MECHANISM OF ACTION AND SPECTRUM OF CELL-LINES SENSITIVE TO A DOXORUBICIN-TRANSFERRIN CONJUGATE

A transferrin-doxorubicin conjugate exhibited greatly increased cytoto xicity relative to unconjugated doxorubicin toward a variety of cultur ed tumor cell lines. An L929 cell line selected for doxorubicin resist ance was as sensitive to the transferrin-doxorubicin conjugate as was the parental unselected line. Quantitative measurements of doxorubicin fluorescence in single L929 cells showed that uptake was similar in a mount when cells were exposed to equivalent concentrations of doxorubi cin presented either free or as the transferrin-doxorubicin conjugate. However, unconjugated drug fluorescence was distributed in membranes, cytoplasm and nucleus; whereas conjugate fluorescence was confined ma inly to the cytoplasmic compartment. In as much as NADPH-dependent fre e radical formation is a known mechanism of doxorubicin cytotoxicity, localization in the vicinity of NADPH production might facilitate this cytotoxic pathway. Neither cytotoxicity nor uptake of the conjugate q uantified by doxorubicin fluorescence was significantly blocked by exc ess free transferrin, and the conjugate was not concentrated in the pl asma membrane at 4 degrees C. These findings suggest that conjugate in ternalization is not entirely dependent on transferrin receptor bindin g.