Malaria drug-susceptibility testing. HRP2-based assays: current data, future perspectives.

In past decades, malaria in-vitro drug-susceptibility assays have become an indispensable tool for the development of novel drugs, as well as for the surveillance of antimalarial drug resistance. The traditional in-vitro assays, however, remain tedious procedures, which, depending on the method employed, require a high degree of expertise, sophisticated laboratory infra-structure, skills and patience. We therefore developed a new drug sensitivity assay for Plasmodium falciparum based on the measurement of histidine-rich protein II (HRP2), a histidine and alanine-rich protein produced by P. falciparum. The assay uses a simple HRP2 double-site sandwich ELISA to quantify parasite growth and its inhibition. The complete ELISA takes about 2-3 hours to perform and requires little technical equipment. In our experiments with laboratory strains of P. falciparum against common antimalarials, the results closely parallel those obtained from the isotope assay and from WHO schizont maturation tests (P < 0.001). Preliminary results using the HRP2 assay in our field studies in 2002 suggest similar outcomes. In this setting the high sensitivity and simplicity of the assay is of particular advantage. The data closely parallel those obtained with the traditional WHO assay (Mean difference on the log scale: 0.033; R = 0.942; P < 0.001). The assay is currently being further validated under field conditions. It has proved to be a valuable tool for a wide range of applications, from epidemiological field studies to the screening of new drugs, and may therefore have the potential to replace traditional in-vitro drug-sensitivity techniques.