Han, Yiqun

Introduction Spleen tyrosine kinase (SYK) is a non-receptor tyrosine kinase involved in immune signaling and has been implicated in tumor-intrinsic resistance to DNA-damaging therapies. Although SYK activity has been associated with poor outcomes in triple-negative breast cancer (TNBC), its role in resistance to combined radiotherapy and immune checkpoint blockade remains unclear. Methods SYK expression and clinical relevance were assessed across multiple breast cancer cohorts, including radiotherapy-treated datasets. Integrated transcriptomic analyses were used to characterize SYK-associated immune states. The functional effects of pharmacologic SYK inhibition were evaluated using in vitro assays and an immunocompetent bilateral 4T1 tumor model in female BALB/c mice treated with radiotherapy and PD-1 blockade. Treatment-associated immune remodeling was assessed by flow cytometry and transcriptomic profiling. Results Across multiple cohorts, elevated SYK expression, particularly in TNBC, was associated with inferior clinical outcomes and immune dysregulation. Pharmacologic SYK inhibition enhanced tumor responses to radiotherapy and PD-1 blockade at both irradiated and distant tumor sites. These effects were accompanied by coordinated remodeling of the tumor immune microenvironment, including suppression of neutrophil-associated signatures and reductions in immunosuppressive myeloid populations, including M2-like macrophages and myeloid-derived suppressor cells. Discussion Pharmacologic SYK inhibition enhances responses to radiotherapy and PD-1 blockade in preclinical TNBC models and is accompanied by remodeling of the immunosuppressive myeloid compartment. These findings support further investigation of SYK-targeted strategies to improve responses to combined radiotherapy and immune checkpoint blockade.

Introduction Antibody–drug conjugates (ADCs) are now widely used in breast cancer across multiple disease subtypes. With increasing clinical use, treatment-related toxicities have become an important factor in therapeutic decision-making. However, comparative safety data among ADCs are limited because direct head-to-head trials are lacking. Methods We conducted a systematic review and Bayesian network meta-analysis of randomized controlled trials evaluating ADCs in breast cancer, with the literature search updated through March 31, 2025. Treatment-related adverse events (TRAEs) and serious adverse events (SAEs) were classified according to the Common Terminology Criteria for Adverse Events. A Bayesian random-effects network meta-analysis was used to compare toxicity risks across regimens. Odds ratios (ORs) with 95% credible intervals (CrIs) were estimated, and surface under the cumulative ranking curve (SUCRA) values were used as descriptive ranking summaries interpreted alongside the corresponding comparative estimates. The protocol was registered in PROSPERO (CRD42024606194). Results Seventeen randomized trials including 8,946 patients and five ADCs—trastuzumab emtansine (T-DM1), sacituzumab govitecan (SG), trastuzumab deruxtecan (T-DXd), datopotamab deruxtecan (Dato-DXd), and ARX788—were analyzed. SG and T-DXd showed the least favorable hematologic safety profiles, with SUCRA values for anemia/neutropenia of 22.6%/17.4% and 12.0%/27.0%, respectively. T-DM1-based regimens had the highest risk of thrombocytopenia. For gastrointestinal toxicities, SG ranked worst for diarrhea (SUCRA 6.0%), whereas T-DXd was associated with higher risks of nausea (2.2%), vomiting (7.9%), and decreased appetite (8.6%). In analyses of SAEs, SG had the highest rates of anemia (11.7%) and neutropenia (8.5%), while gastrointestinal SAEs occurred more frequently with T-DXd. Conclusions This network meta-analysis highlights clinically meaningful differences in ADC safety profiles and may help guide individualized toxicity management in breast cancer.

Our research uncovers a role for ATR in responding to ECM stiffness and promoting epithelial-to-mesenchymal transition (EMT) and metastasis. ATR, when deubiquitinated and upregulated by USP21 under enhanced ECM stiffness conditions, phosphorylates the nuclear protein SUN2, which promotes β-catenin nuclear translocation and EMT. ATM-mediated EMT promotes polymorphonuclear myeloid-derived suppressor cell recruitment and inhibits CD103 + dendritic cells, fostering an immunosuppressive tumor milieu. ATR inhibition disrupts this malignant cascade by promoting mesenchymal-to-epithelial transition to enhance antitumor immunity and mitigate metastases. Consistently, circulating HLA-DR + dendritic cells were also enhanced following treatment with the ATR inhibitor berzosertib in patients with therapeutically resistant early-stage breast cancer. Our data suggest that ATR-targeted therapy may be optimized by considering both DNA damage–dependent and EMT-inducing effects of ATR ATR promotes metastasis and immunosuppression through EMT process

1110 Background: CDK4/6 inhibitors with endocrine therapy are widely used for HR+/HER2– metastatic breast cancer (BC). While studies have shown similar progression-free outcomes across agents, overall survival (OS) data are limited. Since OS is a definitive endpoint and BMI may influence outcomes, this study examines BMI-dependent OS differences among CDK4/6 inhibitors. Methods: A retrospective cohort study was conducted using the Mayo Clinic Platform full AMC1 dataset. Postmenopausal women with BC receiving palbociclib/ribociclib/abemaciclib plus aromatase inhibitor (AI) were included. Patients with prior use of fulvestrant, anthracyclines, radiation therapy, GnRH agonists, or history of MACE, heart failure, myocardial infarction, or stroke before BC diagnosis were excluded. Patients were stratified by BMI and CDK4/6 inhibitor. The primary outcome was 5-year OS. Results: Among 1,989 patients (palbociclib: 868; ribociclib: 465; abemaciclib: 656), palbociclib was associated with significantly worse OS compared to ribociclib in BMI <25 (HR 2.91; 95% CI: 1.76–4.79; P < 0.01) and BMI ≥30 (HR 1.91; 95% CI: 1.21–3.01; P < 0.01). Compared to abemaciclib, palbociclib showed worse OS: BMI <25 (HR 1.73; P < 0.01), BMI 25–30 (HR 1.56; P < 0.01), and BMI ≥30 (HR 1.82; P < 0.01). No significant OS differences were observed between ribociclib and abemaciclib. Palbociclib patients were older, particularly in BMI <25 (Cohen's D = 0.39 vs. ribociclib; 0.27 vs. abemaciclib) and BMI ≥30 (0.28 vs. ribociclib; 0.23 vs. abemaciclib). In the BMI 25–30 group, palbociclib group was also older than ribociclib group (Cohen's D = 0.26). Conclusions: In postmenopausal BC patients on AIs and CDK4/6 inhibitors, palbociclib was associated with worse OS in BMI < 25 and BMI ≥ 30 groups, while ribociclib and abemaciclib showed similar outcomes across BMI groups. Palbociclib patients were older than those on other drugs, which may partially confound results; however, in the BMI 25–30 group, despite similar age differences, no survival difference was observed between palbociclib and ribociclib, suggesting age alone may have limited impact. These findings indicate a potential interaction between BMI and drug choice, and highlight the need for prospective studies to confirm and adjust for these variables. Limitations include the observational design and inability to perform multivariable regression analyses to adjust for confounders. Overall survival comparisons by CDK4/6 inhibitor and BMI subgroup. Comparison BMI Group Hazard Ratio (95% CI) P-value Palbociclib vs Ribociclib <25 2.91 (1.76 – 4.79) < 0.01 25 – 30 1.34 (0.87 – 2.071) 0.17 ≥30 1.91 (1.21 – 3.01) < 0.01 Palbociclib vs Abemaciclib <25 1.73 (1.29 – 2.32) < 0.01 25 – 30 1.56 (1.09 – 2.23) < 0.01 ≥30 1.82 (1.15 – 2.88) < 0.01 Ribociclib vs Abemaciclib <25 1.27 (0.81 – 1.99) 0.29 25 – 30 0.87 (0.54 – 1.43) 0.59 ≥30 1.05 (0.63 – 1.75) 0.85

e15003 Background: Antibody-drug conjugates (ADCs) have shown substantial promise as therapies for metastatic breast cancer (mBC). However, direct comparisons of different ADC-based regimens are limited. We assessed survival outcomes in mBC patients treated with ADCs. Methods: Randomized controlled trials (RCTs) published before October 2024 were searched in Medline, Embase, ClinicalTrials.gov, and Cochrane Library. Progression-free survival (PFS) and overall surcvival (OS) outcomes were extracted from the most updated publications. Participants were stratified based on molecular subtypes, hormone receptor (HR) status, duration of cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) treatment, and sensitivity to endocrine therapy (ET). Bayesian frameworks were deployed, presenting the synthesized outcomes of indirect comparisons using the surface under the cumulative ranking curve (SUCRA), hazard ratio (HR), and 95% credible interval (Crl). This study was prospectively registered on the International Platform of Registered Systemic Reviews and Meta-analysis Protocols (INPLASY) (INPLASY2024110021). Results: A total of 6449 patients from 12 RCTs of ADCs ado-trastuzumab emtansine (T-DM1), sacituzumab govitecan (SG), trastuzumab deruxtecan (T-DXd), and datopotamab deruxtecan (Dato-DXd) were included in the analysis. Compared to chemotherapy, T-DXd significantly improved PFS (HR 0.35, 95% Crl 0.15-0.84) and OS (HR 0.56, 95% Crl 0.32-1.0). SG also significantly extended OS (HR 0.65, 95% Crl 0.43-0.97) but not PFS. In HER2+ mBC, pooled analyses revealed T-DXd was the most effective treatment in terms of PFS, with a SUCRA of 94.2%, followed by T-DM1 (58.9%), and chemotherapy with HER2-directed therapy (34.2%). Synthesized outcomes of OS mirrored the PFS outcomes. In HR+/HER2- mBC (some of which was HER2-low), Dato-DXd (76.1%) showed slightly superior efficacy over SG (67.4%) for PFS, while SG (85.1%) was advantageous over Dato-DXd (49.1%) in terms of OS. Among patients with HR+/HER2+ mBC, T-DXd (80.0%), SG (55.5%), and chemotherapy with HER2-directed therapy (53.1%) were the most effective therapies for prolonging PFS. Regarding OS, SG (70.3%) and T-DM1 (60.3%) were the most effective, with comparable efficacy observed between chemotherapy with HER2-directed therapy (52.1%) and T-DXd (52.0%). In HR-/HER2+ mBC, T-DXd (78.1%), SG (56.1%), and T-DM1 (51.5%) significantly prolonged PFS, while SG (69.4%) had the most substantial benefit for OS, followed by T-DXd (62.8%) and chemotherapy with HER2-directed therapy (57.6%). Dato-DXd showed an advantage over SG in mBC patients received CDK4/6i for > 12 months. Conclusions: This network meta-analysis showed prominent survival benefits with ADC-containing regimens for mBC. Our results provide valuable insights into personalized treatment strategies for mBC patients.

Abstract Poly ADP ribose polymerase inhibitors (PARPi) are being investigated in combination with photon and proton radiotherapy in clinical trials of pediatric and adult malignancies. The precise mechanisms of PARPi radiosensitization, and whether differences in photon and proton-mediated relative efficacy exist, have not been fully elucidated. To study mechanisms of PARPi radiosensitization we investigated the effects of PARPi in combination with photons and two different ionizing densities of protons delivered at the entrance (ENT) or the Bragg peak (BP) of the profile (dose-averaged LET [LETd] 2.2 and 7.0 keV/µm, respectively) of a 76.8 MeV proton beam after observing an extreme tumor and normal tissue response in an 18-year-old female patient with high grade glioma treated veliparib and proton therapy in a Children’s Oncology Group clinical trial. BRCA1/2 wild-type hTERT RPE-1, U2OS, and MDA-MB-231 cells were hypersensitive to the combination of veliparib and proton BP, with enhanced relative biological effectiveness (RBE) at 2 Gy values of 1.7, 1.4, and 1.3, respectively. Similar findings were observed with talazoparib and olaparib, suggesting that PARP trapping efficiency was not a major influence on the proton BP RBE. Further, we did not observe differences in transcription replication conflict between the different proton ionizing densities. Unexpectedly, DNA fiber assays revealed more elongated replication fork tracks in PARPi treated cells 60 minutes after proton BP compared with proton ENT, indicative of accelerated replication fork progression. To assess gap formation in the vicinity of nascent DNA we treated cells with the ssDNA-specific S1 nuclease and found that the lengths of CldU-labeled tracks were significantly reduced in cells treated with the PARPi/proton BP combination, suggestive of greater gap formation at replication forks. We postulated that more ssDNA gaps were being converted to DSBs during replication as we further observed that PARPi/proton BP resulted in increased γH2AX foci at 4- and 24-hours after treatment. Consistent with increased DSB formation, a higher proportion of cells arrested in the G2/M phase of the cell cycle at 24 hours following PARPi/proton BP. Despite being arrested in G2/M when homologous recombination is active, there was a significant increase in 53BP1 and a decrease in RAD51 recruitment, indicative of upregulated repair by more error-prone non-homologous end-joining (NHEJ) repair pathway. The enhanced proton BP RBE in PARPi treated cells was abrogated upon knockdown of XRCC4, a critical component of NHEJ repair pathway, suggesting that toxic NHEJ may be contributing to the enhanced proton BP RBE. In conclusion, PARPi renders cells hypersensitive to high LET BP proton irradiation. Enhancing the LET in combination with PARPi warrants further investigation to overcome therapeutic resistance. Citation Format: Yiqun Han, Qin Zhou, Anna Olson, Cameron M. Callaghan, Bin Chen, Nicholas B. Remmes, Meng Xu Welliver, Zheming Wu, Daniel K. Ebner, Jake A. Kloeber, Zhenkun Lou, Robert W. Mutter. Poly ADP ribose polymerase inhibitors potentiate proton therapy end-of-range relative biological effective dose by increasing replication stress [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4689.