P-glycoprotein (Pgp) is a major ABC transporter responsible for multidrug-resistance (MDR) in cancer chemotherapy. Pre-clinical MDR modulation studies identified promising chemosensitizers, but none are in the clinic yet. Two novel progesterone-derived carbamates (11-carbamic acid N,N-dibenzyl progesterone ester and 11-carbamic acid N,N-dibutyl progesterone ester) were examined as potential chemosensitizers in the Pgp-expressing human colon cancer line HCT-15, applying the classical MDR-drugs paclitaxel and doxorubicin. The major findings were: (1) Pgp was expressed in the HCT-15 cells in both the cell and the nuclear membranes, (2) at the low dose range of 1-5 microM, each new candidate: (i) increased cytotoxicity of doxorubicin (15-fold) and (separately) of paclitaxel (40-fold), (ii) induced an increase in intracellular accumulation, 60% (4h) for doxorubicin and 300% (18h) for paclitaxel, (iii) reduced drug efflux from the cell, 2-fold and 4-fold for doxorubicin and for paclitaxel, respectively. Based on detailed kinetic analysis, using liposomes to model paclitaxel diffusion through cell membranes, efflux slowdown can be attributed to reduction in the rate constant of drug diffusion through Pgp, and not to Pgp blockage. Chemosensitization was consistently-better for paclitaxel (cytosol-operating) than for doxorubicin (nuclear-operating) implying linkage between P-glycoprotein localization and loci of drug action. Mapping intracellular locations of MDR-pumps may assist therapeutic strategies.
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