Negative Metastatic Breast Cancer Research Articles

Prognostic and Predictive Significance of HER2-low Expression in Metastatic Hormone Receptor Positive Breast Cancer Patients Receiving CDK4-6 Inhibitor Therapy

Objective: This study aimed to analyze the predictive and prognostic value of HER2-low expression in hormone receptor (HR) positive human epidermal growth factor receptor-2 (HER2) negative metastatic breast cancer patients receiving cyclin-dependent kinase-4/6 inhibitor (CDK4/6i) therapy. Methods: This retrospective study included patients who received CDK4/6i plus endocrine therapy (ET). The pathological and clinical characteristics and survival times of the patients were compared and analyzed. Results: Our study included 122 patients. There were HER2-zero 88(72%) and HER2-low 34 (28%) patients. The median progression free survival (mPFS) of all patients who received CDK4/6i+ET was 21 (95% confidence interval (CI),18.5–23.5) months, while mPFS was not reached in the HER2-zero group, and mPFS in the HER2-low group was 12 (95%CI, 6.8–17.1) months (p=0.001). The mPFS was shorter in patients with primary endocrine resistance (6 vs. 21 months, p=0.001). There was a change in the HER2-low status of 26(45%) patients with recurrence compared to the first biopsy. In the HER2-zero and HER2-low groups, 22(25%) and 24(71%) patients, respectively, progressed with CDK4/6i+ET (p=0.001). Estrogen receptor (ER) levels less than and greater than 50% resulted different mPFS (6 and 21 months, respectively) (p=0.025). Median PFS differed based on CDK4/6i+ET combination, treatment line, and best treatment response (all p=0.001). In multivariate analysis, HER2 status(p=0.018), chemotherapy status(p=0.006), best response status with CDK4/6i (p=0.001) for PFS, and best response status with CDK4/6i therapy (p=0.007) for OS were significant. Conclusions: In patients with HR+HER- metastatic breast cancer receiving CDK4/6i therapy, the duration of mPFS was lower in the HER2-low group than that in the HER2-zero group. HER2-low expression is a predictive biomarker of response to CDK4/6 inhibitor therapy.

Abstract PO2-04-12: From Clinical Trials to Clinical Practice; Real World Clinical Outcomes of Patients Treated with Ribociclib in Combination with Aromatase Inhibitors or Fulvestrant for HR-Positive, HER2- Negative Metastatic Breast Cancer

Abstract Background: Cyclin-dependent kinase (CDK) 4/6 inhibitors have revolutionized the treatment landscape of hormone receptor-positive (HR+), human epidermal growth factor receptor-2 (HER2)-negative metastatic breast cancer (MBC), with an impressive efficacy and safety profile. Here we report real-world clinical outcomes and toxicity data in patients treated at a tertiary care cancer center. Methods: The study is a retrospective analysis of individual patients’ data. All consecutive patients with HR+ and HER2-negative MBC, treated at our institution with ribociclib plus aromatase inhibitors (AI) or fulvestrant, between June 2017 and May 2020 were reviewed. Data were collected from patients’ electronic medical records. Progression free survival (PFS) was defined as the time from treatment initiation with CDK4/6i until the first documented progression, death from any cause or last follow-up, whichever occurred first. The Overall survival (OS) was defined as the time from treatment initiation with CDK4/6i until the date of death from any cause, or last follow-up. Results: A total of 305 patients, median age 49 (22-87) years were enrolled. Although bone metastasis was reported in 241 (79.0%) patients, bone-only metastasis was identified in 64 (20.9%). Visceral metastasis in the liver and lungs were reported in 22.0% and 25.9 % of patients, respectively. CDK4/6i combined with AI were used in first-line setting in 195 (63.9%) patients and with fulvestrant as a second line (20.7%) or beyond (15.4%). Dose reduction was required in 44 (14.14%) patients, mostly because of neutropenia (n=38, 12.5%) and abnormal liver enzymes (n=12, 3.9%), while 11 (3.6%) discontinued treatment, mostly due cardiac toxicities. Nevertheless, there were no deaths due to toxicity. Patients were followed up for a median of 31.1 months. Overall response rate (ORR) was 51.4% and median PFS, irrespective of the line of therapy, was 19.3 (17.6-23.3) months. PFS was longer in first-line setting; 23.1 (19.7-NR) months compared to 13.9 (10.2-17.6) months in second-line or beyond, p< 0 .0001, and when combined with AI compared to fulvestrant; 28.6 (17.8-23.1) months versus 9.1 (4.4-NR) months, p=0.0001, and in those with bone-only metastasis; 23.1 (19.5-NR) months versus 17.3 (12.2-18.9) months for patients with visceral metastasis, p=0.0008. The median OS was not reached. Conclusions: Despite enrolling sicker patients, and outside the stringent clinical trials settings, our treatment outcomes, in real world settings, are comparable to those reported in major clinical trials, including MONALEESA-2, MONALEESA-3 and MONALEESA-7. Citation Format: Baha' Sharaf, Faris Tamimi, Sarah Edaily, Osama Salama, Hazem Abdulelah, Anas Zayed, Mahmoud Abunasser, Hala Abu-Fares, Hikmat Abdel-Razeq. From Clinical Trials to Clinical Practice; Real World Clinical Outcomes of Patients Treated with Ribociclib in Combination with Aromatase Inhibitors or Fulvestrant for HR-Positive, HER2- Negative Metastatic 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 PO2-04-12.

Abstract PO2-13-03: Spatial HER2 heterogeneity on HER2-PET in HER2-low and negative disease: potential biomarker for antibody-drug conjugate effect

Abstract Background: Anti-HER2 antibody drug conjugates (ADCs) can assert an effect in part of patients with HER2-low breast cancer. In light of patient burden and financial sustainability, biomarkers to select responding patients are urgently needed, but so far, none have been identified. In the IMPACT trial (NCT01957332), patients with newly diagnosed and non-rapidly progressive metastatic breast cancer (MBC) of all subtypes (including HER2-negative, -low, and –positive disease) were included. We assessed the performance of Zirconium-89 (89Zr)-trastuzumab positron emission tomography (HER2-PET) in HER2-low and negative MBC. Methods: All patients received extensive workup, including a metastasis biopsy and HER2-PET at baseline. HER2 status was determined by immunohistochemistry (IHC) and in situ hybridization. ⁸⁹Zr-trastuzumab uptake was quantified as maximum and mean standardized uptake values (SUVmax, SUVmean) in metastases and healthy background tissue. Quantitative ⁸⁹Zr-trastuzumab uptake of all metastases and corresponding biopsied metastasis, was related to HER2 IHC status. Results: In 200 patients, ⁸⁹Zr-trastuzumab uptake was quantified in 5,163 metastases. With increasing HER2 IHC status, uptake was higher (geometric mean SUVmax 7.0, 7.6, 7.3, and 17.4 with HER2 IHC 0, 1, 2, or 3+, in respectively 71, 71, 20, and 24 biopsied metastases (P  0.001 between IHC 0 and 3+). SUVmax in lesions was heterogeneous within and between patients for all HER2 IHC groups. All HER2 IHC groups, also HER2-negative and -low, had uptake exceeding that of the healthy background (geometric mean tumor-to-background ratio SUVmax >1 in all groups, increasing with higher HER2 IHC). Of patients with a HER2 IHC 0 biopsy, 27.8% had at least one metastasis elsewhere in the body, with uptake exceeding the overall 90th SUVmax percentile of 14.6; in HER2 low IHC1+ and 2+, this was 31.9% and 30%. Conclusions: In this largest series with HER2-PET in patients with MBC, we show that ±30% of patients with HER2-low or -negative IHC in the biopsy, in fact, have metastases with high HER2 uptake elsewhere in the body. This spatial heterogeneity provides novel insights into HER2-negative and -low disease compared to standard HER2 IHC of a single biopsy. As HER2-PET uptake is related to ADC response in HER2 positive MBC1, HER2-PET is a potential biomarker for anti-HER2 ADC effect in HER2-low and -negative MBC as well. Reference: 1. Gebhart G, et al. Ann Oncol 2016;27(4): 619-624. Supported by the Dutch Cancer Society grant 2012-5565 Citation Format: Carolien Schroder, Jasper van Geel, Bertha Eisses, Adrienne Brouwers, Sjoerd Elias, Frederike Bensch, Evelien Kuip, Agnes Jager, Andor Glaudemans, Bert van der Vegt, Willemien Menke-van der Houven van Oordt, Elisabeth de Vries. Spatial HER2 heterogeneity on HER2-PET in HER2-low and negative disease: potential biomarker for antibody-drug conjugate effect [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 PO2-13-03.

Abstract PO4-19-09: Comparison of clinical efficacy between letrozole + ribociclib vs. Fulvestrant + letrozole + ribociclib in Hormone receptor positive, HER2 negative metastatic breast cancer – a randomized, phase 2 study (KCSG BR22-20)

Abstract Background The current standard first-line treatment in advanced or metastatic hormone receptor (HR) positive breast cancer is combination of aromatase inhibitor (AI) and cyclin D kinase-4/6 (CDK4/6) inhibitor. Emergence of ESR1 mutation is a key mechanism of resistance during first-line AI + CDK4/6 inhibitor treatment. Selective estrogen receptor downregulator (SERD) such as elacestrant or fulvestrant is suggested as second-line treatment, but the progression-free survival (PFS) outcome of second-line treatment is reported up to 3.78 months in patients who harbour ESR1 mutation. Recently, early switch of AI + CDK4/6 inhibitor to fulvestrant + CDK4/6 inhibitor at the point of rising ESR1 mutation showed prolongation of PFS up to 6 months, suggesting there may be a new strategy to overcome ESR1 mutation by modifying endocrine treatment. SWOG S0226 suggested that upfront fulvestrant + AI is feasible and associated with prolongation of PFS compared to AI alone. After CDK4/6 inhibitor has been introduced to standard treatment, there were no trials evaluating the role of AI + fulvestrant with CDK4/6 inhibitor. In this trial, we investigated triplet combination of AI + fulvestrant + CDK4/6 inhibitor as a treatment to overcome resistant mechanism such as development of ESR1 mutation when compared to current standard treatment of AI + CDK4/6 inhibitor as the first line treatment in HR positive advanced or metastatic breast cancer. Methods This study is a randomized, multicenter, open-label phase II trial comparing AI plus ribociclib versus AI, fulvestrant plus ribociclib in HR-positive advanced or metastatic breast cancer. De novo patients or patients with treatment-free interval (TFI) over 12 months after completion of adjuvant AI can be enrolled. If patients received adjuvant tamoxifen, patients with TFI less than 12 months can be also enrolled for the study. Prior endocrine treatment or cytotoxic chemotherapy at metastatic setting was not allowed. In cases of premenopausal women, gonadotropin releasing hormone agonist (GnRHa) is administered every 4 weeks with treatment. A total of 202 patients will be enrolled for 30 months, and 30 months of follow-up is planned after last-patient enrolment. Patients are randomized to 1:1, stratified based on the presence of visceral metastasis and previous administration of adjuvant AI. Study treatment of the control arm consists of letrozole 2.5mg daily with ribociclib 600mg (Day 1-Day 21 daily by month, 1 week off) until progression. In the experimental arm fulvestrant 500mg intramuscular injection is added (Day 1, Day 15, Day 29 and every 4 weeks thereafter) until progression. For exploratory analysis of ESR1 mutations, circulating tumor DNA (ctDNA) is collected at baseline, and then every 3 months thereafter. Primary endpoint is to evaluate the difference of 24-months PFS rate. Secondary endpoint included PFS, overall survival (OS), emergence and frequency of ESR1 mutation, overall response rate (ORR) and clinical benefit rate (CBR). The analysis is planned after completion of 30 months of follow-up, which provides approximately 90% power to detect superiority 24 months-PFS rate assumed by Kaplan-Meier curve of letrozole + fulvestrant + ribociclib versus letrozole + ribociclib using a log-rank test, assuming a hazard ratio of 0.64 at a two-sided alpha of 0.2. Citation Format: Jieun Lee, Su-Jin Koh, Jung Hye Kwon, Gun Min Kim, Jee Hung Kim, Ho Young Kim, Hee-Jun Kim, Keon-Uk Park, Kyong Hwa Park, Yeon Hee Park, Mi Sun Ahn, Hee Kyung Ahn, Gyeong-Won Lee, Kyoung Eun Lee, Kyung-Hun Lee, Moon Hee Lee, Sung Sook Lee, Jung Lim Lee, Kyung Hae Jung, Joo Young Jung, In Hae Park. Comparison of clinical efficacy between letrozole + ribociclib vs. Fulvestrant + letrozole + ribociclib in Hormone receptor positive, HER2 negative metastatic breast cancer – a randomized, phase 2 study (KCSG BR22-20) [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 PO4-19-09.

Abstract PO1-15-06: Serum thymidine kinase activity as a “real-time” clinical biomarker of tumor response to CDK4/6 inhibition in hormone receptor positive (HR+), HER2 negative (HER2-) metastatic breast cancer

Abstract Background: CDK4/6 inhibitors (CDK4/6i) plus endocrine therapy (ET) are standard first-line (1L) treatment for hormone receptor positive (HR+), HER2-negative (HER2-) metastatic breast cancer (MBC). Despite general improvements in long-term outcome, not all patients benefit equally from these drugs. The average duration of 1L CDK4/6i therapy is ~2 years. However, 10-15% of patients will progress within 6 months of therapy initiation, while others will derive durable clinical benefit of years. Differences in treatment efficacy cannot be easily explained by clinical-pathological features. No predictive biomarkers are currently available to determine tumor sensitivity to CDK4/6i. There is an unmet need for a complementary diagnostic to identify patients that will or will not respond to these treatments in order to better inform therapy management. Thymidine kinase (TK) is an E2F-regulated enzyme that plays a key role in DNA replication during cell division. The expression and activity of TK is strongly linked to the cell cycle. TK is first expressed in late G1, peaks during S-phase, and then is degraded during mitosis. Because of the restricted expression of TK (only during S phase of the cell cycle) combined with the mechanism of action of CDK4/6i, the measurement of TK activity (TKa) may serve as an ideal pharmacodynamic biomarker to assess a tumor’s response to CDK4/6i-based therapy. DiviTum® TKa is an FDA cleared assay that can quantitate TKa from human serum (sTKa) and is intended to be used as a disease progression monitoring tool for patients with HR+, HER2- MBC. TK IMPACT is a prospective, single arm trial designed to assess the impact of incorporation of DiviTum® TKa on the physician’s decision regarding subsequent timing of routine disease monitoring modalities in patients with advanced HR+, HER2- MBC receiving 1L ET plus CDK 4/6i (NCT04968964). As a part of this ongoing study, DiviTum® TKa values are serially assessed relative to imaging findings and other circulating tumor biomarkers, such as CA15-3 and ctDNA. The primary endpoint is any physician-reported intended change in imaging testing interval post TKa by study cohort, within the first 48-week period of study participation, assessments for which are ongoing. Here we report real-world data and examples of the clinical utility of sTKa as a companion diagnostic from patients treated at Washington University Siteman Cancer Center, St. Louis, MO. Methods: Patients with HR+, HER2- MBC receiving 1L ET + any FDA-approved CDK 4/6i (palbociclib, ribociclib, or abemaciclib) had blood serum samples collected at baseline, weeks 2, 4, 6, 8, every 4 weeks up to week 24, and then every 12 weeks thereafter until disease progression. Samples were analyzed for TKa levels in real-time using the DiviTum® TKa assay. Optional repeat TKa within 2-4 wks (+/- 3 days) is permitted in case of rising TKa. Other standard monitoring assessments were carried out according to institutional guidelines and/or treating clinician discretion. Results: Pre-treatment and early on-therapy TKa levels predicted response to CDK4/6i therapy more accurately than a patient’s clinical characteristics. Rising TKa levels also identified patients with clinically occult disease progression ahead of imaging, and in some cases where radiographic interpretation and serum tumor markers such as CA15-3 were ambiguous. Measurement of TKa levels following CDK4/6i dose reduction was able to confirm continued optimal therapy response at the lower dose. A rise in TKa levels aligned with the re-emergence of ESR1 mutation as identified by ctDNA in one patient and may herald molecular progression. Conclusion: sTKa was able to predict tumor sensitivity and resistance (both intrinsic and acquired) to CDK4/6i + ET therapy in patients with HR+, HER2- MBC. We believe these findings help support the role of sTKa as a non-invasive pharmacodynamic biomarker that can assess in real-time a tumor’s response to CDK4/6i based therapy. Citation Format: Nusayba Bagegni, Amy Williams, Isabella Grigsby, Mattias Bergqvist, Katherine Clifton, Cynthia Ma. Serum thymidine kinase activity as a “real-time” clinical biomarker of tumor response to CDK4/6 inhibition in hormone receptor positive (HR+), HER2 negative (HER2-) metastatic 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 PO1-15-06.

Abstract PO4-15-09: Unraveling the influence of progesterone receptor status on endocrine therapy sensitivity: insight from epigenetic and fragmentomics ctDNA profiling in estrogen receptor positive, HER2 negative metastatic breast cancer (MBC)

Abstract Background: In patients (pts) with estrogen receptor (ER) positive (pos), HER2 negative (neg) MBC, the absence of progesterone receptor (PR) expression is generally associated with a poorer endocrine therapy (ET) response and an unfavorable prognosis. However, real-time molecular changes during ET according to PR status are still not fully understood. The aim of this study was to examine the prognostic impact of PR and its association with the methylation levels of ESR1 promoters A (promA) and B (promB) at different timepoints in pts with ERpos HER2neg MBC who received first-line ET. Methods: Pts were enrolled in the prospective multicenter MAGNETIC.1 trial (CRO-2018-56) between January 2018 and January 2023. As first-line ET, pts received either fulvestrant or AIs with or without CDK4/6 inhibitors (CDK4/6i) as first-line ET. ctDNA samples were collected at baseline (T0) and at 3-month intervals (T3 and T6) and analyzed through methylation-specific (MS) droplet digital PCR (ddPCR), ddPCR fragmentomics and next generation sequencing (NGS). Clinico-pathological differences across the PR expression spectrum were analyzed using Fisher’s exact test. Matched pairs variations across timepoints (T0, T3, T6) of ACTBfragments distribution and ESR1 promA and promB were tested through Wilcoxon signed rank test. Results: Out of the 111 enrolled pts, 28 had PRneg MBC, of which 20 had an invasive ductal carcinoma (IDC) histotype, and 10 were diagnosed as de novo disease. Bone was the most common metastatic site both in the PRpos and PRneg subgroups (75% vs 73%), most pts received ET+CDK4/6i (96% vs 95%). ESR1 and PIK3CA mutations were respectively detected in 8% and 6%, and in 23% and 25% pts respectively for PRpos and PRneg. The prognostic impact of PR status was evident for both PFS (median 38 mos and 14 mos respectively for PRpos and PRneg, P = 0.0048) and OS (median not reached and median 33 mos respectively for PRpos and PRneg, P= 0.001). A significant increase in promB was observed at T3 vs T0 and at T6 vs T3 in the PRpos subgroup (respectively P=0.0158 and P< 0.001) but not in the PRneg. When comparing T6 to T0, no significant differences were observed regardless of PR status. At T3, significant decreases were observed in both the PRpos and PRneg populations for ACTBshort (P < 0.001 and P=0.03, respectively), ACTBmedium (P=0.0049 and P= 0.035, respectively) and ACTBlong(P< 0.001 and P< 0.001, respectively), compared to T0. In the comparison of T6 to T3, a significant variation in ACTBshort levels was observed in both subgroups (P< 0.001 for PRpos and P=0.012 for PRneg). while a significant reduction in ACTBmedium levels was observed only in the PRpos population when comparing T6 to T0 (P< 0.001). Conclusions: PRneg status was confirmed as an unfavorable prognostic factor with respect to PRpos both for PFS and OS, potentially due to different ET sensitivity. Epigenetic ctDNA profiling revealed different dynamics of ESR1 promB between PR-positive and PR-negative MBC, suggesting a distinct role of ESR1 in the onset of ET resistance within the two subgroups. Citation Format: Lorenzo Foffano, Elisabetta Molteni, Arianna Dri, Alessandra Franzoni, Linda Cucciniello, Lucia Da Ros, Silvia Buriolla, Silvia Bolzonello, Claudia Noto, Stefania Russo, Simon Spazzapan, Elena Nascimbeni, Brenno Pastò, Giada Targato, Serena Della Rossa, Marta Bonotto, Alessandro Marco Minisini, Giuseppe Damante, Barbara Belletti, Lorenzo Gerratana, Fabio Puglisi. Unraveling the influence of progesterone receptor status on endocrine therapy sensitivity: insight from epigenetic and fragmentomics ctDNA profiling in estrogen receptor positive, HER2 negative metastatic breast cancer (MBC) [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 PO4-15-09.

Abstract PO3-18-09: OPERA-01: A randomized, open-label, phase 3, study of palazestrant (OP-1250) vs standard-of-care treatment for ER+, HER2- advanced or metastatic breast cancer after endocrine and CDK4/6 inhibitor therapy

Abstract Background: In estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer (MBC), adding a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor to endocrine therapy (ET) has improved outcomes and is the current standard-of-care treatment in the first-line setting. However, resistance to first-line treatment develops in most patients (pts), which is often attributed to acquired mutations in ESR1. Limited data exist for ET-based treatment options after progression on prior ET and CDK4/6 inhibitors, and most pts will transition to chemotherapy for further treatment. Therefore, there is a significant unmet need for more effective ET in pts with ER-positive, HER2-negative MBC to improve outcomes and delay time to chemotherapy. Palazestrant (OP-1250) is a small molecule oral complete ER antagonist (CERAN) and selective ER degrader (SERD) that binds the ligand binding domain of ER and completely blocks ER-driven transcriptional activity in both wild-type (ESR1-wt) and mutant (ESR1-mut) forms of ER. In preclinical studies, palazestrant demonstrated better tumor shrinkage in ESR1-wt and ESR1-mut models when compared to fulvestrant; it also showed efficacy in brain metastasis models. In a phase 1/2 monotherapy study in pts with heavily pretreated ER-positive, HER2-negative advanced breast cancer (BC) or MBC (NCT04505826), palazestrant showed a tolerable safety profile, promising antitumor efficacy, and favorable pharmacokinetics (PK) supporting once a day (qd) dosing. The recommended Phase 2 dose of palazestrant is 120 mg qd. Methods: The OPERA-01 trial is an international, multicenter, randomized, open-label, phase 3 clinical trial comparing the efficacy and safety of palazestrant as a single agent to standard-of-care ET (fulvestrant, anastrozole, letrozole, or exemestane) in pts with ER-positive, HER2-negative advanced BC or MBC that has relapsed or progressed on 1 or 2 prior lines of ET for MBC, which includes a CDK4/6 inhibitor. Eligible pts are women (pre- or post-menopausal) or men who have a confirmed diagnosis of ER-positive, HER2-negative locally advanced evaluable BC or MBC not amenable to curative therapy. ER and HER2 status are determined according to the American Society of Clinical Oncology/College of American Pathologists guidelines. Patients must have Eastern Cooperative Oncology Group performance status 0 or 1. Prior treatments must include 1 or 2 prior lines of ET as monotherapy or in combination with a CDK4/6 inhibitor for MBC; must have received CDK4/6 inhibitor in combination with ET and have disease progression during or within 28 days of completion of each line of prior treatment for MBC; most recent ET given for ≥6 months. Prior chemotherapy in the metastatic setting is not permitted. Patients (N∼500) are randomized to palazestrant or standard-of-care ET monotherapy, and are stratified by ESR1 mutation status, prior lines of ET for advanced disease (1 vs 2 lines), and presence or absence of visceral disease. The primary endpoint of progression-free survival will be assessed by blinded independent central review in pts with and without ESR1 mutations in the intent-to-treat population. Secondary endpoints include overall survival, antitumor activity (objective response rate, clinical benefit rate, and duration of response), safety, patient-reported outcomes, and PK in pts with and without ESR1 mutations. The study is planned to be conducted globally. Citation Format: Arlene Chan, Jane Meisel, Komal Jhaveri, Joo Hyuk Sohn, Meritxell Bellet- Ezquerra, Jason Schroeder, Mark Shilkrut, Barbara Pistilli. OPERA-01: A randomized, open-label, phase 3, study of palazestrant (OP-1250) vs standard-of-care treatment for ER+, HER2- advanced or metastatic breast cancer after endocrine and CDK4/6 inhibitor therapy [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 PO3-18-09.

Abstract PO4-17-04: CDK 4/6 inhibitor switching patterns in Swedish patients with metastatic breast cancer: 5-year update from the SIRI study

Abstract Background: CDK 4/6 inhibitors (CDK 4/6i) combined with endocrine therapy is a well-established treatment option in patients with hormone receptor-positive (HR+), human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer (MBC). However, there is limited evidence on switching patterns within this class of drugs. The Swedish Ibrance Registries Insights (SIRI) study investigated CDK 4/6i switching patterns in real-world setting using a nationwide cohort of MBC patients. Methods: This was a retrospective study utilizing population-based Swedish Health Data Registers. The overall cohort included all breast cancer patients ≥ 18 years with ≥ 1 dispensation of palbociclib from January 2017 – June 2022. This subgroup analysis focused on patients with ≥ 1 dispensation of either ribociclib or abemaciclib in addition to palbociclib. Minimum follow-up was 3 months. CDK 4/6-I sequencing patterns in total and over time as well as time from the first CDK 4/6i to the subsequent, was investigated. No information on the reason for switching was available. Results: Out of a total 2314 patients with ≥ 1 dispensation of palbociclib, 256 patients (11%) had ≥ 1 dispensation of either ribociclib or abemaciclib, either prior to (60% of cases) or following (40 % of cases) palbociclib treatment. The share of patients with dispensation of > 1 CDK 4/6i increased over the study period from 7% of the patients initiating treatment in 2017 to 15% in 2021. Of the total 2161 patients initiating CDK 4/6i treatment on palbociclib, about 5% were subsequently prescribed another CDK 4/6i. The median age at treatment initiation in patients with > 1 dispensed CDK 4/6-i was similar to the overall study cohort (67.1 vs 68.4 years). Half of the patients identified with > 1 CDK 4/6i were initiated on ribociclib and prescribed palbociclib subsequently. The second and third most common switches were palbociclib to abemaciclib (25%) and palbociclib to ribociclib (10%), respectively. Seven patients received all three CDK 4/6i. In terms of time to subsequent CDK 4/6i, 108 patients (42%) were prescribed a subsequent CDK 4/6i within three months of treatment initiation with ribociclib-palbociclib being the most common sequence in 74 patients whereas 17%, 8%, and 4% of switches happened 4-6, 7-9, and 10-12 months after treatment initiation, respectively. Seventy-two patients (28%) had a dispensation of a subsequent CDK 4/6i more than 12 months after initiation of the first CDK 4/6i, with palbociclib-abemaciclib being the most common switch. Conclusions: A relatively high proportion of Swedish patients treated with CDK4/6i is switched from one to another CDK 4/6i soon after treatment initiation, implying that this strategy might be applied mainly, but not solely, due to adverse events. The increased trend over time might be associated with the regulatory approval of new CDK4/6i over time. Future studies should focus on the potential impact of switching CDK 4/6i on treatment effectiveness and toxicity as these issues have yet not been clarified. Declaration of Interest: This study is sponsored by Pfizer. AV has reported receiving research funding from Roche. HL has reported receiving consultant/advisory fees from Lilly, Novartis, Daichii, Pfizer, MSD, Pierre Fabre, Astra-Zeneca and research funding from Roche. MJ and DN are employees of Pfizer. RL and ML are employees of Quantify Research and were paid consultants to Pfizer for this research. Table. Table. Citation Format: Henrik Lindman, Antonis Valachis, Rosa Lauppe, Mathias Lilja, Daniel Nyqvist, Maria Jakobsson. CDK 4/6 inhibitor switching patterns in Swedish patients with metastatic breast cancer: 5-year update from the SIRI study [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 PO4-17-04.

Abstract PO1-14-08: Real-world clinical genomics study of HR+/HER2- metastatic breast cancers treated by CDK4/6i plus endocrine therapies revealed a drug resistant tumor segment characterized by ER independence

Abstract Background CDK4/6 inhibitors (CDK4/6i) plus endocrine therapies (ET) are the standard-of-care for hormone receptor–positive/human epidermal receptor 2–negative metastatic breast cancer (HR+/HER2− mBC). However, drug resistance remains a major unmet need. Investigations of drug resistance mechanisms has been hampered by a dearth of tumor molecular profiling data from the post-treatment setting. To address this challenge, we have conducted a real-world clinical genomics study to better understand the molecular mechanism of CDK4/6i resistance as well as to stratify patients based on integrated multi-omics profiles. Methods We retrospectively analyzed a multi-omics dataset of 400 HR+/HER2- mBC patients who had received CDK4/6i plus ET and developed progressive disease (PD) from the de-identified Tempus database. Pre-treatment and post-progression biopsies were taken  1 year prior to starting the CDK4/6i treatment or following PD respectively. Tempus xT next-generation sequencing (DNA-seq of 648 genes) and RNA sequencing assays were performed on 427 tumor FFPE samples, including 200 pre-treatment, 227 post-progression and 26 longitudinal pairs. Results The median age of the patients was 57 (54.9-57.4) and median progression free survival (PFS) is 379 (341-433) days. Two genes were found to harbor a significant increase in genomic alteration frequencies (GAF) after adjusting for FDR at post-progression vs. pre-treatment – ESR1 (41.9% vs. 15%, p=5.4e-10), RB1 (13.2% vs. 3%, p=8.5e-05). ESR1 and RB1 also harbored high frequencies of acquired genomic alterations among 26 paired samples at 34.6% and 11.5% respectively. TP53 mutation at baseline was significantly associated with shorter PFS at baseline (p=4.23e-05, HR=2.081) and TP53 GAF significantly increased after PD (37% vs. 28.5%, p=0.039). BRCA1/2 pathogenic mutations (p=1.63e-04, HR=3.066), APOBEC mutation signature S13 (p=0.0125, HR=1.55) and CCNE1 gene expression (p=0.024, HR=1.46) were significantly associated with shorter PFS. APOBEC signature (p=0.0035) and CCNE1 expression (p=1.33e-06) also significantly increased post-progression. Among the top molecular features associated with longer PFS were markers of estrogen signaling such as PGR gene expression (p=6.76e-04, HR=0.565) and the Hallmark estrogen response signature (p=0.021, HR=0.679). Applying a multi-omics pattern recognition algorithm, we identified a molecularly distinct cluster (IC1) characterized by down-regulation of estrogen signaling. IC1 is significantly associated with shorter PFS (p=3.72e-05, HR=0.22) and increased from 4% pre-treatment to 23% post-progression (p=7.3e-08). Further, IC1 is strongly enriched in markers previously implicated in CDK4/6i resistance including CCNE1 expression, RB1 mutation and MYC/E2F activation. We then developed machine learning models to predict gene-level dependency trained on cancer cell line expression and CRISPR-KO screen data. These models predicted decreased dependency on ESR1 and CDK4 and increased dependency on CDK2 in IC1, strengthening the association between ER independence and CDK4/6i resistance. Conclusions Our real-world clinical genomics study identified a comprehensive list of biomarkers associated with resistance to CDK4/6i plus ET and estimated patient prevalence for these markers in the post-treatment setting. Integrated and machine-learning analyses identified a subset of aggressive tumors with estrogen independence characteristics that are implicated in CDK4/6i resistance and suggested new therapeutic strategies. Citation Format: Zhengyan Kan, Ji Wen, Jennifer Webster, Vinicius Bonato, Whijae Roh, Xinmeng Jasmine Mu, Paul Rejto, Jadwiga Bienkowska. Real-world clinical genomics study of HR+/HER2- metastatic breast cancers treated by CDK4/6i plus endocrine therapies revealed a drug resistant tumor segment characterized by ER independence [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 PO1-14-08.

Abstract PO2-05-02: Real-world experience of patients receiving treatment for hormone receptor-positive/human epidermal growth factor receptor-2 negative metastatic breast cancer: A global analysis of symptoms and side effects

Abstract Background: Advances in treatment for patients (pts) with hormone receptor-positive (HR+)/human epidermal growth factor receptor-2 negative (HER2–) metastatic breast cancer (mBC) have improved overall survival, but treatment resistance is common and median survival remains only a few years. These pts usually need chronic therapy and often experience symptoms and side effects. There is limited knowledge on pts’ experiences outside the clinical trial setting, largely because this information is not routinely reported. This analysis leverages a unique database of physician- and pt-reported data to describe symptoms and side effects among a real-world, geographically diverse population of pts receiving treatment for HR+/HER2– mBC. Methods: This retrospective cohort study used Adelphi Disease Specific Programme Real World Data, comprising validated, linked pt and physician questionnaire data from the US, France, Germany, Italy, Spain, and UK. Eligible pts were ≥18 y, currently receiving treatment for advanced BC and not enrolled in a clinical trial. Data were collected between August 2022 and February 2023. Physician surveys provided data on individual pt demographics and disease characteristics, comorbidities, treatment history, and treatment (including adherence). Physicians also reported presence or absence of symptoms and side effects after treatment initiation for each pt they considered. A subset of pts completed one-time validated quality of life (QOL) instruments, including the European Organisation for Research and Treatment of Cancer Core Quality of Life Questionnaire (EORTC QLQ-C30) and EORTC QLQ-breast (BR23). Results: Data from 1578 pts receiving treatment for HR+/HER2– mBC were analyzed. Mean age was 65-y, 91% were white, 71% had de novo mBC, and 82% had ECOG performance status 0-1 at the start of current treatment. Across all lines of therapy, 71% of pts were receiving endocrine-based therapy, 16% were receiving chemotherapy only, and 13% were receiving other therapies; 40% of all pts were receiving second or later lines of any systemic therapy (2L+). Among all pts, the most common physician-reported symptoms and side effects (in ≥10% of pts) were bone pain (30%), change in appetite (13%), weight loss (12%), neutropenia (11%), and joint/muscle pain (11%). Pts on 2L+ therapy experienced a greater frequency of symptoms and side effects than those on first-line therapy (Table). Of 220 pts who completed QOL questionnaires, the most commonly reported symptoms and side effects (in ≥70% of pts) were feeling tired (87%), worry about future health (80%), general worrying (79%), needing to rest during the day (74%), feeling weak (74%), having trouble doing strenuous activity (71%), and trouble taking a long walk (71%). These were common irrespective of pt age, disease state (recurrent vs de novo), current treatment regimen, and line of therapy. Conclusions: This real-world analysis shows that both physicians and pts identified substantial symptoms and side effects during treatment. These were pervasive across the population and more common in those receiving 2L+ therapy, although it remains unclear which were due to treatment or disease. Thus, it is important to continue to develop treatments and supportive care to improve both clinical outcomes and pt QOL. Citation Format: MK Rehnquist, Ann Partridge, Annabel Lambert, Allison Kurian, Natalia Sadetsky, Emily Freeman, Lisa Carey. Real-world experience of patients receiving treatment for hormone receptor-positive/human epidermal growth factor receptor-2 negative metastatic breast cancer: A global analysis of symptoms and side effects [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 PO2-05-02.

Abstract PO2-05-03: Costs associated with adverse events in patients receiving treatment for hormone receptor positive/human epidermal growth factor receptor-2 negative metastatic breast cancer

Abstract Background:Adverse event (AE)-related health care resource utilization (HCRU) and costs are an important component in understanding the economic burden associated with managing patients (pts) with cancer and can inform treatment decision-making and access for pts, providers, and payers. At present, there are little data on AE-related HCRU and costs among pts receiving late-line treatment for hormone receptor positive (HR+)/human epidermal growth factor receptor-2 negative (HER2–) metastatic breast cancer (mBC). As such, the objective of this analysis was to describe the AE-related HCRU and costs among pts with HR+/HER2– mBC receiving chemotherapy (CT) who have received prior CT and endocrine-based therapies (ET). Methods: This retrospective study included pts who had at least 1 prior ET, had completed at least 2 CTs, had initiated their third (or greater) CT (post-ET) in the metastatic setting between Jan 2016 and Mar 2022, and were included in the Optum Research Database. The start date for the current line of CT was the index date. Pts were followed until disenrollment from health plan, study end date (Jun 2022), or death. AEs included in this study were based on clinical impact; these AEs were then organized into categories. AE-related HCRU and costs were reported as per pt per month (PPPM) and included outpatient visits, office visits, emergency room visits, inpatient stays, other medical costs (eg, laboratory/ancillary costs), and pharmacy costs. Costs were adjusted to 2021 US dollars using the Consumer Price Index. Results: A total of 769 pts were included with a mean age of 64 y (standard deviation: 13 y) and baseline National Cancer Institute comorbidity score of 0 (44%) or 1–2 (39%). 51% were commercially insured. A total of 715 (93%) pts had ≥1 AE during the follow-up period; the mean total PPPM cost associated with AEs was $7421 (Table 1) and the majority of costs were driven by ambulatory visits (office and outpatient visits: $3846 PPPM; Table 1) and inpatient stays ($3042 PPPM; Table 1). Among pts who experienced AEs, AEs that were associated with the highest mean total PPPM costs included infections, hepatoxicity, gastrointestinal toxicity and renal failure events. The costs associated with infections and renal failures were predominantly driven by inpatient stay costs ($5540 and $3298 PPPM, respectively; Table 1). Conclusions: This real-world analysis suggests that there is high prevalence of AEs associated with CTs in late-line (≥3 CTs) HR+/HER2– mBC, which may lead to a substantial economic burden and suboptimal treatment outcomes. Newer, effective therapies with manageable AE profiles may offer a better benefit-to-risk profile and improve outcomes compared with chemotherapies. Citation Format: Anju Shah, Amy Sainski-Nguyen, Kristin Moore, MK Rehnquist, Rita Nanda. Costs associated with adverse events in patients receiving treatment for hormone receptor positive/human epidermal growth factor receptor-2 negative metastatic 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 PO2-05-03.

Abstract PO4-06-02: Frequency of presentation and outcomes of stage IV breast cancer over the past decade: a population-based study

Abstract Background: Over the past decade, there have been major advances for the treatment of patients (pts) with metastatic breast cancer (MBC). For pts with hormone receptor (HR) positive, human epidermal growth factor receptor 2 (HER2) negative MBC, newer treatments include CDK 4/6 inhibitors among other targeted therapies; for HER2+ MBC, we have seen the development of antibody-drug conjugates (ADC) and other anti-HER2 agents; and for triple-negative breast cancer (TNBC) options include immunotherapy, ADCs, and PARP inhibitors. The aims of this study were to evaluate time trends in clinical characteristics, frequency of presentation and outcomes of pts with stage IV breast cancer over the past decade. Methods: Using data from the Surveillance, Epidemiology, and End Results (SEER) program, we identified pts diagnosed with breast cancer between 2010-2020. Pts required to have known HR and HER2 status and known stage at diagnosis. We described the proportion of stage IV disease overall for each tumor subtype and evaluated trends in the proportion of stage IV over time. Outcomes were evaluated by breast cancer-specific survival (BCSS) using Kaplan-Meier. Among stage IV pts, we conducted multivariable cox models to assess changes in BCSS over time, stratified by tumor subtype and adjusted for age at diagnosis, race and ethnicity, sex, tumor grade, histology, surgery, radiation therapy, chemotherapy, sites of distant metastases, marital status, median household income, rurality and US region. We evaluated overall survival as a secondary endpoint. Results: We included 443,631 pts of whom 24,788 (5.59%) had stage IV disease at diagnosis. Pts with stage IV breast cancer had a median age of 61 years and a median follow up of 57 months (IQR 28 – 89 months). When compared to stage I-III, stage IV pts were more often of Black race (10.4% for stage I-III vs 15.7% for stage IV), less often married (56.7% vs 43.5%, respectively) and less often HR+/HER2- (74.1% vs 59.9%, respectively); all p < 0.001. Among pts with stage IV, the frequency of each metastatic site at initial diagnosis was: bone 66.45%, lung 30.47%, liver 25.79%, and brain 7.9%. Across all years (2010-2020), the proportion of stage IV disease varied significantly by tumor subtype: 4.6% in HR+/HER2-, 8.5% in HR+/HER2+, 11.3% in HR-/HER2+, and 7.1% in TNBC; p < 0.001. The proportion of stage IV increased modestly during 2010-2020 in HR+/HER2- tumors (4.6% in 2010 to 5.1% in 2020, p for trend = 0.01), in HR-/HER2+ (10% in 2010 to 13.7% in 2020, p for trend < 0.001), and in TNBC (6.9% in 2010 to 7.8% in 2020, p for trend = 0.001). No significant difference was observed in the proportion of stage IV over time in HR+/HER2+ (p for trend = 0.76). Rate of BCSS at 5 years was: 32.51% for HR+/HER2-, 43.92% for HR+/HER2+, 37.28% for HR-/HER2+, and 10.97% for TNBC; p < 0.001. In adjusted Cox models, BCSS improved significantly over time for each additional year in HR+/HER2- (adjusted Hazard Ratio [HzR] 0.982, p = 0.01), in HR+/HER2+ (adjusted HzR 0.945, p < 0.001), and in HR-/HER2+ (adjusted HzR 0.959, p = 0.03). The improvement in BCSS over time in TNBC was not significant (adjusted HzR 0.982, p = 0.15). Overall survival results were similar to those of BCSS both in 5-year rates as well as Cox model results. Conclusions: In this population-based study, we saw that the proportion of pts presenting with de-novo stage IV varied significantly by tumor subtype, being as low as 4.6% in HR+/HER2- and as high as 11.3% in HR-/HER2+ disease. These proportions have unfortunately increased over time in most subtypes, highlighting a need for additional research to identify the reasons behind these trends. BCSS improved significantly over time in HR+/HER2- and in both HER2+ subtypes, coinciding with a time period of important therapeutic advances. TNBC remains a highly unmet medical need and further follow up is needed to assess the impact of newer treatments. Citation Format: Jorge Avila, Julieta Leone, Nabihah Tayob, Carlos Vallejo, Michael Hassett, Nancy Lin, Sara Tolaney, Jose Leone. Frequency of presentation and outcomes of stage IV breast cancer over the past decade: a population-based study [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 PO4-06-02.

Abstract PO1-17-05: Real-world effectiveness of palbociclib plus aromatase inhibitors (AI) in metastatic breast cancer patients with cardiovascular diseases

Abstract Background Breast cancer survivors are at risk for mortality from cardiovascular diseases (CVD). CVD and their treatments can impact breast cancer treatment selection and clinical outcomes. A cyclin dependent kinase 4/6 inhibitor (CDK4/6i) combined with endocrine therapy (ET) is more effective than ET alone for hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2) metastatic breast cancer (MBC). CDK4/6i plus ET is now the standard of care in the first-line setting for HR+/HER2 MBC. However, data on the effectiveness of CDK4/6i in MBC patients with CVD are limited. We compared overall survival (OS) and real-world progression-free survival (rwPFS) of palbociclib plus AI (PAL+AI) vs AI alone in HR+/HER2 MBC patients with CVD in routine US clinical practices. Methods The Flatiron Health longitudinal database contains electronic health records from >280 cancer clinics, representing >3 million actively treated cancer patients in the US. Using the Flatiron database, we conducted a retrospective analysis of 469 patients with HR+/HER2 MBC and CVD who started PAL+AI or AI as first-line therapy between February 2015 and March 2020. CVD prior to the initiation of PAL+AI or AI were identified based on their definitions within the National Cancer Institute-Comorbidity Index (NCI-CI), including myocardial infarction, congestive heart failure, peripheral vascular diseases, and cerebrovascular diseases. Patients were assessed from start of PAL+AI or AI to September 30, 2020 (data cutoff date), death, or last visit, whichever came first. OS was defined as months from start of PAL+AI or AI to death. rwPFS was defined as months from start of PAL+AI or AI to death or disease progression, evaluated based on clinical assessment or radiographic scan/tissue biopsy. Stabilized inverse probability treatment weighting (sIPTW) as primary analysis was used to balance baseline demographics and clinical characteristics. Cox proportional-hazards models were used to estimate the relative effectiveness of PAL+AI vs AI alone. Results Of the 469 eligible patients, 160 (34.1%) were treated with PAL+AI and 309 (65.9%) were treated with AI alone. Compared with AI-alone patients, those treated with PAL+AI were younger and were more likely to have de novo MBC, ≥2 metastatic sites, and lung/liver involvement. After sIPTW, patient characteristics were generally balanced. After sIPTW, median OS (95% confidence interval [CI]) was 40.7 months (30.956.0) in PAL+AI patients and 26.5 months (23.337.3) in AI patients (hazard ratio [HR]=0.732, 95% CI=0.5370.997, p=0.0476). Median rwPFS (95% CI) was 20.0 months (11.7–27.5) in PAL+AI patients and 12.5 months (9.718.3) in AI patients (HR=0.679, 95% CI=0.5120.900, p=0.0070). Consistent results were observed with multivariate Cox proportional-hazards models as sensitivity analysis. See Table for baseline patient characteristics and outcome results. Conclusions First-line PAL in combination with AI is associated with prolonged OS and rwPFS in patients with HR+/HER2 MBC and CVD in a real-world setting compared with AI alone. Further studies with larger cohorts and comprehensive assessments of comorbidities are needed to provide additional evidence of outcomes and safety of CDK4/6i plus AI for MBC patients with various comorbid conditions in routine clinical practice. Citation Format: Adam Brufsky, Xianchen Liu, Benjamin Li, Lynn McRoy, Connie Chen, Doris Makari, Rachel Layman, Hope Rugo. Real-world effectiveness of palbociclib plus aromatase inhibitors (AI) in metastatic breast cancer patients with cardiovascular diseases [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 PO1-17-05.

Abstract PO3-16-10: Real-world outcomes with first-line ribociclib + endocrine therapy in patients with metastatic HR+, HER2– breast cancer: Fourth interim analysis of REACH AUT trial

Abstract Background: Ribociclib (RIB), a cyclin-dependent kinase 4 and 6 inhibitor (CDK4/6i) in combination with endocrine therapy (ET) yielded significant improvement in progression-free survival (PFS) and overall survival (OS) across three phase III MONALEESA trials including patients (pts) with hormone receptor–positive (HR+), human epidermal growth factor receptor 2–negative (HER2–) metastatic breast cancer (MBC) in first-line (1L) and second-line (2L) setting. In the previous interim analyses of REACH AUT trial, 1L RIB + ET showed tolerable safety and favorable PFS results in a real-world setting. Results from the preplanned fourth interim analysis are reported here. Methods: REACH AUT is a noninterventional study conducted in Austria reporting data on the real-world clinical experience of RIB + ET (aromatase inhibitor [AI] or fulvestrant [FUL]) in premenopausal, perimenopausal, or postmenopausal pts with HR+, HER2– MBC. Pts with no prior ET for advanced disease and maximum up to 1 prior line of chemotherapy (CT) in the advanced setting were included. Results: At data cutoff (March 27, 2023) with a median duration of follow-up of 23.8 months, out of 281 analyzed pts (12.5% of premenopausal/perimenopausal and 85.1% of postmenopausal, 2.5% of unknown status) 31.7% of pts were receiving ongoing treatment. The most common reasons for treatment discontinuations (68.3%) were disease progression (39.1%) and adverse events (AEs, 22.1%). The median age was 63 years (< 65 years, n=156, 55.5%; ≥65 years, n=125, 44.5%) and ECOG PS ≥2: 1.8%. In 106 pts (37.7%), de novo metastatic disease was reported. Visceral metastases (lung, liver) were present in 116 pts (41.3%) and bone-only metastases in 80 pts (28.5%). Prior adjuvant therapy was received by 139 pts (49.5%): AI ± GnRH: n=80 (26.7% without and 1.8% with GnRH); tamoxifen ± GnRH: n=74 (21.7% without and 4.6% with GnRH); CT: n=56 (19.9%); antibody therapy n=3 (1.1%). In the metastatic setting, 1L RIB was prescribed in combination with an AI in 223 pts (79.4%; letrozole: n=139 [49.5%]; anastrozole: n=48 [17.1%]; exemestane: n=47 [16.7%]) and with FUL in 52 pts (18.5%). Prior CT for MBC was received by 5 pts (1.8%). The median PFS was 29.7 months in the overall population; 28.3 months in pts with bone only and 26.9 months in pts with visceral metastatic disease. In pts evaluable for response, the objective response rate and clinical benefit rate were 31.3% and 60.9%, respectively. The OS rates were 91.9%, 81.7%, and 63.7% at 12 months, 24 months, and 36 months, respectively. The trial is ongoing with ~40% of subjects end of documentation and ~27% of deaths. In total, 69.4% of pts had RIB dose interruptions and 48.8% of pts had a RIB dose reduction. The majority of all reported AEs were grade 1 (50.1%) or grade 2 (31.2%); the median time to first AE was 0.5 months. The most common AE was neutropenia (all grades: n=143, 50.9%; grade 1: n=39, 13.9%; grade 2: n=68, 24.2%; grade 3: n=98, 34.9%; grade 4: n=5, 1.8%). QTc prolongation of any grade was observed in 32 pts (11.4%; grade 1: n=24, 8.5%; grade 2: n=11, 3.9%; grade 3: n=1, 0.4%). Hepatobiliary toxicity of any grade was observed in 42 pts (14.9%; grade 1: n=12, 4.3%; grade 2: n=18, 6.4%; grade 3: n=22, 7.8%; grade 4: n=3, 1.1%). Overall, currently 101 pts (35.9%) received a follow-up therapy after progression on RIB + ET. The most common first subsequent therapies were targeted therapies + ET (40.0%), followed by cytotoxic chemotherapy (22.0%) and endocrine monotherapy (19.0%). Conclusion: 1L RIB + ET continues to show favorable efficacy and a tolerable safety profile in the routine clinical practice. The results are consistent with the data from MONALEESA trials. The OS is showing a positive trend but is still immature during this study follow-up. Efficacy Outcomes With RIB + ET in Patients With HR+, HER2– MBC (Overall Population, Nf281) XX XX Citation Format: Christian F. Singer, Daniel Egle, Richard Greil, Edgar Petru, Leopold Oehler, Marija Balic, Laurenz Schöffmann, Georg Pfeiler, Maximilian Marhold, Christine Brunner, Karin Haider, Arik Galid, Ursula Pluschnig, Ferdinand Haslbauer, Michael Hubalek, Andreas Redl, Julia Flatschacher, Shanow Uthman, Bernhard Mraz, Rupert Bartsch. Real-world outcomes with first-line ribociclib + endocrine therapy in patients with metastatic HR+, HER2– breast cancer: Fourth interim analysis of REACH AUT trial [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 PO3-16-10.

Abstract PO2-04-09: Combination of the PLK1 Inhibitor Onvansertib and the PI3Kα Inhibitor Alpelisib Overcomes Palbociclib Resistance in PIK3CA-mutated HR+ Breast Cancer

Abstract In the 1st line setting of hormone receptor-positive HER2 negative (HR+/HER2-) metastatic breast cancer, cyclin dependent kinase 4 and 6 (CDK4/6) inhibitors (palbociclib, abemaciclib and ribociclib) are typically used in combination with endocrine therapy. Though initially effective, acquired resistance to either CDK4/6 inhibitors or endocrine therapy eventually leads to disease relapse in most patients. Activating mutations in PIK3CA, the gene encoding alpha catalytic subunit of phosphatidylinositol-4,5-bisphosphat 3-kinase (PI3K) are found in approximately 40% of HR+ breast cancer patients and have been implicated in mediating resistance to CDK4/6 inhibitors. The PI3Kα inhibitor alpelisib is currently approved for PI3KCA-mutant HR+ breast cancer after progression on CDK4/6 inhibitors. However, drug-associated resistance mechanisms may limit its clinical benefit. Synergistic combination therapies have the potential to improve efficacy and overcome resistance mechanisms. Polo-like kinase 1 (PLK1) is a serine-threonine-protein kinase, key cell cycle regulator that controls G2/M entry and mitotic progression, and that has been shown to mediate resistance to palbociclib in HR+ breast cancer. Onvansertib is an oral and highly specific PLK1 inhibitor currently on clinical development in solid tumors and hematological malignancies. Here we evaluated the potential of onvansertib to increase the activity of alpelisib in PIK3CA-mutant HR+ breast cancer preclinical models. The combination of onvansertib and alpelisib synergistically inhibited cell viability and/or colony formation in three PI3KCA-mutant HR+ breast cancer cell lines, MCF-7 (PIK3CA E545K), EFM-19 (PIK3CA H1047L), and T-47D (PIK3CA 1047R). A significant increase in apoptosis was observed in cells treated with the combination compared to either agent alone. We next tested the combination of onvansertib and alpelisib in patient-derived xenograft (PDX) models. For this purpose, palbociclib-resistant PIK3CA-mutant HR+ breast cancer PDXs were established from primary breast tumor (PDX HBCx-86, PIK3CA E545K) or from metastatic bone biopsies of patients who had progressed on endocrine therapy plus palbociclib (HBCx-180, PIK3CA H1047R) or on PI3Kα inhibitor (HBCx-134palboR31, PIK3CA H1047R). PDX tumors were grafted subcutaneously in nude mice and mice were treated with vehicle, onvansertib (45 mg/kg, oral, 5 days a week), alpelisib (25mg/kg, oral, 5 days a week) or the combination. The combination was well tolerated and showed superior anti-tumor activity than the single agents in the 3 PDX models. In the HBCx-86 PDX, although none of the single agents showed activity, the combination induced potent tumor growth inhibition. In the metastasis-derived PDX HBCx-134palboR31, the combination treatment induced pronounced tumor regression in 62% of mice (5/8), with complete response in 25% (2/8), while mice treated with the single agents showed tumor progression. Finally, in the HBCx-180 PDX, established from a patient with primary resistance to palbociclib, the efficacy of the combination was greater and more durable, with significant survival advantage, compared to the monotherapies. Collectively, our preclinical findings suggest that co-targeting PLK1 and PI3Kα with onvansertib and alpelisib respectively, may constitute a promising therapeutic combination strategy for patients with PIK3CA-mutant HR+ breast cancer failing to respond to first-line standard of care therapies. Citation Format: Sreeja Sreekumar, Pierre Painsec, Davis Klein, Dalia Gonzalez, Laura Sourd, Elodie Montaudon, Tod Smeal, Elisabetta Marangoni, Maya Ridinger. Combination of the PLK1 Inhibitor Onvansertib and the PI3Kα Inhibitor Alpelisib Overcomes Palbociclib Resistance in PIK3CA-mutated HR+ 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 PO2-04-09.

Abstract PO3-05-03: Real-World Data on First-line Treatment of Hormone Receptor-positive, HER2-negative, Metastatic Breast Cancer in Brazil (BRAVE study - LACOG 0221)

Abstract Introduction: Endocrine therapy (ET) plus CDK 4/6 inhibitors is considered the standard of care as first-line therapy in hormone receptor positive (HR+)/HER2 negative (HER2-) metastatic breast cancer (mBC). Nonetheless, the use of chemotherapy (CT) is frequent in clinical practice, and access to CDK 4/6 inhibitors is limited in some countries. We used real-world data to describe treatment patterns, the impact of health insurance type, and outcomes among Brazilian patients with HR+/HER2- mBC treated in the first-line setting. Methods: In this observational retrospective study, female patients ≥ 18 years old diagnosed with de novo or recurrent HR+/HER2- mBC between January 2018 and December 2020 were included. Patients' data were extracted from medical charts or electronic health records (EHR). The primary endpoint was first-line treatment patterns in public and private health system cohorts. Key secondary endpoints were progression-free survival (PFS), defined as time from 1L treatment to progression or death, and overall survival (OS), defined as time from diagnosis of mBC until death or censoring. No formal statistical assumption was performed for sample size, the study recruited 2 cohorts of 150 patients from the public and 150 patients from the private health insurance system. Results: A total of 307 patients were included, 156 from public and 151 from private health insurance system, from 13 sites in Brazil. Median age was 56.8 (range 19.9-94.1), and 32.2% (N=99) had de novo MBC. Treatment patterns are described in Table 1. The most frequent treatment received in 1L was ET alone 42.4%, followed by CT (alone or CT followed by ET +- CDK4/6i) 31.6% and ET + CDK4/6i 25.1%. There was a significant difference between the 1L treatment with ET + CDK4/6i in the private and public systems, 48.3% vs 2.6%, p < .0001, respectively. The most frequently used CDK4/6i in 1L setting was Ribociclib (65.0%), followed by Palbociclib (36.9%) and Abemaciclib (7.1%). There was an increase in the use of CDK4/6i over time, 17% in 2018, 30.0% in 2019, and 27.9% in 2020. With a median follow-up of 32.2 months (95% CI: 29.4 - 34.4), the median 1L PFS was 22.9 months (95% CI: 16.9 - 25.2) in the public and 21.8 months (95% CI: 17.7 - 31.1) in the private health system, p=0.92. Median 1L PFS by treatment received was 13.6 months (95% CI: 6.1-39.0) with CT, 18.4 months (95% CI: 12.1-22.1) with CT followed by ET +-CDK4/6i, 21.9 months (95% CI: 17.3-25.5) with ET and 31.1 months with ET plus CDK 4/6i (95% CI: 15.8-42.8). Overall, median OS was 52.8 months (95%CI 49.4-NR). The 4-year OS rate per 1L treatment received were 50.3% (95% CI: 28.7-68.5) with CT, 39.0% (95% CI: 13.9-63.8) CT followed by ET +- CDK4/6i, 74.4% (95% CI: 58.0-85.1) with ET alone, 71.3% (95% CI: 49.6-85.0) ET + CDK4/6i, 50.3% (95% CI: 28.7-68.5) with CT and 39.0% (95% CI: 13.9-63.8) CT followed by ET +- CDK4/6i. Conclusion: ET is the most commonly used 1L treatment in clinical practice in Brazil, followed by ET + CDK4/6i and CT. A large difference in CDK4/6i use was observed between private and public health systems, reflecting the current limited access to CDK4/6i in Brazil. This study highlights treatment disparities within a nation with a fragmented health insurance system. Table 1. Treatment Patterns in Public and Private Healthcare Systems Citation Format: Gustavo Werutsky, Daniela Rosa, Tomás Reinert, Romualdo Barroso-Sousa, Heloísa Resende, Poliana Signorini, Eduardo Cronenberger, Juliana Góes Martins Fagundes, Jorge Henrique Santos Leal, Marina Dutra Giffoni, Aline Vieira, Luiza Nardin Weis, Jose Marcio Figueiredo, Tatiana Correa, Fernando Castilho Venero, Luisa Mostardeiro Tabajara Franche, Marina Musse Bernardes, Rafaela Jesus, Gustavo Gössling, José Bines. Real-World Data on First-line Treatment of Hormone Receptor-positive, HER2-negative, Metastatic Breast Cancer in Brazil (BRAVE study - LACOG 0221) [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 PO3-05-03.

Abstract PO3-06-09: A phase II study of nivolumab plus ipilimumab and androgen receptor antagonist bicalutamide to stimulate thymic T cell generation in HER2-negative metastatic breast cancer

Abstract Background: Systemic chemotherapy is used commonly in breast cancer and is associated with lymphopenia, potentially limiting efficacy of immune checkpoint blockade. The androgen receptor (AR) serves as a negative regulator of thymic function. AR antagonists, such as bicalutamide, may stimulate thymic production of naïve T-cells, which may help to restore lymphocyte diversity and augment immunotherapy response. The AR is expressed in 50% of hormone receptor negative (HR-) metastatic breast cancer (MBC) and >75% of HR+ MBC, and bicalutamide is clinically active with stable disease as best response. We hypothesize that bicalutamide could be combined with dual immune checkpoint blockade (anti-programmed death 1, nivolumab; plus anti-cytotoxic T-lymphocyte antigen 4, ipilimumab), stimulating T-cell production and resulting in durable clinical response. Methods: This is a phase II Simon 2-stage trial of nivolumab (240 mg IV q2w), ipilimumab (1 mg/kg IV q6w), and bicalutamide (150mg oral daily) for first- or second-line treatment of HER2-negative MBC (NCT03650894). The primary endpoint is iRECIST 24-week clinical benefit rate (CBR), evaluated separately for HR+ MBC (n=15) and AR+/HR- MBC (n=15). An optimum Simon 2-stage design (80% power, 5% one-sided alpha) is employed to evaluate an alternative hypothesis of >50% CBR compared to historical control of < 30% CBR for conventional chemotherapy, with expansion of each cohort planned if ≥6/15 responses are observed. Secondary outcomes include best overall objective response rate (ORR), progression-free and overall survival (PFS,OS), and safety. Exploratory endpoints include assessment of peripheral T-cell counts and T cell receptor excision circles (TREC PCR, Mayo Laboratories, a surrogate of thymic T-cell production) over time. Results: Durable responses were observed, with an ongoing complete response at 41+ months in a patient who discontinued due to toxicity (AR+/HR- cohort, PD-L1 CPS score 3%). Therapy was tolerated with a toxicity profile consistent with prior studies of ipilimumab & nivolumab, with five subjects (19%) requiring treatment discontinuation related to toxicity. Outcomes by cohort and PDL1 are summarized in Table 1. Using mixed effects linear models, therapy was associated with a significant expansion of CD8+ T cells (23/mcL/month, p< .01) but not CD3+ T cells (7/mcL/month, p=.59) or CD4+ T cells (4/mcL/month, p=.65). Baseline TREC counts were higher in younger participants (mean: < 50y 2156/mcL; 50-65y 961/mcL; >65y 311/mcL). TRECs did not increase globally, however TREC expansions were observed in several younger participants. Conclusions: The regimen of ipilimumab, nivolumab, plus bicalutamide is safe and clinically active in 1st/2nd-line HER2-negative MBC. Neither cohort crossed the Simon futility barrier for cohort expansion, however durable responses were observed including in PD-L1-negative patients, highlighting the unmet need for biomarkers to identify candidates for chemotherapy-sparing checkpoint blockade. Further research is indicated to evaluate the impact of AR antagonists on T-cell function and thymic stimulation in MBC. Acknowledgements: Drug and study support was provided by Bristol-Myers Squibb and The BMS International Immuno-Oncology Network (II-ON). Clinical outcomes XX XX HR: hormone receptor; AR: androgen receptor; CPS: PD-L1 combined positive score Citation Format: David Page, Alexander Hall, Krystle Collins, Nicole Moxon, Staci Mellinger, Tracy Kelly, Aysha Kelley, Nicole Fredrich, Amanda Seino, Gabriella Liberatore, Alison Conlin, Zheng Topp, Kelly Perlewitz, Sasha Stanton, Walter Urba, Yaping Wu, Maritza Martel, Zhaoyu Sun, Yoshinobu Kogushi, William Redmond, Tiffany Traina, Ayca Guculp. A phase II study of nivolumab plus ipilimumab and androgen receptor antagonist bicalutamide to stimulate thymic T cell generation in HER2-negative metastatic 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 PO3-06-09.

Abstract PO4-04-10: Selection criteria for second-line treatments after Cyclin-dependent kinase 4/6 inhibitors failure in Hormone Receptor positive metastatic breast cancer patients: the preliminary experience of the HERMIONE-13 trial

Abstract BACKGROUND: Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) represent the standard of care for hormone receptor-positive (HR+)/Human Epidermal Growth Factor 2 negative (HER2-) metastatic breast cancer (MBC); no consensus actually exists for second-line (2L) treatments. The HERMIONE-13 trial evaluates the adopted therapeutic options after CDK4/6i failure and the potential factors that influence these choices. METHODS: HERMIONE-13 is a retrospective and prospective multicentric observational trial involving 15 Italian Centers. Information about therapies after disease progression to CDK4/6i from January 2016 until December 2020 was collected for the present analysis, aiming at evaluating which are the clinical criteria guiding the choice of 2L therapy after CDK4/6i failure. RESULTS: 114 pts have been enrolled: 14 pts (12.3%) received neoadjuvant treatment, 80 (70.2%) adjuvant therapy, of whom 46 (57.5%) chemotherapy (CT)+ endocrine therapy (ET) and 33 (41.3%) ET alone. Among the 79 pts treated with adjuvant ET, 44 pts (55.7%) showed primary or secondary endocrine resistance. 39 pts (34.2%) received first-line (1L) CDK 4/6i in combination with Fulvestrant (F), the remaining with aromatase inhibitors (AIs). Median duration of 1L CDK4-6i was 10.4 months (interquartile range (IQR) 4.7-17.4). Median age at 2L start was 59.5 years (IQR 51-68). As 2L therapy, 79 pts (69.3%) received CT±targeted therapy (TT) and 35 (30.7%) ET±TT. Visceral metastases at the beginning of 2L were documented in 46 pts (40.4%). Comorbidities were present in 48 (42.9%) of the pts, mainly cardiac diseases (25, 22.3%); 71 pts (63.4%) showed a 1L-related adverse events (AEs) (NA: 2 pts), mainly hematological toxicity (60, 53.6%). Among 79 pts treated with adjuvant ET, 60 (76%) were addressed to 2L CT±TT, while 19 (24%) to an ET-based therapy. A longer duration of 1L was observed in pts treated with 2L ET±TT (12.5 months, 7.2-22.2) in comparison with those treated with CT±TT (8.7 months, 4.2-14.9). The majority of pts treated with 1L F+CDK 4/6i underwent 2L CT-based treatment, as summarized in Table 1. Analyzing the population treated with 2L CT±TT, we observed that median age was 58 years (49-67), slightly lower than that of pts treated with ET±TT (63, 54-69). Among pts with visceral metastases, 30 pts (65.2%) were addressed to 2L CT±TT and 16 pts (34.8%) to ET±TT; CT±TT was also the favorite choice for the majority of pts with concomitant disease (33, 68.8%) and with previous CDK4/6i-related toxicity (47, 66.2%). Among age, visceral metastases, comorbidities, adjuvant treatment, duration of 1L therapy, use of 1L F and 1L related AEs, univariate analysis revealed that previous use of adjuvant ET (p=0.021), 1L F (p< 0.001), duration of 1L treatment (p=0.043) and age at 2L start (p=0.053) are all factors influencing the choice of 2L therapy. At the multivariate logistic regression model, only the use of F in association with CDK4/6i as 1L treatment and age at 2L therapy were associated with the choice between ET±TT vs CT± TT, with a backward selection, as summarized in table 2. CONCLUSION: The preliminary results of our study suggest that previous F in combination with CDK4/6i is a statistically confirmed predictor of choice for subsequent CT, whereas the increase in age is significantly associated with the choice for ET. Further research is needed to investigate the optimal sequencing of treatments following CDK4/6i and to determine predictive factors of response to 2L therapies. Table 1 2L choices in 1L F+CDK 4/6i pts. Table 2 Statistical model with the significant variables. Citation Format: Marina Elena Cazzaniga, Emanuela Rossi, Antonella Turla, Massimo Ambroggi, Alice Baggi, Rossana Berardi, Fulvio Borella, Serena Capici, Federica Cicchiello, Luigi Coltelli, Ugo De Giorgi, Antonella Ferro, Ornella Garrone, Monica Giordano, Elisabetta Landucci, Marco Mazzotta, Gabriella Moretti, Raffaella Palumbo, Francesca Fulvia Pepe, Patrizia Vici, Fable Zustovich, Viola Cogliati. Selection criteria for second-line treatments after Cyclin-dependent kinase 4/6 inhibitors failure in Hormone Receptor positive metastatic breast cancer patients: the preliminary experience of the HERMIONE-13 trial [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 PO4-04-10.

Abstract PO1-04-10: Body Mass Index and Treatment Efficacy in advanced Luminal Breast Cancer: Insights from the GEICAM/2013-02 (PEARL) trial

Abstract Background: Endocrine therapy (ET) and treatment directed against molecular targets (CDK 4/6, PI3K, mTOR) constitute the basis of initial treatments for patients with advanced luminal breast cancer. Most of these treatments are oral and have been developed at a fixed dose, without weight adjusted dosing. There is little evidence on the influence that patients’ weight might have on clinical outcomes of these oral drugs in the context of a randomized clinical trial. The present study analyzes the potential impact of the weight of patients treated in the two cohorts of the PEARL trial (NCT02028507) on Progression-Free Survival (PFS), Overall Survival (OS) and the incidence of side effects in each treatment arm. Methods: GEICAM/2013-02 (PEARL) is a multicenter, phase III randomized clinical trial that enrolled patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative metastatic breast cancer (MBC) who progressed to a prior aromatase inhibitor. Patients were randomized 1:1 to receive ET + palbociclib or capecitabine. Patient weight was assessed based on baseline body mass index (BMI) categories: normal (BMI ≤25 Kg/m2), overweight (BMI 25-29.9 Kg/m2) and obesity (BMI ≥30 Kg/m2). Cox's stratified proportional hazard model was used for survival analysis. Adverse events were assessed and graded according to National Cancer Institute common terminology criteria for adverse events (NCI-CTCAE) version 4.0. Results: A total of 600 patients were included in the study and all were evaluable for the weight-based analysis, including 301 in the ET +palbociclib arm and 299 in the capecitabine arm. The distribution of patient’s weights is shown in Table 1. For the Intention-to-treat (ITT) population, no significant difference in the median OS was found between the different weight categories (Normal: 32.8 months vs overweight: 31.9 months, [p=0.467]; obese: 32 months [p=0.407]). Specifically, for patients with normal weight, the median PFS was 9.3 months in the ET + palbociclib group compared to 9.0 months in the capecitabine group (hazard ratio (HR) 0.83, 95% confidence interval [CI]: 0.61-1.13; p=0.23). In the overweight group, the PFS was 7.5 months in the ET+palbociclib group and 11.1 months in the capecitabine group (HR 1.17, 95% CI: 0.87-1.58; p=0.30) while for the obese group, the PFS was 5.7 months vs 11.7 months (HR 1.14, 95% CI: 0.82-1.59; p=0.43), respectively. No substantial differences in the incidence of grade ≥ 3 events were observed when considering weight categories in the different treatment groups (capecitabine arm (Normal: 60%, Overweight: 70.4%, Obese: 61.6%) and ET+palbociclib arm (Normal: 74.5%, Overweight: 76.1%, Obese: 51.6%). Conclusions: Our findings, despite not finding statistically significant differences, suggest a potentially clinically relevant trend towards improved PFS with capecitabine in overweight and obese patients compared to those receiving palbociclib plus ET. The impact of weight on the outcomes with fixed-dose oral drugs warrants further investigation in future studies. Table 1. Citation Format: Manuel Alva, Sara López-Tarruella, Christoph Zielinski, Manuel Ruíz - Borrego, Nicholas Turner, Eva Ciruelos, Montserrat Muñoz, Begoña Bermejo, Mireia Margelí, Antonio Antón, Zsuzsanna Kahan, Tibor Csöszi, Maribel Casas, Laura Murillo, Serafin Morales Murillo, Emilio Alba, Einav Nili-Gal Yam, Angel Guerrero, Lourdes Calvo, Juan de la Haba-Rodríguez, Manuel Ramos, Isabel Álvarez, Andrés García-Palomo, María Koehler, José Ángel García-Sáenz, J. Ignacio Chacón, Claire Swift, Christiane Thallinger, Miguel Gil-Gil, Miguel Martín. Body Mass Index and Treatment Efficacy in advanced Luminal Breast Cancer: Insights from the GEICAM/2013-02 (PEARL) trial [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 PO1-04-10.

Abstract PS15-04: A Phase 1b/2 study of palazestrant (OP-1250), an oral complete estrogen receptor antagonist (CERAN) and selective ER degrader (SERD), with palbociclib in ER-positive, HER2-negative, advanced or metastatic breast cancer patients

Abstract Background: Palazestrant (OP-1250) is a small molecule oral complete estrogen receptor (ER) antagonist (CERAN) and selective ER degrader (SERD) that binds the ligand binding domain of ER and completely blocks ER-driven transcriptional activity in both wild-type (ESR1-wt) and mutant (ESR1-mut) forms of ER. In preclinical studies, palazestrant demonstrated improved tumor growth inhibition and shrinkage when combined with a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor. In a Phase 1/2 monotherapy clinical study in patients with ER-positive, human epidermal growth factor receptor 2-negative metastatic breast cancer, palazestrant was well tolerated with demonstrated antitumor efficacy and favorable pharmacokinetics (PK) supporting once a day (qd) dosing at the recommended Phase 2 dose of 120 mg. In this study, pts received palazestrant at escalating doses of 30, 60, 90, and 120 mg qd in combination with palbociclib (125 mg qd) and no dose-limiting toxicities were observed. Palazestrant did not affect palbociclib PK and no drug–drug interactions (DDIs) occurred. Here we report updates on safety, efficacy, and PK from the ongoing study of palazestrant in combination with palbociclib. Methods: Patients with advanced or metastatic breast cancer with progression on or after ≤1 line of endocrine therapy (prior CDK4/6 inhibitor therapy and ≤1 chemotherapy allowed) were enrolled to receive oral palazestrant 120 mg qd with oral palbociclib 125 mg qd for 21 of 28 days (NCT0526610). Results: As of May 12, 2023, 33 patients have been treated with palazestrant in combination with palbociclib; 12 pts were dosed in the dose-escalation phase and 21 patients were dosed in the dose-expansion phase of the study. Twenty-two patients (67%) received prior CDK4/6 inhibitor therapy; 15 patients received prior palbociclib. Thirteen of 25 patients (52%) with available baseline ctDNA had an ESR1 mutation. The most common (≥20%) treatment-emergent adverse events (TEAEs) were neutropenia, nausea, vomiting, constipation, and diarrhea. Most of these events were grade 1 or 2. The most common grade 3 or 4 related AE was neutropenia (18/33 [55%] were grade 3; 3/33 [9%] were grade 4). The steady-state exposure of palazestrant when dosed with 125 mg palbociclib was consistent with the monotherapy study. Palbociclib exposure at steady-state was comparable to published monotherapy data when combined with palazestrant at all dose levels tested. As of the data cut-off, there were 4 partial responses (2 confirmed) out of 19 response-evaluable patients, with a clinical benefit rate (CBR) of 42% (8/19) across all patients and 67% (6/9) in patients with ESR1 mutations. Conclusions: Palazestrant and palbociclib dosed in combination were well tolerated with a safety profile consistent with the individual profiles of each drug as monotherapy, and there were no DDIs. Tumor responses and clinical benefit were observed in this population of patients, including those who received prior CDK4/6 inhibitors. Expanding on our previous report, these data provide the rationale to continue advancing the clinical development of palazestrant with the approved dose of palbociclib. Updated data will be presented. Citation Format: Arlene Chan, Daphne Day, Phuong Dinh, Michael Slancar, Janine Lombard, Vinod Ganju, Nicole McCarthy, Rosalind Wilson, Demiana Faltaos, Gurpreet Mathauda-Sahota, Caitlin Murphy. A Phase 1b/2 study of palazestrant (OP-1250), an oral complete estrogen receptor antagonist (CERAN) and selective ER degrader (SERD), with palbociclib in ER-positive, HER2-negative, advanced or metastatic breast cancer patients [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 PS15-04.