Plasmodium falciparum drug transporter genes in emerging malaria multidrug resistance

Abstract: Malaria is caused by an intracellular protozoan parasite of the genus Plasmodium. The use of chemotherapy, the foremost tool available for the control of the disease, has been challenged in the last decades by the development and spread of drug resistance among malaria parasites. A clear understanding behind the mechanisms of parasite resistance is required for the improvement of treatment efficacy, policy assessment and in the development of new drugs. A common strategy used by parasites in achieving resistance involves decreasing drug accumulation inside the cell. This is typically accomplished by increasing the availability of transporter proteins that mediate the efflux of the active compound. The goal of this thesis was to better understand the involvement of drug transporter genes in the molecular mechanisms underlying drug susceptibility in Plasmodium falciparum malaria. The approaches involved clinical drug trials, clinical isolates and extensive studies of laboratory P. falciparum parasites. The contribution of pfmrp1 polymorphisms in in vivo parasite drug response was studied in P. falciparum infected patients from drug efficacy clinical trials. After SP treatment, recrudescent infections selected for parasites that had a lysine at amino acid position 1466 in pfmrp1, thus providing the first indication that this transporter gene may have a role in P. falciparum antifolate drug responses in vivo. We examined the effect of the ACT partner drug, mefloquine, on the intra-erythrocytic cell cycle of P. falciparum laboratory parasites having different in vitro drug susceptibilities, while in parallel investigating the expression of four pivotal drug transporter genes: pfcrt, pfmdr1, pfmrp1 and pfmrp2. This study revealed a delay in the cell cycle of the parasite after drug pressure, accompanied by gene induction of the transporter genes studied. The genetic background of the drug transporter genes pfcrt, pfmdr1, pfmrp1 and pfmrp2 were further studied at length in field isolates collected at the Thai-Myanmar border, a historically known epicenter of resistance. The isolates were characterized in vitro for their sensitivity against a broad range of ACT relevant antimalarials. Correlation analyses revealed novel candidate markers for multidrug resistance against structurally unrelated antimalarial drugs used extensively in ACT regimens worldwide. In conclusion, these studies reinforce the concept of malaria drug resistance as a multifactorial and complex phenomenon that may involve not only the parasite’s handling of the incoming drug, but also concomitant responses of its basic physiology.