Chimeric B72.3, composed of the V-regions of murine B72.3 and the constant regions of human immunoglobulin G4 heavy and Klight chain, was administered as a 131I-lahck-d conjugate to 12 patients with metastatic colon cancer. Seven of these patients had an antibody response after initial infusion, and the immune response was primarily directed to the murine V-region, although a small proportion of the antibody response was directed to topographical epitopes requiring the presence of both murine V-region and human CH-1 and Kconstant regions (neo-epitopes). The pharmacokinetics included a plasma disappearance curve best fit by a two-compartmental model with an a t1/2 âo€f¢18 Â±7 h and a, i r,. of 224 Â±66 h. A second infusion of the same dose of I3ll-chimeric B72.3 was administered to four of these patients 8 wk after the first infusion. Two patients who had a high antibody response to initial infusion had an anamnestic antibody response, and the infused ch-B72.3 rapidly disap peared from the circulation with associated immune complexes and free 131I in the plasma. One patient with no initial antibody response had no antibody response and identical pharmacokinetics on second infusion. One patient with a modest transient antibody response to initial infusion had no antibody response on second infusion and a modest shortening of plasma circulation. Thus, the human immunoglobulin G4 isotype chimeric B72J monoclonal antibody has a plasma half-life 6 to 8 times as long as murine B72.3 and retains considerable immunogenicity in some patients which can adversely affect repetitive infusions. © 1991, American Association for Cancer Research. All rights reserved.