{
  "abstract": "Background Immunotherapy has emerged as a promising avenue for malignant tumors treatment, but methods to validate its in vivo antitumor activity remain limited. Granzyme B, a key mediator of immune-mediated cell death, is a potential biomarker for assessing immunotherapy efficacy. This study aimed to develop a radiotracer with high sensitivity and specificity for in vivo imaging of granzyme B.Methods 10 granzyme B targeting inhibitors were synthesized and evaluated to improve binding affinity. The most promising compound was conjugated with 1,4,7-triazacyclononane-N,N',N\"-triacetic acid chelator and radiolabeled with Al 18F to generate radiotracer 18F-G1. Its imaging and pharmacokinetic properties were compared with those of the recently developed radiotracer 68Ga-grazytracer. 18F-G1 was applied to assess T cell-mediated and natural killer (NK) cell-mediated antitumor immunity. In addition, the cyclic guanosine monophosphate AMP synthase-stimulator of interferon genes (cGAS-STING) pathway-targeted therapies were also quantitatively evaluated using 18F-G1.Results 18F-G1 was produced with high radiochemical purity of >98% through a simple radiolabeling procedure. It displayed a 4.2-fold improvement in granzyme B binding affinity compared with grazytracer. In the anti-programmed cell death protein-1 antibody-treated MC38 tumor model, 18F-G1 outperformed 68Ga-grazytracer with significantly higher tumor uptake (2.1-fold increase in maximum tumor uptake) and superior tumor-to-muscle contrast (7.46 vs 2.33). For NK cell-based therapy, 18F-G1 positron emission tomography (PET)/CT successfully monitored granzyme B release following NK-92MI cell injection, with peak signals observed 24 hours postinjection. 18F-G1 PET/CT detected robust immune activation in SR-717-treated tumors compared with other cGAS-STING pathway-targeted drug treatments, which correlated with the highest CD8+ T cell infiltration, granzyme B expression level and tumor growth inhibition.Conclusions 18F-G1 is an effective granzyme B imaging radiotracer. Its superior imaging properties make it a promising tool for non-invasively evaluating the therapeutic efficacy of various types of immunotherapies with high sensitivity.",
  "authors": [
    {
      "affiliations": [
        "Department of Nuclear Medicine, Peking University First Hospital, Beijing, China"
      ],
      "name": "Hongchuang Xu"
    },
    {
      "affiliations": [
        "Department of Nuclear Medicine, Beijing Chaoyang Hospital Affiliated to Capital Medical University, Beijing, China"
      ],
      "name": "Jingming Zhang"
    },
    {
      "affiliations": [
        "Department of MRI, Chinese Academy of Medical Sciences and Peking Union Medical College Fuwai Hospital, Beijing, China"
      ],
      "name": "Zhuxin Wei"
    },
    {
      "affiliations": [
        "Department of Nuclear Medicine, Peking University People’s Hospital, Beijing, China"
      ],
      "name": "Xiao Wang"
    },
    {
      "affiliations": [
        "Department of Nuclear Medicine, Beijing Chaoyang Hospital Affiliated to Capital Medical University, Beijing, China"
      ],
      "name": "Min-Fu Yang"
    },
    {
      "affiliations": [
        "Department of Nuclear Medicine, Peking University People’s Hospital, Beijing, China",
        "Department of Institute of Medical Technology, Peking University Health Science Center, Beijing, China",
        "International Cancer Institute, Peking University Health Science Center, Beijing, China",
        "Yunnan Baiyao Group, Kunming, China"
      ],
      "name": "Xing Yang"
    }
  ],
  "title": "Development of a novel 18F-labeled radiotracer targeting granzyme B for imaging early tumor responses to immunotherapy",
  "uid": "d0bad2d8-9cd7-5bf2-a989-c90194030645"
}