In vitro chemosensitization of Plasmodium falciparum to antimalarials by verapamil and probenecid.

We tested the effect of probenecid and verapamil in chemosensitizing Plasmodium falciparum to 14 antimalarials using the multidrug-resistant strain V1S and the drug-sensitive 3D7. Verapamil chemosensitizes V1S to quinine and chloroquine. Interestingly, probenecid profoundly chemosensitizes V1S to piperaquine. Thus, probenecid could be used to increase piperaquine efficacy in vivo.

Genome-wide screen identifies new candidate genes associated with artemisinin susceptibility in Plasmodium falciparum in Kenya.

Early identification of causal genetic variants underlying antimalarial drug resistance could provide robust epidemiological tools for timely public health interventions. Using a novel natural genetics strategy for mapping novel candidate genes we analyzed >75,000 high quality single nucleotide polymorphisms selected from high-resolution whole-genome sequencing data in 27 isolates of Plasmodium falciparum. We identified genetic variants associated with susceptibility to dihydroartemisinin that implicate one region on chromosome 13, a candidate gene on chromosome 1 (PFA0220w, a UBP1 ortholog) and others (PFB0560w, PFB0630c, PFF0445w) with putative roles in protein homeostasis and stress response. There was a strong signal for positive selection on PFA0220w, but not the other candidate loci. Our results demonstrate the power of full-genome sequencing-based association studies for uncovering candidate genes that determine parasite sensitivity to artemisinins. Our study provides a unique reference for the interpretation of results from resistant infections.

Genome wide adaptations of Plasmodium falciparum in response to lumefantrine selective drug pressure.

The combination therapy of the Artemisinin-derivative Artemether (ART) with Lumefantrine (LM) (Coartem®) is an important malaria treatment regimen in many endemic countries. Resistance to Artemisinin has already been reported, and it is feared that LM resistance (LMR) could also evolve quickly. Therefore molecular markers which can be used to track Coartem® efficacy are urgently needed. Often, stable resistance arises from initial, unstable phenotypes that can be identified in vitro. Here we have used the Plasmodium falciparum multidrug resistant reference strain V1S to induce LMR in vitro by culturing the parasite under continuous drug pressure for 16 months. The initial IC(50) (inhibitory concentration that kills 50% of the parasite population) was 24 nM. The resulting resistant strain V1S(LM), obtained after culture for an estimated 166 cycles under LM pressure, grew steadily in 378 nM of LM, corresponding to 15 times the IC(50) of the parental strain. However, after two weeks of culturing V1S(LM) in drug-free medium, the IC(50) returned to that of the initial, parental strain V1S. This transient drug tolerance was associated with major changes in gene expression profiles: using the PFSANGER Affymetrix custom array, we identified 184 differentially expressed genes in V1S(LM). Among those are 18 known and putative transporters including the multidrug resistance gene 1 (pfmdr1), the multidrug resistance associated protein and the V-type H+ pumping pyrophosphatase 2 (pfvp2) as well as genes associated with fatty acid metabolism. In addition we detected a clear selective advantage provided by two genomic loci in parasites grown under LM drug pressure, suggesting that all, or some of those genes contribute to development of LM tolerance--they may prove useful as molecular markers to monitor P. falciparum LM susceptibility.

Declining responsiveness of Plasmodium falciparum infections to artemisinin-based combination treatments on the Kenyan coast.

BACKGROUND: The emergence of artemisinin-resistant P. falciparum malaria in South-East Asia highlights the need for continued global surveillance of the efficacy of artemisinin-based combination therapies. METHODS: On the Kenyan coast we studied the treatment responses in 474 children 6-59 months old with uncomplicated P. falciparum malaria in a randomized controlled trial of dihydroartemisinin-piperaquine vs. artemether-lumefantrine from 2005 to 2008. (ISRCTN88705995). RESULTS: The proportion of patients with residual parasitemia on day 1 rose from 55% in 2005-2006 to 87% in 2007-2008 (odds ratio, 5.4, 95%CI, 2.7-11.1; P<0.001) and from 81% to 95% (OR, 4.1, 95%CI, 1.7-9.9; P = 0.002) in the DHA-PPQ and AM-LM groups, respectively. In parallel, Kaplan-Meier estimated risks of apparent recrudescent infection by day 84 increased from 7% to 14% (P = 0.1) and from 6% to 15% (P = 0.05) with DHA-PPQ and AM-LM, respectively. Coinciding with decreasing transmission in the study area, clinical tolerance to parasitemia (defined as absence of fever) declined between 2005-2006 and 2007-2008 (OR body temperature >37.5°C, 2.8, 1.9-4.1; P<0.001). Neither in vitro sensitivity of parasites to DHA nor levels of antibodies against parasite extract accounted for parasite clearance rates or changes thereof. CONCLUSIONS: The significant, albeit small, decline through time of parasitological response rates to treatment with ACTs may be due to the emergence of parasites with reduced drug sensitivity, to the coincident reduction in population-level clinical immunity, or both. Maintaining the efficacy of artemisinin-based therapy in Africa would benefit from a better understanding of the mechanisms underlying reduced parasite clearance rates. TRIAL REGISTRATION: Controlled-Trials.com ISRCTN88705995.

In vivo and in vitro efficacy of amodiaquine against Plasmodium falciparum in an area of continued use of 4-aminoquinolines in East Africa.

In light of reports of increasing resistance of parasites to amodiaquine in African countries in which Plasmodium falciparum is endemic as well as the paucity of recent in vitro sensitivity data, we assessed the in vivo and in vitro sensitivity to amodiaquine of P. falciparum isolates from 128 pediatric outpatients (0.5-10 years old) in Pingilikani, Kilifi District, Kenya, who were treated with amodiaquine (10 mg/kg/day for 3 days). The polymerase chain reaction-corrected parasitological cure rate on day 28 (by Kaplan-Meier analysis) was 82% (95% confidence interval [CI], 74%-88%). Twenty-six percent (17/66) of tested pretreatment P. falciparum field isolates had 50% in vitro growth inhibition at concentrations of N-desethyl-amodiaquine (DEAQ)-the major biologically active metabolite of amodiaquine-above the proposed resistance threshold of 60 nmol/L, but baseline median DEAQ 50% inhibitory concentration values were not associated with subsequent risk of asexual parasite recrudescence (29 nmol/L [95% CI, 23-170 nmol/L] and 34 nmol/L [95% CI, 30-46 nmol/L] for patients with and those without recrudescences, respectively). The median absolute neutrophil count dropped by 1.3 X 10(3) cells/microL (95% CI, -1.7 X 10(3) to -0.7 X 10(3) cells/microL) between days 0 and 28. The high prevalence of in vitro and in vivo resistance precludes the use of amodiaquine on its own as second-line treatment. These findings also suggest that the value of amodiaquine combinations as first- or second-line treatment in areas with similar patterns of 4-aminoquinoline resistance should be reassessed.