International Journal of Radiation Oncology*Biology*Physics
Bioreductive TherapiesInvolvement of human cytochromes P450 (CYP) in the reductive metabolism of AQ4N, a hypoxia activated anthraquinone DI-N-oxide prodrug
Introduction
The poorly defined vascular network of many solid tumors creates a subpopulation of hypoxic cells which can be resistant to chemotherapy and radiotherapy 1, 2. Bioreductively activated prodrugs specifically target hypoxic cells by becoming selectively activated to cytotoxic agents under low oxygen tension 3, 4. AQ4N is a di-N-oxide prodrug which is metabolized to AQ4 (Fig. 1) , a DNA binding agent and topoisomerase II inhibitor 5, 6, 7. In a number of murine tumors, AQ4N has impressive activity both as a chemosensitizer and radiation enhancer 8, 9, 10, 11. To date, detailed studies on the enzymology of AQ4N metabolism have not been described. The involvement of cytochrome P450 (CYP) in the reduction of tertiary amine-N-oxides has been described and there is preliminary evidence that rodent CYP can reduce AQ4N (12). Here we have used a panel of human liver microsomes phenotyped with respect to their CYP expression and activities to investigate the involvement of human CYP enzymes in the bioactivation of AQ4N.
Section snippets
Chemicals
All standard laboratory reagents were purchased as the highest commercially available grade and obtained from BDH Ltd, Poole, UK. The metabolites of AQ4N (1,4,-bis-{{2-(dimethylamino-N-oxide)ethyl]amino}-5,8- dihydroxyanthracene-9,10 dione), namely 1-{[2-(dimethylamino-N-oxide) ethyl]amino}-4-{[2-[di-methylamino] ethyl]amino}-5,8-dihydroxyanthra-cene-9,10-dione (AQM) and 1,4-bis-{{2-(dimethylamino)ethyl]amino}-5,8-dihydroxyanthracene- 9,10-dione (AQ4) were synthesized in house and shown to be
Results and discussion
The anaerobic metabolism of AQ4N to AQM and AQ4 was observed in all 17 human livers assayed (Fig. 2). The range (± SE) for total substrate turnover was 14. 26 ± 1.43 nmol/incubate (highest) to 3.65 ± 1.05 nmol/incubate (lowest). Anaerobic metabolism of AQ4N was not detected in the absence of NADPH or microsomes. Under aerobic conditions, formation of AQ4 was undetectable and the production of AQM was inhibited by >96%. Figure 3 shows that the metabolism of AQ4N correlated significantly
Acknowledgements
The authors wish to thank Dr. R. Weaver, Mary Maley, Duncan Webster, and Ketan Ruparelia for their technical assistance. The work was supported by the Cancer Research Campaign, UK.
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