In situ enzyme-linked immunosorbent assay to quantitate in vitro development of Eimeria tenella

Inhibition of Eimeria tenella CDK-related kinase 2: From target identification to lead compounds

Apicomplexan parasites encompass several human- and animal-pathogenic protozoans such as Plasmodium falciparum, Toxoplasma gondii, and Eimeria tenella. E. tenella causes coccidiosis, a disease that afflicts chickens, leading to tremendous economic losses to the global poultry industry. The considerable increase in drug resistance makes it necessary to develop new therapeutic strategies against this parasite. Cyclin-dependent kinases (CDKs) are key molecules in cell-cycle regulation and are therefore prominent target proteins in parasitic diseases. Bioinformatics analysis revealed four potential CDK-like proteins, of which one-E. tenella CDK-related kinase 2 (EtCRK2)-has already been characterized by gene cloning and expression.1 By using the CDK-specific inhibitor flavopiridol in EtCRK2 enzyme assays and schizont maturation assays (SMA), we could chemically validate CDK-like proteins as potential drug targets. An X-ray crystal structure of human CDK2 (HsCDK2) served as a template to build protein models of EtCRK2 by comparative homology modeling. Structural differences in the ATP binding site between EtCRK2 and HsCDK2, as well as chicken CDK3, were addressed for the optimization of selective ATP-competitive inhibitors. Virtual screening and "wet-bench" high-throughput screening campaigns on large compound libraries resulted in an initial set of hit compounds. These compounds were further analyzed and characterized, leading to a set of four promising lead compounds for development as EtCRK2 inhibitors.

The proliferative effects of retinoic acid on primary cultures of adult rat type II pneumocytes depend upon cell density

Retinoic acid (RA) is important for maintaining integrity of alveolar epithelial cells, but the mechanism has not been defined. We cultured type II pneumocytes at confluent, high cell density (10(4) cells/mm(2)) and found that RA (10(-6) M) inhibited thymidine incorporation to 60% of control, despite a dose-dependent increase in epidermal growth factor receptor (EGFR) levels. However, at lower, subconfluent density (10(2) cells/mm(2)), RA stimulated thymidine incorporation to 280% of control. EGF increased thymidine incorporation at concentrations as low as 0.1 ng/mL, but no further increase was observed at higher concentrations up to 100 ng/mL. In subconfluent cells co-treated with EGF (100 ng/mL) and increasing concentrations of RA (10(-8) M-10(-5) M RA), thymidine incorporation was significantly greater at all concentrations than RA alone, with greatest increases observed at 10(-7) (422% of control) and 10(-6) (470% of control) M RA. In summary, the effects of RA on thymidine incorporation are sensitive to changes in cell density. RA inhibits thymidine incorporation at high cell density and stimulates thymidine incorporation at low density. RA increases EGFRs in cultured type II pneumocytes, and EGF stimulates thymidine incorporation independent of the cultured cell density. These data may help to explain how RA mediates lung repair in vivo.

8-Aminoquinolines as anticoccidials--II

During a chemistry program aimed at finding a novel analogue of pentaquine with improved in vivo activity, a number of hypotheses concerning the way this drug acts in the chicken were investigated. Consideration of the products of monoamine oxidase metabolism of pentaquine suggested that pentaquine aldehyde is the likely active metabolite. Although isolation of this unstable compound was not possible, oxime and cyclic acetal and ketal derivatives were obtained and shown to possess in vitro anticoccidial activity.

Efficacy of 101 antimicrobials and other agents on the development of Cryptosporidium parvum in vitro

An in-situ ELISA was used as a primary screen to test the effects of 101 antimicrobials and other agents on the development of Cryptosporidium parvum in vitro. Over 40 of the compounds displayed some form of anticryptosporidial activity, and dose-response curves were generated for 40 of these. The in-situ ELISA makes a highly effective primary, pharmaceutical screen for C parvum, to be used prior to more detailed microscopical, toxicological or in-vivo assays.

Development of a microtitre ELISA to quantify development of Cryptosporidium parvum in vitro

An in situ enzyme-linked immunosorbent assay (ELISA) was developed to evaluate growth of Cryptosporidium parvum in vitro. Ninety-six-well tissue culture microtitre plates were each seeded with 4.0 x 10(4) human ileocecal adenocarcinoma (HCT-8) cells, then infected with CsCl-purified oocysts 24 h later. The growth medium consisted of RPMI 1640 supplemented with 10% fetal bovine serum, 15 mM HEPES (N-2-hydroxyethylpiperazine N'-2-ethanesulfonic acid), 50 mM glucose, 1 microgram ml-1 folic acid, 4 micrograms ml-1 4-aminobenzoic acid, 2 micrograms ml-1 pantothenic acid and 35 micrograms ml-1 ascorbic acid. Incubation conditions were at 37 degrees C in a 5% CO2/95% humidified air incubator. Oocysts were allowed to excyst in situ so that sporozoites could infect cells directly. Monolayers were then washed, new medium added, and infected cells re-incubated. Levels of infection were assessed 48 h later using a rat anti-C. parvum polyvalent antiserum directed against purified parasite membranes, followed by a goat anti-rat IgG conjugated to horseradish peroxidase and 3,3',5,5'-tetramethyl-benzidine as substrate. Using various parasite inoculating doses and incubation times, optimal results were obtained using a 90-min exposure of host cells to 2.5-3.0 x 10(4) oocysts/well. Evaluation of various concentrations of four anti-microbials (monensin, lasalocid, paromomycin and sulfadimethoxine) in the system resulted in the acquisition of precise dose-response curves for each compound.

Selective labeling of intracellular parasite proteins by using ricin

Studies focused on the synthesis by intracellular parasites of developmentally regulated proteins have been limited due to the lack of a simple method for selectively labeling proteins produced by the parasite. A method has now been developed in which ricin is employed to selectively inhibit host-cell protein synthesis. Ricin is a heterodimer composed of two subunits, a lectin and a glycosidase, and it binds to terminal galactose residues on the cell surface via the lectin. Following endocytosis of the intact molecule, a disulfide bond linking the two subunits is cleaved, and only the glycosidase subunit enters the cytoplasm, where it inhibits cytoplasmic protein synthesis by catalyzing the cleavage of the 28S rRNA. Due to the loss of the receptor-binding lectin subunit, ricin cannot permeate host-cell mitochondria or intracellular parasites, and, therefore, protein synthesis within these compartments continues uninterrupted. This system has been used to selectively label parasite proteins from Eimeria tenella and Toxoplasma gondii by using the avian cell line DU-24. In these cells, mitochondrial protein synthesis was inhibited by using chloramphenicol. The use of the avian rho0 cell line DUS-3 provided an additional advantage, because these cells lack mitochondrial DNA. Therefore, those proteins radiolabeled with [35S]methionine/cysteine in ricin-treated, parasite-infected rho0 cells are exclusively those of the intracellular parasite. This technique should be applicable for studying protein synthesis by other intracellular parasites.

CP-72,588, a semisynthetic analog of the polyether ionophore UK-58,582 with increased anticoccidial potency

We have employed semisynthesis to enhance the anticoccidial potency of a polyether ionophore. CP-72,588 is the alpha-methyl analog of the fermentation-derived polyether ionophore UK-58,852. The parent ionophore required a dose of 15 ppm to achieve anticoccidial efficacy in chickens equivalent to that of salinomycin at 60 ppm. CP-72,588 demonstrated substantially improved potency, with efficacy at 5 to 7.5 ppm. The intrinsic antimicrobial potencies of the two ionophores are similar; however, CP-72,588 was found in chicken tissues at higher levels than those of the parent ionophore when each was administered at the same dose (8 ppm). The enhanced potency of CP-72,588 may be partially due to enhanced uptake into tissues.

In vivo expression of in vitro anticoccidial activity

Large-scale screening has led to the identification of several experimental compounds that have very potent intrinsic activity against coccidia, but the lack of translation to in vivo efficacy has been a major hurdle in developing such leads into effective new drugs. We developed methods to explore the impact of oral availability and appropriate distribution in tissue, both of which are potentially important factors in the expression of activity in vivo. For the compounds that we examined, neither oral absorption nor distribution to the site of infection appeared to be the critical barrier to in vivo expression of intrinsic anticoccidial activity. Elucidation of the nature of additional factors that might be involved could assist greatly in the identification of useful new anticoccidial agents.

Eimeria tenella: growth and drug sensitivity in tissue culture under reduced oxygen

For Plasmodium falciparum in culture, growth is enhanced as oxygen tension is lowered and drug susceptibility, particularly susceptibility to 70S ribosomal and mitochondrial inhibitors, changes. Whether similar effects occur in Eimeria tenella was tested as a possible explanation for why certain 70S ribosomal inhibitors, while active in Eimeria-infected birds, are virtually inactive in vitro under ambient oxygen conditions. It was reasoned that perhaps these agents would exhibit good in vitro potency under reduced oxygen conditions. Such proved not to be the case, and it was further found that no positive effect on Eimeria growth occurred under these conditions. Finally, lowering oxygen tension had no substantial effects on sensitivity to anticoccidials or mitochondrial inhibitors.

Further investigation of anticoccidial activity of 7-bromo-N-(2-imidazolidinylidene)-1H-indazol-6-amine

The clonidine analog 7-bromo-N-(2-imidazolidinylidene)-1H-indazol-6-amine exhibits potent activity against Eimeria tenella infections in chickens. Disease control was abrogated by a selective alpha 2 antagonist, which is consistent with the dependence of such activity upon binding to receptors with characteristics of the vertebrate alpha 2 adrenoceptor. Lack of significant activity against the parasite in tissue culture and our inability to detect significant binding of alpha 2 adrenergic ligands to E. tenella imply that the anticoccidial action may be an indirect effect mediated by the host. Efficacy varied, depending upon the Eimeria species, being greatest for the cecal species E. tenella and less for the intestinal species. The effects described differ substantially from previous accounts of adrenergic actions on parasitic protozoa. The evidence suggests that we have observed a new mechanism of action for antiparasitic drugs.

Anticoccidial efficacy of semduramicin in battery studies with laboratory isolates of coccidia

The anticoccidial activity of semduramicin against laboratory isolates of five species of poultry Eimeria was investigated. In laboratory scale battery trials, semduramicin at 20 to 30 ppm demonstrated broad-spectrum anticoccidial efficacy equivalent to salinomycin at 60 ppm. Also, semduramicin at 25 ppm was fed to uninfected cockerels in batteries for 21 days, and growth rate and feed efficiency were found to be equivalent to birds fed salinomycin at 60 ppm. Semduramicin was well tolerated when coadministered with tiamulin. Semduramicin demonstrated the same activity whether produced by semisynthesis or by direct fermentation.