Development of drug resistance is a major limitation of chemotherapeutic treatment of cancer Resistance arises as a consequence of the genomic instability of tumour cells, and usually relies on the perversion of mechanisms used by normal cells to protect themselves against environmental toxic agents. One of these mechanisms is the increased expression of xenobiotic metabolising enzymes, in an attempt to reduce drug activation or enhance its detoxification. Cytochromes P450 are a family of enzymes responsible for biotransformation of both xenobiotic and endogenous compounds. These enzymes can metabolise a number of chemotherapeutic drugs and thus they have the potential to influence the sensitivity of tumour cells to anticancer agents. Incubation of Adriamycin and Vincristine with recombinant cytochrome P450 3A4 (CYP3A4) was shown to decrease their toxic effects on A549 cells Epithelial lung BEAS-2B cells transfected with CYP3A4 cDNA showed a modest increase in resistance to Adriamycin, which was reversed when simultaneously treated with the CYP3A inhibitor 17a-ethynyl oestradiol (17 AEE) Treatment of HL60 cells with the CYP1B1 inducer 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) did significantly increase mRNA expression and ethoxyresorufm-O-deethylase (EROD) activity of this enzyme, but failed to induce changes in the toxicity profiles of 5-fluorouracil, Cisplatin, Taxol or Taxotere Pulse selection of MCF-7 cells with Taxotere increased Taxotere resistance and greatly enhanced expression of CYP1B1 mRNA and EROD activity Knock-down of CYP1B1 expression by siRNA resulted in decreased cell survival, but did not substantially enhance Taxotere toxicity. The role of P450 NADPH reductase was also investigated, and it was found that MDA 231 cells transfected with this enzyme were more sensitive to Mitomycin C, Adriamycin and 5-fluorouracil-induced toxicity. This appeared to be due to increased oxidative stress as evidenced by lower NADPH levels and increased ROS production in MDA R4 cells. Expression of a set of key enzymes involved in glutathione turnover was also found to be altered in a panel of drug-selected cell lines In order to study the mechanisms involved in the development of resistance, two metabolically competent cell lines were pulse-selected with Taxol, Taxotere and Cisplatin. These cells showed increased resistance to the drugs they were pulsed with and also displayed cross-resistance to taxanes and Vincristine Expression of CYP3A4 and CYP3A5 was increased in pulse-selected cells as compared to their parental counterparts, the same was found of MDR1. To analyse the role of these proteins in drug resistance, combination assays were performed with cytotoxic drugs in the presence of a CYP3A inhibitor, an MDR1 inhibitor or both Simultaneous treatment with 17 AEE did not appreciably affect Taxol toxicity in any of the cell lines tested, however, combination of Taxotere with the MDR1 inhibitor GF120918 dramatically enhanced the toxicity of the anticancer drug. The study of xenobiotic metabolism enzymes in tumours could result in the discovery of novel and attractive targets for adjuvant therapy that can maximise the effect of chemotherapeutic agents and circumvent some types of resistance.