Improved Conjugation, 64-Cu Radiolabeling, in Vivo Stability, and Imaging Using Nonprotected Bifunctional Macrocyclic Ligands: Bis(Phosphinate) Cyclam (BPC) Chelators

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
α-amines were more suitable than alkyl amines, e.g., Lys ω-amine
(pKa ∼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.