Reduction - Mediated 99mTc ·· Labeling of Antitumor Monoclonal Antibodies : Effect of Increasing Specific Activity on Antibody Binding Kinetics
VIPA BOONKITTICHAROEN, Ph.D.*, PUTHIPORN CHOUPL YWECH, B.S,:. *, DUANGPEN PUCHINDA, B.Sc. *, WIRA WA T JALA YONDEJA, B.Sc. *
Affiliation : * Division of Nuclear Medicine, Department of Radiology, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Bangkok 10400, Thailand.
Abstract
Reduction-mediated 99mTc-labeling of antibodies has gained widespread acceptance in
preparation of tumor imaging agents. Increased specific activity to enhance detection signals has
raised the question of whether such an attempt would cause change in antibody binding kinetics.
To answer this question, two antitumor monoclonal antibodies, i.e. IOR-CEA (lgG 1) and EMD
(lgG2a) were labeled with 99mTc to yield specific activities ranging from 549-4414 MBq/mg.
Regression analysis of the binding data revealed that the binding kinetics of IOR-CEA were shifted
from monovalent to bivalent binding upon increasing the specific activities. This phenomenon of
affinity enhancement was confirmed by the dissociation study where we found soluble CEA had
greater difficulty in extracting the cell-bound IOR-CEA labeled at higher specific activity. The
bivalent binding was further supported by the finding that IOR-CEA with higher specific activities
delivered less than expected radioactivity to tumor targets despite their immunoreactivities being
well preserved.
For EMD, the kinetics seemed to be shifted from bivalent to monovalent interaction. At
higher specific activities, adverse changes in immunoreactivity were recognized. Breakage of
EMD into 99mTc-Fab fragments was likely to occur and was supported by the observation
that EMD delivered more than expected radioactivities to target cells upon increasing specific
activity. Precaution should be taken when one deals with high specific activity labeling since this
might alter the antibody binding kinetics either favorably or unfavorably.
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