No abstract

BackgroundEffective diagnosis of malaria is a major component of case management. Rapid diagnostic tests (RDTs) based on Plasmodium falciparumhistidine-rich protein 2 (PfHRP2) are popular for diagnosis of this most virulent malaria infection. However, concerns have been raised about the longevity of the PfHRP2 antigenaemia following curative treatment in endemic regions.MethodsA model of PfHRP2 production and decay was developed to mimic the kinetics of PfHRP2 antigenaemia during infections. Data from two human infection studies was used to fit the model, and to investigate PfHRP2 kinetics. Four malaria RDTs were assessed in the laboratory to determine the minimum detectable concentration of PfHRP2.ResultsFitting of the PfHRP2 dynamics model indicated that in malaria naïve hosts, P. falciparum parasites of the 3D7 strain produce 1.4 × 10-13 g of PfHRP2 per parasite per replication cycle. The four RDTs had minimum detection thresholds between 6.9 and 27.8 ng/mL. Combining these detection thresholds with the kinetics of PfHRP2, it is predicted that as few as 8 parasites/μL may be required to maintain a positive RDT in a chronic infection.ConclusionsThe results of the model indicate that good quality PfHRP2-based RDTs should be able to detect parasites on the first day of symptoms, and that the persistence of the antigen will cause the tests to remain positive for at least seven days after treatment. The duration of a positive test result following curative treatment is dependent on the duration and density of parasitaemia prior to treatment and the presence and affinity of anti-PfHRP2 antibodies.

The stage-specific antimalarial activities of a panel of antiretroviral protease inhibitors (PIs), including two nonpeptidic PIs (tipranavir and darunavir), were tested in vitro against Plasmodium falciparum. While darunavir demonstrated limited antimalarial activity (effective concentration [EC 50 ], >50 M), tipranavir was active at clinically relevant concentrations (EC 50 , 12 to 21 M). Saquinavir, lopinavir, and tipranavir preferentially inhibited the growth of mature asexual-stage parasites (24 h postinvasion). While all of the PIs tested inhibited gametocytogenesis, tipranavir was the only one to exhibit gametocytocidal activity.The global distributions of HIV and malaria overlap in many regions of the world (reviewed in reference 17). Although data on the number of individuals with both diseases are unavailable, rates of coinfection are likely to be high (7). Furthermore, coinfection often leads to severe disease (4,12,19,20). While the effects of antiretroviral therapy on the outcome of malaria infection are not understood, defining these interactions is important (11,13,16,18). Understanding the antimalarial activities of the antiretroviral protease inhibitors (PIs) (reviewed in reference 17), for example, may lead to treatment recommendations that improve clinical outcomes and may also result in the identification of a new antimalarial drug target.Current data suggest that PIs kill malaria parasites by inhibiting one or more of the six nondigestive vacuole plasmepsins (reviewed in reference 17). In the present study we investigated the stage-specific effects of the PIs on asexual-and sexual-stage Plasmodium falciparum parasites in order to help define the antimalarial target(s) of these drugs and to help guide partner drug choices in the field. To gain additional structure-activity data and information that may be relevant for coinfected individuals we also examined the activities of the nonpeptidic PIs tipranavir (Aptivus) and darunavir (Prezista), new-generation PIs that are active against HIV-1 strains resistant to first generation PIs (9).The antimalarial activities of saquinavir, lopinavir, ritonavir, tipranavir, darunavir, and chloroquine (diphosphate salt; Sigma) were determined as described previously (18). Concentrations required to achieve 10, 50, and 90% growth inhibition (Ϯ the standard error [SE]) were determined by nonlinear regression curve fitting. Each assay was performed in triplicate on at least two separate occasions. Stage-specific growth inhibition assays were performed on synchronized parasite cultures (8) at 0 (ring), 24 (trophozoite), and 36 h (schizont) postsynchronization. Cultures were washed post-drug exposure, resuspended in drug-free medium, and seeded into tissue culture plates containing 0.5 Ci/well [ 3 H]hypoxanthine for 40 h. Incorporation of [ 3 H]hypoxanthine was compared to that in vehicle controls.Drug-induced effects on gametocytogenesis were examined using Pfs16-GFP parasites (3) as previously described (14). Assays were performed in triplicate on three separat...