Preclinical evaluation of FAP-targeted PET imaging to investigate CAF responses to radiotherapy

Background Cancer-associated fibroblasts (CAFs) are influential elements of the tumor microenvironment with significant roles in tumor progression and therapy resistance. However, the effects that radiotherapy has on CAFs dynamics remains understudied. This study aimed to provide a non-invasive PET imaging approach to quantify radiation-induced changes in cancer-associated fibroblasts in preclinical tumor models. Methods The FAP-specific radiotracer [ ¹⁸ F]AlF-FAPI-74 was used to monitor CAF dynamics following precision external beam radiotherapy (RT) in two syngeneic subcutaneous murine tumor models (LLC and CT26). Tumors were irradiated via two radiation regimens (1x12 Gy or 2x6 Gy), and PET/MR imaging was performed 7 days after RT. Additionally, the dynamics of FAP + CAFs in tumors were quantified ex vivo via flow cytometry and immunohistochemistry. Results Tumor-targeted irradiation led to a significant reduction in tumor size. Uptake of [ ¹⁸ F]AlF-FAPI-74 in subcutaneous tumors was low but significantly above muscle-background values. Quantification of standardized uptake values (SUVs) from static PET images revealed a twofold increase in the PET signal in LLC tumors irradiated with two fractions of medium-dose RT (2x6 Gy). Ex vivo analysis confirmed the low abundance of FAP + cells in tumors and demonstrated similar RT-induced changes in CAF levels across the different models. Conclusions Our findings suggest that CAFs constitute a relatively small cell population in subcutaneously transplanted tumor models, and that fractionated radiotherapy may induce a moderate increase in FAP ⁺ cells in LLC tumors. Additionally, we demonstrated that [ ¹⁸ F]AlF-FAPI-74 is a reliable biomarker for evaluating the number of FAP + stromal cells in tumors and for addressing potential therapy-induced changes in CAFs.