Amplification of pvmdr1 associated with multidrug-resistant Plasmodium vivax

Abstract

Background: Multidrug-resistant strains of Plasmodium vivax are emerging in Southeast Asia.

Methods: In vitro drug susceptibility and pvmdr1 genotype were determined in P. vivax field isolates from Indonesia and Thailand.

Results: Increased pvmdr1 copy number was present in 21% of isolates from Thailand (15/71) and none from Indonesia (0/114; P < .001). Compared with Indonesian isolates, the median IC(50) of Thai isolates was lower for chloroquine (36 vs. 114 nmol/L; P < .001) but higher for amodiaquine (34 vs. 13.7 nmol/L; P = .032), artesunate (8.33 vs. 1.58 nmol/L; P < .001), and mefloquine (111 vs. 9.87 nmol/L; P < .001). In 11 cryopreserved Thai isolates, those with increased pvmdr1 copy number had a higher IC(50) for mefloquine (78.6 vs. 38 nmol/L for single-copy isolates; P = .006). Compared with isolates with the wild-type allele, the Y976F mutation of pvmdr1 was associated with reduced susceptibility to chloroquine (154 nmol/L [range, 4.6-3505] vs. 34 nmol/L [range, 6.7-149]; P < .001) but greater susceptibility to artesunate (1.8 vs. 9.5 nmol/L; P = .009) and mefloquine (14 vs. 121 nmol/L; P < .001).

Conclusions: Amplification of pvmdr1 and single-nucleotide polymorphisms are correlated with susceptibility of P. vivax to multiple antimalarial drugs. Chloroquine and mefloquine appear to exert competitive evolutionary pressure on pvmdr1, similar to that observed with pfmdr1 in Plasmodium falciparum.

Figure 1

Selection of samples analyzed. RT ratio, ring to trophozoite ratio.

Figure 2

In vitro drug-susceptibility profiles for Indonesian (circles) and Thai (diamonds) isolates. The nos. above each group and the horizontal bars indicate median values. P values are for the comparison between Indonesian and Thai isolates.

Figure 3

In vitro susceptibility to mefloquine in thawed Thai isolates according to pvmdr1 genotype: single copy of pvmdr1 (black circles, solid line) vs. amplified pvmdr1 (white circles, dashed line). Data points represent assay wells for 11 isolates with 8 dilutions of mefloquine. The predicted population curves were calculated using the formula $Y=1-[X^{\gamma }∕(X^{\gamma }+{\mathrm{IC}}_{50}^{\gamma }\left)\right]$, where Y is the response compared with the control well, X is the mefloquine concentration, and IC₅₀ and γ are the median values for the IC₅₀ and slope, respectively, derived for each isolate by regression analysis.