%0 Journal Article %A Jyoti Roy %A Elaine Jagoda %A Falguni Basuli %A Colleen Olkowski %A Xiang Zhang %A Karen Wong %A Carolyn woodroofe %A Rolf Swenson %A Peter Choyke %A Frank Lin %T Fluorine-18 labeled fibroblast activation protein alpha targeting ligand: In vitro and in vivo studies. %D 2021 %J Journal of Nuclear Medicine %P 1468-1468 %V 62 %N supplement 1 %X 1468Background: Cancer-associated fibroblasts cells (CAFs) are one of the major cell types constituting the tumor microenvironment (TME). CAFs support tumor growth and development by modulating extracellular matrix, producing immunosuppressive chemokines/cytokines, etc. Collectively these factors contribute towards poor drug penetration and diminished therapeutic efficacy. Fibroblast activation protein alpha (FAP) has emerged as an important biomarker of CAFs. Overexpression of FAP on CAFs and its limited expression on healthy cells have persuaded the development of various FAP-targeted imaging and therapeutic agents. Herein we report on the synthesis and validation (in vitro and in vivo) of fluorine-18 labeled small molecule ligand targeting FAP. Since the current fluorine-18 PET agents cannot identify CAFs burden in a tumor, thus developing a FAP-targeted F18 PET agent can potentially serve as an imaging biomarker. Methods: Fluorine-18 labeled FAP targeted ligand ([18F]FAPL) was prepared by an indirect radiolabeling method using 6-[18F]Fluotonicotinic acid-2,3,5,6-tetrafluorophenyl ester. [18F]FAPL- was evaluated in vitro to determine binding affinity for mouse and human FAP in 3T3-mFAP(transfected with mouse FAP) and HT1080-huFAP (transfected with human FAP) respectively. For in vivo studies female Balb/c nude mice bearing HT1080-huFAP tumor were injected with 50 μCi (i.v) and biodistribution was performed at 15, 30, 60, and 120 min post-injection. For the blocking study HT1080-huFAP tumor bearing mice were injected with [18F]FAPL (50 µCi, i.v) in the presence of an excess of non-radioactive FAP ligand (FAPL), and biodistribution performed at 60 min post-injection. Radioactivity associated with the tissues/organs were determined by gamma counter and used to calculate %injected dose/g (%ID/g), tissue:blood ratio, and tissue:muscle ratios. Results: The overall radiochemical yields of [18F]FAPL were 32-40% (n=30, decay corrected) in 45 min with a radiochemical purity > 98%. [18F]FAPL exhibited binding affinity (Kd) of 4 nM-7 nM for both mouse and human FAP, indicating that the agent recognizes both mouse and human FAP. In vitro uptake of [18F]FAPL in 3T3-mFAP and HT1080-huFAP was blocked in the presence of an excess of FAPL, indicating FAP mediated uptake of [18F]FAPL in the cells. In vivo in HT1080-huFAP tumors showed the highest uptake compared to other organs. Tumor [18F]FAPL %ID/g ranged from 3.7-8.9 % ID/g and declined over 120 min. Whereas, compared to earlier time-points, 120 min biodistribution showed the highest tumor:blood (60.7) and tumor:muscle (67) ratios. A blocking study showed that tumor uptake of [18F]FAPL at 60 min, was blocked in the presence of an excess of FAPL whereas its accumulation in other organs could not be blocked, indicating FAP mediated accumulation in tumors. Conclusions: In vitro and in vivo studies demonstrate that Fluorine-18 labeled FAP targeting ligand [18F]FAPL can be successfully used as a PET imaging agent to determine the tumor burden of FAP positive cells. %U