A Conjugate of 5-Fluorouridine-Poly(L-lysine) and an Antibody Reactive with Human Colon Carcinoma

The ribose moiety of 5-fluorouridine (FUR) was oxidized with periodate and the product was bound through a poly(L-lysine) bridge to monoclonal antibodies, denoted SF25MAb, reactive with a human colon carcinoma LS180. The antibody was linked via its polysaccharide (previously oxidized with periodate) to the poly(L-lysine)-drug conjugate. The linking of FUR-poly(L-lysine) to the antibody markedly increased the latter's binding to the tumor cells. A relatively lower increase was also observed with conjugates of nonrelated antibodies, such as anti-hepatitis B surface antigen and anti-epidermal growth factor receptor antibodies. The pharmacological activity of the specific conjugate FUR-poly(L-lysine)-SF25MAb was higher than that of the drug-substituted polymer alone. The poly(L-lysine) bridge caused toxic effects in vivo, even though substituted both by FUR and by antibody. Therefore, the additional unreacted lysyl residues were blocked by succinylation. Partial blocking of free amino groups on the conjugate rendered it nontoxic but decreased its cell-binding capacity, though to a level still higher than that of the original unmodified antibody. The pharmacological activity of the specific conjugate after blocking was also reduced and necessitated prolonged incubation periods or higher concentrations. Following periodate oxidation and reduction, FUR was as effective as the clinically preferred compound 5-fluoro-2'-deoxyuridine in vitro and in vivo, against the LS180 colon carcinoma. Experiments in nude mice, with LS180 tumor subcutaneous xenotransplants, showed that FUR-poly(L-lysine)-SF25MAb (blocked by succinylation) was not toxic and was effective in the retardation of tumor growth. In a metastatic form of the colon carcinoma, the conjugate caused a marginal delay in the median survival time as well as the prevention of the development of metastases in a small but consistent number of mice. Intraperitoneal injection of the specific conjugate (unblocked by succinylation) was not toxic but was also not effective. The drug-substituted polymer (without the antibody) was not therapeutically effective in the succinylated form. However, in the unblocked form, it diminished the size of an already existing tumor when injected directly into the tumor site.