Use of the NP-40 Detergent-Mediated Assay in Discovery of Inhibitors of β-Hematin Crystallization

The protozoan parasite responsible for malaria affects over 500 million people each year. Current antimalarials have experienced decreased efficacy due to the development of drug-resistant strains of Plasmodium spp., resulting in a critical need for the discovery of new antimalarials. Hemozoin, a crystalline by-product of heme detoxification that is necessary for parasite survival, serves as an important drug target. The quinoline antimalarials, including amodiaquine and chloroquine, act by inhibiting the formation of hemozoin. The formation of this crystal does not occur spontaneously, and recent evidence suggests crystallization occurs in the presence of neutral lipid particles located in the acidic digestive vacuole of the parasite. To mimic these conditions, the lipophilic detergent NP-40 has previously been shown to successfully mediate the formation of beta-hematin, synthetic hemozoin. Here, an NP-40 detergent-based assay was successfully adapted for use as a high-throughput screen to identify inhibitors of beta-hematin formation. The resulting assay exhibited a favorable Z' of 0.82 and maximal drift of less than 4%. The assay was used in a pilot screen of 38,400 diverse compounds at a screening concentration of 19.3 mu M, resulting in the identification of 161 previously unreported beta-hematin inhibitors. Of these, 48 also exhibited >= 90% inhibition of parasitemia in a Plasmodium falciparum whole-cell assay at a screening concentration of 23 mu M. Eight of these compounds were identified to have nanomolar 50% inhibitory concentration values near that of chloroquine in this assay.