Intracellular disposition and cytotoxicity of transferrin-mitomycin C conjugate in HL60 cells as a receptor-mediated drug targeting system.

A macromolecular conjugate of mitomycin C (MMC) with transferrin (TF) which possessed binding ability for TF receptor was synthesized. The conjugate (TF-MMC) was internalized into the human leukemia cell line HL60 cells and distributed into intracellular fractions, then exocytosed into an incubation medium. Although these phenomena were similar to those of TF, part of the internalized TF-MMC was degraded to a trichloroacetic acid (TCA)-soluble fraction. Therefore, the intracellular disposition of the conjugate was analyzed kinetically. The mean time of internalization of TF-MMC (7.14 min) was longer than that of TF (5.46 min). The mean exocytosis time of TF-MMC (22.1 min) was also longer than that of TF (13.0 min). Although elongation of both the internalization and exocytosis steps was responsible for the increase in recycling time of the conjugate, the binding process to the TF receptor in the internalization stage was found to be markedly retarded. The recycling times of TF-MMC and TF were 29.2 and 18.5 min, respectively. The mean decomposition time of TF-MMC was 76.3 min. Proliferation of HL60 cells was inhibited by TF-MMC in vitro. These results indicate that the TF-MMC was internalized via a TF receptor and a part of the internalized TF-MMC was degraded, so the released MMC might represent antitumor activity. TF-MMC was demonstrated to be a useful hybrid as a receptor-mediated targeting system.