Résumé
Background and Importance: Targeting the tumour microenvironment recently gained interest in oncology, as evidenced by the use of fibroblasts activation protein inhibitors (FAPI) for cancer-associated fibroblasts imaging. Among these derivatives, FAPI-04 radiolabeled with gallium-68 emerged as a promising PET diagnostic agent. To date, [⁶⁸Ga] Ga-FAPI-04 is considered an experimental radiopharmaceutical, with a tedious and intricate radiolabeling process. Thus, the development of a single-vial cold kit (SVCK) formulation of FAPI-04 to simplify the preparation of [⁶⁸Ga] Ga-FAPI-04 would be of particular interest.Aim and Objectives: Various parameters involved in the formulation of FAPI-04 as a SVCK were investigated. Then, optimal conditions for successful radiolabeling of [⁶⁸Ga] Ga-FAPI-04 were identified.Material and Methods: Kit vials were conditioned to contain several bulk agents (five tested), buffers (six tested), anti-radiolysis compound (three tested) and FAPI-04. Mixtures were solubilised in water for injection and then lyophilised. The influence of each component on the radiolabeling process was studied, as well as the amount of vector (30, 45 or 60 µg). [⁶⁸Ga] GaCl3 was eluted from a GalliAD® generator directly into the kit vials, subsequently heated for 10 minutes at 97°C. Radiochemical purity (RCP) of each reaction was assessed by radio-TLC and radio- HPLC. The pH was checked by pH strips during the kit’s conditioning and after each reaction, aiming at an optimal value of 3.4 (ideal for ⁶⁸Ga radiolabeling). Results Mannitol (50 mg) was the bulk agent with the best appearance after freeze-drying and was retained in subsequent assays. As expected, the pH of the reaction medium was critical to the success of radiolabeling. HEPES buffer 0.3 M pH 4 allowed RCP of 83.3% by TLC and 78.9% by HPLC, compared with extremely poor results obtained with the five other buffers. Anti-radiolysis agents showed a moderate improvement in RCP (~10% increase with ascorbic acid) which persisted over a period of 4 hours, confirming radiocomplex stability. Using 45 µg FAPI-04 instead of 30 µg in the reaction slightly increased RCP (>96% in TLC, >91% in HPLC).Conclusion and Relevance: The importance of carefully selecting the ingredients of a radiopharmaceutical SVCK was demonstrated, resulting in excellent RCP values for [⁶⁸Ga] Ga-FAPI-04. A terminal purification step would remove HEPES buffer to comply with European Pharmacopoeia requirements.