Clinical pharmacokinetics of halofantrine.
Halofantrine is a phenanthrenemethanol antimalarial that is effective against asexual forms of multidrug-resistant Plasmodium falciparum malaria. It has no action on gametocytes or hypnozoites in the liver. The drug is administered as a racemic mixture but the (+)- and (-)-enantiomers show no difference in activity in vitro. Three formulations for oral administration are available for human use, i.e. tablets, capsules and suspension. Toxicity studies in animals suggest that halofantrine has very low toxicity both in short term and long term animal studies, and there has been no evidence of mutagenicity in these studies. Phase I, II and III clinical trials of halofantrine conducted in several tropical countries found the drug to be well tolerated and effective against multidrug-resistant P. falciparum malaria when 500mg was administered every 6 hours for 3 doses. The majority of clinical adverse effects reported, including nausea, vomiting, abdominal pain, diarrhoea, orthostatic hypotension, prolongation of QTc interval, pruritus and rash, have been mild and transient. There is wide interindividual variation in halofantrine absorption. The maximal plasma concentration (Cmax) is achieved approximately 6 hours after oral administration. Bioavailability is not dose-proportional for doses over 500mg, but there is a dose-proportional increase in Cmax and area under the plasma concentration-time curve (AUC) for doses between 250 and 500mg. In patients with malaria the bioavailability of halofantrine is decreased. The mean half-life of absorption is 4 hours and Cmax is significantly lower than that obtained in healthy individuals. Furthermore, halofantrine absorption is enhanced when the drug is taken with fatty food. Therefore, halofantrine should be taken with food to ensure optimal absorption in patients with malaria. The terminal elimination half-life is 5 days in patients with malaria. Halofantrine is biotransformed in the liver to its major metabolite N-debutyl-halofantrine. Plasma concentrations of this metabolite are observed soon after administration of halofantrine, but in much lower concentrations. The elimination half-life is similar to that of halofantrine. There have been increasing reports of halofantrine treatment failure, particularly in the eastern part of Thailand. The majority of treatment failures have been associated with incomplete drug absorption. The dose-dependent cardiotoxic effects (e.g. cardiac arrhythmia) are a major concern, particularly when the bioavailability of the drug cannot be predicted. Ongoing and future studies should aim at developing more appropriate drug formulation(s) and/or optimising dosage regimens. This will allow therapeutic concentrations to be achieved with minimum adverse effects, particularly cardiotoxicity.