Comparative study on the in vitro activity of lumefantrine and desbutyl-benflumetol in fresh isolates of Plasmodium vivax from Thailand.

The occurrence of chloroquine resistance in Plasmodium vivax underlines the need for monitoring the drug response of this important malaria parasite and for the evaluation of alternative therapeutic agents. In-vitro methods facilitate these tasks. This investigation employed a recently developed in-vitro micro-technique and validated it for lumefantrine and desbutyl-benflumetol, a compound that was initially considered a metabolite of lumefantrine. The studies were conducted in 2001 at Mae Sot, a town situated in northwestern Thailand near the border to Myanmar. Parallel in-vitro tests with lumefantrine and desbutyl-benflumetol were carried out with 53 fresh isolates of P. vivax. For both compounds, the parasite showed a homogenous, log-normal inhibition pattern with nearly parallel log-probit regressions. The geometric mean drug concentrations effecting complete growth inhibition were 2361 nM for lumefantrine and 187 nM for desbutyl-benflumetol. With p=3.264 x 10(-18) the difference was highly significant. The EC50 and EC90 values for lumefantrine, 17.6 nM and 448.5 nM, respectively, were much higher as compared to those determined for desbutyl-benflumetol, with 1.5 nM and 39.7 nM. This difference expressed itself in a highly significant Power Ratio (PR) of 11.0. The activity of desbutyl-benflumetol in P. vivax exceeds that of lumefantrine by one order of magnitude, suggesting a high, hitherto unexploited therapeutic potential of desbutyl-benflumetol.