The mechanisms of resistance to antimalarial drugs in Plasmodium falciparum.

Drug-resistant malaria is primarily caused by Plasmodium falciparum, a species highly prevalent in tropical Africa, the Amazon region and South-east Asia. It causes severe fever or anaemia that leads to more than a million deaths each year. The emergence of chloroquine resistance has been associated with a dramatic increase in malaria mortality among inhabitants of some endemic regions. The rationale for chemoprophylaxis is weakening as multiple-drug resistance develops against well-tolerated drugs. Plasmodium falciparum drug-resistant malaria originates from chromosome mutations. Analysis by molecular, genetic and biochemical approaches has shown that (i). impaired chloroquine uptake by the parasite vacuole is a common characteristic of resistant strains, and this phenotype is correlated with mutations of the Pfmdr1, Pfcg2 and Pfcrt genes; (ii). one to four point mutations of dihydrofolate reductase (DHFR), the enzyme target of antifolates (pyrimethamine and proguanil) produce a moderate to high level of resistance to these drugs; (iii). the mechanism of resistance to sulfonamides and sulfones involves mutations of dihydropteroate synthase (DHPS), their enzyme target; (iv). treatment with sulphadoxine-pyrimethamine selects for DHFR variants Ile(51), Arg(59), and Asn(108) and for DHPS variants Ser(436), Gly(437), and Glu(540); (v) clones that were resistant to some traditional antimalarial agents acquire resistance to new ones at a high frequency (accelerated resistance to multiple drugs, ARMD). The mechanisms of resistance for amino-alcohols (quinine, mefloquine and halofantrine) are still unclear. Epidemiological studies have established that the frequency of chloroquine resistant mutants varies among isolated parasite populations, while resistance to antifolates is highly prevalent in most malarial endemic countries. Established and strong drug pressure combined with low antiparasitic immunity probably explains the multidrug-resistance encountered in the forests of South-east Asia and South America. In Africa, frequent genetic recombinations in Plasmodium originate from a high level of malaria transmission, and falciparum chloroquine-resistant prevalence seems to stabilize at the same level as chloroquine-sensitive malaria. Nevertheless, resistance levels may differ according to place and time. In vivo and in vitro tests do not provide an adequate accurate map of resistance. Biochemical tools at a low cost are urgently needed for prospective monitoring of resistance.