Local distribution and concentration of intravenously injected 131I-9.2.27 monoclonal antibody in human malignant melanoma.

Regional measurements of 131I-9.2.27 distribution in human melanoma tumors were obtained using quantitative autoradiography. Tumors were removed from patients 72-96 h after they had received an i.v. injection of 9.15 mCi (100 mg) of 131I-9.2.27. The autoradiographic images showed that the radioactivity reaching the tumor was heterogeneously distributed. Areas of relative high and low uptake were selected in each tumor. Regions of high activity contained from 51 to 1371 nCi/g, while areas with low uptake had radioactivity ranging from 12 to 487 nCi/g. The reliability of the autoradiographic measurements was demonstrated by the strong positive correlation with direct tissue sample counting (r = 0.994 P less than 0.001). Since comparative immunocytochemistry showed a homogeneous and diffuse staining of target antigen on viable tumor cells, variability of monoclonal antibody uptake within individual tumors was not primarily due to heterogeneity of antigen expression in these cases. However, antigen levels accounted for some of the variation from tumor to tumor. When immunoperoxidase staining was repeated on adjacent sections without the addition of 9.2.27, it confirmed the nonuniform distribution of monoclonal antibody found at autoradiography. Thus, quantitative autoradiography gives information about the distribution and the local concentration of radioactive antibody in tumors allowing calculation of the radiation dose delivered to small regions within tumors.