New chelating agent for attaching indium-111 to monoclonal antibodies: in vitro and in vivo evaluation

111In possesses excellent radiophysical properties suitable for use in immunoscintigraphy of cancerous tissues when attached to an antitumor antibody. However, 111In has a tendency to accumulate in normal tissues such as liver. Instability of the linkage between 111In and antibody may contribute to this problem. To avoid this, we developed a new bifunctional chelating agent, 1,3-bis[N-[N-(2-aminoethyl)-2-aminoethyl]-2-aminoacetamido]-2-(4- isothiocyanatobenzyl)propane-N,N,N',N'',N''',N'''',N''''',N'''''- octaacetic acid (LiLo), that forms a kinetically stable chelate with metal ions such as indium. Using LiLo, indium-111 was conjugated to a human monoclonal antibody, 16.88. Competitive binding analysis revealed that the 16.88-LiLo conjugate is as immunoreactive as the unconjugated native antibody. This conjugate was compared with 111In-16.88, where diethylenetriaminepentaacetic acid dianhydride (DTPAa) was used as the chelating agent. In vitro stability studies showed that 111In was more stably bound to 16.88-LiLo than to 16.88-DTPA. Biodistribution studies in athymic mice bearing colorectal tumor xenografts indicated less liver retention with 16.88-LiLo than with 16.88-DTPA. These results demonstrate that LiLo is superior to DTPAa for attachment of 111In to the monoclonal antibodies.