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Mouse monoclonal antibody TA99 detects a 70-kDa pigmentation-associated glycoprotein in human melanoma cell lines. The antigen cannot be detected on the cell surface by sensitive rosetting techniques or absorption studies, nor can it be detected as a secreted product in culture fluids. Contrary to expectation, I251-labeled TA" specifically localized to pigmented human melanoma transplants in nu/nu mice; no localization to nonpigmented melanoma or control tumors was found. Tumor imaging was initially obscured by circulating I251-labeled TA99 during the first 6 days after antibody injection. With clearance of sI-labeled TA99 from the blood (half-life, 4-7 days), specific tumor images could be clearly dermed by day 13. Due to the persistence of 125I-labeled TA99 at the tumor site (8.9% of the injected dose at 1 week and 4.6% at 8-10 weeks), images were obtainable for up to 10 weeks. At 8-10 weeks, the tumor/blood ratio was 104-105, and the tumor/normal tissue ratio ranged from 102 to 105. In view of these findings, antibodies detecting intracellular antigens may have a role in tumor imaging and therapy.

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