The synergistic anti-tumour activity of ICI 182,780 in combination with docetaxel is mediated by P-glycoprotein inhibition

Docetaxel is a semi-synthetic drug that has been shown to be effective in refractory advanced breast cancer. Its main mechanism of action seems to be to block microtubule depolymerisation. In this study we investigated the interaction between docetaxel and the pure anti-oestrogen ICI 182,780 on different oestrogen receptor-negative cancer cells, some of which express the classical multi-drug resistance (MDR) phenotype. ICI 182,780 potentiated the anti-proliferative effect of docetaxel only in the MDRpositive cells. Isobolic analysis demonstrated that this chemosensitising effect was a synergism. In the same conditions where synergism was detected, cell cycle analysis demonstrated an augmentation of cells blocked at the G2/M phase of the cell cycle, suggesting that ICI 182,780 increases the activity of docetaxel. In order to test this hypothesis, we performed bcl-2 western blot analysis and demonstrated that the addition of ICI 182,780 to docetaxel induced bcl-2 phosphorylation only in MDR-positive cells. The functional inactivation of bcl-2 is probably responsible for the commitment to apoptosis, since the combined docetaxel/ICI 182,780 treatment was able to foster a massive apoptosis in MDR-bearing cells as demonstrated by morphological analysis. Our results suggest that the synergism between docetaxel and ICI 182,870 is due to a block in Pglycoprotein activity, thus determining cell cycle block, bcl-2 inactivation and apoptosis induced by Endocrine-Related Cancer (1998) 5 315-324 docetaxel accumulation. C Ferlini et al.: Synergism between docetaxel and ICI 182,780 on human cancer cells 316 oestrogens as down-modulators of the P-gp function is an attractive approach to improve the efficacy of docetaxelbased anti-tumour therapy. The aim of this study was to investigate the in vitro efficacy of docetaxel in combination with the pure antioestrogen ICI 182,780 on different oestrogen receptor (ER)-negative cancer cell lines, some of which express the multi-drug resistant (MDR) phenotype. Results indicate that, in MDR-bearing cells, the combination treatment of docetaxel/ICI 182,780 presents a significant synergism in terms of growth inhibition, cell cycle block, bcl-2 phosphorylation and induction of apoptosis. Materials and methods