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 h after enrolment, and the concentrations of chloroquine and desethylchloroquine were measured in plasma by high-performance liquid chromatography with fluorescence detection.

Results: Of 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 to 184 ng/ml and from 175 to 827 ng/ml at 24 and 168 hours. For desethylchloroquine, the values ranged from 32 to 144 ng/ml and from 90 to 440 ng/ml at 24 and 168 h. There were no significant correlations between the plasma levels of chloroquine and the doses administered (mg/kg) at 24 and 196 h. 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.