The characteristics of blood-borne radiolabels and the effect of anti-mouse IgG antibodies on localization of radiolabeled monoclonal antibody in cancer patients.

Quantitative and qualitative aspects of the blood survival of 131I and 111In-labeled monoclonal antibody 791T/36 have been examined in patients with colo-rectal carcinoma, ovarian carcinoma and osteogenic sarcoma who were receiving labeled antibody in diagnostic immunoscintigraphy trials. The blood clearance of intact antibody radiolabeled with either 131I or 111In was similar. A bi-phasic decline of both radiolabeled preparations was measured with initial half-lives 0.62 and 0.42 days for 131I and 111In labels and then with 1.85 and 1.40 day half-lives, respectively. The Fab fragment of the antibody was lost more rapidly (initial half-life 0.20 days and then 0.78 days). Blood-borne radioactivity was associated predominantly with plasma rather than cellular elements. Radioactivity was still attached to undegraded, uncomplexed, and immunologically active antibody as demonstrated by molecular filtration, immune precipitation, and antigen binding assays. However, anti-mouse-IgG antibody detected within 7 days of administration of radiolabeled antibody was present for at least 10 mo and has implications for the efficiency of repeated image studies.