Hirst, Gillian L

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.

612 Background: EOP, an I-SPY2 sub-study, evaluates the tolerability and activity of novel endocrine strategies in stage 2/3 breast cancer (BC) patients (pts) predicted to have lower chemotherapy benefit. Laso, a selective estrogen receptor modulator (SERM), has shown favorable toxicity profile and activity in HR+/HER2- endocrine-resistant metastatic BC. Methods: Pts with Stage 2/3 HR+/HER2-, MammaPrint (MP) low risk BC were enrolled. Pts with MP High1 BC were included if clinically node-negative. Pts received oral laso 5 mg daily for six 28-day cycles. Laso was continued until the day prior to surgery. Premenopausal pts received ovarian function suppression (OFS) starting C2D1. The primary endpoint was feasibility (>75% of patients completing >75% study therapy). Baseline (T0), 3-wk (T1) biopsies, and the surgical specimen (T3) was assessed centrally for Ki-67. Breast MRI functional tumor volume (FTV) was performed at T0, T1, 12 weeks (T2), and pre-operatively (T3). Blood was collected for tumor informed ctDNA at T0, T1, T2, T3. Advance event (AE) was assessed using CTCAE V5. Results: From 3/2023 to 5/2024, 20 pts were enrolled. Median age 50.5 years, 50% premenopausal, and 1 male pt. 60% cN0, 80% MP low-risk signature. 18 (90%) pts completed >75% study therapy. Two pts discontinued treatment due to pt preference. Median Ki67 at T0 was 14.7%. At T1, 87.5% of pts remained or suppressed Ki67 to <10% and 37.5% suppressed to <2.7%. Ki67 at T1 was similar between pre-and postmenopausal pts despite OFS (Table). The median MRI FTV was 8.4 cc at T0, and 3.4 cc at T3. Median % FTV reduction from T0 to T3 was -47.5%. 2/20 pts (10%) achieved a modified PEPI score of 0. No patients achieved completed pathological response. Of the 16 pts with RCB results, 2 (12.5%) RCB-1, 6 (37.5%) RCB-2, 8 (50%) RCB-3 disease. 14 pts had ctDNA available at T0. 4/14 were ctDNA+ at T0, 2 of whom became ctDNA negative, and 2 remained ctDNA+. 10/14 pts were ctDNA negative at T0, 8 of whom remained negative, 2 became positive at T1 then cleared. All AEs were grade(G) 1 except 1 pt with G2 hot flashes. Most common AEs include hot flashes (85%), constipation (50%), fatigue (50%), and nausea (35%). One pt had G3 hypersensitivity and hypertension, both unrelated to therapy. Conclusions: Neoadjuvant laso demonstrates a favorable AE profile and promising anti-tumor activity in suppressing 3-wk Ki67 and MRI FTV change in pts with HR+ HER2-negative early BC. Ki67 suppression in premenopausal pts was seen in the absence of OFS. Clinical trial information: NCT01042379 . Ki67 expression at pre-treatment, and 3-wk time point. All Patients(n=20) Premenopausal (n=10) Postmenopausal (n=9) Median Ki67 expression Baseline 10.0% 12.5% 10.0% 3-wk 5.1% 3.0% 6.0% Number of pts with Ki67 expression <10% at 3-wk 87.5% 1 87.5% 85.7% Number of pts with Ki67 expression <2.7% at 3-wk 37.5% 50% 28.6% 1 Include the one male pt.

504 Background: Axillary surgery in breast cancer is used for staging and therapeutic purposes, but axillary lymph node dissection (ALND) confers high risk of complications including lymphedema. Accordingly, trials have focused on right-sizing axillary surgery, with options ranging from complete omission of surgery, sentinel lymph node (SLN) surgery, targeted axillary dissection (TAD), or ALND. Identifying a biomarker to reliably predict nodal burden would facilitate accurate selection of surgical management. We evaluated whether the presence or absence of ctDNA in the blood pre and post NAC is associated with residual nodal burden. Methods: I-SPY2 is a prospective, multicenter NAC trial for patients with clinical stage II-III high-risk breast cancer. Patients are randomized to novel NAC agents, with pathologic complete response being the primary endpoint. As part of the trial, serial ctDNA is assessed with a highly sensitive tumor-informed assay using up to 16 patient-specific tumor mutations (Signatera) at baseline, 3 weeks, 12 weeks, and post-NAC. We determined whether ctDNA positivity or negativity post-NAC, and the change in ctDNA status baseline/post-NAC (-/-, -/+, +/+, +/-) are associated with ypN category (N0, N1, N2). Results: ctDNA status was available post-NAC in 495 patients and change in ctDNA status from baseline was available in 493. At baseline, ctDNA was detected in 160/220 (72.3%) cN0 patients and 227/273 (83.2%) cN+ patients (p=0.006). Post-NAC, ctDNA was detected in 11/220 (5%) cN0 patients and 34/273 (12.5%) cN+ patients (p=0.004). While baseline ctDNA status was not associated with ypN category, there was a significant association between post-NAC ctDNA status and ypN category. For ctDNA + patients post-NAC, 33.3% were ypN0, 31.1% were ypN1, and 35.6% were ypN2 at surgery; in contrast, for ctDNA - patients post-NAC, 67.1% were ypN0, 23.1% were ypN1, and 9.8% were ypN2 (p<0.0001). Dynamic ctDNA changes were also associated with ypN category, with significantly more ypN0 patients among ctDNA -/- or ctDNA +/- cases, and more ypN2 patients in those who did not clear ctDNA (+/+) (p=0.0001, Table). Conclusions: To our knowledge this is the first study to demonstrate a significant relationship between ctDNA and ypN category, which has important surgical implications. CtDNA negativity was associated with low likelihood of ypN2 disease, making these patients excellent candidates for SLN surgery or TAD. Ongoing analyses will incorporate receptor subtype and timing of clearance of ctDNA. CtDNA in breast cancer may help tailor surgical management of the axilla, potentially reducing patient morbidity without compromising prognostic information. ctDNA baseline/post-NAC ypN0 (n=317) ypN1 (n=116) ypN2 (n=60) ctDNA -/- (n=105) 72.4% 19.1% 8.6% ctDNA -/+ (n=1) 0.0% 100.0% 0.0% ctDNA +/+ (n=44) 34.1% 29.6% 36.4% ctDNA +/- (n=343) 65.9% 23.9% 10.2%

3008 Background: Numerous studies have demonstrated the prognostic value of ctDNA analysis after neoadjuvant chemotherapy for early-stage high-risk breast cancer. Few studies have characterized ctDNA in pts receiving NET for early-stage hormone receptor positive (HR+)/HER2- BC that is predicted to benefit less from chemotherapy. Methods: Cell-free DNA (cfDNA) was isolated from 432 plasma samples from 108 pts enrolled in the I-SPY2 EOP trial. Pts had Stage 2/3 HR+/HER2-, MammaPrint low or high risk 1 BC. Pts were randomized to one of 7 neoadjuvant-based treatment arms including arms containing AI, Z-endoxifen, Lasofoxifene, vepdegestrant (ARV-471), and Abemaciclib. Pts were treated for 6 months prior to surgery. Blood was collected at baseline (T0), 3 weeks (T1), 12 weeks (T2), and 6 months (T3). A personalized ctDNA test (Signatera) was designed to detect up to 16 patient-specific mutations (from whole-exome sequencing of pretreatment tumor) in cfDNA by ultra-deep sequencing. The chi-square test was used to assess associations between categorical variables, and the Wilcoxon rank-sum test was used to evaluate differences in medians. Results: ctDNA information was available for 101 patients at baseline (T0) (Table 1). At T0, 36 (35.6%) patients were ctDNA-positive. 23/36 (63.9%) became ctDNA-negative and 13/36 (36.1%) remained ctDNA-positive. At T0, 65/101 patients (64.4%) were ctDNA-negative. Of these, 57 (87.7%) remained negative, while 8 (12.3%) became ctDNA-positive at T1 before reverting to ctDNA-negative. A higher percentage of ctDNA-positive patients at T0 were cN+ compared to ctDNA-negative pts (p = 0.036, 64% vs. 40%). Additionally, ctDNA-positivity at T0 was strongly associated with higher Ki67 (p = 0.03) and larger functional tumor volume by MRI at baseline (p = 0.03). T3/T4 and high-grade tumors at baseline were also associated with having ctDNA positivity at baseline, though this was not statistically significant (p = 0.34 and 0.058, respectively). Conclusions: In this study of pts with Stage 2/3 HR+ HER2- BC with largely MammaPrint low risk signatures, over one-third of pts had detectable ctDNA at baseline. Detectable ctDNA at baseline was associated with cN+ disease, larger FTV, and high baseline Ki67. The majority of pts with positive ctDNA at baseline cleared the ctDNA on NET. Clinical trial information: NCT01042379 . Clinicopathologic characteristics of EOP patients. All Patients(N=101) Age at screening 55 (27-80) years* Clinical N Stage Node+ (cN+) 49 (48.5%) Node- (cN-) 52 (51.5%) SET status High 84 (83.2%) Low 14 (13.9%) Missing 3 (2.97%) MammaPrint (MP) risk** High risk 1 (H1) 15 (14.9%) Low risk 86 (85.1%) *Mean(min-max). **H1: MP score between 0 and -0.57; low: MP score between 0 and 0.355.

Abstract Background: Neoadjuvant immunotherapy (IO) has become standard of care for early-stage triple negative breast cancer (TNBC). I-SPY2 was the first randomized trial to examine the efficacy of IO therapy in high-risk HR+HER2- breast cancer, where most IO arms showed improved efficacy relative to control. We also previously showed immune-gene expression signatures associate with pathologic complete response (pCR) in HR+HER2- treated with IO; and developed a clinically applicable Immune classifier (ImPrint) predicting response to IO that is now being used in I-SPY2.2 as part of the Response Predictive Subtypes. Here we report the performance of ImPrint in HR+HER2- patients from 5 IO arms. Methods: 204 HR+HER2- (MammaPrint high risk) patients from 5 IO arms (anti-PD1, anti-PDL1/PARPi, anti-PD1/TLR9 dual-IO, and anti-PD1 +/- LAG3 dual-IO) and 191 patients from the control arm were included in this analysis. Patients were classified ImPrint+ (likely sensitive) vs. ImPrint- (likely resistant), by Agendia, Inc, using pre-treatment mRNA. Performance of ImPrint for predicting pCR to IO was characterized and compared to that of MammaPrint (ultra) High2 risk (MP2) and tumor grade (III). Based on the DRFS of pCR vs. non-pCR HR+HER2-/ImPrint+ patients (median follow up ~4.5yr), we predicted the DRFS in the IO vs. control arms using an exponential model as a function of the pCR rates. Results: Overall, the pCR rate over the 5 IO arms was 33%. 28% of HR+HER2- patients were ImPrint+; and pCR rates with IO were 76% in ImPrint+ vs. 16% in ImPrint-, with the highest pCR rate &gt;90% in a dual-IO arm. In the control arm, pCR rates were 33% in ImPrint+ and 8% in ImPrint-. Based on the pCR rates in the IO vs. control (76% vs. 33%), the predicted DRFS of HR+HER2-/ImPrint+ patients are 91% vs 80% at 5 years, respectively. This translates to a risk reduction of 52% for the IO arm relative to standard chemotherapy in HR+HER2-/ImPrint+ patients. We also compared MP2 and tumor grade (III) to ImPrint+ as pCR predictors, as these markers have been used or proposed as selection markers for neoadjuvant IO trials in HR+HER2-. The pCR rates for MP2, Grade III, at 56%, 45%, respectively, are much smaller than that for ImPrint+ at 75% pCR; thus ImPrint appears to be a more precise predictive biomarker for neoadjuvant IO therapy in the HR+HER2- breast cancers. Conclusion: These results suggest that a subset of high risk HR+HER2- breast cancers is highly sensitive to immunotherapy, and by using a specific and sensitive selection strategy patients could achieve pCR rates similar to what is seen with best neoadjuvant therapies in TNBC and HER2+ (i.e., pCR rate &gt; 65-70%). ImPrint, an FDA IDE-enabled assay currently being further evaluated in I-SPY2, may represent the way to identify patients for IO that best balances likely benefit vs risk of serious immune-related adverse events. Citation Format: Denise M. Wolf, Christina Yau, Lajos Pusztai, Rita Nanda, Jo Chien, Erica Stringer-Reasor, Rebecca Shatsky, Claudine Isaacs, MInetta Liu, Hyo Han, Hatem Soliman, Michael Campbell, Annuska Glas, Andrei Barcaru, Lorenza Mittempergher, Midas Kuilman, Lamorna Brown Swigart, Gillian Hirst, Hongmei Yu, Philip Beineke, Amrita Basu, I-SPY2 Investigators, Douglas Yee, W. Fraser Symmans, Angela DeMichele, Jane Perlmutter, Amy Delson, Laura Huppert, Hope Rugo, Amy Clark, Nola Hylton, Paula Pohlmann, Laura Esserman, Laura van 't Veer. Immune subtyping identifies a subset of HR+HER2- early-stage breast cancer patients with a very high likelihood of response to neoadjuvant immunotherapy (IO): Results from 5 IO arms of the I-SPY2 TRIAL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1207.

ImportanceThere has been little consideration of genomic risk of recurrence by breast cancer subtype despite evidence of racial disparities in breast cancer outcomes.ObjectiveTo evaluate associations between clinical trial end points, namely pathologic complete response (pCR) and distant recurrence–free survival (DRFS), and race and examine whether gene expression signatures are associated with outcomes by race.Design, Setting, and ParticipantsThis retrospective cohort study used data from the Investigation of Serial Studies to Predict Your Therapeutic Response With Imaging and Molecular Analysis 2 (I-SPY 2) multicenter clinical trial of neoadjuvant chemotherapy with novel agents and combinations for patients with previously untreated stage II/III breast cancer. Analyses were conducted of associations between race and short- and long-term outcomes, overall and by receptor subtypes, and their association with 28 expression biomarkers. The trial enrolled 990 female patients between March 30, 2010, and November 5, 2016, with a primary tumor size of 2.5 cm or greater and clinical or molecular high risk based on MammaPrint or hormone receptor (HR)-negative/ERBB2 (formerly HER2 or HER2/neu)–positive subtyping across 9 arms. This data analysis was performed between June 10, 2021, and October 20, 2022.ExposureRace, tumor receptor subtypes, and genomic biomarker expression of early breast cancer.Main Outcomes and MeasuresThe primary outcomes were pCR and DRFS assessed by race, overall, and by tumor subtype using logistic regression and Cox proportional hazards regression models. The interaction between 28 expression biomarkers and race, considering pCR and DRFS overall and within subtypes, was also evaluated.ResultsThe analytic sample included 974 participants (excluding 16 self-reporting as American Indian or Alaska Native, Native Hawaiian or Other Pacific Islander, or multiple races due to small sample sizes), including 68 Asian (7%), 120 Black (12%), and 786 White (81%) patients. Median (range) age at diagnosis was 47 (25-71) years for Asian, 49 (25-77) for Black, and 49 (23-73) years for White patients. The pCR rates were 32% (n = 22) for Asian, 30% for Black (n = 36), and 32% for White (n = 255) patients (P = .87). Black patients with HR-positive/ERBB2-negative tumors not achieving pCR had significantly worse DRFS than their White counterparts (hazard ratio, 2.28; 95% CI, 1.24-4.21; P = .01), with 5-year DRFS rates of 55% (n = 32) and 77% (n = 247), respectively. Black patients with HR-positive/ERBB2-negative tumors, compared with White patients, had higher expression of an interferon signature (mean [SD], 0.39 [0.87] and −0.10 [0.99]; P = .007) and, compared with Asian patients, had a higher mitotic score (mean [SD], 0.07 [1.08] and −0.69 [1.06]; P = .01) and lower estrogen receptor/progesterone receptor signature (mean [SD], 0.31 [0.90] and 1.08 [0.95]; P = .008). A transforming growth factor β signature had a significant association with race relative to pCR and DRFS, with a higher signature associated with lower pCR and worse DRFS outcomes among Black patients only.Conclusions and RelevanceThe findings show that women with early high-risk breast cancer who achieve pCR have similarly good outcomes regardless of race, but Black women with HR-positive/ERBB2-negative tumors without pCR may have worse DRFS than White women, highlighting the need to develop and test novel biomarker-informed therapies in diverse populations.

Abstract Purpose: The neutralizing peptibody trebananib prevents angiopoietin-1 and angiopoietin-2 from binding with Tie2 receptors, inhibiting angiogenesis and proliferation. Trebananib was combined with paclitaxel±trastuzumab in the I-SPY2 breast cancer trial. Patients and Methods: I-SPY2, a phase II neoadjuvant trial, adaptively randomizes patients with high-risk, early-stage breast cancer to one of several experimental therapies or control based on receptor subtypes as defined by hormone receptor (HR) and HER2 status and MammaPrint risk (MP1, MP2). The primary endpoint is pathologic complete response (pCR). A therapy “graduates” if/when it achieves 85% Bayesian probability of success in a phase III trial within a given subtype. Patients received weekly paclitaxel (plus trastuzumab if HER2-positive) without (control) or with weekly intravenous trebananib, followed by doxorubicin/cyclophosphamide and surgery. Pathway-specific biomarkers were assessed for response prediction. Results: There were 134 participants randomized to trebananib and 133 to control. Although trebananib did not graduate in any signature [phase III probabilities: Hazard ratio (HR)-negative (78%), HR-negative/HER2-positive (74%), HR-negative/HER2-negative (77%), and MP2 (79%)], it demonstrated high probability of superior pCR rates over control (92%–99%) among these subtypes. Trebananib improved 3-year event-free survival (HR 0.67), with no significant increase in adverse events. Activation levels of the Tie2 receptor and downstream signaling partners predicted trebananib response in HER2-positive disease; high expression of a CD8 T-cell gene signature predicted response in HR-negative/HER2-negative disease. Conclusions: The angiopoietin (Ang)/Tie2 axis inhibitor trebananib combined with standard neoadjuvant therapy increased estimated pCR rates across HR-negative and MP2 subtypes, with probabilities of superiority &gt;90%. Further study of Ang/Tie2 receptor axis inhibitors in validated, biomarker-predicted sensitive subtypes is warranted.

Abstract Background: Treatment of estrogen receptor (ER)-positive breast cancer with selective estrogen receptor degraders (SERDs) frequently results in the loss or reduction of ER expression. Whether these changes are due to on-target effects of SERDs degrading ER or arise as a mechanism of tumor resistance with associated changes in cellular phenotypes remains unknown. It is critical to distinguish between these possibilities to accurately assess treatment response and determine the most appropriate subsequent therapy. To this end, we created and conducted molecular analyses on patient-derived organoid cultures from post-treatment tissue in patients receiving neoadjuvant SERD therapy for early-stage ER+ breast cancer in the I-SPY2 Endocrine Optimization Protocol (EOP). Methods: The I-SPY2 EOP study is a prospective, randomized substudy within the I-SPY TRIAL testing the oral SERD amcenestrant alone or in combination with letrozole or abemaciclib in stage 2/3 ER+ Her2-negative breast cancer. Enrollment is ongoing, with patients receiving amcenestrant neoadjuvantly for 6 months until the day before surgery. Tumor tissue is collected at baseline, 3 weeks, and at surgery. Organoids were generated from post-treatment surgical samples. Organoid cultures were optimized based on established methods (Dekkers et al., Nature Protocols, 2021) to assess ER levels and activity. Pre- and post-treatment tissue samples were also assessed for ER, PR, Ki67, and GATA3, a luminal marker and transcription factor that is functionally linked with ER, via immunohistochemistry. Results: In 7 patients with both pre- and post-treatment tissue samples including fresh surgical samples for organoid generation, the ER in baseline tumor tissue was &gt;=90% in all patients, PR ranged from 40-90%, and Ki67 ranged from 5-30%. In post-treatment surgical tissue from these cases, ER ranged from 0-30%, PR from 0-50%, Ki67 from &lt; 1%-10%, and GATA3 was positive in 5 of 5 cases tested to-date. The creation of organoids from residual disease at surgery was attempted for these 7 patients, with organoids successfully propagated in 5 cases thus far. 3 of 5 organoid cultures were ready for analysis and in all cases strong ER and PR expression in organoids was observed after culture for &gt; 1 month in the absence of amcenestrant. Detailed gene expression profiling (including Mammaprint/Blueprint) and gene set enrichment analyses (GSEA) to assess for intrinsic breast cancer subtype and ER activity in each sample and corresponding organoid culture are in progress and will be reported with the full dataset. Conclusion: Patient-derived organoid culturing of residual disease after neoadjuvant endocrine therapy is feasible. Neoadjuvant treatment with a SERD can render ER and PR low or absent at the time of surgical resection, which does not necessarily imply the presence of endocrine therapy resistant disease. The use of organoids and additional IHC markers (GATA3) demonstrate that receptor negativity may be an indicator of the drug hitting its target, suggesting ER signaling is still intact. In general, patient-derived tumor organoid cultures modeling residual disease states can be a useful adjunct to existing methods used to monitor the effects of neoadjuvant endocrine therapy and is being explored in the I-SPY EOP trial. Citation Format: Jennifer Rosenbluth, Christopher J. Schwartz, Tam Binh Bui, Shruti Warhadpande, Pravin Phadatare, Sigal Eini, Michael Bruck, Julissa Molina-Vega, Kami Pullakhandam, Nicole Schindler, Lamorna A. Brown Swigart, Christina Yau, Gillian Hirst, Rita Mukhtar, Karthik V. Giridhar, Olufunmilayo I. Olopade, Kevin Kalinsky, Cheryl A. Ewing, Jasmine M. Wong, Michael D. Alvarado, Laura Van’t Veer, Laura J. Esserman, Jo Chien. Characterization of residual disease after neoadjuvant selective estrogen receptor degrader (SERD) therapy using tumor organoids in the I-SPY Endocrine Optimization Protocol (EOP) [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-09-01.

Abstract Background Transcriptomic immune-related gene signatures have been associated with achievement of pathologic complete response (pCR) and prognosis in the neoadjuvant setting. I-SPY 2 is a multicenter, phase 2 platform trial using response-adaptive randomization within subtypes defined by receptor status (HR/HER2) and MammaPrint (MP) risk to evaluate novel agents as neoadjuvant therapy for women with high-risk breast cancer. Given racial disparities in mortality from breast cancer and the paucity of racial demographic data from clinical trials, we aimed to evaluate the association between racial groups and baseline characteristics, including expression-based subtypes and immune signatures, treatment response, and prognosis of patients enrolled in the I-SPY 2 TRIAL. Methods Our study population included 990 I-SPY 2 patients. 15 patients identified as part of a racial group with &lt;10 patients enrolled in the trial and were excluded from analysis. Pre-treatment expression data was available for 971 patients. Follow-up data was available for 907 patients; median follow-up time of 4.4 yrs. Chi-square test was used to assess associations between racial groups and pre-treatment SBR grade, HR/HER2 defined subtypes, intrinsic subtype (defined by BluePrint 80-gene molecular subtyping) and residual cancer burden (RCB) class. Logistic regression was used to evaluate race association with pCR. Cox proportional hazard modeling was used to assess the association between racial groups and event free survival (EFS) in a univariate setting, adjusting for pCR status. Association between racial groups and 28 expression signatures related to immune, proliferation, ER and HER2 pathway was analyzed using ANOVA with post-hoc Tukey test in the overall population and in each receptor subtype. Results Of 975 patients included in our analysis, 787 (81%) were White, 68 (7%) were Asian, and 120 (12%) were Black or African American. No significant associations between race and pre-treatment SBR grade (p=0.49), HR/HER2 defined subtypes (p=0.09), or expression-based subtypes (p=0.25) were observed. pCR rates do not significantly differ by racial groups (Odds ratio of pCR relative to White: 1.00 for Asian and 0.89 for Black or African American); and no significant differences in RCB class distribution by race was observed (p=0.88). Event free survival was not associated with patient racial group in a univariate Cox model (Hazard ratio relative to White: 1.10, p=0.73 for Asian and 1.37, p=0.13 for Black or African American). Among the 28 expression signatures evaluated, four were differentially expressed among racial groups within the overall population (F-test p&lt;0.05): IFN module, B cell signature, Dendritic cell signature, and Mitotic score. Pairwise comparisons between racial groups with post-hoc Tukey test identified significant differences in IFN module expression between Black or African American vs. White (p=0.019) and Dendritic cell signature expression between Asian vs White (p=0.047). Among patients in the TNBC subtype, three signatures (dendritic cell signature, macrophage signature and ERBB2 module) were differentially expressed between Black or African American and White patients (p=0.002, 0.016 and 0.007). Conclusion Our analysis demonstrates that among women with high risk breast cancer, race does not affect subtype specific response rates nor event free survival. Distribution of subtypes previously shown to be associated with pCR in the I-SPY2 trial did not significantly differ among racial groups indicating race is less likely than tumor biology to predict response. The decreased expression of immune signatures observed in Black or African American women with TNBC suggests possible differential sensitivity to immunotherapy plus combination chemotherapy. Tumor immune multiplex studies are underway to further investigate. Citation Format: Beverly Kyalwazi, Christina Yau, Olufunmilayo Olopade, A. Jo Chien, Anne Wallace, Andres Forero-Torres, Lajos Pusztai, Erin Ellis, Kathy Albain, Anne Blaes, Barbara Haley, Judy Boughey, Anthony Elias, Amy Clark, Claudine Isaacs, Rita Nanda, Hyo Han, Rachel Yung, Debu Tripathy, Kristen Edmiston, Rebecca Viscusi, Donald Northfelt, Qamar Khan, Ashish Sanil, Scott Berry, Smita Asare, Amy Wilson, Gillian Hirst, Nola Hylton, Michelle Melisko, Jane Perlmutter, Hope Rugo, Fraser Symmans, Laura van ‘t Veer, Donald Berry, Laura Esserman. Analysis of clinical outcomes and expression-based immune signatures by race in the I-SPY 2 trial [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr GS4-02.

Abstract Background: There is no clinical equipoise on the best upfront management of patients with early-stage hormone receptor-positive (HR+)/HER2-negative (HER2-) breast cancer (BC) that is high-risk by clinicopathologic criteria, and low-risk based on molecular profiling. These patients are unlikely to respond to chemotherapy. However, these patients still have risk, often risk of late recurrence, despite standard adjuvant endocrine therapy. Novel endocrine-based strategies that are more effective and tolerable than current standard therapies are needed for this population. Next-generation orally-bioavailable selective estrogen receptor degraders (oSERDs) with improved pharmacokinetic (PK) properties are promising potential therapies for HR+ BC. The oSERD amcenestrant has demonstrated a favorable safety profile and encouraging efficacy in a phase I/II dose escalation and expansion trial for heavily pre-treated patients with HR+ metastatic BC and is an attractive agent for assessment in the neoadjuvant BC setting. The neoadjuvant setting offers a unique opportunity to study novel agents and to assess early biological endpoints. However, one of the challenges in studying endocrine-based strategies in the neoadjuvant setting is the lack of a robust surrogate endpoint to reliably predict response and benefit. The I-SPY2 Endocrine Optimization Protocol (EOP) is a pilot sub-study within the main I-SPY2 TRIAL that will test amcenestrant alone or in combination with abemaciclib or letrozole. EOP will test the feasibility of using the I-SPY2 platform to test novel endocrine-based strategies in the neoadjuvant setting in patients with clinical high-risk, molecular low-risk, HR+/HER2- tumors, and will generate a rich database of imaging, molecular, and pathologic correlative endpoints that may potentially inform the improved assessment of response to neoadjuvant endocrine therapy. Trial Design/Eligibility/Accrual: The I-SPY2 EOP is a prospective, randomized, open-label trial specifically for patients with HR+/HER2-negative MammaPrint (MP) low-risk tumors that are at least 2.5 cm in size. Eligible patients are identified during the screening process for the parent I-SPY2 trial. The planned total accrual for the EOP is 120 patients. Patients are randomized 1:1:1 to one of 3 oral treatment arms: 1) amcenestrant 200 mg daily; 2) amcenestrant 200 mg daily + abemaciclib 150 mg bid; 3) amcenestrant 200 mg daily + letrozole 2.5 mg daily. Patients are treated for 6 months prior to surgery. Premenopausal women must receive concomitant monthly ovarian suppression. Serial breast MRIs, breast biopsies, blood, and patient reported outcomes (PROs) are being collected, and patients will be followed for 10 years for recurrence and survival. Serial dedicated breast PET (dbPET) scans and PKs will be assessed in a subset of patients. Objectives/Statistics: The primary objective of the EOP is to investigate the feasibility of enrolling and treating molecularly-selected patients with early stage HR+/HER2- MP low-risk BC in a randomized neoadjuvant trial using an oral-SERD backbone. Treatment will be determined to be feasible if ≥75% of enrolled patients complete ≥75% of assigned study therapy. Secondary objectives include the safety, tolerability, PROs, and PKs related to amcenestrant +/- abemacilcib and letrozole, as well as the assessment of imaging, pathologic, and molecular correlative endpoints as potential biomarkers of response to neoadjuvant endocrine therapy. Status: This study opened in May 2021. Accrual is ongoing. Citation Format: A. Jo Chien, Kevin M Kalinsky, Julissa Molina-Vega, Rita Mukhtar, Karthik Giridhar, Olufunmilayo I Olopade, Amrita Basu, Smita M Asare, Paul Henderson, Gillian Hirst, Ruixiao Lu, Ella Jones, Nola Hylton, Lamorna Brown-Swigart, Laura J van 't Veer, Douglas Yee, Ingrid Mayer, Laura J Esserman. I-SPY2 endocrine optimization protocol (EOP): A pilot neoadjuvant endocrine therapy study with amcenestrant as monotherapy or in combination with abemacicilib or letrozole in molecularly selected HR+/HER2- clinical stage 2/3 breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT1-10-02.