Cytochrome P450 CYP1B1 activity in renal cell carcinoma.

Renal cell carcinoma (RCC) is the most common malignancy of the kidney and has a poor prognosis due to its late presentation and resistance to current anticancer drugs. One mechanism of drug resistance, which is potentially amenable to therapeutic intervention, is based on studies in our laboratory. CYP1B1 is a cytochrome P450 enzyme overexpressed in a variety of malignant tumours. Our studies are now elucidating a functional role for CYP1B1 in drug resistance. Cytochrome P450 reductase (P450R) is required for optimal metabolic activity of CYP1B1. Both CYP1B1 and P450R can catalyse the biotransformation of anticancer drugs at the site of the tumour. In this investigation, we determined the expression of CYP1B1 and P450R in samples of normal kidney and RCC (11 paired normal and tumour and a further 15 tumour samples). The O-deethylation of ethoxyresorufin to resorufin was used to measure CYP1B1 activity in RCC. Cytochrome P450 reductase activity was determined by following the reduction of cytochrome c at 550 nm. The key finding of this study was the presence of active CYP1B1 in 70% of RCC. Coincubation with the CYP1B1 inhibitor alpha-naphthoflavone (10 nM) inhibited this activity. No corresponding CYP1B1 activity was detected in any of the normal tissue examined (n=11). Measurable levels of active P450R were determined in all normal (n=11) and tumour samples (n=26). The presence of detectable CYP1B1, which is capable of metabolising anticancer drugs in tumour cells, highlights a novel target for therapeutic intervention.

The role of cytochrome P450 in cytotoxic bioactivation: future therapeutic directions.

The cytochrome P450s are an essential group of enzymes involved in metabolism of drugs, foreign chemicals, arachidonic acid, cholesterol, steroids and other important lipids. The cytochrome P450 enzyme system is responsible for much of the phase I metabolism of chemotherapeutic agents. At the simplest level the detoxification properties of the cytochrome P450s are used to help clear a cytotoxic before it results in serious irreversible toxicity to the patient while at other levels the cytochrome P450s are involved to varying extents in drug bioactivation. This metabolism primarily occurs in organs and tissues of the body known to express cytochrome P450 ubiquitously (i.e. liver and gastrointestinal tract), but there is also evidence to suggest that it occurs within the tumor microenvironment due to localized, tumor specific expression of certain P450 isoforms. Several of today's currently prescribed cytotoxics (e.g. cyclophosphamide and tamoxifen) undergo systematic bioactivation by cytochrome P450, which often results in toxicity to the patient. The realization that many tumors have differential cytochrome P450 expression when compared to the corresponding normal tissue has allowed the rational design of the next generation of cytotoxic around cytochrome P450 enzymology. Several new agents now entering clinical trials (e.g. Phortress and AQ4N) are specifically designed to exploit tumor cytochrome P450, resulting in local bioactivation of the cytotoxic at the tumor site. Specific activation of pro-drugs by isoforms whose expression or particular catalytic activity is limited to cancer cells offers the possibility of truly targeted chemotherapy with minimized systemic toxicity.