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

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 outcome 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. <b>Methods:</b> TEFAPI-06 and TEFAPI-07 were synthesized and labeled with ⁶⁸Ga, 86Y and ¹⁷⁷Lu 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 ¹⁷⁷Lu-TEFAPI-06 and ¹⁷⁷Lu-TEFAPI-07 have been evaluated and the comparative study with ¹⁷⁷Lu-FAPI-04 has also been performed in pancreatic cancer PDX-bearing mice. <b>Results:</b> The binding affinity (Kd) to FAP of ⁶⁸Ga-TEFAPI-06 and ⁶⁸Ga-TEFAPI-07 is 10.16 ± 2.56 nM and 7.81 ± 2.28 nM, respectively, which were comparable with that of ⁶⁸Ga-FAPI-04. Comparative PET imaging of HT-1080-FAP and HT-1080 tumor–bearing mice and blocking study showed the FAP targeting ability in vivo of these two tracers. Compared with ¹⁷⁷Lu-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 ¹⁷⁷Lu-TEFAPI-06 and ¹⁷⁷Lu-TEFAPI-07 have been observed, while the control groups and the group treated by ¹⁷⁷Lu-FAPI-04. <b>Conclusion:</b> Two albumin binder-conjugated FAPI radiopharmaceuticals have been developed and evaluated in vitro and in vivo. Notably improved tumor uptake and retention have been observed compared to the original FAPI tracer. Both ¹⁷⁷Lu-TEFAPI-06 and ¹⁷⁷Lu-TEFAPI-07 showed remarkable growth inhibition to PDX tumors while the side effect is negligible, showing that they are promising for further clinical translational studies.

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