Ex vivo activity of Proveblue, a methylene blue, against field isolates of Plasmodium falciparum in Dakar, Senegal from 2013–2015

•Proveblue, a methylene blue preparation, was highly active against Plasmodium falciparum isolates (8.1 nM).•Proveblue was more active than standard antimalarial drugs, but less active than artemisinin derivatives.•A low significant positive correlation was found between Proveblue and artemisinin derivatives and piperaquine.•The correlation between Proveblue and artemisinin derivatives and piperaquine was too low to indicate cross-resistance.•Proveblue is to be considered in triple artemisinin-based combination therapy for treatment of multidrug-resistant malaria. Resistance to most antimalarial drugs has spread from Southeast Asia to Africa. Accordingly, new therapies to use with artemisinin-based combination therapy (triple ACT) are urgently needed. Proveblue, a methylene blue preparation, was found to exhibit antimalarial activity against Plasmodium falciparum strains in vitro. Proveblue has synergistic effects when used in combination with dihydroartemisinin, and has been shown to significantly reduce or prevent cerebral malaria in mice. The objectives of the current study were to evaluate the in vitro baseline susceptibility of clinical field isolates to Proveblue, compare its activity with that of other standard antimalarial drugs and define the patterns of cross-susceptibility between Proveblue and conventional antimalarial drugs. The Proveblue IC50 of 76 P. falciparum isolates ranged from 0.5 nM to 135.1 nM, with a mean of 8.1 nM [95% confidence interval, 6.4–10.3]. Proveblue was found to be more active against P. falciparum parasites than chloroquine, quinine, monodesethylamodiaquine, mefloquine, piperaquine, doxycycline (P