Methylene blue retains in vitro activity against early ring-stage artemisinin-resistant Plasmodium falciparum

Artemisinin resistance in Plasmodium falciparum is now well established in three continents and challenges the efficacy of antimalarial drug regimens. This study examined whether methylene blue, an ancient antimalarial drug with a broad spectrum of activity against Plasmodium blood stages, retains its activity against the young rings of artemisinin-resistant parasites. Clinical isolates carrying kelch13 wild type (n = 3), R561H (n = 9), or P441L (n = 10) genotypes were tested with a modified ring survival assay whereby 0- to 3-h post-invasion rings were exposed to a range of methylene blue concentrations. Ring survival was analysed with a Bayesian mixed effects E_max model accounting for variability across isolates and experimental replicates. Methylene blue suppressed ring-stage survival at low nanomolar concentrations with no evidence of kelch13-mediated cross-resistance: the mean 50% inhibitory concentration (IC₅₀) estimates were 23 nM (95% credible interval [CrI]: 15 to 37) for wild type, 27 nM (95% CrI: 21 to 36) for R561H and 14 nM (95% CrI: 10 to 20) for P441L. These findings indicate that methylene blue remains active in vitro against ring-stage artemisinin-resistant parasites.