Mode of action of exocarpic acid against mycobacterium tuberculosis

Natural products continue to be a driving force in drug development, particularly for the treatment of infectious diseases. A stunning one third of the world's population (2 billion individuals) is infected by Mycobacterium tuberculosis, the causative agents of pulmonary tuberculosis. While most of these cases are quiescent, about 8 million people suffer from active disease, resulting in a death toll of 2 million people annually worldwide. Exocarpic acid, a polyacetylenic fatty acid isolated from Exocarpos latifolia Santalacea R.Br., native to Papua New Guinea, has shown promising activity against Mycobacterium tuberculosis in vitro. Since several active exocarpic acid analogs were also found in Exocarpos extracts, the decision was made to develop exocarpic acid further as a pharmacophore. The central theme of this dissertation concerns the mechanism of action of exocarpic acid, since it needed to be elucidated before exocarpic acid can be further developed. Mechanistic lead information was developed through microarray data. Inhibition of fatty acid degradation, fatty acid biosynthesis, and amino acid starvation were the primary gene families induced. Physico-chemical consideration also suggested a potential effect on the mycobacterial cell membrane. Experiments designed to address these mechanisms revealed that exocarpic acid exhibits all the signs of a fatty acid biosynthesis system II inhibitor, without any recognizable effects on the mycobacterial membrane. In order to generate lead information for the next step in exocarpic acid drug development, several amide derivatives of exocarpic acid were synthesized. Some of these compounds showed acceptable activity against Mycobacterium tuberculosis without concomitant toxicity against eukaryotic cells. In order to provide initial quantitative structural activity data, a set of exocarpic acid analogs, which lack an unsaturated bond found in exocarpic acid, were also tested. These exhibited a wide array of mechanisms, indicating that the chain-length and unsaturated bond positions are crucial for activity against Mycobacterium tuberculosis. Exocarpic acid's conjugated double-bond acetylenic bond structure may therefore be a valuable pharmacophore for further development.