Membrane therapy for chagas' disease

Currently, benznidazole and nifurtimox are the only drugs available for the specific treatment of Chagas’ disease. Both are limited by low efficacy in the chronic stage of the disease and considerable toxicity, which is why there is an urgent need for drugs that provide safe and efficient treatment for Chagas’ disease. Trypanosoma cruzi, the causative agent of Chagas’ disease, requires specific endogenous sterols and is therefore very sensitive to sterol biosynthesis inhibitors (SBIs). SBIs are widely used as antifungals and lend themselves to drug repurposing. Sterols are an essential class of lipids in eukaryotes, where they serve as structural components of membranes and play important roles as signaling molecules. The most abundant sterol in vertebrates is cholesterol, whereas fungi synthesize ergosterol, which has a greater degree of unsaturation and an additional methyl group at C24. Like fungi, trypanosomes require the presence of ergosterol and other 24-alkylated sterols; their similar sterol content is the rationale for testing inhibitors of ergosterol synthesis against trypanosomes.
In the framework of this PhD thesis various aspects of sterol anabolism in eukaryotes and its potential exploitation as drug target in parasites were analysed. First, using genome profiling, I did a comparative genomics study of sterol biosynthesis (SB) focusing on eukaryotic parasites. In vitro testing of known SBIs and quantifying the expression levels of SB genes during the different life stages of T. cruzi and Trypanosoma brucei completed this part of the thesis. Then, I used genetically modified yeast strains as a tool to assess selectivity of SBIs to ergosterol-containing cells. Finally, integrating the results from my work led to a specific proposition how to advance drug development in Chagas’ disease.
For the genome profiling an in silico pipeline was developed to globally evaluate sterol metabolism and perform comparative genomics. Hidden Markov model-based profiles for 42 SB enzymes allowed to represent the genomic makeup of a given species as a numerical vector. Hierarchical clustering of these vectors functionally grouped eukaryote proteomes and revealed convergent evolution, in particular metabolic reduction in obligate endoparasites. The only obligate endoparasites found to possess SB genes were the trypanosomatids, Trypanosoma spp. and Leishmania spp. However, the origin of SB genes in trypanosomatids remains obscure, as there was no evidence for horizontal transfer.
SBIs are generally thought to act by inhibition of ergosterol anabolism. To investigate this more closely, I developed an assay using genetically modified yeast strains that either synthesize ergosterol or cholesterol. Different efficiencies of a given molecule in inhibiting ergosterol- or cholesterol-producing yeast can thus be attributed to sterol content. Nystatin concentrations required to inhibit growth in the cholesterol-producing yeast strain were 10-fold higher than in the ergosterol-producing strain, demonstrating the validity of the approach. Like amphotericin B, nystatin binds to ergosterol and forms pores in the membrane that lead to death of the target cell. This clear-cut result was only observed for molecules that bind to the finished end product of SB. Inhibitors of enzymes involved in SB did not exclusively inhibit growth of ergosterol-producing yeast strain, showing that the selectivity of SBIs for fungi is not based on differences between cholesterol and ergosterol anabolism. Two possible explanations why SBIs are selective inhibitors of fungal and trypanosomatid growth are brought forward: i) mammalian cells can salvage cholesterol from the environment and thus circumvent inhibition of sterol de novo synthesis whereas trypanosomatids and fungi require the presence of ergosterol and other 24-alkylated sterols, which cannot be replaced by the host’s sterols or ii) fungal and protozoan orthologs of SB enzymes are more susceptible to SBIs than the respective mammalian orthologs.
Even though azoles have been used as antifungals for decades, their use against trypanosomatids is still not implemented. Even worse, the most advanced candidate – posaconazole – could not confirm its initial potential in a recent phase II clinical trial for chronic Chagas’ disease. Based on my findings and integrating the work of others, posaconazole should not be abandoned but partnered with another drug for combination therapy. In the concluding chapter I elaborate on why a sphingolipid biosynthesis inhibitor is probably the best match.