Apicomplexan parasites encompass several human-pathogenic as well as animal-pathogenic protozoans like Plasmodium falciparum, Toxoplasma gondii, and Eimeria tenella. E. tenella is the causative agent of coccidiosis a disease of chickens, which causes tremendous economic losses to the world poultry industry. Considerable increase of drug resistance makes it necessary to develop and pursue new therapeutic strategies. Cyclin-dependent kinases (CDKs) are key molecules in the regulation of the cell cycle and are therefore prominent target proteins in parasitic diseases. Bioinformatic analysis revealed four potential CDK-like proteins of which one -E. tenella CDKrelated kinase 2 (EtCRK2) -is already cloned, expressed and characterized. [1] Using the CDK specific inhibitor Flavopiridol in EtCRK2 enzyme assays and schizont maturation assays we could chemically validate CDK-like proteins as potential drug targets. An X-ray crystal structure of human CDK2 (HsCDK2) served as template to built protein models of EtCRK2 by comparative homology modeling. Structural differences in the ATP-binding site between EtCRK2 and HsCDK2 as well as chicken CDK3 have been addressed for the optimization of selective ATPcompetitive 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 inhibiting EtCRK2.

The poultry disease coccidiosis, caused by infection with Eimeria spp. apicomplexan parasites, is responsible for enormous economic losses to the global poultry industry. The rapid increase of resistance to therapeutic agents, as well as the expense of vaccination with live attenuated vaccines, requires the development of new effective treatments for coccidiosis. Because of their key regulatory function in the eukaryotic cell cycle, cyclin-dependent kinases (CDKs) are prominent drug targets. The Eimeria tenella CDC2-related kinase 2 (EtCRK2) is a validated drug target that can be activated in vitro by the CDK activator XlRINGO (Xenopus laevis rapid inducer of G2/M progression in oocytes). Bioinformatics analyses revealed four putative E. tenella cyclins (EtCYCs) that are closely related to cyclins found in the human apicomplexan parasite Plasmodium falciparum. EtCYC3a was cloned, expressed in Escherichia coli and purified in a complex with EtCRK2. Using the non-radioactive time-resolved fluorescence energy transfer (TR-FRET) assay, we demonstrated the ability of EtCYC3a to activate EtCRK2 as shown previously for XlRINGO. The EtCRK2/EtCYC3a complex was used for a combined in vitro and in silico high-throughput screening approach, which resulted in three lead structures, a naphthoquinone, an 8-hydroxyquinoline and a 2-pyrimidinyl-aminopiperidine-propane-2-ol. This constitutes a promising starting point for the subsequent lead optimization phase and the development of novel anticoccidial drugs.

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The inside cover picture shows the validated drug target EtCRK2, a cyclin-dependent kinase of Eimeria tenella homologous to human CDK2 and chicken CDK3. A potential EtCRK2 inhibitor is docked into the active site of the protein.