The microculture tetrazolium assay (MTA): another colorimetric method of testing Plasmodium falciparum chemosensitivity

Malarial lactate dehydrogenase (LDH), which uses 3-acetyl pyridine adenine dinucleotide as coenzyme in a reaction leading to the formation of pyruvate from L-lactate, may be used to study the susceptibility of Plasmodium falc iparum to a drug in vitro. Several methods to determine the activity of thi s enzyme are available. One, the colorimetric method of Makler and colleagu es, was modified slightly, by using sodium-2,3-bis-[2-methoxy-4-nitro-5-sul phophenyl]-2H-tetrazolium-5-carboxanilide (XTT) and following the reaction by measuring the optical density at 450 nm. Using two, culture-adapted stra ins of P. falciparum, this LDH assay was compared with the unmodified Makle r's assay and with the isotopic microtest based on the incorporation of tri tium-labelled hypoxanthine. Fresh, clinical P. falciparum isolates mere als o tested in the presence of several drugs, including chloroquine, mefloquin e, quinine, halofantrine, atovaquone and qinghaosu derivatives. The results of the three assays were correlated for all the drugs tested except atovaq uone. The two enzymatic assays are non-radioactive, rapid, reliable, inexpe nsive to perform and semi-automatic. However, they do require an initial pa rasitaemia of 2% with a haematocrit of 1.8%.