This longitudinal study aims to evaluate the performance of 68 Ga-FAPI-04 and 18 F-FDG and to profile the dynamic process of tumor metastasis in a preclinical 4T1 breast cancer model. Although both of these two radioligands are wildly used in clinic, no study was reported on their performance in the longitudinal monitoring of tumor metastasis. Also, no correlation between the expression level of fibroblast activation protein (FAP) and the development of tumor metastasis has been elucidated previously. In this study, we evaluated the performance of 68 Ga-FAPI-04 and 18 F-FDG PET during the entire process of tumor metastasis, and their potential for the early diagnosis of tumor metastasis. We also clarified the correlation of uptakes as well as the signal-to-background (S/B) ratios between these two probes at different stages of tumor metastasis.Methods: Forty 4T1 metastatic breast cancer murine model were established using female BALB/c mice, followed by the longitudinal imaging with 68 Ga-FAPI-04 and 18 F-FDG once a week for up to six weeks. In vitro Hematoxylin & Eosin (H&E) and immunochemistry (IHE) staining were performed to evaluate FAP expression on the metastatic lesions. Further statistical analysis was performed to evaluate the correlation of 68 Ga-FAPI-04 and 18 F-FDG uptake (%ID/cc) at different stages of the metastasis.Results: 68 Ga-FPAI-04 holds an advantage over 18 F-FDG with higher sensitivity at the early stage of tumor metastasis. However, with the progress of tumor metastasis, uptake of 68 Ga-FAPI-04 decreases and becomes less sensitive than 18 F-FDG. There is also no direct correlation between uptake or S/B ratios of 68 Ga-FAPI-04 and 18 F-FDG during this dynamic process. Conclusion:68 Ga-FAPI-04 is more sensitive than 18 F-FDG in detecting the early stage of tumor metastasis, but becomes less sensitive than 18 F-FDG at the late stage of tumor metastasis. We envision this result would be meaningful for the explanation of the 18 Ga-FAPI-04 and 18 F-FDG imaging both in the future clinic and preclinic studies.

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Purpose: 68Ga-labeled fibroblast activation protein inhibitors, such as [68Ga]Ga-DOTA-FAPI-04 and [68Ga]Ga-DOTA-FAPI-46, have been successfully applied in positron emission tomography imaging of various tumor types. To broaden the PET tracers of different positron nuclides for imaging studies of FAP-dependent diseases, we herein report the radiosynthesis and preclinical evaluation of two 11C-labeled FAP inhibitors, 11C-RJ1101 and 11C-RJ1102. Methods: Two phenolic hydroxyl precursors based on a quinoline amide core coupled with a 2-cyanopyrrolidine moiety were coupled with [11C]CH3I to synthesize 11C-RJ1101 and 11C-RJ1102. In vivo small-animal PET and biological distribution studies of 11C-RJ1101 and 11C-RJ1102 compared to [68Ga]Ga-DOTA-FAPI-04 were conducted in nude mice bearing U87MG tumor xenografts at 30, 60, and 90min, respectively. Results: 11C-RJ1101 and 11C-RJ1102 were synthesized in over 15% radiochemical yields, with specific activities of 67 GBq/μmol and 34 GBq/μmol, respectively, at the end of synthesis and radiochemical purities greater than 99%. In U87MG tumor xenograft PET studies, the three tracers experienced higher specific uptake at the tumor site. However, because of significant differences in metabolism and clearance, [68Ga]Ga-DOTA-FAPI-04 experienced high uptake in the kidney, whereas 11C-RJ1101 and 11C-RJ1102 showed high uptake in the liver and intestine. Biodistribution studies revealed significant hepatobiliary excretion of 11C-RJ1101 and 11C-RJ1102. 11C-RJ1102 showed higher specific tumor uptake in U87MG xenografts (1.71 ± 0.08% injected dose per Gram of tissue [ID/g]) than 11C-RJ1101 (1.34 ± 0.10%ID/g) and [68Ga]Ga-DOTA-FAPI-04 (1.29 ± 0.04%ID/g) after 30 min p. i. In orthotopic glioma models, the uptake values were 0.07 ± 0.03% ([68Ga]Ga-DOTA-FAPI-04) and 0.16 ± 0.03% (11C-RJ1102), respectively. Conclusion: 11C-RJ1101 and 11C-RJ1102 are interesting candidates for translation to the clinic, taking advantage of the shorter half-life and physical imaging properties of C-11.