TARGETING OF MURAMYL DIPEPTIDE TO MACROPHAGES BY GELATIN CONJUGATION TO ENHANCE THEIR IN-VIVO ANTITUMOR-ACTIVITY

This work describes the active targeting of a muramyl dipeptide (MDP) to macrophages (M phi) by the chemical modification with gelatin. The targeting ability of gelatin to M phi was investigated in terms of M phi uptake of MDP-gelatin conjugates and the consequent M phi activation in comparison with that of other proteins such as bovine serum albumin (BSA), bovine immunoglobulin (IgG) and human fibronectin (FN). When M phi activation was assessed as the activity to suppress the in vitro growth of mouse Meth A fibrosarcoma cells (R1), the antitumor activity of M phi pretreated with conjugates increased with an increase in M phi uptake. Gelatin conjugation was effective in enhancing the ingested amount of MDP compared with conjugation of other proteins, leading to an augmented MDP-induced M phi activation. After intraperitoneal (i.p.) injection of MDP-protein conjugates in mice, uptake of the conjugates by M phi in the peritoneal cavity was influenced by the type of proteins used for MDP conjugation. The MDP-protein conjugates were more strongly ingested by M phi in the order of gelatin much greater than IgG=FN much greater than BSA. The i.p. injection of MDP-gelatin conjugates activated M phi to suppress the in vitro growth of R1 cells to a higher extent than that of other MDP-protein conjugates. The order of protein to enhance M phi activation was in good accordance with that for M phi uptake. These findings demonstrate that gelatin was effective in enhancing the amount of MDP ingested by M phi, leading to the augmentation of MDP-induced antitumor function of M phi. In addition, when M phi were activated by i.p. injection of the MDP-gelatin conjugate, the MDP amount required for M phi activation was smaller and the activated state was maintained for a longer period than that of other MDP-protein conjugates. Therapeutic experiments demonstrated that the MDP-gelatin conjugate strongly suppressed the in vivo growth of R1 cells compared with free MDP at a 1000 times larger MDP dose. It may be concluded that gelatin is a promising polymer as a carrier capable of targeting drugs to M phi.