Transferrin conjugates of triazacyclononane-based bifunctional NE3TA chelates for PET imaging: Synthesis, Cu-64 radiolabeling, and in vitro and in vivo evaluation

Abstract

Three different polyaminocarboxylate-based bifunctional NE3TA (7-[2-[carboxymethyl)amino]ethyl]-1,4,7-triazacyclononane-1,4-diacetic acid) chelating agents were synthesized for potential use in copper 64-PET imaging applications. The bifunctional chelates were comparatively evaluated using transferrin (Tf) as a model targeting vector that binds to the transferrin receptor overexpressed in many different cancer cells. The transferrin conjugates of the NE3TA-based bifunctional chelates were evaluated for radiolabeling with 64Cu. In vitro stability and cellular uptake of 64Cu-radiolabeled conjugates were evaluated in human serum and prostate (PC-3) cancer cells, respectively. Among the three NE3TA–Tf conjugates tested, N-NE3TA–Tf was identified as the best conjugate for radiolabeling with 64Cu. N-NE3TA–Tf rapidly bound to 64Cu (>98% radiolabeling efficiency, 1min, RT), and 64Cu–N-NE3TA–Tf remained stable in human serum for 2days and demonstrated high uptake in PC-3 cancer cells. 64Cu–N-NE3TA–Tf was shown to have rapid blood clearance and increasing tumor uptake in PC-3 tumor bearing mice over a 24h period. This bifunctional chelate presents highly efficient chelation chemistry with 64Cu under mild condition that can be applied for radiolabeling of various tumor-specific biomolecules with 64Cu for potential use in PET imaging applications.