Direct site-specific radiolabeling of an affibody protein with 4-[18F]fluorobenzaldehyde via oxime chemistry

Purpose: In this study, we introduce a methodology for preparing F-18-labeled Affibody protein, specifically F-18-Anti-HER2 dimeric Affibody (14 kDa), for in vivo imaging of HER2neu with positron emission tomography (PET). Procedures: We have used 4-[F-18] fluorobenzaldehyde as a synthon to prepare F-18-Anti-HER2 Affibody. Aminooxy-functionalized Affibody (Anti-HER2-ONH2) was incubated with 4-[F-18] fluorobenzaldehyde in ammonium acetate buffer at pH 4 in the presence of methanol at 70 C for 15 min. The resulting F-18-labeled Affibody molecule was evaluated as a PET probe in xenograft models expressing HER2. Results: We have successfully prepared F-18-Anti-HER2 dimeric Affibody (14 kDa), N-(4-[F-18]fluorobenzylidine)oxime-Anti-HER2 Affibody, [F-18]FBO-Anti-HER2, in 26-30% radiochemical yields ( decay corrected). High-contrast small-animal PET images with relatively moderate tumor uptake (1.79 +/- 0.40% ID/g) were observed for the F-18-Anti-HER2 Affibody. Conclusion: Site-specific F-18-labeled Affibody against HER2 has been synthesized via chemoselective oxime formation between an aminooxy-functionalized Affibody and F-18-fluorobenzaldehyde. The results have implications for radiolabeling of other affibodies and macromolecules and should also be important for advancing Affibody imaging with PET.