Study on the biochemical basis of mefloquine resistant Plasmodium falciparum

Increase in drug detoxification and alteration of drug uptake and efflux of Plasmodium falciparum were investigated for their possible association with mefloquine (MQ) resistance in five different clones of P. falciparum from Thailand (T994b3, K1CB2, PR70CB1, PR71CB2 and TM4CB8-2.2.3). Fifty percent inhibitory concentration (IC50) values from these five clones varied between 30- and 50-fold. Regarding the detoxification mechanism, the ability of P. falciparum clones to biotransform MQ was shown in vitro by parasite microsomal protein prepared from parasite infected red blood cells protein (30 μg), NADPH (1 nM) and phosphate buffer pH 7.4, carried out at 37 °C with agitation. Radiolabelled unmetabolized MQ and possible metabolite(s) generated from the reaction was extracted into ethylacetate and separated by radiometric-HPLC after 1 h. All clones were capable of converting MQ into carboxymefloquine (CMQ), which is the main metabolite in human plasma. In addition, another unidentified metabolite eluted at 4.2 min on the chromatograph could be detected from the incubation reaction. This metabolite has never been detected in human liver microsomes before. There was no significant difference in the percentages of CMQ formed in the resistant (T994b3, PR70CB1, PR71CB2) and sensitive (TM4CB8-2.2.3, K1CB2) clones. Another possible mechanism, i.e., alteration in the accumulation of MQ in the parasites was investigated in vitro using [14C]MQ as a tracer. The time courses of [14C]MQ uptake and efflux were generally characterized by two phases. A trend of increased efflux of [14C]MQ was observed in the resistant compared with sensitive clones.