Uptake and Metabolism of n'In-labeled Monoclonal Antibody B6.2 by the Rat Liver1

When '"In-labeled murine monoclonal antibodies are used in radioscintigraphic diagnostic procedures, a large fraction of the injected radionuclide is sequestered by the liver. Neither the cells responsible for the uptake nor the mechanism of uptake are known. Little is known about either the site within the liver of antibody metabolism or the form of the products of metabolism. In these studies, the uptake and metabolism of a monoclonal antibody, B6.2 radiolabeled with '"In or '"I (either intact B6.2 or F(ab')2| were determined in rats. One h after injection of either '"Ior '"In-diethylenetriaminepentaacetic acid ('"In-DTPA)-labeled B6.2, the predominant liver cell in which the radionuclide was found was the parenchyma! cell. At this time, the absolute uptake of I25Iin the liver was 0.23 ±0.06% (SD) of the injected dose compared to 0.61 ±0.06% when the radionuclide was '"In. Removal of the Fc portion of the antibodyreduced the absolute liver uptake of '"I to 0.10 ±0.01 and the absolute uptake of "'In to 0.16 ±0.06. Both radionuclides were still associated predominantly with the parenchyma! cell. Using size exclusion high performance liquid chromatography analysis of liver supernatants the metabolism of radiolabeled B6.2 was followed for 24 h. Of the radioac tivity recovered, 47.9% of the '"I was precipitable by centrifugation (and presumed bound to cell membranes) while 15.4% was attached to B6.2 found in the cytosol. In contrast, when '"In-DTPA-B6.2 was adminis tered, 16.0% of "'In recovered from the liver was precipitable by cen trifugation, and 6.5% was attached to B6.2 found in the cytosol. Sixty % of the "'In was recovered as a low molecular weight (less than 1000) component in the cytosol. This metabolite was not immunoreactive, nor did it comigrate with ferritin, and was resolved into four components by ion exchange high performance liquid chromatography. Of these, only a minor component cochromatographed with an '"In-DTPA standard. These data suggest that the large accretion of radionuclide by the liver is due to uptake of monoclonal antibodies by an Fc receptor-mediated mechanism and the subsequent accumulation of low molecular weight metabolites, presumably '"In-DTPA, attached to one or more amino acids. The reasons for the entrapment of metabolites in the liver are under investigation.