Selection of parasites with diminished drug susceptibility by amodiaquine-containing antimalarial regimens in Uganda

Abstract

Background: Amodiaquine (AQ) is paired with artesunate (AS) or sulfadoxine-pyrimethamine (SP) in recommended antimalarial regimens. It is unclear how readily AQ resistance will be selected with combination chemotherapy.

Methods: We collected 61 Plasmodium falciparum samples from a cohort of Ugandan children randomized for treatment with AQ-SP, AS-AQ, or artemether-lumefantrine (AL) for uncomplicated malaria. In vitro susceptibility to monodesethylamodiaquine (MDAQ) was measured with a histidine-rich protein 2-based enzyme-linked immunosorbent assay, and potential resistance-mediating polymorphisms in pfmdr1 were evaluated.

Results: Parasites collected from patients treated with AQ-SP or AS-AQ within the prior 12 weeks were less susceptible to MDAQ (n = 18; mean of the median inhibitory concentration [IC(50)], 62.9 nmol/L; range, 12.7-158.3 nmol/L) than were parasites from those not treated within 12 weeks (n = 43; mean IC(50), 37.5 nmol/L; range, 6.3-184.7 nmol/L; P=.009) or only from those patients in the treatment arm that did not receive AQ (n = 12; mean IC(50), 28.8 nmol/L; range, 6.3-121.8 nmol/L; P = .004). The proportion of strains with polymorphisms expected to mediate diminished response to AQ (pfmdr1 86Y and 1246Y) increased after AQ therapy, although differences were not statistically significant.

Conclusions: Prior therapy selected for diminished response to MDAQ, which suggests that AQ-containing regimens may rapidly lose efficacy in Africa. The mechanism of diminished MDAQ response is not fully explained by known mutations in pfmdr1.

Figure 1. Sensitivity of clinical samples to MDAQ

Parasites were placed into culture and sensitivities were assessed by comparing levels of HRP-2 with those of untreated controls with an ELISA assay.

Figure 2. Selection of parasites with diminished response to AQ

Sensitivities were determined as described in Methods, and results are plotted for parasites from all patients that did not receive an AQ-containing regimen within 12 weeks (No Prior AQ; A) or only patients in the AL treatment arm, who did not receive an AQ-containing regimen for the course of the study (No Prior AQ; B), in both cases compared to results for subjects who did receive an AQ-containing regimen within 12 weeks prior to the time of this analysis (Prior AQ). Means are indicated by horizontal lines. Differences for both comparisons were significant, as described in the text.

Figure 3. Selection of pfmdr1 alleles

The proportions of samples with mutations at the alleles indicated are shown for the two comparisons described in figure 2.

Figure 4. Correlation of in vitro sensitivity to MDAQ with pfmdr1 genotypes

For each sample studied, the sequence at the three alleles of interest and the in vitro sensitivity to MDAQ are shown.