Albumin Binder–Conjugated Fibroblast Activation Protein Inhibitor Radiopharmaceuticals for Cancer Therapy

Abstract

Fibroblast activation protein (FAP) has become an attractive target for diagnosis and therapy, and a series of FAP inhibitor (FAPI) based radiotracers have been developed and performed excellent diagnosis outcomes in clinical applications. Yet, their fast clearance and insufficient tumor retention have hampered their further clinical applications for cancer treatment. In this study, we developed two albumin binder-conjugated FAPI radiotracers, TEFAPI-06 and TEFAPI-07. They are derived from FAPI-04, and optimized by conjugating two types of well-studied albumin binders, 4-(p-iodophenyl) butyric acid moiety (TEFAPI-06) and truncated Evans blue moiety (TEFAPI-07), to try to overcome the above limitations at the expense of prolonging the blood circulation.Methods: TEFAPI-06 and TEFAPI-07 were synthesized and labeled with 68 Ga, 86 Y and 177 Lu successfully. A series of cell assays were performed to identify the binding affinity and FAP specificity in vitro. PET imaging, SPECT imaging and biodistribution study were performed to evaluate the pharmacokinetics in the pancreatic cancer patient-derived xenografts (PDX) animal models. The cancer treatment efficacy of 177 Lu-TEFAPI-06 and 177 Lu-TEFAPI-07 have been evaluated in pancreatic cancer PDX-bearing mice. Results:The binding affinity (Kd) to FAP of 68 Ga-TEFAPI-06 and 68 Ga-TEFAPI-07 is 10.16 ± 2.56 nM and 7.81 ± 2.28 nM, respectively, which were comparable with that of 68 Ga-FAPI-04. Comparative PET imaging of HT-1080-FAP and HT-1080 tumor-bearing 2 mice and blocking study showed the FAP targeting ability in vivo of these two tracers.Compared with 177 Lu-FAPI-04, PET imaging, SPECT imaging and biodistribution studies of TEFAPI-06 and TEFAPI-07 have demonstrated their remarkably enhanced tumor accumulation and retention, respectively. Notable tumor growth inhibitions of 177 Lu-TEFAPI-06 and 177 Lu-TEFAPI-07 have been observed, while the control groups and the group treated by 177 Lu-FAPI-04. Conclusion:Two albumin binder-conjugated FAPI radiopharmaceuticals have been developed and evaluated in vitro and in vivo. Significantly improved tumor uptake and retention have been observed compared to the original FAPI tracer. Both 177 Lu-TEFAPI-06 and 177 Lu-TEFAPI-07 showed remarkable growth inhibition to PDX tumors while the side effect is almost negligible, showing that they are promising for further clinical translational studies.purpose of the study is to evaluate whether the modification improves tumor retention in vivo, and which albumin binder matches FAPI molecules better. A series of detailed experiments and comparisons including cell binding assay, PET imaging, biodistribution and radiotherapy study have been performed. And the results demonstrated the high FAP binding affinity and specificity, the enhanced tumor retention and the improved radiotherapy efficacy of these two albumin binders conjugated FAPI radiotracers. MATERIALS AND METHODS Ligands and RadionuclidesThe synthesis route and chemical characterization of TEFAPI-06 and TEFAPI-07 were described in the s...

Rational Design and Pharmacomodulation of Protein-Binding Theranostic Radioligands for Targeting the Fibroblast Activation Protein

“…The albumin-binding motif (ABM) for the in vivo “albumin hitchhiking” approach has been used as a reliable strategy for increasing the tumor uptake of radiopharmaceuticals, which may help bolster the use of small-molecule drugs for long-acting therapeutics. − For this purpose, the p -iodophenyl moiety, truncated Evans blue (EB), ibuprofen, and fatty acids have been developed as albumin-binding entities. , Typically, based on the albumin-binding effect, the long-circulating PSMA/folate-targeted radiotracers with appropriate albumin binders offer distinct advantages, including low background interference, enhanced tumor uptake, prolonged tumor residence, and better therapeutic effect. − , Most recently, some studies have attempted to pharmacomodulate FAP-targeted radioligands and have gained considerable headway. , …”

Fibroblast activation protein-based theranostics in pancreatic cancer

“…However, small sample populations and certain disparities among studies have led to difficulties in drawing definitive conclusions. Given the widely reported potential for oncological diagnosis, application of the [ 68 Ga]Ga-FAPI-04 probe, for example, has been reported in clinical trials (NCT04554719 and NCT04605939) with the uptake of [ 68 Ga]Ga-FAPI-04 in benign pancreatic lesions being demonstrated (64). Nevertheless, in low [ 18 F]F-FDG-avid tumors, such as in PAAD, [ 68 Ga]Ga-FAPI PET/CT shows high sensitivity in detecting primary pancreatic tumors, involved lymph nodes, and metastases, and is superior in terms of TNM staging ( 65), thereby indicating the potential utility of [ 68 Ga]Ga-FAPI-PET/CT as an effective imaging tool.…”

Research progress on the role of fibroblast activation protein in diagnosis and treatment of cancer

“…Compared to FAPI-04, tumor uptake and retention were improved. Both 177 Lu-TEFAPI-06 and 177 Lu-TEFAPI-07 can significantly inhibit the growth of patient-derived tumor xenograft tumors [65]. Similarly, they modified FAPI-04 with two fatty acids, lauric acid and palmitic acid, to obtain FAPI-C12 and FAPI-C16 precursors.…”