Testing of Natural Products and Synthetic Molecules Aiming at New Antimalarials

The search for new antimalarials, which in the past relied on animal models, is now usually performed with cultures of Plasmodium falciparum (PF) blood parasites by evaluation of parasite growth inhibition. Field isolates of PF human malaria parasite, parasite strains and clones, well characterized for their susceptibility to chloroquine and other standard antimalarials are available for the in vitro tests. The simplest method to evaluate parasite growth is the determination of parasitemias in Giemsa stained blood smears through light microscopy. Other methodologies have proven to be more precise and allow mass screening of new compounds against PF blood stages, such as: (i) measuring the incorporation of radioactive hypoxanthine by the parasites; (ii) indirect colorimetric assays in which specific parasite enzyme activities, and histidine-rich protein II (HRP2) production are measured with the help of monoclonal antibodies; (iii) the β- haematin formation, and; (iv) assays using green fluorescent protein (GFP) in gene-expressing parasites. The advantages and disadvantages of the different in vitro screening methods, as well as the different in vivo models for antimalarial tests, are described in this review. Such tests can be used for the evaluation of medicinal plants, synthetic and hybrid molecules or drug combinations.