Synthesis and Biological Evaluation of a Bisphosphonate–Tazobactam Conjugate for Bone-Targeted β-Lactamase Inhibition

Background/Objectives: Osteomyelitis and implant-associated bone infections remain difficult to treat because of persistent bacterial colonization, biofilm formation, limited antibiotic exposure in infected bone, and antimicrobial resistance. Bisphosphonate conjugation has been explored for bone-targeted antibiotic delivery, but targeting a β-lactamase inhibitor itself to our knowledge has not been established. This study aimed to synthesize and evaluate an ester-linked bisphosphonate–tazobactam (BP-Tazo-ester) conjugate as a putative bone-targeted β-lactamase inhibitor prodrug. Methods: BP-Tazo-ester (compound 5) was synthesized and characterized by nuclear magnetic resonance spectroscopy and mass spectrometry. β-lactamase inhibition was assessed using a nitrocefin-based in vitro assay. Hydroxyapatite binding was evaluated using a sample derived from compound 5. Piperacillin combinations were evaluated against β-lactamase-producing Escherichia coli HS and planktonic Staphylococcus aureus CCUG 25925. The same S. aureus strain was used in an ex vivo bone-slice early-colonization model. Results: Compound 5 was obtained in 61% yield and retained measurable β-lactamase inhibitory activity, although its observed activity was lower than that of free tazobactam. TMSBr-treated material derived from compound 5 exhibited high apparent hydroxyapatite binding. Compound 5 supported piperacillin-mediated growth inhibition against E. coli HS. Against S. aureus CCUG 25925, piperacillin–compound 5 and piperacillin–tazobactam showed the same observed MIC, both lower than that of piperacillin alone, while compound 5 alone exhibited limited antibacterial activity. In the bone-slice model, piperacillin–compound 5 reduced recoverable S. aureus counts below the assay limit of detection during 24 h of treatment co-initiated with inoculation. Conclusions: These findings provide preliminary proof-of-concept for a bisphosphonate-based strategy targeting a β-lactamase inhibitor rather than the antibiotic partner. Further research on linker-cleavage kinetics, bone biodistribution, pharmacokinetics, and in vivo efficacy is warranted.