Molecular imaging reveals the heterogeneous progression of tumor cells and tumor stroma: a practice of FDG PET and FAPI PET in diagnosing PSMA-negative bone metastases of progressive prostate cancer.

Tumors are often with complex and heterogeneous biological processes, such as glycometabolism and fibrosis, which are the main biochemical pathways that determine therapeutic effects. Specifically, this study aims to assess the diagnosing performance of ¹⁸F-FDG and ⁶⁸Ga-FAPI-04 PET for different stages of progressive bone metastases with PSMA-negative pathology. Bone metastatic mouse model of prostate cancer was constructed via intra-bone injection of PSMA-negative prostate cancer PC3 cells. Cellular uptakes of ¹⁸F-FDG and ⁶⁸Ga-FAPI-04 were separately performed on PC3, NIH-3T3 (FAP-positive) and a mixture. ⁶⁸Ga-PSMA-11, ¹⁸F-FDG and ⁶⁸Ga-FAPI-04 PET/CT imaging were performed at 2, 4 weeks after tumor cell transplantation. Furthermore, PSMA and FAP expression in bone metastases were assessed by immunohistochemistry, and then compared with the imageological findings. On the cellular level, the independent tracer uptake on the basis of glycometabolism and fibrosis was observed. For animal imaging, ⁶⁸Ga-PSMA-11 imaging showed weak or absent tracer uptake in PSMA-negative bone metastatic lesions. In contrast, ⁶⁸Ga-FAPI-04 PET of bone metastases had a higher uptake and tumor-to-muscle (T/M) ratio than ¹⁸F-FDG PET that was relative steady during the observation, but T/M ratio of fibrosis gradually decreased with increasing tumor growth, which ranged from 5.11 ± 1.26 at 2 weeks to 3.54 ± 0.23 at 4 weeks, revealing the delayed formation of tumor stroma in rapid proliferation. In addition, PET imaging results were corroborated by immunohistochemical assessment. In conclusion, molecular imaging approach revealed the heterogeneous progression of tumor cells and tumor stroma of bone metastasis of prostate cancer, and further confirming the necessity of multi-molecular imaging in cancer imaging.