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Abstract Background PARP inhibitors (PARPi) are active in patients (pts) with germline BRCA1/2 or PALB2 mutated (gBRCA1/2; gPALB2m) breast cancer (BC). Preclinical data suggest PARPi leads to intratumoral activation of the proinflammatory cGAS/STING pathway, recruiting CD8+ T cells, and sensitizing gBRCAm cancers to immune-targeted therapy. Prolonged preoperative treatment with PARPi monotherapy has demonstrated notable pathologic complete response (pCR) rates. The randomized phase II TBCRC 056 study evaluates the PARPi niraparib (N) with the anti-PD-1 antibody dostarlimab (D) as neoadjuvant treatment of gBRCAm or gPALB2mHER2-negative BC, with cohorts for both triple negative BC (TNBC, Arms A and B) and estrogen receptor positive (ER+) BC (Arm C). Results from Arms A and B are being presented. Methods Eligible pts (gBRCA1/2m/gPALB2m BC, T >1.0 cm, ER- (<10%), HER2-) were randomized to either Arm A: upfront combination N 200 mg orally QD and D 500 mg IV every 3 weeks (wks) for 18 wks (1 cycle = 3 wks), or Arm B: N monotherapy for 3 wks, followed by combination N plus D for 15 wks, with baseline (BL) and wk 3 tumor biopsies. After 18 wks, pts either underwent surgery or received additional preoperative systemic therapy (per MD choice) after an end of treatment biopsy if residual disease present. The primary study objectives are to evaluate pCR in Arms A and B combined, as well as change in stromal tumor infiltrating lymphocytes (sTILs) from BL to 3 wks in each Arm separately; 46 pts and a target pCR rate of 50% provides 85% power to reject the null hypothesis of pCR < 30%. Results A total of 46 pts with TNBC enrolled to Arms A and B (38 gBRCA1m (82.6%), 8 gBRCA2m (17.4%), 0 gPALB2m). Median age was 39.3 years (range 24.8-72.8); 84.8% self-reported White, 6.5% Black, 4.3% Asian, and 8.7% Hispanic. Clinical stage distribution was 37.0% stage I, 45.7% stage II, and 17.4% stage III; most had ER- BC (42 ER <1%, and 4 ER 1-10%). Among all pts, 38 (82.6%) completed the target number of cycles of D, with mean cycles received 5.1. Five pts discontinued D early (3 inadequate response/progression, 2 unacceptable AE: LFTs, rash). A total of 38 pts (82.6%) completed 6 cycles of N, with mean cycles received 5.7. Seven pts discontinued N early (3 inadequate response/progression, 4 unacceptable AE: LFTs, anemia, rash (2)). Most common AEs (> grade 2) with D+N were anemia (26.1%), fatigue (21.7%), hypertension (15.2%), hypothyroidism (15.2%), and neutropenia (15.2%). Most common Grade >3 AEs with D+N were anemia (17%) and neutropenia (6.5%). Among all pts, 23 (50%) had a pCR at surgery (90% CI 37.1% - 62.9%), 12 (26.1%) had residual disease (4 RCB-I, 5 RCB-II, 2 RCB-III, 1 RCB not calculable), and 11 (23.9%) crossed over to additional preoperative therapy. The pCR rate exceeded the protocol efficacy criterion of >43%, a significant increase over null hypothesis. pCR rates were the same on Arms A (50%, 90% CI 31.1% - 68.9%) and B (50%, 90% CI 31.9% - 68.1%). Among 15 pts on Arm A with evaluable sTILs at BL and 3 wks, mean sTILs at BL was 16% (range 1% - 40%), and at 3 wks was 27.4% (range 0.5% - 60%), for an average increase of 11.4% (p=0.009). Among 22 pts on Arm B with evaluable sTILs at BL and 3 wks, mean sTILs at BL was 19.5% (range 1% - 75%), and at 3 wks was 42.1% (range 1% - 85%), for an average increase of 22.7% (p=0.0003). Among all pts, BL sTILs (continuous) were significantly higher among those who achieved pCR than those who did not (median 15% vs. 5%, p=0.03). There was no relationship between pCR and BL PD-L1 score, or pCR and ER-0 vs. ER-low. Conclusions In pts with gBRCAm early TNBC, 18 wks of targeted therapy using PARPi and anti-PD1 agents, with or without 3-week PARPi lead-in, resulted in a pCR rate of 50%, and a statistically significant increase in sTILs from BL to 3 wks. Further ongoing correlative work may identify best candidates for this non-chemotherapy-based approach. Citation Format: E. L. Mayer, N. Graham, R. A. Leon-Ferre, M. Rozenblit, C. A. Santa-Maria, S. Isakoff, J. Specht, N. Tung, V. Abramson, J. Desrosiers, A. Schubert, B. Koca, S. M. Tolaney, E. P. Winer, I. E. Krop, A. C. Wolff, G. I. Shapiro, N. Tayob, J. L. Guerriero. Tbcrc 056: a phase 2 study of neoadjuvant niraparib with dostarlimab for patients with BRCA- or PALB2-mutated breast cancer: results from the TNBC cohorts [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 RF5-02.

Abstract Background: Most patients with estrogen receptor-positive (ER+), metastatic breast cancer (mBC) treated with endocrine therapy (ET) will ultimately develop resistance. A large unmet need exists especially when resistance occurs following treatment with a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i), which is potentially driven by a mutation in the ERα-coding gene, ESR1. Lasofoxifene (LAS), an oral, next-generation ET and ER breast antagonist, was evaluated in two phase 2 studies of women with ER+/HER2- mBC and an ESR1 mutation who had disease progression on previous ET and CDK4/6i. In the ELAINE 1 trial, LAS monotherapy provided numerically greater progression-free survival (PFS, median 5.6 mos vs 3.7 mos; HR: 0.669 [95% CI, 0.445‒1.125], P=0.138), objective response rate (ORR, 13% vs 3%; P=0.124), and clinical benefit rate (CBR, 37% vs 22%; P=0.117) compared with the ER degrader fulvestrant (fulv), with a favorable safety profile (Goetz MP et al. Ann Oncol 2023;34:1141‒51). The single-arm, ELAINE 2 trial showed that LAS combined with abemaciclib (Abema) was well tolerated with a median PFS of ∼13 mos, ORR of 56%, and CBR of 66% (Damodaran S et al. Ann Oncol 2023;34:1131‒40). The phase 3, registrational, ELAINE 3 trial was initiated based on these promising earlier-phase data. Recent results from postMONARCH confirmed superior efficacy of fulv/Abema over fulv/placebo (mPFS 6.0 mos vs 5.3 mos, HR 0.73 [0.57‒0.95], nominal P-value 0.017) in non-biomarker‒selected, mBC patients after progression on a prior CDK4/6i and aromatase inhibitor (AI), with similar HR in an ESR1-mutated cohort (Kalinsky K et al. J Clin Oncol 2025;43:1101‒12). Additionally, recent data from EMBER-3 demonstrated meaningful PFS with a novel ET combined with abemaciclib after progression on AI/CDK4/6i (Jhaveri KL et al. N Engl J Med 2025;392:1189‒202). The ELAINE 3, global trial will compare LAS/Abema with fulv/Abema in a post-CDK4/6i, ESR1-mutation‒selected, mBC population. Methods: ELAINE 3 (NCT05696626) is an open-label, phase 3, multicenter study evaluating the efficacy, safety, and tolerability of LAS plus Abema versus fulv plus Abema. Patients are being enrolled at approximately 152 sites in 13 countries (Australia, Belgium, Canada, China, France, Italy, Israel, Poland, Romania, Spain, Taiwan, United Kingdom, United States). Key inclusion criteria are pre- and postmenopausal women and men aged ≥18 yrs; ER+/HER2-, locally advanced and/or mBC (measurable and/or non-measurable disease); ≥1 acquired ESR1 mutation; progression on an AI plus palbociclib or ribociclib as their first hormonal treatment for advanced/mBC; and ≤1 line of chemotherapy in the advanced/metastatic setting. Patients will be randomized 1:1 to receive LAS 5 mg/day plus Abema 150 mg BID, or fulv 500 mg IM (days 1, 15, and 29, then once monthly) plus Abema 150 mg BID. Treatment will continue until progression, death, unacceptable toxicity, or withdrawal from the study. The primary endpoint is PFS per RESIST 1.1 by blinded independent central review (BICR); key secondary endpoints are ORR, overall survival, CBR, duration of response, and time to response. Safety, time to cytotoxic chemotherapy, and quality of life (including vaginal health) will also be evaluated. Blood samples for circulating tumor DNA (ctDNA) will be collected for genomic analyses at screening, at weeks 4 and 8 and every 8 weeks thereafter, and at the final visit. Outcomes with LAS/Abema and fulv/Abema will be compared using a stratified Cox proportional hazards model and stratified logrank test with an expected PFS hazard ratio of 0.68 at final analysis. Enrollment is currently ongoing to meet the target sample size of 500 patients. Citation Format: M. P. Goetz, S. A. Wander, T. Bachelot, G. Curigliano, A. de Nonneville, E. N. Gal-Yam, K. Jhaveri, S. L. Sammons, S. Shen, C. J. Twelves, G. Gasper, P. V. Plourde, D. J. Portman, S. Damodaran. Open-label, randomized, multicenter, phase 3, ELAINE 3 study of the efficacy and safety of lasofoxifene plus abemaciclib for treating ER+/HER2-, locally advanced or metastatic breast cancer with an ESR1 mutation [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 PS5-07-30.

Abstract Background: Endocrine therapy (ET) with aromatase inhibitors (AIs) plus ovarian function suppression (OFS) is a standard endocrine strategy for premenopausal women with ER+/HER2- breast cancer. However, poor tolerability to AI+OFS limits its use, leaving tamoxifen as the only alternative. Prospective studies demonstrated that premenopausal women treated with neoadjuvant ET whose tumors exhibit Ki-67 ≤ 10% following 4 weeks of therapy achieve 5-year distant disease-free survival > 96%. Problematically, only 41% of patients reach this threshold with tamoxifen compared to 78% with AI+OFS (Nitz JCO 2022). (Z)-endoxifen (ENDX) is a potent selective estrogen receptor modulator that dually targets ERα and PKCβ1. At plasma concentrations of 3-5 ng/mL, it inhibits ERα; at ≥ 500 ng/mL, it targets PKCβ1, leading to AKT suppression. The EVANGELINE trial aims to examine endocrine sensitivity in terms of Week 4 Ki-67 in premenopausal women whose baseline Ki-67 > 10%, and objective response rate after 24 weeks of ENDX + goserelin in premenopausal women with baseline Ki-67≤ 10%. Methods: EVANGELINE (NCT05607004) is a multicenter, open-label, Phase 2 study comprised of three parts. Eligible patients are premenopausal women with Stage IIA/IIB ER+/HER2- breast cancer. • Part 1 (PK Run-in): assessed 40mg vs 80mg ENDX (+/- OFS) in 22 patients. • Part 2: patients with baseline Ki-67 >10% were randomized to 40 mg ENDX + goserelin or exemestane + goserelin for 6 months with the primary objective being 4-week ESD rate. Patients with a baseline Ki-67 ≤10% were assigned to a single-arm cohort of 40 mg ENDX monotherapy (no OFS) to evaluate the 24-week endocrine sensitivity disease rate. • Part 3 (replaces Part 2): all patients are treated with ENDX 40mg daily + goserelin every 28 days. A two stage Phase II design was chosen to assess outcomes as follows: Part 3a: Up to 45 patients with baseline Ki-67> 10% to assess Week 4 Ki-67≤ 10% rate is at least 65%. If 9 or more of these patients have a Week 4 Ki-67>10%, enrollment will be stopped due to futility. Otherwise, enrollment will continue. Part 3b: Up to 20 patients enrolled with baseline Ki-67≤ 10% to assess Week 24 objective response rate. Ki-67 levels are determined by central laboratory and images are centrally reviewed. Secondary objectives include safety, tolerability, surgical outcomes, and biomarker analysis using paired biopsies (baseline, week 4) and surgical specimens. Results: The PK run-in (completed Fall 2024) enrolled 22 patients and assessed ENDX doses (40 mg daily vs 80 mg daily (+/-OFS)). The Week 4 Ki-67 ≤ 10% rate with ENDX was 86% and did not differ according to dose. Early safety, PK, efficacy and compliance data supported the selection of 40 mg ENDX daily as the optimal dose. Enrollment in Part 2 began in May 2025. Part 3 will replace Part 2 with simplification of the trial design. Conclusions: EVANGELINE is the first trial to evaluate (Z)-endoxifen + OFS as neoadjuvant therapy in premenopausal ER+/HER2- breast cancer. By targeting both ER and PKC pathways, ENDX may offer a well-tolerated alternative to AI-based regimens and expand endocrine therapy options for this population. Clinical Trial Registration: NCT05607004 Citation Format: M. P. Goetz, V. J. Suman, C. Lopez, H. Erickson, L. Beaulieu, S. S. Hammer, L. Mina, R. Leon-Ferre, K. N. Hunt, M. Piltin, A. Degnim, J. N. Ingle, J. C. Boughey, B. A. Sarah, J. M. Reid, M. Schellenberg, J. R. Hawse, P. Advani, K. Giridhar, F. Batalini, D. Flora, J. M. Jones, N. A. Bagegni, J. Jakub, V. Abramson, V. Dabak, W. J. Irvin, E. Sima, K. Seema, S. C. Quay. A Phase 2 Trial of (Z)-endoxifen + Goserelin as Neoadjuvant Treatment for Premenopausal Women with ER+, HER2-, Breast Cancer (EVANGELINE) [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 PS5-12-16.

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: 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 Introduction: Metaplastic breast cancer (MpBC) is a rare and histologically diverse subtype of breast cancer (BC) that is frequently triple negative (TN) and exhibits poor responsiveness to chemotherapy (CT). Our group previously reported a low pathologic complete response (pCR) rate among patients (pts) with MpBC treated with neoadjuvant CT in the pre-immunotherapy (IO) era. The standard neoadjuvant approach for pts with stage 2-3 TNBC has evolved with the results of the KEYNOTE-522 (KN522) trial, which demonstrated improved pCR rates, event-free survival (EFS), and overall survival (OS) with the addition of pembrolizumab to CT. However, pts with MpBC were not well-represented in KN522, and their response to IO remains poorly characterized. In this study, we examine the pCR rate with the addition of IO to neoadjuvant CT in pts with TN MpBC. Methods: Pts diagnosed with MpBC and treated at Miami Cancer Institute between 2017-2024 were retrospectively identified by COTA real-world Analytics® platform following IRB approval. Eligible pts had stage 2-3 ER-negative (defined as ER < 10%), PR-negative, HER2-negative MpBC, received neoadjuvant systemic therapy, and subsequently underwent definitive surgery. Demographic, clinical, treatment and outcome data were extracted from the electronic health record. pCR was defined as the absence of residual invasive cancer in the breast and axillary lymph nodes at the time of surgery. Kaplan-Meier method was used to estimate median disease-free survival (mDFS) and median OS (mOS). Comparison between treatment groups for pCR were performed using Fisher’s exact test. No multivariable or survival modeling was performed due to limited statistical power in this small cohort. All p values were two-sided. Results: A total of 27 pts were included in the analysis; 16 pts received CT (in the pre-IO era) with 19.2 months (m) median follow up and 11 pts received CT + IO with 15.5m median follow up. Spindle cell histology was reported in 18.5% of tumors, non-spindle cell in 59.3% and 22.2% were not further specified. Most patients (81.5%) had clinical stage 2 disease, 18.5% had stage 3; median tumor size was 3.2 cm. Anthracycline-based CT was administered to 81.5% of pts, and the remainder received non-anthracycline regimens. pCR was achieved in none of the 16 pts (0.0%) who received CT alone and in 3 of 11 pts (27.3%) who received CT + IO (p=0.046). Among the 3 pts who achieved pCR, 2 had non-spindle cell histology and 1 unspecified. All 3 pCR pts had stage 2 disease, received IO and anthracycline, and experienced immune-related adverse events. mDFS was 23.9m in the CT group and 14.6 m in the CT + IO group (p = 0.641), though follow-up duration differed between groups. Among all pts, mDFS was 15.3 m in those who did not achieve pCR and was not reached in those that did (p = 0.226). mOS was not reached in the CT group and was 20.7m in the CT + IO group (p = 0.192). Conclusion: In this small, retrospective real-world analysis of pts with early-stage TN MpBC, the addition of IO to neoadjuvant CT was associated with a higher pCR rate compared to CT alone. Although the observed pCR rate of 27.3% with CT+IO is lower than that reported in KN522 for unselected TNBC, this likely reflects the small sample size and the inherently CT-resistant biology of MpBC. None of the pts in the CT alone group achieved pCR, suggesting that IO may help to overcome CT resistance in MpBC. Differences in mDFS and mOS were not statistically significant, possibly due to limited follow up and sample size. Despite these limitations, our findings highlight the importance of incorporating IO into treatment for MpBC. Future multicenter analyses are ongoing to validate these results in a larger, more diverse population and to further explore the role of histologic subtypes and immune biomarkers in predicting response. Citation Format: N. Dempsey, L. Hodgson, A. Sandoval-Leon, L. Carcas, M. Ahluwalia, C. Wang, H. Kim, P. Advani, R. Mahtani. Pathologic complete response rates in early-stage triple negative metaplastic breast cancer in the neoadjuvant immunotherapy era: a retrospective review [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 PS4-10-07.

Abstract Introduction Serial assessments of circulating tumor cells (CTCs) in metastatic breast cancer (MBC) provide independent prognostic significance for progression free survival and overall survival. A low CTC burden (<5 CTCs / 7.5mL blood) at baseline is favorable prognostically, and reduction of CTCs during treatment indicates response to treatment (Janni, CCR 2025). Despite increasing evidence supporting its clinical validity, real-time serial CTC enumeration has not been utilized broadly in clinical practice in the U.S. The ACT-MBC study is a prospective observational study to evaluate the impact of serial CTC testing on treatment decisions and response assessment (NCT05662345). Provider (physician or advanced practitioner) perception of CTC testing is measured with questionnaires at baseline (PQ1), and for each pt during the first restaging visit (PQ2), and at the end of the study (PQ3). We previously reported PQ1 results (Giridhar, SABCS 2024). Here, we report on results of PQ2. Methods ACT-MBC prospectively enrolled patients (pts) at Mayo Clinic Rochester and Mayo Clinic Health Systems sites (Mankato, Albert Lea and Eau Claire). Pts with MBC were enrolled at time of progressive disease during any line of therapy (TNBC) or ≥2nd line metastatic therapy for ER+/HER2- disease. Pts were tested for CTCs using the FDA-cleared CellSearch® assay which included enumeration and biomarker testing on CTCs for ER, HER2, and PD-L1. The treating clinician determined the regimen and timing of restaging scans. CTC testing was performed once per cycle (every 3-4 weeks) with results returned to the treating provider within 7 days. We descriptively summarized PQ2 responses from providers who had at least one on-treatment CTC result available by the first restaging visit. We report the proportion (90% CI) of providers who agreed or strongly agreed that assessing CTCs enhanced their ability to determine response to therapy and that the CTC results helped guide their decision to continue or change therapy (co-primary endpoints) while accounting for within provider correlation. Results As of June 25, 2025, 15 providers enrolled a median of 2 pts each (range: 1 to 15 pts) and completed PQ2 surveys on 40 pts. 7 providers (47%) practiced in the community setting and 4 (27%) were nurse practitioners. Most pts were female (98%), White (98%), and had ER+/HER2- MBC (80%). Baseline (C1) CTC results were elevated (≥5 CTCs) in 24 (60%) pts (CTC range 0-609). At C2 (n=39), CTC values remained undetected in 7 pts, decreased in 21 pts, increased in 8 pts, and remained unchanged in 3 pts. At PQ2 (n=40), based on clinician interpretation of radiographic findings, results showed 2.5% of pts with complete response, 20% with partial response, 25% with stable disease, 17.5% with mixed response, and 35% with progressive disease. Clinicians reported that CTC results were concordant with imaging results in 34 (85%) responses, discordant with imaging in 7.5% of responses, and unclear in the remaining 7.5% of responses. Most clinicians (82.5%) agreed or strongly agreed that assessing CTCs enhanced their ability to determine response to therapy (90% CI: 68.3%, 96.7%). Further, 70% of providers agreed or strongly agreed that the CTC results helped guide their decision to continue or change therapy (90% CI: 50.8%, 89.2%). 32.5% of the time, CTCs results influenced the timing of the first restaging scans. In 11/25 pts without progressing disease, CTC results influenced the timing of future scans. In the 7/8 pts with a mixed response at first restaging scans with decreasing CTCs, 4 remained on the same systemic therapy. Conclusions Oncologists in academic and community practice responded that serial CTC assessments enhance ability to determine response of therapy, guide decisions to continue or change treatments, and influence timing of radiographic assessments. Citation Format: K. V. Giridhar, D. Zahrieh, M. Hanna, A. Singh, E. Al-Hattab, P. Schumacher, J. Carroll, J. Hoppenworth, T. Haddad, A. Tevaarwerk, B. Rudder, G. M. Choong, S. Basu, R. A. Leon-Ferre, T. O'Brien, A. Lewis, F. de Snoo, L. Stork-Sloots, F. K. Kuhr, S. Lazaro, R. Prendergast, D. S. Spinner, M. P. Goetz. Assessing the impact of real-time serial circulating tumor cell results on the management of patients with metastatic 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 PS4-03-10.

Abstract Background: Hispanic/Latinx (HL) and African American (AA) breast cancer (BC) patients (pts) have increased prevalence of high-risk features such as incidence of BC at age <50 and estrogen receptor negativity. We previously showed that AA and HL had a lower fraction of neutrophils and M2 macrophages (Mɸ) despite higher percentage of PD-L1 positive cases. This study aims to further characterize differences in the tumor immune microenvironment between racial and ethnic groups, with a focus on how these differences vary by site of biopsy. Methods: We analyzed 19,459 BC samples tested by NGS (592, NextSeq; WES, NovaSeq) and WTS (NovaSeq; Caris Life Sciences). Race/ethnicity data were self-reported by pts. Immune cell proportions were estimated using WTS deconvolution (Quantiseq). Gene expression was analyzed for T-cell inflammed score (TIS), MAPK Activation Score (MPAS) and interferon-gamma (IFNy) score. Real-world overall survival (OS) data were derived from insurance claims, calculated from tissue collection or treatment initiation to last contact, and analyzed using Kaplan-Meier. Statistical significance was assessed by chi-square and Mann-Whitney U test with p-values adjusted for multiple comparisons (q<0.05). Results: There were 9674 Non-Hispanic Whites (NHW) (primary BC, defined as biopsy (bx) from breast) pBC: n=3,537, 36.6%; (metastatic BC, defined as biopsy outside of the breast) mBC: n=6,137 63.4%, 1800 AA (pBC: n=750, 42%, mBC: n=1,050, 58%), and 1795 HL (pBC: n=842, 47%; mBC n=953, 53%) samples. There were 2364 TNBC (66% NHW, 19% NHB, 15% HL), 4318 HR+/HER2- (76% NHW, 11.6% NHB, 12.4% HL), and 893 HER2+ (68% NHW, 15% NHB, 17% HL). In pBC overall, AA had lower median cell fraction (%) of B cells (4.8% vs 5.3%), M2 Mɸ (3.4% vs 4.4%), but higher median cell fraction of dendritic cells (DC) (2.8% vs 2.6%) compared to NHW while HL had lower median cell fraction of M2 Mɸ (3.9% vs 4.4%) and higher median cell fraction of CD8+ T cell (0.4% vs 0.2%), all q<0.05. In pBC, HL had higher IFNy score (-0.3 vs -0.34), while AA had lower MPAS (-1.2 vs -0.96), all q<0.05. In lymph node bx, AA had lower median % of M2 Mɸ (3.4% vs 4.4%, q<0.05). No significant differences were seen in the liver bx by race but notably there were lower median cell fractions of CD4+, CD8+, and regulatory T cells (0.0 across NHW, AA, HL) across all race/ethnicities vs. other biopsy sites. By subtype, TNBC primary bx yielded much of the significant changes with AA having lower median cell fraction of M2 Mɸ (2.49% vs 3.04%, q= 0.01) compared to NHW. In primary bx, AA (35.6 m vs 48.5 m, HR 1.4, 95% CI 1.2-1.5, p<0.001) and HL (42.4 m vs 48.5 m, HR 1.2, 95% CI 1.1-1.4, p<0.001) had worse median OS versus NHW. In lymph node bx, AA also had worse OS (29.2 m vs 37.2 m, HR 1.2, 95% CI 1-1.5, p=0.03) compared to NHW. In liver bx, there was worse mOS overall but no difference by race (NHW 18.65m, AA 18.79m, HL 20.6m, p =0.73). In HR+/HER2- liver bx AA had worse OS (14.2 m vs 20.9 m, HR 1.3, 95% CI 1-1.7, p=0.03) versus NHW. In HER2+ primary bx, HL had worse OS (43.2m vs 66.9m, HR 1.7, 95% CI 1.1-2.6, p=0.009). There was a trend towards worse mOS in TNBC AA patients receiving pembrolizumab (18.42m vs HL 23.69m vs NHW 35.30m; p=0.16) and HER2+ AA patients receiving trastuzumab deruxtecan (AA: 16.06m vs HL: 26.81m vs NHW: not reached; p = 0.05). Conclusions: We found race and biopsy site specific differences in the breast cancer immune microenvironment. AA pts had lower M2 Mɸ in primary and lymph node bx while NHW pts showed higher CD8+ T cell in primary tumors. Liver biopsies exhibited low T cell infiltration across all groups. AA pts had worse mOS in primary and lymph node bx compared to NHW but not in liver bx of which we saw worse mOS in all races. These results highlight the need to consider both race and bx site in treatment decisions. Citation Format: R. Hsu, S. Deshmukh, N. Gyabaah-Kessie, E. Rallos, B. Al-Zubeidy, E. Gonzalez, A. G. Baugh, S. Carrel, M. Li, D. Zavala, A. Martynova, P. Jayachandran, D. Stewart, D. Spicer, S. Wu, J. Xiu, G. W. Sledge Jr., R. L. Mahtani, A. C. Sandoval Leon, S. Gandhi, S. Chumsri, M. Lustberg, M. C. Stern, E. T. Roussos Torres. Immune microenvironment and survival differences among Hispanic and African American breast cancer cases by biopsy site [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 PS3-12-06.

Abstract Background: Tumor-infiltrating lymphocytes (TILs) in HER2+ and triple-negative breast cancer (BC) are prognostic and associated with therapy response; however, there are few data regarding distribution and significance of TIL values in ER+ HER2-negative (HER2-) MBC patients (pts) treated with CDK4/6 inhibitors (CDK4/6i). CDK4/6 inhibition may promote antitumor immunity by production of type III interferons and tumor antigen presentation, and by inhibition of regulatory T cells (Goel, Nature 2017). Further, the addition of immune checkpoint therapy to palbociclib numerically prolonged PFS in the 2nd line (2L) setting (Mayer, JCO 2024). We aimed to investigate the prognostic role of TILs derived from fresh metastatic tumor biopsies in pts with ER+HER2- MBC prior to starting CDK4/6i and endocrine therapy (ET) in the first line (1L) and 2L setting. Methods: PROMISE (NCT0281902) is a multicenter, prospective study that enrolled women commencing the CDK4/6i palbociclib (Pb) with letrozole (1L) or fulvestrant (2L) for HR+HER2- MBC. The primary goal was to identify novel genomic variants and pathways associated with PFS. Here, we performed a secondary analysis to quantitate stromal TILs using protocol mandated pretreatment tumor biopsies and determine their association with PFS. Using the Kaplan-Meier method, we reported median PFS (mPFS) and 3-year PFS percentage (%) (95% confidence intervals [CIs]) by TILs [≥20% vs <20%] separately within 1L and 2L. As sample TIL values were generally concentrated at zero (27/58 = 46.6%), coupled with wide spread of sample TIL values ≥20% [range: 20%- 80%], we applied a ≥20% threshold for PFS analysis (high vs low TIL group) as described by Luen, Salgado et al. Lancet Oncol 2017. Statistical significance was based on the Wilcoxon test of the null hypothesis that there was no difference in the PFS curves by TILs. PFS is reported at median follow-up of 21 months. Results: Pretreatment tumor biopsies from 58/68 (85.3 %) pts were evaluable (42 in 1L and 16 in 2L). Median [Q1, Q2] TIL values were higher in 1L (5% [0, 10]) vs 2L (0% [0, 15]). The most common biopsy sites were bone (44.8%), lymph node (17.2%), liver (17.2%) and breast (12%). Overall, TILs frequency (>20%) by the 4 most common biopsy sites were bone (2/20, 10%), lymph node (4/7, 57%), liver (2/6, 33%), and breast (1/7, 14%). In 1L, although PFS was not significantly different (P = 0.237), mPFS (44 vs 34 months) and 3-year PFS 75.0%, [95% CI: 50.3%, 100%] vs 46.4% [95% CI: 31.0%, 69.5%]) were longer in pts with high versus low TILS, respectively. In 2L, the 3-year PFS was numerically similar comparing high and low TILs groups, (25.0%, [95% CI: 4.6%, 100%] vs 22.5% [95% CI: 6.7%, 76.1%]) with no significant difference in PFS (P = 0.761). Conclusions: In pts with ER+HER2- MBC treated with Pb and ET, PFS was numerically longer comparing high vs low TIL values in 1L but not 2L settings. Future studies evaluating TILs in larger datasets are needed to validate these findings. Genomic analysis (DNA/RNA) and its association with TIL levels is underway and will be presented at the meeting. Funding: The Center for Individualized Medicine, Mayo Clinic; Mayo Clinic Breast Specialized Program of Research Excellence (SPORE P50CA116201); Mayo Clinic Cancer Center (CA15083-40A2); TEMPUS (https://www.tempus.com/) and Conquer Cancer Foundation of ASCO (Career Development Award 2018, Dr. O’Sullivan) Citation Format: C. C. O'Sullivan, A. M. Moyer, A. Nassar, D. M. Zahrieh, V. Suman, X. Tang, K. J. Thompson, R. C. Hentz, J. J. Harrington, A. Moreno-Aspitia, D. W. Northfelt, T. C. Haddad, S. Chumsri, P. P. Peethambaram, K. J. Ruddy, K. V. Giridhar, R. A. Leon-Ferre, R. Weinshilboum, L. Wang, K. R. Kalari, M. P. Goetz. Tumor-infiltrating lymphocytes in advanced ER-positive, HER2-negative breast cancer prior to treatment with palbociclib and endocrine therapy: a retrospective analysis of the PROMISE study [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 PS1-10-22.

Abstract Introduction: Metaplastic breast cancer (MpBC) represents a rare subtype, constituting <1% of all breast cancer cases. It is pathologically characterized by epithelial and mesenchymal cellular components exhibiting a basal molecular pattern, hormone receptor and HER2-negativity, and high grade. Clinically, MpBC exhibits an aggressive course with early disease progression and poor survival. Antibody-drug conjugates (ADCs) have been shown to penetrate the central nervous system (CNS); however, little is known about the response of ADCs in MpBC, particularly in patients with CNS disease. Here we aimed to describe features of CNS disease in a subset of patients (pts) who received an ADC for MpBC . Methods: We conducted a multi-institutional retrospective observational study of pts with advanced MpBC treated with ADCs at 4 NCI-designated cancer centers (abstract #A-345). This analysis includes a subset of this cohort with CNS metastases, with the aim of examining the incidence, characteristics, and treatment patterns. Clinical outcomes were evaluated for pts with information available, including CNS median progression-free survival (mPFS; defined from initiation of systemic therapy to time of further documented CNS progression or death) and median real world PFS (rwPFS; defined from time of treatment initiation to documented disease progression or death). Results: We identified 48 pts with MpBC who received an ADC. Among them, we identified 15 (31%) who developed CNS metastases. Of these, the majority (n=11, 73%) had triple-negative breast cancer, and 4 (27%) had HER2-positive breast cancer. The median age was 50 years (32-75 years). The median interval from the initial MpBC diagnosis to CNS metastasis was 13 months (0-54 months). Excluding those with de novo CNS disease, the median interval from time of metastatic diagnosis to development of CNS metastasis [mets] was 3 months (0-22 months). 4 (27%) pts had de-novo metastatic disease, of which 2 (50%) presented with CNS disease. 4 (27%) pts had screening CNS imaging prior to their diagnosis of CNS metastatic disease. At the time of CNS disease diagnosis, 9 (60%) pts had oligometastatic intraparenchymal CNS disease (defined as <5 CNS mets), 5 (33%) had diffuse (>5 CNS mets) CNS disease, and 1 (6%) had leptomeningeal and intraparenchymal disease. Most pts 14(93%) with CNS disease were symptomatic at time of diagnosis. One pt underwent resection, 9 (60%) underwent stereotactic radiosurgery (SRS), 6 (40%) underwent whole brain radiation therapy (WBRT), and 1 pt received intrathecal chemotherapy with methotrexate. The median number of lines in the metastatic setting prior to documented CNS disease was 1 (range 0-3). The majority 13 (87%) underwent a change in systemic therapy at time of CNS disease diagnosis, with 60% changing to an ADC at the time of CNS diagnosis or progression. Among pts with progression of CNS disease warranting change in systemic therapy, the median time from first CNS event to further CNS progression was 6.5 months. Median CNS PFS was 3.5 months (range 1 -74 months) and median rwPFS was 4.4 months. For those patients who used ADCs after diagnosis of metastatic CNS diagnosis (12/15), median rwPFS was 4 months. Conclusions: Within this CNS sub-cohort study, we found that 31% of pts with MpBC developed CNS metastasis. The majority of pts had neurological symptoms at diagnosis, but most pts did not undergo routing screening for CNS disease. Given the high rate of CNS mets within a short timeframe from diagnosis with poor outcomes, CNS screening should be considered. The extremely short median CNS PFS and low median rwPFS of this cohort highlight the guarded prognosis associated with MpBC. Given the small sample size of this cohort, larger studies are warranted to validate these findings. Citation Format: S. Premji, S. Jacob, A. LeVee, S. Marion, S. Fisch, L. Huppert, J. Mortimer, D. Schmolze, A. Nayak, A. DeMichele, P. Shah, R. Leon-Ferre, D. Idossa. Central nervous system disease in patients with metaplastic breast cancer receiving antibody drug conjugates: a multi-institutional study [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 PS1-09-07.