THE MECHANISM OF HEPATIC-UPTAKE OF A RADIOLABELED MONOCLONAL-ANTIBODY

Clinical and experimental scintigraphic studies have found that radiolabelled antibodies are not only taken up by tumour(s) but also by normal liver. The accumulation of radionuclides in this organ poses a major problem to the use of radiolabelled antibodies as diagnostic and therapeutic tools. In an attempt to understand the mechanism of hepatic uptake and clearance of radiolabelled antibodies, the intrahepatic biodistribution of an In-111-labelled MAb (HMFG1), was determined following i.v. administration to normal male rats. Two hours after administration the liver contained 15% of the injected dose, with most of the remaining radioactivity in the blood. The hepatic burden of the In-111 MAb remained constant over the next 72 hr in the face of decreasing blood levels of radioactivity as well as its urinary and faecal excretion. At 2 and 72 hr after injection, 50% and 10% respectively of the hepatic radiolabel was due to blood borne antibody. Following a collagenase-cell isolation procedure, only 23% of the amount remaining in the liver at 2 hr was found to be cell-associated; 66% was lost during the cell isolation and purification procedure. Cellular uptake increased with time so that, by 72 hr after administration, 58% was cell-associated and 29% freely removable. At all timepoints, the parenchymal cells contained more activity than non-parenchymal cells. No evidence of antibody-receptor interactions could be obtained either in vivo or in cultures of hepatic parenchymal and non-parenchymal cells. Our data suggest that the bulk of the hepatic burden of In-111 MAb results from extravascular pooling of the antibody.