Background While trastuzumab deruxtecan (T-DXd) demonstrates intracranial efficacy, the potential for radionecrosis (RN) when combined with stereotactic radiosurgery (SRS) remains a concern, given the established risk with other antibody-drug conjugates like T-DM1. This study evaluated the safety and efficacy of T-DXd and SRS in patients with HER2-positive or -low breast cancer brain metastases (BCBM). Methods We conducted a multi-center retrospective analysis of 113 patients (461 SRS treatments) treated with SRS and anti-HER2 agents. Patients were stratified into T-DXd(+) (n=29 patients, 61 treatments) and T-DXd(-) (n=84 patients, 400 treatments) groups. Endpoints included RN, radionecrosis-free survival (RNFS), and intracranial control outcomes (any intracranial progression, local failure, and distant intracranial metastasis). Results No cases of RN were observed in the T-DXd(+) group, compared with 11 cases in the T-DXd(-) group (p=0.028). On multivariate analysis, T-DXd(+) status remained significantly associated with improved RNFS (HR 0.31, p=0.009). In the treatment-level analysis, the 1-year cumulative incidence of RN was 0% for T-DXd(+) versus 4.3% for T-DXd(-) (p=0.009). Additionally, T-DXd(+) was associated with significantly better 1-year outcomes for any intracranial progression (40% vs. 76%, p<0.001), local failure (6.6% vs. 29%, p=0.002), and distant intracranial metastasis (40% vs. 66%, p=0.009). All efficacy endpoints remained significant on multivariate analysis. Conclusion Combining T-DXd with SRS demonstrated a favorable safety profile without increasing the risk of radionecrosis. Furthermore, this combination was associated with superior intracranial control, encompassing both local and distant outcomes, supporting the potential of T-DXd combined with SRS as an effective and well-tolerated approach for HER2-positive or -low BCBM.

Importance:Clonal hematopoiesis of indeterminate potential (CHIP) is linked to an increased incidence of cardiovascular and malignant diseases. Objective:To determine whether CHIP is associated with cardiotoxic effects in patients with breast cancer receiving trastuzumab. Design, Setting, and Participants:Analyses of human cohorts with complementary animal experimentation were performed using a nationwide population-based cohort (UK Biobank), 1 tertiary referral center (Seoul National University Hospital [SNUH]), and a controlled laboratory setting. UK Biobank participants were enrolled between 2006 and 2010, and SNUH patients were enrolled from January 2004 to March 2024. Data were analyzed from March 2021 to February 2026. Main outcome measures:Incident heart failure (HF) in the UK Biobank cohort and trastuzumab-related cardiotoxic effects in the SNUH cohort were the main outcome measures. Trastuzumab-related cardiotoxic effects were defined using established clinical criteria (European Society of Cardiology [ESC], Canadian Trastuzumab Working Group, and Cardiac Review and Evaluation Committee [CREC]). Fine-Gray competing risk models were used to account for death and myocardial infarction; multivariable models were adjusted for age and cardiovascular risk factors in both cohorts, with additional adjustment for anthracycline use in the SNUH cohort. Changes in left ventricular ejection fraction (LVEF) were assessed in Tet2-deficient bone marrow chimeric mice following trastuzumab exposure. Results:Overall, 15 729 patients with breast cancer from the UK Biobank cohort (mean [SD] age, 58.8 [7.3] years; 107 [0.68%] male) and 454 female patients with breast cancer who received trastuzumab from the SNUH cohort (mean [SD] age, 52.0 [9.6] years) were included. The corresponding 2-year cumulative incidence values for trastuzumab-related cardiotoxic effects were 15.7% vs 5.0% by ESC criteria (Gray test P = .001), 19.9% vs 10.8% by Canadian criteria (P = .01), and 20.9% vs 11.3% by CREC criteria (P = .02). Using the ESC definition, CHIP positivity (variant allele frequency ≥1.0%) was associated with cardiotoxic effects in multivariable competing risk analysis (adjusted subdistribution hazard ratio, 1.91; 95% CI, 1.32-2.76). Tet2-deficient mice demonstrated a significant LVEF reduction following trastuzumab treatment (effect size, -4.2%; 95% CI, -7.91 to -0.49; P = .03); other experimental groups showed no significant change. Conclusions and Relevance:In this cohort study, the presence of CHIP was associated with increased susceptibility to trastuzumab-related cardiotoxic effects.

Abstract Background: Poly (ADP-ribose) polymerase (PARP) 1, a multifunctional protein, is widely recognized for its critical role in the HR-mediated DNA repair, and its inhibition has therefore been introduced into the treatment of various solid tumors. Besides DNA damage response (DDR), PARP1 has also emerged as a key regulator of immune modulation. This is attributed to the accumulation of cytosolic nucleic acids (NAs), such as dsDNA or dsRNA, which are sensed by innate immune pathways like cGAS-STING. However, the precise mechanisms by which PARP1 inhibition induces these cytosolic NAs and activates innate immune response remain to be elucidated. Thus, we investigated the anti-tumor and immune modulatory effects of AZD5305 in gastric cancer cells to explore the underlying mechanism. Methods: To evaluate the in vitro antitumor effect, colony formation assay (CFA) was performed for 14 days with increasing concentrations of AZD5305 (dose range: 0-5nM). The in vivo antitumor activity was assessed using an SNU-601 xenograft model by monitoring tumor growth. Cell cycle distribution was analyzed by flow cytometry and apoptosis was assessed by annexin-V/PI staining. DNA damage was detected by comet assay and the cell immunofluorescence assay (IFA). To investigate immune modulation, the expression of immune-related molecules was quantified by qRT-PCR and western blot, while the formation of dsRNA was detected by cell IFA. RNAseq was performed using the cell lines for differentially expressed gene and gene set enrichment analyses. Results: The RAD51C-deficient SNU-601 cells were highly sensitive to AZD5305 (IC50: 0.25 nM), unlike the moderately sensitive SNU-668 (IC50: 2.05 nM) or resistant KATOIII cells (IC50>5 nM). Moreover, AZD5305 demonstrated potent antitumor activity in an SNU-601 xenograft model (52.4% TGI, p=0.003). After treatment, SNU-601 cells exhibited G2/M arrest and apoptosis, confirmed by an increased sub-G1 population, positive annexin-V staining, and cleavage of PARP and caspase-7, but not in KATOIII. This cytotoxicity was driven by the accumulation of DNA damage, as indicated by elevated p-RPA, γH2AX, and comet tail formation. Furthermore, AZD5305 increased cytosolic dsRNA and cGAS-STING signaling in SNU-601, triggering the innate immune response. Consistent with this, RNAseq confirmed enrichments of activated IFN-γ response pathways and corresponding increases in the expression of interferon-stimulated genes (ISGs) in SNU-601, but not in KATOIII. Conclusion: AZD5305 exhibits potent cytotoxic effects in HR-deficient gastric cancer cell lines in vitro and in vivo, by inducing DNA damage and apoptotic cell death. Immune modulation by AZD5305 was characterized by innate immune activation with increased interferon signaling, and our findings suggest the formation of cytosolic dsRNA as a key mediator of this effect. Citation Format: Sujin Ham, Hae Min Hwang, Youlim Noh, Jiwon Koh, Chaeyoung Lee, Seohyeon Lim, Minyoung Jeong, Yu-Jin Kim, Minyoung Lee, Changhee Park, Dae-Won Lee, Kyung-Hun Lee, Seock-Ah Im. AZD5305, a PARP1 selective inhibitor, exhibits antitumor effects and stimulates immune response in homologous recombination (HR)-deficient gastric cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 513.

Abstract Introduction: Trastuzumab deruxtecan (T-DXd) is an antibody-drug conjugate (ADC) that is effective for both HER2-positive and HER2-low metastatic breast cancers (MBC). While correlation between HER2 expression levels and response to T-DXd was identified, effective biomarkers and mechanisms of sensitivity or resistance remain elusive, partly due to complex pharmacokinetics of T-DXd. Our goal is to delineate spatial transcriptomic (ST) features which is associated with therapeutic response and resistance to T-DXd. Methods: We performed ST with the 10x Genomics Visium HD on formalin-fixed paraffin-embedded tumor tissues from patients with MBC treated with T-DXd. Responders to T-DXd were defined as patients who experienced objective response or stable disease for more than 6 months, and non-responders were defined as otherwise. Fragments of pre-treatment biopsy samples from patients classified as responders were designated as "sensitive fragment” while that of pre-treatment biopsy samples from non-responders or post-progression biopsy samples from responders were defined as “resistant fragment”. Various transcriptome analyses were performed including the compartment modeling-based methods for estimating antibody and payload concentrations from T-DXd pharmacokinetic profiles. Results: A total of 20 tumor tissues from 13 patients were available including matched pre- and post-treatment biopsied in 5 patients. After quality check, 11 T-DXd-sensitive (19 fragments) and 7 resistant tissue samples (27 fragments) were included for analysis. Utilizing autocorrelation-related indices, spatial heterogeneity of ERBB2 gene expression influenced outcomes, with more dispersed ERBB2 gene expression distributions by cancer cells correlating with improved response. In HER2-positive tumors, resistant fragments showed downregulation of ERBB2 gene expression and activation of PI3K and EGFR pathways, with autocrine amphiregulin-EGFR signaling emerging as a candidate resistance mechanism. In HER2-low tumors, resistance was notably associated with pharmacokinetic barriers: resistant fragments exhibited increased cancer-vessel distance, reduced colocalization of ERBB2 expression with cathepsin linker-cleaving enzymes and diminished predicted tumor-to-non-tumor payload concentration ratio. Exploratory longitudinal analyses of paired pre- and post-treatment samples revealed temporal increases in vessel-cancer distance at resistance. Conclusion: These findings highlight potential determinants of T-DXd efficacy. Our study demonstrates the utility of ST for uncovering ADC response mechanisms in clinical samples and potential novel therapeutic strategies. Citation Format: Changhee Park, Minki Choi, Jiwon Koh, Sungwoo Bae, Hongyoon Choi, Kwon Joong Na, Dae-Won Lee, Kyung-Hun Lee, Han Suk Ryu, Seock-Ah Im. Spatial transcriptomics uncovers pharmacokinetic barriers and tumor-intrinsic determinants of resistance to trastuzumab deruxtecan in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1211.

Abstract Brain metastases (BM) represent a significant unmet need in ER+/HER2– mBC. Although endocrine therapy (ET) plus CDK4/6i is the 1L systemic treatment, most agents have limited BBB penetration, and tumors eventually develop resistance to ET, leading to disease progression. Elacestrant is the only single-agent oral SERD to significantly improve PFS versus standard-of-care ET in the EMERALD trial in both the overall population (HR = 0.70; 95% CI:0.55-0.88; P = 0.0018) and in patients with ESR1-mutant tumors (HR = 0.55; 95% CI:0.39-0.77; P = 0.0005), with a manageable safety profile (Bidard, 2022). Preclinical data demonstrate both elacestrant and abemaciclib cross the BBB (Conlan, 2020; Tolaney, 2020), providing a rationale for evaluating this combination in patients with BM. ELECTRA (NCT05386108) is an open-label, multicenter, phase 1b/2 study evaluating elacestrant plus abemaciclib in patients with BM from ER+/HER2– breast cancer. Eligibility includes locally advanced/mBC with ≥1 active, measurable BM (RECISTv1.1), prior therapy in the metastatic setting including ≥1 ET, ≤2 chemotherapy regimens, and 0-2 prior CDK4/6i (excluding abemaciclib). Phase 1b established the RP2D. Phase 2 primary endpoint is ORR (RECISTv1.1). Secondary endpoints include iORR, DoR, CBR, PFS, OS, PK, and QoL. Exploratory endpoints include CSF PK of elacestrant plus abemaciclib. This analysis reports phase 2 CSF PK results. In evaluable patients (n = 7), elacestrant achieved clinically meaningful CSF concentrations, with levels exceeding the target engagement threshold in 50% of patients. Abemaciclib attained CSF levels sufficient for CDK4/6 inhibition. These findings confirm that the combination achieves pharmacologically relevant drug exposure in the CNS. Updated safety, and clinical efficacy data, including intracranial-response assessments, will be presented (n = 33). Both elacestrant and abemaciclib successfully penetrated the BBB, achieving clinically meaningful CSF concentrations in most patients. These PK data support the continued evaluation of this all-oral BBB-penetrant combination for patients with ER+/HER2– mBC and BM. The phase 2 portion of ELECTRA is actively enrolling worldwide.

Abstract Background: Trophoblast cell surface antigen-2 (TROP2), Tumor-associated calcium signal transducer 2 (TACSTD2) is a transmembrane glycoprotein expressed in various cancer types. Datopotamab deruxtecan (Dato-DXd) is an antibody-drug conjugate (ADC) composed of an anti-TROP2 antibody (Datopotamab) linked via a cleavable peptide linker to DXd, a Topoisomerase-I inhibitor. Topoisomerase-I maintains DNA topological stress by resolving DNA supercoils during replication and transcription. We sought to determine whether this DNA damage mechanism classically induced by topoisomerase I inhibitors emerges when delivered through an ADC, and to further elucidate the previously uncharacterized molecular mechanisms of DNA damage response (DDR) and cell death by Dato-DXd. Methods: Five established human breast cancer cell lines and one patient-derived breast cancer (PDC) cell line (SNU-3171) were used in this study. MCF7 and SNU-3171 were luminal breast cancer cell lines, and the others were triple-negative breast cancer (TNBC) cell lines. Sensitivity of each cell to Dato-DXd was assessed by colony formation assay (CFA) for 14 days (0.25-5 nM). Internalization and cell cycle analysis were performed by flow cytometry. Apoptosis was detected by Annexin V assay. The protein expression of TROP2, DNA damage, and repair molecules was analyzed by Western blotting and immunofluorescence. Results: Based on CFA results, HCC1806, HCC70, and SNU-3171 were identified as sensitive cells (IC50<0.5 nM), whereas MCF7, MDA-MB-157, and HCC1395 as less sensitive cells (IC50>2.5 nM) to Dato-DXd. Dato-DXd was internalized within 2 hours in TROP2-positive cells. Following internalization, topoisomerase-I cleavage complexes (Top1ccs) were formed in all TROP2-positive cells; however, co-localization of Top1ccs and ɣ-H2AX was observed only in sensitive cells. Dato-DXd induced G2/M arrest in all TROP2-positive cells. In sensitive cells, the expression of TDP1, XRCC1, DNA Ligase III, and PNKP was decreased, while phospho-Chk1 (S345) and ɣ-H2AX were increased, indicating a reduction in single-strand DNA damage repair capacity and activation of DDR. Moreover, increased doses of Dato-DXd were associated with higher number of Annexin V-positive cells and sub-G1 populations, as well as higher expression of cleaved PARP, caspase-3, and caspase-7 in sensitive cells. Conclusion: Dato-DXd demonstrated efficient internalization in TROP2-positive breast cancer cells including PDC, and impaired DNA single-strand break (SSB) repair, which consequently induce apoptosis in sensitive breast cancer cells. In particular, formation of Top1ccs and G2/M phase accumulation are induced in most of TROP2-positive cells. Further experiments are required to elucidate how impaired SSB repair contributes to the induction of DNA double-strand breaks and apoptosis. Citation Format: Seohyeon Lim, Sujin Ham, Hae Min Hwang, Youlim Noh, Jiwon Koh, Chaeyoung Lee, Minyoung Jeong, Yu-Jin Kim, Minyoung Lee, Sohyeon Kim, Changhee Park, Dae-Won Lee, Kyung-Hun Lee, Seock-Ah Im. Antitumor effect of TROP2-targeted antibody drug conjugate, datopotamab deruxtecan (Dato-DXd), in association with DNA damage response in breast cancer cell lines [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2943.

Background The impact of treatment delay on breast cancer survival across disease stages remains inconsistent across studies. We evaluated the association between time to treatment initiation (TTI) and overall survival by disease stage among Korean women with breast cancer. Methods A retrospective cohort study was conducted using the Breast Cancer Collaborative Stage-Linked Database (2012–2019), which was established under the Korean Clinical Data Utilization for Research Excellence (K-CURE) project. TTI was categorized as ≤ 30 days (reference), 31–60 days, ≥ 61 days, or no treatment within 1 year. Stage-stratified Cox proportional hazards models were used to estimate adjusted hazard ratios (aHRs) for overall survival. Subgroup analyses were performed according to molecular subtype and comorbidity status. Results A total of 16,495 women were included. Among treated patients, TTI was not significantly associated with overall survival in patients with stage I, II, or IV disease. In patients with stage III disease, a delay of ≥ 61 days was associated with a 50% higher risk of death (aHR, 1.50; 95% CI, 1.04–2.16). In the overall cohort, this delay was associated with worse survival among patients with triple-negative breast cancer (aHR, 1.75; 95% CI, 1.10–2.78) and HR+/HER2 + tumors (aHR, 1.41; 95% CI, 1.04–1.92). Among patients with stage III disease, the association was especially pronounced for those with preexisting cardiovascular disease (aHR, 5.28; 95% CI, 1.78–15.68). Conclusions Among Korean women with stage III disease, a treatment delay of ≥ 61 days was associated with worse overall survival. The association was more pronounced in aggressive subtypes and in those with preexisting cardiovascular disease, highlighting the need to minimize avoidable delays in these high-risk groups.

In the Asian international PATHWAY trial, palbociclib–tamoxifen demonstrated improved progression-free survival compared with placebo–tamoxifen in patients with HR+/HER2− locally advanced or metastatic breast cancer (ABC). This analysis compared quality of life (QoL) between treatment arms. Pre/peri or postmenopausal women with HR+/HER2− ABC were randomly assigned 1:1 to receive palbociclib–tamoxifen or placebo–tamoxifen. Patient-reported outcomes were assessed on day 1 of cycles 1 (baseline), 4, and 7, and at the end of treatment (EOT) using the EORTC QLQ-C30 and QLQ-BR23 questionnaires. Least-square mean (LSM) changes in scores from baseline were calculated. Kaplan–Meier method was used to calculate time to deterioration (TTD) in the minimally important difference (MID) in the pain subscale. In the comparison between treatment arms, no significant differences were observed in LSM changes in scores from baseline in global QoL (cycle 4: 0.70 [95

LBA1018 Background: In Asia, BC incidence is increasing and peaks before menopause; tamoxifen is commonly used for pre/perimenopausal BC. However, tamoxifen was not approved for use in combination with CDK4/6 inhibitors in many countries. PATHWAY previously showed significantly improved progression-free survival (PFS) for palbociclib plus tamoxifen (± goserelin) at the primary analysis (data cut-off 15 Sep 2022) with immature OS. Here, we report updated PFS and final OS. Methods: This phase 3 trial was conducted in Japan, Korea, Taiwan, and Singapore (NCT03423199). Eligible patients were women with locally advanced or metastatic HR-positive, HER2-negative BC who were candidates for 1st- or 2nd-line tamoxifen-based endocrine therapy and had not received a prior CDK4/6 inhibitor. Patients were randomized 1:1 to receive palbociclib (125 mg daily, days 1–21 of a 28-day cycle) or placebo, each with tamoxifen 20 mg daily. Pre/perimenopausal patients also received goserelin. Randomization was stratified by line of therapy and menopausal status. Investigator-assessed PFS (primary endpoint) and OS were analyzed using Kaplan–Meier methods and compared using a stratified log-rank test with 1-sided p values; hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated using a stratified Cox proportional hazards model. Results: 184 patients were randomized (Feb 2018–Jul 2019) at 22 sites. Overall, 28.3% were pre/perimenopausal and 71.7% postmenopausal; 60.9% received study treatment as 1st-line and 39.1% as 2nd-line endocrine therapy. At the end of study in October 2025, 154 PFS events and 105 deaths occurred. Updated PFS remained significantly improved with palbociclib plus tamoxifen vs placebo plus tamoxifen (HR 0.594; 95% CI, 0.427–0.825; P < 0.001), with median PFS 24.4 months (95% CI, 13.1–32.4) vs 11.1 months (95% CI, 7.4–14.6), respectively. PFS benefit was observed regardless of menopausal status (pre/perimenopausal: HR 0.343; 95% CI, 0.175–0.675; postmenopausal: HR 0.697; 95% CI, 0.479–1.012). Final OS also favored palbociclib plus tamoxifen (HR 0.772; 95% CI, 0.525–1.134; P = 0.093), with median OS 71.7 months (95% CI, 57.1–not estimable) vs 62.0 months (95% CI, 49.9–73.1); OS results were consistent across menopausal subgroups (pre/perimenopausal: HR 0.747; 95% CI, 0.339–1.647; postmenopausal: HR 0.772; 95% CI, 0.497–1.198). Palbociclib plus tamoxifen was generally well tolerated; adverse events were manageable with dosing interruptions/dose reductions, and no new safety findings. Conclusions: With longer follow-up, palbociclib plus tamoxifen (± goserelin) provided durable PFS benefit and a favorable OS trend with manageable safety, supporting this regimen as an effective option regardless of menopausal status. Clinical trial information: NCT03423199 .

PURPOSE:Sustained cell proliferation and cell cycle acceleration in cancer cells inherently increase DNA damage, which interferes with homeostatic replication and transcription. Ataxia telangiectasia and Rad3-related (ATR) is crucial for initiation of the DNA damage response, and ATR inhibitors, such as elimusertib, induce increased replication stress and DNA damage. We investigated the anti-tumor effects of elimusertib and its mechanism of action in relation to replication stress. MATERIALS AND METHODS:Anti-tumor effects were evaluated by MTT assay and colony formation assay in breast cancer cell lines in vitro, in breast cancer cell xenografts in vivo, and in patient-derived xenograft models. Cell cycle was assessed by flow cytometry and BrdU assay was used to measure replicating cells and S-phase progression. Alkaline and neutral comet assay was used to measure single and double-stranded DNA damages, respectively. RESULTS:Elimusertib delayed S-phase progression in MDA-MB-453 and MDA-MB-231 cells and induced caspase-7-dependent apoptosis. Furthermore, the increase in sub-G1 population in the fluorescence-activated cell sorting analysis and Annexin V assay also confirmed apoptotic cell death. In the BrdU assay, single-stranded DNA (ssDNA) increased in sensitive cells and aberrant ssDNA induced DNA damage in S-phase and eventually caused replication catastrophe. Finally, these anti-tumor effects were proven in in vivo xenograft and patient-derived xenograft models. CONCLUSION:Elimusertib had anti-tumor effects and induced replication catastrophe in breast cancer cells with a high replication rate. Moreover, cells under high DNA replication stress were sensitive to elimusertib. Further studies and treatment strategies with elimusertib are warranted for cancers with a high replication rate.

Cyclin-dependent kinase 9 (CDK9) is a crucial regulator of transcriptional progression of RNA polymerase-II (RNAP2). RNA polymerases trapped in DNA can be a source of transcription-replication conflict (T-R conflict), which is a common source of replication stress. AZD4573, a highly selective CDK9 inhibitor, has been shown to induce apoptosis in leukemia cell lines, while its anti-tumor potential in breast cancer has yet to be elucidated. To evaluate the cytotoxicity of AZD4573 in vitro, MTT assays were performed. The expression of signal transduction molecules was determined using Western blotting, immunoprecipitation, and immunofluorescence. Apoptotic cell death was verified by the annexin-V assay. DNA strand breaks and repair efficacy were evaluated through the alkaline comet assay. The siRNA knock-down system was used to confirm the action mechanism. AZD4573 induced T-R conflicts during S-phase, increasing replication stress and DNA strand breaks, resulting in apoptosis by induction of caspase-3. Furthermore, we identified Dead-box 25 (DDX25) helicase as a key mediator in resolving the T-R conflicts. Nuclear translocation of DDX25 correlated with reduced sensitivity to AZD4573 by the resolution of T-R conflicts. Inhibition of CDK9 by AZD4573 induces the accumulation of DNA damage through T-R conflicts. DDX25 helicases were identified as a key mediator in resolving T-R conflicts and the reduced sensitivity to AZD4573.

Background: In the PATHWAY trial, palbociclib plus tamoxifen demonstrated improved progression-free survival compared with placebo plus tamoxifen in hormone receptor-positive, HER2-negative (HR+/HER2−) advanced breast cancer. Quality of life (QOL) data was also collected and evaluated as a secondary objective. This analysis compared QOL between the two treatment groups. Methods: Pre-, peri-, or postmenopausal women with locally advanced or metastatic HR+/HER2− breast cancer who were candidates to receive tamoxifen as first-line or second-line endocrine treatment for advanced disease were randomly assigned 1:1 to receive palbociclib–tamoxifen or placebo–tamoxifen in Japan, Republic of Korea, Taiwan, and Singapore. Patients continued to receive the assigned treatment until progressive disease (PD), clinically diagnosed symptomatic deterioration, unacceptable toxicity, death, or patient’s refusal, whichever occurred first. The study was conducted as a Clinical Research Collaboration with the National Cancer Center Hospital as the regulatory study sponsor and Pfizer providing drug and financial support (NCT03423199). Patient-reported outcomes were assessed on Day 1 of Cycles (C) 1 (baseline), 4, and 7, and at the end of treatment (EOT) using EORTC QLQ-C30 and EORTC QLQ BR23. Least-square means (LSM) changes from baseline were calculated. Kaplan-Meier Plot was used to calculate time to deterioration (TTD). A composite definition of deterioration based on death, PD, clinically diagnosed symptomatic deterioration, and minimally important difference (MID) was used for TTD. MID was defined as the increase in QOL score of 10 points or greater from baseline. Results: The EORTC QLQ-C30 and the EORTC QLQ-BR23 were completed by 88 patients in the palbociclib plus tamoxifen arm and 93 patients in the placebo plus tamoxifen arm at baseline, and by 64 (72.7%) to 80 (90.9%) patients in the palbociclib plus tamoxifen arm and by 59 (63.4%) to 79 (84.9%) patients in the placebo plus tamoxifen arm, from C4 to EOT. In the EORTC QLQ-C30, the global QOL subscale, the LSM changes from baseline for C4, C7, and EOT were -0.22 (Standard error [SE]: 1.99) and -0.92 (SE: 2.09), 2.83 (SE: 2.21) and 0.66 (SE: 2.30), and -12.31 (SE: 2.40) and -7.12 (SE: 2.21) in the palbociclib plus tamoxifen arm and in the placebo plus tamoxifen arm, respectively. The difference in the LSM between treatments at C4, C7, and EOT were 0.70 (95% CI: -5.00, 6.41), 2.17 (95% CI: -4.14, 8.48) and -5.19 (95% CI: -11.64, 1.26). For either the functional subscales or symptom scales, no notable trends or differences between the treatment arms were found in the LSM changes from baseline. In the pain symptom scale, a total of 68 patients (77.3%) in the palbociclib plus tamoxifen arm and 81 patients (87.1%) in the placebo plus tamoxifen arm had documented PD, clinically diagnosed symptomatic deterioration, or MID at the data cutoff as of 15 Sept 2022. The median TTD was 10.3 months (95% CI: 5.8, 24.4) for the palbociclib plus tamoxifen arm and 5.5 months (95% CI: 3.6, 7.6) for the placebo plus tamoxifen arm with the hazard ratio of 0.601 (95% CI: 0.433, 0.834). In the analysis of the EORTC QLQ-BR23: no notable trends or differences between the treatment arms were found in the LSM changes from baseline for either the functional subscales or symptom scales. Conclusion: The addition of palbociclib demonstrated a prolongation of TTD in the pain symptom scale of EORTC QLQ-C30 compared with placebo. No differences were found between treatment arms in other QOL analyses. Palbociclib plus tamoxifen allowed patients to maintain QOL while experiencing delayed disease progression. Citation Format: Joohyuk Sohn, Kazuki Sudo, Takashi Yamanaka, Hirofumi Mukai, Naohito Yamamoto, Chi-Feng Chung, Yen-Shen Lu, Kyung-Hun Lee, Soo-Chin Lee, Tsutomu Iwasa, Hiroji Iwata, Kenichi Watanabe, Kyung Hae Jung, Yuko Tanabe, Seok Yun Kang, Hiroyuki Yasojima, Kenjiro Aogi, Eriko Tokunaga, Sung Hoon Sim, Yoon Sim Yap, Koji Matsumoto, Ling-Ming Tseng, Yoshiko Umeyama, Emi Noguchi, Tomomi Hata, Aya Kuchiba, Taro Shibata, Kenichi Nakamura, Kenji Tamura, Kan Yonemori. Quality of life with palbociclib plus tamoxifen in hormone receptor-positive, HER2-negative advanced breast cancer: results from NCCH1607/PATHWAY, an Asian international double-blind randomized phase 3 trial [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P4-07-19.

Background The phase 2 randomised Young-PEARL study demonstrated that palbociclib plus exemestane with ovarian function suppression significantly prolonged progression-free survival compared with capecitabine in premenopausal women with hormone receptor-positive, HER2-negative metastatic breast cancer. Here, we report results of the protocol-specified secondary endpoint of overall survival. Methods Young-PEARL was a multicentre, randomised, open-label, phase 2 study conducted at 14 institutions in South Korea. Premenopausal women aged 19 years or older with histologically confirmed hormone receptor-positive, HER2-negative metastatic breast cancer that recurred or progressed during or after previous tamoxifen treatment, who were aromatase inhibitor naive, and had an Eastern Cooperative Oncology Group performance status of 0-2 were eligible. One previous line of chemotherapy was permitted in the metastatic setting. Eligible patients were randomly assigned (1:1), using block randomisation (block size of two) stratified by previous chemotherapy for metastatic breast cancer and presence of visceral metastasis, to receive either palbociclib (orally, 125 mg per day on a 3-weeks-on, 1-week off schedule) plus exemestane (orally 25 mg daily) with leuprorelin (subcutaneously 375 mg on day 1 of each 28-day cycle) or capecitabine (orally, 1250 mg/m(2) twice a day on a 2-weeks-on, 1-week-off schedule) until disease progression or unacceptable toxicity). The primary endpoint was progression-free survival. Overall survival was a secondary endpoint. All analyses were done in the modified intention-to-treat population (ie, included all patients randomly assigned to treatment who had at least one post-baseline CT scan and excluded those who did not receive study medication and who had any major violation of the eligible criteria). Safety was assessed in all patients who received any study treatment. This study is registered with ClinicalTrials.gov, NCT02592746, and is now complete. Findings Between June 15, 2016, and Dec 10, 2018, 189 patients were enrolled. 184 patients were randomly assigned to the palbociclib plus endocrine therapy group (n=92) or the capecitabine group (n=92), of whom 174 were included in the modified intention-to-treat population (n=90 in the palbociclib plus endocrine therapy group and n=84 in the capecitabine group). All patients were female and ethnicity data were not collected. As of data cutoff (Feb 29, 2024), median follow-up was 540 months (IQR 341-744). Median progression-free survival was 195 months (90% CI 143-222) for palbociclib plus endocrine therapy and 140 months (117-187) for capecitabine (hazard ratio 074 [90% CI 057-098]; one-sided log-rank p=0036). 52 (58%) of 90 patients in the palbociclib plus endocrine therapy group and 48 (57%) of 84 in the capecitabine group died, with a median overall survival of 548 months (95% CI 489-771) in the palbociclib plus endocrine therapy group versus 578 months (463-892) in the capecitabine group (hazard ratio 102 [95% CI 069-151]; p=092). The most common grade 3 or worse adverse event was neutropenia (59 [64%] of 92 in the palbociclib plus endocrine therapy group vs 15 [18%] of 85 in the capecitabine group) . No treatment-related deaths occurred. Interpretation With extended follow-up, palbociclib plus exemestane with ovarian function suppression continued to show a significant benefit in progression-free survival compared with capecitabine in premenopausal patients with hormone receptor-positive, HER2-negative metastatic breast cancer who had been previously treated with tamoxifen; however, no improvement in overall survival was seen. Given the progression-free survival benefit, the upfront use of palbociclib plus endocrine therapy is the preferred option for premenopausal women, although a capecitabine-first strategy might be an alternative treatment strategy for maintaining overall survival in resource-limited settings. Copyright (c) 2025 Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.

593 Background: In the KEYNOTE(KN)-756 study, adding pembrolizumab to NAC increased pCR in high-risk HR+ HER2- BC. There is an unmet need to discover who will benefit from adding immune checkpoint inhibitors (ICIs). This study aims to investigate the role of pCR in HR+ HER2- BC and to identify whether AI-powered TIL analysis could predict pCR in patients treated with NAC without ICIs. Methods: This is a single-center study conducted in Seoul National University Hospital, Korea. H&E whole-slide images (WSIs) of archival breast tumor tissues at diagnosis were analyzed by Lunit SCOPE IO, an AI-powered spatial TIL analyzer. Tumors with a high proportion of area with high intratumoral TIL were classified as immune-inflamed, those with a high proportion of area with low intratumoral but high stromal TIL as immune-excluded, and the remaining as immune-desert. Recurrence risk prediction AI model was trained and validated on independent 1,552 H&E WSIs to predict OncotypeDx score. Patients with histologic grade 3 and tumor size ≥2 cm with node-positive status, or tumor size ≥5 cm were classified as those eligible for KN-756 study. Results: A total of 425 BC patients who were treated with NAC without ICIs between January 2015 and October 2018 were included. The median age was 47 (range 24-80), 67.1% had stage III disease, 93.6% had node-positive disease, and 23.5% were eligible for KN-756 study. pCR was achieved in 57 (13.4%) patients and was higher in patients with whom were eligible for KN-756 study (20% vs 11.4%, p= 0.041). Patients who achieved pCR had better 5-year event free survival (EFS, 92.9% vs 77.7%, p= 0.010). AI-powered spatial analysis was performed in 340 patients. There were 125 (36.8%) with immune-desert tumor, 138 (40.6%) with immune-excluded tumor, and 77 (22.6%) with immune-inflamed tumor. Patients with inflamed tumor had higher pCR rate compared to those with immune-excluded or immune-desert in the whole population (Table) and in patients not eligible for KN-756 study (25.6% vs 11.9% vs 5.7%, p = 0.013). Patients with inflamed or excluded tumor had better EFS compared to desert group (80.9% vs 71.2%, p = 0.0275). In addition, recurrence prediction model showed that those who were predicted to have OncotypeDx score ≥ 26 had higher pCR rate (18.2% vs 4.3%, p= 0.002). Conclusions: Achieving pCR was associated with favorable EFS in HR+ HER2- patients treated with NAC. Patients with immunogenic tumor microenvironment had higher pCR rate and better EFS. Investigating the role of immune checkpoint inhibitors according to tumor microenvironment may be promising. Desert(N=125) Excluded(N=138) Inflamed(N=77) p-value pCR 8 (6.4%) 18 (13.0%) 23 (29.9%) <0.001 Stage III 85 (68.0%) 88 (63.8%) 51 (66.2%) 0.32 LN + 114 (91.2%) 132 (95.7%) 72 (93.5%) 0.34 HG 3 21 (16.8%) 31 (22.5%) 38 (49.4%) <0.001 KN-756 eligible 20 (16.0%) 29 (21.0%) 34 (44.2%) <0.001

Background & Aims:We performed a meta-analysis of data from the KEYNOTE-240 and KEYNOTE-394 studies to obtain a more precise estimate of the pembrolizumab treatment effect in participants with previously treated advanced hepatocellular carcinoma (HCC). Methods:Participants with confirmed HCC and disease progression after treatment with or intolerance of sorafenib or oxaliplatin-based chemotherapy (KEYNOTE-394 only), Barcelona Clinic Liver Cancer stage C or B disease not amenable to or refractory to locoregional therapy, and one or more measurable lesion per Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 were randomly assigned 2:1 to receive pembrolizumab or placebo for ≤35 cycles. Data from the KEYNOTE-240 and KEYNOTE-394 intention-to-treat populations were pooled, and the treatment effect was evaluated for pembrolizumab and placebo separately. Results:In total, 578 and 288 participants who received pembrolizumab and placebo, respectively, were included in this analysis. Compared with placebo, pembrolizumab improved overall survival (hazard ratio 0.79, 95% CI 0.67-0.93), progression-free survival (per RECIST v1.1 by blinded independent central review [BICR]; hazard ratio 0.76, 95% CI 0.64-0.89), and objective response rate (per RECIST v1.1 by BICR; 15.4% vs. 2.8%, for an estimated treatment difference of 12.5%; 95% CI 8.8-16.2). Subgroup analyses showed that the treatment effect of pembrolizumab was generally similar across baseline participant characteristics, including viral status, Barcelona Clinic Liver Cancer stage, and geographic region. Conclusions:Meta-analysis of KEYNOTE-240 and KEYNOTE-394 showed that pembrolizumab provides clinically meaningful improvement in overall survival, progression-free survival, and objective response rate. This analysis expands on findings from each study individually and provides further evidence of the global benefit of pembrolizumab as second-line therapy for advanced HCC after prior sorafenib- or oxaliplatin-based therapy. Impact and implications:To obtain a more precise estimate of the pembrolizumab treatment effect in participants with previously treated advanced hepatocellular carcinoma, we performed a meta-analysis of efficacy using pooled participant data from the phase III KEYNOTE-240 and KEYNOTE-394 studies. Subgroup analyses showed that the treatment effect of pembrolizumab was generally similar across baseline characteristics, including viral status, Barcelona Clinic Liver Cancer stage, and geographic region. This meta-analysis provides further evidence of the global benefit of pembrolizumab as second-line therapy for advanced hepatocellular carcinoma. Clinical Trials Registration:Registered at ClinicalTrials.gov NCT02702401 (KEYNOTE-240) and NCT03062358 (KEYNOTE-394).

Resistance to chemotherapy remains a major clinical challenge in triple-negative breast cancer (TNBC), an intrinsic subtype with limited available therapeutic options. The expression of moesin (MSN) is upregulated in TNBC patients, but little is known about the role of MSN in breast carcinogenesis. We investigated the MSN-dependent autocrine loop between extracellular interleukin 6 (IL-6) and NF-κB, along with a signaling cascade involving GTPase-mediated STAT3 phosphorylation. Various in vitro and in vivo assays were used to evaluate tumor initiation, growth, and stemness properties in TNBC models. High MSN expression was correlated with shorter overall and disease-free survival in TNBC patients. In vivo, MSN promotes tumor initiation and growth. Mechanistically, MSN-mediated IL-6/NF-κB autoregulatory feedback enhances IL-6 transcription. IL-6 binding to LPAR1 activated MSN phosphorylation, which then sequentially phosphorylated the CDC42-PAK4 complex, triggering nuclear translocation of the pSTAT3-MSN complex. This led to pSTAT3-mediated activation of cancer stemness genes (IGFN1, EML1, and SRGN), contributing to Adriamycin resistance. Notably, combination treatment with the FDA-approved STAT3 inhibitor Atovaquone and Adriamycin restored drug sensitivity. Our findings uncover the critical role of MSN in regulating STAT3-mediated cancer stemness via the IL-6/NF-κB signaling axis. These results provide a strong rationale for repositioning STAT3 inhibitors such as Atovaquone as a therapeutic strategy in Adriamycin-resistant TNBC patients exhibiting pSTAT3-MSN complex upregulation.

1016 Background: In HER2+ advanced breast cancer (ABC), standard treatment has been anti-HER2 therapy with chemotherapy, regardless of hormone receptor status. While prior studies support the use of CDK4/6 inhibitors with anti-HER2 and endocrine therapy in pretreated HR+/HER2+ ABC, data on their first line use without chemotherapy are limited. This study investigates ribociclib, trastuzumab, and letrozole as a first-line combination in HR+/HER2+ ABC. Methods: This multicenter, single-arm, prospective trial was conducted across 17 academic institutions in South Korea (NCT03913234). Eligible patients were HR+/HER2+ ABC with no prior systemic therapy for metastatic disease. The Phase IB study used a 3+3 design to determine the recommended Phase II dose (RPIID) of ribociclib with fixed doses of letrozole (2.5 mg QD) and trastuzumab (8 mg/kg loading, then 6 mg/kg every 3 weeks). The Phase II trial evaluated efficacy and safety at the RPIID. The primary endpoint was progression-free survival (PFS), targeting an improvement from 8 to 12 months. Secondary endpoints included overall survival (OS), objective response rate (ORR), duration of response (DOR), and safety. PAM50 testing assessed correlations between intrinsic subtype and treatment efficacy. Results: Phase IB (n = 13) identified the RPIID as ribociclib 600 mg QD, with one dose-limiting toxicity (Grade 3 ALT elevation) at 400 mg. In Phase II, 77 patients were enrolled, with a median age of 61 years (range 31–85), 18.2% (14/77) premenopausal, 66.2% (51/77) HER2 IHC 3+ and recurrent disease in 64.9% (50/77). 66.2% (51/77) had visceral metastases. At a median follow-up of 15.8 months (95% CI: 12.9-19.1) months, the median PFS was 30.4 months (95% CI: 19.6–NA), meeting the primary endpoint. The median OS was not reached. The ORR was 61.1%, including 3 complete and 41 partial responses, with a DOR of 11.8 months (95% CI: 7.6-13.4). Common adverse events included neutropenia (66.7%), pruritus (24.4%), and nausea (22.2%). There was a death reported due to aortic aneurysm. PAM50 analysis in 75 patients (phase IB/II) showed no significant correlation between intrinsic subtype and efficacy. Conclusions: Ribociclib, trastuzumab, and letrozole as first-line therapy in HR+/HER2+ ABC demonstrated a median PFS of 30.4 months with a manageable safety profile, supporting its potential as a chemotherapy-free option. Clinical trial information: NCT03913234 .