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Clinical observations have shown that pyrimethamine resistance develops rapidly in endemic countries where antifolate drugs are used massively for the treatment of Plasmodium falciparum infections. To analyze this phenomenon, the in vitro response of clinical isolates to pyrimethamine and the dihydrofolate reductase (dhfr) gene sequence were analyzed in 2000–2001 and compared with the results obtained since 1994 in Yaounde, Cameroon. Of 139 samples obtained in 2000–2001, 10 (7.2%) isolates were of the wild-type, 116 had pure mutant alleles (2 [1.4%] with a single mutation, 11 [7.9%] with double mutations, and 103 [74.1%] with triple mutations), and 13 (9.4%) had mixed alleles. With the exception of a single isolate with triple mutations (50% inhibitory concentration [IC50] = 84.3 nM), all isolates with pure wild-type dhfr alleles (IC50 < 100 nM) and those with pure mutant dhfr alleles (between 1 and 3 point mutations; IC50 
100 nM) were clearly distinguished by in vitro drug sensitivity assays. The results of the two methods are highly correlated, and both methodologic approaches indicate an increasing proportion of pyrimethamine-resistant isolates in Yaounde over the past eight years (42–45% in 1994–1995, 63–67% in 1997–1998, and 88–92% in 2000–2001). At present, clinical isolates carrying triple dhfr mutations predominate in Yaounde. This situation calls for a regular surveillance of the efficacy of antifolate drugs by all available means, including clinical evaluation, in vitro drug sensitivity assays, molecular markers, and pharmacologic studies.
Received September 7, 2002. Accepted for publication April 26, 2003.
Acknowledgment: I thank Sister Marie-Solange Oko, the personnel of the Nlongkak Catholic Missionary Dispensary, and Delphine Ngo Ndombol for their aid in recruiting patients.
Financial support: This study was supported by the French Ministry of Research (Program VIHPAL/PAL+, Actions 2000 and 2001) and the European Union (INCO-DEV Program, contract no. ICA4-CT-2001-10078).
Author’s address: Leonardo K. Basco, Unité de Recherche Paludologie Afro-Tropicale, Institut de Recherche pour le Développement, Laboratoire de Recherche sur le Paludisme, Organisation de Coordination pour la Lutte contre les Endémies en Afrique Centrale, BP 288, Yaounde, Cameroon, Telephone: 237-223-22-32, Fax: 237-223-00-61, E-mail: Leonardo.Basco{at}ibaic.u-psud.fr or lkbasco{at}yahoo.fr.
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