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Background. Estrogen receptor (ER) expression is the key determinant for endocrine treatment of breast cancer (BC). In clinical practise ER analysis is currently almost entirely by immunohistochemistry (IHC). ASCO/CAP recommends cut-offs of 79%). 770 baseline FFPE samples were analysed (582 HER2-, 188 HER2+). IHC used the NCL-L-ER6F11 (Novocastra, Leica) antibody with the DAKO Flex kit and were scored centrally. mRNA was assessed by rtPCR. Values were expressed according to an arbitrary scale (“A units”). Results. ER IHC was available from 517 HER2- and 188 HER2+ tumors and ER mRNA from 376 HER2- and 173 HER2+ tumors. The correlation between ER levels by IHC and rtPCR was moderately high (Rho=0.616, p10% tumors median (range) mRNA levels were 1.290 (0.188-11.346), 2.368 (0.575-13.863) and 111.150 (0.826-1001.423), respectively. The tables show the %age of PR, IR and GR for each category of ER by IHC or mRNA. The number of cases was adjusted to cater for the analysis of only 30% of the HER2- IR and GR , by multiplying the HER2- IR and GR number of cases by 3.3, resulting in this not being an integer for some categories. IHC: 87% of cases had >60% of cells +ve and only 11-12% of these were PRs. For cases with 10% IHC (%)0 < 1≥1 < 10≥10 < 20≥20 < 40≥40 < 60≥60 < 80≥80No. of cases42.32922.625.671.7239.81010.6PR87.596.566.462.541.811.710.8IR12.53.419.020.741.640.933.1GR0.00.014.616.816.647.456.1 mRNA (A units) Citation Format: Elena Lopez Knowles, Simone Detre, Margaret Hills, Gene F Schuster, Maggie CU Cheang, Holly Tovey, Lucy Kilburn, Judith Bliss, John Robertson, Ian Smith, Mitch Dowsett, POETIC investigators. Estrogen receptor expression thresholds by IHC and mRNA for Ki67 response to aromatase inhibition: A POETIC study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD15-06.

Aims 1. To determine the variability of mutational profiles and sub-clonality in core-cut biopsies from ER+ BC and the impact of 2-weeks’ AI therapy on these. 2. To identify mutations or patterns of mutations associated with poor anti-proliferative response to AI treatment. Background DNA alterations may lead to de novo and acquired resistance to medical therapies including AIs. Assessing this requires single time-point or sequential sampling usually with core-cuts but there is little information on their ability to represent mutational profiles or sub-clonal structure. We studied this in paired biopsies from ER+ BC primaries in 60 selected postmenopausal patients from the Peri-Operative Endocrine Therapy for Individualising Care (POETIC) trial (CRUK/07/015) before and after 2-weeks’ non-steroidal AI or no AI (randomised 2:1). Methods DNA was extracted from RNAlater-preserved diagnostic and surgical 14-gauge core-cut samples and peripheral blood from 20 no AI (Control) and 40 AI-treated patients (15 poor and 25 good Ki67-responders [PR and GR, respectively]). Patients with low ER+ BC or unsuppressed estradiol on treatment were not considered. Exome sequencing (Illumina HiSeq 2000) achieved >60% coverage across the exome at 15x depth. Variants were validated by re-sequencing (median >100x) together with 79 genes of interest curated from COSMIC and selected publications. Statistically significant genes (SMGs) were determined using MuSiC. Sub-clonality was analysed by SciClone. Results Good quality exomes were obtained on 102 samples including 44 pairs (control n=14; PR n=10; GR n=20). There were 5684 mutations (including 3616 missense and 1322 silent) affecting 3261 genes. SMGs in this series were PIK3CA (35.3%), TP53 (27.5%), CDH1 (13.8%), HEATR7B2 (8.8%), GATA3 (5.9%), CENPF (5.9%), MAP3K1 (5.9%), MAP2K4 (4.9%), HTR1A (2.9%) and C22orf23 (1%). PR had more mutations than GR (median 65 vs 36, p=0.04). More PR than GR were HER2+ (5/14 vs 1/24, p=0.019) and/or TP53-mutated (5/10 vs 3/20, p=0.08) but similar proportions were PIK3CA-mutated. The correlation of diagnostic vs surgical variant allele frequencies was strong for the control (r=0.75) and treated (r=0.89) groups (for treated GR r=0.83; for treated PR r=0.65). In the treated group there were fewer mutations at surgery vs diagnosis (p Conclusion This is the largest reported study of exome reproducibility in ER+ BC for mutation profiles based on core-cut biopsy. Multiple sub-clones are identifiable in ER+ primary BC. In c.20% tumours, a single core-cut does not allow inference of all sub-clonal populations, probably due to spatial heterogeneity. TP53 mutations but not PIK3CA mutations are associated with PR. Large numbers of BC will be needed to identify any associations of lower frequency mutations with resistance. A trend to fewer mutations after just 2 weeks AI needs confirmation. Citation Format: Pascal Gellert, Corrinne V Segal, Qiong Gao, Tiandao Li, Christopher A Miller, Elaine Mardis, Lesley-Ann Martin, Christopher Holcombe, Anthony Skene, Judith Bliss, John Robertson, Ian Smith, Mitch Dowsett, POETIC Trial Management Group and Trialists. Exome sequencing of post-menopausal ER+ breast cancer (BC) treated pre-surgically with aromatase inhibitors (AIs) in the POETIC trial (CRUK/07/015) [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr S1-04.

Introduction: TACT2 (CRUK/05/019), a multicentre randomized phase III trial in patients with node +ve or high risk node -ve invasive EBC with E-CMF as control tested two hypotheses in a 2x2 factorial design, with previously presented results showing: i) no evidence of a benefit from accelerated 2-weekly epirubicin (aE) compared to standard 3-weekly epirubicin (E) (Cameron 2012); and ii) capecitabine (X) gives equivalent efficacy but preferential side-effect profile to CMF (Canney 2014). Studies suggest that Androgen receptor (AR) expression may be associated with improved outcome in early breast cancer. We performed an analysis of AR expression in TACT2 patients to test the hypothesis that AR would represent an independent predictor of residual risk following adjuvant therapy. Methods: Tumour samples were collected prospectively from 3803/4391 TACT2 patients, Tissue micro-arrays were constructed as per published guidelines and central AR, ER, PgR, HER2, Ki67, CK5/6, EGFr and BCL2 staining performed and quantified by imaged analysis for ER, PgR, HER2, Ki67 and BCL2. EGFr and CK5/6 were dichotomised by light microscopy evaluation of cores. After planned biomarker analyses had been performed, a subset of TACT2 cases for whom remaining tissue was available were analysed for AR expression. Data for 1878 cases was available for this analysis (49% of those consenting to tissue collection). AR expression was dichotomised as AR-ve (10%). Log-rank tests were used to explore the prognostic value of AR on time to recurrence (TTR). Cox-regression models were used to test the independent prognostic value of AR in the presence of tumour size, grade, nodal status, and biological subtypes. Marker by treatment interaction terms were included in models to test the predictive value of AR for both randomised treatment comparisons. Results: 1398/1878 (74%) patients were classed as AR+ve and 480/1878 (26%) as AR-ve. The proportion of AR+ve patients differed significantly between biological subtypes (Luminal A 269/293 (92%); Luminal B 784/928 (84%); HER+ve 86/111 (77%); Basal-like 37/216 (17%); 5-marker-ve 37/93 (40%); χ2 p 117/480 (24%) AR-ve patients had a TTR event compared with 183/1398 (13%) AR+ve patients (HR for AR-ve compared to AR+ve =2.05 95%CI 1.63-2.59; p No differential treatment effect between AR subtypes was observed for either randomised treatment comparison (E/aE or CMF/X). Conclusion: AR is an independent prognostic marker for residual risk following chemotherapy in this large study which included patients with both luminal and non-luminal cancers. AR expression patterns differ between molecular subtypes of early breast cancer, with evidence suggesting the impact of AR on residual risk may also differ between subtypes. AR is currently being explored as another potential target for therapy so these data could have future clinical relevance. Citation Format: Jane Bayani, James Morden, Sunil Skaria, Peter Bliss, Robert Grieve, Adrian Harnett, Chris Bradley, Diana Ritchie, Peter Barrett-Lee, Peter Canney, David Cameron, Judith Bliss, John Bartlett. Androgen receptor expression is an independent marker of lower residual risk in the TACT2 trial (CRUK/05/019) [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P4-11-03.