Albumin Binders for Tumor Enhanced FAPI Radioligand Retention

72 Objectives: Great success in fibroblast activation protein (FAP) inhibitor imaging of different types of cancer makes FAP associated cancer therapy be of great hopefulness. 68Ga-FAPI-04 PET examination has been applied to the cancer patients in clinical applications and gained expected results. However, short blood half-life and poor tumor retention of FAPI-04 is still a limitation to efficient radionuclide therapy. An established means of improving drugs9 pharmacokinetic properties is to use albumin as a drug carrier. In this study, using FAPI-04 as a model FAP inhibitor, we synthesized a series of new long-acting FAPI-04 derivatives (denoted as TEFAPI) based on the albumin binder. Methods: The FAP conjugate TEFAPI could be radiolabeled with 68GaCl3, 86YCl3, and 177LuCl3. The blood half-life of TEFAPI in mice was measured via 68Ga-TEFAPI dynamic PET imaging. Binding affinities of the TEFAPI for FAP were measured with membrane competition binding assays. Long time stability was monitored by HPLC. In vivo small-animal 86Y-TEFAPI PET imaging was conducted from the patient-derived xenograft (PDX) model. The therapeutic anticancer effect of 177Lu-TEFAPI (3.7MBq, three times) in PDX models was investigated in different groups of mice (n=7) by monitoring tumor volume, mice weight and the survival time of treated mice. Results: TEFAPI radiolabeled at a specific activity of 40 GBq/μmol, a radiochemical yield of more than 98%, and stability of more than 95% over 7 d in PBS. The blood half-life of TEFAPI was extended to 400 min compared with 26 min of FAPI-04. A long blood half-life resulted in high tumor retention in the PDX model. It showed high tumor uptake and clear background in 36 h post-injection PET imaging. The SUVmax of the tumor was 1.58 ± 0.12 while the SUVmax of the kidney was 0.03 ± 0.01 at 36 h after injection. Lu-177 radionuclide therapy (3.7 MBq) revealed almost complete remission of tumors and a significantly prolonged survival time, compared with the control group. Conclusion: TEFAPI with albumin binder modification represented long blood half-life and enhanced tumor retention with low background uptake. The therapy effect of TEFAPI made it a promising tracer for targeted therapy of tumors with a high content of activated fibroblasts.