Verapamil-sensitive transport of quinacrine and methylene blue via the plasmodium falciparum chloroquine resistance transporter reduces the parasite's susceptibility to these tricyclic drugs

Background. It is becoming increasingly apparent that certain mutations in the Plasmodium falciparum chloroquine resistance transporter (PfCRT) alter the parasite's susceptibility to diverse compounds. Here we investigated the interaction of PfCRT with 3 tricyclic compounds that have been used to treat malaria (quinacrine [QC] and methylene blue [MB]) or to study P. falciparum (acridine orange [AO]). Methods. We measured the antiplasmodial activities of QC, MB, and AO against chloroquine-resistant and chloroquine sensitive P. falciparum and determined whether QC and AO affect the accumulation and activity of chloroquine in these parasites. We also assessed the ability of mutant (PfCRT^Dd2) and wild-type (PfCRT^D10) variants of the protein to transport QC, MB, and AO when expressed at the surface of Xenopus laevis oocytes. Results. Chloroquine resistance-conferring isoforms of PfCRT reduced the susceptibility of the parasite to QC, MB, and AO. In chloroquine-resistant (but not chloroquine-sensitive) parasites, AO and QC increased the parasite's accumulation of, and susceptibility to, chloroquine. All 3 compounds were shown to bind to PfCRT^Dd2, and the transport of QC and MB via this protein was saturable and inhibited by the chloroquine resistance-reverser verapamil. Conclusions. Our findings reveal that the PfCRT^Dd2-mediated transport of tricyclic antimalarials reduces the parasite's susceptibility to these drugs.