In vivo bacteria-targeted imaging with vancomycin-based positron emission tomography and optical tracers in an orthopaedic trauma implant infection model.

Abstract

Prosthetic joint- and fracture-related infections are major postoperative complications. Early, specific, and preferably non-invasive diagnostic tools, such as bacteria-targeting imaging agents, are essential for optimal treatment, since current approaches, including white blood cell scintigraphy and [¹⁸F]FDG-PET, are accurate, but may lack specificity. Vancomycin, an antibiotic targeting Gram-positive bacteria, has been successfully applied as a molecular scaffold for bacteria-specific tracer development. This study evaluated two novel vancomycin-based PET tracers, [¹⁸F]BODIPY-FL-vancomycin and [¹⁸F]VE1-PQ-vancomycin, along with the fluorescent tracer vancomycin-IRDye800CW, for diagnosing biofilm-associated orthopaedic implant infections. We employed a preclinical in vivo model of orthopaedic implant infection, where C57BL/6 mice (n = 72) received stainless-steel Kirschner (K)-wire implants in the distal femur, which were inoculated with bioluminescent Staphylococcus aureus or Escherichia coli bacteria, or with PBS (sterile control). At days 13-14 post-implant infection, animals were injected with one of the vancomycin tracers or [¹⁸F]FDG (benchmark control) and underwent PET/CT or fluorescence imaging. Tracer accumulation at the infection site was quantified, followed by biodistribution analysis and bacterial load assessment. Both vancomycin-based PET tracers showed significantly higher uptake in S. aureus-infected legs compared to sterile controls. The differences in binding between S. aureus or E. coli were less distinct. In this model, [¹⁸F]FDG did not distinguish K-wire infection from sterile controls. Vancomycin-IRDye800CW accumulated specifically at S. aureus infection sites, but not at E. coli infection sites or sterile controls, indicating accurate bacteria-targeted imaging. Vancomycin-based PET and optical tracers differentiated bacterial infection from the sterile K-wire controls in a murine model. Optical imaging distinguished Gram-positive from Gram-negative bacterial infection, whereas PET tracers did not. These preclinical findings support further investigation of vancomycin-based tracers for non-invasive imaging of prosthetic joint and fracture-related infections.