Nucleoside transport as a potential target for chemotherapy in malaria

Malaria constitutes an enormous drain on the health and economies of many countries and causes more than a million deaths annually. Moreover, resistance to existing antimalarial drugs is a growing problem, rendering the search for new targets urgent. Protozoan parasites of the genus Plasmodium that cause malaria lack the ability to synthesise the purine ring de novo and so are reliant upon salvage of purines, including hypoxanthine, inosine and adenosine, from the host. The transport systems responsible for uptake of these precursors are therefore promising targets for novel antimalarial drugs. In humans, purine uptake into many cell types is mediated by members of the Equilibrative Nucleoside Transporter (ENT) family, in particular hENT1 and hENT2. Genome sequencing has revealed that P. falciparum and P. vivax, the species responsible for the majority of malaria cases, each also possesses four members of this family, and in P. falciparum transcripts of each are expressed in the erythrocytic stages of the parasite responsible for clinical disease. One of the proteins, PfENT1, is known to be present in the parasite plasma membrane, and the kinetic properties of the heterologously expressed transporter are consistent with its representing the major purine uptake system in the trophozoite. Importantly, its inhibitor specificity and permeant selectivity differ from those of the host. In this review we discuss the possibility of exploiting these differences to develop novel antimalarial drugs that either selectively inhibit purine uptake into the pararasite or are selectively delivered by the transporter to the parasite cytoplasm.