Basco L K, Ringwald P
Institut de Recherche pour le Développement, Laboratoire Associé Francophone 302, Organisation de Coordination pour la lutte contre les Endémies en Afrique Centrale, Yaounde, Cameroon.
Am J Trop Med Hyg. 2000 Feb;62(2):271-6. doi: 10.4269/ajtmh.2000.62.271.
Pyrimethamine and cycloguanil, the major human metabolite of proguanil, are inhibitors of dihydrofolate reductase that play a key role in the treatment and prevention of chloroquine-resistant Plasmodium falciparum infections in sub-Saharan Africa. Resistance to these antifolate drugs has emerged in some areas of Africa. Earlier molecular studies have demonstrated that point mutations at key positions of the dihydrofolate reductase-thymidylate synthase gene are strongly associated with antifolate resistance. However, whether the same or distinct mutations are involved in the development of resistance to both pyrimethamine and cycloguanil has not been well established in naturally occurring P. falciparum isolates. In this study, the in vitro responses to both antifolate drugs were measured in 42 Cameroonian isolates and compared with the complete sequence of the dihydrofolate reductase domain of the gene (from 34 of 42 isolates) to analyze the genotype that may distinguish between pyrimethamine and cycloguanil resistance. The wild-type profile (n = 11 isolates) was associated with low 50% inhibitory concentrations (IC50s) ranging from 0.32 to 21.4 nanamole for pyrimethamine and 0.60-6.40 nM for cycloguanil. Mutant isolates had at least one amino acid substitution, Asn-108. Only three mutant codons were observed among the antifolate-resistant isolates: Ile-51, Arg-59, and Asn-108. The increasing number of point mutations was associated with an increasing level of pyrimethamine IC50 and, to a much lesser extent, cycloguanil IC50. These results support a partial cross-resistance between pyrimethamine and cycloguanil that is based on similar amino acid substitutions in dihydrofolate reductase and suggest that two or three mutations, including at least Asn-108, may be necessary for cycloguanil resistance, whereas a single Asn-108 mutation is sufficient for pyrimethamine resistance.
乙胺嘧啶和环氯胍（氯胍的主要人体代谢产物）是二氢叶酸还原酶抑制剂，在撒哈拉以南非洲地区治疗和预防对氯喹耐药的恶性疟原虫感染中发挥关键作用。在非洲一些地区已出现对这些抗叶酸药物的耐药性。早期的分子研究表明，二氢叶酸还原酶-胸苷酸合成酶基因关键位置的点突变与抗叶酸耐药性密切相关。然而，在自然发生的恶性疟原虫分离株中，对乙胺嘧啶和环氯胍耐药性的产生是否涉及相同或不同的突变尚未明确。在本研究中，测定了42株喀麦隆分离株对这两种抗叶酸药物的体外反应，并与基因二氢叶酸还原酶结构域的完整序列（42株中的34株）进行比较，以分析可能区分乙胺嘧啶和环氯胍耐药性的基因型。野生型谱（n = 11株分离株）与低50%抑制浓度（IC50）相关，乙胺嘧啶的IC50范围为0.32至21.4纳摩尔，环氯胍为0.60 - 6.40 nM。突变分离株至少有一个氨基酸取代，即Asn-108。在抗叶酸耐药分离株中仅观察到三个突变密码子：Ile-51、Arg-59和Asn-108。点突变数量的增加与乙胺嘧啶IC50水平的升高相关，而与环氯胍IC50的升高程度小得多。这些结果支持乙胺嘧啶和环氯胍之间基于二氢叶酸还原酶中相似氨基酸取代的部分交叉耐药性，并表明包括至少Asn-108在内的两个或三个突变可能是环氯胍耐药所必需的，而单个Asn-108突变足以导致乙胺嘧啶耐药。
