Citations

Abstract Background Ceftriaxone-resistant Escherichia coli infections are increasingly common, partially because of the emergence of E. coli sequence type 131 (ST131), including its subclade C2/H30Rx, which produce extended-spectrum β-lactamases. Methods A prospective cohort including 14 US sites, which enrolled monomicrobial ceftriaxone-resistant and susceptible E. coli bloodstream infection (BSI) cases in a 1:1 ratio, was used to compare ST131 versus non-ST131 E. coli BSI, with specific attention to E. coli ST131 C2/H30Rx. Desirability of outcome ranking was determined at 30 days after infection onset. Results This analysis included 282 patients with E. coli BSI; 43% (121/282) were E. coli ST131 and 23% (66/282) belonged to the C2/H30Rx subclade. Resistance to ceftriaxone was present in 79% (96/121) ST131, 86% (57/66) E. coli ST131 C2/H30Rx, and 27% (43/161) E. coli non-ST131. Compared to patients with non-ST131 E. coli BSI, patients with ST131 BSI were older (median 70 years, [Q1 62, Q3 76] years vs 65 years, [51, 74]; P = .005) and more often admitted from long-term care facilities (21/121 [17%] vs 7/161 [4%], P < .001). Overall and empiric carbapenem use was more frequent in the treatment of patients with ST131 BSI compared with non-ST131 BSI (overall 89/121 [74%] vs 50/161 [31%]; empiric: 58/121 [48%] vs 31/161 [19%], P < .001). Desirability of outcome ranking outcomes were similar between groups. Conclusions Most ceftriaxone-resistant E. coli from US patients with E. coli BSI belong to ST131, particularly E. coli ST131 C2/H30Rx, serving as an important driver of carbapenem use.

BACKGROUND: Left ventricular (LV) dysfunction is a potential cardioembolic source of ischemic stroke, but its role in recurrent stroke risk and treatment response remains unclear. We explore whether LV injury associates with the risk of recurrent stroke and modifies the association between anticoagulation and stroke recurrence using real-world data. METHODS: We performed a multicenter, retrospective study of Cardiac Abnormalities in Stroke Prevention and Recurrence cohort across 27 US sites. Patients with LV ejection fraction ≥20% were included. LV injury, defined as LV ejection fraction 20% to 40% and wall motion abnormality, was the primary exposure and treatment effect modifier. The treatment of interest was anticoagulant versus antiplatelet therapy. The composite outcome included recurrent stroke, major bleeding, or death. Outcomes were evaluated using unadjusted and inverse probability weighting adjusted Cox proportional hazards models, with treatment effect modification tested by LV injury status. RESULTS: Among 2685 patients enrolled, 2328 with complete data were analyzed (median age, 65 years; 49.8% female; median follow-up, 1.6 years). LV injury was present in 310 patients (13.3%). Overall, 535 events occurred: 258 recurrent ischemic strokes, 28 hemorrhagic strokes, 67 major hemorrhages, and 256 deaths. LV injury was associated with a higher unadjusted risk of the primary outcome (hazard ratio [HR], 1.51 [95% CI, 1.21–1.87]), though nonsignificant after inverse probability weighting adjustment (adjusted HR, 1.29 [95% CI, 0.97–1.70]). In the LV injury subgroup, anticoagulation versus antiplatelet therapy was associated with a lower risk of the primary outcome (adjusted HR, 0.24 [95% CI, 0.10–0.59]), relative to the non-LV injury subgroup (adjusted HR, 1.28 [95% CI, 0.83–1.95]; p[LV-interaction], 0.001). Similar interactions were seen for EF 20% to 40% (versus >40%; adjusted HR, 0.19 [95% CI, 0.04–0.86]; p[LV-interaction], 0.001) and wall motion abnormality (versus no wall motion abnormality; adjusted HR, 0.33 [95% CI, 0.15–0.73]). CONCLUSIONS: After cryptogenic stroke, anticoagulation in those with LV injury was associated with lower rates of recurrent stroke, major bleeding, and death. These findings warrant confirmation in a dedicated randomized controlled trial. REGISTRATION: URL: https://www.clinicaltrials.gov ; Unique identifier: NCT06398366.

Abstract Background: (Z)-endoxifen (ENDX) is an oral SERM that dually targets ERα (> 5 ng/ml) and PKCβ1 (>500 ng/ml). It has been tested in the endocrine resistant setting at doses of 20-360 mg/day; however, the optimal dose in an endocrine naïve population is not defined. We explored 10 mg/day dose of neoadjuvant ENDX in the Endocrine Optimization Pilot (EOP) of I-SPY2. Methods: EOP eligibility included pts with Stage II/III HR+ HER2- breast cancer, MammaPrint (MP) low. Pts with MP High1 scores were eligible if clinically node-negative. Pts received ENDX 10 mg orally once daily for 6 cycles (each cycle = 28 days). Premenopausal pts started ovarian function suppression on cycle 2, day 1. ENDX was continued until the day prior to surgery. The primary study endpoint was feasibility, defined as >75% of pts completing >75% of ENDX. Baseline (T0), 3-wk (T1) biopsies, and the surgical specimen (T3) were assessed centrally for Ki-67. Rates of modified preoperative endocrine prognostic index (mPEPI) 0 were reported, defined as pT1-2 pN0, Ki67< 2.7%, or pathologic complete response. A genomic index for sensitivity to endocrine therapy (SET) index was done at T0. Breast MRI measurement of functional tumor volume (FTV) was performed at T0, T1, 12 weeks (T2), and pre-operatively (T3). Blood was collected for tumor informed ctDNA at T0, T1, T2, T3 and for pharmacokinetic (PK) steady state concentrations (Css) at T1. Safety data using CTCAE V5 is reported. Results: Between March 2023 and May 2024, 20 pts were enrolled in this arm. The median age was 52.5 years (range 32 -73), 13 (65%) were premenopausal, and 55% had clinically positive lymph nodes. Tumors were MPHigh1 in 3 pts (15%) and SET2,3 index high in 16 pts (80%). The primary endpoint was met with 95% of study participants completing ≥75% of therapy. The T0 median Ki-67 was 10.5% (range 2-60%). At T1 (n=19) and T3 (n=18), median Ki-67 was 5% (range 0-35%) and 5% (range 0-45%), respectively. The 3-week rate of Ki-67 ≤10% at T1 was 61.5% in premenopausal and 71.4% in postmenopausal women. The rate of Ki-67 ≤10% at surgery was 81.8% in premenopausal and 57.1% in postmenopausal women. An mPEPI score of 0 was observed in 1 (5%) of participants. The median T0 FTV was 8.9 ccs (range 0.9 to 251.1). The median T3 FTV was 1.3 ccs (range 0 to 11.2) and the median change in FTV from T0 to T3 was -72.0% (range -88.5% to -61.6%). ctDNA analysis was performed in 17 pts longitudinally across 64 blood samples.10 pts had positive ctDNA at T0, with 7 converting to ctDNA undetectable during treatment with ENDX. 1/7 pts who was ctDNA negative at T0 became positive at T1 and then reverted to undetectable ctDNA. The other 6 pts remained ctDNA negative at all timepoints. The median ENDX T1 Css was 79.3 ng/mL (range 73.6 - 906.4 ng /mL). The most common side effects reported were Grade 1 hot flush (70%), nausea (40%), fatigue (40%), and insomnia (35%). Grade 2 side effects included fatigue (15%) and night sweats (5%). No dose reductions or discontinuations due to toxicity occurred. Conclusions: ENDX at 10 mg was well tolerated and demonstrated reductions in Ki-67, MRI FTV, and ctDNA. The ISPY2 EOP is currently enrolling patients to ENDX 40 mg/day (intended to dually target ERα and PKCβ1) in combination with abemaciclib (with and without OFS). Citation Format: K. V. Giridhar, M. P. Goetz, C. Yau, A. M. Wallace, C. Falkson, J. Boughey, M. Arora, E. Stringer-Reasor, C. Nangia, C. Gallagher, A. Elias, A. Zimmer, M. S. Trivedi, C. Omene, L. van 't Veer, L. Brown Swigart, G. L. Hirst, W. Symmans, A. D. Borowsky, N. Onishi, N. Hylton, C. Vakalvas, M. Wei, J. M. Reid, S. Alhafaji, N. Chen, R. M. Mukhtar, L. Huppert, O. Opopade, L. J. Esserman, A. Chein. Low dose (Z)-endoxifen in the I-SPY2 Endocrine Optimization Pilot [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 PS3-07-10.

Abstract Background: In MBC, CTC enumeration is highly prognostic for both progression free survival (PFS) and overall survival (OS). Multiparametric technologies enable for expanded phenotypic profiling of CTCs, including ER and HER2 assessment. Limited information is available on the concordance of CTC ER and HER2 levels compared to synchronously obtained tissue. We assessed the prognostic performance of CTC enumeration and phenotypic profiling of ER and HER2 on CTCs. Methods: The prospective, multi-institutional Aurora study enrolled pts with MBC of any subtype either at diagnosis of metastatic disease or at time of progression on any line of systemic therapy. Pts underwent a metastatic site biopsy, consented to archival primary tissue collection and donated blood samples serially every 3-4 months until progression. CTCs, from blood collected at study entry, were identified as nucleated, EpCAM+/ cytokeratin+/ CD45- cells using an automated imaging system (RareCyte, Seattle). ER and HER2 expression levels were measured by mean fluorescence intensity (MFI), with thresholds above 80 (ER) and 160 (HER2) considered positive. Comparisons were made to total RNAseq from paired metastatic tissue biopsies. PFS and OS were calculated from the time of study entry to last follow up or death. Results: Baseline CTC enumeration was performed in 154 pts. The most common clinical subtype was HR+/HER2- (59.7%), followed by HER2-positive (16.8%) and TNBC (15.5%). The median lines of prior therapy were 1 (range 0-10). At least 1 CTC was identified in 68/154 pts (44%), 30 (19%) had ≥ 5 CTCs, and 15 (9.7%) had CTC clusters. The median CTC value was 0 (0-877). The presence of CTCs (≥1 or ≥5) and CTC clusters was adversely associated with PFS and OS (Table 1 Table 1. CTC survival analyses Number Median PFS PFS HR (95% CI) Median OS OS HR (95% CI) Presence of CTC 0 CTC 86 7.37 (0.03 - 43.1) reference 20.72 (0.03 - 50.6) reference ≥ 1 CTC 68 4.7 (0.43 - 19.5) 1.65 (1.16-2.36) 12.04. (0.43-44.1) 2.13 (1.44-3.16) Presence of ≥ 5 CTC <5 CTC 124 7.7 (0.03 - 43.1) reference 19.31 (0.03 - 50.6) reference ≥ 5 CTC 30 2.88 (0.43 - 18.7) 1.77 (1.15 - 2.71) 5.74 (0.43 - 29.3) 3.59 (2.26-5.70) Presence of clusters 0 clusters 139 7.37 (0.03 - 43.1) reference 18.06 (0.03 - 50.6) reference ≥ 1 cluster 15 1.91 (0.43 - 18.7) 2.06 (1.16 - 3.68) 3.42 (0.43 - 18.7) 5.44 (2.94-10.09) ). Across all subtypes, the median CTC ER-MFI across was 36.2 (range 4.2-10,016.6) and the median HER2-MFI was 139.6 (range 4.2-2480.5). We observed high intra-patient heterogeneity of ER-MFI (CV: 0.65 [0.34-1.25]) and HER2-MFI (CV: 0.79 [0.42-1.54]). Clinical metastatic subtype from metastasis at study entry was available in 54/68 pts with CTCs. Pts with clinical ER+ MBC had a higher median ER-MFI compared to clinical TNBC (111.9 vs 68.6, p = 0.02). However, 11 pts (26.8%) with clinical ER+ MBC had all CTCs with an ER-MFI <80. Lower ER expression was observed in CTCs collected after prior exposure to a SERD (median ER-MFI: post-SERD [n=8] 27 vs post-aromatase inhibitor [n=4] 252, p<2.22e-16). No correlations were observed between the median CTC ER-MFI or HER2-MFI and paired tissue ESR1 or ERBB2 RNA expression (n=66). Conclusions: CTCs are strongly predictive of PFS and OS. CTC ER- and HER2-MFI were highly heterogeneous without clear correlation with bulk tissue RNA expression. The clinical impact of discordant clinical receptor status and CTC receptor status warrants further exploration. Citation Format: K. V. Giridhar, W. C. Nenad, H. Ye, G. L. Wheeler, T. Hinoue, A. R. Michmerhuizen, B. M. Felsheim, U. R. Chandran, B. J. Kelly, E. R. Mardis, P. W. Laird, J. M. Balko, A. C. Garrido-Castro, S. Coppens, J. Bowen, R. Nanda, N. U. Lin, C. K. Anders, C. Isaacs, J. J. Tao, J. D. Anampa, N. Jahan, L. Huppert, A. DeMichele, B. H. Park, S. Vinayak, A. L. Delson, M. M. Magbanua, M. Balic, H. A. Parsons, A. V. Lee, L. A. Carey, L. Norton, A. C. Wolff, N. E. Davidson, I. Krop, T. A. King, C. M. Perou, K. A. Hoadley, Aurora US Metastases Network. Characterizing circulating tumor cell (CTC) enumeration and phenotypic profiling in blood with synchronously obtained tissue in metastatic breast cancer (MBC) in TBCRC /AURORA US [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 PS2-07-17.

Abstract Individuals who have serum elevations of anti-cyclic citrullinated protein (anti-CCP) antibodies are at risk for developing rheumatoid arthritis (RA), yet immunologic factors that lead to a transition from pre- to clinical RA remain unclear. Here, we used materials from anti-CCP antibody-positive individuals enrolled in a clinical trial that evaluated the efficacy of hydroxychloroquine to prevent clinical RA, and performed multi-modal single-cell profiling (transcriptome, surface proteins, T/B-cell receptor sequencing, and chromatin accessibility) on samples obtained at baseline and at RA onset in those who developed clinical RA (Converters) or follow-up point in matched Nonconverters. At both baseline and follow-up, Converters had expansions of peripheral helper T (Tph) cells and CD8 + T cells expressing GZMK and GZMB , along with elevated potentially autoreactive T-cell receptors in CD4 + T cells compared to Nonconverters. Induction of age-associated B cell signatures was observed in B cells of Converters prior to RA onset. Epigenetic profiling further identified chromatin accessibility changes in Converters over time, particularly within myeloid and NK cells. Lastly, predictive modeling using baseline immune features, including Tph cells, GZMK + XCL1 + CD8 + , and GZMB + CD57 + CD8 + T cells, together with clinical features such as anti-CCP3 levels, RF-positivity, and HLA shared epitope status, stratified RA risk and predicted time to onset. These findings define immune endotypes in pre-RA that could serve as targets for future preventive interventions and be used to stratify the risk of developing clinical RA in anti-CCP antibody-positive individuals.

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 Background: Aurora kinase A (AURKA) plays an integral role in the mitotic cell cycle, regulating spindle assembly, centrosome function, chromosome alignment, and mitotic entry. In addition to its mitotic role, activated AURKA has been shown to induce epithelial-mesenchymal transition reprogramming and contribute to endocrine resistance in ER+ breast cancer (BC) models. Amplification/overexpression of AURKA is linked to poor prognosis across multiple tumor types, including ER+/HER2- BC. Alisertib is a selective, reversible, ATP-competitive AURKA inhibitor that has been shown to disrupt spindle formation, leading to mitotic defects and cell death. In xenografts, the potent inhibition of AURKA by alisertib led to antitumor activity across a variety of tumor types. Preliminary clinical data with alisertib have shown significant antitumor activity and a manageable safety profile in patients with various cancers, including ER+/HER2- BC. This study aims to better characterize the role of alisertib in patients with BC to provide a rationale for future clinical studies. Methods: Three studies evaluating alisertib in patients with ER+/HER2- MBC have been completed, including a phase I study of alisertib + fulvestrant (Haddad 2018), a phase II alisertib monotherapy study (Melichar 2015), and a phase II study of alisertib ± fulvestrant (TBCRC 041; Haddad 2023). Descriptive summary statistics were utilized for patient characteristics, as well as clinical safety and efficacy data. Results: A total of 126 patients were enrolled and evaluable in the 3 clinical studies, as per Table 1. All patients had MBC and received prior endocrine therapy and chemotherapy. All patients in TBCRC 041 also received a prior CDK4/6 inhibitor (CDK 4/6i). In these heavily pretreated patients, tumor objective response rates (ORR) were ∼20%, and 6-month clinical benefit rates (CBRs) were 29-78%. The most common grade 3/4 adverse events (AEs) across studies were hematologic AEs (neutropenia, leukopenia, and anemia), and fatigue. In TBCRC 041, ctDNA- and tumor-derived RNA-Seq-based biomarker assessments are ongoing to investigate associations with clinical benefit from alisertib alone or combined with fulvestrant. Conclusions: Alisertib has shown efficacy and safety across multiple clinical studies in patients with ER+/HER2- MBC and could provide a novel option for treating those who have progressed on endocrine therapy and a CDK 4/6i. These findings provide a strong rationale for further development of alisertib in this patient population. The ongoing phase II ALISCA-Breast1 study (NCT06369285) is investigating the optimal dose of alisertib in combination with standard endocrine therapies in this setting. Citation Format: T. C. Haddad, K. McCann, P. Advani. The Aurora Kinase A Inhibitor Alisertib Alone or in Combination with Endocrine Therapy for Patients with Estrogen Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative (ER+/HER2-) Metastatic Breast Cancer (MBC) [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 PS1-10-03.

Abstract Introduction: With emerging antibody drug conjugates (ADC) as treatment of metastatic breast cancer (mBC), it is key to understand the mechanisms that drive resistance to inform optimal ADC selection and sequencing. The AURORA (AUR) US Network was established to characterize paired primary and metastatic (met) samples via multiplatform profiling, with a prospective phase of met tissue collection that included pre and post-ADC samples. Methods: Fresh frozen or FFPE met and archival primary tumors were analyzed across 3 platforms: DNA low pass whole genome and exome sequencing (WES), DNA methylation arrays (Methyl) and RNA sequencing (RNA seq). The molecular cohort included pts with data from ≥1 platform. Pairwise gene expression comparisons were performed in R using t-tests, and pathway analysis with GSEA. Results: A total of 168 pts comprised the molecular cohort: WES, 168; Methyl, 163; RNA seq, 168. Median age at mBC diagnosis was 56 (28-88). 21 pts (12.5%) had known germline BRCA1/2 mutations; 37 (22.0%) presented with de novo mBC. Subtype at mBC (if not available, at primary) diagnosis was: 108 (64.3%) HR+/HER2-, 22 (13.1%) HER2+, 33 (19.6%) TNBC and 5 (3.0%) unknown. Among 139 pts with known subtype in the first AUR met collected, 95 (68.3%) were HR+/HER2-, 17 (12.2%) HER2+ and 27 (19.4%) TNBC. 72 pts received ≥1 ADC with 1 (0-4) median line of chemotherapy for mBC prior to ADC1. As ADC1, 42 pts received HER2 ADC (35 trastuzumab deruxtecan, T-DXd; 7 trastuzumab emtansine), 29 TROP2 ADC (28 sacituzumab govitecan, SG; 1 datopotamab deruxtecan) and 1 NECTIN4 ADC. 22 pts received ≥2 ADC (16 immediate sequence; 6 intervening therapy); 11 T-DXd and 11 SG as ADC2. Median duration of ADC1 was 141 days (6-522); 81 (7-425) for ADC2. Progressive disease was the most common reason for ADC discontinuation, denoting resistance in post-ADC samples. 99 samples were collected pre-ADC1 and 24 post-ADC1, of which 5 were pre-ADC2. ERBB2 expression did not differ between samples after HER2 (n=9) vs non-HER2 ADC (n=8) (p=0.55), nor did TACSTD2 after TROP2 (n=8) vs non-TROP2 ADC (n=9) (p=0.16). In pts with HER2+ subtype at mBC diagnosis or AUR collection who received HER2 ADC, ERBB2 expression significantly decreased in post (n=4) vs pre-treatment (n=7) samples (p=0.043). ERBB2 downregulation was observed in 5 pts with paired pre vs post-HER2 ADC samples. In pts who received TROP2 ADC, TACSTD2 expression did not differ in post (n=10) vs pre-treatment (n=47) samples (p=0.63). GSEA of post-ADC samples revealed significant pathway differences by payload, including actin cytoskeleton in post-microtubule inhibitor (MTi) vs non-MTi ADC samples (p=0.001) and ATM-dependent DNA damage response after topoisomerase I inhibitor (TOP1i) vs non-TOP1i ADCs (p=0.043). Lysosomal vesicle biogenesis (p=0.014) and calcium release (p=0.05) pathways significantly differed between ADC naïve vs treated metastases. ERBB2 hypermethylation was not detected in the cohort. TACST2 promoter methylation was observed in 10 samples (β value >0.3) without association with TROP2 ADC exposure. ERBB2 or TACSTD2 mutations were not identified in the ADC-treated cohort. A novel loss-of-function (LOF) mutation in MED12 was detected in one pt immediately post-T-DXd. MED12 LOF has been reported to promote PARP inhibitor and platinum resistance in BRCA-deficient tumors and may represent a novel resistance mechanism to TOP1i. Conclusions: Multiplatform molecular characterization of mBC treated with ADC revealed potential mechanisms of resistance related to downregulation of target expression, payload (e.g., microtubule formation, DNA damage repair) and lysosomal processing. Additional analyses integrating these multimodal data are ongoing. Strategies to overcome resistance, e.g. modifying ADC target, are being evaluated in clinical trials. Citation Format: A. C. Garrido-Castro, W. C. Nenad, J. M. Balko, T. Hinoue, G. L. Wheeler, B. J. Kelly, M. Singha, B. M. Felsheim, A. R. Michmerhuizen, U. Chandran, A. V. Lee, L. A. Carey, T. A. King, E. Mardis, P. W. Laird, K. V. Giridhar, K. A. Hoadley, C. M. Perou. Molecular characterization of resistance to antibody drug conjugates in metastatic breast cancer: a prospective analysis from the AURORA US Network [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 PD3-09.

Abstract Background: Patients with metastatic breast cancer (MBC) often have short survival, and clinical disease management is challenging due to tumor heterogeneity, treatment resistance, and unique contributions of organ-specific metastatic microenvironments. Methods: The retrospective and prospective phases of the AURORA US Metastasis Project have profiled samples from 220 MBC patients using RNA-seq, DNA-seq, and DNA methylation arrays. RNA-seq has been performed on 392 specimens from 143 primary tumors and 249 metastases. In total, there are 162 primary-metastasis pairs from 134 patients (13 patients have 2+ metastases profiled). The 249 metastatic samples include 116 liver, 29 lymph node, 17 lung, 14 bone, 10 brain, and 8 skin metastases. Each specimen was assayed to determine its clinical and molecular subtypes. Here we investigate the molecular causes of clinical and PAM50 subtype switching in primary tumor-metastasis pairs using multi-platform data. Results: Of the patients with paired data in the AURORA dataset, 38/134 (28%) had clinical subtype (HR+/HER2+, HR-/HER2+, HR+/HER2-, TNBC) switches between the primary tumor and paired metastasis. The most common clinical subtype switch was loss of hormone receptor expression (16/38 patients) in the metastatic tumor. Many of these observations have been confirmed by concomitant RNA and/or DNA changes. In addition, 59/134 (44%) patients had PAM50 molecular subtype switches. 26/59 (44%) patients had a Luminal A primary tumor which switched either into Luminal B (16/26) or HER2-Enriched (7/26) subtypes, suggesting recurrent progression mechanisms during metastasis. For example, patient ARRP had a Luminal A primary tumor and a HER2-Enriched bone metastasis. Using DNA clonality data, we identified a clonal mutation in the DNA binding domain of FOXA1 (H247L) as well as mutations in CDH1 (frameshift) and NLRC3 (Q650H) that originated in the primary tumor. A distinct, dominant clone arose in the metastasis, harboring oncogenic activating mutations in PIK3CA (H1047R) and ERBB2 (L869R). There was also an increase in RNA expression of ERBB2 and FGFR4 but no change in ERBB2 copy number. This is in alignment with previous publications demonstrating a shift to the HER2-Enriched subtype, often without gain of HER2 amplification. Interestingly, there were 21 patients that had both clinical and PAM50 subtype switches. Patient ANFC had a HR+/HER2- primary tumor (Luminal A) and a TNBC liver metastasis (HER2-Enriched). Notably, the metastasis lost clinical expression of ER and PR (by IHC), and this was also observed through a decrease in ESR1 and PGR mRNA expression. Using DNA clonality data, we identified 5 new clones in the metastasis that were not present in the primary tumor. Finally, we observed that basal-like primary tumors with a PAM50 subtype switch in the paired metastasis had a significantly lower correlation to the basal-like centroid compared to primary tumors with a basal-like paired metastasis (p=0.0053). This suggests that either these tumors had significant intra-tumoral heterogeneity in the primary tumor and the metastasis originated from a non-basal-like portion of the tumor, or the switches from a basal-like primary tumor are a byproduct of the PAM50 bioinformatics algorithm (i.e. the tumor is very close to 2 subtype centroids). Ongoing analyses will precisely determine which subtype switches can be attributed to genetic or epigenetic causes, organ site biases, and/or bioinformatic methodologies. Conclusions: Using multi-platform molecular analyses, we identified cases of clinical and molecular subtype switching in primary-metastasis pairs. These findings begin to elucidate the underlying molecular drivers of tumor progression and subtype switching in patients with MBC, which may have implications for clinical treatment. Citation Format: A. R. Michmerhuizen, B. M. Felsheim, G. L. Wheeler, B. J. Kelly, M. Singha, T. Hinoue, W. C. Nenad, A. C. Garrido-Castro, J. M. Balko, K. Giridhar, U. Chandran, A. V. Lee, L. A. Carey, T. A. King, K. A. Hoadley, P. W. Laird, E. R. Mardis, C. M. Perou, AURORA US Metastasis Network. Molecular causes of subtype switching in primary and metastatic breast tumor pairs from the AURORA US Project [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 PD2-06.