610 Background: The adverse event (AE) landscape in oncology is changing due to the introduction of immunotherapy and antibody drug conjugates. These AEs come with both short and long-term symptoms that significantly impact patient quality of life. Monitoring for early onset of symptoms could optimize therapy for a particular patient, maximizing potential efficacy while mitigating toxicity. It is also possible that some toxicities are directly associated with drug sensitivity. We sought to identify symptoms associated with pathologic complete response (pCR) using patient-reported outcomes (PROs) in early-stage high-risk breast cancer patients. Methods: Our study population included 288 stage II/III high-risk breast cancer patients enrolled on the I-SPY2 trial from 2021-2024, who received novel neoadjuvant therapies ± standard paclitaxel. pCR was assigned if tumor was absent in breast and nodes at surgery following neoadjuvant treatment. Patients (n = 288, pCR rate = 29%, 89% administered immunotherapy) were sent electronic PROs. 33 patient-reported AEs were measured using NCI's Patient Reported Outcomes - Common Terminology Criteria for Adverse Events (PRO-CTCAE). Each symptom was evaluated using severity, frequency, and interference on a Likert Scale. Presence of early PRO symptoms (cycles 1-3 of treatment) were binarized (at least one of moderate or greater), and odds ratios were computed with pCR as outcome. To assess whether higher grade AEs were enriched in patients that achieved a pCR, we also performed the Wilcoxan rank sum test using maximum (worst) symptom severity. Results: Of 288 patients included in our analysis (median age = 48 years, range = 20-78), 203 (70.5%) were White, 17 (5.9%) were Asian, 33 (11.5%) were Black or African American, and 35 (12.2%) were Hispanic. PRO analysis revealed that patients that had moderate to severe muscle pain (27% vs 10% OR = 3.15, p < 0.05), joint pain (22% vs 8% OR = 3.23, p < 0.05), headache (27% vs 12.5% OR = 2.59, p < 0.05), or mouth/throat sores (16% vs 5% OR = 3.56, p < 0.05) within weeks 1-3 had higher odds of achieving a pCR. When we looked at maximum severity between weeks 1-3, patients that achieved a PCR had higher grade muscle pain (p = 0.04), heart palpitations (p = 0.035), and significantly lower grade numbness and tingling (p = 0.002). Beyond 6 weeks, associations were weaker or insignificant. Conclusions: Our study utilizes an analysis framework that was able to determine sentinel symptoms such as muscle and joint pain, mouth/throat sores and palpitations as early as weeks 1-3 associated with increased efficacy. This may suggest an early immune reaction in patients that eventually respond favorably to treatment. Our work can help provide earlier proactive monitoring to mitigate toxicities, treatment redirection if needed, and a potential symptom-based early understanding to personalize treatment efficacy. A similar analysis is underway to predict immune related AEs. Clinical trial information: NCT01042379 .

625 Background: I-SPY 2 is a multicenter phase II adaptive platform trial evaluating novel agents for stage II-III high-risk breast cancer using response-predictive subtypes (RPS) and a sequential neoadjuvant treatment design of up to three blocks (A/B/C). Investigational regimens are given in Block A, followed by response-guided escalation to standard-of-care (SOC) therapy (tx) in subsequent blocks or early surgery for responders. Topoisomerase I (TOPO1) inhibition plus PARP inhibition may enhance DNA damage and tumor response. DAN-222 is a tumor-targeted polymeric camptothecin conjugate designed to improve the therapeutic index of TOPO1 inhibition. Based on favorable phase 1 safety and preliminary activity in metastatic breast cancer, DAN-222 plus niraparib was evaluated as Block A tx in HER2− disease. Notably, the neoadjuvant regimen used a higher dose of niraparib (200 mg daily) than the prior phase 1 metastatic study. Methods: Patients (pts) with HER2− breast cancer were randomized to DAN-222 plus niraparib for up to 12 weeks in Block A. Response was assessed by serial MRI and biopsy. Inadequate responders escalated early to SOC tx in Blocks B/C. Primary endpoint was pathologic complete response (pCR). Secondary endpoints included residual cancer burden, safety, early discontinuation, and treatment-strategy performance versus subtype-specific SOC tx controls. Given established PARPi sensitivity in BRCA mut disease, the key objective was to assess clinically meaningful activity of DAN-222 plus niraparib in BRCA wt pts. Accordingly, a prespecified Bayesian interim futility analysis focused on BRCA wt and tested the probability of the true pCR rate after Block A alone exceeded 15% after 50 pts. Results: Fifty pts enrolled from Sep to Dec 2024. In BRCA wt (n=47), estimated Block A pCR was 3%, with posterior probability of 0.02 that that true pCR exceeded 15%, meeting futility criteria and prompting arm closure. Across all blocks, pCR rate was 57% (8/14) for HR-HER2−; 11% (4/36) for HR+ HER2−, largely after SOC tx. 2/3 BRCA mut pts achieved pCR. Treatment strategies initiating with DAN-222 plus niraparib underperformed subtype-specific controls. 68% discontinued Block A early, mainly due to toxicity. Grade ≥3 cytopenias were common: neutropenia (n=25; 50%), anemia (24%), and thrombocytopenia (18%). Cystitis occurred in 36% (1 grade ≥3). Conclusions: Neoadjuvant DAN-222 plus niraparib showed insufficient activity and no treatment-strategy benefit in BRCA wt HER2− breast cancer, underscoring challenges for TOPO1–PARP inhibitor combinations. Although not supported for further development in this setting, these results highlight the strength of I-SPY 2 in rapidly enrolling patients and quickly distinguishing ineffective from promising therapeutic strategies to accelerate progress toward improved outcomes for patients with curable breast cancer. Clinical trial information: NCT01042379 .

Background Immune checkpoint inhibitors can result in serious, long-lasting immune-related adverse events (irAEs). Early identification of symptoms predictive of irAEs could enhance monitoring and timely intervention. This study assessed whether symptoms within the first 8 weeks of treatment could predict subsequent development of immune-related adrenal insufficiency(AI) or hypothyroidism. Methods This retrospective cohort study analyzed prospectively collected data from the I-SPY2 trial, a phase 2 platform trial for high-risk stage II/III breast cancer across 30 U.S. sites. The cohort included 482 women treated with experimental immunotherapy agents concurrent with weekly paclitaxel neoadjuvant chemotherapy. The primary outcomes were grade ≥ 1 hypothyroidism or AI, adjudicated by an independent safety group, up to 1-year post-treatment. Symptoms and irAEs were assessed using the Common Terminology Criteria for Adverse Events. Symptom burden was quantified as area under the curve (AUC) based on symptom grade and duration. Predictive modeling was performed using logistic regression and ROC analysis; symptom enrichment between cases and controls was evaluated using Fisher’s exact tests. Results Among 482 participants, 107 (22.2%) developed irAEs, with hypothyroidism (n = 61, 12.7%) occurring more frequently than AI (n = 38, 7.9%) at medians of 99 and 105 days from treatment initiation, respectively. Symptom enrichment analysis identified early predictive symptoms. Fatigue (17.2% vs 6.8%, p = 0.011) and rash (20.7% vs 7.8%, p = 0.0037) were predictive of hypothyroidism, while diarrhea (45.9% vs 31%, p = 0.048), constipation (5.4% vs 0.2%, p = 0.018), and taste changes (5.4% vs 0.5%, p = 0.034) were associated with AI. A predictive model demonstrated moderate performance (AUC 0.65 for AI, p < 0.0001; AUC 0.61 for hypothyroidism, p = 0.012). Model accuracy in an external validation cohort was 72.8% for AI and 74.7% for hypothyroidism. Conclusions This study presents a predictive framework to identify patients at risk for adrenal insufficiency and hypothyroidism as irAEs, enabling personalized care and proactive intervention to improve treatment outcomes and safety.

e12755 Background: Pathologic complete response (pCR) is an early endpoint in neoadjuvant breast cancer considered in accelerated approval decisions by the FDA. However trial-level odds ratios of pCR and mature hazard ratios of disease free survival are only weakly associated in meta-analyses, resulting in approvals being delayed until survival evidence becomes available. We used data from the I-SPY 2 trial to develop early endpoints with better trial level correlations to support accelerated approvals. Methods: The I-SPY 2 trial treated 2117 patients with early stage high-risk breast cancer across 26 regimens between 2010 and 2022. When we separate the regimens into groups by subtype, there were 47 total experimental groups (19 HR+HER2-, 19 HR-HER2- and 9 HER2+). Each group is matched to concurrent control group enrolled within 90 days of the enrollment period, forming a experimental/control comparison. Treatment groups with less than 10 patients or insufficient follow-up were excluded, leaving 27 comparisons across 1577 patients. Hazard ratios for event free survival and negative log odds ratios for various early endpoints (e,g, pCR, RCB Class 0 or1) are computed for each experimental/control comparison. Novel early endpoints, including a binary subtype-specific RCB cutoffs, were developed on a training subset, with 5 comparisons held out for validation. The weighted trial-level correlation (R) was calculated overall and in HRHER2 subgroups. Subtype specific RCB cutoffs were chosen to optimize the overall and within subgroup weighted correlation, while maintaining C indices > .70 for individual survival association. More sophisticated models with RCB index as a continuous variable that incorporated baseline T stage, nodal status and response predictive subtypes were also explored. Results: Overall weighted correlations (R) for PCR, RCB 0/1 and Subtype Specific RCB cutoffs are shown in Table 1. Optimal cutoffs of RCB of 1.1 for HER2+, 1.7 for HR-HER2-, and 2.6 for HR+HER2- were identified giving a correlation of .70 overall in the training set and .54 in training and validation combined. The C index (C) was greater than 0.8 for all endpoints indicating good individual association. Preliminary results using more sophisticated models will be presented. Conclusions: Subtype-specific RCB cutoffs show a modest but clear improvement as an early endpoint over RCB-0/1 and pCR while maintaining a strong patient-level association with survival. External validation efforts are underway. Integrative approaches using continuous RCB improve trial level correlation and will be presented. Trial-level weighted correlation and individual C indices across all 3 subtypes and within each HRHER2 subtype separately. Measure OverallR,C HER2+R,C HR+HER2-R,C HR-HER2-R,C PCR .23, .84 .31, .80 .13, .84 .18, .87 RCB 0-1 .31, .83 .37, .80 .38, .85 .16, .85 Subtype Specific Cutpoint .54, .80 .37, .77 .40, .75 .54, .87 R = correlation; C = C index.

592 Background: Historically, young women with early-stage breast cancer (EBC) have had worse outcomes than older women. This is based largely on population-based or adjuvant studies, with limited neoadjuvant data. Methods: Patients (pts) with Stage 2-3 EBC treated with neoadjuvant chemotherapy +/- immunotherapy (Cx-IT) on the I-SPY2 trial between 2010-2022 were included (data cutoff 07/2025). Pts were stratified by age at trial screening (<40 vs ≥40 yrs), and clinical/molecular subtypes, including ImPrint, an immune signature predictive of Cx-IT response. Pathologic complete response (pCR), residual cancer burden (RCB), and event-free survival (EFS) were evaluated by age and disease subtypes using Kaplan-Meier methods and Cox proportional hazard ratios (HR). Results: Among 2117 evaluable pts, 510 (24%) were <40 yrs and 1607 (76%) were ≥ 40 yrs. Overall, 43% of tumors were HR+/HER2-, 35% HR-/HER2- (TN), 22% HER2+. 34% / 66% were ImPrint +/- (27% / 73% for HR+; 44% / 56% for HR-), with similar rates of ImPrint+/- between age groups. Rates of pCR (31% vs 35 %) and RCB 0/1 (48% vs 51%) were similar between pts <40 and ≥40. Overall, pts < 40 had worse EFS (p < 0.01) vs pts ≥ 40; however, in pts who achieved pCR, there was no difference in EFS in all biologic subgroups (HR 0.90, p=0.77). In contrast, in pts with residual disease, those <40 had worse EFS than pts ≥40 (HR 0.72, p<0.01). The worse EFS in pts<40 with residual disease was limited to pts with ImPrint- disease, in both HR+/HER2- (HR 0.64, p=0.02) and TN (HR 0.62, p=0.04) subtypes, where response rates were lower. In ImPrint+ pts, no differences in EFS by age were observed (HR 1.01, p=0.98). Similar results were seen for pts with HR+/HER2- disease when stratifying by RCB (RCB 0/I vs 2/3). There was no difference in EFS by age for pts with RCB 0/1 disease in any subtype (HR 0.66, p=0.09). For ImPrint-, pts<40 with RCB 2/3 disease had worse EFS than pts ≥40 (HR 0.61, p=0.01). For HR+/HER2-, 64% of tumors were MammaPrint (MP) High 1 vs 36% MP High 2, and 74% were Luminal. When stratifying by pCR or RCB 0/1, similar age-related EFS differences were observed in HR+/HER2- Luminal or MP High 1 tumors, as in ImPrint- tumors. Conclusions: Overall, young pts <40 yrs with high-risk EBC in the I-SPY2 trial had worse 5-yr EFS than pts ≥40; however, when taking response into account, there were no age-related EFS differences for pts who had an excellent response to neoadjuvant Cx-IT (i.e. pCR or RCB 1). The EFS difference was driven by pts with residual disease, specifically in the subset with ImPrint- disease, which is associated with less response to Cx-IT. Whereas, for pts with ImPrint+ disease, there was no difference in EFS by age, regardless of pCR or RCB. This highlights the unmet need for more effective therapies for immune-negative tumors. Our results reinforce the importance and prognostic value of neoadjuvant therapy, particularly in our youngest pts, to better risk-stratify and optimize treatment based on tumor biology and response to therapy. Clinical trial information: NCT01042379 .

Importance Although adding immune checkpoint inhibitors to neoadjuvant chemotherapy improves outcomes in high-risk early-stage breast cancer, opportunities remain to further enhance response. Dual checkpoint blockade offers a potential strategy to further enhance efficacy. Objective To evaluate the combination of anti−programmed cell death 1 protein (PD-1) cemiplimab and anti−lymphocyte activation gene 3 (LAG-3) added to neoadjuvant therapy in ERBB2 -negative early-stage, high-risk breast cancer. Design, Setting, and Participants The I-SPY2 (Investigation of Serial Studies to Predict Your Therapeutic Response With Imaging and Molecular Analysis 2) is an ongoing randomized clinical platform trial being conducted at multiple US clinical sites including patients with early-stage (II or III) ERBB2 -negative, high-risk breast cancer. Participants, continuously enrolled since 2010, were adaptively randomized from February 2, 2020, to December 9, 2021, to one of several experimental neoadjuvant therapies or control groups based on receptor subtypes defined by hormone receptor (HR), ERBB2 status, and MammaPrint (Agendia Inc) molecular risk, categorized as high (MP1) or ultrahigh (MP2). Data were analyzed from January 1, 2022, to August 5, 2025. Interventions Both groups received weekly paclitaxel for 12 weeks, then doxorubicin and cyclophosphamide followed by surgery; concomitant with paclitaxel, the intervention group also received 4 doses of cemiplimab and fianlimab (PCF) every 3 weeks. Main Outcomes and Measures Pathologic complete response (pCR). Treatments graduated when they achieved 85% bayesian probability of success in a subtype-specific phase 3 trial. Pathway-specific biomarkers were assessed for response prediction. Results A total of 78 participants (mean [SD] age, 47 [39-54] years) were randomized to the intervention group, with 350 participants (mean [SD] age, 48 [39-57] years) randomized to the historical control population. PCF graduated in all clinical signatures, with pCR rates vs control of 44% (95% CI, 34%-53%) vs 21% (95% CI, 17%-25%) in all ERBB2 , 53% (95% CI, 39%-67%) vs 29% (95% CI, 22%-36%) in triple-negative, and 36% (95% CI, 23%-49%) vs 14% (95% CI, 9%-19%) in HR-positive and ERBB2 -negative disease. Among the total participants, 16 (21%) experienced adrenal insufficiency, including hypophysitis (11% grade 3 or 4), mostly occurring after immunotherapy completion. PCF was found to be highly effective in the subset of patients with immune signature positive status (ImPrint positive). Conclusions and Relevance In this randomized clinical trial, the combination of PD-1 and anti−LAG-3 inhibition with standard NAC was effective in early-stage ERBB2 -negative breast cancer, particularly in patients displaying a positive ImPrint immune signature. These results warrant further definitive trials. Trial Registration ClinicalTrials.gov Identifier: NCT01042379

Background:Circulating tumor DNA (ctDNA) is an emerging biomarker of treatment response and recurrence risk, while residual cancer burden (RCB) after neoadjuvant treatment (NAT) is a well-established risk factor for distant recurrence. Here, we examined the association between high ctDNA concentration at diagnosis and risk of distant recurrence after neoadjuvant treatment (NAT), in the context of RCB. Methods:The study included 712 patients with high-risk breast cancer in the neoadjuvant I-SPY2 trial. Tumor- informed ctDNA test results at diagnosis were used to stratify patients into ctDNA-negative and ctDNA-positive groups. For this analysis, the ctDNA-positive group was divided into tertiles (low, intermediate, high) based on ctDNA concentration reported as mean tumor molecules per mL [MTM/mL] of plasma. Correlations between MTM/mL at diagnosis and ctDNA dynamics during NAT, residual cancer burden (RCB), and distant recurrence-free survival (DRFS) were examined across all subtypes. Results:In all subtypes, high ctDNA concentration at diagnosis was associated with worse DRFS, whereas low ctDNA concentration or ctDNA-negative status was associated with improved DRFS, even with high tumor burden after NAT (RCB-II/RCB-III). We also found that patients with high ctDNA concentration, regardless of subtype, were less likely to experience early ctDNA clearance; however, those who did had a significantly higher likelihood of achieving a favorable response (RCB-0/RCB-I) than those with late or no ctDNA clearance. Furthermore, across all subtypes, patients with early ctDNA clearance, including those with substantial residual cancer (RCB-II/RCB-III) after NAT, had improved DRFS, irrespective of the ctDNA concentration at diagnosis. Conclusions:Across all subtypes, pathologic response and ctDNA clearance reduce the risk of distant recurrence associated with high ctDNA concentration at diagnosis. ctDNA concentration at diagnosis and ctDNA clearance dynamics during NAT may facilitate the prediction of treatment response and further stratify the risk of metastatic recurrence in non-responders. Trial Registration: NCT01042379.

Abstract Introduction: Pathological complete response (pCR) is a strong prognostic marker, but survival outcomes comparing treatment to control do not reliably align with treatment-control differences in pCR rates in breast cancer. A novel Bayesian hierarchical framework models treatments within trials, allowing us to predict treatment effects on distant recurrence-free survival (DRFS) from pCR with greater accuracy. Methods: We analyzed 12 neoadjuvant breast cancer trials (6,000 patients, all HR/HER2 subtypes; including I-SPY2). The framework of Burzykowski, Molenberghs & Buyse (2005) is extended to a novel arm-based hierarchical structure providing a distribution of pCR and DRFS treatment effects controlling for subtype (HR/HER2), N and T stage, grade, and calendar year. Three held-out trials (877 patients; 26 regimens; med follow-up >4 years) validate predictions of DRFS treatment benefit from pCR. All data was used to estimate treatment-effect correlation and the surrogate threshold effect (STE). Analyses were repeated for Residual Cancer Burden Index (binary RCB01, continuous RCB). Results: Predicted probability of DRFS benefit closely matched actual DRFS follow-up (mean absolute error 0.06; Pearson r = 0.9). Across all trials, pCR showed moderate surrogacy (ρ = 0.82, R2 = 0.67). A 60% increase in pCR odds achieves ≥95% probability of DRFS benefit (STE = OR of 1.60). RCB outperforms pCR across all metrics, with 92% sensitivity and 93% specificity for detecting DRFS benefit in 26 validation regimens (Table 1). Conclusion: This novel Bayesian meta-analytic framework reveals that pCR, RCB01 and continuous RCB reliably predict survival benefit across heterogeneous trials, with RCB performing the best in an external validation. This provides a statistical foundation for accelerated approval decisions using robust early biomarkers in modern neoadjuvant trial designs by accurately predicting survival at the treatment arm level. Citation Format: Keli S. Santos-Parker, Jessica R. Santos-Parker, W. Fraser Symmans, Laura J. Esserman, Christina Yau, Angie DeMichele, Laura van't Veer, Doug Yee, Fabien Reyal, Helena Earl, Jean Abraham, David Cameron, Peter Hall, Judy Boughey, Matthew Goetz, Gabe Sonke, Miguel Martín, Sara López-Tarruella, Priyanka Sharma, Rachel Freiberg, Jane Perlmutter, Aditya Bardia, Martin Eklund, Rachel Freiberg, Lajos Pusztai. A novel statistical framework for surrogate endpoint prediction of survival in neoadjuvant breast cancer trials [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 1401.

Importance:Although adding immune checkpoint inhibitors to neoadjuvant chemotherapy improves outcomes in high-risk early-stage breast cancer, opportunities remain to further enhance response. Dual checkpoint blockade offers a potential strategy to further enhance efficacy. Objective:To evaluate the combination of anti-programmed cell death 1 protein (PD-1) cemiplimab and anti-lymphocyte activation gene 3 (LAG-3) added to neoadjuvant therapy in ERBB2-negative early-stage, high-risk breast cancer. Design, Setting, and Participants:The I-SPY2 (Investigation of Serial Studies to Predict Your Therapeutic Response With Imaging and Molecular Analysis 2) is an ongoing randomized clinical platform trial being conducted at multiple US clinical sites including patients with early-stage (II or III) ERBB2-negative, high-risk breast cancer. Participants, continuously enrolled since 2010, were adaptively randomized from February 2, 2020, to December 9, 2021, to one of several experimental neoadjuvant therapies or control groups based on receptor subtypes defined by hormone receptor (HR), ERBB2 status, and MammaPrint (Agendia Inc) molecular risk, categorized as high (MP1) or ultrahigh (MP2). Data were analyzed from January 1, 2022, to August 5, 2025. Interventions:Both groups received weekly paclitaxel for 12 weeks, then doxorubicin and cyclophosphamide followed by surgery; concomitant with paclitaxel, the intervention group also received 4 doses of cemiplimab and fianlimab (PCF) every 3 weeks. Main Outcomes and Measures:Pathologic complete response (pCR). Treatments graduated when they achieved 85% bayesian probability of success in a subtype-specific phase 3 trial. Pathway-specific biomarkers were assessed for response prediction. Results:A total of 78 participants (mean [SD] age, 47 [39-54] years) were randomized to the intervention group, with 350 participants (mean [SD] age, 48 [39-57] years) randomized to the historical control population. PCF graduated in all clinical signatures, with pCR rates vs control of 44% (95% CI, 34%-53%) vs 21% (95% CI, 17%-25%) in all ERBB2, 53% (95% CI, 39%-67%) vs 29% (95% CI, 22%-36%) in triple-negative, and 36% (95% CI, 23%-49%) vs 14% (95% CI, 9%-19%) in HR-positive and ERBB2-negative disease. Among the total participants, 16 (21%) experienced adrenal insufficiency, including hypophysitis (11% grade 3 or 4), mostly occurring after immunotherapy completion. PCF was found to be highly effective in the subset of patients with immune signature positive status (ImPrint positive). Conclusions and Relevance:In this randomized clinical trial, the combination of PD-1 and anti-LAG-3 inhibition with standard NAC was effective in early-stage ERBB2-negative breast cancer, particularly in patients displaying a positive ImPrint immune signature. These results warrant further definitive trials. Trial Registration:ClinicalTrials.gov Identifier: NCT01042379.

LBA514 Background: Preclinical and clinical data show that antibody-drug conjugates (ADCs) such as T-DXd may synergize with immunotherapy. Rilvegostomig (R) is a monovalent, Fc-reduced, dual checkpoint bispecific IgG1 monoclonal antibody against PD-1 and TIGIT receptors. R + T-DXd was evaluated as neoadjuvant therapy for up to 4 cycles. Methods: I-SPY2.2 is a phase II neoadjuvant trial with three neoadjuvant sequences (Blocks A/B/C). All HER2-negative subtypes were eligible for R + T-DXd in Block A. Predicted responders at the end of Block A or B could undergo surgery; otherwise, they continued to Block B +/- C. Block B pts received paclitaxel +/- carboplatin +/- pembrolizumab and Block C pts doxorubicin + cyclophosphamide (AC) +/- pembrolizumab, HR+ immune+ received IO in Blocks B/C. Results are analyzed by receptor and response predictive subtypes (RPS), that include expression-based immune, and luminal signatures with hormone receptor (HR) and HER2 status. Some pts were randomized to a control regimen (skipping R + T-DXd in block A and starting in block B). To minimize risk of interstitial lung disease (ILD) that could lead to treatment discontinuation, pts underwent pulmonary function tests and high-resolution chest CTs every 6 weeks. Abnormalities were reviewed in real time. The primary endpoint was pathologic complete response (pCR), within each RPS and HR/HER2 signature, with estimated pCR (EpCR) rates compared to subtype-specific block A goal rates and control pts in those groups. Other endpoints included safety and treatment discontinuation. Results: 105 pts were treated with R + T-DXd in block A, while 31 pts were randomized to the control regimen. Block A EpCR rate was 57% in the 12 HR+Immune+ pts vs a pre-specified goal of 15% (posterior probability, PP > .99). In 35 HR-Immune+ pts EpCR was 52% vs a pre-specified goal of 40% (PP = .87). No other groups (Immune-) had a PP > .85 for their respective pCR goal rate. HER2-IHC was 0 (27.6%), 1+ (49.5%) and 2+ (22.9%) in the treatment arm vs. 37.7%, 37.7%, and 24.7% in the control arm, with no difference in distribution between arms in immune subtypes. The most common adverse events in Block A were fatigue (84.8%), nausea (81.0%) and alopecia (58.1%). ILD was the most common adverse event of special interest in Block A and overall, occurring in 12 pts (11.4%; 7G1, 4G2, 1G3) during Block A and in 3 pts (G1, 2.9%) in other Blocks. 8 pts discontinued T-DXd + R due to ILD and proceeded to the next Block. Conclusions: When considering the full treatment strategy including block B and C regimens, experimental strategies had similar EpCR rates vs. controls in immune+ subtypes but block A rates exceeded pre-defined goal thresholds. This infers that treatment beginning with R + T-DXd has equivalent efficacy to standard of care for immune+ pts, but 64% pts with pCR skip chemotherapy (Block B) and 97% avoid AC (Block C). R + T-DXd participants avoided persistent ILD. Clinical trial information: NCT01042379 .

618 Background: In neoadjuvant cancer trials, early endpoints that predict treatment effect on survival identify promising agents early and support accelerated regulatory approval. However, binary endpoints like pathologic complete response inadequately characterize the full distribution of residual disease in breast cancer, while promising continuous biomarkers like MRI derived functional tumor volume (FTV) are associated with survival outcomes but not widely collected. Using a Bayesian hierarchical model, long-term treatment effects on distant recurrence free survival (DRFS) can be predicted from continuous MRI-derived functional tumor volume (FTV) in the I-SPY 2 platform trial. Methods: I-SPY 2 treated 2117 patients from 2010-2022 (12 weeks of paclitaxel ± experimental agent followed by 4 cycles of doxorubicin + cyclophosphamide), and 1,859 underwent dynamic contrast-enhanced MRI at baseline and after 12 weeks of neoadjuvant therapy. MRI-derived functional tumor volume provided volumetric quantification of dynamic tissue enhancement. ΔFTV was defined as the ratio of 12-week to baseline FTV. A Bayesian joint hierarchical model (brms) fit treatment effects on ΔFTV and DRFS for each treatment regimen by HR/HER2 subtype, controlling for clinical nodal status, clinical T stage, grade, and calendar year. Arms with < 8 subjects are excluded. Performance was assessed using cross-validation, predicting DRFS treatment effects in one held out fold at a time from the learned ΔFTV-DRFS association in the remaining data, then comparing predicted to actual DRFS treatment effect. Sensitivity analyses on priors will be presented. Results: Across 1753 patients and 45 treatment–subtype combinations, the estimated treatment effects on ΔFTV and DRFS were highly correlated (posterior correlation -0.91; 95% CrI -1.00 to -0.23). Predicted DRFS treatment effect from ΔFTV demonstrated strong concordance with actual DRFS treatment effects (Pearson r = 0.94 in TNBC; 0.97 HER2+; 0.80 HR+HER2-). The top 5 treatment-subtype regimens ranked by predicted and actual DRFS were identical. 20 regimens predicted to have > 70% probability of DRFS benefit over subtype specific controls showed DRFS improvement, yielding 100% specificity and 69% sensitivity at this decision threshold. Conclusions: We demonstrate internally validated prediction of neoadjuvant treatment effect on DRFS from MRI-derived change in functional tumor volume in the I-SPY 2 trial of high-risk early breast cancer. This suggests continuous imaging measures capture a range of response to therapy while Bayesian approaches can be effective for predicting treatment effects. This encourages collecting MRI biomarkers in trials to facilitate validation as early endpoints supporting decisions in screening platform trials as well as regulatory accelerated approval.

PURPOSE:Neoadjuvant immunotherapy (IO) has become the standard of care for early-stage triple-negative breast cancer (TNBC), but not yet for other subtypes. We previously developed a clinical-grade mRNA-based immune classifier (ImPrint) predicting response to IO that is now being used in I-SPY2.2 as part of the response predictive subtypes. We report the performance of ImPrint in hormone receptor-positive and human epidermal growth factor receptor 2-negative (HR+HER2-) patients from five IO arms. METHODS:A total of 204 HR+HER2- (MammaPrint high-risk) patients from five IO arms (anti-PD-1, anti-PD-L1/poly [ADP-ribose] polymerase inhibitor combination, anti-PD-1/toll-like receptor 9 dual-IO combination, and anti-PD-1 ± lymphocyte activation gene 3 dual-IO combination) and 191 patients from the chemotherapy-only control arm were included in this analysis. Patients were classified as ImPrint+ (likely sensitive) versus ImPrint- (likely resistant), using pretreatment mRNA. Performance of ImPrint for predicting pathologic complete response (pCR) to IO-containing arms was characterized and compared with tumor grade (III), MammaPrint (ultra) High2 risk (MP2), and estrogen receptor (ER)-low (ER ≤ 10%). RESULTS:Overall, the pCR rate across the five IO arms was 33%. 26% of HR+HER2- patients were ImPrint+, and pCR rates with IO were 75% in ImPrint+ versus 17% in ImPrint-, with the highest pCR rate >90% in a dual-IO arm. In the control arm, pCR rates were 33% in ImPrint+ and 8% in ImPrint-. Tumor grade (III), MP2, and ER-low showed pCR rates in IO of 45%, 56%, and 63%, respectively, with lower pCR odds ratios (OR < 7.5) compared with ImPrint (OR = 14.5). CONCLUSION:Using an accurate selection strategy, HR+HER2- patients could achieve pCR rates similar to what is seen with best neoadjuvant therapies in TNBC and HER2+ (ie, pCR rate >65%-70%). ImPrint, an Food and Drug Administration IDE-enabled assay, may represent a way to identify HR+HER2- patients for IO that best balances likely benefit versus risk of serious immune-related adverse events.

Neoadjuvant chemotherapy (NAC) may allow de-escalation of axillary surgery; yet treatment disparities persist. We aimed to assess race-based disparities in use of axillary lymph node surgery (ALND) among patients who achieve a nodal response in the context of a large, multicenter NAC trial. We conducted a retrospective analysis of the I-SPY 2 trial. All patients received NAC, but type of surgery was not mandated. Multivariable logistic regression was used to predict odds ratio (OR) of undergoing ALND by race while adjusting for clinical and demographic confounders, including age, region, tumor receptor subtype, clinical and pathologic node status (cN and ypN +/−, respectively), and clinical and pathologic tumor size (cT and ypT, respectively). Among 1394 patients, 79.4

Symptoms are often underdetected during cancer treatment. To determine if symptom monitoring with electronic patient-reported outcomes (PROs) improves clinical outcomes, we conducted a cluster-randomized trial in which 52 oncology practices were assigned to PRO or usual care. At PRO practices, patients with metastatic cancer were invited to complete weekly symptom surveys. Severe or worsening symptoms generated alerts to the care team. The primary outcome was overall survival, and secondary outcomes included emergency visits, time to deterioration of physical function, symptoms, health-related quality of life (HRQL) and patient satisfaction with PRO. Among 1,191 enrolled patients, there was no difference in survival (hazard ratio (HR) 0.99 (95% confidence interval (CI), 0.83-1.17); P = 0.86). Time to first emergency visit was significantly prolonged with PRO compared to usual care (HR 0.84 ((95% CI, 0.71-0.98); P = 0.03), with a 6.1% reduction in the cumulative incidence of emergency visits and fewer mean visits at 12 months with PRO (1.02 versus 1.30; P < 0.001). Benefits also significantly favored PRO for delayed deterioration of physical function (median 12.6 versus 8.5 months, HR 0.73; P = 0.002), symptoms (12.7 versus 9.9, HR 0.69; P < 0.001) and HRQL (15.6 versus 12.2, HR 0.72; P = 0.001), which remained significant when considering deaths in analyses. Most patients felt that PRO improved discussions with the care team (77.0% (188/244)), made them feel more in control of their care (84.0% (205/244)) and would recommend it to other patients (91.4% (223/244)). Patients completed 91.5% (20,565/22,486) of expected weekly symptom surveys. These findings demonstrate that symptom monitoring with PRO meaningfully improves clinical outcomes, the patient experience and utilization of services and should be included as a standard part of quality cancer clinical care. Future studies of PRO in clinical care should focus on these outcomes rather than mortality as primary endpoints. ClinicalTrials.gov registration: NCT03249090

Neoadjuvant chemotherapy (NAC) is widely used to treat high-risk breast cancer. However, the optimal time to surgery (TTS) following NAC remains undefined. This study investigates the impact of TTS on oncologic outcomes using the I-SPY 2 Trial cohort. A retrospective analysis of 1877 patients with breast cancer enrolled in the I-SPY 2 Trial was performed. Patients were grouped by TTS post-NAC: 1–4 weeks, 5 weeks, 6–8 weeks, and 9 + weeks. Baseline demographic, clinical, imaging, and treatment response data were collected. Event-free survival (EFS) and local recurrence-free interval (LRFI) were evaluated using Kaplan–Meier analyses and Cox models. Subgroup analyses were performed by tumor receptor subtypes (hormone receptor [HR]+ human epidermal growth factor receptor 2 [HER2]−, HER2 +, and triple-negative breast cancer [TNBC]) and residual cancer burden (RCB) class. Among 1877 patients, 526 (28.0

Treatment options for early-stage aggressive breast cancer are improving, particularly with the introduction of neoadjuvant PARP inhibitors, immune oncology agents (IO), and dual HER2 targeting. In the neoadjuvant trial I-SPY2 we previously established a schema of molecular ‘Response Predictive Subtypes’ (RPS), where for 4 out of 6 subtypes optimal tumor biology-treatment matching is predicted to result in 60-90% pathologic complete response (pCR), associated with long-term survival benefit (PMID:35623341), which also includes the prospectively validated immune-sensitive subtype (PMID:39277672). However, one third of 2300 I-SPY2 patients treated 2010-2024 across 28 treatments had very low pathologic complete response (pCR) rates in all arms (∼10% on average). These patients have HER2-negative tumors with low expression of immune genes and low DNA repair deficiency (DRD) (HER2-/Immune-/DRD- subtype). Here, we characterized the tumor biology of I-SPY patients with non-responding subtypes to identify novel targets of vulnerability, which we tested for efficacy in patient-derived organoids. We analyzed pre-treatment gene expression (GE) and phospho-protein (RPPA) data of 1628 HER2-patients across 20 treatment arms in I-SPY2 for canonical metabolic, signaling, and regulatory pathways from the Molecular Signatures Database (n=2274). We compared 1) HR+HER2-/Immune-/DRD- (RPS1;n=617) vs other (Immune+ and/or DRD+) HR+HER2- tumors (n=272) and 2) TN/Immune-/DRD- (RPS2;n=177) vs other (Immune+ and/or DRD+) TN tumors (n=562) using Wilcoxon rank-sum tests with Benjamini-Hochberg adjustment, ‘adjusted p-values’ <0.05 were considered significant. Organoids derived from HR+HER2- and triple-negative (TN) breast cancers grown in 3D cultures were used to perform drug screens with inhibitors of candidate targets from the Omics analyses. GE and RPPA data from the RPS HER2-/Immune-/DRD- subtypes RPS1(HR+) and RPS2(TN) tumors infer that they are characterized by a dormant-like, low proliferation state with significantly altered epigenetic modulator activity, WNT-signaling, and various metabolic pathways. Concomitantly, there is an observed increase of potential targets such as CyclinD1, Androgen Receptor, PI3K/AKT/mTOR, HDAC5, bcl2, and low HER2, potentially mediated in part by RUNX3. Our organoid studies on a set of chemo-resistant TN organoids show that pro-apoptotic agents, including bcl-2 inhibitors and IAP inhibitors, as well as HDAC inhibitors, can overcome resistance to cisplatin, as assessed by both chemical cell viability assays and imaging-based assays. The dormant-like state of HER2-/Immune-/DRD- non-responding subtypes (RPS1/RPS2) combined with our organoid treatment studies suggests that senolytics and HDAC inhibitors may be potential therapeutic strategies. The I-SPY2 neoadjuvant trial provides the opportunity to test these new concepts in the upfront treatment block where new therapies are tested without standard chemotherapy. Laura van t Veer, Tam Binh V. Bui, Denise Wolf, Emanuel Petricoin, Julia Wulfkuhle, Jean-Philippe Coppé, Renée van Amerongen, Nadine Goldhammer, Annuska Glas, Douglas Yee, Jane Perlmutter, Rosalyn Sayaman, Christina Yau, Lajos Pusztai, Paula Pohlmann, Laura Esserman, Jennifer Rosenbluth. Emerging Novel Targets to treat non-responding HER2-negative breast cancer patients: The neoadjuvant I-SPY2 TRIAL 2300 patient Omics data and organoid studies [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr C051.

Clinical trials of cancer immunotherapy (IO) were historically based on a drug development paradigm built for chemotherapies. The remarkable clinical activity of programmed cell death protein 1/programmed death ligand 1 blockade, chimeric antigen receptor-T cells, and T cell engagers yielded new insights into how the mechanistic underpinnings of IO are reflected in the clinic. These insights and the sheer number of novel immunotherapies currently in the pipeline have made it clear that our strategies and tools for IO drug development must adapt. Recent innovations like engineered T cells and tumor-infiltrating lymphocytes demonstrate that immune-based treatments may rely on real-time manufacturing programs rather than off-the-shelf drugs. We now recognize adoptively transferred cells as living drugs. Progression criteria have been redefined due to the unique response patterns of IO. Harnessing the power of both biomarkers and the neoadjuvant setting earlier in drug development is of broad interest. The US Food and Drug Association is increasingly impacting the design of trials with respect to dose optimization and clinical endpoints. The use of novel endpoints such as pathologic complete/major response, treatment-free survival, and minimal residual disease is becoming more common. There is growing acceptance of using patient-reported outcomes as trial endpoints to better measure the true clinical benefit and impact of novel IO agents on quality of life. New opportunities created by modern data science and artificial intelligence to inform and accelerate drug development continue to emerge. The importance of streamlining the clinical research ecosystem and enhancing clinical trial access to facilitate the enrollment of diverse patient populations is broadly recognized. Patient advocacy is critical both to drive the science of IO, and to promote patient satisfaction. To capitalize on these opportunities, the Society for Immunotherapy of Cancer (SITC) has established a goal of at least 100 new, unique IO approvals over the next 10 years. Accordingly, SITC has developed initiatives designed to integrate the viewpoints of diverse stakeholders and galvanize the field in further adapting clinical trials to the unique features of IO, moving us closer to our ultimate goal of using IO to cure and prevent cancer.

Invasive lobular carcinoma (ILC) has lower response rates to neoadjuvant chemotherapy (NAC) than invasive ductal carcinoma. While ILC often has low-risk biology, there is a high-risk subset within this heterogeneous tumor type. We compared surgical treatment and response rates by histology in I-SPY2, a multicenter NAC trial. We evaluated 1329 patients with stage II–III breast cancer and high-risk 70-gene assay. Patients with classic, pleomorphic, or mixed lobular/ductal histology were included in the lobular cohort. We evaluated rates of mastectomy, positive margins, axillary dissection, and conversion from clinical node-positive (cN+) to pathologic node-negative (ypN−) status after NAC. Overall, 124 patients (9.3