Abstract S1-8: Outcome According to CYP2D6 Genotype among Postmenopausal Women with Endocrine-Responsive Early Invasive Breast Cancer Randomized in the BIG 1-98 Trial
Authors: Leyland-Jones B , Regan MM , Bouzyk M , Kammler R , Tang W , Pagani O , Maibach R , Dell'Orto P , Thürlimann B , Price KN , Viale G. 
Published: April 2011

Background: Available endocrine therapies, tamoxifen or aromatase inhibitors (AIs), are effective on average for the large population of postmenopausal women with steroid hormone-receptor positive early breast cancer, but many patients still relapse and die of their disease. Treatment selection may be improved by determining patient and disease features that suggest one or another endocrine therapy regimen is better for the individual patient. Recent clinical studies have presented conflicting evidence about whether postmenopausal women receiving adjuvant tamoxifen who carry genetic variants associated with low or absent CYP2D6 activity have a worse disease outcome.

BIG 1-98 is a double-blind randomized trial comparing five years of the AI letrozole (Letx5), tamoxifen (Tamx5) and sequences of Let and Tam as adjuvant endocrine therapy for postmenopausal women with endocrine-responsive breast cancer. BIG 1-98 enrolled 8010 patients between 1998 and 2003. Results at 6 years median follow-up showed that, on average, Let was superior to Tam for disease-free and overall survival, and for time to distant recurrence. The sequential arms (Tamx2 → Letx3; Letx2 → Tamx3 had similar results compared with Let. Genotyping is underway using DNA extracted from tumor blocks of patients in trial BIG 1-98 to elucidate genetic determinants of interindividual variability in efficacy and side effects of adjuvant endocrine therapies. We hypothesize that variants of CYP2D6 are associated with shorter breast cancer-free interval on tamoxifen monotherapy, and that the relation will not be observed with letrozole monotherapy as a control. The association with sequential therapies is unknown and will be explored. Methods: Formalin-fixed, paraffin-embedded primary tumor specimens were reviewed in the IBCSG Central Pathology Laboratory and DNA isolated from one or two 1mm cores. Single nucleotide polymorphisms (SNPs) of CYP2D6 {including CYP2D6*4, *2, *3A, *6, *7, *10, *17, *41} were genotyped on the Beckman SNPstream or the ABI 7900 TaqMan system.

The association of CYP2D6 variants with breast cancer-free interval and onset of hot flashes/night sweats will be investigated. The role of adjuvant chemotherapy, which was administered in about one-third of trial patients prior to randomization to adjuvant endocrine therapy, will also be investigated.

Results: At this time, CYP2D6*4 has been genotyped from FFPE material of 2000 patients, with a success rate greater than 75%. The frequency of homozygous and heterozygous CYP2D6*4 was 8% and 18%, respectively, which is in line with anticipated allele frequency in a predominantly Caucasian population. Genotyping of additional CYP2D6 alleles is ongoing for these patients, as well as full genotyping for up to another 2000 patients.

Conclusions: Trial BIG 1-98 provides a large, homogeneously-treated and followed cohort of patients treated with tamoxifen and other endocrine therapy regimens with available material for genotyping. Results of CYP2D6 genotyping (8 alleles) for all available patients will be presented at the meeting.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr S1-8.