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Recent studies on resistance to blood schizontocides in Plasmodium falciparum give a rational basis for the use of artemisinins combined with arylaminoalcohols for the treatment of uncomplicated chloroquine-resistant malaria in Africa. In areas where such combinations are introduced, there is reason to believe that the continued use of chloroquine in the community will help protect the new drugs from resistance. In view of several laboratory studies, combinations of artemisinins with antifolates or chloroquine pose a risk of antagonistic interaction. This can be avoided by use of the artemisinin and the companion drug sequentially.

The declining efficacy of chloroquine and pyrimethamine/sulphadoxine in the treatment of human malaria has led to the use of newer antimalarials such as mefloquine and artemisinin. Sequence polymorphisms in the pfmdr1 gene, the gene encoding the plasmodial homologue of mammalian multidrug resistance transporters, have previously been linked to resistance to chloroquine in some, but not all, studies. In this study, we have used a genetic cross between the strains HB3 and 3D7 to study inheritance of sensitivity to the structurally unrelated drugs mefloquine and artemisinin, and to several other antimalarials. We find a complete allelic association between the HB3-like pfmdr1 allele and increased sensitivity to these drugs in the progeny. Different pfmdr1 sequence polymorphisms in other unrelated lines were also associated with increased sensitivity to these drugs. Our results indicate that the pfmdr1 gene is an important determinant of susceptibility to antimalarials, which has major implications for the future development of resistance.

With the spread of resistance to chloroquine, the combination of sulphadoxine and pyrimethamine is growing in importance for the treatment of infection with the malaria parasite Plasmodium falciparum. Mutations in the dhfr gene of P. falciparum have been associated with resistance to pyrimethamine. Recently, several polymorphisms have been identified in the P. falciparum dhps gene which may correlate with sulphadoxine-resistance. Simple and rapid tests have been developed to detect these polymorphisms, using PCR followed by restriction digestion. These tests can accurately identify all the polymorphisms described to date at codons 16, 51, 59, 108, and 164 in the dhfr gene and those at codons 436, 437, 540, 581, and 613 in the dhps gene. A nested system has been developed which allows the accurate detection of these polymorphisms in samples of fingerprick blood collected on glass fiber membranes and filter papers, some with very low parasitaemias.

To assess the level of resistance to chloroquine (CQ) of Plasmodium falciparum in The Gambia in 1995-1996 we measured susceptibility in vivo by quantifying parasitaemia of children with mild malaria on days 4 and 8 after treatment. Pretreatment blood samples were used for susceptibility testing in vitro by the World Health Organization microtest and the prevalence of the tyrosine (tyr)86 allele of the Pfmdr1 gene was assessed by the polymerase chain reaction and restriction fragment length polymorphism analysis. Seven of 42 children (17%) treated with CQ remained parasitaemic on day 4 and required a change of antimalarial treatment. Susceptibility assays in vitro were performed on 50 P. falciparum isolates obtained from eligible children before treatment; 36 (72%) were resistant to CQ (> or = 1.6 mumol/L). The median minimum inhibitory concentration (MIC) of artemether was 3.38 nmol/L (range 0.42-13.51 nmol/L) and the median MIC of dihydroartemisinin was 0.88 nmol/L (range 0.22-14.04 nmol/L). Susceptibility in vitro to CQ and the Pfmdr1 genotype were determined for 31 fresh isolates. The allele was present in 12 of 22 isolates found to be resistant to CQ in vitro, but in none of the 9 isolates which were susceptible (Fisher's exact test, P = 0.005). All P. falciparum isolates with the tyr86 allele were CQ resistant in vitro, but since only half of all resistant isolates contained the allele, additional explanations for CQ resistance are required.

A nested polymerase chain reaction was used to amplify a fragment of the gene for dihydrofolate reductase of Plasmodium falciparum containing codon 108, where a point mutation, causing a serine to asparagine change, occurs in pyrimethamine resistant parasites. The presence of the mutation was detected by restriction enzyme digestion. Parasites in blood samples collected from asymptomatic children before, and 3 weeks after, treatment with pyrimethamine-sulfadoxine or chlorproguanil-dapsone were analysed. Parasites in the samples taken at 3 weeks carried only the asparagine mutant.