Granzyme B PET Imaging for Assessment of Disease Activity in Inflammatory Bowel Disease

Developing a noninvasive imaging method to detect immune system activation with a high temporal resolution is key to improving inflammatory bowel disease (IBD) management. In this study, granzyme B (GZMB), typically released from cytotoxic T and natural killer cells, was targeted using PET with ⁶⁸Ga-NOTA-GZP (where GZP is β-Ala–Gly–Gly–Ile–Glu–Phe–Asp–CHO) to detect early intestinal inflammation in murine models of colitis. Methods: Bioinformatic analysis was used to assess the potential of GZMB as a biomarker for detecting IBD and predicting response to treatment. Human active and quiescent Crohn disease and ulcerative colitis tissues were stained for GZMB. We used IL-10^−/− mice treated with dextran sulfate sodium (DSS) as an IBD model, wild-type C57BL/6J mice as a control, and anti–tumor necrosis factor as therapy. We used a murine GZMB-binding peptide conjugated to a NOTA chelator (NOTA-GZP) labeled with ⁶⁸Ga as the PET tracer. PET imaging was conducted at 1, 3, and 4 wk after colitis induction to evaluate temporal changes. Results: Bioinformatic analysis showed that GZMB gene expression is significantly upregulated in human ulcerative colitis and Crohn disease compared with the noninflamed bowel by 2.98-fold and 1.92-fold, respectively; its expression is lower by 2.16-fold in treatment responders than in nonresponders. Immunofluorescence staining of human tissues demonstrated a significantly higher GZMB in patients with active than with quiescent IBD (P = 0.032).⁶⁸Ga-NOTA-GZP PET imaging showed significantly increased bowel uptake in IL-10^−/− mice with DSS-induced colitis compared with vehicle-treated IL-10^−/− mice (SUV_mean, 0.75 vs. 0.24; P < 0.001) and both vehicle- and DSS-treated wild-type mice (SUV_mean, 0.26 and 0.37; P < 0.001). In the IL-10^−/− DSS-induced colitis model, the bowel PET probe uptake decreased in response to treatment with tumor necrosis factor–α (SUV_mean, 0.32; P < 0.001). There was a 4-fold increase in colonic uptake of ⁶⁸Ga-NOTA-GZP in the colitis model compared with the control 1 wk after colitis induction. The uptake gradually decreased to approximately 2-fold by 4 wk after IBD induction; however, the inflamed bowel uptake remained significantly higher than control at all time points (week 4 SUV_mean, 0.23 vs. 0.08; P = 0.001). Conclusion: GZMB is a promising biomarker to detect active IBD and predict response to treatment. This study provides compelling evidence to translate GZMB PET for imaging IBD activity in clinical settings.