124I-huA33 Antibody Uptake Is Driven by A33 Antigen Concentration in Tissues from Colorectal Cancer Patients Imaged by Immuno-PET

A33 is a cell surface glycoprotein abundantly expressed in 95% of colon carcinomas and by epithelial cells of the normal intestine but not by other normal tissues (1–3). The A33 antigen is a member of the immunoglobulin superfamily, with homology to cell adhesion and tight junction–associated proteins (4,5). The antigen has been purified from human colon cancer cells, the protein sequence determined, the complementary DNA cloned, and the mouse homolog identified (4,6). Because of its restricted tissue localization and high level of expression, the A33 antigen is appealing as a therapeutic target, and a variety of anti-A33–based approaches are under preclinical investigation as potential therapies for colon cancer (7–11). Early clinical studies (12,13) were performed with murine anti-A33 monoclonal antibody radiolabeled with 131I or 125I to assess its potential for radioimmunotherapy. Among the findings of these studies was that antibody localization at primary and metastatic tumor sites was extremely persistent, with significant retention beyond 6 wk after administration, whereas the initially high uptake in normal intestine gradually diminished with time. The applicability of murine A33 was restricted by the development of an antimouse IgG immune response and, in an attempt to overcome this limitation, a fully humanized complementarity determining region-grafted A33 IgG1 (huA33) was developed (14). However, subsequent studies showed that repetitive administrations of huA33 could elicit a human antihuman antibody response (15). Humanized antibody A33 (huA33) has been shown to be equivalent to the mouse antibody in competitive binding assays and localization studies in animal models (14) and can be radioiodinated with retention of immunoreactivity. The relatively long half-life (4.2 d) of 124I allows PET to be performed for up to a week after the administration of 124I-labeled antibody (16), with radiation doses to normal tissues comparable with those of the 131I-labeled antibody. It has been shown that quantitative noninvasive imaging of 124I is possible using a dedicated PET system (17,18). We performed a clinical study of 124I-huA33 PET in patients with colorectal cancer. In 15 patients, for whom surgery was prescheduled as a standard of care, a PET/CT scan was acquired approximately 1 wk after antibody administration, immediately before the surgical removal of tumor and elements of neighboring normal tissues. Surgically excised tissues were subsequently processed for ex vivo quantification of antibody uptake, antigen density determination, digital autoradiography (DAR), and histologic or immunohistochemical staining. In this paper, we describe the findings of this study, focusing on the ex vivo measurements and their clinical implications. The clinical PET/CT is described only to the extent that it has an impact on these. A more comprehensive description of the clinical imaging study of 124I-huA33 is provided elsewhere (19).