d‑[5-<sup>11</sup>C]-Glutamine Positron\nEmission Tomography Imaging for Diagnosis and Therapeutic Monitoring\nof Orthopedic Implant Infections

Orthopedic implant infections (OIIs) present diagnostic\nand therapeutic\nchallenges, owing to the lack of methods to distinguish between active\ninfection and sterile inflammation. To address this unmet need, d-amino-acid-based radiotracers with unique metabolic profiles\nin microorganisms have emerged as a novel class of infection-specific\nimaging agents. Given the pivotal role of d-glutamine in\nbacterial biofilm formation and virulence, herein, we explored the\npotential of positron emission tomography (PET) imaging with d-[5-<sup>11</sup>C]-Glutamine (d-[5-<sup>11</sup>C]-Gln)\nfor early detection and treatment monitoring of OIIs. <i>In vitro</i> studies confirmed an active uptake of d-[5-<sup>11</sup>C]-Gln by Staphylococcus aureus (S. aureus) biofilm commonly associated with OIIs. <i>In vivo</i> evaluations included PET imaging comparisons with d-[5-<sup>11</sup>C]-Gln <i>vs</i> l-[5-<sup>11</sup>C]-Gln or 2-deoxy-2-[<sup>18</sup>F]-fluoroglucose ([<sup>18</sup>F]-FDG) in a rat OII model with tibial implantation of sterile\nor S. aureus<i>-</i>colonized\nstainless-steel screws before and after treatment. These studies demonstrated\nthat the uptake of d-[5-<sup>11</sup>C]-Gln was significantly\nhigher in the infected screws than that in sterile screws (∼3.4-fold, <i>p</i> = 0.008), which displayed significantly higher infection-to-background\nmuscle uptake ratios (∼2-fold, <i>p</i> = 0.011)\nwith d-[5-<sup>11</sup>C]-Gln as compared to l-[5-<sup>11</sup>C]-Gln. Following a 3 week vancomycin treatment, imaging\nwith d-[5-<sup>11</sup>C]-Gln showed a significant reduction\nin uptake at the infected sites (∼3-fold, <i>p</i> = 0.0008). Further regression analyses revealed a superior correlation\nof residual infection-associated radiotracer uptake with the postimaging <i>ex vivo</i> bacterial counts for d-[5-<sup>11</sup>C]-Gln (<i>k</i> = 0.473, <i>R</i><sup>2</sup> = 0.796) <i>vs</i> [<sup>18</sup>F]-FDG (<i>k</i> = 0.212, <i>R</i><sup>2</sup> = 0.434), suggesting that d-[5-<sup>11</sup>C]-Gln PET had higher sensitivity for detecting\nresidual bacterial burden than [<sup>18</sup>F]-FDG PET. Our results\ndemonstrate the translational potential of d-[5-<sup>11</sup>C]-Gln PET imaging for noninvasive detection and treatment monitoring\nof OIIs.