Currently this project focuses on five key areas: (1) chemical synthesis of lead molecules and series identified by high-throughput screening against whole Mycobacterium tuberculosis (MTb), (2) the lead development of an oxazolidinone with optimized activity against MTb, (3) the synthesis and evaluation of inhibitors of the inosine 5'-monophosphate dehydrogenase, (4) structure-based design and screening of inhibitors of NAD synthetase and fumarase and (5) identification of environmental microbes that produce anti-tubercular secondary metabolites. Most of our effort currently is devoted to Project (1) in which we are screening large compounds libraries obtained from global collaborators including several large pharmaceutical companies to identify inhibitors of MTb growth under in vivo relevant conditions, performing dose-titration follow-up of hits and synthesizing or purchasing chemically similar compounds. These series are evaluated using secondary screens with a battery of conditions that are thought to be relevant during in vivo growth of MTb. We selected carbon source as an in vivo relevant variable since numerous studies have shown the metabolism of glucose, cholesterol and/ or other lipids are critical for growth and survival of this pathogen in host tissues. Since September 2016, we have diminished the amount of primary screening performed with only a few small focused libraries screened over the last reporting period. In contrast, we have expanded our efforts in following up on prioritized hits. We have performed formal hit assessment by our hit prioritization screens and counter-screens of the top confirmed hits from the new 250,000 compound deck from the Medicines for Malaria Venture (MMV), the 60,000 new compound deck from the Dundee Drug Discovery Unit and the 69,000 compound deck the Global Chemical Diversity Library using our 4-tiered hit prioritization approach that bins compounds into major mechanistic classes, in particular highlighting those compounds that hit well-known targets in cell wall synthesis or respiration and excluding generally cytotoxic compounds. Every attempt is made to progress as many chemo-types as possible to increase the likelihood of hitting a diversity of targets. Hit series with multiple members showing activity for the scaffold with low-complexity, acceptable solubility and promising physicochemical properties for profiling are prioritized for follow-up to determine if the desirable balance of potency and ADME (absorption, distribution, metabolism and excretion) properties could be achieved in Lead Optimization. In contrast, series with structural alerts suggesting toxicophores are deprioritized. To rapidly expand the SAR for the prioritized chemotypes, commercially available analogs are purchased and tested in MIC assays. Target identification for prioritized series is initiated by mutation frequency analysis, whole genome resequencing of resistant isolates, microarray and metabolomics analyses. In addition, for top hits of interest where SAR indicates that certain positions on the molecule can be modified while retaining anti-tubercular activity, we have chemically modified the compounds by addition of a linker that can be UV-crosslinked onto the putative targets, as well as a linker moiety that provides a handle allowing purification of the resultant ligand-protein complexes. This chemical biology approach is guiding our efforts in target identification. Kinetic and thermodynamic solubility determinations and microsomal stability assays are also done to further develop the information that will be essential to facilitate go / no-go progression into lead optimization. A major reason for the low success rates in TB drug development is that the leads ultimately fail to reduce bacterial burdens in TB patients. Failure of a drug to reach sufficient levels at the site of disease is a leading cause of treatment failure. We are performing human alveolar epithelial transport assays in an attempt to develop an assay that will prioritize those compounds that we predict will accumulate in the epithelial lining fluid providing a reservoir of drug that will penetrate into adjacent granulomas. In project 2, we are working on developing an oxazolidinone with an increased potency against Mtb combined with a lower ability to inhibit mitochondrial protein synthesis in order to decrease the toxicities associated with linezolid chemotherapy. We are developing SAR based on anti-tubercular potency and lack of mitochondrial toxicity using mitobiogenesis assays and screening for antibacterial spectrum using B. subtilis as an indicator strain tested in parallel. We have identified an oxazolidinone that is selectively active against mycobacteria by capitalizing on metabolic pathways that are unique to this genus. Compounds with optimized properties including good PK values are progressed to evaluation in marmosets. In Projects 3-4 we are taking a variety of approaches to develop inhibitors and potential lead series against specific enzyme targets that are considered well validated as druggable in TB. In project 5, we have identified environmental reservoirs that are rich in mycobacteria that compete with other environmental bacteria for limited nutrients. Specifically, sphagnum peat bogs have been reported to support diverse bacterial communities including slow-growing mycobacteria closely related to Mtb that compete for nutrients under conditions that recapitulate some of the defining characteristics of human granulomas including an acidic pH, hypoxia as well as nutrient limitation. We have screened microbes from these peat bogs and identified novel bacteria that produce secondary metabolites capable of inhibiting the growth of Mtb. Genome sequencing of some of these bacteria has allowed us to identify several genome clusters that encode enzymes known to be involved in secondary metabolite production and the purification and characterization of these metabolites is currently in progress. We are expanding our collection of microbes from these peat bogs to expand the repertoire of antibiotic-producing bog isolates with activity not only against Mtb but also other important bacterial pathogens and will be using genome-guided hit deconvolution to identify potential encoded metabolites.
Showing the most recent 10 out of 61 publications
