The extent of chloroquine underdosing in adult patients with malaria by Plasmodium vivax from an endemic area of the Brazilian Amazon basin.

OBJECTIVES To evaluate the extent of chloroquine underdosing and to measure the concentrations of chloroquine and desethylchloroquine in adult patients with P. vivax malaria in the Brazilian Amazon basin. METHODS Prospective study of cases in male adult patients with malaria by Plasmodium vivax treated with a total dose of 1500 mg chloroquine over three days and a short course of primaquine. Patients were weighed at admission and the dose per mg/kg was determined. Blood samples were collected at 24 and 168 hours after enrollment, and the concentrations of chloroquine and desethylchloroquine were measured in plasma by high-performance liquid chromatography with fluorescence detection. RESULTS 61 patients were included in the study, and 60% received a total dose of chloroquine below 25 mg/kg. Plasma chloroquine concentrations ranged from 90-184 ng/ml and from 175-827 ng/ml at 24 and 168 hours. For desethylchloroquine, the values ranged from 32-144 ng/ml and from 90-440 ng/ml at 24 and 168 hours. There were no significant correlations between the plasma levels of chloroquine and the doses administered (mg/kg) at 24 and 196 hours. Similar results were found for desethylchloroquine. CONCLUSION There is widespread suboptimal dosing of chloroquine that is probably due to the dosing regimen based on patient age, which reduces the drug exposure with a possible influence on parasite clearance.