Rationale and objectives: Monoclonal antibody 81C6 reacts with the extracellular matrix antigen, tenascin, present on gliomas and other tumors, as well as several normal tissues, including spleen and liver tissue. Single photon emission computed tomography (SPECT) and I-123-labeled 81C6 at various protein doses were used to maximize tumor to normal tissue uptake ratios.
Methods: The distribution of I-123-labeled monoclonal antibody 81C6 was determined in 16 patients with recurrent gliomas, using SPECT: Between 3.5 and 11.5 mCi of I-123 were administered to each patient, and the antibody doses were between 10.0 and 100.0 mg. Blood was obtained for pharmacokinetic studies, and patients were imaged 1 hour and 18 hours after antibody administration.
Results: All tumors were visualized readily on the SPECT study in areas that corresponded to the contrast, enhancing abnormalities on anatomic neuroimaging studies. The half-life in blood of the I-123 81C6 ranged from 16 to 37 hours. Radiation dosimetry calculations suggest that it might be possible to administer more than 700 cGy to intracranial glioma with I-131 labeled 81C6 under optimal conditions with acceptable non-neurologic organ radiation exposure.
Conclusions: SPECT imaging with I-123 81C6 identified all tumors and suggests that, with this antibody, more favorable tumor-to-liver and tumor-to-spleen radiation dose ratios are obtained at higher protein doses.