Design and synthesis of α-aminoxy acid dipeptides as cation transporters and their applications as anti-bacterial agents

Abstract

Staphylococcus aureus is a deadly human pathogen that widely transmits in hospitals and communities. In recent years, the emergence of the multidrug resistant bacteria that carry antibiotics resistant genes have posed a great threat to public health due to the overwhelming failures of traditional antibiotics therapy. Apart from antibiotics resistance, bacterial persisters, which undergo phenotypic change instead of genetic change, is a distinct strategy adopted by bacteria that make them highly recalcitrant to antibiotics challenge. These slow-growing variants have reduced metabolism. Once antibiotics are removed, they regrow and lead to the relapse of bacterial infections. Moreover, they also account for antibiotics tolerance in bacterial biofilm. Currently, our arsenal to combat multi-drug resistant S. aureus and their persisters is still very limited and the replenishment of new antibiotics is urgently needed. Targeting ion homeostasis is an attractive antibacterial strategy since it disrupts proton motive force. However, previously reported ion transporters showed limited value due to indiscriminate toxicity to both mammalian cells and bacteria. In this thesis, a series of cation transporters containing α-aminoxy acid motif have been designed and synthesized to combat S. aureus. Compound 2.16 was found to be active against multidrug resistant S. aureus with minimal inhibitory concentration of 3.13 μΜ and it exhibited low toxicity against mammalian cells. In contrast to conventional antibiotics which are ineffective in killing persisters, 2.16 successfully eradicated them. 2.16 showed excellent capacity not only to inhibit biofilm formation but also to reduce established biofilm. Moreover, it can suppress the secretion of the virulence proteins from S. aureus. Mechanistic study revealed that 2.16 was an ion transporter that mediated the antiport of proton and potassium ion. It deceased the cytoplasmic pH and led to potassium ion efflux in S. aureus. The disruption of ion homeostasis resulted in membrane hyperpolarization and calcium ion influx. These series of events accelerated bacterial autolysis, which ultimately led to osmotic cell burst. At the same time, 2.16 reduced biofilm formation and the secretion of virulence proteins through repression of the bacterial transcriptional regulatory protein WalR. Further structural optimization of 2.16 yielded 4.15 with better antibacterial activity and minimum inhibitory concentration of 0.78 μΜ. 4.15 also showed low toxicity to mammalian cells and was active against S. aureus persisters and biofilm. In line with 2.16, it triggered autolysis and the osmotic cell burst of S. aureus. This series of compounds containing α-aminoxy acids as building blocks lay the foundation for eradicating S. aureus persisters and biofilm through disruption of ion homeostasis.