Molecular epidemiology of malaria in Yaounde, Cameroon IV. Evolution of pyrimethamine resistance between 1994 and 1998

Pyrimethamine, in combination with sulfadoxine, is currently one of the major alternative drugs used for the treatment of chloroquine-resistant Plasmodium falciparum malaria infections in Africa. The mechanism of pyrimethamine resistance has been strongly associated with a single, key point mutation in the dihydrofolate reductase-thymidylate synthase gene, resulting in the substitution of the wild-type allele Ser-108 by either Asn-108 or Thr-108. The pyrimethamine-resistant phenotype and/or genotype were determined in 273 Cameroonian clinical isolates obtained in Yaounde by in vitro assays and polymerase chain reaction-restriction fragment length polymorphism over a 5-year period. The in vitro assays showed that 42% (18 of 43) and 63% (69 of 110) of the isolates obtained in 1994-1995 and 1997-1998, respectively, were resistant to pyrimethamine (50% inhibitory concentration [IC50] > 100 nM). The polymerase chain reaction showed that 43% (55 of 127) and 59% (50 of 85) of the isolates in 1994-1995 and 1997-1998, respectively, had the mutant Asn-108 allele. The pyrimethamine-resistant genotype (Asn-108) corresponded with the pyrimethamine-resistant phenotype (IC50 > or = 100 nM) in a large majority (> 95%) of the isolates. The results of our study suggest an increasing prevalence of pyrimethamine resistance in Yaounde. Our study further suggests that pyrimethamine resistance can be monitored by a technique that can be adopted by malaria research centers in Africa.