Northfelt, Donald W.

Abstract Endocrine therapies (ET) with cyclin-dependent kinase 4/6 (CDK4/6) inhibition are the standard treatment for estrogen receptor-α-positive (ER+) breast cancer, however drug resistance is common. In this study, proteogenomic analyses of patient-derived xenografts (PDXs) from patients with 22 ER+ breast cancer demonstrated that protein kinase, membrane-associated tyrosine/threonine one (PKMYT1), a WEE1 homolog, is estradiol (E2) regulated in E2-dependent PDXs and constitutively expressed when growth is E2-independent. In clinical samples, high PKMYT1 mRNA levels associated with resistance to both ET and CDK4/6 inhibition. The PKMYT1 inhibitor lunresertib (RP-6306) with gemcitabine selectively and synergistically reduced the viability of ET and palbociclib-resistant ER+ breast cancer cells without functional p53. In vitro the combination increased DNA damage and apoptosis. In palbociclib-resistant, TP53 mutant PDX-derived organoids and PDXs, RP-6306 with low-dose gemcitabine induced greater tumor volume reduction compared to treatment with either single agent. Our study demonstrates the clinical potential of RP-6306 in combination with gemcitabine for ET and CDK4/6 inhibitor resistant TP53 mutant ER+ breast cancer.

Abstract Purpose: We previously reported that postmenopausal women with estrogen receptor-α–positive breast cancer receiving adjuvant anastrozole 1 mg/day (ANA1) with estrone (E1) ≥1.3 pg/mL and estradiol (E2) ≥0.5 pg/mL [inadequate estrogen suppression (IES)] had a threefold increased risk of a breast cancer event. The objective of this study was to determine if increasing anastrozole to 10 mg/day (ANA10) could result in adequate estrogen suppression (AES: E1 <1.3 pg/mL and/or E2 <0.5 pg/mL) among those with IES on ANA1. Patients and Methods: Postmenopausal women with estrogen receptor-α–positive breast cancer planning to receive adjuvant ANA1 were eligible. E1 and E2 were assessed pre- and post-8 to 10 weeks of ANA1. Those with IES were switched to 8- to 10-week cycles of ANA10 followed by letrozole 2.5 mg/day. E1 and E2 were assessed after each cycle. Anastrozole concentrations were measured post-ANA1 and post-ANA10. Primary analyses included patients who documented taking at least 80% of the planned treatment (adherent cohort). Results: In total, 132 (84.6%) of 156 eligible patients were ANA1 adherent. IES occurred in 40 (30.3%) adherent patients. Twenty-five (78.1%) of 32 patients who began ANA10 were adherent, and AES was achieved in 19 (76.0%; 90% confidence interval, 58.1%–89.0%) patients. Anastrozole concentrations post-ANA1 and post-ANA10 did not differ by estrogen suppression status among adherent patients. AES was maintained/attained in 21 (91.3%) of 23 letrozole-adherent patients. Conclusions: Approximately 30% of ANA1-adherent patients had IES. Among those who switched to ANA10 and were adherent, 76% had AES. Further studies are required to validate emerging data that ANA1 results in IES for some patients and to determine the clinical benefit of switching to ANA10 or an alternative aromatase inhibitor.

Abstract BACKGROUND: The amplification of genes PDL-1, PDL-2, and JAK2 on chromosome 9p24, known as the PDJ amplicon, has been identified in 15% of triple negative breast cancer (TNBC) and is associated with tumor infiltrating lymphocytes and elevated JAK2 mRNA, suggesting a coordinated inflammatory response in TNBC. Adding pembrolizumab to chemotherapy in KEYNOTE trials 355 and 522 has validated the use of PD-1 blockade in TNBC and redefined the standard of care. Despite preclinical data suggesting a role for targeting the IL- 6/JAK2/STAT3 pathway, the JAK1/JAK2 inhibitor ruxolitinib has not shown efficacy as a single Agent in breast cancer. This is the first phase I clinical trial evaluating the combination of pembrolizumab and ruxolitinib in the treatment of metastatic TNBC. METHODS: We conducted a phase I clinical trial with accrual from 2018 to 2023 at Mayo Clinic Arizona. Patients (n=12) received 200 mg of IV pembrolizumab per cycle (every 3 weeks) with 3+3 dose escalation of oral ruxolitinib twice a day (dose level 0: 10 mg PO BID, level 1: 15 mg PO BID, level 2: 20 mg PO BID). The primary endpoint was to determine the maximum tolerated dose (MTD) of ruxolitinib and secondary endpoints were treatment safety and efficacy. Clinical response was assessed by computed tomography of the chest, abdomen, and pelvis every 3 cycles. PD-L1 expression in tumor was measured by immunohistochemistry (IHC). The effect of the combined therapies on T and B cell populations were assessed by flow cytometry and antibody profiling of 695 tumor antigens and 8 viral antigens by nucleic acid protein programmable array. RESULTS: Twelve female patients with chemotherapy refractory metastatic TNBC were treated on study. All received 200 mg IV pembrolizumab every cycle. Due to rapid disease progression of three patients early after treatment initiation, eligibility was restricted to prohibit patients with extensive pulmonary disease or elevated LDH. Those patients were excluded from efficacy or correlative analysis. The most common toxicities were transaminitis (n=12), anemia/leukopenia (n=12), and fatigue (n=9). The majority of adverse events were grade 1 and occurred at dose level 2. There were 5 adverse events grade 3 or higher: respiratory or thoracic complications (n=3), hypertension (n=1), and abdominal pain (n=1), but MTD was not established. The median number of cycles was 4 for each dose level (range n=2.0-9.0) and the median time to progression was 3 months (range n=1.4-6.3). No patient had tumor response by RECIST v1.1 criteria and there was no association of disease stability with PD-L1 tumor expression by IHC. Circulating antibodies to tumor antigens remained stable, and viral-specific antibodies mildly decreased by month 2 in all patients at dose levels 0 and 1. Of the 7 patients reactive to TRIM21 autoantibodies, 2 patients had a transient increase at 2 months. CONCLUSIONS: The combination of pembrolizumab with ruxolitinib was well tolerated at all dose levels, and the number but not the severity of adverse events increased with higher ruxolitinib dose. Two of six patients in dose level 0 and 1 had stable disease for six months on treatment, suggesting that the lower dose levels of ruxolitinib (i.e. 10 mg PO BID) may have more activity in combination with pembrolizumab. Citation Format: Anelia Kassi, Jacqueline Carmona, Sarah M. Green, Heidi Kosiorek, Michael T. Barrett, Barbara A. Pockaj, Donald Northfelt, Karen S. Anderson. Phase I trial of combination pembrolizumab and ruxolitinib in metastatic triple negative breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-18-07.

Abstract Background: Residual disease after neoadjuvant chemotherapy (NAC) in breast cancer (BC) confers poor outcomes. Trastuzumab emtansine (T-DM1) is approved for HER2+ patients with residual disease after NAC. However, many patients still develop recurrence. H2NVAC is a multi-epitope CD4 helper T cell activating vaccine that includes a pool of 4 degenerate HER2-derived HLA-DR epitopes admixed with GM-CSF. Our previous phase I trial showed that this vaccine was safe and generated robust, long-lasting T and B cell immune responses. Method: We conducted a safety run-in phase of H2NVAC in combination with T-DM1 in stage II-III HER2+ BC with residual disease after NAC. Patients were treated with H2NVAC and T-DM1 for 6 cycles, followed by 2 boosters at 3 and 12 months. Dose-limiting toxicity (DLT) was defined as any grade (G) ≥ 3 toxicity occurring within 21 days after the first vaccination. Results: 20 patients were enrolled (median age 51; 27-69 years), including 80% White and 15% Black. 90% had hormone receptor-positive disease. There was no DLT observed in the safety run-in cohort. All adverse events (AEs) during the DLT period were grades 1 and 2, with fatigue being the most common (60% G1, 5% G2), followed by peripheral sensory neuropathy (55% G1, 5% G2), and injection site reaction (50% G1). Across all treatment cycles, the 10 most common AEs that were at least possibly related to treatment in all cycles are summarized in the table below. The injection site reaction was the most common AE, with 85% G1 and 15% G2. Conclusion: H2NVAC, in combination with T-DM1, was safe, with a favorable side effect profile. There was no DLT observed. The most common AE was injection site reaction, with the majority being G1. A multicenter, randomized, placebo-controlled, phase II trial of H2NVAC vs. placebo in combination with trastuzumab emtansine is currently ongoing through the ACCRU consortium (NCT04197687). Adverse Events Grades 1 2 N % N % Injection site reaction 17 85.0 3 15.0 Aspartate aminotransferase increase 13 65.0 Alanine aminotransferase increase 9 45.0 Fatigue 7 35.0 2 10.0 Peripheral sensory neuropathy 7 35.0 2 10.0 Alkaline phosphatase increase 8 40.0 Nausea 7 35.0 1 5.0 Arthralgia 7 35.0 Myalgia 5 25.0 1 5.0 Platelet count decrease 6 30.0 Citation Format: Saranya Chumsri, Sharmila Giri, Andy J. Ness, David W. Hillman, Nadine Norton, Davitte Cogen, Brian M. Necela, Aziza Nassar, Donald W. Northfelt, Pooja Advani, Rohit Rao, Kostandinos Sideras, Alvaro Moreno-Aspitia, Brenda J. Ernst, Kathryn J. Ruddy, Matthew P. Goetz, Keith L. Knutson. Safety and tolerability of multi-epitope HER2 peptide vaccines in combination with trastuzumab emtansine in HER2-positive breast cancer patients with residual disease after neoadjuvant chemotherapy [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 5005.

Adaptive therapy, an ecologically inspired approach to cancer treatment, aims to overcome resistance and reduce toxicity by leveraging competitive interactions between drug-sensitive and drug-resistant subclones, prioritizing patient survival and quality of life instead of killing the maximum number of cancer cells. In preparation for a clinical trial, we used endocrine-resistant MCF7 breast cancer to stimulate second-line therapy and tested adaptive therapy using capecitabine, gemcitabine, or their combination in a mouse xenograft model. Dose modulation adaptive therapy with capecitabine alone increased survival time relative to MTD but not statistically significantly (HR = 0.22, 95% CI = 0.043–1.1, p = 0.065). However, when we alternated the drugs in both dose modulation (HR = 0.11, 95% CI = 0.024–0.55, p = 0.007) and intermittent adaptive therapies, the survival time was significantly increased compared to high-dose combination therapy (HR = 0.07, 95% CI = 0.013–0.42, p = 0.003). Overall, the survival time increased with reduced dose for both single drugs (p < 0.01) and combined drugs (p < 0.001), resulting in tumors with fewer proliferation cells (p = 0.0026) and more apoptotic cells (p = 0.045) compared to high-dose therapy. Adaptive therapy favors slower-growing tumors and shows promise in two-drug alternating regimens instead of being combined.

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.

365 Background: Automated scalp cooling (ASC) is utilized in breast cancer patients to reduce chemotherapy-induced alopecia. Nursing staff and patient perspectives on ASC are limited regarding impact on care delivery and outcomes. Methods: A survey-based study of patient and chemotherapy nurse perspectives of ASC was conducted at three multi-regional Mayo Clinic sites. One-time surveys were sent to breast cancer patients and nurses for self-reporting of the ASC experience. Patients were identified via ICD-10 codes for breast cancer with subsequent chart review to confirm relevant chemotherapy regimen and ASC use. Chemotherapy nurses were identified by their respective chemotherapy unit nurse managers. Patient survey responses were compared between groups that did or did not use ASC by use of Pearson chi-square test for frequency data and the Kruskal-Wallis rank sum test for continuous measures. P values <0.05 were considered statistically significant. Results: Surveys were sent to 181 nurses with 61 (33.7%) completing them. Nurses reported ASC administration to 10-50 patients (n=43, 63.9%) or ≥50 patients (n=18, 29.5%). Most felt they had little or no technical support available (67.2%), witnessed scheduling difficulties (52.5%), were distressed by workload intensity (52.5%), and did not receive adequate training (67.2%). Most felt that ASC demanded greater nursing time compared to other duties (57.4%). Most would not recommend ASC to a friend or family member (88.6%). Surveys were sent to chemotherapy patients with 93 of 465 (20.0%) ASC patients and 56 of 341 (16.4%) without ASC (control) completing them. Controls were significantly more likely to experience hair loss, shave their head, and wear a wig (all p<0.001). There was no significant difference between groups for the EORTC QLQC30, EORTC BR23, and Body Image Scale results or frequency of headaches. Patients who used ASC for less than half of their chemotherapy sessions reported significantly more cold discomfort ( p=0.040) and dizziness ( p=0.003). Time investment (33.3%) and side effects (31.2%) were the most frequent burdens cited. Conclusions: Most nurses indicated that ASC increased nursing workload compared with other duties, and they lacked adequate training and technical support. Increasing formal nurse education and access to technical support documents and live technical support may be options for improving nursing satisfaction with ASC. Patient responses suggest qualitative effectiveness of ASC in reducing hair loss. While those with ASC were less likely to shave or use a wig, this did not translate to significant differences in quality of life or psychosocial body image compared to controls. Cold discomfort and dizziness were the most frequent side effects among those with lower utilization of ASC.

Abstract Purpose: Paclitaxel is a widely used anticancer therapeutic. Peripheral neuropathy is the dose-limiting toxicity and negatively impacts quality of life. Rare germline gene markers were evaluated for predicting severe taxane-induced peripheral neuropathy (TIPN) in the patients of European ancestry. In addition, the impact of Cytochrome P450 (CYP) 2C8, CYP3A4, and CYP3A5 metabolizer status on likelihood of severe TIPN was also assessed. Experimental Design: Whole-exome sequencing analyses were performed in 340 patients of European ancestry who received a standard dose and schedule of paclitaxel in the adjuvant, randomized phase III breast cancer trial, E5103. Patients who experienced grade 3–4 (n = 168) TIPN were compared to controls (n = 172) who did not experience TIPN. For the analyses, rare variants with a minor allele frequency ≤ 3% and predicted to be deleterious by protein prediction programs were retained. A gene-based, case–control analysis using SKAT was performed to identify genes that harbored an imbalance of deleterious variants associated with increased risk of severe TIPN. CYP star alleles for CYP2C8, CYP3A4, and CYP3A5 were called. An additive logistic regression model was performed to test the association of CYP2C8, CYP3A4, and CYP3A5 metabolizer status with severe TIPN. Results: Cytochrome P450 oxidoreductase (POR) was significantly associated with severe TIPN (P value = 1.8 ×10−6). Six variants were predicted to be deleterious in POR. There were no associations between CYP2C8, CYP3A4, or CYP3A5 metabolizer status with severe TIPN. Conclusions: Rare variants in POR predict an increased risk of severe TIPN in patients of European ancestry who receive paclitaxel.

Abstract Hereditary factors play a role in the development of several cancers. Identification of germline predisposition can have implications for treatment and cancer prevention. While previous studies have found that universal multigene panel testing in patients with cancer yields increased detection of actionable heritable variants compared to targeted testing, patients belonging to racial and ethnic minority groups have been underrepresented in these studies. We undertook a prospective multi-site study of germline alternations among solid tumor cancer patients from under-represented minority (URM) populations receiving care at Mayo Clinic Comprehensive Cancer Centers between January 2020 and November 2022. Patients with a new or active cancer diagnosis (all stages) not selected for family history of cancer were tested with a &gt;80-gene next generation sequencing panel. Of 532 patients (mean [SD] age, 57.2 [12.7] years; 320 (60.2%) female), 247 (46.4%) were Hispanic/Latino, 130 (24.4%) Black/African American, 80 (15.0%) Asian, 66 (12.4%) American Indian/Alaskan Native, 2 (0.4%) Native Hawaiian/Pacific Islander, and 7 (1.3%) other. PGV were found in 43 patients (8.1%), including 29 moderate- or high- penetrance cancer susceptibility genes. Variants of uncertain significance (VUS) were found in 272 patients (51.1%). Of those with a PGV, 29 (67.4%) had incremental clinically actionable findings that would not have been detected by established testing criteria, of which 16 were moderate- or high-penetrance variants. Younger age of diagnosis, gender, race, ethnicity, and stage of disease were not predictive of PGV. In this study of under-represented minority cancer patients, universal genetic testing found that 1 in 12 patients harbor a PGV, more than two-thirds of whom would remain undetected using current guidelines. The paucity of genomic data in URM populations also results in a large proportion of VUS which may lead to under-estimation of disease associated variants. Table 1. Clinical and Demographic Characteristics of Included Patients Positive Negative VUS Total N (%) 43 217 272 532 (8.1%) (40.8%) (51.1%) Age - mean (SD) 53.7 57.0 57.8 57.2 (12.7) (12.6) (12.8) (12.7) Gender - Female 26 132 162 320 (60.5%) (60.8%) (59.6%) (60.2%) Race/Ethnicity Hispanic/Latino 24 110 113 247 (55.8%) (50.7%) (41.5%) (46.4%) Black/African 5 56 69 130 American (11.6%) (25.8%) (25.4%) (24.4%) Asian 10 (23.3%) 23 (10.6%) 47 (17.3%) 80 (15.0%) American Indian/Alaskan Native 4 (9.3%) 26 (12.0%) 36 (13.2%) 66 (12.4%) Native Hawaiian/Pacific Islander 0 (0.0%) 0 (0.0%) 2 (0.7%) 2 (0.4%) Other 0 (0.0%) 2 (0.9%) 5 (1.8%) 7 (1.3%) Tumor Types Breast 19 (44.2%) 80 (36.9%) 110 (40.4%) 209 (39.3%) CNS 1 (2.3%) 2 (0.9%) 1 (0.4%) 4 (0.8%) GI 7 (16.3%) 46 (21.2%) 58 (21.3%) 111 (20.9%) GU 8 (18.6%) 57 (26.3%) 62 (22.8%) 127 (23.9%) GYN 0 (0.0%) 8 (3.7%) 10 (3.7%) 18 (3.4%) Head/Neck 5 (11.6%) 8 (3.7%) 6 (2.2%) 19 (3.6%) Lung 0 (0.0%) 6 (2.8%) 11 (4.0%) 17 (3.2%) Melanoma 0 (0.0%) 2 (0.9%) 1 (0.4%) 3 (0.6%) Sarcoma 2 (4.7%) 4 (1.8%) 4 (1.5%) 10 (1.9%) Other 1 (2.3%) 4 (1.8%) 9 (3.3%) 14 (2.6%) Citation Format: Kevork A. Khadarian, Sailaja Pisipati, Jeremy C. Jones, Gerardo Colon-Otero, Donald W. Northfelt, Lindsey Gary, Idara U. Ekpoh, Giovanna G. Moreno, Edward D. Esplin, Robert Nussbaum, Brandie Heald, Margaret A. Klint, Kathleen A. Barrus, Tanios S. Bekaii-Saab, Aleksandar Sekulic, Michael A. Golafshar, Katie L. Kunze, Cheryl L. Willman, Konstantinos N. Lazaridis, Niloy Jewel Samadder. Universal genetic testing for hereditary cancer syndromes in an under-represented minority population [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1921.

Abstract Introduction: We performed matched case-control studies utilizing cohorts of postmenopausal women with ER+ breast cancer receiving adjuvant aromatase inhibitors (AI) on MA.27 [anastrozole, exemestane] or PreFace [letrozole] to assess the association between estrogen suppression after 6 months of treatment and an early breast cancer (EBC) event within 5 years of AI initiation (Clin Cancer Res 2020;26:2986-98). We found a significant 3.0-fold increase in risk of an EBC event for those taking anastrozole with levels of estrone (E1) ≥1.3 pg/mL and estradiol (E2) ≥0.5 pg/mL, but not for exemestane or letrozole. Given these findings we designed a prospective pharmacodynamic (PD) study to evaluate the impact of anastrozole (1 mg/day: ANA1) on E1 and E2 levels, and among those with inadequate estrogen suppression (IES: E1 ≥1.3 pg/mL and E2 ≥0.5 pg/mL), to evaluate the safety and PD efficacy of high-dose anastrozole (10 mg/day: ANA10), which has been found to be safe in prior clinical trials (Cancer 1998;83:1142-52). Methods: Post-menopausal women with stage I-III, ER ≥1% positive/HER2-negative breast cancer who were candidates for anastrozole were eligible after completion of locoregional therapy and chemotherapy, as clinically indicated. Women who were pre-menopausal at diagnosis were not eligible. All patients received 8-10 weeks of ANA1, after which those with adequate estrogen suppression (AES: E1&lt; 1.3 pg/mL or E2&lt; 0.5 pg/mL) came off study. Those with IES went on to receive ANA10 for 8-10 weeks, followed by letrozole (2.5 mg/day: LET) for 8-10 weeks. All patients were managed at their treating oncologist’s discretion following study discontinuation. E1 and E2 blood levels were measured pre-treatment and after completion of each treatment cycle by a CLIA-approved liquid chromatography with tandem mass spectrometry in the Immunochemical Core Laboratory at Mayo Clinic. With a sample size of 29 patients with IES after ANA1, a one-sided binomial test of proportions with a significance level of 0.05 will have an 87% chance of rejecting the proportion with AES after ANA10 is at most 25% (Ho) when the true proportion is at least 50%. Specifically, the null hypothesis is rejected if the number of women with AES after ANA10 is 12 or more. Data lock was July 6, 2022. Results: Of the 161 women enrolled from April 2020 through May 2022, 3 withdrew consent prior to start of ANA1 and 2 were ineligible; thus, 156 women comprised the study cohort. Median patient age was 64 years (range 44-86), 10% of patients were of Hispanic ethnicity and/or non-white race, and 15% received chemotherapy. Six patients remain on ANA1, and 10 discontinued ANA1 due to refusal (7), adverse event (AE) (2), or COVID-19 (1). Forty-one of the remaining 140 patients (29.3%; 95%CI: 21.9-37.6%) had IES with ANA1. Nine of these 41 patients did not go on to ANA10 due to refusal (6) or AE (3). Of the 32 patients who started ANA10, 8 remain on treatment, 5 discontinued due to refusal (3) or AE (1-grade 2 urinary tract infection; 1-grade 1 palpitations), and 19 had a blood draw 45 days or more after starting ANA10. No grade 3-5 AEs or grade 2 hot flashes or arthralgias were reported. Of these 19 patients, 14 achieved AES with ANA10 (73.7%; 95%CI: 48.8-90.9%). All 19 patients switched to LET of which 3 remain on treatment, 1 is missing E1/E2 data, and 15 had a blood draw 45 days or more after starting LET. Of these 15 patients, 10 maintained AES, 2 acquired AES with LET, and 3 no longer had AES. Anastrozole and letrozole drug levels will be reported at the meeting. Conclusions: Approximately 29% of postmenopausal women with ER+/HER2- BC receiving adjuvant anastrozole 1 mg/daily had IES. A majority of these patients achieved AES with dose escalation to ANA10 without tolerability issues. E1 and E2 levels are logical biomarkers given the mechanism of action of anastrozole, and further study utilizing them to determine the optimal dose of anastrozole for a given patient should be performed. Citation Format: Tufia C. Haddad, Vera Suman, Karthik V. Giridhar, Alvaro Moreno-Aspitia, Donald Northfelt, Brenda Ernst, Kostandinos Sideras, Ciara C. O’Sullivan, Ravinder Singh, Zeruesenay Desta, Jodi Taraba, Barbara Goodnature, Matthew P. Goetz, Liewei Wang, James N. Ingle. Anastrozole dose escalation for optimal estrogen suppression in postmenopausal early stage breast cancer: A prospective 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 OT1-04-02.