Citations

505 Background: SWOG S1007 was a phase 3 randomized trial to evaluate the benefit of endocrine therapy alone (ET) or with chemotherapy (CET) in pts with HR+/HER2- invasive breast cancer with recurrence score (RS) ≤ 25. There was no invasive disease-free survival (IDFS) benefit for CET compared to ET in postmenopausal women However, in premenopausal women CET had an IDFS improvement compared to ET (HR=0.60; 95% 0.43-0.83). The objective was to further refine patient subgroups who benefit from CET, we measured baseline hormone levels in S1007 pts to quantify their menstrual status by serum hormone levels and associated clinical outcomes. Methods: Serum estradiol (E), progesterone (P), follicular stimulating hormone (FSH), luteinizing hormone (LH), anti-Müllerian hormone (AMH), and inhibin B (IB) were assessed on baseline serum samples from 1,015 pts who self-reported being premenopausal and age < 55. Magnetic fluorescent bead-based immunoassays ( i.e., Luminex assays) or colorimetric ELISAs were performed at the Biomarker Discovery Laboratory at the University of Kansas Medical Center to measure hormone levels. Associations of the markers (both continuous and dichotomized) with IDFS and distant relapse-free survival (DRFS) were tested for prognosis and prediction of CET benefit using Cox regression. Results: Single baseline measurement of IB, P, E, LH, FSH, or combination of E/LH/FSH, was not prognostic for IDFS or predictive for chemotherapy benefit. However, AMH levels (≥10 pg/mL) showed significant interaction with chemotherapy benefit (p=0.019), and risk increased with continuous AMH with ET alone, but not CET (p=0.03). Compared to ET alone, CET was beneficial in the 79% of premenopausal patients with AMH levels ≥10 pg/mL, a standard cutoff to define normal ovarian reserve (HR=0.48; 95% 0.33-0.69, adjusting for RS). CET was not beneficial in the 21% with AMH < 10 (HR=1.21; 95% 0.60-2.43). Absolute 5-year IDFS benefit of CET for RS ≤ 25 was 7.8% (95.2%-87.4%) for AMH ≥ 10 vs. -1.7% (89.1%-90.8%) for AMH < 10. DRFS showed a similar pattern with CET benefit in AMH ≥10 pg/mL (5-year DRFS benefit: 4.4%; HR=0.41; 95% 0.25-0.68, adjusting for RS) and no benefit in AMH < 10 (5-year DRFS benefit: -0.9%, HR=1.50; 95% 0.62-3.63). Conclusions: Among self-reported premenopausal patients < age 55, pts with baseline serum AMH levels ≥10 pg/mL showed significant benefit from CET compared to patients with AMH levels <10 pg/mL. AMH levels were a better indicator of CET benefit than self-reported menopause status, age, or E/LH/FSH levels. Use of AMH-based selection of patients may inform who most benefits from the addition of adjuvant chemotherapy to ET. Clinical trial information: NCT01272037 .

ImportanceLittle is known about regional nodal irradiation (RNI) practice patterns or rates of locoregional recurrence (LRR) with and without RNI in patients with limited nodal disease and favorable biology treated with modern surgical and systemic therapy, including approaches that de-escalate those latter treatments.ObjectiveTo investigate how often patients with low–recurrence score breast cancer with 1 to 3 nodes involved receive RNI, incidence and predictors of LRR, and associations between locoregional therapy and disease-free survival.Design, Setting, and ParticipantsIn this secondary analysis of the SWOG S1007 trial, patients with hormone receptor-positive, ERBB2-negative breast cancer, and a Oncotype DX 21-gene Breast Recurrence Score assay result of no more than 25, were randomized to endocrine therapy alone vs chemotherapy then endocrine therapy. Prospectively collected radiotherapy information was collected from 4871 patients treated in diverse settings. Data were analyzed June 2022 to April 2023.ExposureReceipt of RNI (targeting at least the supraclavicular region).Main Outcome(s) and Measure(s)Cumulative incidence of LRR was calculated by locoregional treatment received. Analyses were assessed for associations between invasive disease-free survival (IDFS) and locoregional therapy, adjusted for menopausal status, treatment group, recurrence score, tumor size, nodes involved, and axillary surgery. Radiotherapy information was recorded in the first year after randomization, so survival analyses were landmarked as starting at 1 year among those still at risk.ResultsOf 4871 female patients (median [range] age, 57 [18-87] years) with radiotherapy forms, 3947 (81.0%) reported radiotherapy receipt. Of 3852 patients who received radiotherapy and had complete information on targets, 2274 (59.0%) received RNI. With a median follow-up of 6.1 years, the cumulative incidence of LRR by 5 years was 0.85% among patients who received breast-conserving surgery and radiotherapy with RNI; 0.55% after breast-conserving surgery with radiotherapy without RNI; 0.11% after mastectomy with postmastectomy radiotherapy; and 1.7% after mastectomy without radiotherapy. Similarly low LRR was observed within the group assigned to endocrine therapy without chemotherapy. The rate of IDFS did not differ by RNI receipt (premenopausal: hazard ratio [HR], 1.03; 95% CI, 0.74-1.43; P = .87; postmenopausal: HR, 0.85; 95% CI, 0.68-1.07; P = .16).Conclusions and RelevanceIn this secondary analysis of a clinical trial, RNI use was divided in the setting of biologically favorable N1 disease, and rates of LRR were low even in patients who did not receive RNI. Disease-free survival was not associated with RNI receipt; omission of chemotherapy among patients similar to those enrolled in the S1007 trial is not an independent indication for use of RNI.

LBA520 Background: I-SPY 2 is a multicenter trial using response-adaptive randomization within biomarker subtypes including MammaPrint (MP) risk to evaluate novel neoadjuvant agents in high-risk breast cancer. Oral paclitaxel and encequidar (OPE) is an oral combination of paclitaxel (P) with a p-glycoprotein pump inhibitor, encequidar, to enhance gastrointestinal (GI) absorption. Dostarlimab (D) is an anti-PD-1 monoclonal antibody. Methods: Women with tumors ≥ 2.5cm and MP high-risk cancers were screened and treated starting Oct 5, 2020. Treatment included Oral Paclitaxel 205mg/m2 and encequidar 12.9 mg on days 1-3, Carboplatin (Cb) AUC 1.5 on day 1 weekly x 12, and Dostarlimab (D) 500 mg every 3 weeks x 4, followed by doxorubicin/ cyclophosphamide (AC) every 2-3 weeks x 4. The control arm was IV P weekly x 12 followed by AC every 2-3 weeks x 4. For patients with HER2+ disease, weekly Trastuzumab (T) was administered during the first 12 weeks. The arm was eligible for graduation [> 85% chance of success in a 300-person phase 3 neoadjuvant trial with a pathologic complete response (pCR) endpoint] in 10 predefined signatures. Results: 106 patients (44 HR+HER2-, 56 HR-HER2- (TN), 6 HER2+) received OPE/Cb/D ± T. The control arm included 388 historical controls (201 HR+HER2-, 156 TN, 31 HER2+). 22 patients (20 HR+, 2 HR-) in OPE/Cb/D were MP1 (MP high) and 84 patients (29 HR+, 55 HR-) were MP2 (MP ultra-high). OPE/Cb/D graduated in the TN signature and accrual was stopped. Conclusions: OPE/Cb/D graduated in the TN signature with a higher predicted pCR rate compared to control. OPE and D have been, individually, shown to have efficacy in other settings. The lower-than-expected pCR rate and lower irAEs with triplet therapy (taxane, platinum, immune checkpoint inhibitor [ICI]) in TN compared to prior IV chemo + ICI arms on I-SPY 2 suggests a possible interference of OPE with ICI. Changes in gut microbiome is being investigated as an explanation. Clinical trial information: NCT01042379 . [Table: see text]

Abstract Background: I-SPY2 is a multicenter, phase 2 trial using response-adaptive randomization within biomarker subtypes defined by hormone-receptor (HR), HER2, and MammaPrint (MP) status to evaluate novel agents as neoadjuvant therapy for high-risk breast cancer. The primary endpoint is pathologic complete response (pCR). Cemiplimab (Cemi) is a PD-1 inhibitor approved for the treatment of NSCLC, cutaneous basal, and squamous cell cancer. Here, we report current efficacy rates of Cemi in combination with paclitaxel followed by AC. Methods: Women with tumors ≥ 2.5cm were eligible for screening. Only HER2 negative (HER2-) patients were eligible for this treatment; HR positive (HR+) patients had to be MP high risk. Treatment included paclitaxel 80 mg/m2 IV weekly x 12 and Cemi 350 mg IV given q3weeks x 4, followed by doxorubicin/cyclophosphamide (AC) every 2 weeks x 4. The control arm was weekly paclitaxel x 12 followed by AC every 2-3 weeks x 4. All patients undergo serial MRI imaging; and imaging response (at 3 weeks, 12 weeks and prior to surgery) were used along with accumulating pCR data to continuously update and estimate pCR rates for trial arms. Analysis was modified intent to treat. Patients who switched to non-protocol therapy count as non-pCR. The goal is to identify (graduate) regimens with ≥85% Bayesian predictive probability of success (i.e. demonstrating superiority to control) in a future 300-patient phase 3 neoadjuvant trial with a pCR endpoint within responsive signatures. Cemi was eligible to graduate in 3 pre-defined signatures: HER2-, HR-HER2-, and HR+HER2-. To adapt to changing standard of care, we constructed “dynamic controls” comprising ‘best’ alternative therapies using I-SPY 2 and external data and estimated the probability of Cemi being superior to the dynamic control. Results: 60 HER2- patients (28 HR+ and 32 HR-) received Cemi arm treatment. The control group included 357 patients with HER2- tumors (201 HR+ and 156 HR-) enrolled since March 2010. Cemi graduated in HR-/HER2- signature. Estimated pCR rates (as of June 2022) are summarized in the table. Immune-related endocrine disorders include: hypothyroid (14.5%), adrenal insufficiency (10%), hyperthyroid (4.8%),) and thyroiditis (3.2%). Only one grade 3 adrenal insufficiency was observed. All immune related AE’s were manageable. Additional biomarker analyses are ongoing and will be presented at the meeting. Response predictive subtypes (Immune+ vs Immune-) and additional predictive biomarkers were assessed. Associations with pCR will be presented at SABCS. Conclusion: The I-SPY 2 study aims to assess the probability that investigational regimens will be successful in a phase 3 neoadjuvant trial. Anti-PD-1 therapy with Cemi resulted in a higher predicted pCR rate in HR-/HER2- 55 rate% disease compared to control at 29%. Immune-mediated AE’s were observed. This data is consistent with previously published data using check point inhibitors in early-stage HR-/HER2- breast cancer. Estimated pCR rates Citation Format: Erica Stringer-Reasor, Rebecca A. Shatsky, Jo Chien, Anne Wallace, Judy C. Boughey, Kathy S. Albain, Hyo S. Han, Rita Nanda, Claudine Isaacs, Kevin Kalinsky, Zahi Mitri, Amy S. Clark, Christos Vaklavas, Alexandra Thomas, Meghna S. Trivedi, Janice Lu, Smita Asare, Ruixiao Lu, Maria Pitsouni, Amy Wilson, Jane Perlmutter, Hope Rugo, Richard Schwab, W. Fraser Symmans, Nola M. Hylton, Laura Van ’t Veer, Douglas Yee, Angela DeMichele, Donald Berry, Laura J. Esserman, I-SPY Investigators. PD11-01 Evaluation of the PD-1 Inhibitor Cemiplimab in early-stage, high-risk HER2-negative breast cancer: Results from the neoadjuvant I-SPY 2 TRIAL [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 PD11-01.

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: I-SPY2 is a multicenter, phase 2 trial using response-adaptive randomization within biomarker subtypes defined by hormone-receptor (HR), HER2, and MammaPrint (MP) status to evaluate novel agents as neoadjuvant therapy for high-risk breast cancer. The primary endpoint is pathologic complete response (pCR). Cemiplimab is an anti-PD-1 inhibitor approved for the treatment of NSCLC and cutaneous basal and squamous cell CA. Lymphocyte activation gene 3 (LAG-3) binds MHC class II leading to inhibition of T-cell proliferation and activation and is often co-expressed with PD-1. REGN3767 is a fully humanized mAb that binds to LAG-3 and blocks inhibitory T-cell signaling. Concurrent blockade of LAG-3 with an anti-PD-1 may enhance efficacy of an anti-PD-1. Methods: Women with tumors ≥ 2.5cm were eligible for screening. Only HER2 negative (HER2-) patients were eligible for this treatment; HR positive (HR+) patients had to be MP high risk. Treatment included Paclitaxel 80 mg/m2 IV weekly x 12 and Cemiplimab 350 mg and REGN3767 1600 mg both given q3weeks x 4, followed by doxorubicin/cyclophosphamide (AC) every 2 weeks x 4. The control arm was weekly paclitaxel x 12 followed by AC every 2-3 weeks x 4. Cemiplimab/REGN3767 was eligible to graduate in 3 of 10 pre-defined signatures: HER2-, HR-HER2-, and HR+HER2-. The statistical methods for evaluating I-SPY 2 agents has been previously described. To adapt to changing standard of care, we constructed “dynamic controls” comprising ‘best’ alternative therapies using I-SPY 2 and external data and estimated the probability of Cemiplimab/REGN3767 being superior to the dynamic control. Response predictive subtypes (Immune+ vs Immune-) were assessed using pre-treatment gene expression data and the ImPrint signature. Results: 73 HER2- patients (40 HR+ and 33 HR-) received Cemiplimab/REGN3767 treatment. The control group included [357 patients with HER2- tumors (201 HR+ and 156 HR-) enrolled since March 2010. Cemiplimab/REGN3767 graduated in both HR-/HER2- and HR+/HER2- groups; estimated pCR rates (as of June 2022) are summarized in the table. Safety events of note for Cemiplimab/REGN3767 include hypothyroidism 30.8%, adrenal insufficiency (AI) 19.2%, hyperthyroidism 14.1%, pneumonitis 1.3%, and hepatitis 3.8%. All were G1/2 except for 6 (7.7%) G3 AI and 3 (3.8%) G3 colitis. Rash occurred in 62.8%, 9% G3 and 2 pts (2.6%) had pulmonary embolism. X% of adrenal insufficiency cases required replacement therapy. 40 patients (11 HR+ and 29 HR-) in Cemiplimab/REGN3767 were predicted Immune+; 32 (29 HR+ and 3 HR-) were predicted Immune-. In the HR+ group pCR was achieved in 10/11 (91%) patients with Immune+ subtype compared with 8/29 (28%) with Immune- subtype. Additional biomarker analyses are ongoing and will be presented at the meeting. Conclusion: The I-SPY 2 study aims to assess the probability that investigational regimens will be successful in a phase 3 neoadjuvant trial. Dual immune blockade with a LAG-3 inhibitor and anti-PD1 therapy resulted in a high predicted pCR rate both in HR-/HER2- (60%) and HR+/HER2- (37%) disease. The novel Imprint signature identified a group of HR+ patients most likely to benefit from this active regimen. Table 1: Estimated pCR rates Citation Format: Claudine Isaacs, Rita Nanda, Jo Chien, Meghna S. Trivedi, Erica Stringer-Reasor, Christos Vaklavas, Judy C. Boughey, Amy Sanford, Anne Wallace, Amy S. Clark, Alexandra Thomas, Kathy S. Albain, Laura C. Kennedy, Tara B. Sanft, Kevin Kalinsky, Hyo S. Han, Nicole Williams, Mili Arora, Anthony Elias, Carla Falkson, Smita Asare, Ruixiao Lu, Maria Pitsouni, Amy Wilson, Jane Perlmutter, Hope Rugo, Richard Schwab, W. Fraser Symmans, Nola M. Hylton, Laura Van’t Veer, Douglas Yee, Angela DeMichele, Donald Berry, Laura J. Esserman, I-SPY Investigators. Evaluation of anti-PD-1 Cemiplimab plus anti-LAG-3 REGN3767 in early-stage, high-risk HER2-negative breast cancer: Results from the neoadjuvant I-SPY 2 TRIAL [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 GS5-03.

Abstract Background: We previouslyreported that invasive disease-free survival (IDFS), the primary outcome, and distantdisease-free survival (DDFS), a secondary outcome, differed by menopausalstatus in a pre-specified analysis. Here,we report updates on IDFS and DDFS with additional follow-up, as well as distantrecurrence-free interval (DRFI) and post hoc analyses in premenopausal women.Methods: Eligibilitycriteria included women &gt; 18 years of age with HR+, HER2-BC and 1-3 +LN and no contraindications to taxane and/or anthracycline basedCT. DRFI was defined as time to distant recurrence or death from breast cancer.Analyses were intent-to-treat among eligible pts. We performed a post hocanalysis evaluating IDFS between treatment arms in premenopausal pts with pN1mi.In addition, we conducted a two-year landmarked IDFS analysis by ovarianfunction suppression (OFS) or not in the ET arm, as well as by regularmenstrual periods or not in both treatment arms.Results: Among the 4,984eligible pts, there were 553 IDFS events and median follow-up of 6.1 years.Postmenopausal women do not have any IDFS or DDFS benefit with CT; however, 5-yearabsolute benefits for IDFS and DDFS with CT for premenopausal pts were 5.9% and3.3%, respectively. In pre-menopausal pts, CT was associated with improved DRFIfor all RS values &lt; 25, with an absolute improvement of 2.3% for RS0-13 and of 2.8% for RS 14-25. Among premenopausal pts, 12.4% (n=206) had pNmi)disease. In a post hoc analysis, there was a trend for CT benefit for thosewith pNmi [hazard ratio (HR)=0.44, confidence interval (CI)=0.18-1.08]. Therewere only 22 IDFS events. Only 17.2% of premenopausal pts assigned to ETunderwent OFS in the first 24 months and in two-year landmarked analysis, therewas no IDFS difference in those who underwent OFS or not (HR=0.88,CI=0.47-1.63). In premenopausal women assigned to ET, 58.9% stopped havingperiods within the first 24 months, and have anumerically improved IDFS compared to those who continued regular periods (HR=1.48,CI: 0.92-2.40). Among premenopausal assigned to CT followed by ET, 80.8%stopped having periods within the first 24 months and have a numericallyimproved IDFS compared to those who continue to have regular periods (HR=1.56, CI:0.85-2.86).Discussion: In accordancewith the differential IDFS and DDFS benefit based upon menopausal status inS1007, premenopausal pts with 1-3+LN and RS &lt; 25 had a statisticallysignificant improvement in DRFI with the addition of CT. A small proportion of S1007premenopausal participants underwent OFS. The role of OFS as it relates to CTbenefit cannot be determined from this study. A future randomized trial should address the clinical question if OFScan replace CT in premenopausal pts with HR+/HER2-, node-positive breast cancer.Funding: Supported by National Cancer Institute grants U10CA180888, U10CA180819,U10CA180820, U10CA180821, U10CA180868, U10CA180863, and in part by Susan G.Komen for the Cure® Research Program, The Hope Foundation for Cancer Research,Breast Cancer Research Foundation, and Genomic Health, Inc. Citation Format: Kevin M Kalinsky, William E Barlow, Julie R Gralow, Funda Meric-Bernstam, Kathy S Albain, Daniel F Hayes, Nancy U Lin, Edith A Perez, Lori A Goldstein, Stephen KL Chia, Sukhbinder Dhesy-Thind, Priya Rastogi, Emilio Alba, Suzette Delaloge, Miguel Martin, Catherine M Kelly, Manuel Ruiz-Borrego, Miguel Gil-Gil, Claudia H Arce-Salinas, Etienne GC Brain, Eun Sook Lee, Jean-Yves Pierga, Begoña Bermejo, Manuel Ramos-Vasquez, Kyung Hae Jung, Jean-Marc Ferrero, Anne Schott, Steven Shak, Priyanka Sharma, Danika Lew, Jieling Miao, Debasish Tripathy, Lajos Pusztai, Gabriel Hortobagyi. Updated results from a phase 3 randomized clinical trial in participants (pts) with 1-3 positive lymph nodes (LN), hormone receptor-positive (HR+) and HER2-negative (HER2-) breast cancer (BC) with recurrence score (RS) ≤ 25 randomized to endocrine therapy (ET) +/- chemotherapy (CT): SWOG S1007 (RxPONDER) [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 GS2-07.

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.

508 Background: I-SPY 2 is a multicenter, phase 2 trial using response-adaptive randomization within molecular subtypes defined by receptor status and MammaPrint (MP) risk to evaluate novel agents as neoadjuvant therapy for women with high-risk breast cancer. SD-101 is an investigational Toll-like receptor 9 (TLR9) agonist CpG-C class oligodeoxynucleotide that stimulates the production of IFN-α and interleukin (IL)-12, functional maturation of plasmacytoid dendritic cells, and production of cytotoxic antibodies. IT SD-101 was combined with systemic anti-PD-1 antibody Pb to investigate the antitumor and immunologic activity of this novel immunotherapeutic strategy. Methods: Women with tumors ≥ 2.5cm were eligible for screening. Only pts (pts) with HER2- disease were eligible for this treatment. Treatment included weekly P x 12 in combination with IT SD-101 2 mg/ml (1 ml for T2 tumors, 2 ml for >T3 tumors) weekly x 4, then q3 weeks x 2, and IV Pb q3 weeks x 4, followed by doxorubicin/cyclophosphamide (AC) q2-3 weeks x 4 (SD-101+Pembro 4). Pts in the control arm received weekly P x 12 followed by AC q2-3 weeks x 4. The I-SPY 2 methods have been previously published. This investigational arm was eligible for graduation (>85% chance of success in a 300-person phase 3 neoadjuvant trial) in 3 of 10 pre-defined signatures: HER2-, hormone receptor (HR)+/HER2- and HR-/HER2-. Results: 75 pts were randomized and evaluable in SD-101+Pembro 4 treatment arm. The control arm included 329 historical controls enrolled since April 2010. The study arm was stopped due to maximal patient accrual. Pt characteristics were balanced; 56% HR+, 44% HR-. The probability that SD-101+Pembro4 was superior to control exceeded 97% for all eligible tumor signatures, but did not reach the threshold for graduation in any of the signatures. However, it is notable that the rate of pCR/Residual Cancer Burden 1 (RCB1) in the HR+/HER2- signature was 51%. Preliminary safety events for SD-101+Pembro 4 include increased rates of fever, neutropenia, febrile neutropenia, transaminitis, and immune-related events, including adrenal insufficiency. Conclusions: The SD-101+Pembro 4 regimen was active but did not meet the pre-specified threshold for graduation in I-SPY 2. pCR/RCB 1 analysis suggests improved response in the HR+/HER-negative signature compared to control. The clinical significance of these findings needs to be weighed against the potential risk of immune-related toxicities. Clinical trial information: NCT01042379. [Table: see text]