Philosophiae Doctor - PhD / Over the years, microorganisms have become resistant to commonly used antimicrobial agents leading to multidrug resistance. This is believed to occur even with new classes of therapeutic agents thus creating a challenge on the global healthcare system. The study of medicinal plants allows for their possible use as alternative therapeutic agents. Galenia africana (G. africana) is a South African medicinal plant with numerous health benefits. The purpose of this study was to evaluate the potential antimicrobial, antifungal and anticancer properties of the ethanolic extract of G. africana. Prior to evaluating these properties, in vitro and in vivo acute toxicity studies were conducted to assess the toxicity profile of G. africana. The toxicity profile of the G. africana extract was evaluated using acute toxicity studies conducted in animal and reconstituted human epidermis skin models. The results of the acute oral and dermal toxicity studies revealed that the median lethal dosage (LD50) for G. africana extract in Sprague-Dawley rats was considered to exceed 2000 mg/kg. In the dermal sensitization study, the stimulation index (SI) values for the mice treated with the G. africana extract at concentrations of 25% (50 mg/ml), 50% (100 mg/ml), and 100% (200 mg/ml), when compared to the control group, were 1.3, 0.9 and 1.3, respectively which did not result in an SI value of ? 3 in any group. Hence, it did not elicit a hypersensitivity response. In the irritation test, the G. africana (concentrate) and G. africana (in-use dilution) extracts were non-irritant on the reconstituted human epidermis.

Various extracts of Entada abyssinica stem/root bark have been shown to have antimicrobial activity using organisms : Staphylococcus aureus, S. epidermidis, Escherichia coli, Pseudomonas aeruginosa, Bacillus megaterium, Candida albicans, Aspergillus niger and Tricophyton mentagrophytes on solid media. Chromatographic analysis of the bark extracts of different trees (according to age), showed them to contain in common the components responsible for the observed antimicrobial activity with differences observed in minor bands not of antimicrobial importance.

During the past few decades, analogs of purine nucleosides have been described that are modified either in the heterocyclic base, sugar moiety or both, and many of these modified nucleosides display antiviral and/or antineoplastic activity. The Class II mesoionic purinones are isosteric with their non~mesoionic purinone counterparts. It is conceivable that the mesoionic purinone nucleosides might constitute an entirely novel class of modified nucleosides with potential chemotherapeutic activity. That the mesoionic heterobases are bioisosteric as well as isosteric with non-mesoionic purinones was realized by demonstrating that certain mesoionic xanthine derivatives, such as Anhydro-8-ethylw-5-hydroxy-7-oxo-l,3,4-thiadiazolo[3,2-a]pyrimidinium hydroxide and Anhydro-6-p-chlorobenzyl-8-ethyl-5-hydroxy-7-oxo—l,3,4—thiadiazolo [3,2-a]pyrimidinium hydroxide were comparable in potency to test compound, theophylline as inhibitors of adenosine binding at the A1 site. Three different types of mesoionic nucleosides were subsequently designed and synthesized as potential antineoplastic agents. Mesoionic thiadiazolopyrimidine nucleosides, i.e. Anhydro-6-ethyl-8-(2’,3',5'-tri-O-acetyl-D-ribofuranosyl)MS-hydroxy-7-oxo-l,3,4~thiadiazolo{3,2-a]pyrimidinium hydroxide and Anhydro-8-(2',3'.5’-tri-O-acetyl-D-ribofuranosyl) -5-hydroxy-7-oxo-l,3,4-thiadiazolo[3,2-a]pyrimidinium hydroxide were designed to serve as potential pro-drugs of 2-amino-l,3,4~thiadiazole mononucleotide which has been reported to be a potent inhibitor of inosine monophosphate dehydrogenase. The O~acylated derivatives of the target compounds were prepared by the acid catalyzed condensation of D-ribose with 2-ATD followed by protection of the hydroxyl groups and subsequent cyclization to the mesoionic products; anomeric separation was achieved by column chromatography; All attempts to deprotect the hydroxyl groups of the mesoionic nucleosides resulted in hydrolytic ring-opening of the mesoionic heterobase. The 0-acetyl derivatives of the mesoionic thiadiazolopyrimidine nucleosides were evaluated for antineoplastic activity but were found to be inactive. The mesoionic thia-zolinopyrimidine nucleoside, i.e. Anhydro-6-ethyl-8-(D-2‘-deoxyribo-furanosyl-5-hydroxy-7-oxo-2,3-dihydrothiazolo[3,2-a]pyrimidinium hydrox- ide, prepared in a similar fashion to the mesoionic thiadiazolopyrimie- dine nucleosides, was designed as a potential inhibitor of the enzyme thymidylate synthetase. The mesoionic thiazolinopyrimidine nucleoside, was obtained as the a anomer and was not evaluated for antineoplastic activity. The mesoionic imidazothiazine nucleoside, i.e. Anhydro-l- (2',3',5'-tri-O-acetyl-D-ribofuranosyl)-5-hydroxy-7-oxoimidazo[2,l-b] thiazinium hydroxide was prepared as a potentially useful agent, due to its structural and isosteric similarity with purine nucleosides. The mesoionic imidazothiazine nucleoside was prepared by a cyclization reaction between the tri-0-acetyl-D-ribofuranosyl imidazole-2-thione and carbon suboxide. The mesoionic imidazothiazine nucleoside was not stable at room temperature or in aqueous Solution. While the results of this study on the chemotherapeutic utility of mesoionic nucleosides was rather discouraging, knowledge has been.gained that might be of value for the future design and synthesis of useful mesoionic nucleosides.

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The methanol extract of Helichrisum pedunculatum was screened for antimicrobial activity up to a concentration of 5 mg/ml using the agar dilution technique. A number of test bacterial isolates, comprising both Gram negative and Gram positive organisms were susceptible to the crude extract of the plant. The minimum inhibitory concentrations (MICs) of the extract ranged between 1 and 5 mg/ml for the susceptible organisms. The MICs of the selected antibiotics, chloramphenicol and penicillin, ranged between 2 and 4 mg/L, and 2 and 32 mg/L respectively against Bacillus cereus, Proteus vulgaris and Staphylococcus aureus OKOH1. Bactericidal activity was determined by the time kill assay. The methanol extract of the plant was not bactericidal at 1 × MIC for B. cereus, P. vulgaris and Staph. aureus OKOH1. At 2 × MIC the extract was bacteriostatic against B. cereus but bactericidal against P. vulgaris and Staph. aureus OKOH1. Combination studies were done at 1/2 × MIC, 1 × MIC and 2 × MIC of the plant extract with 1 × MIC of the antibiotics. Combinations of the plant extract and chloramphenicol resulted in mostly indifferent interactions against P. vulgaris and Staph. aureus OKOH1 but synergistic interactions at higher concentration of the plant extract for B. cereus. Penicillin combinations gave synergistic interactions at lower concentrations of the plant for P.vulgaris and Staph. aureus OKOH1 but was mostly indifferent for B. cereus.

SMAC (Secondary Mitochondria-derived Activator of Caspases) mimetics have generated significant interest as potential chemotherapeutic compounds via their ability to promote apoptosis in cancer cells. These molecules target several Inhibitor of Apoptosis Proteins (IAPs) whose elevated expression is ubiquitous with tumorigenesis. We report several novel SMAC based peptidomimetics which appear to mirror the anti-IAP activity of SMAC in vitro. Elements of reported SMAC mimetics were combined with unique structural features to design novel, efficacious IAP antagonists. Our approach included modifications to the 2nd and 4th residues of the AVPI peptide sequence, the motif responsible for SMAC 's interaction with IAPs. Cell-based compound testing against MDA-MB-231 breast cancer cells identified several promising leads possessing nanomolar cytotoxic effects. Apoptotic activity was confirmed via capsase-3/7 activation, a hallmark of regulated cell death. Our experimental data suggests we have developed selective, potent anti-cancer compounds to be further developed in the pursuit of new anti-cancer therapeutics.

Covalent bond-forming reactions between small molecules and proteins are ubiquitous. These reactions play a central role in the diversification and functionalization of proteins, enabling normal cell growth and life. Scientists routinely employ electrophilic compounds to modify proteins by exploiting the intrinsic nucleophilicity found on amino acid side-chains. These modifications permit a wide variety of experiments and allow for new insights and a deeper understanding of the chemistry and biology of living systems. The three research projects described in this thesis employ electrophilic, protein-modifying agents to meet unique goals. The first study (Chapter 2) details the development of a novel class of compounds that enhance the efficacy of therapeutic oncolytic viruses specifically in cancer cells. A medicinal chemistry-based approach was used to understand, measure and improve physicochemical and pharmacological properties of these small molecules. Inspired by the unique scaffold identified in Chapter 2, the second study of this thesis (Chapter 3) explores the bioactivity of the structurally related armeniaspirole natural products. Chemical synthesis enabled the uncovering of structure-activity relationships and ultimately allowed for the design of an activity-based probe. The final study (Chapter 4) details investigations of the terminal thioesterase involved in the biosynthesis of valinomycin. Small molecules substrates for the enzyme were synthesized and used to reveal details of the enzymatic mechanisms.

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Tuberculosis (TB) is a life-threatening infectious disease caused by Mycobacterium tuberculosis (Mtb). Globally, TB is a major public health burden with an estimated 10.4 million new cases and 1.8 million deaths reported in 2015. Although TB is curable, the treatment options currently available are beset by numerous shortcomings such as lengthy and complex treatment regimens, drug-drug interactions, drug toxicities, as well as emergence of widespread multi-drug resistance. Therefore, there is an urgent and compelling need to develop new, more effective, safer drugs with novel mechanisms of action, and which are capable of shortening treatment duration. This study focused on hit-to-lead optimization of two new classes of compounds with potential anti-TB properties: 2-aminoquinazolinones (AQZs) and benzoxazole-based oximes (BZOs). A hit compound for each of these classes with low micromolar antimycobacterial activity had previously been identified through phenotypic whole-cell in vitro screening.