In vitro assessment of methylene blue on chloroquine-sensitive and -resistant Plasmodium falciparum strains reveals synergistic action with artemisinins.

Methylene blue (MB) represents a promising antimalarial drug candidate for combination therapies against drug-resistant parasite strains. To support and facilitate the application of MB in future field trials, we studied its antiparasitic effects in vitro. MB is active against all blood stages of both chloroquine (CQ)-sensitive and CQ-resistant P. falciparum strains with 50% inhibitory concentration (IC50) values in the lower nanomolar range. Ring stages showed the highest susceptibility. As demonstrated by high-performance liquid chromatography-tandem mass spectrometry on different cell culture compartments, MB is accumulated in malarial parasites. In drug combination assays, MB was found to be antagonistic with CQ and other quinoline antimalarials like piperaquine and amodiaquine; with mefloquine and quinine, MB showed additive effects. In contrast, we observed synergistic effects of MB with artemisinin, artesunate, and artemether for all tested parasite strains. Artemisinin/MB combination concentration ratios of 3:1 were found to be advantageous, demonstrating that the combination of artemisinin with a smaller amount of MB can be recommended for reaching maximal therapeutic effects. Our in vitro data indicate that combinations of MB with artemisinin and related endoperoxides might be a promising option for treating drug-resistant malaria and should be studied in future field trials. Resistance development under this drug combination is unlikely to occur.