Tuberculosis (TB) is the leading cause of death due to a single infectious disease and remains a major threat to global public health. The increasing emergence of multi-drug resistance to current anti-TB drugs, exacerbated by the long treatment duration, highlights the need for new effective treatments or strategies to shorten the treatment duration, improve patient adherence and curb the alarming rates of resistance. A key challenge to current strategies employed in the development of anti-TB drugs is the complexity in TB disease pathology which presents as a wide spectrum of lesions in patients presenting with the disease. These lesions occur at different anatomical loci in the same individual and at different stages as the disease progresses. In addition, the interaction between the causative agent, Mycobacterium tuberculosis, and its obligate human host induces physiologic and metabolic changes in the infecting bacillus that are specific to each lesion compartment, and dynamic. This is likely to influence M. tuberculosis susceptibility to antibiotic treatment and, consequently, affect treatment duration and possibly the development of drug resistance. A major limitation in current strategies to address this problem is translation of in vitro compound potency to in vivo efficacy. To reach the target site, a drug must first distribute and accumulate in the lesion microenvironments where bacteria reside: the macrophage host cell and the caseum. This thesis focused on the development of an in vitro infection model that could be used to predict drug penetration into M. tuberculosis-infected macrophages. Of particular interest was the extent to which host intracellular drug concentrations translate into effective antimycobacterial activity. To this end, the thesis comprised three key aspects: (i) characterization of physicochemical properties, antimycobacterial activities and M. tuberculosis-mediated metabolism of selected antiTB compounds; (ii) determination of intracellular drug permeation in resting, activated and foamy macrophages; and (iii) determination of the correlation (or not) between intracellular drug concentration and effective M. tuberculosis growth inhibition. The highly lipophilic natural product, fusidic acid (FA), its known human metabolite, 3-ketofusidic (3-ketoFA or GKFA37), and two C-3 alkyl esters (GKFA16 and GKFA17) as FA prodrugs were utilized in the study. In addition, another chemical class, the less lipophilic benzoxazole-based oxime derivatives were also investigated. Moxifloxacin (MXF), levofloxacin (LVF), bedaquiline (BDQ), rifampicin (RIF) and clofazimine (CFZ) were included for reference as known anti-TB drugs with varying lipophilicities. In chapter 2, FA and derivatives showed potent antimycobacterial activity (~1 µM) with selectivity indices (SI) >20 against the THP-1 macrophage cell line. Predicted artificial membrane permeability assay (PAMPA) results suggested that FA and derivatives would readily permeate the cell membrane. M. tuberculosis metabolized the C-3 alkyl-ester prodrug GKFA17 to form both FA and 3-ketoFA, with complete hydrolysis of the prodrug. FA was metabolized to 3-ketoFA, but the low levels of the metabolite suggested that another unidentified metabolite, presumed to be 3-epifusidic acid (3-epiFA), was formed. In vitro assays revealed that the potent benzoxazole-based oxime carbamates (PMN1-201, PMN1-136 and PMN2-09) were rapidly hydrolyzed by M. tuberculosis and were also susceptible to spontaneous degradation in media, forming the poorly active corresponding free oximes (PMN1-199, PMN1-135 and PMN1-157). In chapter 3, the in vitro macrophage drug uptake assay showed that FA C-3 alkyl prodrugs, GKFA16 and GKFA17, accumulated in significantly higher amounts in resting macrophages in comparison to FA and GKFA37. Accumulation of MXF was comparable to the least accumulated FA derivative, GKFA37, and showed steady state intracellular concentrations over a 6-day period. While GKFA16 and GKFA17 showed continued increasing accumulation, intracellular concentrations of FA and GKFA37 decreased after 48 hours, suggesting a likely susceptibility to macrophage efflux. In infected macrophages, the presence of intracellular bacteria or increasing bacterial burden did not affect the host cell ability to accumulate the drugs. FA and derivatives exhibited bacteriostatic inhibition of intracellular mycobacterial growth. MXF showed a potent bactericidal effect, reducing intracellular bacterial counts significantly at 10x MIC, with complete sterilization at 50x MIC even though MXF accumulation was significantly less than that of FA alkyl esters. These results suggested that both the inherent activity of a compound and ability to accumulate within host cells drive cellular efficacy. Given that the C-3 alkyl ester prodrugs accumulated at significantly higher concentrations than FA and GKFA37, this demonstrates the limitations of this assay in ascertaining the impact of intracellular concentration on drug efficacy for bacteriostatic drugs while highlighting its ability to correlate drug penetration and intracellular activity for cidal drugs. The prodrug GKFA17 was shown to undergo metabolism in resting host cells and during infection to form FA and then 3-ketoFA. Therefore, the prodrug strategy could be used to increase intracellular exposure of FA as GKFA17 showed superior macrophage accumulation. Benzoxazole-based oxime carbamates and their corresponding free oximes failed to accumulate in host macrophages and this was corroborated by their failure to control host cell bacterial growth despite the potent in vitro activity against M. tuberculosis of the carbamates, suggesting that they are poorly permeable. Chapter 4 investigated drug permeation in different macrophage phenotypes known to exist in the granuloma during TB disease, including foamy and activated macrophages. The activation state of the host cell did not affect the ability to accumulate anti-TB drugs such as RIF and BDQ. However, FA and its prodrug GKFA17 were significantly reduced in M1 activated macrophages. Despite the significantly reduced intracellular concentration, activated macrophages treated with FA and derivatives showed superior intracellular M. tuberculosis growth inhibition, suggesting that macrophage activation potentiates the activity of these compounds. In order to assess the effect of foamy macrophage lipid bodies (LBs) on drug uptake and intracellular localization, oleic acid-induced foamy macrophages were treated with selected antiTB drugs and experimental compounds. FA and derivatives showed early increased accumulation in foamy cells compared to resting macrophages, while MXF, BDQ and RIF levels were not significantly changed. Intracellular:extracellular (I/E) ratios increased with increase in lipophilicity, with FA C-3 alkyl prodrugs exhibiting the highest I/E ratios of >100. Despite exhibiting increased foamy macrophage concentrations, FA and derivatives exhibited a similar reduction (bacteriostatic) in bacterial counts in both resting and foamy macrophages. The intracellular activity of RIF was also not affected by presence of LBsin foamy macrophages. BDQ, LVF and MXF, however, showed reduced intracellular efficacy against M. tuberculosis in foamy macrophages compared to resting macrophages, suggesting a role for LBs to impact intracellular drug distribution. In conclusion, this thesis demonstrates the potential utility in combining advanced analytical methods and an in vitro infection model to determine cellular drug permeation profiles that might be applied to prioritize compounds and combinations optimized for distribution to target bacterial populations. This will facilitate well-informed decision-making processes in progression of lead compounds in pre-clinical development and, therefore, may offer the potential to reduce high rates of attrition of compounds which enter clinical phase of development.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/33768 |
| Date | 13 August 2021 |
| Creators | Mabhula, Amanda N |
| Contributors | Chibale, Kelly, Warner, Digby F |
| Publisher | Faculty of Science, Department of Chemistry |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, Doctoral, PhD |
| Format | application/pdf |
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