D-Glutamine PET for Early Detection and Precision Treatment of Orthopedic Implant Infection

About This Grant

This research addresses the critical unmet clinical need for accurate diagnosis and treatment monitoring of orthopedic implant infection (OII), which can be caused by either Gram-positive (Gram+) or Gram-negative (Gram-) bacteria. Current positron emission tomography (PET) imaging radiotracers have limitations. For instance, 2-deoxy-2-[18F]-fluoroglucose ([18F]FDG) struggles to differentiate infection from inflammation, and 2- deoxy-2-[18F]-fluorosorbitol ([18F]FDS) is only effective at detecting Gram- bacteria. To overcome these limitations, efforts have been seen to develop D amino acid (DAA)-derived radiotracers, leveraging bacteria’s unique ability to consume DAAs. While three carbon-11-labeled DAAs - D-alanine (D-[3-11C]Ala), D-methionine (D-[methyl-11C]Met), and D-glutamine (D-[5-11C]Gln) - have been reported, a direct comparative study is lacking. To bridge the gap, we synthesized all three 11C-DAA radiotracers with high and comparable radiochemical purity and enantiopurity. Our preliminary in vitro studies revealed significant differences in DAA consumption across various bacteria, following the order: D-[methyl-11C]Met (Gram+/Gram- = 3.8) > D-[3-11C]Ala (Gram+/Gram- = 2.5) > D-[5-11C]Gln (Gram+/ Gram- = 1.1). These results suggest that D-Gln-based radiotracers may offer superior potential for surveying a broad spectrum of bacteria responsible for OII. Furthermore, preliminary in vivo studies demonstrated that, among the three DAA radiotracers, D-[5-11C]Gln exhibited the lowest systemic background, the best target-to-background contrast, and highest imaging sensitivity for detecting the lowest number of colonized bacteria. Notably, D-[5-11C]Gln effectively visualized both Gram+ and Gram- bacteria- mediated infections in soft tissue and metal screw-mediated bone infections, outperforming [18F]FDG and [18F]FDS. These compelling findings underscore the superior potential of D-[5-11C]Gln and highlight the significance of our proposed work to further research and compare D-Gln-derived radiotracers for PET imaging of OII. Our ongoing efforts include producing D-[5-11C]Gln, D-[3-11C]Ala, and D-[methyl-11C]Met at comparable levels of molar activity and enantiopurity. This will facilitate comprehensive comparative in vitro and in vivo studies. Concurrently, we will complete the design and synthesis of fluorine-18-labeled D-Gln (D-[3/4-18F]FGln) radiotracers (Aim 1) to broaden the clinical applicability of our developed imaging approach. Utilizing both in vitro and in vivo models (Aim 2), we will carry out preclinical studies to: 1) investigate and compare the roles of D-Gln, D-Ala, and D-Met in the physiology of Gram+/- bacterial infections and in the pathological activities of bacteria colonizing implant surfaces using in vitro biofilm models; 2) evaluate and optimize the performance of the D-[3/4-18F]FGln radiotracers in comparison with D-[5-11C]Gln, D-[3-11C]Ala and D-[methyl-11C]Met for detecting slime-positive/slime-negative clinical isolates causing implant-related bone infection; and 3) validate the sensitivity and specificity of two leading radiotracers in detecting and monitoring antimicrobial treatment outcomes using a rat bone screw-mediated infection model.