Deradicalising Giardiasis Treatment

Sammanfattning: Giardia intestinalis is a unicellular parasite causing the disease giardiasis. This disease is mostly prevalent in low resource settings, mandating low-cost treatment options. Treatment is mainly based on antiparasitics from the classes of 5-nitroimidazoles and benzimidazoles, which both target parasite DNA and other cellular macromolecules based on radicals generated from the drugs. In recent years resistance to those classes and cross-resistance in between them has become a problem, hence alternative antigiardials are needed.In this thesis we enhanced our understanding of the crucial differentiation process of encystation, which produce the environmentally stable and infective form of the parasite, the cyst (Paper I). Resistance development has been slow in G. intestinalis until now and it has been shown that resistance to the main treatment option, metronidazole, can be lost after en- and excystation so an enhanced understanding of this process can help us to identify the cause of this loss of drug tolerance. In the second study (Paper II) we followed this up by analysing two metronidazole resistant lines and one revertant on their ability to produce infective cysts, while cross checking with growth rates, metronidazole resistance level, transcriptomics and proteomics. We found the resistant cells lines to have deeply disturbed cellular pathways with the main resistance mechanisms being a reduction of uptake, a reduction of activation rate and an upregulation of oxidative stress responses. Cyst production and growth rates were highly reduced giving those lines a clear disadvantage when no drug pressure is applied. Most changes, phenotypically and expression wise, were reset in the revertant. As next step we evaluated alternative antigiardials and their targets. In Paper III we characterised the giardial thymidine kinase, on which this parasite depends completely to supply thymidine for DNA synthesis. We identified a nucleoside analogue, azidothymidine, that is targeting this enzyme and efficiently inhibits growth and encystation of trophozoites both in vivo and in vitro. Azidothymidine is currently used in HIV treatment, has a good safety profile and is comparatively cheap, which makes it a good candidate for treatment of giardiasis. In conclusion, this study has focussed on several aspects of nitroimidazole resistance in G. intestinalis throughout the life cycle as well as repurposing of antibiotics from other drug classes that could be used to fill our arsenal of antigiardials with new alternatives.