Fibroblast activation protein (FAP) is higher expressed on cancer-associated fibroblasts (CAFs) in most malignant epithelial neoplasms, which is lower expressed in normal tissues. As a promising small molecular probe, FAP inhibitor (FAPI) shows the specific binding to FAP. This study aimed to explore a novel molecular probe [99mTc]Tc-HYNIC-FAPI targeting CAFs. The in vitro characteristics of the probe were also evaluated. The FAPI targeting FAP was designed, synthesized and conjugated with the chelator 6-hydrazinylnicotinic acid (HYNIC) for radiolabeling with 99mTc. The radiolabeling yield, radiochemical purity and stability were evaluated by Instant thin-layer chromatography (ITLC) and High performance liquid chromatography (HPLC). Lipophilicity was performed by the distribution coefficient test. The binding and migration ability of the probe was assessed using the FAP transfected tumor cell line. The radiolabeling yield of [99mTc]Tc-HYNIC-FAPI was (97.29 ± 0.46) %. The radiochemical purity was more than 90% and kept stable until 6 h. The radioligand was shown as lower lipophilicity, of which logD7.4 value was − 2.38 $$\pm$$ ± 0.13. In vitro experiments, the results indicated that the probe showed binding properties, and inhibited the migration of tumor cells. The novel [99mTc]Tc-HYNIC-FAPI probe was successfully radiosynthesized and exhibited good radiochemical purity, stability and in vitro binding ability to tumor cells. The [99mTc]Tc-HYNIC-FAPI will be a promising SPECT/CT imaging probe.

Introduction: Bone metastasis of malignant pheochromocytoma is a rare disease. We report a patient with a 10-year history who underwent 18F-FDG PET/CT to detect bone metastasis and receive radiotherapy and chemotherapy with complete response for bilateral iliac pain.Case presentation: A 48-year-old male patient complained of dizziness, hypertension, and bilateral iliac pain for 2 months. The patient had a history of resection of bilateral malignant adrenal pheochromocytoma 10 years earlier, and all complaints were relieved immediately after operation. 18F-FDGPET/CT showed abdominal lymph node uptake and multiple bone uptake, as well as multiple brown fat uptake. A biopsy of the left ilium confirms the metastasis of malignant pheochromocytoma.Discussion: In our literature review, we discuss the metastasis of pheochromocytoma reported by some scholars, and the role of radionuclides such as 18F-FDG PET/CT, 18F-DOPA PET/CT, I-123MIBG, and 68Ga-DOTATATE PET, in the diagnosis of malignant pheochromocytoma. The patient above is a good case for clinicians in the diagnosis and treatment of metastatic pheochromocytoma, especially in some hospitals with only 18F-FDG imaging agents.Conclusion: A review of this case and similar rare cases in the literature illustrates the importance of 18F-FDG PET/CT in the diagnosis of malignant pheochromocytoma.

Background Fibroblast activation protein (FAP) is higher expressed on cancer-associated fibroblasts (CAFs) in most malignant epithelial neoplasms, which is lower expressed in normal tissues. As a promising small molecular probe, FAP inhibitor (FAPI) shows the specific binding to FAP. This study aimed to explore a novel molecular probe 99mTc-HYNIC-FAPI targeting CAFs. The in vitro characteristics of the probe were also evaluated. Methods The FAPI targeting to FAP was designed, synthesized and conjugated with the chelator 6-hidrazinylnicotinic acid (HYNIC) for radiolabeling with 99mTc. The radiolabeling yield, radiochemical purity and stability were evaluated by Instant thin-layer chromatography (ITLC) and High performance liquid chromatography (HPLC). The binding and migration ability of the probe were assessed using the tumour cell line. Results The radiolabeling yield of 99mTc-HYNIC-FAPI were (97.29 ± 0.46) %. The radiochemical purity was more than 90% and keep stable until 6 h. In vitro experiments, the results indicated that the probe showed binding properties, and inhibited the migration ability of tumor cells. Conclusion The novel 99mTc-HYNIC-FAPI probe was successfully radiosynthesized and exhibited good radiochemical purity, stability and in vitro biding ability to tumor cells. The 99mTc-HYNIC-FAPI will be a promising SPECT/CT imaging probe.