Treating cryptosporidiosis: A review on drug discovery strategies

Abstract

Despite several decades of research on therapeutics, cryptosporidiosis remains a major concern for human and animal health. Even though this field of research to assess antiparasitic drug activity is highly active and competitive, only one molecule is authorized to be used in humans. However, this molecule was not efficacious in immunocompromised people and the lack of animal therapeutics remains a cause of concern. Indeed, the therapeutic arsenal needs to be developed for both humans and animals. Our work aims to clarify research strategies that historically were diffuse and poorly directed. This paper reviews in vitro and in vivo methodologies to assess the activity of future therapeutic compounds by screening drug libraries or through drug repurposing. It focuses on High Throughput Screening methodologies (HTS) and discusses the lack of knowledge of target mechanisms. In addition, an overview of several specific metabolic pathways and enzymatic activities used as targets against Cryptosporidium is provided. These metabolic processes include glycolytic pathways, fatty acid production, kinase activities, tRNA elaboration, nucleotide synthesis, gene expression and mRNA maturation. As a conclusion, we highlight emerging future strategies for screening natural compounds and assessing drug resistance issues.

Keywords: Cryptosporidium; Drug repurposing; Drug screening; Metabolic pathways.

Graphical abstract

Fig. 1

Metabolic pathways targeted by anti-cryptosporidial drugs. Several drugs against Cryptosporidium exhibit inhibiting effects on nuclear- (in grey) as well as cytoplasmic-located biochemical processes. In this figure, a Cryptosporidium parasite is represented in its intracellular but extracytoplasmic host location within the parasitophorous vacuole. Molecule exchanges between host cell and the parasite take place through the electron dense connection. Surrounding the parasite, the purple text refers to the name of compounds inhibiting specific metabolic pathways. In the nucleus, the histone acetylation process (via histone deacetylase) as well as mRNA maturation (via polyadenylation specificity factor 3) are both inhibited by drugs. In the cytoplasm the glycolytic pathway (in orange) could be impeded by the action on three different enzymes (phosphoglucose isomerase, pyruvate kinase and lactate dehydrogenase). Regarding the fatty acids pathway (in yellow), inhibitors were identified as acting on acyl-CoA synthetase). Calcium regulated processes (in purple) were also described as target sites by the inhibition of calcium dependent kinases. In the phosphatidyl inositol phosphorylating process, the inhibition of a kinase (CpPi(4)K) exhibited a strong activity by reducing parasite growth (in Pink). In the mechanism linking amino acids (phenylalanyl, lysyl or methionyl)) to their specific tRNA, actions of tRNA synthetases are also altered by therapeutic drugs (in blue). Finally, the guanosine synthesis pathway (in green) is inhibited by action on the inosine-5 monophosphate dehydrogenase. In red are the enzymes seen to be targeted by anticryptosporidial drugs. In order to make the figure readable, the glycolytic pathway has been reduced to the set of enzymes affected by anti-parasitic compounds. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)