Effect of isotype on the biodistribution of radioiodinated immunoglobulins

A series of monoclonal antibodies of different isotypes developed against the same arsenate hapten were labeled by the iodogen bead method and employed to define the importance of isotype on biodistribution. Studies were performed with IgG1, IgG2a, IgG2b, IgG3, and IgM (pentamer). An ELISA assay performed before and after labeling demonstrated retained antigen combining activity. Groups of 6 rats each were injected with 12-18 μg/kg antibody and killed 1.5, 24, 48, 72 and 144 hours later. The concentration of radio-iodine in blood, heart, lung, liver, spleen, kidney, stomach, GI tract, and skeletal muscle were determined and expressed as a percent injected dose/organ. The mean half-time of blood clearance was less than 24 hours for all isotypes. Retention in the blood at 24 hours averaged 28% of the injected dose/organ for the IgG but only 9.4% with IgM. No focal localization was observed in any organ. Whole body counts indicated a 2 component clearance with an initial half-time between 48 and 72 hours for IgG, while IgM had a half-time of less than 24 hours. These data suggest that IgG isotypes have similar biodistributions, while IgM has a shorter biological half-time