Monoclonal antibody-dendrimer conjugates enable radiolabeling of antibody with markedly high specific activity with minimal loss of immunoreactivity

For the purpose of radioimmunotherapy, labelling of monoclonal antibody with high specific activity is often necessary, especially when using a radionuclide with a shorter half-life. Polyamine dendrimers (PAMAM) are novel synthetic polymeric molecules with large numbers of amine residues on their spherical surface. In order to bind large numbers of radiometals to single antibody molecules, the generation-4 PAMAM (G4), which has 64 amines, was conjugated with 43 molecules of 2-(p-isothiocyanatobenzyl)-6-methyl-diethylene triamine penta-acetic acid (1B4M), a derivative of DTPA. This product [G4-(1B4M)(43)] was then conjugated with OST7, a murine monoclonal IgC(1). We evaluated the achievable specific activity for In-111 labeling, immunoreactivity, biodistribution, and tumor targeting in mice of the In-111- or Gd-153-OST7-G4-(1B4M)(43) as compared with radiolabeled OST7-1B4M or 56C-1B4M. The maximum specific activity of In-111-OST7-G4-(1B4M)(43) and In-111-OST7-1B4M was 470 and 8.7 GBq/mg (12,700 and 263 mCi/mg), respectively. Immunoreactivity of radiolabeled OST7-G4-(1B4M)(43) and OST7-1B4M, as determined by the binding to KT005 cells expressing the antigen, was respectively 91% and 84% of that of I-125-labelled OST7, Biodistribution studies for preparations with maximum specific activity in normal mice 3 h after injection showed that In-111- or Gd-153-OST7-C4(1B4M),, cleared faster from the blood and accumulated more in the liver than did In-111- or Gd-153-OST7-1B4M. The dendrimer 1B4M [G4-(1B4M)(64)] itself showed similar saturation effects with metals. The radioactivity in all the other organs reflected the rapid clearance of radioactivity from the blood. Gd-153-OST7-G4-(1B4M)(43) showed specific accumulation ill the KT005 tumor. In conclusion, we could successfully bind 49 times as many metal atoms to an antibody molecule as is possible with conventional metal labeling for indium and gadolinium, and did so with minimal loss of immunoreactivity. When we achieved radiolabeling with maximum specific activity, Gd conjugate showed better biodistribution than In conjugate.