Citations

Abstract Introduction: An Our Practice Advisory (OPA), formerly known as Best Practice Advisory (BPA), is an electronic health record (EHR) tool. OPAs identify key parameters to alert providers of potential interventions that could enhance quality of care. Such functionality is increasingly relevant given the rapid pace of new drug approvals. Within the Mayo Clinic system, we previously identified that amongst Mayo Clinic patients (pts) meeting the MonarchE criteria, 28% of pts did not undergo a documented discussion with their provider regarding the role of adjuvant abemaciclib (Premji, SABCS 2024). Further, a Flatiron study identified that only 40% of eligible pts received abemaciclib (Sandoval, Miami Breast 2025). To address this practice gap, in 2024 we began to implement EHR clinical decision support by creating an OPA trigger for pts eligible for an adjuvant CDK 4/6 inhibitor (CDK 4/6i). Here we report on ‘action taken’ versus not for the OPA and the most common reasons for re-prompts. Methods: The Mayo Breast Disease Group designed an OPA including pts who are within 14 months of a clinical or pathologic diagnosis of high-risk early-stage breast cancer. We leveraged structured data captured in the Epic staging forms (Epic Systems) to identify pts with clinical or pathologic T-stage, N-stage, grade, ± Ki-67/ multigene signature to identify ER+/HER2- pts eligible for adjuvant ribociclib or abemaciclib based on FDA approvals. The OPA was triggered by a patient encounter in oncology when endocrine therapy (but not CDK4/6i) was on the active medication list resulting in a hard stop. The OPA gathered limited details about whether adjuvant CDK4/6 inhibitors were discussed and whether the provider followed a link that resulted in prescribing one—considered an indicator of action taken. In this IRB-exempt quality improvement project, we describe the responses captured in the OPA trigger utilizing Epic tools. Results: The OPA went live on 1/10/2025. Over a 5-month period post OPA initiation, we identified 517 OPA alerts among 332 unique pts. Their median age was 62.5 years, and the majority were female (97%). The median time to resolve the OPA was 17.5 seconds (range 2-567 seconds). 12.6% (65/517) of the time there was ‘action taken’ by selecting the hyperlink to potentially start a treatment plan. Providers selected the following OPA pre-set responses: ‘re-prompt in one month’ 248 (48%), ‘patient not a candidate for therapy’ 48 (9%), ‘patient declines’ 69 (13%), ‘outside oncologist will prescribe’ 36 (7%), ‘not discussed’ 44 (9%), and unknown 72 (14%). The most common free text comments among the 517 OPA alerts (summarized) included: ‘revisit in future’ 108 (21%), ‘too early’ 65 (13%), ‘patient declines’ 66 (13%), ‘outside oncologist will start’ 31 (6%), ‘not a candidate’ 26 (5%), and ‘co-morbidities’ 18 (3%). Erroneous OPA prompting occurred in 14 (3%) who previously failed a CDK4/6i due to toxicity and in 18 (3%) for pts currently on a CDK4/6i. 30 (6%) OPA comments noted that they discussed or started therapy that day. Conclusion: The OPA offered clinical decision support and ensured that clinicians considered prescribing CDK 4/6i in all pts identified as eligible, and captured deferrals. Refinements are needed to ensure appropriate timing of OPA and minimize erroneous firing. The OPA remains active and captures the reason for deferral of adjuvant CDK 4/6i prescription. This data pairs real world decision making to better understand clinically meaningful application of improved invasive disease-free survival (iDFS). Citation Format: S. Premji, P. Advani, S. Chumsri, L. Mina, J. Taraba, J. Carroll, J. Hoppenworth, E. Cathcart-Rake, T. Haddad, B. Rudder, G. Choong, D. Idossa, C. O’Sullivan, S. Yadav, K. Ruddy, P. Peethambaram, T. Hobday, M. Goetz, R. Rao, B. Ernst, F. Batalini, R. Leon-Ferre, A. Tevaarwerk, K. Giridhar. Implementing an Our Practice Advisory (OPA) to provide clinical decision support for adjuvant CDK4/6 inhibitors in early-stage hormone receptor positive breast cancer: The Mayo Clinic Experience [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 PS5-11-10.

Abstract Background: In conflict zones like Syria, accessing cancer care presents immense challenges. However, there is a critical lack of data on the specific barriers patients face. This study aimed to document these obstacles for breast cancer patients in Syria following 14 years of civil war. We surveyed patients presenting for cancer care to better understand the challenges they faced. Methods: This cross-sectional study included adult patients with a confirmed breast cancer diagnosis presenting to Al-Bairouni University Hospital for care. Data was collected through face-to-face interviews using a structured questionnaire adapted for the Syrian context. The survey covered six key areas: Demographic & Clinical, Healthcare System Barriers, Financial Barriers, Geographic & Security barriers, Reasons for Diagnostic Delays, and Psychosocial Impact. Multivariate analysis via logistic regression was conducted to determine which responses were associated with a higher odds ratio (OR) of having a late-stage (III/IV) cancer diagnosis. Results: A total of 109 patients diagnosed with breast cancer were enrolled. The mean age was 51.4 years (±15.2 SD). Advanced diagnostics (CT/MRI/biopsy) and radiation therapy were inaccessible for 58% (n=63) and 63% (n=69) of patients. Systemic therapy drugs were "rarely" or "never" available for 33% (n=36) of patients, and 14% (n=15) reported waiting over three months for an appointment. The costs of medications "greatly" or "very greatly" impacted 86% (n=94) of patients. Over two-thirds of patients reported that essential pain control medications (e.g., morphine) were "rarely" or "never" available (66%, n=72). Nearly two-thirds (67%, n =73) reported that security concerns "greatly" or "somewhat" affected their ability to access treatment. An overwhelming 89% (n=97) of patients reported a negative psychological impact from their treatment course. Patient responses associated with a higher OR of late-stage cancer diagnosis are present in Table 1. Conclusions: Breast cancer patients in Syria confront overwhelming systemic, financial, and security barriers that compromise their care and contribute to advanced diagnoses. The findings highlight a profound humanitarian crisis and underscore the urgent need for targeted public health and aid interventions to support cancer care in conflict settings. Citation Format: A. Al-Bitar, A. Kouli, F. Nahhat, M. Saifo. Systemic Barriers to Breast Cancer Care in Conflict-Affected Syria [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 PS5-11-14.

Abstract Background: Breast cancer (BC) chemoprevention with selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AIs) remains underutilized among high-risk women. Improving knowledge about BC risk and chemoprevention among high-risk patients and their healthcare providers may enhance informed decision-making about this BC risk reduction strategy. Methods: We conducted a cluster randomized controlled trial to evaluate the effectiveness of web-based patient and provider decision support, RealRisks and BNAV, to improve chemoprevention informed choice among women with atypical hyperplasia (AH) or lobular carcinoma in situ (LCIS). Thirty-one sites, including academic and community practices across the U.S., were randomized to standard patient educational materials alone or combined with RealRisks and BNAV. A total of 412 patients and 210 healthcare providers were enrolled. Patient surveys were administered at baseline, 6 and 12 months. The primary outcome at 6 months was chemoprevention informed choice, defined as adequate chemoprevention knowledge and attitudes congruent with decision. Secondary endpoints included BC risk perceptions, worry, chemoprevention knowledge, decision conflict and chemoprevention decision. Assuming 10% loss to follow up (LTFU), roughly equal accrual across sites, and an intraclass coefficient (ICC) of 0.02, we had 90% power to detect a 15% absolute difference in informed choice if the event rate in the control arm was 10%. Results: Among 24 evaluable sites, 14 were randomized to the intervention arm and 10 to the control arm. Among 210 enrolled providers, median age was 46 (range, 28-83) and 78% were female; 75% physicians and 25% advanced practice providers/nurses; 53% medical oncologists, 29% surgeons, and 10% gynecologists/family practitioners/internists; and 83% had previously prescribed chemoprevention. Among 412 enrolled patients, median age was 53 (range, 35-74), including 77% White, 8% Black, and 10% Asian race, and 14% Hispanic ethnicity. In terms of BC risk factors, 77% had AH and 23% LCIS, 34% had a first-degree family history of BC, and 65% had heterogeneously or extremely dense breasts on mammography. Thirty percent were unevaluable for the primary endpoint (1% ineligible, 26% LTFU, 3% missing data). Among 288 evaluable patients (148 at intervention sites, 140 at control sites), the proportion of patients exercising informed choice at 6 months in the intervention and control arms was 35% vs. 27%, respectively (odds ratio [OR]=1.44, 95% confidence interval [CI]=0.84-2.46, p=0.19; observed ICC=0.006). Compared to patients in the control arm at 6 months, those in the intervention arm were more likely to have accurate BC risk perceptions (OR=1.80, 95% CI=1.11-2.91) and adequate chemoprevention knowledge (OR=1.62, 95% CI=1.00-2.61). We observed no significant differences in BC worry, decision conflict, or chemoprevention decision between study arms. At 6 months, 49% of women in the control arm and 51% in the intervention arm decided to take a SERM or AI for BC chemoprevention (p=0.58), a rate higher than anticipated. Conclusions: We did not observe a significant increase in informed choice among patients and providers assigned to chemoprevention decision support compared to standard educational materials alone. Among women with AH or LCIS seen mainly by breast specialists who prescribed chemoprevention, about half decided to take a SERM or AI. Therefore, relatively high chemoprevention uptake may be achieved among women with high-risk breast lesions managed by breast specialists. Chemoprevention decision support may improve decision antecedents, such as accurate BC risk perceptions and chemoprevention knowledge, but may have a limited role in changing health behaviors. Citation Format: K. D. Crew, G. L. Anderson, K. B. Arnold, A. Michel, M. T. DeLucie, C. W. Law, S. Pruthi, A. C. Sandoval Leon, R. Shirley, M. T. Grosse Perdekamp, S. V. Colonna, S. L. Krisher, T. King, L. D. Yee, T. J. Ballinger, C. Braun-Inglis, D. A. Mangino, K. B. Wisinski, C. A. DeYoung, M. Ross, J. D. Floyd, A. Kaster, L. VanderWalde, T. Saphner, C. Zarwan, S. Lo, C. Graham, A. K. Conlin, K. J. Yost, D. M. Agnese, C. Jernigan, D. L. Hershman, M. L. Neuhouser, J. A. Zell, B. Arun, R. Kukafka. Making informed choices on incorporating chemoprevention into care (MiCHOICE): Cluster randomized controlled trial of decision support for breast cancer chemoprevention, SWOG 1904 [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 PS3-01-15.

Abstract Uptake of tamoxifen is limited to 5-15% of all eligible women due to concerns regarding medication side effects combined with the lack of ability to predict which women will benefit from the therapies. We used a unique retrospective sample of n=92 high-risk women with paired benign biopsies obtained before (PreTam) and during tamoxifen (OnTam) to discover tissue gene expression biomarkers of biologic response, defined as change (Δ) in lobular Ki67. Ki67 was digitally quantified in normal lobules from PreTam and OnTam benign tissue samples and samples with PreTam Ki67 = 0 were excluded. Data were analyzed from 51 women with both PreTam and OnTam Ki67 (median age = 49 years and median BMI = 25.5 kg/m2 at PreTam biopsy). OnTam biopsies were collected a median of 11.5 months after PreTam biopsy (median 6.5 months after starting tamoxifen; dose 20 mg/day in 86%). Median Ki67 significantly decreased overall (Δ Ki67 = -0.29%, p=0.003), but the effect was restricted to women with baseline Ki67 ≥ 0.5% (n=38, Δ Ki67 = -0.78%, p=0.0002); no significant change occurred with baseline Ki67 < 0.5 (Δ Ki67 = 0.19%, p=0.23). Δ Ki67 was inversely proportional to baseline Ki67. RNA-seq was successful for 171 benign tissue samples (including 53 PreTam-OnTam pairs). After edgeR normalization (>14,600 genes), PreTam vs OnTam comparison across 33 pairs with baseline Ki67 ≥ 0.5% yielded 66 differentially expressed genes (fold-change > 2 and p<0.01). At p<0.01, ranked gene set enrichment analysis revealed 27 gene sets enriched for reduced expression with Tam (with top-ranked gene set related to steroid hormone biosynthesis) and 59 gene sets enriched for increased expression with Tam (with top-ranking gene sets related to B cell and T cell receptor signaling, regulation of cell cycle, and other immune-related pathways). To identify predictors of response, we correlated baseline PreTam gene expression with Δ Ki67 in 37 women with available baseline PreTam RNAseq and Δ Ki67 data. Partial correlation analysis (via ‘ppcor’ package, adjusted for library qubit) revealed 73 genes with significant positive correlation (rho > 0.50) and 14 genes with significant negative correlation (rho < -0.50) with Δ Ki67 (p<0.01). At p<0.01, ranked gene set enrichment analysis yielded 67 gene sets enriched with negative correlation (i.e. increased expression associated with a reduction in Ki67), including pathways related to ribosome biogenesis, DNA recombination, and nucleic acid metabolic processes; there were 62 gene sets enriched for positive correlation (i.e. decreased expression associated with a reduction in Ki67), including pathways such as VEGF and ERBB signaling. Our data support (i) baseline Ki67 ≥ 0.5 % and (ii) an 87-gene baseline signature as complementary biomarkers of tamoxifen response. Prospective validation of this combined assay could target tamoxifen to the women most likely to benefit, potentially overcoming a major barrier to chemoprevention uptake. Citation Format: S. J. Winham, C. Wang, Y. Liu, B. M. McCauley, D. J. Tabrizi, D. L. Gehling, J. L. Fischer, L. R. Seymour, M. H. Solanki, M. E. Sherman, K. R. Lohani, T. L. Hoskin, D. C. Radisky, A. C. Degnim. Gene expression signatures of beneficial response to tamoxifen for breast cancer prevention [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 PS3-01-18.

Abstract Breast cancer is the most common female malignancy in the U.S. and the leading cause of cancer-related deaths in women worldwide. Nearly 80% of breast tumors express estrogen receptor alpha (ERα) which drives tumor progression. While CDK4/6 inhibitors in combination with endocrine therapy improve overall survival in the metastatic setting and disease-free survival in the adjuvant setting, many patients eventually relapse/progress with resistant forms of disease. In the post-CDK4/6 inhibitor setting, second line endocrine therapy responses are limited, highlighting the need for alternative treatment approaches. Through a genome-wide CRISPR knockout screen, we identified Lemur Tail Kinase 2 (LMTK2) as the top hit exhibiting synthetic lethality in the presence of the tamoxifen metabolite, endoxifen, in both endocrine-sensitive and -resistant cell line models. We further studied multiple other endocrine resistant models and identified that LMTK2 mRNA and protein expression were both increased in ERα cell lines resistant to estrogen deprivation, tamoxifen, fulvestrant, abemaciclib and palbociclib, as well as in endocrine resistant patient derived models. We then studied LMTK2 in patient samples and identified copy number gain/amplification in 67% of ERα+ human metastatic breast tumors, while only 1% of primary tumors harbored these genomic alterations. From a prognostic standpoint, elevated tumoral LMTK2 expression was associated with worse recurrence-free survival and overall survival in early-stage breast cancer. Through proteomic studies, we identified the MET-PI3K-AKT pathway as overactivated in LMTK2 overexpressing cells. Unlike activating PIK3CA mutations that drive AKT signaling, LMTK2 was found to employ an alternative mechanism to activate AKT signaling by inhibiting PP1α, an AKT-inhibitory phosphatase. Further, LMTK2 overexpression was sufficient to drive endocrine and CDK4/6 inhibitor resistance in vitro and resulted in dramatic increases in AKT and ERα phosphorylation and activity. While knockout of LMTK2 in treatment naïve ERα+ breast cancer cells neither altered AKT/ERα phosphorylation/activity nor endocrine/CDK4/6 inhibitor responsiveness, attempts to knockout LMTK2 in endocrine/CDK4/6 inhibitor-resistant models failed, further confirming its essentiality in this setting. In summary we have uncovered LMTK2 as a novel biomarker of endocrine/CDK4/6 inhibitor resistance that is mechanistically linked to the AKT pathway. LMTK2 may be a novel drug target for the effective treatment of resistant and advanced forms of ERα+ breast cancer. Citation Format: A. M. Detry, S. Marigliano, J. Heinen, A. Bass, C. Jones, R. S. Muthyala, M. P. Goetz, J. R. Hawse. Lmtk2: a novel driver of endocrine resistance in ER+ breast cancer [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 PS2-11-01.

Abstract Benign breast disease (BBD) confers elevated risk for breast cancer, yet current tools lack precision in identifying lesions likely to progress. Immune mechanisms may influence progression, but the spatial and molecular features of immune responses in BBD versus invasive cancer remain poorly characterized. We applied Digital Spatial Proteomics (DSP) to evaluate immune-related protein expression across 1,540 regions of interest (ROIs) from breast tumors (n=58, Carolina Breast Cancer Study [CBCS]), a UNC BBD cohort (n=20), and a Mayo BBD cohort (n=177). Using Consensus Clustering in CBCS and UNC BBD, we identified two stable immune phenotypes: “High Immune,” enriched in tumors and high-risk benign lesions, and “Quiet Immune,” predominant in low-risk BBD. ROIs were classified by immune marker expression, and patients were labeled as “High” if ≥50% of their ROIs were High Immune. In CBCS, High Immune profiles were associated with ER-negative status (OR=2.92 [0.99-8.65] participant-level; OR=3.28 [1.34-8.04] ROI-level). In BBD, High Immune profiles trended toward higher odds of high-risk lesions (OR=1.00 [0.15-6.70] participant-level; OR=1.68 [0.57-4.97] ROI-level), though this pattern was not replicated in the Mayo cohort. BBD samples exhibited substantial spatial heterogeneity in immune expression (40% within-patient ROI agreement vs. 74% in tumors), potentially limiting biomarker reliability. Nonetheless, four immune proteins (CD44, CD80, CD14, CD66b) were significantly associated with future cancer development in the Mayo cohort, though these findings require validation due to within-person variability. Our findings highlight the challenges posed by spatial heterogeneity in assessing immune biomarkers in BBD and suggest that immune activation may emerge later in disease progression. These results raise important questions about the timing and consistency of immune responses in the natural history of breast cancer. Citation Format: A. Zhang, A. Cozzo, L. Olsson, E. Kirk, X. Gao, S. Nyante, B. Calhoun, R. Vierkant, M. Sherman, M. Troester. Immune Heterogeneity in Breast Cancer Natural History: Benign Breast Disease to Breast Cancer [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 PS2-05-23.

Abstract Background PALLAS trial compared palbociclib plus endocrine therapy (ET) vs. ET alone in stage II-III HR+/HER2- breast cancer. SETER/PR index quantifies transcriptional activity related to estrogen and progesterone receptors and serves as a measure of endocrine sensitivity. In CALGB 9741, low SETER/PR (< 0.75) - reflecting endocrine resistance - predicted survival benefit from adjuvant anthracycline/taxane-based treatment. In PALLAS low SETER/PR was associated with worse prognosis and early recurrence. Early recurrences driven by endocrine-resistant tumors may obscure the benefit of palbociclib by disproportionately contributing events from a more resistant population. We hypothesized that palbociclib efficacy might vary by endocrine sensitivity and prior chemotherapy regimen, with greatest benefit in patients with intermediate SETER/PR index. Methods Invasive disease-free survival (iDFS) was summarized with Kaplan-Meier plots and the treatment arms were compared with a log-rank test. Cox models estimated the hazard ratio (HR) with 95% confidence interval (95%CI) (significance level used was a two-sided 0.05). SETER/PR index was successfully obtained for 3,090 PALLAS patients (TransPALLAS ITT cohort, Metzger et al. ASCO 2024). SETER/PR index cut points (0.75 and 1.50) were proposed from previous studies comparing different adjuvant chemotherapy regimens. Results Table 1 summarizes the efficacy of adjuvant palbociclib by prior (neo)adjuvant chemotherapy regimen. Palbociclib was associated with longer iDFS in the group who received prior anthracycline-paclitaxel chemotherapy (AC-P), though the chemo regimen x palbociclib interaction term was not significant (Table 1). In the 1,132 patients from the TransPALLAS cohort who received AC-P chemotherapy, the 518 (46%) patients with intermediate SETER/PR (0.75≤ SETER/PR <1.50) had longer iDFS with palbociclib (HR 0.58; 95% CI: 0.40-0.84; p=0.004; 7-yr iDFS 80.2% vs. 68.4%), but this subset did not predict palbociclib benefit relative to all others, i.e., 206 (18%) patients with low SETER/PR (<0.75) and 408 (36%) high SETER/PR (≥1.50); P interaction=0.11. Exploratory analysis of SETER/PR index as a continuous variable in patients who received prior AC-P chemotherapy revealed a non-linear relationship with the hazard ratio favoring palbociclib observed in the subset with SETER/PR values between 1.1 and 1.8, compared to all others (P interaction=0.01). Conclusions In PALLAS, we observed a numerical benefit favoring palbociclib among pts treated with adjuvant AC-P; in this subset a moderate level of endocrine transcriptional activity (SETER/PR index between 1.1 and 1.8) predicted palbociclib benefit. These results are hypothesis-generating. Citation Format: O. Metzger, P. O'Brien, K. Ballman, M. Gnant, M. Watson, E. Chen, K. Tran, D. Hlauschek, M. Martin, J. Balko, Z. Nowecki, O. Hahn, C. Denkert, C. Curtis, F. Liu, A. Dueck, C. Fesl, E. Mayer, A. De Michele, W. Symmans. Exploratory analysis of palbociclib benefit in the PALLAS trial by SETERPR index and prior chemotherapy regimens (ABCSG-42/AFT-05/BIG-14-13) [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 PS3-08-13.

Subglottic stenosis (SGS) may arise from both mechanical and inflammatory airway injury. Patients with mast cell activation syndrome (MCAS) are uniquely vulnerable, due to both direct effects of systemic inflammation and indirect effects of repeated intubation for anaphylaxis that can converge to cause life-threatening airway compromise. We describe a sentinel case of a 34-year-old female with MCAS and multiple prior intubations who presented with progressive dyspnea. Flexible laryngoscopy revealed grade III SGS. She underwent urgent endoscopic intervention, including balloon dilation, carbon dioxide (CO 2 ) laser radial incisions, and steroid injection. Airway patency was restored, and postoperative care focused on optimizing systemic mast cell control with cromolyn, fexofenadine, montelukast, and omalizumab. Literature review supports a mechanistic role for mast cell mediators in airway fibrosis and impaired wound healing, compounding the effects of mechanical trauma in MCAS. This sentinel case highlights clinically significant risk of SGS in patients with MCAS. By integrating case findings with mechanistic insights, this report underscores the urgent need for interdisciplinary awareness, proactive airway evaluation, and research into surveillance and prevention strategies in MCAS and related disorders. Level of Evidence: Level V/Case Report and Narrative Review.

Abstract Background: Metformin may impact tumorigenesis directly, through effects on AMP kinase, and indirectly, through reduction in circulating growth factors such as insulin. To examine the potential preventive effects of metformin, we analyzed MD, a known risk factor for breast cancer, in participants enrolled in the NCIC MA.32 trial, a randomized, placebo-controlled trial investigating the effects of 5 years of metformin on invasive disease-free survival in women with early breast cancer. The primary aim of this companion to MA.32 (Alliance A211201) was to examine the effects of 1 year of metformin vs placebo on MD in women treated for estrogen receptor negative (ER-) breast cancer. Methods: Eligible women were enrolled on MA.32, had ER- breast cancer, had >25% MD at baseline, and had an unaffected contralateral breast. MD was calculated on the contralateral breast using Cumulus software and analyzed at baseline, 1 and 2 years. The aim was to achieve a sample size of 274 (137 per arm), which would have at least 85% power to detect a 4% difference between study arms, i.e., 5% change from baseline to one year of treatment for metformin vs. 1% for placebo, using a two-sided two-sample t-test at a significance level of 5%. The current study was activated in the United States on 8/22/2012. Enrollment was halted on 10/13/2017 with 177 participants enrolled. Results: 146 women were evaluable (66 on metformin and 80 on placebo). The mean age of evaluable women was 53.6 years (standard deviation [SD] 9.4), 88.4% were white, 31.5% had a body mass index greater than or equal to 30 kg/m2, 99.3% of participants received chemotherapy. The proportions of white women and HER2+ participants in the metformin group were higher compared to placebo ([white 92.4% vs. 85.0%] and [HER2+ 21.2% vs. 11.3%], respectively). The mean MD at baseline was 22.7% (SD 14.1%) for the group as a whole (24.5% (SD 14.5%) for the metformin group and 21.2% (SD 13.6%) for the placebo group. This is lower than the eligibility criteria (>25%MD), due to difference in methods of calculation of MD (BIRADs for eligibility and Cumulus for study analysis). At 1 year, MD decreased by 0.7% (SD 7.6%) in the metformin group and 0.2% (SD 7.3%) in the placebo group (p = 0.58). At 2 years, MD decreased by 0.7% (SD 8.0%) in the metformin group and 0.8% (SD 7.1%) in the placebo group (p=0.60). Conclusions: In a cohort of women with early ER- breast cancer, there was no significant difference in the change in MD in women treated with metformin vs. placebo. There was a non-significant trend toward decreased MD while on metformin after 1 year of treatment. The arms were well balanced, though with more white and HER2+ participants in the metformin arm. The sample size for this study was about half of what was intended thus limiting power to detect a significant difference. Additional limitations include low mean MD, which may have limited our ability to detect the benefit in those at the highest risk, and further, the Cumulus method of MD determination can be imprecise. Interestingly, trends toward decreased MD with metformin were seen in Alliance A211102, and it might be reasonable to consider further study of metformin in a select population (highest MD or atypical hyperplasia). Support: UG1CA189823, U10CA180863, CCS 707213; https://acknowledgments.alliancefound.org.Clinicaltrials.gov ID: NCT01666171. Citation Format: M. Wood, M. I. Elsaid, L. J. Grimm, H. Liu, C. Oswold, C. S. Kuzma, I. Bedrosian, J. Ligibel, P. J. Goodwin. Change in mammographic density (MD) with metformin use: Alliance A211201, A companion study to NCIC study MA.32 [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 PS3-01-30.

Abstract Introduction Triple-negative breast cancer (TNBC) is an aggressive subtype lacking estrogen (ER), progesterone (PR), and HER2 expression. It accounts for ∼15% of breast cancers and is more common in Black women, younger patients, and those with BRCA1 mutations. TNBC carries a poor prognosis with 5-year survival rates of 92% (localized), 66.8% (regional), and 14.3% (metastatic). Patients with high-risk of recurrence can be treated with different adjuvant therapies based on results of 3 trials (capecitabine, pembrolizumab, olaparib), but data on adverse events (AE) on their combination is limited.High-risk, stage II-III TNBC is treated with curative intent using neoadjuvant chemotherapy with pembrolizumab, followed by surgery and radiation. Pembrolizumab, an anti-PD-1 checkpoint inhibitor, was FDA-approved in 2021 based on KEYNOTE-522. This phase III trial showed that pembrolizumab plus chemotherapy significantly improved pathologic complete response (pCR) and event-free survival (EFS). 3-year EFS was 84.5% vs. 76.8% with placebo (HR 0.63); 5-year follow-up confirmed sustained benefit, with OS of 86.6%. Patients achieving pCR had better outcomes, while those with residual disease remained at high risk for recurrence, justifying further adjuvant therapy.In KEYNOTE-522, grade ≥3 treatment-related (AEs occurred in 77.1% receiving pembrolizumab-chemotherapy vs. 73.3% with placebo-chemotherapy. Pembrolizumab was linked to more serious AEs (34.1% vs. 20.1%) and higher discontinuation rates (23.3% vs. 12.3%). Before pembrolizumab, capecitabine was standard of care for TNBC with residual disease, based on the CREATE-X trial (HR 0.52 for OS benefit). Common AEs included hand-foot syndrome, fatigue, and liver enzyme elevation. Notably, KEYNOTE-522 excluded adjuvant capecitabine, and CREATE-X predated the use of pembrolizumab. Olaparib, a PARP inhibitor, was FDA-approved in 2022 for high-risk early TNBC with germline BRCA mutations, based on OlympiA. It improved invasive disease-free and OS. Grade ≥3 AEs included anemia, neutropenia, and fatigue. OlympiA did not include pembrolizumab or capecitabine, limiting insight into optimal sequencing or combination. Currently, no trials compare pembrolizumab, capecitabine, or olaparib directly, nor do they guide how to sequence or combine these therapies. Despite NCCN guidelines allowing combined/sequential use in high-risk patients, real-world safety and outcome data are lacking. This study aims to evaluate and compare the safety and effectiveness of capecitabine, olaparib, pembrolizumab, and their combinations. We will describe AE rates, onset, discontinuation, and dose modifications to inform clinical management and help reduce early termination of effective therapies. Methods We conducted a retrospective observational study across all Mayo Clinic sites. Eligible patients were ≥18 years with early-stage TNBC who initiated adjuvant therapy with capecitabine, pembrolizumab, olaparib, or combinations (capecitabine + pembrolizumab or olaparib + pembrolizumab) from October 2018 to September 2024. Patients with metastatic disease, other active malignancies, or receiving concurrent investigational therapy were excluded. The primary outcomes were incidence and timing of treatment-related AEs. Secondary outcomes included time to AE resolution, immune-related AEs, treatment interruptions/modifications, early discontinuation, and EFS. Time-to-event data will be analyzed using Kaplan-Meier and Cox proportional hazards models. Subgroup analyses by age, DPYD status, regimen, and dosing will be conducted. Data was abstracted from electronic health records and managed using REDCap. Results Data collection is completed. Final analyses are being conducted, and results will be presented at SABCS 2025. Citation Format: R. Garza-Morales, F. Raheem, D. Alton, S. Sharma, B. Murad, W. Harris, E. Terwilliger, F. Batalini. Analysis of Adverse Events and Outcomes of Adjuvant Therapies in TNBC as Monotherapy and Combination: Pembrolizumab, Capecitabine, and Olaparib [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 PS4-10-06.