Identification of Anti-tubercular Compounds in Marine Organisms from Aotearoa
Tuberculosis (TB) is responsible for more than one million human deaths per year globally, more than any other disease caused by a bacterial pathogen. Although effective treatments exist, the long duration of the current treatment regimen as well as associated drug toxicities can lead to patient non-compliance. This has resulted in the spread of drug resistance which is highlighted by the isolation of extensively drug-resistant strains of Mycobacterium tuberculosis complex in all regions of the globe. The management of drug-resistant TB cases requires an extended treatment duration of 18 months minimum post sputum-culture conversion, using drugs that are more frequently linked to adverse side-effects. Therefore, there is a need to identify new drugs that shorten treatment time, and also exhibit better tolerance in patients.
This thesis describes the screening of crude marine extract libraries for the identification of novel anti-tubercular compounds. The libraries consisted of extracts from New Zealand marine organisms that were screened for growth inhibitory activity using a surrogate TB model organism M. smegmatis, and subsequently were validated against M. tuberculosis. Bioassay-guided fractionation led to the identification of two new compounds as well as a mixture of known compounds. In addition to anti-mycobacterial activity, the spectrum of activity of the isolated compounds was also investigated.
Fractionation of the liver extract of the blobfish Psychrolutes marcidus led to the identification of a mixture of four common fatty acids as the major bioactive components. Testing each of the fatty acids individually identified palmitic acid as the most active component against M. tuberculosis H37Ra. This is at odds with current understanding of the fatty acid parameters required for bioactivity in terms of chain length and degree of unsaturation.
Bioassay-guided fractionation of a Latrunculia sp. sponge extract led to the isolation of a novel taurinated terpene rimarikiamide A (98). This compound was initially believed to be responsible for the observed anti-mycobacterial activity, however, further purification led to partitioning of the anti-mycobacterial activity away from 98. The taurinated terpene rimarikiamide A was nevertheless tested for activity in mammalian cell lines and was found to exhibit higher levels of growth inhibition towards HL-60 cells with respect to HEK cells.
Working with Xiphophora chondrophylla, a New Zealand brown alga, led to the identification of the amino alcohol 3-epi-xestoaminol C (156) a stereoisomer of a known compound, as the active principle. This compound displayed the same level of activity in all microorganisms tested, however it proved to be more active against HL-60 cells. To further investigate the mode of action of 156, chemical genetics was employed to indicate the relevant genetic networks that the compound interacts with. This led to the identification of expected pathways such as lipid metabolism as well as cytoskeletal organization, which have been linked to this class of compounds. Furthermore this work also identified new processes such as RNA catabolism, protein targeting and phosphorylation in addition to several unknown processes.