Evaluation of bifunctional chelates for the development of gallium-based radiopharmaceuticals.

Ga radioisotopes, including the generator-produced positron-emitting isotope (68)Ga (t1/2 = 68 min), are of increasing interest for the development of new radiopharmaceuticals. Bifunctional chelates (BFCs) that can be efficiently radiolabeled with Ga to yield complexes with good in vivo stability are needed. To this end, we undertook a systematic comparison of four BFCs containing different chelating moieties: two novel BFCs, p-NO2-Bn-Oxo (1-oxa-4,7,10-triazacyclododecane-4,7,10-triacetic acid) and p-NO2-Bn-PCTA (3,6,9,15-tetraazabicyclo [9.3.1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid), and two more commonly used BFCs, p-NO2-Bn-DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and p-NO2-Bn-NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid). Each BFC was compared with respect to radiolabeling conditions, radiochemical yield, stability, and in vivo clearance properties. p-NO2-Bn-PCTA, p-NO2-Bn-Oxo, and p-NO2-Bn-NOTA were all more efficiently radiolabeled with Ga compared to p-NO2-Bn-DOTA. p-NO2-Bn-DOTA required longer reaction time, higher concentrations of BFC, or heating to obtain equivalent radiochemical yields. Better stability was observed for p-NO2-Bn-NOTA and p-NO2-Bn-PCTA compared to p-NO2-Bn-DOTA and p-NO2-Bn-Oxo, especially with respect to transmetalation to transferrin. Ga-radiolabled p-NO2-Bn-Oxo was found to be kinetically labile and therefore unstable in vivo. Ga-radiolabeled p-NO2-Bn-NOTA and p-NO2-Bn-PCTA were relatively inert, while Ga-radiolabeled p-NO2-Bn-DOTA had intermediate stability, losing >20% of Ga in less than one hour when incubated with apo-transferrin. Similar stability differences were seen when incubating at pH 2. In vivo PET imaging and biodistribution studies in mice showed that (68)Ga-radiolabeled p-NO2-Bn-PCTA, p-NO2-Bn-NOTA, and p-NO2-Bn-DOTA all cleared through the kidneys. While there was no statistical difference in the biodistribution results of (68)Ga-radiolabeled p-NO2-Bn-PCTA and p-NO2-Bn-DOTA, (68)Ga-radiolabeled p-NO2-Bn-NOTA cleared more rapidly from blood and muscle tissue but retained at up to 5 times higher activity in the kidneys.

Evaluation of novel bifunctional chelates for the development of Cu-64-based radiopharmaceuticals.

BACKGROUND: Currently available bifunctional chelates (BFCs) for attaching Cu-64 to a targeting molecule are limited by either their radiolabeling conditions or in vivo stability. With the goal of identifying highly effective BFCs, we compared the properties of two novel BFCs, 1-oxa-4,7,10-triazacyclododecane-S-5-(4-nitrobenzyl)-4,7,10-triacetic acid (p-NO(2)-Bn-Oxo) and 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-S-4-(4-nitrobenzyl)-3,6,9-triacetic acid (p-NO(2)-Bn-PCTA), with the commonly used S-2-(4-nitrobenzyl)-1,4,7,10-tetraazacyclododecanetetraacetic acid (p-NO(2)-Bn-DOTA). METHODS: p-NO(2)-Bn-DOTA, p-NO(2)-Bn-Oxo and p-NO(2)-Bn-PCTA were each radiolabeled with Cu-64 under various conditions to assess the reaction kinetics and robustness of the radiolabeling. Stability of each Cu-64 BFC complex was evaluated at low pH and in serum. Small animal positron emission tomography imaging and biodistribution studies in mice were undertaken. RESULTS: p-NO(2)-Bn-Oxo and p-NO(2)-Bn-PCTA possessed superior reaction kinetics compared to p-NO(2)-Bn-DOTA under all radiolabeling conditions; >98% radiochemical yields were achieved in <5 min at room temperature even when using near stoichiometric amounts of BFC. Under nonideal conditions, such as low or high pH, high radiochemical yields were still achievable with the novel BFCs. The radiolabeled compounds were stable in serum and at pH 2 for 48 h. The imaging and biodistribution of the Cu-64-radiolabeled BFCs illustrated differences between the BFCs, including preferential clearance via the kidneys for the p-NO(2)-Bn-PCTA Cu-64 complex. CONCLUSIONS: The novel BFCs facilitated efficient Cu-64 radiolabeling under mild conditions to produce stable complexes at potentially high specific activities. These BFCs may find wide utility in the development of Cu-64-based radiopharmaceuticals.