Sensitivity of multidrug-resistant MCF-7 cells to a transferrin-doxorubicin conjugate

Two multidrug-resistant breast cancer cell lines (MCF-7/AdrVp and MCF-7/D.40) each expressing a different membrane protein, involved in the drug resistance, have been treated with a transferrin-doxorubicin conjugate. Conjugates have shown an increase of activity over free doxorubicin on these resistant cell lines. Growth inhibition of doxorubicin-resistant cells, as evaluated by the MTT-assay, was higher for conjugates than for free doxorubicin especially for a 4-day contact period. I D 50 were twice and 10-fold lower for the conjugate than for free doxorubicin on resistant cells. MCF-7/AdrVp seemed to be particularly affected by the conjugate even if its intracellular content of doxorubicin was similar. With the Trf-Dox conjugate, an inverted correlation does exist between the drug-DNA content and the cytotoxicity of the conjugate. Verapamil influenced the uptake of free doxorubicin but not the uptake of Trf-Dox conjugate, thus showing a different mechanism of entry.