The malaria parasite’s chloroquine resistance transporter: An exploration of its interactions with drugs and of its evolution as a drug transporter

Abstract

The malaria parasite’s chloroquine resistance transporter: An exploration of its interactions with drugs and of its evolution as a drug transporter Abstract: Initially identified as the primary determinant of chloroquine resistance in the malaria parasite Plasmodium falciparum, mutations in the ‘chloroquine resistance transporter’ (PfCRT) can influence the parasite’s susceptibility to diverse molecules. The ability of PfCRT to affect the activity of so many compounds is likely to be a product of its location at the membrane of the parasite’s digestive vacuole – an acidic compartment in which many types of drugs accumulate, act, and/or are activated. The Xenopus laevis oocyte system enables the functional expression of PfCRT and has been used to demonstrate that a mutant isoform of PfCRT mediates the efflux of chloroquine from the vacuole (i.e., away from its site of action), whereas the wild-type protein lacks this activity. However, the evolution of chloroquine transport activity by PfCRT has yet to be explored, and little is known of how PfCRT interacts with diverse compounds. The overarching aim of this study was to understand how mutations in PfCRT confer chloroquine transport activity and alter the parasite’s susceptibility to diverse pharmacons.

A kinetic analysis of the inhibition of PfCRT-mediated chloroquine transport by verapamil, a compound which partially restores the activity of chloroquine against drug-resistant parasites, was undertaken in the oocyte system. The findings of this work revealed verapamil to be a partial-mixed-type inhibitor of the transporter, and suggested that the mutations required for chloroquine transport introduce multiple substrate-binding sites into PfCRT.

A series of complementary assays were then applied to examine the interactions of PfCRT with a range of compounds to identify, and distinguish between, PfCRT substrates and inhibitors. Using the oocyte system, two new classes of compounds were identified as potent inhibitors of the PfCRT-mediated transport of chloroquine. Transgenic parasite lines that are isogenic except for their pfcrt allele were employed, in conjunction with an assay that indirectly detects the transport of drugs out of the parasite’s digestive vacuole, to further characterise these compounds. The resulting data revealed that most of these molecules are not substrates of the mutant transporter. Furthermore, parasite proliferation assays demonstrated that the compounds possessed enhanced activities against parasites harbouring mutant PfCRT. Structure-activity relationships were consistent with these compounds binding to multiple points of attachment within PfCRT via lipophilic and electrostatic interactions.

Measurements of chloroquine transport via diverse isoforms of PfCRT (as well as by a series of chimeric proteins) were also undertaken in the oocyte system. These analyses revealed that multiple mutational pathways lead to saturable chloroquine transport via PfCRT. The finding that diverse PfCRT variants are all limited in their capacity to transport chloroquine suggests that resistance could be overcome by re-optimising the chloroquine dosage. Moreover, the results of this study indicated that the remodelling of PfCRT for chloroquine transport required a complex reorganisation of interacting residues.

These studies support the idea that, in addition to being a mediator of multidrug resistance, PfCRT is itself a viable drug target. Antimalarial therapies could be formulated to exert opposing selection forces upon PfCRT, thereby exploiting a key resistance mechanism and prolonging drug efficacy against this important human pathogen.