Both in clinical routine and in preclinical research, the established standard procedure for the final purification of radiometal-labeled peptide radiopharmaceuticals is cartridge-based reversed-phase (RP) solid phase extraction (SPE). It allows the rapid and quantitative separation of the radiolabeled peptide from hydrophilic impurities and easy integration into automated synthesis procedures. However, product elution from RP cartridges necessitates the use of organic solvents and product recovery is sometimes limited. Thus, an alternative purification method based on commercially available size exclusion cartridges was investigated.
Since most peptide radiopharmaceuticals have a molecular weight > 1 kDa, Sephadex G10 cartridges with a molecular size cut-off of 700 Da were used for the final purification of a broad palette of ⁶⁸ Ga-, ⁶⁴ Cu- and ^99m Tc-labeled experimental peptide radiotracers as well as the clinically relevant ligand PSMA-617. Results (radiochemical purity (RCP, determined by ITLC), recovery from the solid support) were compared to the respective standard RP-SPE method. Generally, retention of unreacted ⁶⁸ Ga, ⁶⁴ Cu and ^99m Tc salts on the G10 cartridges was quantitative up to the specified elution volume (1.2 mL) for ⁶⁸ Ga and ^99m Tc and 99.6% for ⁶⁴ Cu. Even at increased elution volumes of 1.5-2 mL, RCPs of the eluted ⁶⁸ Ga- and ^99m Tc -radiopeptides were > 99%. For all peptides with a molecular weight ≥ 2 kDa, product recovery from the G10 cartridges was consistently > 85% upon respective adjustment of the elution volume. Product recovery was lowest for [ ⁶⁸ Ga]Ga-PSMA-617 (67%, 1.2 mL to 84%, 2 mL). The pH of the final product solution was found to be volume-dependent (1.2 mL: pH 6.3; 1.5 mL: pH 5.9; 2 mL: pH 5.5). Notably, the G10 cartridges were reused up to 20 times without compromising performance, and implementation of the method in an automated radiosynthesis procedure was successful.
Overall, size exclusion purification yielded all peptide radiopharmaceuticals in excellent radiochemical purities (> 99%) in saline within 10-12 min. Although product recovery is marginally inferior to classical SPE purifications, this method has the advantage of completely avoiding organic solvents and representing a cost-effective, easy-to-implement purification approach for automated radiotracer synthesis.