8-Hydroxyquinolines Are Boosting Agents of Copper-Related Toxicity in Mycobacterium tuberculosis

Abstract

c Copper (Cu) ions are likely the most important immunological metal-related toxin utilized in controlling bacterial infections. Impairment of bacterial Cu resistance reduces viability within the host. Thus, pharmacological enhancement of Cu-mediated antibacterial toxicity may lead to novel strategies in drug discovery and development. Screening for Cu toxicity-enhancing antibacterial molecules identified 8-hydroxyquinoline (8HQ) to be a potent Cu-dependent bactericidal inhibitor of Mycobacterium tuberculosis. The MIC of 8HQ in the presence of Cu was 0.16 M for replicating and nonreplicating M. tuberculosis cells. We found 8HQ's activity to be dependent on the presence of extracellular Cu and to be related to an increase in cell-associated labile Cu ions. Both findings are consistent with 8HQ acting as a Cu ionophore. Accordingly, we identified the 1:1 complex of 8HQ and Cu to be its active form, with Zn, Fe, or Mn neither enhancing nor reducing its Cu-specific action. This is remarkable, considering that the respective metal complexes have nearly identical structures and geometries. Finally, we found 8HQ to kill M. tuberculosis selectively within infected primary macrophages. Given the stark Cu-dependent nature of 8HQ activity, this is the first piece of evidence that Cu ions within macrophages may bestow antibacterial properties to a Cu-dependent inhibitor of M. tuberculosis. In conclusion, our findings highlight the metal-binding ability of the 8-hydroxyquinoline scaffold to be a potential focus for future medicinal chemistry and highlight the potential of innate immunity-inspired screening platforms to reveal molecules with novel modes of action against M. tuberculosis.

8‐Hydroxyquinolines are bactericidal against Mycobacterium tuberculosis

“…To compare cell lines, all 26 analogs were tested for cytotoxicity against Vero cells; all active analogs were cytotoxic in this cell line as well (Table 4). These cytotoxicity data are in contrast to results from other groups, where cytotoxicity for 8-hydroxyquinoline (1) and related compounds was not noted (Ananthan et al, 2009;Darby & Nathan, 2010;Shah et al, 2016). The differences in these data may reflect the status of cells in the cytotoxicity assay, since, similar to our findings, Shen et al found HQ analogs to be cytotoxic via inhibition of DNA synthesis in HepG2 cells (Shen, Chen, & Roffler, 1999).…”

“…Three analogs (14,15,18) showed no measurable cytotoxicity against HepG2 cells, but none of these were active against M. tuberculosis. These cytotoxicity data are in contrast to results from other groups, where cytotoxicity for 8-hydroxyquinoline (1) and related compounds was not noted (Ananthan et al, 2009;Darby & Nathan, 2010;Shah et al, 2016). To compare cell lines, all 26 analogs were tested for cytotoxicity against Vero cells; all active analogs were cytotoxic in this cell line as well (Table 4).…”

“…The HQ are active against several bacterial species including Staphylococcus aureus and Staphylococcus epidermidis (Abouelhassan et al, 2014(Abouelhassan et al, , 2015Basak et al, 2016;Garrison et al, 2017;Lam et al, 2014), Enterococcus faecium (Basak et al, 2016;Garrison et al, 2017), Burkholderia pseudomallei (Wangtrakuldee et al, 2013), Neisseria gonorrhoeae, (Lawung et al, 2018), Listeria monocytogenes (Cherdtrakulkiat et al, 2016), and Mycobacterium avium (Hongmanee, Rukseree, Buabut, Somsri, & Palittapongarnpim, 2007;Kos et al, 2015). In addition, activity of the HQ compounds against replicating and nonreplicating Mycobacterium tuberculosis has been demonstrated in medium containing copper and in infected guinea pigs (Ananthan et al, 2009;Darby & Nathan, 2010;Hongmanee et al, 2007;Shah et al, 2016;Tison, 1952;Urbanski, Slopek, & Venulet, 1951). Despite these studies, the structure-activity relationship (SAR) of HQs for antitubercular activity has not been thoroughly explored.…”

Kunkel Lecture: Fundamental immunodeficiency and its correction

“…Even more encouraging are antibiotics that can kill bacteria extensively, not only when they are replicating but also when they are not replicating and are phenotypically tolerant to other antibiotics. With respect to TB, this has been reported with 8-hydoxyquinolines ( Darby and Nathan, 2010 ; Shah et al, 2016 ) and nitazoxanide, an antibiotic approved for other indications ( de Carvalho et al, 2009 ). In vitro, the nitroimidazole PA-824 (pretomanid) kills both replicating and nonreplicating Mtb to comparable extents and at comparable concentrations ( Singh et al, 2008 ; Gold and Nathan, 2017 ).…”

Targeting Phenotypically TolerantMycobacterium tuberculosis

“…Compound 30 , a 5-chloro, 7-bromo-substituted 8-hydroxyquinoline inhibitor of methionine aminopeptidase, killed M. tuberculosis that had been in stationary phase for two months (114). Some 8-hydroxyquinolines may mediate toxicity by chelating metals essential for mycobacterial survival, or by forming metal-quinoline complexes that engage target enzymes (211). …”