Overview: clinical pharmacology of antimalarials

The effectiveness of antimalarials depends on its pharmacodynamics ie inhibitory effect on the parasites and unwanted effects on the host. It also depends on the pharmacokinetics of the drugs. The ideal antimalarials are drugs that show curative activity in the absence of toxicity to the host. Recommendation for antimalarial dosage regimens should be based on pharmacokinetic and pharmacodynamic studies in appropriate populations ie ethnic groups, adults children, and in pregnancy. Chloroquine remains the drug of choice for treating malaria caused by Plasmodium species other than P. falciparum. Even in the presence of chloroquine resistance the drug may still be quite useful, especially in areas with high communal immunity. In general sulfadoxine/pyrimethamine (S/P) should be used as an alternative antimalarial when chloroquine fails. The decision to change to S/P from chloroquine depends on many factors. Quinine still remains the drug of choice for severe chloroquine-resistant falciparum malaria. Resistance to mefloquine has appeared the exact mechanism being unknown. In general, before the use of any combination of antimalarial drugs the superiority (efficacy and side-effects) over each of the individual drugs should be clearly demonstrated. The combination of mefloquine with sulfadoxine/pyrimethamine was made on the grounds that the combination would delay the resistance to mefloquine. Desferrioxamine will hardly be an agent to be used on its own for treating malaria due to the high recrudescent rate. However, a recent report indicated that its association with antimalarial drugs in the management of severe and complicated falciparum malaria shortens fever and parasite clearance time and resolves complications faster than the standard antimalarial drug alone. Clinical trials with halofantrine has been done in several countries in the region from 1988 to the present with diverse results. Further studies on a larger scale should be carried out to ascertain whether these are due to variation in drug absorption or drug resistance. An improved formulation of halofantrine must be developed to ensure adequate absorption and bioavailability. The artermisinin group of antimalarials is known to be highly effective and independent, in its mode of action, from standard malaria drugs but associated with high recrudescent rate. Phase II studies are needed for determining/optimizing therapeutic dose regimens and to ensure safer and more effective use of these compounds.