Several monoclonal antibodies (MAbs), reactive with tumor-associated antigens, selectively persist on tumor sites in vivo for many days. If biotinylated, such highly specific tags on tumor cells could become targets for radioactive avidin, administered after suitable intervals. The proposed strategy is based on a number of assumptions concerning the ability of avidin t o preserve i t s biological properties in heterologous in vivo environments, on i t s lack of toxicity and on i t s biodistribution. A preliminary study has been carried out in rabbits, using biotinylated nitrocellulose and polystyrene targets. The results of this study indicate that in rabbits I ) avidin can be administered i.v. and i.p. without adverse reactions, 2) it does not show any preferential localization, 3) it is eliminated with a biological half-life of 24 hr, 4) i t s biological properties are not impaired by in vivo conditions, since it accumulates at biotinylated targets only, 5) CEA-bearing targets can be biotinylated in vivo by biotin-labelled anti-CEA MAbs and 6) the biotin-avidin chain can be further extended in vivo since bound avidin is still able t o bind biotinylated radioactive proteins.Several MAbs used in immunoscintigraphy of human tumors persist a long time on the surface of tumors, giving the best localization ratios at 72-96 hr after their administration (e.g., Buraggi et al., 1987). A much slower loss of labelled antibodies from tumor sites as compared to non-specific sites has been documented in both animals and humans (Colcher et al., 1984(Colcher et al., , 1987. Surgical specimens removed 10-20 days after administration still bear evidence of antibody with very high selectivity for tumor cells. Although these antibodies represent a very minute fraction of the injected dose, they can effectively tag the tumor if they are labelled with long-lived isotopes, such as 1251, a property that has been exploited to develop radio-immuno-guided surgery (Martin et al., 1985).Our purpose was to develop a method to post-label anti-tumor MAbs in vivo, exploiting their differential persistence at tumor sites. Our strategy is to biotinylate MAbs, to inject them cold and to leave an appropriate interval before the administration of radioactive avidin. The aim of the study is to evaluate if the very high affinity and specificity of the avidin-biotin reaction (Green, 1975) can be exploited in vivo in laboratory animals, as it has been widely exploited in vitro in a variety of diagnostic applications. MATERIAL AND METHODS Material and animalsImmuno-affinity-purified CEA from an hepatic metastasis, MAb F023C5 (reactive with a protein epitope of CEA) and F023C5-coated beads were from SORIN Biomedica. Pure hen egg avidin and biotinyl-eaminocaproic acid Nhydroxysuccinimide ester and MAb L16B8 (reactive with hen egg lysozyme) were from Societh Prodotti Antibiotici. Na13'I (specific activity 16 Ci/mol) and ' "InC13 were purchased from ORIS/CEA, Saclay (France) and Amersham (UK), respectively. Outbred rabbits were used for all experiments. Dep...

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.

No abstract

Ten patients with bulky brain glioblastoma, recurring after surgery, radiotherapy or chemotherapy, underwent direct intralesional radioimmunotherapy (RIT) using a monoclonal antibody (MAb), BC-2, raised against tenascin and labelled with 131I. Tenascin, the BC-2-recognized glycoprotein, is an antigen expressed by the stroma of malignant gliomas but not by normal cerebral tissue. Preliminary studies in animals have demonstrated the ability of anti-tenascin radiolabelled MAbs to detect and reduce tumours. A mean MAb dose of 1.93 mg (corresponding to 551.3 MBq of 131I) was injected directly into the tumour by means of a stereotaxic technique. Both systemic and local toxicity were negligible. After 24 hr, average tumour BC-2 uptake was 4.9% per gram and its effective half-life in neoplastic tissue was 66.5 hr: a mean radiation dose to target tissue of 36.48 cGy per MBq of injected 131I was delivered. Normal brain tissue and the major organs were spared. Most patients underwent multiple injections, reaching a cumulative tumour radiation ranging from 7,000 to 41,000 cGy. RIT failed to achieve any result in 4 of the 10 patients; in 3, the disease was stabilized; in the remaining 3, CT scan or NMR revealed 2 partial remission (greater than 50% reduction in tumour volume; PR) and I complete remission (CR). One patient with PR relapsed after II months; the other 2 patients were still maintaining their responses at the time of writing, 17 (CR) and 12 (PR) months after injection.

No abstract