Drug Recovery: Effect of Astemizole - Methylene Blue Combination Therapy against Plasmodium Strains in vitro

Plasmodium species are protozoa from the Apicomplexa phylum which cause malaria. In the tropics and sub-tropics, approximately 3.3 billion people are threatened by this disease. Artemisinin Combination Therapy, has been reported to have a possible emergence of resistance. Therefore, there is an urgent need for new drug formulations. Drug repurposing offers an appealing alternative to de novo drug development. Although astemizole and methylene blue have been reported to have anti-malarial properties, their efficacy when used in combination has not been studied. Five concentrations ranging from 7.81 µg/ml to 125 µg/ml were combined in various ratios and assessed against two Plasmodium strains in vitro. Parasite load (per µl of blood) was determined by microscopy. The results were represented as mean ± standard error. ANOVA analysis was used to determine differences in the treatment groups at p<0.05. Antiplasmodial activity was observed in all drugs that were cultured with P. falciparum chloroquine sensitive (3D7) and chloroquine resistant (W2) strains. Strain dependent differences were observed in the efficacy scores of the tested drugs. Astemizole-methylene blue combinations of ratios 1:1, 3:1 and 1:3 interacted antagonistically. The least antagonistic interactions were 3:1 and 1:3 ratios at 31.25 µg/ml against the Plasmodium strains (FIC of 2.2 and 2.6 respectively). Astemizole antagonized methylene blue in the combinations. This study provided information on the importance of astemizole-methylene blue combination therapy against malaria and emphasized on the relevance of drug repurposing in malaria. This study shows that the drugs work better as monotherapies and that combinations in these ratios have insignificant antiplasmodial activity.