Radiolabeling and Stability of FAP- Seeking Radiopharmaceuticals for Radio-Molecular Imaging and Therapy

1129 Introduction: Cancer-associated fibroblast activation protein is a high potential target for radio-molecular imaging and therapy. This protein is expressed on cancer-associated fibroblasts. The concentration in normal tissue is low. A few quinoline-based PET tracers have been developed which act as FAP inhibitors (FAPIs). The study of the biodistribution of 68Ga labeled FAPIs showed an uptake which is comparable with 18F-FDG but also a significant washout effect between 1-3 h after injection. Another approach, namely the use of FAP-affine peptides such as the peptide FAP-2286, opens the opportunity for radio-molecular therapy due to significantly longer residence times. Furthermore, the PET tracers based on FAP-2286 seem to show a higher tumor uptake compared to 18F-FDG. The purpose of this study was to evaluate the synthesis and stability of 68Ga and 177Lu labeled DOTA-conjugated peptide FAP-2286 to implement the radiopharmaceutical production in the clinical routine for imaging and treatment for patients with different tumor diseases. Methods: Radiolabeling of 3BP-2286 with 68Ga:The peptide 3BP-2286 was labeled with 68Ga using the NaCl based labeling procedure as previously described in (1). For the automated radiopharmaceutical production, a Modular-Lab PharmTracer Modul (Eckert&Ziegler) was used as described in (2). In detail, up to four 68Ge/68Ga generators were eluted and the eluate (1.2 - 2.6 GBq) was passed through a preconditioned SCX-cartridge. The collected 68Ga was subsequently eluted using 0.5 mL of 5 M NaCl, spiked with 12.5 µL 5.5 M of HCl. The 68Ga-eluate of the SCX-cartridge was then added to a solution of 150 µg 3BP-2286, 5 mg L-ascorbic acid and 1.2 mg of L-methionine dissolved in 350 µl 1 M sodium acetate buffer (adjusted with HCl and acetic acid to pH=4.5) and 2.3 mL of water for injection. The reaction mixture was incubated for 8 minutes at 95 °C. After labeling the mixture was diluted with 2 mL of water for injection, neutralized using 2 mL of sterile sodium phosphate buffer (BRAUN-Melsungen) and sterile filtered. Samples were taken for quality control, endotoxin test and test for sterility. The labeling efficiency and the radiochemical purity was determined by iTLC and HPLC, respectively. Radiolabeling of 3BP-2286 with 177Lu: The peptide 3BP-2286 was labeled with 177Lu using the following procedure: The automated labeling was carried out using an SYKAM-synthesis module (SYKAM-Germany). To a solution of 12.7 GBq of 177Lu in 300 µL 0.05 M HCl a solution of 10 mg L-ascorbic acid, 5 mg L-methionine, and 600 µg 3BP-2286 in 1 mL 1 M sodium acetate buffer (adjusted with HCl to pH=5.5) was added. The mixture was incubated for 35 minutes at 95 °C. After the labeling reaction, the radiochemical purity was determined by HPLC which is integrated into the automated SYKAM-synthesis module. The mixture was diluted with 0.9 % saline solution and sterile filtered. Samples were taken for quality control, endotoxin test and test for sterility. Results: The radiolabeling yield of 68Ga labeled FAP-2286 was 80 % (n.d.c.) and the radiochemical purity was consistently higher than 95 % as determined by iTLC and HPLC, respectively. The addition of the radical scavengers L-methionine and L-ascorbic acid can prevent radiolysis. No significant formation of byproducts could be determined within two hours. After the labeling of FAP-2286 with 177Lu the radiochemical purity was higher than 98 % as determined by HPLC. Side products could be determined by HPLC after 24 hours. Conclusions: The radiopharmaceutical production of 68 Ga- and 177Lu-labeled FAP-2286 could be easily included in the clinical routine for diagnosis and therapy for patients with different tumor diseases. Furthermore, we could show that the radiopharmaceuticals were sufficiently stable for in-house applications. Acknowledgments: The DOTA-conjugated peptide was provided by 3B Pharmaceuticals GmbH. We acknowledge the team of 3B Pharmaceuticals GmbH for supporting this study.