The toxicity of antifolates in Babesia bovis

Extensive Shared Chemosensitivity between Malaria and Babesiosis Blood-Stage Parasites

The apicomplexan parasites that cause malaria and babesiosis invade and proliferate within erythrocytes. To assess the potential for common antiparasitic treatments, we measured the sensitivities of multiple species of Plasmodium and Babesia parasites to the chemically diverse collection of antimalarial compounds in the Malaria Box library. We observed that these parasites share sensitivities to a large fraction of the same inhibitors and we identified compounds with strong babesiacidal activity.

Environmental risk and toxicology of human and veterinary waste pharmaceutical exposure to wild aquatic host-parasite relationships

Pollution of the aquatic environment by human and veterinary waste pharmaceuticals is an increasing area of concern but little is known about their ecotoxicological effects on wildlife. In particular the interactions between pharmaceuticals and natural stressors of aquatic communities remains to be elucidated. A common natural stressor of freshwater and marine organisms are protozoan and metazoan parasites, which can have significant effects on host physiology and population structure, especially under the influence of many traditional kinds of toxic pollutants. However, little is known about the effects of waste pharmaceuticals to host-parasite dynamics. In order to assess the risk waste pharmaceuticals pose to aquatic wildlife it has been suggested the use of toxicological data derived from mammals during the product development of pharmaceuticals may be useful for predicting toxic effects. An additional similar source of information is the extensive clinical studies undertaken with numerous classes of drugs against parasites of human and veterinary importance. These studies may form the basis of preliminary risk assessments to aquatic populations and their interactions with parasitic diseases in pharmaceutical-exposed habitats. The present article reviews the effects of the most common classes of pharmaceutical medicines to host-parasite relationships and assesses the risk they may pose to wild aquatic organisms. In addition the effects of pharmaceutical mixtures, the importance of sewage treatment, and the risk of developing resistant strains of parasites are also assessed.

Cloning, expression, and characterization of Babesia gibsoni dihydrofolate reductase-thymidylate synthase: inhibitory effect of antifolates on its catalytic activity and parasite proliferation

Dihydrofolate reductase-thymidylate synthase (DHFR-TS) is a well-validated antifolate drug target in certain pathogenic apicomplexans, but not in the genus Babesia, including Babesia gibsoni. Therefore, we isolated, cloned, and expressed the wild-type B. gibsoni dhfr-ts gene in Escherichia coli and evaluated the inhibitory effect of antifolates on its enzyme activity, as well as on in vitro parasite growth. The full-length gene consists of a 1,548-bp open reading frame encoding a 58.8-kDa translated peptide containing DHFR and TS domains linked together in a single polypeptide chain. Each domain contained active-site amino acid residues responsible for the enzymatic activity. The expressed soluble recombinant DHFR-TS protein was approximately 57 kDa after glutathione S-transferase (GST) cleavage, similar to an approximately 58-kDa native enzyme identified from the parasite merozoite. The non-GST fusion recombinant DHFR enzyme revealed K(m) values of 4.70 +/- 0.059 (mean +/- standard error of the mean) and 9.75 +/- 1.64 microM for dihydrofolic acid (DHF) and NADPH, respectively. Methotrexate was a more-potent inhibitor of the enzymatic activity (50% inhibition concentration [IC(50)] = 68.6 +/- 5.20 nM) than pyrimethamine (IC(50) = 55.0 +/- 2.08 microM) and trimethoprim (IC(50) = 50 +/- 12.5 microM). Moreover, the antifolates' inhibitory effects on DHFR enzyme activity paralleled their inhibition of the parasite growth in vitro, indicating that the B. gibsoni DHFR could be a model for studying antifolate compounds as potential drug candidates. Therefore, the B. gibsoni DHFR-TS is a molecular antifolate drug target.

Development of a microtitre-based spectrophotometric method to monitor Babesia divergens growth in vitro

Babesia divergens multiplication cycle involves erythrocyte invasion, intracellular division, and erythrocyte lysis with the simultaneous liberation of hemoglobin. We have decided to set up a spectrophotometric protocol based on hemoglobin concentration in the culture supernatants to monitor B. divergens in vitro growth. After the selection of 405 nm as the most appropriate endpoint hemoglobin wavelength in our conditions (hemoglobin concentration in the supernatant), cultures were standardized [1 x 10(9) red blood cell (RBC)/ml, 1-2.5 x 10(5) infected red blood cell (iRBC)/ml] to allow their monitoring over 3 days. The protocol was then compared to the most commonly used growth measurement methods: parasitemia counting and [(3)H]hypoxanthine incorporation. An excellent correlation was demonstrated between A(405) of the culture supernatant and parasitemia of the iRBC, whatever the RBC concentration used in the medium. This correlation was also evidenced between A(405) and [(3)H]hypoxanthine incorporation for [(3)H]hypoxanthine concentrations lower than 4 microCi/ml. Our assays also highlighted the inhibitory effect of [(3)H]hypoxanthine on B. divergens growth even when used at low concentrations (0.8 microCi/ml) and for a short incorporation duration (24 h). This effect was confirmed by both A(405) and parasitemia counting. In conclusion, A(405) measurement of B. divergens culture supernatant represents a simple, rapid, safe, and reliable way to measure the in vitro growth of this parasite. Generation times of three different B. divergens strains were then determined by the protocol described here and varied between 8 h 36 min and 13 h 8 min.

An amino acid substitution in the Babesia bovis dihydrofolate reductase-thymidylate synthase gene is correlated to cross-resistance against pyrimethamine and WR99210

The genomic locus and cDNA encoding Babesia bovis dihydrofolate reductase-thymidylate synthase (DHFR-TS) were cloned and sequenced. A single dhfr-ts gene, composed of four exons, encodes a 511 aa protein that is most closely related to Plasmodium falciparum DHFR-TS. The genomic locus is characterized by the presence of four other genes of which at least three are expressed during the erythrocytic cycle. Three of the genes were highly conserved in closely related Theileria species and for two of the genes and dhfr-ts, gene synteny was observed between B. bovis and Theileria parva, B. bovis in vitro cultures displaying approximately 10-20-fold decreased sensitivity towards the antimalarial drugs WR99210 and pyrimethamine were selected repeatedly after prolonged growth in presence of drugs. Five cultures examined in detail were shown to encode a DHFR-TS carrying amino acid substitution S125F. Three-dimensional-modelling, using the P. falciparum DHFR structure as a template, suggests that substitution S125F protrudes into the binding site of NADPH. The S125F mutant could be isolated by growth under pyrimethamine or WR99210 pressure conferring cross-resistance to both drugs. Although opposing selection for pyrimethamine or WR99210 resistance was reported recently using P. falciparum or P. vivax strains carrying wildtype dhfr, the results obtained here are reminiscent of a quadruple mutant of P. falciparum dhfr displaying strong resistance to pyrimethamine and 10-fold enhanced resistance against WR99210. Wildtype B. bovis DHFR carries three mutations present in this mutant possibly explaining the low sensitivity to pyrimethamine and the ease by which moderately WR99210 resistant mutants could be isolated.

Growth-inhibitory effects of artesunate, pyrimethamine, and pamaquine against Babesia equi and Babesia caballi in in vitro cultures

Three antimalarial drugs, artesunate, pyrimethamine, and pamaquine, were evaluated for their growth-inhibitory effects against Babesia equi and Babesia caballi in in vitro culture. B. equi was more resistant to pyrimethamine than B. caballi. B. equi was also found to be more sensitive to artesunate and pamaquine than B. caballi. Of the three compounds, pyrimethamine gave the most promise for in vivo effectiveness.