A Novel Method of 18F Radiolabeling for PET

Abstract

Small biomolecules are typically radiolabeled with ¹⁸F by binding it to a carbon atom, a process that usually is designed uniquely for each new molecule and requires several steps and hours to produce. We report a facile method wherein ¹⁸F is first attached to aluminum as Al¹⁸F, which is then bound to a chelate attached to a peptide, forming a stable Al¹⁸F-chelate-peptide complex in an efficient 1-pot process. Methods: For proof of principle, this method was applied to a peptide suitable for use in a bispecific antibody pretargeting method. A solution of AlCl₃·6H₂O in a pH 4.0 sodium-acetate buffer was mixed with an aqueous solution of ¹⁸F to form the Al¹⁸F complex. This was added to a solution of IMP 449 (NOTA-p-Bn-CS-d-Ala-d-Lys(HSG)-d-Tyr-d-Lys(HSG)-NH₂) (NOTA-p-Bn-CS is made from S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid; HSG is histamine-succinyl-glycine) and heated to 100°C for 15 min. In vitro and in vivo stability and targeting ability of the Al¹⁸F-IMP 449 were examined in nude mice bearing LS174T human colonic tumors pretargeted with an anti-CEACAM5 bispecific antibody (TF2). Results: The radiolabeled peptide was produced in 5%−20% yield with an estimated specific activity of 18,500–48,100 GBq (500–1,300 Ci)/mmol. The Al¹⁸F-IMP 449 was stable for 4 h in serum in vitro, and in animals, activity isolated in the urine 30 min after injection was bound to the peptide. Nonchelated Al¹⁸F had higher tissue uptake, particularly in the bones, than the chelated Al¹⁸F-IMP 449, which cleared rapidly from the body by urinary excretion. Tumor uptake was 30-fold higher with TF2-pretargeted Al¹⁸F-IMP 449 than with the peptide alone. Dynamic PET showed tumor localization within 30 min and rapid and thorough clearance from the body. Conclusion: The ability to bind highly stable Al¹⁸F to metal-binding ligands is a promising new labeling method that should be applicable to a diverse array of molecules for PET.