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Aim/Introduction: In recent years quinoline-based small molecules targeting the fibroblast activation protein alpha (FAP) have gained interest for imaging a variety of tumor entities (1). A number of radiotracers for SPECT and PET have been developed, but so far only three FAP inhibitor (FAPI) radioligands have been reported to be radiolabeled with fluorine-18 (2,3,4). This study shows the optimization and automation of the radiofluorination of FAPI-74. Materials and Methods: (S)-(4-Carboxymethyl-7-{2-[4-(3-{4-[2-(2-cyano-pyrrolidin-1-yl)-2-oxoethylcarbamoyl]-quinolin-6-yloxy}-propyl)-piperazin-1-yl]-2-oxo-ethyl}-[1,4,7]triazonan-1-yl)-acetic acid, commonly referred to as FAPI-74, was radiolabeled using the [18F]AlF chelation method. Starting from [18F]fluoride, the reaction with AlCl3, chelate formation and subsequent purification was initially optimized by manual syntheses. Optimization included the examination of different anion exchangers (QMA light, PSHCO3) and elution solutions (NaOAc buffer pH4, 0.9% NaCl) as well as careful adjustment of the reaction parameters time (0-20 min), temperature (r.t. to 100°C), amount of AlCl3 and NaOAc buffer pH4, solvents (DMSO, EtOH), and precursor concentration (1-350 µM). Radiochemical conversion (RCC) was determined by radio-UPLC of the crude reaction mixtures. Selected reaction mixtures were analyzed after decay using UPLC-MS to identify non-radioactive byproducts. The optimized radiosynthetic procedure was transferred to a fully automated radiosynthesizer (TRACERlab FXFN) and the final product was purified and formulated using semi-preparative HPLC and SPE. Results: Under optimized conditions, RCC of [18F]AlF-FAPI-74 of > 99% was still observed at precursor concentrations as low as 12 µM FAPI-74 after reaction in a 1:1 molar ratio with AlCl3 in DMSO/sodium acetate buffer at pH 4 at 80°C for 15 minutes. Transfer of optimized conditions and upscaling was successfully achieved and delivered radiochemical pure [18F]AlF-FAPI-74 formulated in EtOH suitable for further preclinical experiments. Work on a more rapid SPE-purification and full characterization according to GMP guidelines is in progress. Conclusion: The radiosynthesis of [18F]AlF-FAPI-74 was optimized and automated, which in the future will allow the production of large quantities and the distribution of this promising radiotracer to other (clinical) centers. References: (1) Altmann A et al. The latest developments in imaging fibroblast activation protein (FAP). J. Nucl. Med. 2021, 62(2) 160-167. (2) Giesel FL et al. FAPI-74 PET/CT Using Either 18F-AlF or Cold-Kit 68Ga Labeling: Biodistribution, Radiation Dosimetry, and Tumor Delineation in Lung Cancer Patients. J. Nucl. Med. 2021, 62(2) 201-207. (3) Jiang X et al. FAPI-04 PET/CT using [18F]AlF Labeling Strategy: Automatic Synthesis, Quality Control, and in vivo Assessment in Patient. Front. Oncol. 2021, 11:649148. |
