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

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 remain 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. The FAP-specific radiotracer 18FAlF-FAPI-74 was used to monitor CAFs dynamics following precision external beam radiotherapy (RT) in two syngeneic subcutaneous murine tumor models (LLC and CT26). Tumors were irradiated via two radiation regimens (1 × 12 Gy or 2 × 6 Gy), and dynamic 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. Tumor-targeted irradiation led to a significant reduction in tumor size. Uptake of 18FAlF-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 (2 × 6 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. 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 18FAlF-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.