The design and synthesis of drug-like trypanosome alternative oxidase inhibitors for the treatment of African trypanosomiasis

Trypanosome alternative oxidase (TAO) is the sole terminal oxidase responsible for the aerobic respiration of the parasite T. b. brucei. Specific strains of this parasite cause the neglected tropical disease Human African trypanosomiasis (HAT), and thus TAO is an interesting target for the potential treatment of this disease. Inhibition of TAO with the natural product inhibitors colletochlorin B or ascofuranone has been shown to clear infections of T. b. brucei in mice at high concentrations. However, these natural product inhibitors contain undesirable chemical functionality and have poor physicochemical properties, preventing adequate drug exposure to effectively treat HAT. Robust protocols for the expression and purification of recombinant TAO were developed, which enabled the development of biochemical assays to identify inhibitors of TAO function. Single point inhibition screening of the Medicines Malaria Venture ‘kinetoplastid collection’ of 400 compounds identified a range of micro-molar inhibitors of TAO. A program of chemical optimisation was carried out around the natural product inhibitor colletochlorin B, with the aim to improve the physicochemical properties and retain inhibitory potency against TAO. The structure activity relationships generated over the course of this exploration identified a dependency on high lipophilicity to retain potent TAO inhibition. The TAO inhibitors synthesised were also assessed for parasite growth inhibition and mammalian cell cytotoxicity to correlate inhibition data with cellular efficacy, in collaboration with Novartis. The physicochemical properties of these novel compounds showed improvement over the natural product colletochlorin B and prompted further assessment of leading compounds in advanced parasite kill kinetic and parasite clearance assays at Novartis. The data generated in these assays for compounds synthesised in this thesis determined that TAO inhibition results in a trypanostatic response, and not a preferred trypanocidal response in T. b. brucei.