Chlorproguanil/dapsone for the treatment of non-severe Plasmodium falciparum malaria in Kenya: a pilot study

A rapid, semiautomated microdilution method was developed for measuring the activity of potential antimalarial drugs against cultured intraerythrocytic asexual forms of the human malaria parasite Plasmodium falciparum. Microtitration plates were used to prepare serial dilutions of the compounds to be tested. Parasites, obtained from continuous stock cultures, were subcultured in these plates for 42 h. Inhibition of uptake of a radiolabeled nucleic acid precursor by the parasites served as the indicator of antimalarial activity. Results of repeated measurements of activity with chloroquine, quinine, and the investigational new drug mefloquine demonstrated that the method is sensitive and precise. Several additional antimalarial drugs and compounds of interest were tested in vitro, and the results were consistent with available in vivo data. The use of P. falciparum isolates with known susceptibility to antimalarial drugs also permitted evaluation of the cross-resistance potential of each compound tested. The applications and expectations of this new test system within a drug development program are discussed.

The standard checkerboard titration for detecting synergy between antibiotics is practicable for combinations of two antibiotics, laborious for combinations of three, and not feasible for combinations of four or more. Nevertheless, methods for testing of combinations of several antibiotics are urgently needed because some combinations might be superior to those in use and enable the successful treatment of infections resistant to current therapy. A simple method for measurement of synergy (or antagonism) with combinations of any number of agents has been developed which requires less effort than the standard checkerboard titration of two agents. With this method, the concentrations of each of n agents producing some specified effect (such as minimal inhibitory concentration or minimal bactericidal concentration) are determined. A reference combination made up of 1/n of each of these concentrations is titrated to find a dilution that produces the specified effect. The degree of dilution required is equal to the sum of the fractional inhibitory concentrations (concentration of each agent in combination/concentration of each agent alone) as conventionally determined by checkerboard titrations; sums of less than 1, 1, and greater than 1 indicate synergy, additivity, and antagonism, respectively.

For the past 300 years antimalarial dosage regimens have not been based on pharmacokinetic information. However, now that this information is available, it is appropriate to examine current recommendations for prophylaxis and treatment. In healthy subjects, the cinchona alkaloids (quinine and quinidine), primaquine and proguanil (chloroguanide) are all rapidly eliminated with half-lives (t1/2 beta) of between 6 and 12 hours. Hepatic biotransformation accounts for approximately 80, 96 and 50% of their total clearance, respectively. In malaria, the pharmacokinetic properties of quinine and quinidine are significantly altered with a decrease in the apparent volume of distribution (Vd), prolongation of the elimination half-life, and a reduction in systemic clearance (CL) that is proportional to the severity of infection. Red cell concentrations and plasma protein binding are both increased in severe disease. Parenteral quinine or quinidine should be given by slow intravenous infusion rather than by intravenous or intramuscular injection, and a loading dose is necessary in severe infections. Chloroquine (t1/2 beta 6 to 50 days) and mefloquine (t1/2 beta 6.5 to 33 days) have extensive tissue distribution and prolonged activity after a single dose. Both drugs are concentrated in erythrocytes and 55% of chloroquine and 98% of mefloquine in plasma is bound to protein. The pharmacokinetics of chloroquine are complex and, because of the extremely long beta phase, difficult to accurately define. Pyrimethamine (t1/2 35 to 175 hours) has more limited tissue distribution, plasma and erythrocyte concentrations are similar, and 85% of the drug in plasma is bound to plasma proteins. The clearance of quinine, mefloquine and pyrimethamine appears to be higher in children than in adults. Currently, most of the information available on disposition of antimalarial drugs in humans is derived from studies in healthy adult subjects. More information is required on their pharmacokinetics in malaria, pregnancy, and in young children.