Investigation of Siderophore-Platinum(IV) Conjugates Reveals Differing Antibacterial Activity and DNA Damage Depending on the Platinum Cargo

The growing threat of bacterial infections coupled with the dwindling arsenal of effective antibiotics has heightened the urgency for innovative strategies to combat bacterial pathogens, particularly Gram-negative strains, which pose a significant challenge due to their outer membrane permeability barrier. In this study, we repurpose clinically approved anticancer agents as targeted antibacterials. We report two new siderophore-platinum(IV) conjugates, both of which consist of an oxaliplatin-based Pt(IV) prodrug (oxPt(IV)) conjugated to enterobactin (Ent), a triscatecholate siderophore employed by Enterobacteriaceae for iron acquisition. We demonstrate that l/d-Ent-oxPt(IV) (l/d-EOP) are selectively delivered into the Escherichia coli cytoplasm, achieving targeted antibacterial activity, causing filamentous morphology, and leading to enhanced Pt uptake by bacterial cells but reduced Pt uptake by human cells. d-EOP exhibits enhanced potency compared to oxaliplatin and l-EOP, primarily attributed to the intrinsic antibacterial activity of its non-native siderophore moiety. To further elucidate the antibacterial activity of Ent-Pt(IV) conjugates, we probed DNA damage caused by l/d-EOP and the previously reported cisplatin-based conjugates l/d-Ent-Pt(IV) (l/d-EP). A comparative analysis of these four conjugates reveals a correlation between antibacterial activity and the ability to induce DNA damage. This work expands the scope of Pt cargos targeted to the cytoplasm of Gram-negative bacteria via Ent conjugation, provides insight into the cellular consequences of Ent-Pt(IV) conjugates in E. coli, and furthers our understanding of the potential of Pt-based therapeutics for antibacterial applications.

Conjugation to Enterobactin and Salmochelin S4 Enhances the Antimicrobial Activity and Selectivity of β-Lactam Antibiotics against Nontyphoidal Salmonella

The pathogen Salmonella enterica is a leading cause of infection worldwide. Nontyphoidal Salmonella (NTS) serovars typically cause inflammatory diarrhea in healthy individuals, and can cause bacteremia in immunocompromised patients, children, and the elderly. Management of NTS infection poses a challenge because antibiotic treatment prolongs fecal shedding of the pathogen and is thus not recommended for most patients. In recent years, the emergence of antibiotic resistance in NTS has also become a major issue. Thus, new therapeutic strategies to target NTS are needed. Here, we evaluated whether six siderophore-β-lactam conjugates based on enterobactin (Ent) and salmochelin S4 (digulcosylated Ent, DGE) provide antimicrobial activity against the two highly prevalent NTS serovars Typhimurium and Enteritidis by targeting the siderophore receptors FepA and/or IroN. The conjugates showed 10- to 1000-fold lower minimum inhibitory concentrations against both serovars Typhimurium and Enteritidis compared to the parent antibiotics under iron limitation and were recognized and transported by FepA and/or IroN. NTS treated with the Ent/DGE-β-lactam conjugates exhibited aberrant cellular morphologies suggesting inhibition of penicillin-binding proteins, and the conjugates selectively killed NTS in coculture with Staphylococcus aureus. Lastly, the DGE-based conjugates proved to be effective at inhibiting growth of NTS in the presence of the Ent-sequestering protein lipocalin-2. This work describes the successful use of siderophore-antibiotic conjugates against NTS and highlights the opportunity for narrowing the activity spectrum of antibiotics by using Ent and DGE to target enteric bacterial pathogens.