In vitro and in vivo antitumor activity of immunoconjugates prepared by linking 5-fluorouridine to antiadenocarcinoma monoclonal antibody.

5-Fluorouridine (5-FUr), a cytotoxic antitumoral agent not in clinical use because of its systemic toxicity, and AR-3, a monoclonal antibody specific to a human colorectal adenocarcinoma, were covalently linked via two different strategies. 5-FUr was 5' succinilated after protection of the secondary hydroxyl groups and the carboxylate derivative was then activated as N-hydroxysuccinimidyl ester in order to react with the amino groups present in the monoclonal antibody, giving an amide linkage. Alternatively, a 5-FUr immunoconjugate containing an acid-cleavable hydrazone bond was formed from the reaction between an acyl hydrazide derivative of 5-FUr and a periodate oxydized antibody with approximately 12 aldehyde groups in its carbohydrate region. An average of 9 to 12 drug molecules were attached to the antibody. In a cytotoxic assay on the human colorectal carcinoma cell line HT-29, the hydrazone containing drug conjugate was equally active as the succinylamido conjugate and the free drug. However, ELISA showed that while in the case of the succinylamido conjugate the Mab immunoreactivity was not affected after conjugation, there was a significant loss of reactivity in the acid cleavable conjugate. In a model of a disseminated intraabdominal carcinomatosis by HT-29 intraperitoneal graft in nude mice, the 5-FUr immunoconjugate selected was more effective than the unconjugated drug in medium-term therapy (21 days after the graft and 16 days after drug treatment), albeit in the longer period the efficacy of the two formulations was similar. The toxic effect of the drug-conjugate in vivo was much weaker, demonstrating its clear advantage over the drug, in terms of pharmacological efficacy.