Efficient method for site-specific ¹⁸F-labeling of biomolecules using the rapid condensation reaction between 2-cyanobenzothiazole and cysteine

An efficient method based on a rapid condensation reaction between 2-cyanobenzothiazole (CBT) and cysteine has been developed for ¹⁸F-labeling of N-terminal cysteine-bearing peptides and proteins. An ¹⁸F-labeled dimeric cRGD ([¹⁸F]CBTRGD₂) has been synthesized with an excellent radiochemical yield (92% based on radio-HPLC conversion, 80% decay-corrected, and isolated yield) and radiochemical purity (>99%) under mild conditions using ¹⁸F-CBT, and shown good in vivo tumor targeting efficiency for PET imaging. The labeling strategy was also applied to the site-specific ¹⁸F-labeling of a protein, Renilla lucifierase (RLuc8) with a cysteine residue at its N-terminus. The protein labeling was achieved with 12% of decay-corrected radiochemical yield and more than 99% radiochemical purity. This strategy should provide a general approach for efficient and site-specific ¹⁸F-labeling of various peptides and proteins for in vivo molecular imaging applications.