma concentration range for halofantrine is 1.67-2.98 microM. Channel inhibition by halofantrine exhibited time-, voltage- and use-dependence. Halofantrine did not alter the time course of channel activation or deactivation, but inactivation was accelerated and there was a 20 mV hyperpolarizing shift in the mid-activation potential of steady-state inactivation. Block was enhanced by pulses that render channels inactivated, and channel blockade increased with increasing duration of depolarizing pulses. We conclude that HERG channel inhibition by halofantrine is the likely underlying cellular mechanism for QT prolongation. Our data suggest preferential binding of halofantrine to the open and inactivated channel states.