Tumor pretargeting: role of avidin/streptavidin on monoclonal antibody internalization

Abstract

Radioimmunodetection of tumor can be improved by introducing a two-step system in which radiolabeled streptavidin is administrated after the injection of a biotinylated monoclonal antibody (MAb) (two-step) or radiolabeled biotin is injected after biotinylated MAb and avidin (three-step). The anti-carcinoembryonic antigen (CEA) MAb FO23C5 has been recently exploited in a three-step protocol based on the avidin-biotin system. The anti-folate receptor (FR) MAb MOv18 has proven suitable for radioimmunodetection of ovarian cancer using directly radiolabeled MAb or in a two-step method. In this study, we analyzed the suitability of MOv18 in a three-step protocol in ovarian carcinoma patients and the internalization events after formation of the MOv18-avidin complex.

Methods: Selected patients with documented metastatic lesions were enrolled in a three-step radioimaging analysis with biotinylated MOv18 and FO23C5, avidin and (111)In-labeled biotin. Two-step internalization experiments were conducted in vitro with MOv18 and MOv19 MAbs on the FR-overexpressing IGROV1 cell line and with the anti-CEA MAb FO23C5 on the LS174T cell line. Cells were incubated sequentially with biotinylated MAb and 125I-labeled streptavidin or with 125I-biotinylated MAb and cold streptavidin.

Results: In the in vivo study, SPECT revealed the majority of metastatic lesions in patients injected with biotinylated MOv18; however, the tumor-to-background ratio was relatively low. In the in vitro study, a consistent internalization was induced by antigen-biotinylated MAb-streptavidin complex formation at the cell surface in both antigenic systems analyzed. However, the extent of internalization was lower in the CEA model.

Conclusion: The internalization ability of avidin suggests its potential clinical application for delivering toxic agents in a two-step approach (biotinylated MAb + avidin conjugate). The suitability of a given MAb for three-step clinical applications (biotinylated MAb + avidin + biotin) should be previously investigated by using appropriate in vitro experiments.