Storniolo, Anna Maria

Abstract Background: In the treatment of ER+/HER2- metastatic breast cancer (MBC), randomized phase II/III trials testing selective estrogen receptor degraders (SERDs), proteolysis-targeting chimeras (PROTACs), complete estrogen receptor antagonists (CERANs) and selective ER modulators (SERMs) (mostly compared to fulvestrant) have demonstrated modest (1-2 months) improvements in progression-free survival (PFS), mainly limited to patients with ESR1-mutated MBC. In these studies, the median PFS (mPFS) ranges from 3 to 6 months, with shorter mPFS in patients with prior progression on CDK 4/6 inhibitor (CDK 4/6i). Here we report the final results of a randomized phase II study that compared Z-endoxifen (Endx) 80 mg/day with tamoxifen (Tam) 20 mg/day in postmenopausal women with estrogen receptor (ER)- positive/ HER2-negative MBC, and report outcomes according to ESR1, PIK3CA, PTEN and AKT alterations (Alliance A011203; NCT02311933). Methods: Postmenopausal women with unlimited prior endocrine regimens (including CDK 4/6i) and up to two chemotherapy regimens for MBC, with tumor biopsy confirmation of ER+ (>10% nuclear staining) and negative for HER2 amplification/overexpression were eligible. Prior adjuvant Tam was allowed if relapse was > 1 year following Tam completion. Patients randomized to Tam could cross over to Endx at progression. The primary end point was PFS analyzed by the interval censored data analytic approach. ctDNA was analyzed for ESR1 and PIK3CA/PTEN/AKT alterations using Guardant360 CDx 74-gene next-generation sequencing-based assay (Guardant Health). Results: Among 108 women who preregistered, 76 (Endx: 40; Tam: 36) were randomized and eligible. Patients were heavily pretreated; all patients had prior exposure to an aromatase inhibitor (AI), 50% had ≥ 2 AI based regimens in the metastatic setting and 50% in each arm had received fulvestrant. Approximately half on each arm received at least one chemotherapy regimen in the metastatic setting. Prior CDK 4/6i therapy was imbalanced (48% on Endx and 28% on Tam). Endx did not significantly prolong PFS compared to Tam (median PFS [mPFS] 4.3 vs 1.7 months; HR 0.72, p=0.168). Because arms were imbalanced according to prior CDK 4/6i exposure, we performed an unplanned analysis demonstrating that Endx improved PFS relative to Tam in CDK 4/6i-naïve patients (mPFS 8.3 vs 2.1 months; HR 0.39, p=0.0097) but not in patients treated with prior CDK 4/6i (mPFS 3.0 vs 1.5 months). In the subset of patients who consented for ctDNA testing (n=28), the mPFS for those with either an activating alteration in the PIK3CA/PTEN/AKT pathway or with an ESR1 mutation was similar as the overall trial. For the 24 patients who progressed on Tam and crossed-over to Endx, the median PFS was 3.5 months (range 1.5-15.0 months) with a 6-month clinical benefit rate of 33.3% (8/24; 90% CI: 17.8-52.1%). Conclusions: Endx did not meet the prespecified threshold for concluding that it significantly improved PFS relative to Tam in a cohort of heavily pretreated MBC. However, the observations of a > 6-month improvement in mPFS in CDK 4/6i-naïve patients, anti-tumor activity following crossover to Tam, and mPFS estimates similar to other novel endocrine agents in development all support the ongoing development of Endx. Support: U10CA180821, U10CA180882, U24CA196171, U10CA180820 (ECOG-ACRIN) and 5P50CA116201. Clinicaltrials.gov Id: NCT02311933. https://acknowledgments.alliancefound.org. Citation Format: M. P. Goetz, V. Suman, J. Reid, T. Dockter, C. Strand, W. F. Symmans, J. Hawse, A. Storniolo, J. Doroshow, J. M. Collins, H. Streicher, M. M. Ames, A. Tarnower, O. M. Hahn, J. N. Ingle, L. Carey, A. Partridge. Final results from the Randomized Phase II study of Z-Endoxifen and Tamoxifen in Endocrine Refractory, Estrogen Receptor Positive, HER2 negative Metastatic Breast Cancer (Alliance A011203) [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-08-07.

PURPOSE Predictive biomarkers to better tailor therapy for patients with early-stage human epidermal growth factor 2 (HER2)-positive breast cancer are a priority. We hypothesized that HER2 and immune-based biomarkers would be predictive of pathologic complete response (pCR) to preoperative trastuzumab/pertuzumab (HP). MATERIALS AND METHODS Patients with stage II/III, estrogen receptor (ER)-negative, HER2-positive breast cancer received neoadjuvant HP in the TBCRC026 clinical trial. The pCR after receiving HP alone was 22% (18/83). Tumor biopsies were performed at baseline. Secondary correlative objectives were to determine the relationship between HER2-based biomarkers and immune processes with pCR. NanoString code sets BC360 and IO360 were used to compare differential gene expression in baseline tumors that underwent pCR versus no pCR. NanoString GeoMx digital spatial profiling was used to assess immune protein abundance in intraepithelial and stromal segments. Stromal tumor-infiltrating lymphocytes and Ki67 were evaluated by hematoxylin and eosin and immunohistochemistry, respectively. RESULTS Intraepithelial HER2 protein abundance was significantly associated with pCR ( P = .001). Low HER2 abundance tumors were primarily basal-like, and essentially all (19/20) failed to achieve pCR. High HER2 abundance tumors that achieved pCR (14/51) exhibited a high degree of immune cell activity, whereas high HER2 abundance tumors that failed to achieve pCR tumors (37/51) were enriched for M-phase processes and epidermal growth factor receptor signaling. Baseline Ki67 was significantly higher in nonresponders ( P = .04). CONCLUSION ER-negative, HER2-positive breast cancer has unique molecular and immunologic features that may predict pCR after neoadjuvant HP. Validation of these potential biomarkers and composite biomarker analyses may guide design of future clinical trials.

530 Background: Robust biomarkers are needed to personalize treatment in early stage HER2-positive breast cancer. The TBCRC026 trial (n=83) demonstrated that early metabolic changes on FDG-PET/CT [>40% decline in SULmax at day 15 post treatment initiation (C1D15); C1D15 SULmax] were associated with response to neoadjuvant trastuzumab/pertuzumab (HP), without chemotherapy, in an ER-negative, HER2-positive cohort (NCT01937117). We investigated the association of RNA sequencing (RNAseq)-based gene expression signatures (GES) at baseline and on-treatment (C1D15), with early metabolic change on PET/CT and clinical outcomes. Methods: RNAseq (664 GES) was performed on baseline (n=56) and C1D15 (n=31) tumor biopsies (25 paired samples available). Association with pCR following HP (secondary), PET/CT parameters and survival (exploratory) was studied. Gene expression changes between baseline and C1D15 samples were tested by Wilcoxon signed-rank test. Association between GES and PET/CT parameters and GES with pCR were evaluated using logistic regression models. Cox regressions were performed to study the association between PET/CT SULmax at C1D15 and a B cell signature and relapse-free survival (RFS). The likelihood ratio test was used to compare the prognostic ability of two nested Cox models. Results: Both RNAseq and clinical outcomes were available in 53/83 patients. HER2-enriched (HER2-E) intrinsic subtype comprised 71% (40/56) of cases at baseline and 61% (19/31) at D15; basal subtype for 27% (15/56) and 39% (12/31), respectively. pCR rate was 19% (10/53) with all but one exhibiting HER2-E subtype. One basal-like sample underwent pCR, with downregulation of HER2 amplicon and IgG signatures slightly more prominent. Gene expression changes from baseline to C1D15 were analyzed; ERBB2was downregulated after HP (p = 0.007), B cell signatures, CD4, CD8, natural killer cells and Treg cells were significantly upregulated at C1D15. GES associated with 40% decline in SULmax at C1D15 included ERBB2 amplicon genes ( ERBB2, GRB7) and correlation to the HER2-E subtype, whereas basal-like signatures were significantly associated with lack of response on PET at C1D15. Univariate logistic regression analysis suggested that FOS-JUN, fibroblast and B cell GES at baseline were significantly associated with pCR (p=<0.05). Finally, the addition of a B cell signature improved the prognostic value of C1D15 SULmax on PET/CT for RFS (p < 0.05). Conclusions: In this unique cohort with ER-negative, HER2-positive breast cancer treated with neoadjuvant HP without chemotherapy, HER2-E tumors and those with increased immune signaling were more likely to respond to HER2-directed therapy. Combining C1D15 SULmax on PET/CT and a B cell signature provided improved prognostic information and should be further explored as candidate biomarkers in this setting. Clinical trial information: NCT01937117 .

Abstract BACKGROUND: Anti-HER2 therapies have transformed the trajectory for patients with HER2-positive (HER2+) MBC. A subset of patients with HER2+ MBC do exceptionally well and remain on maintenance anti-HER2 therapy for many years. However, current therapy is non-curative and patients are treated indefinitely. While these therapies often are well-tolerated, they may cause significant toxicities, are expensive, and typically require frequent infusion and provider visits. Case reports and retrospective series demonstrate that some patients may do well after stopping anti-HER2 treatment. We currently lack prospective data on the likelihood of clinical impact of successful treatment discontinuation, outcomes among patients who progress and restart HER2-directed therapy, and tools to predict who may safely stop therapy. In this study, we will enroll pts with an exceptional response to anti-HER2 therapy. Pts may elect to either continue or stop anti-HER2 therapy and will be followed clinically and radiographically on a protocol-specified schedule. Considering the central role of patients in choosing the study intervention arm, and the unique nature of stopping therapy as a treatment intervention, patient advocates have been deeply involved in the study design to ensure patient concerns are being heard and addressed. We will retrospectively assess minimal residual disease (MRD), measured in plasma, as a biomarker of ongoing response, in a step toward shifting the treatment paradigm for pts with HER2+ MBC. METHODS: STOP-HER2 is a prospective, non-randomized, phase II, multicenter study of either continuation (cohort 1) or cessation (cohort 2) of anti-HER2 therapy in exceptional responders with HER2+ MBC. Eligible patients must have biopsy-proven HER2+ MBC by ASCO/CAP guidelines and must have been receiving first-line anti-HER2 therapy for MBC for at least 3 years without evidence of progressive disease (PD) as determined by the treating investigator. Patients with prior brain-only PD or oligo-PD outside the brain are eligible at least 2 years after local treatment to the central nervous system (CNS) and in absence of subsequent PD. Up to fifty-two participants will be enrolled into cohort 2, and an additional thirty participants enrolled into the cohort 1, at 10 US sites within the Translational Breast Cancer Research Consortium. Co-primary endpoints are the 1-year progression-free survival (PFS) in cohorts 2 and 1, separately. Secondary endpoints include the clinical benefit rate upon re-initiation of anti-HER2 therapy for patients in cohort 2 experiencing PD after treatment discontinuation; and 3-year overall survival in both cohorts. Correlative endpoints include MRD status at baseline and the correlation between MRD dynamics and outcomes. Patient-reported outcomes will be collected and analyzed, including quality of life, illness intrusiveness, financial toxicity, anxiety, and decision regret. The study will use a two-stage design to assess the primary objective of estimating the rate of participants who are free of progression at one year after discontinuing anti-HER2 therapy. If 50% or less are free of progression at one year, stopping HER2 therapy would not be considered a viable treatment plan (null hypothesis). The study is designed to have 80% power to reject the null hypothesis at the 10% type I error rate when 68% of exceptional responders are free of progression at one year after stopping HER2 therapy. STOP-HER2 began enrollment in Q2 2023 (NCT05721248). Citation Format: Heather Parsons, Kathryn Ruddy, Stefania Morganti, Karen Smith, Victoria Attaya, Eliza Kallfelz, Michelle DeMeo, Jamie LaScala, Shirley Mertz, Teri Pollastro, Patricia Spears, Adam Brufsky, Chau Dang, Susan Dent, Ahmed Elkhanany, WIlliam Gwin, Ciara O'Sullivan, Kathy Miller, Sara Nunnery, Elaine Walsh, Anna Maria Storniolo, Sara Tolaney, Nabihah Tayob, Antonio Wolff, Eric Winer, Mothaffar Rimawi, Ian Krop, Nancy Lin. The STOP-HER2 Trial: A Phase 2 Study of Stopping Trastuzumab - Outcomes in Patients with HER2+ Metastatic Breast Cancer (TBCRC 062) [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-20-01.

Abstract Purpose: Clinical trials reported 25% to 30% pathologic complete response (pCR) rates in HER2+ patients with breast cancer treated with anti-HER2 therapies without chemotherapy. We hypothesize that a multiparameter classifier can identify patients with HER2-“addicted” tumors who may benefit from a chemotherapy-sparing strategy. Experimental Design: Baseline HER2+ breast cancer specimens from the TBCRC023 and PAMELA trials, which included neoadjuvant treatment with lapatinib and trastuzumab, were used. In the case of estrogen receptor–positive (ER+) tumors, endocrine therapy was also administered. HER2 protein and gene amplification (ratio), HER2-enriched (HER2-E), and PIK3CA mutation status were assessed by dual gene protein assay (GPA), research-based PAM50, and targeted DNA-sequencing. GPA cutoffs and classifier of response were constructed in TBCRC023 using a decision tree algorithm, then validated in PAMELA. Results: In TBCRC023, 72 breast cancer specimens had GPA, PAM50, and sequencing data, of which 15 had pCR. Recursive partitioning identified cutoffs of HER2 ratio ≥ 4.6 and %3+ IHC staining ≥ 97.5%. With PAM50 and sequencing data, the model added HER2-E and PIK3CA wild-type (WT). For clinical implementation, the classifier was locked as HER2 ratio ≥ 4.5, %3+ IHC staining ≥ 90%, and PIK3CA-WT and HER2-E, yielding 55% and 94% positive (PPV) and negative (NPV) predictive values, respectively. Independent validation using 44 PAMELA cases with all three biomarkers yielded 47% PPV and 82% NPV. Importantly, our classifier's high NPV signifies its strength in accurately identifying patients who may not be good candidates for treatment deescalation. Conclusions: Our multiparameter classifier differentially identifies patients who may benefit from HER2-targeted therapy alone from those who need chemotherapy and predicts pCR to anti-HER2 therapy alone comparable with chemotherapy plus dual anti-HER2 therapy in unselected patients.

Abstract Purpose: We previously demonstrated that high levels of circulating methylated DNA are associated with subsequent disease progression in women with metastatic breast cancer (MBC). In this study, we validated the clinical utility of a Liquid Biopsy-Breast Cancer Methylation (LBx-BCM) prototype assay using the GeneXpert® cartridge system for early assessment of disease progression in MBC. Study Design: The 9-marker, LBx-BCM prototype assay was evaluated in TBCRC-005, a prospective biomarker study, using plasma collected at baseline, week 4 and week 8 from 144 MBC patients. Results: At week 4 MBC patients with high cumulative methylation (CM) had a significantly shorter median PFS (2.88 months v 6.60 months, p = 0.001) and OS (14.52 months v 22.44 months, p=0.005) compared to those with low CM. In a multivariable model, high versus low CM was also associated with shorter PFS (HR = 1.90, 95%CI 1.20-3.01; p =0.006). Change in CM from baseline to week 4 (OR = 4.60, 95%CI 1.77, 11.93; p = 0.002) and high levels of CM at week 4 (OR = 2.78, 95%CI 1.29, 5.99; p = 0.009) were associated with progressive disease at the time of first restaging. A robust risk model (AUC = 0.733) based on week 4 circulating CM levels was developed to predict disease progression as early as 3 months after initiating a new treatment. Conclusion: The easy to perform and automated LBx-BCM prototype assay is a promising clinical tool for detecting disease progression early after initiating treatment in women with MBC. Further validation in larger clinical datasets is needed. Citation Format: Kala Visvanathan, Leslie Cope, Mary Jo Fackler, Michael Considine, Lisa Carey, Andres Forero-Torres, James N. Ingle, Nancy U. Lin, Rita Nanda, Anna Maria Storniolo, Suzana Tulac, Natalie C. Wu, Sudhakar Marla, Neesha R. Venkatesan, Antonio C. Wolff, Saraswati Sukumar. Evaluation of the Automated Liquid Biopsy-Breast Cancer Methylation (LBx-BCM) Cartridge Assay for Predicting Early Disease Progression and Survival: TBCRC-005 Prospective Trial [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-02-09.

Abstract Purpose: We previously demonstrated that high levels of circulating methylated DNA are associated with subsequent disease progression in women with metastatic breast cancer (MBC). In this study, we evaluated the clinical utility of a novel liquid biopsy-breast cancer methylation (LBx-BCM) prototype assay using the GeneXpert cartridge system for early assessment of disease progression in MBC. Experimental Design: The 9-marker LBx-BCM prototype assay was evaluated in TBCRC 005, a prospective biomarker study, using plasma collected at baseline, week 4, and week 8 from 144 patients with MBC. Results: At week 4, patients with MBC with high cumulative methylation (CM) had a significantly shorter median PFS (2.88 months vs. 6.60 months, P = 0.001) and OS (14.52 months vs. 22.44 months, P = 0.005) compared with those with low CM. In a multivariable model, high versus low CM was also associated with shorter PFS (HR, 1.90; 95% CI, 1.20–3.01; P = 0.006). Change in CM from baseline to week 4 (OR, 4.60; 95% CI, 1.77–11.93; P = 0.002) and high levels of CM at week 4 (OR, 2.78; 95% CI, 1.29–5.99; P = 0.009) were associated with progressive disease at the time of first restaging. A robust risk model based on week 4 circulating CM levels was developed to predict disease progression as early as 3 months after initiating a new treatment. Conclusions: The automated LBx-BCM prototype assay is a promising clinical tool for detecting disease progression a month after initiating treatment in women with MBC undergoing routine care. The next step is to validate its clinical utility for specific treatments.

Abstract Arachidonic acid (AA) metabolizing enzymes, including cyclooxygenases (COX), lipoxygenases, and cytochrome P450s (CYP450), are expressed in breast cancer (BC). Elevated levels of metabolites of COX-derived, pro-inflammatory prostaglandin (PGs) metabolites in urine have been linked to increased risk of developing breast cancer (BC) in two large prospective cohorts. Further, CYP450-derived EETs in BC tumors are associated with metastasis. We hypothesize that measuring levels of AA-derived oxylipins in breast tissues could enable improved BC risk prediction, provide insights into pro-carcinogenic inflammatory mechanisms, and serve as potential surrogate endpoints in chemoprevention trials. Thus, we performed a pilot study to assess the feasibility of measuring these analytes in small breast tissue samples. We analyzed two sample sets, collected under Institutional Review Board (IRB) approved protocols: 50 normal breast tissue cores donated to the Komen Tissue Bank (KTB) via vacuum-assisted biopsy and 10 tissues removed surgically at Mayo Clinic beyond resection margins of lumpectomy samples performed for atypical ductal hyperplasia (ADH). Tissues were frozen and maintained at -80°C. Normal KTB tissue cores were split in half. Tissues for analysis were weighed, extracted in cold methanol containing a cyclooxygenase inhibitor, and assayed for a panel of 42 oxylipins using validated mass spectrometric methods with a limit of detection per analyte of 0.010 ng/mL in supernatant and CVs of approximately &lt;11%. Analyte measurements in paired cores (ng/g tissue) were compared with Spearman correlations and intra-class correlation coefficients. Measurements in surgical samples were assessed descriptively and levels were compared with normal KTB breast tissues.Median weight of the 50 split KTB cores was 0.055 grams. 17 oxylipins were detected in the tissue samples. PGD2 demonstrated the weakest correlation between split cores (r=0.12; p=0.40), whereas 15-HETE demonstrated the strongest correlation (r=0.68; p&lt;0.0001). ICCs ranged from non-significant to nearly perfect, with highest values for 12-HETE (ICC=0.98, 95%CI: 0.97-0.99). Exploratory analyses of the KTB cores demonstrated significant associations between two BC risk factors, alcohol use and family history of breast/ovarian cancer, and several analytes. Levels of most eicosanoids tested were substantially and highly significantly elevated in tissues surrounding ADH versus KTB normal breast tissues. We conclude that eicosanoids can be readily measured in small frozen breast tissue samples obtained via percutaneous biopsy or at surgery. Future studies to assess the associations of eicosanoid levels with BC risk factors and development of BC after a benign biopsy may inform risk prediction and potentially inhibitable inflammatory mechanisms of breast carcinogenesis. Citation Format: Ginger L. Milne, Robert A. Vierkant, Amy C. Degnim, Anna Maria Storniolo, Jill E. Henry, Laura M. Pacheco-Spann, Derek C. Radisky, Mark E. Sherman. Quantification of oxylipins as biomarkers of inflammation and cancer risk in breast tissue [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6333.

525 Background: Homologous recombination deficiency (HRD) status can be used to identify patients who are eligible for treatment with DNA damaging agents. Using a 3-biomarker Genomic Instability Score (GIS) threshold of ≥42, studies have previously examined the association between HRD status and outcomes in patients with triple negative breast cancer (TNBC). However, evidence suggests that a GIS threshold of ≥33 may be more appropriate. Here, we conducted an exploratory analysis evaluating the ability of ≥33 and ≥42 GIS thresholds to predict response to platinum-based treatment in patients with TNBC. Methods: Patients across 5 cohorts (TBCRC0301, TBCRC0082, NCT013725793, PrECOG 01054, combined cisplatin cohort4) were included in this analysis if they had a primary TNBC diagnosis, received neoadjuvant platinum-based treatment, had a valid GIS, and had known pathologic complete response (pCR) status. GIS was determined by a combination of loss of heterozygosity, telomeric-allelic imbalance, and large-scale state transitions.4,5 BRCA mutation status was defined by loss of function resulting from a pathogenic variant in BRCA1 or BRCA2. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV), were calculated by comparing binary threshold status and binary pCR status. Results: A total of 204 tumors (158 BRCAwt; 33 BRCAm; 13 unknown) were included; pCR to platinum-based treatment occurred in 55 cases (39 BRCAwt; 14 BRCAm; 2 unknown). Sensitivity, specificity, PPV, and NPV were comparable between the ≥33 and ≥42 GIS thresholds, with the ≥33 threshold producing higher sensitivity values. This was true when thresholds were applied to all samples and to BRCAwt samples only (Table). Among patients who achieved pCR in response to platinum-based treatment, 5.5% of patients in the full cohort and 7.7% of those in the BRCAwt cohort had a GIS between 33-41. Conclusions: To ensure that the majority of patients likely to benefit from treatment are identified, a GIS of ≥33 may be the most appropriate threshold to predict response to platinum-based treatment in patients with TNBC; however, a prospective trial will be needed to confirm these findings. Additional studies will be important to determine whether this threshold may be appropriate to determine eligibility for other DNA-damaging agents such as PARP inhibitors. 1. Ann Oncol. 2020;31(11):1518-25 2. J Nucl Med. 2015;56(1):31-7. 3. Breast Cancer Res Treat. 2015;151(3):629-38. 4. Clin Cancer Res. 2016;22(15):3764-73. 5. Breast Cancer Res Treat. 2014;16(6):1-9. [Table: see text]

Abstract Background: Patients with homologous recombin ation (HR) deficient tumors may benefit from treatment with DNA damaging agents. Markers of genomic instability can be used to identify HR deficiency, including a 3-biomarker Genomic Instability Score (GIS). For patients with ovarian cancer (OC), the FDA-approved GIS threshold for identifying HR deficiency is 42, set as the 5th percentile for BRCA deficient tumors. Recently, a lower 1st percentile cutoff of 33 was explored in OC; this threshold was significantly associated with improved outcome after platinum-based treatment.1,2 Determining an optimal GIS threshold for different types of tumors is crucial, as the GIS distribution may vary between different cancers and even between different cancer subtypes. We propose GIS thresholds for breast cancer separately for triple-negative breast cancer (TNBC) and estrogen receptor-positive (ER+) breast cancer, using the exploratory threshold of 33 for OC as a comparator. Methods: GISs in BRCA deficient tumors were determined for patients newly diagnosed with varying stages of OC, TNBC, or ER+ breast cancer across 5 cohorts (Timms et al,3 TCGA,4 Abkevich et al,5 TBCRC008,6 the OlympiaD trial7). GIS was determined as a combination of loss of heterozygosity, telomeric-allelic imbalance, and large-scale state transitions. BRCA deficiency was defined by loss of function resulting from a pathogenic variant in BRCA1 or BRCA2 or by methylation of the BRCA1 promoter region, with loss of heterozygosity in the affected gene. GIS distributions in different cancer types and subtypes were compared using the Kolmogorov-Smirnov test. A normal distribution was fit to GISs in BRCA deficient ER+ breast tumors. The 1st percentile of the fitted distribution was chosen as the threshold. Results: A total of 561 OC tumors (190 BRCA deficient), 99 TNBC tumors (44 BRCA deficient), and 406 ER+ breast tumors (76 BRCA deficient) were included across the 5 cohorts. When score distributions were evaluated for BRCA deficient tumors, the GIS distribution within ER+ breast cancer was significantly different than for OC (p=9.6x10-5) and TNBC (p=3.2x10-4). This indicates that different GIS thresholds are appropriate for breast cancer subtypes and that the GIS threshold developed for OC is not appropriate for ER+ breast cancer. The 1st percentile of a normal distribution fit in BRCA deficient ER+ breast cancer tumors yields a threshold of 24. Using this threshold, 45.1% (183/406; 75 BRCA deficient, 108 BRCA intact) of ER+ breast tumors were HR deficient. In contrast, the GIS distribution for TNBC was not significantly different than for OC (p=0.77). Using the exploratory threshold of 33, 63.6% (63/99; 44 BRCA deficient, 19 BRCA intact) of TNBC tumors were HR deficient. Conclusions: When compared to OC, the distribution of GIS in BRCA deficient tumors was different for ER+ breast cancer, but not for TNBC. These findings are consistent with the fact that OC and TNBC are known to have similar molecular signatures.8 Exploratory thresholds of 24 for ER+ breast cancer, and 33 for TNBC and OC could be examined to determine if these cutoffs are associated with a benefit from treatment with DNA targeting agents. Clinical validity and utility of these more inclusive 1% thresholds would require demonstration of correlation with clinical outcomes. The threshold difference observed between these cancer subtypes also suggests that cancer or cancer subtype specific thresholds may be needed as evaluations of HR deficiency expands beyond OC to identify candidates for PARP inhibitors. References: 1 Mol Cancer Res. 2018;16(7):1103-11. 2 Cancers. 2021;13(5):946. 3 Br J Cancer. 2012;107(10):1776-82. 4 Nature. 2012;490(7418):61-70. 5 Breast Cancer Res. 2014;16(145):1-9. 6 J Nucl Med. 2015;56(1):31-7. 7 NEJM. 2017;377(17):1700. 8 Int J Mol Sci. 2016;17(5):759. Citation Format: Kirsten M Timms, Lauren Lenz, Chris Neff, Cara Solimeno, Darl Flake, Judy C Boughey, Matthew P Goetz, Andrea Richardson, Anna Maria Storniolo, Alexander Gutin, Roisin M Connolly, Vered Stearns, Jerry S Lanchbury. Identifying homologous recombination deficiency in breast cancer: Genomic instability score thresholds differ in breast cancer subtypes [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 P5-13-09.