Preparation and biological evaluation of ¹¹¹In-, ¹⁷⁷Lu- and ⁹⁰Y-labeled DOTA analogues conjugated to B72.3

Abstract 

Three 1,4,7,10-tetraazacyclododecane-N,N′,Nʺ,Nʺ′-tetraacetic acid (DOTA) analogues were evaluated for relative in vivo stability when radiolabeled with ¹¹¹In, ⁹⁰Y and ¹⁷⁷Lu and conjugated to the monoclonal antibody B72.3. The DOTA analogues evaluated were “NHS-DOTA” [N-hydroxysuccinimdyl (NHS) group activating one carboxylate], “Arm-DOTA” (also known as MeO-DOTA; with a p-NCS, o-MeO-benzyl moiety on the methylene group of one acetic acid arm) and “Back-DOTA” (with a p-NCS-benzyl moiety on a backbone methylene group of the macrocycle). The B72.3 was conjugated to the DOTA analogues to increase the retention time of the radioloabeled conjugates in vivo in mice. The serum stability of the various radiometalated DOTA conjugates showed them to have good stability out to 168 h (all >95% except ¹¹¹In-NHS-DOTA-B72.3, which was 91% stable). Hydroxyapatite stability for the ¹¹¹In and ¹⁷⁷Lu DOTA-conjugates was >95% at 168 h, while the ⁹⁰Y DOTA-conjugates were somewhat less stable (between 90% and 95% at 168 h). The biodistribution studies of the radiometalated DOTA-conjugates showed that no significant differences were observed for the ¹¹¹In and ¹⁷⁷Lu analogues; however, the ⁹⁰Y analogues showed lower stabilities, as evidenced by their increased bone uptake relative to the other two [2–20% injected dose per gram (% ID/g) for ⁹⁰Y and 2–8% ID/g for ¹¹¹In and ¹⁷⁷Lu]. The lower stability of the ⁹⁰Y analogues could be due to the higher beta energy of ⁹⁰Y and/or to the larger ionic radius of Y³⁺. Based on the bone uptake observed, the ¹⁷⁷Lu-NHS-DOTA-B72.3 had slightly lower stability than the ¹⁷⁷Lu-Arm-DOTA-B72.3 and ¹⁷⁷Lu-Back-DOTA-B72.3, but not significantly at all time points. For ⁹⁰Y, the analogue showing the lowest stability based on bone uptake was ⁹⁰Y-Arm-DOTA-B72.3, perhaps because of the metal's larger ionic radius and potential steric interactions minimizing effective complexation. The ¹¹¹In analogues all showed similar biological distributions at the various time points. This study suggests that care must be taken when evaluating ⁹⁰Y-labeled antibodies and in using NHS-DOTA-antibody conjugates with ¹⁷⁷Lu. All evaluations should be extended to time points relevant to the half-life of the radiometal and the therapy applications.