Clinical Study of Trap (FAPI) 3 PET Imaging in Noninvasive Diagnosis of Malignant Tumors

Positron Emission Tomography (PET) is a functional imaging technology for diagnosing and monitoring tumors. In clinical applications for malignant tumors, PET includes staging, response assessment, and prognosis prediction, providing attractive semi-quantitative biomarkers for both clinical and research platforms. Understanding the fundamentals of imaging science and evaluating the strengths and limitations of imaging modalities is crucial for optimizing research assessments. With the widespread use of functional techniques and the development of novel PET biomarkers, PET-based research evaluations will be further enhanced. The field of oncology has undergone a revolution through molecular imaging, which evaluates tumor biology, while traditional radiological imaging focuses on morphological anatomy . Molecular imaging employs non-invasive visualization at cellular or subcellular levels to observe physiological or pathological processes, whereas PET/CT serves as a hybrid imaging tool that provides complementary information on both function and structure . [18F] Fluorodeoxyglucose (FDG), first developed in the late 1970s as a tracer for brain region metabolism, remains the most widely used PET tracer with diverse applications in both oncology and non-oncology fields . Despite its undeniable clinical utility, FDG uptake serves as an alternative indicator for glucose transport/metabolism rather than being specific to malignant tumors. Continuous exploration of cellular targets has led to the discovery of fibroblast activation protein (FAP). Cancer-associated fibroblasts are present in many tumors, particularly those with strong fibrotic-promoting responses such as breast cancer, colorectal cancer, pancreatic cancer, prostate cancer, and lung cancer. Consequently, FAP expression has been observed in over 90% of epithelial tumors. To date, FAP expression has been associated with poor tumor prognosis in colorectal cancer, pancreatic cancer, hepatocellular carcinoma, and ovarian cancer . Although more research is needed, this advantage makes cancer-associated fibroblasts an ideal target for anti-tumor therapy. In practical applications, low FAP expression in fibroblasts or healthy tissues facilitates imaging of subtle pathological changes. Cancer-associated fibroblasts (CAFs), a key component of the tumor microenvironment, account for over half the mass in various tumor types. Previous studies indicate that CAFs play significant roles in tumor growth, immune suppression, and cancer invasion . Therefore, CAFs may become emerging targets for tumor diagnosis and treatment. Fibroblast activation protein (FAP) is overexpressed in CAFs of multiple epithelial cancers but shows weak expression in healthy tissues, making it a promising target in cancer research. Recent years have seen expanded molecular imaging studies targeting FAP in tumor diagnosis . The excellent tumor-targeting efficacy of FAP has been confirmed through multiple clinical trials . The results clearly establish FAPI as a highly promising tumor-targeting ligand with significant potential applications in translational oncology. However, its therapeutic efficacy remains under investigation. An ideal radiopharmaceutical for cancer treatment should demonstrate excellent targeting specificity and relatively long tumor retention time. Previous studies have shown that radiolabeled FAPI variants (FAPI-04 and FAPI-46) accumulate rapidly and satisfactorily in tumors while showing low physiological uptake in normal tissues. However, previous FAP-related tracers exhibited relatively short tumor retention times . Our aim is to design a FAPI trimer, Trap-(FAPI)3, to optimize pharmacokinetics and evaluate whether this novel drug demonstrates superior advantages over its monomeric analogs in tumor imaging diagnosis and staging.