A crucial case for radionuclide-based clinical administration is the synthesis procedure manually or through an automated system. The majority of the therapeutic radiopharmaceuticals are still prepared manually although this process fundamentally causes radiation exposure and risk of contamination (Meyer et al. 2004). An automatically synthesis of radiopharmaceuticals donates standardization, safety dose, stability, reproducibility and high yield (Velikyan et al. 2015). Moreover, this process provides a GMP-compliance production in clinical studies and disposable cassette systems are utilized to prevent cross contamination coming from tubing systems, which leads to an exact sterility and high purity. (Boschi et al 2013).
ML Eazy synthesis device is a fully user-defined system combined by valves, sensors, pump and other equipment. This practical design provides a flexibility option for preparation of various radiopharmaceuticals and it is frequently utilized for [68Ga]Ga and [177Lu]Lu based radiosynthesis (Persico et al. 2020). More recently, Spreckelmeyer et al. has successfully described the synthesis of [68Ga]Ga-FAPI-46 on a ML Eazy synthesis module (Spreckelmeyer et al. 2020). Considerable attention has been devoted to theranostic studies in nuclear medicine, therefore we have developed a fully automated synthesis method for [177Lu]Lu-FAPI-04 and [177Lu]Lu-FAPI-46 on the same module (ML Eazy, Fig. 1). Thus, further multi-center pre-clinical and clinical trials on FAPI based radiopharmaceutical can be easily applied for theranostic purposes in the same commercially available synthesizer.
In our experiments, amount of precursor, pH medium were kept constant due to the previously optimized parameters for well-known [177Lu]Lu-PSMA and DOTATATE synthesis. Table 1 summarizes the results after radiolabeling process. The radiochemical yield was around 85–90% with absolute radiochemical purity (99%). R-HPLC and R-TLC analyses indicated there was trace amount of free and colloidal [177Lu]Lu after completion of the reaction (Fig. 3). Citrate buffer mobile phase was exclusively afforded as a mobile phase and different RF values were well recorded on TLC analysis. All reactions were tried as three times for validation of radiochemical yield and radiochemical purity.
As known that, specific uptake, biodistribution, and longer tumor retention time are vital requirements for an administration of [177Lu]Lu, which is well known therapeutically effective longer-lived radionuclide. For this reason, within the scope of stability studies, radiochemical purity analyzes were comprehensively investigated by R-TLC and R-HPLC for up to 4 days (Fig. 4,5). Stability studies were divided into two parts; in laboratory medium at 24 ºC and in human serum at 37 ºC. First, FAPI-04 and FAPI-46 based compounds were respectively submitted to stability experiments at room temperature. Radiochemical purity results indicated those compounds are highly stable at room temperature up to 4 days confirmed by both R-TLC and R-HPLC analysis (Fig. 4). Similar results were also observed regarding serum stability.