Synergistic antiproliferative activity of tamoxifen and docetaxel on three oestrogen receptor-negative cancer cell lines is mediated by the induction of apoptosis

Abstract

Summary The taxanes are a promising family of anti-tumour drugs that block cell cycle replication by interfering with the microtubule network. The clinical use of these drugs involves some problems related to their low solubility and occurrence of resistance, which is mainly dependent on the multidrug-resistant (MDR) phenotype. To investigate the possible interaction between docetaxel and tamoxifen (TAM), three oestrogen receptor-negative cancer cell lines, MDR-MDA-MB 231, MDR + CEM-VBLr and MCF-7 ADRr, were used. In all three cell lines, the combination of docetaxel and TAM was more effective in terms of growth inhibition than single drug exposure. Isobolic analysis confirmed the presence of synergism in all cell lines when docetaxel was used at 0.2 gM and TAM at a dose equal to or higher than 1 gM. Flow cytometric DNA analysis performed on the three cell lines showed that TAM was able to increase the G/M blocking activity of docetaxel. This blocking activity was followed by an increased flow cytometric DNA fragmentation suggestive of the presence of apoptosis, which was confirmed by DNA gel fragmentation and morphological analysis. While an antagonistic effect on P-glycoprotein (P-gp) activity may contribute to the synergistic effect of tamoxifen and docetaxel on CEM-VBLr and MCF-7 ADRr, other mechanisms must be involved, as the synergistic effect is also apparent with a P-gp-negative cell line.Keywords: docetaxel; tamoxifen; apoptosis; cell cycleThe taxanes are a new class of cytotoxic agents the cellular target of which is the microtubule network. Their mechanism of action is considered to be the enhancement of tubuline polymerization in both the initiation and elongation of microtubules (Schiff et al, 1979). In this way, taxanes induce extensive formation of microtubule bundles, thereby blocking cell replication at a checkpoint between G2 and M phase of the cell cycle. Paclitaxel (formerly called taxol) and docetaxel (formerly called taxotere) were the first members of the taxane family to be developed and have been approved by the US Food and Drug Administration for the treatment of ovarian and breast cancer respectively. Phase II trials are now in progress to extend their application to a wide variety of carcinomas, including lung, colon, head and neck, prostate, cervical and brain cancer (Mastropaolo et al, 1995). However, despite this encouraging therapeutic potential, the clinical use of these drugs involved some problems related to the solubility, toxicity and development of drug resistance, the last-named mainly dependent on an increased MDR activity (Bhalla et al, 1994). In an attempt to minimize these problems, two complementary strategies can be employed: paclitaxel and docetaxel analogues with a better therapeutic efficacy and less toxicity can be developed and possible synergism with other chemotherapeutics can be investigated.The aim of this study was to test the in vitro efficacy of docetaxel in association with the anti-oestrogen tamoxifen (TAM). (Leonessa et al, 1994) and vinblastine (Trump...

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