Chloroquine bioassay of plasma specimens obtained from soldiers on chloroquine plus doxycycline for malaria prophylaxis.

In this study we describe the application of a bioassay for measuring chloroquine equivalent concentrations in plasma samples obtained from soldiers on chloroquine (300 mg weekly) and doxycycline (50 or 100 mg daily) for malaria prophylaxis. Chloroquine, its principal metabolite monodesethylchloroquine and doxycycline were also measured by high performance liquid chromatography (HPLC). Physiological concentrations of doxycycline did not interfere with the measurement of chloroquine equivalent concentrations. The correlation between bioassay and HPLC was rs = 0.88, with a median bioassay/HPLC chloroquine concentration ration of 1.1 (range 0.6-2.4, n = 26). The bioassay is a valuable method, particularly under field conditions, for measuring chloroquine concentrations and can be very helpful in distinguishing drug failures from either poor compliance or inadequate drug absorption.