Identification and Characterization of Potential New Anticancer Drugs from Natural Sources

In 2014, the World Health Organization (WHO) reported that cancer was the leading cause of death worldwide, accounting for 8.2 million deaths in 2012. According to the National Cancer Institute (NCI) and the Centers for Disease Control and Prevention (CDC), cancer ranks second among all causes of death in the United States. The American Cancer Society estimates that there will be 1,685,210 new cases of cancer in 2016, and 595,690 cancer related deaths.14 Cancer chemotherapy is an established adjuvant to surgery and radiation to treat successfully solid tumors, lymphomas, and leukemias. Although many anticancer drugs are approved and clinically available, the problems of resistance to and toxicity of existing agents necessitates the discovery of new drug leads. For decades, secondary metabolites from plants, fungi and bacteria have been found to exhibit potent anticancer activity. Natural products are selected by evolution to reach their molecular target within cells and to interact with biologically active protein folds. Secondary metabolites may serve their producing organisms by improving their competitive strength. Natural products have been essential components of drug discovery and are direct sources of small molecule therapies and scaffolds for semisynthesis. Driven by the need for new anticancer targets, this thesis focuses on exploring new molecules for the novel regulation in cell death to find compounds with good specificity and potency against cancer cells without altering normal cells. Silvestrol is a cyclopenta[b]benzofuran that was isolated from the fruits and twigs of Aglaia foveolata, which is indigenous to the island states of Southeast Asia. Previous testing of silvestrol revealed that it is a potent inhibitor of protein synthesis and hascytotoxic activity similar to or more potent than many Food and Drug Administration (FDA) approved anticancer agents. Silvestrol treatment of cultured human cell lines derived from melanoma (MDA-MB-435) or colon cancer (HT-29) induced cell cycle arrest at the G2 stage. Silvestrol treatment also induced caspase-3 activation and apoptotic cell death in a time- and concentration-dependent manner. Treatment of ATG7-null mouse embryonic fibroblast (MEF) cells with silvestrol resulted in impaired transformation of LC3-I to LC3-II. This suggests that silvestrol caused a conventional autophagy response that ultimately progressed to apoptosis. Strebloside was isolated from the stem bark of Streblus asper in Southeast Asia, and its chemical structure shares a common cardiac glycoside structural motif. Strebloside was studied in a hollow fiber assay to determine if it had any potency in vivo. Strebloside was able to inhibit the growth of ovarian cancer cell model OVCAR3 and also triple negative breast cancer cell model MDA-MB-231. From these data, ovarian cancer models were chosen due to its high sensitivity to strebloside. Strebloside was active in OVSAHO, OVCAR4 and OVCAR5, but was most potent in OVCAR3. Furthermore, strebloside blocked cell cycle progression at the G2-phase in OVCAR3 cells after 72 h treatment at 100 nM. Strebloside treatment also induced PARP cleavage indicating apoptosis activation in OVCAR3 after 72 h at 100 nM. Strebloside potently inhibited cell growth and induced cell death in human ovarian cancer cells through induction of caspase-mediated apoptosis. Strebloside also behaved in a manner consistent with being a cardiac glycoside, which might limit its clinical use. Natural products have been essential components of anticancer drug discovery; they serve both as a direct source of small molecule therapies and as an inspiration for the semisynthesis of novel, biologically active derivatives. Bioassay-guided fractionation and isolation provide the screening platform to identify unique chemical structures. Extracts and fractions from collected plants, fungi, cyanobacteria and actinomycetes were screened in a panel of cancer cell lines derived from melanoma, colon cancer, breast cancer and ovarian cancer. Several potential drug leads have been isolated and their structures elucidated. Among these potential drug leads, phyllanthusmin and verticillin A showed potent anticancer activity. However, their mechanisms of action require further investigation. The mechanism by which silvestrol and strebloside block cell proliferation and induce apoptosis deserves further study. The targets of both silvestol and strebloside are different from conventional chemotherapeutic agents. The possibility of use new targets for anticancer therapies can decrease drug resistance, providing better healthcare to human beings. Based on the cytotoxic potential of these drug leads, they may provide a novel and promising strategy to improve cancer therapy. Structurally complex natural products are being isolated from diverse biological organisms living in aquatic and terrestrial ecosystem all over the world. Overall, anticancer drug discovery from natural sources remains an important approach for novel drug development.