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Background  Chloroquine (CQ), a cost effective antimalarial drug with a relatively good safety profile and therapeutic index, is no longer used by itself to treat patients with  Plasmodium falciparum  due to CQ-resistant strains.  P. vivax , representing over 90% of malaria cases in Brazil, despite reported resistance, is treated with CQ as well as with primaquine to block malaria transmission and avoid late  P. vivax  malaria relapses. Resistance to CQ and other antimalarial drugs influences malaria control, thus monitoring resistance phenotype by parasite genotyping is helpful in endemic areas.    Methods  A total of 47  P. vivax  and nine  P. falciparum  fresh isolates were genetically characterized and tested for CQ, mefloquine (MQ) and artesunate (ART) susceptibility  in vitro . The genes  mdr1  and  pfcrt , likely related to CQ resistance, were analyzed in all isolates. Drug susceptibility was determined using short-term parasite cultures of ring stages for 48 to 72 hour and thick blood smears counts. Each parasite isolate was tested with the antimalarials to measure the geometric mean of 50% inhibitory concentration.    Results  The low numbers of  P. falciparum  isolates reflect the species prevalence in Brazil; most displayed low sensitivity to CQ (IC50 70 nM). However, CQ resistance was rare among  P. vivax  isolates (IC 50  of 32 nM). The majority of  P. vivax  and  P. falciparum  isolates were sensitive to ART and MQ. One hundred percent of  P. falciparum  isolates carried non-synonymous mutations in the pfmdr1 gene in codons 184, 1042 and 1246, 84% in codons 1034 and none in codon 86, a well-known resistance mutation. For the  pfcrt  gene, mutations were observed in codons 72 and 76 in all  P. falciparum  isolates. One  P. falciparum  isolate from Angola, Africa, showing sensitivity to the antimalarials, presented no mutations. In  P. vivax , mutations of  pvmdr1  and the multidrug resistance gene 1 marker at codon F976 were absent.    Conclusion  All  P. falciparum  Brazilian isolates showed CQ resistance and presented non-synonymous mutations in  pfmdr1  and  pfcrt . CQ resistant genotypes were not present among  P. vivax  isolates and the IC 50  values were low in all samples of the Brazilian West Amazon.

Plasmodium vivax and Plasmodium falciparum ex vivo susceptibility to anti-malarials and gene characterization in Rondônia, West Amazon, Brazil