Preparation of [61Cu]DTPA complex as a possible PET tracer

Copper-61 (T_1/2 = 3.33 h) produced via the ⁶⁴Zn(p, α)⁶¹Cu nuclear reaction, using a natural zinc target, was separated from the irradiated target material by two ion exchange chromatography steps and was used for the preparation of [⁶¹Cu]- diethylenetriaminepentacetate ([⁶¹Cu]DTPA) using freshly-prepared DTPA cyclic dianhydride. An electroplated natural zinc layer on a gold-coated copper backing was irradiated with 22 MeV protons (22.12 MeV on the target, 180 μA irradiation, 3.2 h, final activity 220 GBq of ⁶¹Cu ²⁺, RCY > 95%, radionuclidic purity > 99%, ⁶⁰Cu as the only radionuclidic impurity; T_1/2 = 23.7 min). Colorimetric methods showed that traces of chemical impurities in the product were below the accepted limits. The solution of [⁶¹Cu]DTPA was prepared with a radiochemical yield of more than 80% starting with ⁶¹CuOAc ligand at room temperature after 30 min. RTLC showed the radiochemical purity of more than 99%. The specific activity obtained was about 9.1 TBq/mmol. The tracer was shown to be stable in the final product and in the presence of human serum at 37°C up to 3 h.