Potential Value of FAPI PET/CT in the Detection and Treatment of Fibrosing Mediastinitis: Preclinical and Pilot Clinical Investigation

Fibrosing mediastinitis (FM) is a rare proliferative disease within the mediastinum that leads to pulmonary hypertension, which has been regarded as a major cause of death. This study aims to evaluate the potential value of fibroblast activation protein inhibitor (FAPI)-PET/CT in the integration of diagnosis and treatment of FM through targeting FAPI in fibrosis rats and provide a theoretical basis for clinical management of FM patients. By performing a ¹⁸F-FAPI PET/CT scan, the presence of FAPI-avid in the fibrotic lesion was determined. Through a fibrosis rat model, ¹⁸F-FAPI-74 was used for lesion imaging and ¹⁷⁷Lu-FAPI-46 was utilized to investigate the potential therapeutic effect on FM in vivo. In addition, biodistribution analysis and radiation dosimetry were carried out. With the ¹⁷⁷Lu-FAPI-46 pharmacokinetic data of rats as the input, the estimated dose for female adults was computed, which can provide some useful information for the safe application of radiolabeled FAPI in the detection and treatment of FM in patients. Then, major findings on the use of FAPI PET/CT and SPECT/CT in FM were presented. ¹⁸F-FAPI-74 showed a high-level uptake in FM lesions of patients (SUVmax 7.94 ± 0.26), which was also observed in fibrosis rats (SUVmax 2.11 ± 0.23). Consistently, SPECT/CT imaging of fibrosis rats also revealed that ¹⁷⁷Lu-FAPI-46-avid was active for up to 60 h in fibrotic lesions. In addition to this robust diagnostic performance, a possible therapeutic impact was evaluated as well. It turned out that no spontaneous healing of lesions was observed in the control group, whereas there was complete healing on day 9, day 11, and day 14 in the 30, 100, and 300 MBq groups, respectively. With a significant difference in the free of event rate in the Kaplan-Meier curve among four groups (P < 0.001), a dose of 300 MBq displayed the best therapeutic effect, and no obvious damage was observed in the kidney. Furthermore, organ-absorbed doses and an effective dose (0.4320 mSv/MBq) of ¹⁷⁷Lu-FAPI-46 presumed for patients were assumed to give a preliminary indication of its safe use in clinical practice. In conclusion, ¹⁸F-FAPI-46 PET/CT can be a potentially valuable tool for the diagnosis of FM. Of note, ¹⁷⁷Lu-FAPI-46 may be a novel and safe radiolabeled reagent for the integration of diagnosis and treatment of FM.