Biological disposition of intravenously administered 131I-labeled anti-EGF-receptor antibody (RG 83852) in the rat.

RG 83852 is a murine monoclonal antibody that preferentially inhibits the high-affinity binding of epidermal growth factor (EGF) to its receptor. Since overexpression of EGF receptor has been implicated in some human malignancies, the antibody is under investigation as a potential anticancer agent. The present work characterized the tissue distribution and elimination of 131I-labeled antibody in rats following i.v. administration. 131I-RG 83852 was given in a 2.22 mg/kg dose to rats, and 4, 24, 48, and 72 h afterwards 131I activity excreted in the urine and feces and that present in various tissues was determined. The plasma contained the highest concentration of radioactivity at all times. At 4 h the plasma contained about 12% of the injected dose (ID)/ml, and radioactivity in this compartment accounted for almost 70% ID. The plasma elimination of 131I-derived activity occurred linearily at a rate of about 0.48% ID/h. Except in the thyroid, the concentration of 131I activity in all tissues was much lower than in the plasma (tissue-to-plasma ratio < or = 0.1). In the thyroid, accumulation of radioactivity (4% ID at 24 h) was presumably due to trapping of 131I released from the antibody as a result of biodegradation. The urinary excretion occurred at a rate of about 0.5% ID/h; the fecal excretion was minimal. The biodistribution results are consistent with the protein structure of the antibody. Based on the available disposition data, it is proposed that elimination of the antibody involves degradation, a process that follows zero-order kinetics, followed by excretion of the labeled product(s) in the urine.