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

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 Y 3+. 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.