Intraperitoneal radio‐localization of tumors pre‐targeted by biotinylated monoclonal antibodies

We describe 2‐step and 3‐step strategies for intraperitoneal tumor radio‐localization by means of monoclonal antibodies (MAbs). Nude mice bearing intraperitoneal human colon carcinoma tumors were injected i.p. with biotinylated MAb AUAI, followed 24 hr later by radioiodinated streptavidin (2‐step). The uptake of radioactivity in tumor and normal tissues was measured 4 hr after injection of radioactive compound. A 3‐step strategy consisted in administering biotinylated antibody, cold avidin after 24 hr and ¹¹¹In‐labelled biotin after a further 4 hr; mice were then killed 2 hr later. Tumor localization of intraperitoneally‐administered biotinylated antibody and direct targeting of radioactive streptavidin to biotinylated antibody bound to tumor sites were demonstrated using immunohistochemistry and autoradiography. Our results show that (i) the 2‐step approach increased the percentage of radioactivity uptake by tumor with respect to directly labelled antibodies (24% vs. 6%) and improved the tumor/non‐tumor ratio; (ii) the 3‐step approach allowed faster blood clearance of the radioactive probe (¹¹¹In‐biotin) and yielded high tumor/non‐tumor ratios. “Pre‐targeting” methods appear to have advantages over the conventional 1‐step approach with directly radiolabelled antibody.