Trypanocidal activity of methylene blue. Evidence for in vitro efficacy and in vivo failure

Human African trypanosomiasis remains a difficult health problem to treat because of the few compounds available nowadays and their toxicity. The disease also affects animals and is therefore responsible for economic difficulties and zoonotic risks. There is an urgent need to develop new drugs for treatment of African trypanosomiasis. Methylene blue is a safe and easy-to-use drug employed in human therapy. It is also known to have antimalarial activity. In this study, methylene blue trypanocidal activity was found in vitro but it failed to cure trypanosome infection in mice when administered at 300 mg/kg p.o.or at 200 mg/kg i.p. Differences between in vitro and in vivo activities are discussed, and further in-depth studies are warranted.