[¹⁸F]pFBC, a Covalent CLIP-Tag Radiotracer for Detection of Viral Reporter Gene Transfer in the Murine Brain

Preclinical models of neurological diseases and gene therapy are essential for neurobiological research. However, the evaluation of such models lacks reliable reporter systems for use with noninvasive imaging methods. Here, we report the development of a reporter system based on the CLIP-tag enzyme and [F-18]pFBC, an F-18-labeled covalent CLIP-tag-ligand synthesized via a DoE-optimized and fully automated process. We demonstrated its specificity using a subcutaneous xenograft model and a model of viral vector-mediated brain gene transfer by engineering HEK293 cells and striatal neurons to express membrane-tethered CLIP-tag protein. After in vitro characterization of the reporter, mice carrying either CLIP-tag expressing or control subcutaneous xenografts underwent dynamic [F-18]pFBC PET imaging. The CLIP-tag expressing xenografts showed a significantly higher uptake than control xenografts (tumor-to-muscle ratio 5.0 vs 1.7, p = 0.0379). In vivo, metabolite analysis by radio-HPLC from plasma and brain homogenates showed only one radio-metabolite in plasma and none in the brain. In addition, [F-18]pFBC showed fast uptake and rapid clearance from the brain in animals injected with adeno-associated virus (AAV)-CLIP in the right striatum but no right-to-left (R-L) uptake difference in the striata in the acquired PET data. In contrast, autoradiography showed a clear accumulation of radioactivity in the AAV-CLIP-injected right striatum compared to the sham-injected left striatum control. CLIP-tag expression and brain integrity were verified by immunofluorescence and light sheet microscopy. In conclusion, we established a novel reporter gene system for PET imaging of gene expression in the brain and periphery and demonstrated its potential for a wide range of applications, particularly for neurobiological research and gene therapy with viral vectors.