Date of Award
Ph.D. in Pharmaceutical Sciences
MARK T HAMANN
James D. McChesney
Nature has proven to be an endless source of abundant diversity of chemical entities with varying biological activities. The World Health Organization (WHO) estimates that 80% of the world's population depends on traditional medicine for treating their everyday health problems. According to Gordon Cragg, former director of the natural products branch of the NCI, ″Nature has produced wonderfully complex molecules that no synthetic chemist could ever dream up.″ Moreover, he emphasized that over 60% of cancer therapeutics have been derived from nature whether directly through isolation or indirectly through semi-synthesis. Various examples have been reported such as extracts from Catharanthus roseus (Rosy Periwinkle) used for treatment of Hodgkin's disease and leukemia, digitalis from Digitalis purpurea (foxglove) for treatment of heart disease, galantamine from Galanthus caucasicus (snowdrops) for treatment of Alzheimer's disease, and paclitaxel from the bark of the Pacific yew tree to treat cancer.
During the course of these studies, we researched the potential of natural products to provide drugs for treatment of human and plant diseases for which no adequate therapy exists. For instance, methicillin-resistant Staphylococcus aureus (MRSA) is a destructive pathogen with a high patient mortality rate. Over 50% of the Staph. aureus infections around the world are caused by MRSA. In the first chapter, we will address the bioassay-guided isolation and characterization of a series of anti-MRSA active glycosides that exhibit a high degree of selectivity for MRSA from the leaves of the common American sycamore, Platanus occidentalis. The isolated metabolites; E,E-platanoside [kaempferol 3-O-α-L-(2'',3''-di-E-pcoumaroyl)rhamnoside, IC50 2.0 µg/mL], E,Z-platanoside [kaempferol 3-O-α-L-(2''-E-pcoumaroyl-3''-Z-p-coumaroyl)rhamnoside, IC50 0.8 µg/mL], Z,E-platanoside [kaempferol 3-O-αL-(2''-Z-p-coumaroyl-3''-E-p-coumaroyl)rhamnoside, IC50 0.7 µg/mL], and Z,Z-platanoside [kaempferol 3-O-α-L-(2'',3''-di-Z-p-coumaroyl)rhamnoside, IC50 0.4 µg/mL] have been shown to prevent growth of MRSA on surfaces and systemically.
The in vitro anti-MRSA activity of these metabolites indicated that changing the configuration of the double bond in the p-comaroyl units greatly affects the MRSA activity. This, in turn, encouraged us to establish economic methods to transform the less active E,Eplatanoside to the more active Z, Z-platanoside. Light-mediated isomerization in platanosides provides a precise, noninvasive, and “green” approach for controlling MRSA activity. In the second chapter, the development of a simple “green” chemistry approach is described that uses light induced photoisomerization for transforming the thermodynamically stable E,E-platanoside to the Z,E-, E,Z- and Z,Z-platanosides. This provided support for the light mediated photoisomerization over the enzymatic control theory for the biosynthesis formation of the Zisomers.
American sycamore is significant to the forest products industry due to its use as pulpwood for making paper and fiberboard, and sawlogs for manufacturing butcher's blocks, furniture, interior trim, boxes, and flooring. Historically, American sycamore has held, and continues to hold, good potential as a dedicated biofuels crop grown on short rotations in plantations. However, the growth and productivity of sycamore plantations is hampered by bacterial leaf scorch disease (BLS) caused by Xylella fastidiosa. Strains of this bacterium are the major cause of various plant diseases such as phony peach disease (PPD), plum leaf scald (PLS), and Pierce's disease (PD) of grapes, citrus variegated chlorosis (CVC), and leaf scorch of almond, oleander, and pear. The isolated anti-MRSA metabolites are found in greater concentrations in healthy sycamore trees versus diseased trees suggesting an ecological link between tree health and glycoside content. The third chapter describes this ecological link and presents data that demonstrates the ability of the metabolites to inhibit X. fastidiosa at nanomolar concentrations (MIC ≤ 17-67 nM). The presence of these metabolites in sycamore has the potential to be a useful analytical tool for selecting disease-resistant strains.
Depression is the most common disease of the central nervous system affecting approximately 17% of Americans each year. According to the National Institute of Mental Health (NIMH), every year about 40 million American adults suffer from anxiety disorders that frequently co-occur with other psychiatric illnesses like depression. Recently, our group reported the isolation of several marine indole alkaloids and evaluated their biological activity in the forced swim (FST) and locomotor activity tests, revealing their potential to become new antidepressant drug leads. Among the isolated compounds, 5-bromo-N,N-dimethyltryptamine was found to display a high sedative effect in the locomotor activity test. Owing to the limited amounts of this compound obtained from isolation, a synthetic approach was adopted to prepare 5-bromo-N,N-dimethyltryptamine and its derivatives to study the structure activity relationships (SAR) and complete the dose-response experiments. In the fourth chapter, the synthesis is described of a series of 2-(5-halo-1H-indol-3-yl)-N,N-dimethylethanamines, as is the evaluation of their in vivo antidepressant and sedative activities using the FST and locomotor activity tests. Of the synthetically prepared compounds, 2-(1H-indol-3-yl)-N,N-dimethyl-2-oxoacetamide, 2- (5-chloro-1H-indol-3-yl)-N,N-dimethyl-2-oxoacetamide, 2-(1H-indol-3-yl)-N,Ndimethylethanamine, 2-(5-chloro-1H-indol-3-yl)-N,N-dimethylethanamine, and 2-(5-iodo-1Hindol-3-yl)-N,N-dimethylethanamine have been shown to possess antidepressant-like action. Dose response studies were completed for the active compounds.
Manzamine A is an alkaloid that contains a unique multi-heterocyclic ring system coupled to a β-carboline moiety. Manzamine A showed potent in vitro antimalarial activity against P. falciparum D6 and W2 (0.017-0.020 µM), but its toxicity hindered its development as a drug lead. Malaria, a major tropical infectious disease caused primarily by the protozoan parasite Plasmodium falciparum, is responsible for the death of more than 1.12 million individuals every year. Recently, Ihara reported that the π-delocalized lipophilic cationic (DLC) structures of various β-carboline natural products, display reasonable antimalarial activity against P. falciparum with low cytotoxicity against mammalian cells. The fifth chapter describes the synthesis of mono- and di-methylated quaternary carbolinium cations of manzamine A and evaluations of their in vitro antimalarial and antimicrobial activities, cytotoxicity, and also their potential for glycogen synthase kinase (GSK-3β) inhibition using molecular docking studies. Among the analogs, 2-N-methylmanzamine A exhibited antimalarial activity (IC50 0.7 – 1.0 µM), but was less potent than manzamine A. However, the compound was significantly less cytotoxic to mammalian kidney fibroblasts with a selectivity index in the same range as manzamine A.
One of the most serious threats to humans is hepatitis C virus (HCV) infections affecting approximately 4 million individuals in the United States, and 200-300 million individuals worldwide, causing over 8,000 deaths each year. HCV can progress to severe liver disorders such as fibrosis, cirrhosis, and hepatocellular carcinoma. A rare Latin American plant extract provided through collaboration with the National Cancer Institute (NCI) repository (No. 77951) showed potent anti-HCV activity.
Diplostephium rhododendroides (family Asteraceae) is only found between Ecuador and Colombia and has not been subjected to phytochemical screening. The last chapter describes the isolation and identification from this rare plant of anti-HCV oleanane-triterpenes complex glycosides with m/z of 1700-1800. The isolated rhododendrosaponins I-III have shown exceptional activity (EC50 0.2 µg/mL) against HCV and warrant further evaluations.
Ibrahim, Mohamed Ali Mohamed Ali, "Natural Products Chemistry of Leads Acting on MRSA, HCV, Malaria, and Depression Disorders" (2011). Electronic Theses and Dissertations. 1461.