The State of Para is responsible for a large amount of malaria notifications in the Brazilian Amazon region. Plasmodium vivax is the most prevalent specie and the evaluation of its in vitro sensitivity to chloroquine is essential to verify if this drug keeps its efficacy. However, this kind of evaluation was not feasible due to unavailability of methodologies for short-term in vitro culture and for evaluation of growth/maturation of this parasite. Recently, a method for shortterm in vitro culture of P. vivax was introduced, as well as a reliable methodology (DELI-test) for measurement of this parasite’s growth/maturation, based on the detection of the parasite enzyme Lactate Dehydrogenase (pLDH), allowing studies of sensitivity of this plasmodial specie to antimalarials. The aim of this study was then to evaluate the chemosensitivity of P. vivax to chloroquine under field conditions in the municipality of Tucurui, State of Para, Brazil. A total of 45 patients with vivax malaria were enrolled in the study, after microscopic diagnosis by thick smear, and 5mL of blood were withdrawn from each patient. An erythrocyte suspension was prepared for each patient’s blood, using a special culture medium (mixing of RPMI and Waymouth media, AB+ serum, 1.8% hematocrit), and put in short-term (48 hours) culture at 37°C and low tension of oxygen, under a range of concentrations (2,34-600ng/mL) of chloroquine. Plates were then frozen and, later, parasites were lysed by freezing-thawing, releasing the pLDH, which was measured by a sandwich-ELISA (DELI-test), using for capture the monoclonal antibody (MAb) 6C9 (anti-Plasmodium pLDH) e 11D (anti-P. vivax pLDH) and for detection the MAb19G (anti-Plasmodium pLDH). The optical density values allowed in most cases the drawing of curves of sensitivity to the drug and calculation of the 50% inhibitory concentration (IC50). Nearly half (53.8%) of the samples showed better curves with the MAb 11D. Out of the 44 samples, 26 (59.2%) allowed the drawing of interpretable curves of sensitivity for chloroquine. The parasite growth performance of 59% of the samples can be considered satisfactory, taking into account the known difficulty in culturing P. vivax in vitro, and the poor laboratory conditions for carrying out the cultures. Thirteen (46.2%) of the 26 interpretable samples showed an IC50 above the threshold of 100nM of chloroquine, being considered resistants. This high frequency of samples with low sensitivity to chloroquine is of concern and indicates that this kind of evaluation should be continued and extended to other localities in order to have a more clear picture of the situation. The DELI-test is currently the only able to evaluate in vitro resistance of P. vivax and its use in fild conditions can contribute for helping definitions in the malaria therapeutic strategies in Brazil.
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