Monoclonal antibodies penetrate bulky tumors poorly after intravenous administration, in part because of specific binding to the target antigen. Experiments presented here demonstrate an analogous phenomenon in micrometastases; poor antibody penetration, attributable to a "binding-site barrier" phenomenon, can be seen in guinea pig micrometastases as small as 300 ,um in diameter. Increasing the dose of antibody can partially overcome this limitation, but at a cost in specificity. Six and 72 hr after injection, animals were sacrificed by CO2 inhalation. Organs were rapidly removed and weighed, and tissue radioactivity was determined. Lung tissues with metastases were frozen immediately in OCT compound (Tissue-Tek, Miles) and then processed for autoradiography and immunostaining. For high-dose experiments, 1000 ,ug of unlabeled D3 was added to 30 ,ug of labeled D3. Autoradiography and Immunostaining. Serial 20-,um sections were cut from the frozen lung tissues. For autoradiography, sections were placed on SB-5 film (Eastman Kodak) and exposed for 1-4 days. When 125I-labeled BL3 was coinjected with I311-labeled D3, images of the latter were obtained shortly after sacrifice, images of the former after eight 131I half-lives.The avidin-biotin peroxidase complex (ABC) method (29) was used to compare Ab and Ag distribution. Briefly, tissue sections were acetone-fixed for 10 min and incubated with blocking serum for 20 min. To assess Ag expression, sections were incubated with D3 Ab (20 ,ug/ml) for 1 hr at room temperature, biotinylated horse anti-mouse IgG for 45 min, ABC for three 30-min periods, and diaminobenzidine (Vector Laboratories). When injected D3 was assessed, incubation with D3 was omitted. Some sections were also stained for blood vessels by use of polyclonal rabbit anti-factor VIII and an alkaline phosphatase substrate, Vector Red (30).
RESULTS AND DISCUSSIONIn this study, we have assessed the distribution of i.v. administered mAbs in experimental guinea pig lung micrometastases. Two weeks after i.v. injection of L10 cells, multiple metastases 100-1000 ,um in diameter could be seen in the lungs of all animals. This model (metastasis of guinea pig cancer in guinea pigs) is in many ways more relevant to the Abbreviations: Ab, antibody; mAb, monoclonal Ab; Ag, antigen.