Natural Products as Novel Therapies for Tuberculosis

Abstract

Tuberculosis (TB) continues to devastate the global population as the leading cause of death from an infectious disease. TB infects over 10 million people annually and is estimated to cost the world’s economy over US$984 billion in healthcare and productivity over the next 15 years. The current TB vaccine is only partially effective in preventing infection and any protection afforded wanes over time as the recipients reach adolescence. Furthermore, current treatment programs for TB are lengthy, complex and problematic with regards to patient non-adherence and drug misuse. As TB is endemic in areas of low socioeconomic status, these issues have led to a global pandemic of multiple drug-resistant TB. There is an urgent need for a more effective vaccine and new drugs to combat the burden of TB disease. Throughout history, natural products have been the richest source of bioactive compounds as novel vaccines and therapies. In this study, samples from a library of marine organisms and extracts from sponge-associated microflora were screened for antimycobacterial activity against Mycobacterium tuberculosis, the causative agent of TB. Samples displaying inhibitory activity were then further assessed for potency, cytotoxicity, selectivity and efficacy against drug-resistant strains of M. tuberculosis. One species of marine sponge of the Tedania genus was found to yield samples with exceptionally potent antimycobacterial activity against drug-susceptible and drug-resistant strains of M. tuberculosis. These samples were found to be non-toxic against four human cell lines and yielded twelve antimycobacterial fractions upon purification via high performance liquid chromatography. Mass spectroscopy and nuclear magnetic resonance analysis of one of these fractions identified bengamide B as a potential compound for novel TB drug lead development, being highly potent, able to work in synergy with existing TB drugs, and able to inhibit intracellular and drug-resistant M. tuberculosis. Additional screening of extracts from sponge-associated microflora resulted in the identification of two chemical classes, the mycalazals and oroidins, as potent antimycobacterial compounds. They were found to be non-cytotoxic against three human cell lines and retained antimycobacterial activity at micromolar concentrations. Overall this study has identified three chemical scaffolds from the marine biosphere, the bengamides, mycalazals and oroidins, with potential for further development as novel antimycobacterial agents. Similarly, natural products such as tree bark saponin extracts and microbial components have long been used as adjuvants to boost the immunogenicity of protein subunit vaccines. The plant-derived polysaccharide, Advax™, is an immunomodulating δ-inulin formulation which was found to enhance the protective effect of the novel CysVac2 multistage fusion protein vaccine when administered in conjunction with CpG7909 oligonucleotide and murabutide. This vaccine, termed Advax4/CysVac2, was able to induce potent antigen-specific IFNγ+/TNF+/IL-2+ triple positive polyfunctional CD4+ T cell responses and significantly reduce bacterial burden in the lungs of M. tuberculosis-infected mice at both short- and long-term timepoints post-infection. The protective efficacy of Advax4/CysVac2 also correlated with the rapid influx of neutrophils, macrophages and monocytes to the site of vaccination and the expansion of CD44+ CD8+ T cells in the lungs. When dimethyldioctadecylammonium bromide (DDA) was added to Advax4/CysVac2, it significantly increased the frequency of antigen-specific IFNγ+/TNF+/IL-2+ triple positive polyfunctional CD4+ T cell responses, and resulted in lung and splenic protection at a level on par with that conferred by BCG. Both Advax4/CysVac2 and Advax4-DDA/CysVac2 are strong candidates for further preclinical evaluation as potential lead TB vaccines.