Tamoxifen (T) is a pro-drug that undergoes CYP2D6-mediated metabolic activation to metabolites that more potently inhibit estrogen stimulated growth compared to the parent drug. While many studies have examined the role of CYP2D6 genotype in T-treated postmenopausal women, the role of CYP2D6 metabolism in premenopausal women (pre-MW) receiving T, with or without ovarian function suppression (OFS) or exemestane (E) and OFS is unknown. Methods: SOFT randomized 3066 (pre-MW) from 2003-2011 in 27 countries, stratified according to prior receipt or nonreceipt of chemotherapy and nodal status, to receive 5 years of T, T+OFS, or E+OFS. We designed a pharmacogenetics substudy (activated October 2010) to collect blood DNA from North American (NA) patients (pts) or to extract non-tumor DNA from available formalin fixed paraffin embedded (FFPE) tissue blocks. For pts with a blood sample, CYP2D6 was genotyped beginning with the Luminex Tag-It Mutation Detection Kit and when needed, with a copy number variation assay and/or sequencing assays. For pts with FFPE-derived DNA, CYP2D6 genotyping for *3, *4, *6, *9, *10, *17 and *41 was performed using a Taqman Allelic Discrimination Assay. CYP2D6 phenotypes were called by classifying pts on the basis of a combination of poor (PM: *3, *4, *5, *6, *7, *8), slow (SM: *10), intermediate (IM: *9, *17, *29, *41) and extensive metabolizer alleles (EM; all others). Activity scores (AS) from phenotypes assigned for each allele: 0 if PM, 0.25 if SM, 0.5 if IM and 1 if EM allele, and multiplied x2 or x3 if duplicate or triplicate. With concomitant use of potent CYP2D6 inhibitor, AS=0; use of weak inhibitor subtracted 0.5. Metabolizer status was defined by CYP2D6 genotype alone or in combination with CYP2D6 inhibitor use at randomization from the AS: extensive (AS 1.25 to 3), intermediate (AS >0.5 to <1.25), slow/poor (AS 0 to 0.5) metabolizer status. The laboratory was blinded to all clinical data. The substudy primary objective was to assess the association between disease-free survival (DFS) and CYP2D6 metabolizer status in the T arm, and secondarily in the T + OFS, and E + OFS arms. A Cox model estimated hazard ratios comparing status according to treatment assignment, with prespecified prognostic factors. Results: 1200/3047 (39%) randomized pts in the intention-to-treat (ITT) population had successful CYP2D6 genotyping and 50% received prior chemotherapy. Following randomization, 435/1023 (42%) NA pts provided a blood sample and CYP2D6 genotypes were derived in 435/435. Non-tumor tissue was macrodissected from 1277 available FFPE blocks, resulting in DNA concentrations of > 0.3 ng/ml in 1053, and successfully derived CYP2D6 genotypes for 765/3047 pts (25%). 182 (15%) pts had DFS events after 8 yrs median follow-up. Metabolizer status from genotype was 57% extensive, 29% intermediate, 15% slow/poor. Metabolizer status was not associated with DFS in pts assigned T alone (P=0.60; Table), nor in pts assigned T+OFS (P=0.41) or E+OFS (P=0.30). 11% of pts used CYP2D6 inhibitors concomitantly at randomization; for 8% it changed the metabolizer status. The results using this definition were consistent. Conclusion: This retrospective-prospective SOFT pharmacogenetics substudy found no relation of CYP2D6 metabolizer status with DFS in premenopausal pts receiving T, T + OFS, or E + OFS. Given that 50% were pretreated with chemotherapy, further study is needed regarding the role of CYP2D6 metabolism in patients treated with T monotherapy.

Table

Treatment GroupN pts (N events)Comparison (N pts)Hazard Ratio95% CI
Tamoxifen 324 (56) Intermediate (114) vs Extensive (210) 0.78 0.43-1.39 
Tamoxifen 265 (52) Slow/Poor (55) vs Extensive (210) 1.11 0.58-2.13 
Tamoxifen + OFS 357 (46) Intermediate (122) vs Extensive (235) 0.73 0.38-1.40 
Tamoxifen + OFS 299 (45) Slow/Poor (64) vs Extensive (235) 1.25 0.64-2.43 
Exemestane + OFS 344 (46) Intermediate (107) vs Extensive (237) 0.59 0.28-1.22 
Exemestane + OFS 293 (47) Slow/Poor (56) vs Extensive (237) 1.13 0.56-2.27 
Treatment GroupN pts (N events)Comparison (N pts)Hazard Ratio95% CI
Tamoxifen 324 (56) Intermediate (114) vs Extensive (210) 0.78 0.43-1.39 
Tamoxifen 265 (52) Slow/Poor (55) vs Extensive (210) 1.11 0.58-2.13 
Tamoxifen + OFS 357 (46) Intermediate (122) vs Extensive (235) 0.73 0.38-1.40 
Tamoxifen + OFS 299 (45) Slow/Poor (64) vs Extensive (235) 1.25 0.64-2.43 
Exemestane + OFS 344 (46) Intermediate (107) vs Extensive (237) 0.59 0.28-1.22 
Exemestane + OFS 293 (47) Slow/Poor (56) vs Extensive (237) 1.13 0.56-2.27 

Citation Format: Matthew P. Goetz, Gini F. Fleming, Mary Kuffel, John R. Hawse, John L. Black, Richard Weinshilboum, James N. Ingle, Patrizia dell’Orto, Olivia Biasi, Roswitha Kammler, Sherene Loi, Marco Colleoni, Giuseppe Viale, Prudence A Francis, Meredith M Regan. The role of CYP2D6 mediated tamoxifen metabolism in the suppression of ovarian function trial (SOFT) [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD2-09.