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Background Controversy exists regarding CYP2D6 genotype and tamoxifen efficacy. Methods A matched case-control study was conducted utilizing the Austrian Breast and Colorectal Cancer Study Group Trial 8 that randomized post-menopausal women with estrogen receptor positive breast cancer to tamoxifen for 5 years (Arm A) or tamoxifen for 2 years followed by anastrozole for 3 years (Arm B). Cases had disease recurrence, contralateral breast cancer, second non-breast cancer, or died. For each case, controls were identified from the same treatment arm of similar age, surgery/radiation, and TNM stage. Genotyping was performed for alleles associated with no (PM; *3, *4, *6); reduced (IM; *10, and *41); and extensive (EM: absence of these alleles) CYP2D6 metabolism. Findings The common CYP2D6 *4 allele was in Hardy Weinberg Equilibrium. In Arm A during the first 5 years of therapy, women with 2 poor alleles (PM/PM: OR=2.45, 95% CI: 1.05–5.73, p=0.04) and women with one poor allele (PM/IM or PM/EM: OR=1.67, 95% CI: 0.95–2.93, p=0.07) had a higher likelihood of an event than women with two extensive alleles (EM/EM). In years 3–5 when patients remained on tamoxifen (Arm A) or switched to anastrozole (Arm B), PM/PM tended towards a higher likelihood of a disease event relative to EM/EM (OR= 2.40, 95% CI: 0.86–6.66, p=0.09) among women on Arm A but not among women on Arm B (OR= 0.28; 95% CI: 0.03–2.30). Conclusion In ABCSG8, the negative effects of reduced CYP2D6 metabolism were observed only during the period of tamoxifen administration, and not after switching to anastrozole.

The present studies were performed to elucidate the mechanism of cytotoxicity of the aminoflavone analog (5-amino-2,3-fluorophenyl)-6,8-difluoro-7-methyl-4H-1-benzopyran-4-one (AF; NSC 686288), a novel flavone with potent in vitro and in vivo antiproliferative activity against a number of human tumor cell lines and with a unique pattern of antiproliferative activity in the National Cancer Institute tumor cell line screen. AF was extensively metabolized by cytochrome P450 (P450) 1A1 and 1A2 to several metabolites, one of which was identified by mass spectrometry as a potentially reactive hydroxylamine. Radiolabeled AF was converted by rat and human microsomes, by recombinant CYP1A1 and CYP1A2, and by sensitive human tumor cell lines to species that covalently bound macromolecules. Treatment of sensitive human MCF7 cells with AF resulted in increased CYP1A1 mRNA and CYP1A1/1A2 protein followed by covalent binding of an AF metabolite to DNA, phosphorylation and stabilization of p53, and increased expression of the p53 transcriptional target p21. Covalent binding of the AF metabolite was increased by pretreatment with the CYP1A inducer 3-methylcholanthrene and decreased by coincubation with the CYP1A inhibitor ␣-naphthoflavone. In contrast, induction of CYP1A1 and covalent binding of the AF metabolite did not occur in AF-resistant M14-MEL cells. These observations suggest that AF is uniquely able to induce its own metabolic activation via CYP1A1/1A2 in duction to cytotoxic DNA-damaging species directly in tumor cells. AF, and possibly other agents, may offer a treatment strategy for tumors responsive to CYP1A1/1A2 induction, such as breast, ovarian, and renal cancers. AF (Fig.