Comparative in Vivo Stability of Copper-64-Labeled Cross-Bridged and Conventional Tetraazamacrocyclic Complexes

The increased use of copper radioisotopes in radiopharmaceutical applications has created a need for bifunctional chelators (BFCs) that form stable radiocopper complexes and allow covalent attachment to biological molecules. The chelators most commonly utilized for labeling copper radionuclides to biomolecules are analogues of 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA); however, recent reports have communicated the instability of the radio-Cu(II)-TETA complexes in vivo. A class of bicyclic tetraazamacrocycles, the ethylene "cross-bridged" cyclam (CB-cyclam) derivatives, form highly kinetically stable complexes with Cu(II) and therefore may be less susceptible to transchelation than their nonbridged analogues in vivo. Herein we report results on the relative biological stabilities and identification of the resulting radiolabeled metabolites of a series of ⁶⁴Cu-labeled macrocyclic complexes. Metabolism studies in normal rat liver have revealed that the ⁶⁴Cu complex of 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane ( ⁶⁴Cu-CB-TE2A) resulted in significantly lower values of protein-associated ⁶⁴CU than ⁶⁴Cu-TETA [13 ± 6% vs 75 ± 9% at 4 h]. A similar trend was observed for the corresponding cyclen derivatives, with the ⁶⁴CU complex of 4,10-bis(carboxymethyl)-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane ( ⁶⁴Cu-CB-DO2A) undergoing less transchelation than the ⁶⁴Cu complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (⁶⁴Cu-DOTA) [61 ± 14% vs 90.3 ± 0.5% protein associated ⁶⁴Cu at 4 h]. These data indicate that the structurally reinforcing cross-bridge enhances in vivo stability by reducing metal loss to protein in both the cyclam and cyclen cross-bridged ⁶⁴Cu complexes and that ⁶⁴Cu-CB-TE2A is superior to ⁶⁴Cu-CB-DO2A in that regard. These findings further suggest that a bifunctional chelator derivative of CB-TE2A is a highly desirable alternative for labeling copper radionuclides to biological molecules for diagnostic imaging and targeted radiotherapy.