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In Vivo Kinetics of Radiolabeled Monoclonal Anti-CEA Antibodies in Animal Models

Veterans Administration Medical Center and University of California, San Diego
Hybritech, Inc., La Jolla, California

Correspondence: For reprints contact: Samuel E. Halpern, MD, Dept. of Nuclear Medicine, VA Medical Center, 3350 La Jolla Village Dr. San Diego, CA 92161.

ABSTRACT

Studies were performed to determine the effect of the radiolabel and circulating carcinoembryonic antigen (CEA)on the pharmacodynamics of monoclonal anti-CEA antibodies (MoAbs).The studies were performed in normal BALB/c mice and in nude mice bearing human colon tumors. Three different tumors were used, each of which produced CEA levels characteristic of that particular tumor's secretory rate. The CEJ-326 MoAb labeled with either ¹¹¹In or ¹²⁵I was used in all studies. arculating CEA induced the removal of ¹²⁵I and ¹¹¹In MoAbs from the vascular compartment Liver concentrations of ¹¹¹In increased and ¹²⁵I levels decreased as the CEA secretory rate of the tumor rose. This indicates that circulating CEA complexes form in the vascular compartment which, in an animal model, are removed by the liver and spleen. This results in decreased tumor uptake of the labeled MoAb. The iodinated MoAb complexes are dehalogenated while the ¹¹¹In is retained by the liver. This dehalogenation may account for the relatively low liver activity observed in radioimmunoimaging with intact radioiodinated anti-CEA MoAbs, provided the CEA complexes are similarly removed from the vascular compartment by the human liver.