Anti-malarial Drug Discovery from Australian Flora

Abstract

Malaria is a mosquito-borne disease caused by the parasitic protozoan Plasmodium that is responsible for approximately half a million deaths every year. The vast majority of these deaths are caused by P. falciparum in Sub-Saharan Africa (SSA). Although most cases of P. falciparum malaria can currently be treated effectively using artemisinin-based combination therapies (ACTs), resistance to ACTs is beginning to emerge in South-East Asia. This resistance is likely to proliferate and spread into SSA, after which a public health catastrophe is likely to follow. There is currently no drug poised to replace ACTs as the front-line treatment for malaria and there is a need for the discovery of new drugs. Historically, natural products from plants have been our best source of anti-malarial drugs: the alkaloid quinine (from the bark of the Cinchona tree) and the sesquiterpene lactone artemisinin (from the leaves of Artemisia annua) have formed the backbone of anti-malarial chemotherapeutics for centuries. The primary goal of this thesis was to respond to the need for new anti-plasmodial compounds. This was achieved by collecting and screening a library of Australian Rutaceae species against P. falciparum, selecting species that showed high bioactivity and performing large-scale natural product purification. Isolated natural products were screened against chloroquine-resistant and sensitive P. falciparum and human embryonic kidney (HEK-293) cells to evaluate bioactivity and parasite selectivity. This forms the majority of the thesis (Chapters 2-6). Chapter 2 reports the initial collection, screening and fingerprinting of a library of 30 Australian Rutaceae species. Chemical fingerprinting using LC-MS was used to identify species that were most likely to contain new natural products. From these results, four species were selected for investigation: Clausena brevistyla (Chapter 2) Flindersia pimenteliana (Chapters 3-4), Acronychia pubescens (Chapter 5) and Pitaviaster haplophyllus (Chapter 6). This chapter also reports the isolation of two known pyranocoumarins from C. brevistyla. One of the pyranocoumarins showed potent and selective activity against P. falciparum, with IC50 values between 466 – 822 nM. Chapter 3 reports the chemical investigation of F. pimenteliana leaf material. From this plant, a new class of ascorbic-acid adduct indole alkaloids, pimentelamines A-C, were isolated along with one new indole alkaloid, 2-isoprenyl-N,N-dimethyltryptamine. Five known compounds were also isolated. Although the new natural products did not show strong bioactivity, three of the isolated bis-indole alkaloids, borreverine, 4-methylborreverine and dimethylisoborreverine, showed potent activity with IC50 values between 190 – 670 nM against P. falciparum Chapter 4 reports the isolation of three new isoborreverine-type alkaloids, 10,10’- dimethoxydimethylisoborreverine, 10-methoxydimethylisoborreverine and 10’- methoxydimethylisoborreverine from the bark of F. pimenteliana. Two known borreverinetype alkaloids were also isolated. The moderate anti-plasmodial activity of these alkaloids is reported, with IC50 values ranging from 959 – 2407 ng/mL. Further insights into structureactivity relationships of borreverine-type alkaloids are also discussed. Chapter 5 reports the chemical investigation of the roots of A. pubescens, from which a highly unusual oxidized furo[2,3-c]xanthene, acrotrione, was isolated along with two known acetophenones. Acrotrione is the first natural product of its class to be isolated. Moderate anti-plasmodial activity for the natural products is reported, with IC50 values ranging from 1.7 to 4.7 µM. Chapter 6 reports the isolation of one new quinoline alkaloid, leptanoine D, from P. haplophyllus. Nine known alkaloids were also isolated. The chemotaxonomic relationships between the monotypic Pitaviaster genus and the related Australian genera Euodia, Melicope and Acronychia are discussed. The secondary goal of this thesis was to investigate the factors that influence diversity of natural products in Australian plants. In recent years, natural product-driven drug discovery has seen a decrease in popularity in the pharmaceutical industry, part of which has been caused by the repeated isolation of known natural products. In response to this, there is a requirement for the development of new ideas that expedite the discovery of new natural products. Some recent publications have noted that natural product diversity is positively correlated with diversity of plant-herbivore communities. This may suggest that plants in regions of high biotic stress (i.e. rainforests) should be the focal point of terrestrial plant natural product drug discovery. We aimed to validate this hypothesis by using the Australian Rutaceae genus Flindersia as a case study. Contrary to expectations, our results showed that Flindersia species growing in arid regions of central Australia produced a significantly higher number of structurally unique alkaloids than rainforest species. These unexpected results highlight the potential of the Australian arid zone as a source of new natural products.