Polymorphisms of the pfmdr1 but not the pfnhe-1 gene is associated with in vitro quinine sensitivity in Thai isolates of Plasmodium falciparum

Eighty-five Thai isolates of P. falciparum from the Thai-Myanmar and Thai-Cambodia borders from 2003-2008 were determined for in vitro QN sensitivity using radioisotopic assay. Polymorphisms of the pfmdr1 and pfnhe-1 gene were determined by PCR-RFLP and sequence analysis. Associations between the in vitro QN sensitivity and the polymorphisms of the pfmdr1 and pfnhe-1 gene were evaluated.The mean QN IC50 was 202.8 nM (range 25.7-654.4 nM). Only four isolates were QN resistant when the IC50 of >500 nM was used as the cut-off point. Significant associations were found between the pfmdr1 mutations at codons N86Y and N1042D and in vitro QN sensitivity. However, no associations with the number of DNNND, DDNNNDNHNDD, and NHNDNHNNDDD repeats in the microsatellite ms4760 of the pfnhe-1 gene were identified.Data from the present study put doubt regarding the pfnhe-1 gene as to whether it could be used as the suitable marker for QN resistance in Thailand. In contrast, it confirms the influence of the pfmdr1 gene on in vitro QN sensitivity.The emergence of anti-malarial resistance in Plasmodium falciparum is a major public health threat worldwide, especially in tropical developing countries. The situation of multidrug-resistant falciparum malaria is most serious along the Thai-Myanmar and Thai-Cambodia borders [1]. To handle this situation, WHO recommends artemisinin derivative-based combination treatment (ACT) for the treatment of uncomplicated falciparum malaria [2]. Artesunate-mefloquine combination has been used as the first-line treatment in Thailand for more than 15 years [3]. Quinine (QN)-tetracycline/doxycycline has been used as the second-line treatment for uncomplicated falciparum malaria in Thailand. In addition, QN monotherapy is the first-line treatment for pregnancy [3,4]. Unfortunately reduced in vitro and in vivo response to QN has been reported in Southeast Asia [5,6]. Investigations have been carried out to identify the mechanisms of QN resistance. At least th