Copper(II)-Bis(Thiosemicarbazonato) Complexes as Antibacterial Agents: Insights into Their Mode of Action and Potential as Therapeutics

Abstract

There is increasing interest in the use of lipophilic copper (Cu)-containing complexes to combat bacterial infections. In this work, we showed that Cu complexes with bis(thiosemicarbazone) ligands [Cu(btsc)] exert antibacterial activity against a range of medically significant pathogens. Previous work using Neisseria gonorrhoeae showed that Cu(btsc) complexes may act as inhibitors of respiratory dehydrogenases in the electron transport chain. We now show that these complexes are also toxic against pathogens that lack a respiratory chain. Respiration in Escherichia coli was slightly affected by Cu(btsc) complexes, but our results indicate that, in this model bacterium, the complexes act primarily as agents that deliver toxic Cu ions efficiently into the cytoplasm. Although the chemistry of Cu(btsc) complexes may dictate their mechanism of action, their efficacy depends heavily on bacterial physiology. This is linked to the ability of the target bacterium to tolerate Cu and, additionally, the susceptibility of the respiratory chain to direct inhibition by Cu(btsc) complexes. The physiology of N. gonorrhoeae, including multidrug-resistant strains, makes it highly susceptible to damage by Cu ions and Cu(btsc) complexes, highlighting the potential of Cu(btsc) complexes (and Cu-based therapeutics) as a promising treatment against this important bacterial pathogen.

Interplay between tolerance mechanisms to copper and acid stress inEscherichia coli

“…Nevertheless, Cu may become more bioavailable during growth at pH 5 due to protonation of thiols and amines, leading to a decrease in the Cu buffering capacity of the extracellular medium or intracellular milieu. To report for bioavailable Cu, we used a lacZ transcriptional reporter fused to the copA promoter (P copA -lacZ) (12). The latter is activated by CueR, the primary Cu sensor in E. coli (Fig.…”

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

“…As a class, Cudependent inhibitors are known to act through a wide range of mechanisms. They can act as ionophores, overloading cells with Cu ions (e.g., 8HQ against Cryptococcus neoformans [28]); as liberators, growing the pool of bioavailable (and toxic) Cu without an overall increase (e.g., disulfiram against M. tuberculosis [45]); and even as traditional antibiotics, disrupting specific enzymatic targets (e.g., glyoxal-bis[N(4)-methylthiosemicarbazonato]copper(II) complex against Neisseria gonorrhoeae [55]). To gain insight into the mechanism of mycobacterial inhibition, we utilized PhenGreen FL (PGFL), a fluorescent metal sensor, to determine net changes in the amounts of cell-associated labile metal ions upon treatment with Cu or 8HQ alone or cotreatment with Cu ions and 8HQ.…”

Combinatorial phenotypic screen uncovers unrecognized family of extended thiourea inhibitors with copper-dependent anti-staphylococcal activity

“…GTSM, for example, acts upon the electron transport chain of N. gonorrhoeae , with its spectrum of activity hypothesized to be explained through a particular bacterium’s reliance on aerobic respiration 17, 25 . Yet, GTSM is also a potent copper-dependent inhibitor of S. pneumoniae ( 25 ; unpublished observations), a facultative anaerobe lacking an electron transport chain. Thus, additional modes of action must be at play, indicating a multi-faceted mechanism of activity.…”

“…The allure of copper’s anti-bacterial properties has not gone unnoticed 17–20, 25, 47–49 . Unfortunately, free copper ions have little therapeutic value due to their erratic reactivity 50–53 .…”

“…Example inhibitors have been described for Gram negatives 17 , Gram positives 18 , and mycobacteria 19 , as well as eukaryotic targets including pathogenic fungi 20 and cancer cells 21–23 . The broad range of potential targetable pathogens, coupled with the wide array of novel mechanisms of action, has generated interest in exploiting copper’s antibiotic properties 24, 25 . However, until now, discoveries have been largely serendipitous, or based upon established metal-binding motifs; though well suited for chemical probes, these scaffolds are often poorly adaptable to therapeutic uses.…”