26. Glutamate Carboxypeptidase II (GCPII) Expression Changes in Denervated and Reinnervated Muscles

PURPOSE: No imaging modality can reliably identify muscle denervation acutely after nerve injury and monitor for muscle reinnervation in the chronic period after injury or repair. We recently identified glutamate carboxypeptidase II (GCPII) expression as a novel biomarker for muscle denervation and demonstrated that an FDA approved PET imaging agent, 18F-DCFPyL, could detect changes in muscle GCPII expression. In this study we define how long 18F-DCFPyL remains elevated in denervated muscles after nerve injury and reinnervated muscles after nerve repair. METHODS: A total of 64 male Lewis rats underwent right sciatic to femoral nerve transfer. This allowed for simultaneous monitoring of denervated sciatic nerve-innervated muscles and reinnervating femoral nerve-innervated muscles over time. The animals were divided into 9 postoperative timepoints: 2-, 4-, 8-, 12-, 16-, 20-, 24-, 30-, and 36-weeks (n=4-5 rats/timepoint). At each timepoint, we performed ex vivo biodistribution studies after intravenous injection of 18F-DCFPyL. Muscle uptake was calculated using an automated gamma counter and expressed as percent injected dose per gram of wet tissue (%ID/g). We additionally performed near-infrared imaging (NIR) using a fluorescent GCPII ligand, YC27, at no surgery and the 9 postoperative timepoints (n=10). Characterization of muscle GCPII expression were evaluated using immunohistochemistry (n=18). For clinical translatability, a PET 18F-DCFPyL scan of a 55-year-old female who sustained a closed radial nerve injury after a left humeral shaft fracture was imaged 15-weeks post nerve injury. RESULTS: The denervated gastrocnemius muscle had statistically greater 18F-DCFPyL uptake compared to unoperated gastrocnemius muscle all timepoints until 30-weeks post-surgery. The reinnervated vastus medialis (VM) muscle had statistically greater uptake compared to the unoperated VM at all timepoints until 8-weeks post-surgery. NIR imaging showed an increase in YC27 uptake in the denervated gastrocnemius muscle until 30-weeks post-surgery and decrease in uptake of the anterior thigh musculature after 8-weeks post-surgery. On histology, denervated gastrocnemius muscle demonstrated predominance of CD68+/GCPII+ cells in the first 4-weeks of the surgery with a shift to CD68-/GCPII+ cells at the later timepoints. Uptake of 18F-DCFPyL was demonstrated in the radial nerve injury patient with increased uptake in the left mobile wad and forearm extensor compartment muscles relative to the uninjured right side. CONCLUSION: GCPII expression remains elevated in denervated rodent muscles for at least 30-weeks after nerve injury. Reinnervation reduces GCPII expression in rodent muscles by 8-weeks after nerve repair. 18F-DCFPyL is a promising PET agent for non-invasive diagnosis and monitoring of nerve injuries.