3.2. Radiosynthesis of [11C]BPTES

During the production of [¹¹C]CO₂, benzylmagnesium chloride (BnMgCl, 1 M in THF, 500 μL) was passed through a polyethylene loop (0.75 mm i.d. and 1.6 mm o.d. × 200 mm) cooled between −5 and 0 °C and previously flushed with N₂ gas [32]. N₂ gas (50 mL/min) was passed through the loop for 30 s to remove the excess BnMgCl solution and to leave a thin film of BnMgCl on the surface of the loop. After irradiation, [¹¹C]CO₂ was carried from the target with a stream of N₂ gas and trapped in the stainless-steel coil cooled by liquid N₂. After trapping of [¹¹C]CO₂, this coil was heated at 50 °C and the released [¹¹C]CO₂ was transferred in an N₂ stream (3.0 mL/min) into the loop coated with BnMgCl. By passing a solution of ClCOOiBu/iPr₂NEt/THF (5 μL/5 μL/200 μL) through the loop containing the Grignard’s reaction mixture, the total mixture was transferred into an empty reaction vessel and let stand for 3 min at room temperature to produce the mixed anhydride [11C]2. After the formation of the mixed [¹¹C]2, a solution of 1/iPr₂NEt/NMP (1.5 mg/5 μL/300 μL) was added and heated at 80 °C for 5 min.

After the reaction, the reaction mixture was diluted by the HPLC solvent (1 mL) and applied onto a semipreparative HPLC column. The preparative HPLC conditions were as follows: column, CAPCELL PAK UG80 C₁₈ (10 mm i.d. × 250 mm, OSAKA Soda, Osaka, Japan); mobile phase, CH₃OH/50 mM ammonium acetate / DMSO (70/30/0.1); flow rate, 5.0 mL/min; retention time (t_R), 7.9 min). The HPLC fraction corresponding to [¹¹C]BPTES was collected in a sterile flask containing polysorbate 80 (75 μL), EtOH (0.3 mL), and 25% ascorbic acid (100 μL). All the solvents were removed under reduced pressure. Physiological saline (5 mL) was added in the flask to dissolve the residue. The resulting solution was sterilized using a Millex-GV filter (Millipore) to obtain [¹¹C]BPTES as a final product.

HPLC analysis was performed to determine radiochemical purity, identity, and molar activity of [¹¹C]BPTES. The analytic conditions were as follows: column, Capcell Pak UG80 C₁₈ (4.6 mm i.d. × 250 mm, OSAKA Soda); mobile phase, CH₃OH/50 mM ammonium acetate/DMSO (70/30/0.1); flow rate: 1.0 mL/min; UV detection, 254 nm; t_R, 8.3 min. The identity of [¹¹C]BPTES was confirmed by co-injection with BPTES. The molar activity was measured and calculated by comparing the assayed radioactivity with the mass measured at UV (254 nm). The logD_7.4 value of [¹¹C]BPTES was measured in an octanol/buffer mixture at room temperature.