Non-dehalogenation mechanisms for excretion of radioiodine after administration of labeled antibodies.

In patients or mice with cancer the pharmacokinetic behavior of radioiodinated and radiometal chelated antibodies has been observed to be different. Rapid clearance from the tissues and excretion into the urine can occur after injection of radioiodinated antibodies. These observations have been interpreted to reflect in vivo dehalogenation of the antibody. This publication describes a variety of other mechanisms that can underlie these phenomena. These mechanisms include receptor uptake and catabolism of antibody and instability of the labeled antibody due to the labeling conditions. Specifically, the relative masses of chloramine-T and antibody in the iodination reaction mixture, the level of iodination of the antibody, and the amount of antibody administered to the recipient are all factors which can influence the clearance of radioiodinated antibody from the recipient. The final determinant for the different behavior of radioiodinated and In-111 metal chelated antibody relate to the different biologic pathways of indium when compared to iodine.