Synthesis of β-lactam-zidovudine pronucleosides as potential selective narrow-spectrum antibacterial agents.

Since the discovery of penicillin, the forerunner of the most widely used class of antibiotics (i.e. β-lactams), natural compounds and their derivatives represented a major source of antibacterial therapeutic products whose availability enabled modern medical practices (invasive surgery, organ transplant, etc.). However, the relentless emergence of resistant bacteria is challenging the long-term efficacy of antibiotics, also decreasing their economic attractiveness for big pharma, leading to a significant decay in antibacterial development in the 21^st century and an increased use of last-resort drugs such as carbapenems or colistin. Indeed, bacteria evolved an arsenal of resistance mechanisms, leading to the emergence of totally-drug resistant isolates, already sporadically isolated among Gram-negative bacterial species. To face this deadly peril, it is fundamental to explore new ground-breaking approaches. In view of the significance of both β-lactam antibiotics and the production of one or more β-lactamases as the major resistance mechanism (especially in Gram-negative bacteria), we implemented an original approach to selectively deliver antibacterial zidovudine (AZT) exploiting the β-lactamase-mediated hydrolysis of a β-lactam-conjugate prodrug. The synthesis of the targeted pronucleosides was performed in 5-7 steps and based on an original Pd-catalyzed cross-coupling reaction. Enzymatic and microbiological evaluations were performed to evaluate the synthesized pronucleosides, yielding new insights into molecular recognition of β-lactamase enzymes. This approach would potentially allow a targeted and selective eradication of antibiotic-resistant β-lactamase-producing (opportunistic) pathogens, as the inactive prodrug is unable to harm the commensal microbial flora.