Improved Conjugation, 64-Cu Radiolabeling, in Vivo Stability, and Imaging Using Nonprotected Bifunctional Macrocyclic Ligands: Bis(Phosphinate) Cyclam (BPC) Chelators
Abstract
Bifunctional derivatives of bis(phosphinate)bearing cyclam (BPC) chelators bearing a carboxylate, amine, isothiocyanate, azide, or cyclooctyne in the BP side chain were synthesized. Conjugations required no protection of phosphinate or ring secondary amine groups.
The ring amines were not reactive (proton protected) at pH < ~8. For isothiocyanate coupling, oligopeptide N-terminal alpha-amines were more suitable than alkyl amines, e.g., Lys ω-amine (pK(a) ~7.5-8.5 and ~10-11, respectively) due to lower basicity.
The Cu-64 labeling was efficient at room temperature (specific activity ~100 GBq/μmol; 25 °C, pH 6.2, ~100 ligand equiv, 10 min). A representative Cu-64-BPC was tested in vivo showing fast clearance and no nonspecific radioactivity deposition.
The monoclonal anti-PSCA antibody 7F5 conjugates with thiocyanate BPC derivative or NODAGA were radiolabeled and studied in PC3-PSCA tumor bearing mice by PET. The radiolabeled BPC conjugate was accumulated in the prostate tumor with a low off-target uptake, unlike Cu-64-labeled NODAGA-antibody conjugate.
The BPC chelators have a great potential for theranostic applications of the Cu-64/Cu-67 matched pair.