[S1-7] Lack of Correlation between Gene Variants in Tamoxifen Metabolizing Enymes with Primary Endpoints in the ATAC Trial.

Rae JM, Drury S, Hayes DF, Stearns V, Thibert JN, Haynes BP, Salter J, Pineda S, Cuzick J, Dowsett M. University of Michigan, Ann Arbor; University of Michigan; Royal Marsden Hospital, United Kingdom; Breakthrough Breast Cancer Research Centre, United Kingdom; Johns Hopkins University; CRUK Centre for Epidemiology, Mathematics and Statistics, United Kingdom

Background. A number of studies have provided widely heterogeneous results for and against the hypothesis that genetic variants in drug metabolizing enzymes can influence patient response to tamoxifen. Most of these studies are confounded by relatively small numbers of patients, lack of comprehensive genotype information, lack of detailed clinical outcome data, and patient selection biases. The ATAC trial was a prospective randomized double-blind placebo-controlled trial designed to compare the adjuvant use of anastrozole versus tamoxifen for 5 years. The trial's detailed efficacy and safety data, long term (10-year) follow-up and high number of events make it an ideal setting for pharmacogenetic analyses. Here we report our initial findings testing for correlations with SNPs in CYP2D6 and UGT2B7, the primary enzymes responsible for the presumed activation and inactivation of tamoxifen, respectively, with clinical outcomes including any disease recurrence, distant recurrence, and overall survival.
Methods: CYP2D6 genotype data for the 7 most common alleles were used to assign each patient a 'score' based on predicted allele activities from 0 (no activity) to 2 (high activity). Patients were also tested for the UGT2B7*2 variant, which has lower enzymatic activity and has been shown to correlate with higher serum endoxifen concentrations. All genotype determinations were made without knowledge of the patient's treatment assignment or outcomes.
Results: Comprehensive CYP2D6 genotype data were obtained on 588 and 615 patients randomized to receive tamoxifen, and anastrozole, respectively. After median 10 years of follow-up, there were 115 recurrences in the tamoxifen cohort. We found no associations between any of the CYP2D6 scores and rates of recurrence in tamoxifen treated patients. A test for trend across CYP2D6 scores was not significant. In the anastrozole cohort, there were 92 recurrences, and there was no evidence of an association between CYP2D6 score with rates of recurrence. Likewise, there was no detectable association in either the tamoxifen or anastrozole group with UGT2B7 genotype alone or in combination with CYP2D6 score. Concomitant SSRI usage was recorded in these patients and is being assessed for possible impact on these results.
Conclusion: These data from a large prospective clinical trial, with detailed outcome data and long-term follow-up do not support the hypothesis that patient with decreased CYP2D6 enzymatic activity receive less benefit from tamoxifen therapy compared to wild-type CYP2D6 patients. Variants in drug targets and estrogen signaling pathways may be the genetic basis for patient variability in anti-estrogen risk versus benefit profiles.

Thursday, December 9, 2010 10:45 AM

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