HER2-negative Metastatic Breast Cancer Research Articles

Abstract PO4-04-12: A Phase 1b/2 study of palazestrant (OP-1250) in combination with ribociclib or alpelisib in patients with estrogen receptor-positive, human epidermal growth factor receptor 2-negative, advanced and/or metastatic breast cancer

Abstract Background: Endocrine therapy (ET) when administered with cyclin-dependent kinase 4/6 (CDK4/6) inhibitors has improved outcomes in patients (pts) with estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer (BC) or metastatic breast cancer (MBC) and is the current standard of care for first-line treatment. Addition of the PIK3 inhibitor alpelisib to ET significantly improved progression-free survival in pts with PIK3CA-mutated, ER-positive, HER2-negative MBC, and alpelisib + fulvestrant is the standard of care for second or later-line treatment. Most pts will acquire resistance to ET, with mutations in ESR1 constituting the most common mechanism of resistance. 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 in combination with ribociclib demonstrated activity in both ESR1-wt and ESR1-mut xenograft models and showed efficacy in brain metastasis xenograft models. In a Phase 1/2 monotherapy study in pts with ER-positive, HER2-negative MBC, palazestrant was well tolerated with demonstrated antitumor efficacy and favorable pharmacokinetics (PK) supporting once a day (qd) dosing at the recommended Phase 2 dose (RP2D) of 120 mg. The aim of this study is to evaluate the safety, PK, and antitumor activity of palazestrant + ribociclib or alpelisib in pts with ER-positive, HER2-negative advanced BC or MBC. Methods: Eligible pts have evaluable ER-positive, HER2-negative advanced BC or MBC; ≤2 prior ETs (prior CDK4/6 inhibitors allowed); and ≤1 prior line of chemotherapy. In the alpelisib arm, pts must have a PIK3CA mutation in the tumor or ctDNA. Patients are administered oral palazestrant at escalating doses of 30, 60, and 120 mg qd in combination with the approved dose of oral ribociclib (600 mg qd; days 1–21 of 28-day cycle) or approved dose of oral alpelisib (300 mg qd) in a 3+3 design to identify the RP2D. The dose-expansion part will assess additional safety and PK parameters and the antitumor activity of palazestrant in combination with ribociclib or alpelisib (NCT05508906). Results: As of June 15, 2023, 10 pts have received 30 mg and 60 mg doses of palazestrant in combination with ribociclib (n=6) or alpelisib (n=4). No dose-limiting toxicities have been observed. After administration in the first two combination dose levels of 30 and 60 mg, palazestrant exposure was consistent with monotherapy administration; exposure data show no drug–drug interactions (DDI) when compared to published exposure parameters. Most reported treatment-emergent adverse events (TEAEs) were grade 1 or 2 and consistent with the known safety profiles of all 3 drugs. In the ribociclib arm, the TEAEs occurring in ≥2 pts included grade 1 nausea (n=4), grade 1 constipation (n=2), and neutropenia (grade 2, n=1; grade 3, n=3). For the 3 pts in the 30 mg palazestrant + alpelisib cohort, the only TEAE occurring in ≥2 pts was grade 1 diarrhea. One pt had grade 1 hyperglycemia. Conclusions: In this ongoing study, palazestrant in combination with ribociclib or alpelisib was well tolerated, and enrollment to the palazestrant 120 mg dose with ribociclib or alpelisib is ongoing. No new safety signals were observed for the 3 drugs, and no clinically significant DDIs were observed between palazestrant and ribociclib or alpelisib at the doses evaluated. Exposure of each drug was consistent with observed monotherapy exposure levels. Updated data will be presented. Citation Format: Virginia Borges, Carlos Alemany, Nancy Lin, Sara Nunnery, Cynthia Ma, Margaret Tonda, Morena Shaw, Arlene Chan. A Phase 1b/2 study of palazestrant (OP-1250) in combination with ribociclib or alpelisib in patients with estrogen receptor-positive, human epidermal growth factor receptor 2-negative, advanced and/or 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 PO4-04-12.

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 PO4-04-13: Impact of dose reduction of CDK4/6 inhibitors on progression-free survival in Mexican patients with hormone receptor-positive, HER2-negative metastatic breast cancer: a retrospective single-center study

Abstract Background Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) have been demonstrated to prolong survival in women with hormone receptor-positive, HER-2 negative, metastatic breast cancer. However, these treatments often require dose adjustments due to adverse effects, resulting in a reduction in 30-50% of cases. There is evidence suggesting a relationship between dose intensity and progression-free survival (PFS), but data in the Hispanic population are limited. Therefore, the objective of the study was to explore the association between dose reduction of CDK4/6/i inhibitors and PFS in patients with metastatic breast cancer. Methods We conducted a retrospective single-center study involving patients diagnosed with hormone receptor-positive, HER-2 negative metastatic breast cancer (MBC) between 2020 and 2023. Eligible patients were those who received at least one cycle of palbociclib, abemaciclib, or ribociclib and had a follow-up period of more than three months since the initiation of CDK4/6 inhibitor (CDK4/6i) treatment. Patient data were collected from electronic medical records. We analyzed demographic and clinicopathological characteristics and estimated the PFS in patients who had any dose reduction within the first 12 weeks compared to those who did not, using Kaplan-Meier plots and compared the differences in survival using the log-rank test. A Cox regression analysis was performed to estimate survival hazard ratios (HR). Results Among 41 patients, the mean age at CDK4/6i initiation was 61.0 years (SD 12.9), 22 (53.7%) were postmenopausal at diagnosis, 25 (61%) had recurrent disease, and 22 (53.7%) were receiving CDK4/6i in the first-line setting. Bone metastases were present in 30 (73.2%), and visceral metastases in 22 (53.7%). Regarding type of CDK4/6i, 33 (80.5%) received palbociclib, 7 (17.1%) ribociclib, and 1 (2.4%) abemaciclib. At 12 weeks, 12 patients (29.3%) had a dose reduction (Table 1). With a median follow-up since CDK4/6i initiation of 407 days (95% confidence interval (CI) 272-542), 19 events were recorded, with a median PFS of 12.6 months (95% CI 8.0-17.3) for the overall population. Median PFS was 9.7 months (95% CI 0.6-18.8) in patients who had a dose reduction at 12 weeks and 35.2 months (95% CI 0-72.2) in those who did not (p=0.014). The univariate Cox proportional hazard model showed that dose reduction was associated with a worse PFS (hazard ratio 3.1; 95% CI: 1.20–7.80, p = 0.0019) Conclusion In this cohort of Mexican patients with hormone receptor-positive, HER2-negative metastatic breast cancer we found that dose reduction of CDK4/6 inhibitors within the first 12 weeks of treatment was associated with worse PFS. Given the retrospective nature of this analysis, we believe that these findings should be replicated in prospective studies. Table. Patient characteristics Citation Format: Andres Meraz-Brenez, Alejandro Aranda-Gutierrez, Hector Raul Gonzalez-Sanchez, Bertha Alejandra Martinez-Cannon, Leonardo Verduzco-Rodriguez, Haydee Cristina Verduzco-Aguirre. Impact of dose reduction of CDK4/6 inhibitors on progression-free survival in Mexican patients with hormone receptor-positive, HER2-negative metastatic breast cancer: a retrospective single-center 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 PO4-04-13.

Abstract PO4-05-07: Differential dynamics of fragmentomic and epigenetic circulating tumor DNA (ctDNA) features in first-line Palbociclib and Ribociclib treatment for Hormone Receptor (HR) positive, HER2 negative metastatic breast cancer (MBC)

Abstract Background: Cyclin-dependent kinase inhibitors (CDK4/6i) in combination with endocrine therapy (ET) represent the cornerstone for the management of HR positive, HER2 negative MBC. However, the lack of prospective cohorts comparing the distinctions among palbociclib (PAL), ribociclib (RIB) and abemaciclib (ABE) poses a significant clinical knowledge gap. The aim of this ancillary analysis was to explore the differential clinical impact of CDK4/6i by leveraging new ctDNA biomarkers in a prospective clinical study. Methods: A total of 114 patients (pts) with HR positive, HER2 negative MBC were prospectively enrolled in the multicenter MAGNETIC.1 study (CRO-2018-56) from January 2018 to January 2023. At baseline (BL) and every 3 months (mos.), ctDNA samples were collected concurrently with radiological restaging. The samples were then analyzed through methylation-specific (MS) droplet digital PCR (ddPCR), ddPCR fragmentomics, and next generation sequencing. Matched pairs of variations were tested through Wilcoxon signed rank test, and survival was analyzed by log-rank test. Results: Out of the 114 pts enrolled, 59 received PAL, 48 RIB, and 16 ABE. In the de novo subgroup, 51.4 % of pts were treated with PAL, 42.9% with RIB, and 5.7% with ABE. Among the pts who received PAL, 7 harbored an ESR1 mutation at BL (D538G: 2, H377R: 1, Y537N: 1, Y537S: 3). Additionally,12 pts had PIK3CA mutations, and 2 pts had AKT1 mutations. In the RIB group, 5 and 2 pts, respectively, had a PIK3CA- and AKT-mutated MBC, while no ESR1 mutations were detected. When comparing matched pairs of variations, it was found that at BL, at first radiological restaging (T1), and at 6 mos. after initiation of CDK4/6i (T2), only PAL showed significantly higher levels of promoter A and B methylation (promA and promB) at T1 (P = 0.0005 and P = 0.0056, respectively for promA and promB), A significant decrease in promA and promB levels was detected at T2 for PAL (P = 0.0005 and P < 0.0001 respectively for promA and promB) but not for RIB. Both PAL and RIB demonstrated a significant decrease in ctDNA short fragments at T1 (P = 0.0263 for PAL and P = 0.0001 for RIB), However, PAL showed a significant rebound at T2, with an increase in 80% of samples (P = 0.0014). The same was not seen with RIB. After a median follow-up of 34.8 mos., there were no significant differences between PAL and RIB in terms of PFS (P= 0.2573) and OS (P= 0.3783). The PFS rates at 12 and 24 mos. were 69% and 48% for PAL, and 76% and 64% for RIB. The OS rates at 12 and 24 mos. were 94% and 84% for PAL, compared to 97% and 82% of RIB, respectively. Conclusions This analysis of MAGNETIC.1 highlighted significant differences in ctDNA biomarkers between palbociclib and ribociclib, suggesting their potential impact on the development of endocrine resistance. Further biomarker-driven clinical trials leveraging this concept are needed to refine the sequencing algorithms for CDK4/6i. Citation Format: Claudia Noto, Linda Cucciniello, Elisabetta Molteni, Silvia Bolzonello, Alessandra Franzoni, Lucia Da Ros, Silvia Buriolla, Arianna Dri, Lorenzo Foffano, Simon Spazzapan, Stefania Russo, Brenno Pastò, Giulia Cudia, Giada Targato, Serena Della Rossa, Marta Bonotto, Alessandro Marco Minisini, Giuseppe Damante, Barbara Belletti, Lorenzo Gerratana, Fabio Puglisi. Differential dynamics of fragmentomic and epigenetic circulating tumor DNA (ctDNA) features in first-line Palbociclib and Ribociclib treatment for Hormone Receptor (HR) 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 PO4-05-07.

Abstract PO1-09-09: The journey of HR positive, HER2 negative metastatic breast cancer’s patients: heterogeneities and barriers in Brazilian public health system- a national survey

Abstract Background: Breast cancer is the most common neoplasm in Brazilian women, with high percent of patients diagnosed in advanced stages. Metastatic breast cancer (mBC) is a challenge in the country, where 75% of the population is covered by public health system (Sistema Único de Saúde, SUS). We conducted an on-line survey to describe the journey of human epidermal growth factor receptor 2-negative (HER2-), hormone receptor positive (RH+) mBC patients across public scenario in Brazil. Methods: An on-line survey comprised of 39 to 48 (depending on the kind of some questions, which could open another question in case of affirmative answer) objective questions were sent by e-mail to180 oncologists working in public and private health care system. Questions were focused on assessing wait time for first treatment after admission in the hospital in both systems, availability of oncological drugs within SUS according to essential medicines list by World Health Organization, and presence of palliative and multidisciplinary teams in both systems. Continuous variables were measured by median and mean. The level of statistical significance adopted was 5% with Teste p two-sided. Analyses were performed using SAS statistical software (version 25.0). Results: We had 150 respondents working in SUS and private health system, which fulfilled the inclusion criteria to participate in this survey. The other 30 respondents were excluded from the study because they worked exclusively in SUS or exclusively in the private system, and their answers were not analyzed. Median wait time for surgery was 60 days in the SUS (N=150) and 30 days in the private health system (N=150) e (p < 0,0001). Regardless of chemotherapy provided, median waiting time was 30 and 15 days in the SUS (N=150) and the private health system (N=150), respectively (p < 0,0001). Most oncologists in the SUS (N=125, 83.3%) pointed to endocrine therapy as their first-choice treatment in mBC. Concerning endocrine therapy in the SUS, Gonadotropin-Releasing Hormone Agonist (GnRHa) was available, and it was prescribed by 54 (36.0%) and Fulvestrant for 72 (48.0%) of respondents. Considering chemotherapy, weekly paclitaxel alone or combined with platin were available and they were prescribed by 124 (82.6%) and 109 (72.6%) of oncologists respectively. Vinorelbine was available and it was prescribed according to oncologists’ report by 109 (72.6%) and Pegylated liposomal doxorubicin (PLD) by 38 (25.3%) of oncologists. Oncologists answered agree or strongly agree that they had a multidisciplinary care team in SUS 77,3% (N =116) versus 87,3% (N=131) in the private system (p=0.022). Concerning palliative care 66,09% (N= 99) in the SUS versus 82,0% (N=123) in the private system answered that they agreed or strongly agreed that this service was available (p=0.001). Conclusions: The Brazilian government has continuously improved delivery of services and medicines via the public system (SUS), but there are still significant differences within this system and versus the private sector. Unavailability of endocrine therapy agents as GnRHa and Fulvestrant, and some chemotherapy agents as PLD and Vinorelbine for high percentage of patients, raises the existence of disparities within SUS. Also, longer waiting times for treatment in the SUS, lower availability of support teams such multidisciplinary and palliative care, they point out disparities between SUS and private health systems. The lack of a National Cancer Control Program and low health investment by Brazilian government might explain the difference in cancer patients’ access. Citation Format: Heloísa Resende, Igor Soares, Angélica Renó, Ana Cunha, Vinícius Aguiar, Letícia Tureta, Viviane Pereira. The journey of HR positive, HER2 negative metastatic breast cancer’s patients: heterogeneities and barriers in Brazilian public health system- a national survey [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 PO1-09-09.

Abstract RF01-06: Efficacy and safety of toripalimab plus metronomic chemotherapy in HER2 negative metastatic breast cancer: a multicenter phase II trial based on a Bayesian adaptive randomized design

Abstract Background: Combining Immune checkpoint blockade (ICB) with chemotherapy may significantly improve efficacy in patients with breast cancer. Metronomic chemotherapy is based on more frequent and low-dose drug administrations compared with conventional chemotherapy. Some clinical studies with small samples have tried the combination of metronomic chemotherapy and ICB in patients with advanced tumors. However, whether metronomic chemotherapy is more suitable for ICB than conventional chemotherapy is still unclear. Thus, we performed the first clinical trial, to our knowledge, of metronomic chemotherapy combined with PD-1 blockade comparing with the efficacy of combined conventional chemotherapy and ICB. Methods: This study is a multicenter randomized phase 2 trial using a multi-arm design with Bayesian adaptive randomization and efficacy monitoring. Eligible patients have advanced HER2 negative breast cancer, with no more than one prior line of standard chemotherapy. Patients were randomized to 5 groups: ① metronomic vinorelbine (NVB, 40 mg/day, TIW) monotherapy (the control cohort); ② NVB + Toripalimab (anti-PD1 antibody, 240 mg Q3W); ③Bevacizumab (5 mg/kg Q3W) + NVB+ Toripalimab (the BEV cohort); ④ Cisplatin (50mg/m Q3W) + NVB + Toripalimab (the DDP cohort), ⑤Cyclophosphamide (50mg/day, QD) + Capecitabine (500 mg, TID) + NVB+ Toripalimab (the VEX cohort). The primary endpoint is disease control rate (DCR), and secondary endpoints are objective response rate (ORR), progression free survival (PFS), and overall survival (OS). The safety profile has also been assessed. Mass cytometry time-of-flight (CyTOF) analyses of paired blood samples were performed to demonstrate dynamic changes in systemic immune profile. Results: A total of 103 patients were randomized. Characteristics were as expected for advanced breast cancer and balanced between cohorts. Adverse events of any grade occurred in 64 patients, with 5.2% grade >3. The rate of nausea was significantly higher in the cisplatin cohort than in the others (P< 0.001). Toripalimab was not associated with any previously unreported toxicity. Among the five treatment cohorts, the VEX cohort and the cisplatin cohort had the highest DCR, 69.7% (95% CI 51.7–85.9%) and 73.7% (95% CI 56.1–88.7%), respectively. Followed by the bevacizumab cohort, DCR is 55.7% (95% CI 37.4%–73.6%). Most objective responses were observed in the VEX cohort (ORR: 18.5%; 95% CI 5.6–34.1%), followed by cisplatin (ORR 14.5%; 95% CI 3.3–29.0%). It is worth noting that the PFS of patients in the VEX cohort was the longest, reaching 6.6 months (95% CI 4.0-5.9), followed by 4.0 months (95% CI 2.4-10.4) in patients of the bevacizumab cohort. The PFS of patients receiving the cisplatin regimen was relatively short, only 3.5 months (95% CI 2.2-5.3). Similarly, patients in the cisplatin cohort had shorter OS than those in the VEX and bevacizumab cohorts. In the TNBC subgroup, again, patients in the VEX cohort had the highest DCR (74.1%, 95%CI 47.9%-95.4%) and longest PFS (9.8 months, 95%CI 3.8-21.9). We obtained over 63 million cells in total and clustered CD45+ immune cells into 32 clusters. The systemic immune profile of patients changed dramatically over 2 cycles of PD-1 blockade. However, only the change of cluster 30 differed between responders and non-responders. This is a group of intermediate monocytes with a high expression of CD38, which decreased in responders but significantly increased in non-responders. CD38-NAD+ axis regulates the PD1+ exhausted T cell function. Meanwhile, the overall expression of CD38 in monocytes was significantly increased by DDP and BEV treatment compared with baseline, but not in the VEX groups. DDP significantly increased the expression level of CD38 on NK cells, while BEV or VEX treatment did not affect it. Conclusions: These data suggest promising clinical efficacy and evidence of cooperativity between metronomic VEX chemotherapy and PD-1 blockade. Citation Format: Hongnan Mo, Xiaoying Sun, Jingtong Zhai, Jiashu Han, Hewei Ge, Yuhan Wei, Xiuwen Guan, Haili Qian, Fei Ma. Efficacy and safety of toripalimab plus metronomic chemotherapy in HER2 negative metastatic breast cancer: a multicenter phase II trial based on a Bayesian adaptive randomized design [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 RF01-06.

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 PO1-16-06: LuciA-15 - A real-world prospective study of PARP inhibitors for the treatment of Latin American patients with HER-2 negative metastatic breast cancer with mutation of BRCA1/2 genes or in the Homologous Recombination Repair genes

Abstract Background PARP inhibitors (PARPi) improves progression free survival among patients with HER2 negative (HER2-ve) advanced breast cancer (ABC) and a BRCA1 or BRCA2 mutation compared to physician choice of chemotherapy (CT). The AIM of this prospective study was to evaluate the clinical benefit of PARPi treatment in terms of response, outcomes and survival by breast cancer type and treatment in a Latin-American population. Methods We analyzed data of patients with HER2-ve ABC with germline and/or somatic mutation of BRCA 1 or BRCA 2 or in the Homologous Recombination Repair genes, treated with olaparib or talazoparib in daily clinical practice by oncologist from Argentina and México. From September 2019 and April 2023, we collected baseline characteristics, previous systemic treatments, type and pattern of use, treatment beyond PARPi progression, and patient predisposition. Objective responses, best response rate, real-world progression-free survival (rwPFS) and real-world overall survival (rwOS) were analyzed with R software and RStudio version 14.0. Results After a median follow-up of 18.07 months (CI95% 10.53 - 30.07), 51 patients were treated with PARPi. Mean age at start treatment was 47,08 years. 62.7% had ER+ve/HER2-ve and 35.3% had triple negative breast cancer (TNBC). 62.7% and 37.3% of patients received talalaparib and olaparib, respectively. BRCA 1 and 2 germline mutations were the most common alterations found in 96% of patients. 37.5% of patients received platinum-based CT (PbCT) in the (neo)adjuvant/metastatic setting. 57.5% had visceral metastasis and the median number of metastatic sites were 2 (range 1-4). Median number of lines at the time to start PARPi was 2 (range 0-8), 23.5% and 31.4% received PARPi in the 1st line and 2nd line, respectively. The rwORR was 47.0%, and the median rwPFS1 was 7.77 months (CI95% 5.67-14.7). There was a tendence of better rwPFS1 in patients without previous CT versus previous CT in the advance setting, 14.30 months (CI95% 6.47-NR) versus 6.37 months (CI95% 5.03-8.73), respectively (p 0.084). The median rwOS was 26.97 months (CI95% 13.50-NR) and higher in the subgroup of patients naïve for CT versus previous exposure to CT in the advance setting, the median rwOS was 32.1 months (CI95% 27.0-NR) versus 13.0 (IC95%10.1-NR), respectively (p 0.022). 23.5% of patients progressed on PARPi during the first 6 months of treatment, 32.4% with visceral compromise, 27.8% with visceral crisis, and 26.5% with ≥2 metastatic sites. CT was the treatment of choice in most patients (55.3%). The medium rwPFS2 (next treatment after PARPi failure) was 4.00 months (CI95% 3.43 – 7.13). Treatment was discontinued for adverse events in 4.0 % of patients. Conclusion This is the first Latin-American evidence that replicate the data already published in randomized clinical trials and other scanty real-world evidence studies in this field, positive results in rwORR and rwPFS, encouraging data in rwOS. Notably, high proportion of patients with visceral progression even with visceral crisis and need for CT. Interestingly, similar rwOS results among subgroups (TNBC versus ER+ve/HER2-ve, talazoparib versus Olaparib, etc). <graphic> Citation Format: Fernando E. Petracci, Cynthia Villarreal-Garza, Facundo Argañaraz, Gonzalo Gómez Abuin, José Peñaloza, Marcos A. Flores, Luciano Piazzoni, Cecilia Riggi, Lucía Fabiano, Lucía González, Belén Cieplinski, Sergio Rivero, Ernesto Korbenfeld, Pablo Mandó. LuciA-15 - A real-world prospective study of PARP inhibitors for the treatment of Latin American patients with HER-2 negative metastatic breast cancer with mutation of BRCA1/2 genes or in the Homologous Recombination Repair genes [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 PO1-16-06.

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 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 PO2-27-12: Genomic determinants of benefit to nivolumab (NIVO) plus low dose ipilimumab (IPI) among patients (pts) with hypermutated HER2-negative metastatic breast cancer (MBC): results of NIMBUS trial

Abstract Background: Genomic determinants of benefit to immune checkpoint inhibitors (ICI) among pts with MBC and high tumor mutational burden (TMB-H) are largely unknown. NIMBUS was an open-label, single-arm, multicenter, phase 2 study assessing the efficacy of NIVO 3 mg/kg intravenously every 14 days plus IPI 1 mg/kg IV every 6 weeks in pts with TMB-H HER2-negative MBC. The objective of this current study was to evaluate genomic profiling of pts included in NIMBUS and its association with benefit to ICI. Methods: In NIMBUS, eligible pts were required to have measurable HER2-negative MBC, TMB ≥9 Mut/Mb assessed by a cancer-gene panel evaluating > 300 genes and performed in a CLIA-certified laboratory, and 0-3 prior lines of chemotherapy in the advanced setting. The primary objective was overall response rate (ORR) according to RECIST 1.1. Secondary objectives include progression-free survival (PFS), and overall survival (OS). We performed genomic analyses on the pre-treatment tissue, and on baseline and on treatment plasma from pts run on the Foundation Medicine Inc (Boston, MA) FoundationOne®CDx (F1CDx®) and FoundationOne®Liquid CDx (F1LCDx®) panel, respectively. Results: A total of 30 pts (21 with estrogen receptor (ER)-positive and 9 with triple-negative breast cancer) were included in this study. After a median follow-up of 30.6 months, 6 pts (20%) achieved partial response (PR) for a confirmed ORR of 20% (95% CI: 7.7-38.6%). Exploratory analyses did not reveal differences in ORR based on ER status, PD-L1 status, or stromal TIL. The median PFS was 1.4 months (95% CI: 1.3 – 4.6), and the median OS was 16.9 months (95% CI: 7.1-not reached). Pts who achieved PR had a 3-year OS of 75% versus 25% in non-responders. A total of 29 pts (6 responders and 23 non-responders) had at least one pre-treatment genomic panel performed: 16 had both, 5 had pretreatment F1CDx only and 8 had F1LCDx panel only. Blood TMB (bTMB) was highly correlated with tissue TMB by F1CDx (r = 0.95). Using data from the most recent genomic panel available, (24 for baseline blood, and 5 for pretreatment tissue), the most frequently mutated genes ( > 30%) in this population were: TP53 (66%); PIK3CA (45%), ESR1 (35%), CDH1 (35%), ROS1 (31%). ESR1 and PTEN mutations were associated with absence of PR while PALB2 was associated with objective response. We observed a significant correlation between ESR1 mutations and a decrease in OS among pts with ER+ disease. Among the entire 29 pts, a negative correlation between PTEN with PFS and OS, and positive correlation between PALB2 with PFS was observed (Table). Among the 24 pts with baseline F1LCDx panel evaluable, the median TMB was 7.6 Mut/Mb ([interquartile range 0.0–113.8). Median TMB was 8.9 Mut/Mb and 41.7 Mut/mB among pts without and with PR, respectively. Baseline TMB > 20 mut/Mb was found in all pts with PR and in one of those without response. Paired serial blood samples demonstrated that among responding pts, TMB was significantly reduced from baseline to end of treatment (EOT) compared to those with no clinical benefit [n=16, p=0.0015]. Lastly, the tumor fraction (TF) estimates from 7 paired baseline to EOT samples demonstrated that all responders had a decrease or no change in TF compared to an increase in non-responders [n=7, p=0.017]. Conclusion: In the NIMBUS trial, all pts with PR to NIVO plus low dose IPI had a TMB > 20 Mut/Mb and had a decrease in TMB during treatment. ESR1 and PTEN mutations were associated with lack of benefit, while PALB2 mutation was associated with increased benefit. If validated, these results could help tailor the use of ICI among pts with TMB-H MBC. Association between genomic alterations in ESR1, PTEN and PALB2 and benefit to nivolumab and low dose ipilimumab among patients with HER2-negative MBC and TMB-H HR: hazard ratio; MBC: metastatic breast cancer; mOS: median overall survival (months); mPFS: median progression-free survival (months); TMB-H: tumor mutational burden above > 9 mutations/megabase. Citation Format: Romualdo Barroso-Sousa, Tyler Chinsky, Tianyu Li, Sangeetha Reddy, Leisha Emens, Beth Overmoyer, Saud AlDubayan, Hoyin Chu, Paulina Lange, Julie Kasparian, Ameer Basta, Molly DiLullo, Victoria Attaya, Melissa Hughes, Nancy Lin, Nabihah Tayob, Rinath Jeselsohn, Elizabeth Mittendorf, Sara Tolaney. Genomic determinants of benefit to nivolumab (NIVO) plus low dose ipilimumab (IPI) among patients (pts) with hypermutated HER2-negative metastatic breast cancer (MBC): results of NIMBUS 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 PO2-27-12.

Abstract PO1-05-12: Pooled clinical trial data analyses comparing the biology of HER2-low vs HER2-0 breast cancer in patients with metastatic breast cancer following treatment with standard single agent chemotherapy

Abstract Background: Prior analyses of observational data suggest that for patients with metastatic breast cancer (MBC), tumor biology does not vary by HER2-low vs. HER2-0 expression given similar overall survival (OS) times after accounting for patients’ clinicopathologic features including hormone receptor (HR) status. We sought to evaluate potential differential biology by HER2-low status by (1) studying data pertaining to patients treated in historic clinical trials (i.e., data collected to measure associations between protocol defined treatments and outcomes) and (2) evaluating the endpoint of progression-free survival (PFS) in addition to OS following treatment with standard single agent chemotherapy for MBC. Methods: Pooling anonymized clinical trial data from studies within the Medidata Enterprise Data Store, we identified 142 women with HER2-negative MBC who received treatment with an NCCN recommended single agent chemotherapy in the context of a clinical trial. Using patient-level immunohistochemistry (IHC) results, we categorized patients as either HER2-low (IHC 1+/2+ and not amplified by in situ hybridization) or HER2-0 (IHC 0). We compared patients’ baseline demographic and clinicopathologic features according to HER2-low vs HER2-0 status. We used Cox proportional-hazard models stratified by HR status to estimate OS and PFS according to HER2-low vs HER2-0 classification while adjusting for patients' baseline demographic and clinicopathologic attributes. Results: Patients with HER2-low disease represented 20% (28/142) of the HER2-negative cohort. Twenty-five percent (7/28) of HER2-low patients had tumors that expressed hormone receptors compared with 17% (19/114) of HER2-0 patients (p=0.31). In this cohort, the maximum follow up time was 38.4 months. For HER2-low patients vs HER2-0 patients, the median OS was 10.7 vs. 12.7 months (p=0.37), and the median PFS was 3.5 vs. 2.9 months (log rank test, p=0.53) respectively. In Cox proportional-hazard models that adjusted for patient demographic and clinicopathologic features and were stratified by HR status, patients with HER2-low tumors had an 16% elevated (non-significant) hazard of death compared with patients with HER2-0 tumors (HR 1.16, 95% CI: 0.69-1.95) and a 22% reduced (non-significant) hazard of progression or death (HR 0.78, 95% CI: 0.45-1.35). Conclusions: Analyses of pooled historic clinical trial data pertaining to women with HER2-negative MBC who were treated with single agent chemotherapy revealed no meaningful clinical differences in either OS or PFS endpoints according to HER2-negative subtypes (i.e., HER2-low vs. HER2-0) after accounting for patient demographic and clinicopathologic features. These results support prior findings from observational research suggesting that there are no biological differences associated with HER2-negative subtypes. Table. Multivariable Cox Proportional Hazards Models for OS and PFS Stratified by Hormone Receptor Status, (N=134) Variable OS PFS HR* 95% CI HR* 95% CI Age (decade) 0.84 0.70-1.01 0.86 0.72-1.01 Race White 1.00 (referent) 1.00 (referent) Black 1.75 0.84-3.63 1.57 0.66-3.73 Asian 0.93 0.47-1.80 1.33 0.71-2.51 Other 1.69 0.45-6.38 2.10 0.62-7.15 Ethnicity Non-Hispanic or Latino 1.00 (referent) 1.00 (referent) Hispanic or Latino 2.10 0.95-4.64 1.02 0.44-2.35 HER2 Characterization HER2-0 1.00 (referent) 1.00 (referent) HER2-1+/2+ 1.16 0.69-1.95 0.78 0.45-1.35 Prior Lines of Chemotherapy for Metastatic Disease 0 1.00 (referent) 1.00 (referent) 1 0.82 0.46-1.46 1.13 0.65-1.95 2 0.69 0.35-1.36 1.00 0.52-1.93 3 0.59 0.10-3.42 2.18 0.45-10.50 ECOG Performance Status 0 1.00 (referent) 1.00 (referent) 1 1.65 1.07-2.54 1.57 1.02-2.43 Legend: OS=overall survival; PFS=progression-free survival; HR=hazard ratio; HER2=human epidermal group factor receptor 2; ECOG=Eastern Cooperative Oncology Group. *Analyses adjusted for clinical trial membership (coefficients not reported) Citation Format: Elizabeth Lamont, Emily Stein, Paolo Tarantino, Sara Tolaney, Corinne Ahlberg, Krishna Chinnathambu, Jiezhi Qi, Kala Tummagunta, Jackie Bilan, Ruthanna Davi, Lisa Ensign. Pooled clinical trial data analyses comparing the biology of HER2-low vs HER2-0 breast cancer in patients with metastatic breast cancer following treatment with standard single agent chemotherapy [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 PO1-05-12.

Abstract PO1-06-05: Heterogeneity and prognostic characteristics of HER2-low breast cancer: A retrospective analysis of patients with HER2-negative metastatic breast cancer

Abstract Background: It remains uncertain as to whether low levels of HER2 positivity have any prognostic implications in breast cancer patients, whether HER2 levels can be inconsistent between primary tumors and metastatic lesions in individuals with advanced HER2-negative breast cancer, and how such inconsistencies may impact patient prognosis. Methods: A retrospective analysis of advanced breast cancer patients admitted to our hospital from from January 1st, 2010 to January 1st, 2019 was performed, with all patients that underwent at least one metastatic lesion biopsy being screened. The hormone receptor (HR) and HER2 profiles of both primary and metastatic lesions from these patients were confirmed, and HER2-positive patients (HER2 3+ immunohistochemistry [IHC] results or HER2 2+ IHC results with positive in situ hybridization [ISH] results) were excluded from further analyses. To examine the prevalence of HER2-low status across primary and metastatic cancers, the current study set out to detect inconsistencies between these two tumor compartments with respect to HER2-low status and to examine the prognostic implications of these findings in patients. Results: The median follow-up duration for this study at the cutoff date (December 31, 2021) was 49.3 (45.8-52.8) months. The current study examined 1,031 participants with HER2 and HR status data from both primary and metastatic tumors. The proportion of HER2-low expression was significantly higher in metastatic lesions than primary tumors (34.7% vs 25.7%), with a corresponding drop in the proportion of HER2-zero metastatic lesions (FIGURE 1). In a multivariate analysis, HER-2 upregulation was linked to HR status, which was established as an independent influencing factor, with such upregulation being most common in HR-positive individuals (P= 0.004). With respect to HER2 status of primary lesions, the medianl survival of patients in the HER2-zero was similar to HER2-low patients, regardless of HR status [44.8 months (95% CI 38.2-54.1) vs 41.5 months (95% CI 35.0-48.0)] (P = 0.954)(FIGURE 2A). However, when the HER2 status of metastatic lesions was examined, patients with HER2-low expression had a greater median OS than HER2-zero patients [47.6 months (95% CI 39.6-48.0) vs. 32.2 months (95% CI 24.6-40.0)] (P < 0.001)(FIGURE 2B). The median OS of patients that exhibited HER2 upregulation (zero→low) was longer than that of patients without such upregulation (zero-zero) (55.8 months (95% CI 45.0-66.6) vs. 32.4 months (95% CI 23.8-41.0)) (P < 0.001)(FIGURE 3A). Among HR-positive patients, HER2 upregulation was associated with significantly prolonged OS relative to patients without such upregulation [56.6 months (95% CI 44.6-68.6) vs. 41.7 months (95% CI 31.1-52.3)] (P=0.006)(FIGURE 3B), whereas survival outcomes were similar in these two groups in the HR-negative subgroup [44.8 months (95% CI 25.6-64.0) and 25.3 months (95% CI 20.4-30.3)] (P=0.103)(FIGURE 3C). However, independent of HR status, the median OS of patients who had HER2 downregulation (low→zero) was similar to those without downregulation (low→low)(FIGURE 3 D, E, F). Conclusion: These results support inconsistencies in HER2-low expression status between primary and metastatic lesions, with low HER2 levels in metastatic tumors being associated with improved survival outcomes. HR-positive patients are more likely to exhibit HER2 upregulation within metastatic lesions and experience corresponding prognostic benefits. Keywords: HER2-low, inconsistency, survival, prognosis, advanced breast cancer Citation Format: Huimin Lv, Min Yan, Mengwei Zhang, Limin Niu, Huiai Zeng, Huihui Sun, Shengnan Zhao, Jing Wang. Heterogeneity and prognostic characteristics of HER2-low breast cancer: A retrospective analysis of patients with 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 PO1-06-05.

Abstract PO3-05-10: Efficacy and safety of eribulin plus gemcitabine in second-line or beyond for patients with HER2-negative metastatic breast cancer (MBC): A multi-center, open-label, single-arm, phase II study

Abstract Background: Eribulin is a halichondrin non-taxane inhibitor of microtubule dynamics approved in China for advanced breast cancer patients who have received at least two prior chemotherapy treatments. The combination of eribulin and gemcitabine has demonstrated a similar progression-free survival (PFS) benefit as paclitaxel plus gemcitabine, with less neurotoxicity, for patients with MBC who have not received prior cytotoxic chemotherapy. However, the effect of eribulin plus gemcitabine on PFS in second line or beyond remains unclear. Methods: This open-label, single-arm, phase II study (NCT05263882) was conducted at 13 institutions in China. Eligible patients had histologically confirmed HER2-negative MBC and had received at least one prior taxane-containing chemotherapy regimen for advanced disease, and anthracycline-containing regimens in the adjuvant setting. Patients received intravenous infusions of eribulin (1.4 mg/m2) and gemcitabine (1.0 g/m2) on days 1 and 8 of a 21-day cycle. Efficacy outcomes, including PFS, objective response rate (ORR), and disease control rate (DCR), were assessed using RECIST v1.1. Adverse events (AEs) were graded according to NCI-CTC version 5.0. Results: A total of 65 patients were enrolled from November 2021 to May 2023; 47 (72.3%) had HR+/HER2- and 18 (27.7%) had triple-negative MBC. The median patient age was 50 years (range: 31-68), and the sites of metastasis were the bone (70.8%), liver (58.5%), lymph nodes (50.8%), lung (43.1%) and brain (10.8%). Patients had received a median of 3 prior lines of systemic treatment, 2 lines of chemotherapy, and 1 line of endocrine treatment. Among all patients, the ORR was 52.3%, the DCR was 92.3% and the median PFS was 7.6 months (Table). For the HR-positive subgroup, the median PFS was 8.4 months, while for the triple-negative subgroup, it was 7.6 months. Among patients who had received prior CDK4/6 inhibitor treatment, the median PFS was 7.2 months. In the subgroup of patients who had not received CDK4/6 inhibitor treatment, the median PFS had not been reached. The most common grade 3-4 AEs were hematological, including neutropenia (41.6%), leukopenia (33.9%), anemia (23.1%), and thrombocytopenia (15.4%). No grade ≥3 perceived AEs were reported. Conclusion: Eribulin plus gemcitabine was effective in heavily pretreated patients with HER2- MBC, while maintaining a predictable and manageable safety profile. Table. Summary of efficacy outcomes Citation Format: PeiJian Peng, Xiao-Lu Xu, Jin-Cai Zhong, Jin-Hui Ye, Zhi-Hui Wang, Hong Wang, Hong Lin, Cai-Wen Du, Guorong Zou, Jie Ouyang, Ying-Ying Shi, Fei Xu, Geng-Sheng Yu, Yong-Kui Lu, Yong-Xia Wang, Shi-En Cui, Lu-Zhen Li. Efficacy and safety of eribulin plus gemcitabine in second-line or beyond for patients with HER2-negative metastatic breast cancer (MBC): A multi-center, open-label, single-arm, phase II 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 PO3-05-10.

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 PS04-02: BRCA testing and PARP inhibitor utilization in real-world HER2-negative metastatic breast cancer

Abstract Background: PARP inhibitors (PARPi) have demonstrated progression-free survival benefits vs. chemotherapy in patients (pts) with metastatic breast cancer (mBC) and germline BRCA mutations (gBRCAm). NCCN guidelines recommend that all pts with mBC are offered gBRCAm testing to aid in systemic treatment decisions with PARPi. Understanding real-world gBRCAm testing patterns and PARPi use would be beneficial for improving clinical outcomes of pts with gBRCAm mBC. Methods: This retrospective cohort study identified pts ≥ 18 years old with HER2-negative mBC, diagnosed from 2014 to 2022 (US Flatiron Electronic Health Record database). gBRCAm testing patterns in mBC over time were examined by BC subtype (hormone receptor-positive [HR+] or triple-negative breast cancer [TNBC]) and disease stage at diagnosis [de novo or recurrent]). Timing of gBRCAm testing and subsequent initiation of first-line or later PARPi in pts with gBRCAm HR+ and TNBC mBC was assessed in pts diagnosed after 2018 when PARPi were approved in mBC. Prevalence of gBRCAm was calculated overall and by mBC subtype. Demographic and clinical characteristics were described by gBRCAm testing and PARPi initiation (yes/no). Real-world overall survival (OS) by PARPi use since mBC diagnosis was estimated using the Kaplan-Meier method and a log-rank test. Results: Of 15,006 total pts, 4654 (31.0%) had a gBRCAm test. Compared with pts who were not tested, gBRCAm-tested pts were younger at initial mBC diagnosis (median age: 53.5 vs. 62.0 years), had a shorter median disease-free interval before mBC diagnosis (821 vs. 959 days), and were less likely to have de novo mBC (28.0% vs. 31.2%). gBRCAm testing prevalence was 6.1% in the HR+ group (n=11,945) and 8.9% in the TNBC group (n=2430). gBRCAm testing rates mostly increased from 2014 to 2022 and were lowest in HR+ tumor subtypes; < 2/3 of all pts had a gBRCAm test from 2020 onwards (Table). Most pts with recurrent mBC (HR+: 68.3%; TNBC: 76.1%) were gBRCAm tested before diagnosis, whereas pts with de novo mBC were tested at a later timepoint ( > 60 days after mBC diagnosis – HR+: 48.3%; TNBC: 38.4%). Of all pts with gBRCAm diagnosed from 2018 onwards (n=187), 44.4% initiated PARPi. A higher proportion of pts with gBRCAm in the TNBC group initiated PARPi (52.9%) than in the HR+ group (42.4%). Pts with gBRCAm in the TNBC group received PARPi earlier than pts with gBRCAm in the HR+ group (mean days from mBC diagnosis to first PARPi use: 173 and 411 days, respectively). Fewer pts with de novo mBC (35.7%) initiated PARPi than pts with recurrent mBC (50.8%). With a median follow-up of 15.3 months, pts with gBRCAm initiated on PARPi had numerically longer median OS (32.3 months [95% CI: 24.2–47.4]) than PARPi non-initiators (21.4 months [95% CI:18.4–43.5]). OS results should be regarded with caution due to the small sample size (n=187). Conclusions: Despite NCCN guideline gBRCAm-testing recommendations, < 2/3 of pts with HER2-negative mBC in our study were gBRCAm tested from 2020 onwards. gBRCAm testing was lower in pts with HR+ disease and was performed at a later timepoint for pts with de novo disease. Wider and timelier implementation of gBRCAm testing to identify pts that could benefit from PARPi is warranted. Fewer pts with gBRCAm who had HR+ tumors or de novo disease received PARPi than pts with TNBC. Our data suggest that pts who initiated PARPi had numerically longer OS than PARPi non-initiators; further research into OS trends and PARPi use in gBRCAm mBC would be of interest. Editorial acknowledgment: Medical writing assistance was provided by Leigh-Ann Booth, Ph.D. from BOLDSCIENCE Inc., funded by AstraZeneca and Merck Sharp & Dohme LLC, a subsidiary of Merck & Co., Inc., Rahway, NJ, US Legal entity responsible for the study: AstraZeneca. Funding: This study was supported by AstraZeneca and Merck Sharp & Dohme LLC, a subsidiary of Merck & Co., Inc., Rahway, NJ, US, who are codeveloping olaparib. Table. gBRCAm testing rates over time in pts with HER2-negative mBC HER2-negative mBC is defined as IHC 0, or 1-positive, or IHC2-positive/ISH-negative. *Olaparib approved in mBC; †Olaparib approved in eBC. eBC, early breast cancer; gBRCAm, germline breast cancer gene mutation; HER2, human epidermal growth factor receptor 2; HR, hormone receptor; IHC, immunohistochemistry; ISH, in situ hybridization; mBC, metastatic breast cancer; pt, patient; TNBC, triple-negative breast cancer. Citation Format: Siddhartha Yadav, Sam Hillman, Linlin Luo, Weiyan Li, Jagadeswara Earla, Xiaoqing Xu. BRCA testing and PARP inhibitor utilization in real-world 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 PS04-02.

Abstract PO4-15-04: Differential genomic profiling of progesterone receptor (PR) negative, estrogen receptor (ER) positive HER2-negative metastatic breast cancer (MBC) through circulating tumor DNA

Abstract Background: MBC is usually classified based on ER and HER2 status. Although the absence of PR expression is commonly associated with a poorer response to endocrine therapy (ET) and an unfavorable prognosis, data regarding the underlining molecular features that occur in this subgroup are still unclear. The aim of this study was to investigate genomic differences between ER/PRpos HER2neg (PRpos) and ERpos PR/HER2neg (PRneg) MBC through circulating tumor DNA (ctDNA) profiling in a large multicenter consortium. Methods: This retrospective study analyzed a cohort of 1089 patients at Weill Cornell Medicine, Northwestern University, Massachusetts General Hospital, and Washington University in St. Louis with HER2neg MBC and ctDNA testing by Guardant360. Oncogenic pathway status (i.e., RTK, RAS, RAF, MEK, NRF2, ER, WNT, MYC, P53, cell cycle, notch, and PI3K) was defined based on previous research (Sanchez-Vega et al. Cell, 2018). Associations across single nucleotide variations (SNVs), copy number variations (CNVs), pathway classification, and ER/PR status were tested by multinomial logistic regression and corrected for significant clinical characteristics (i.e., lines of treatment, metastatic sites). Prognosis was analyzed through Cox regression for overall survival (OS) defined from time of ctDNA collection. Results: Among the 1089 analyzed patients, PRpos was the most represented subtype (N:580, 53%), followed by PRneg (N:300, 28%) and Triple Negative (TNBC) (N:209, 19%). In the hormone receptor positive subgroup, bone (N:663, 75%), liver (N:323, 37%) and lymph nodes (N:314, 36%) were the main sites of distant involvement. The TNBC group was exposed to significantly fewer lines of therapy (RRR:0.37, P= 0.005 for ≥5 lines). As compared to PRneg, TNBC had less frequent and PRpos had more frequent bone involvement (RRR:0.44, P< 0.001 and RRR:1.80, P< 0.001, respectively) and less frequent liver involvement (RRR:0.64, P=0.019 and RRR:0.71, P=0.020, respectively for TNBC and PRpos). After multivariable analysis, a significant difference with respect to PRneg was observed in SNVs for the PI3K and P53 pathways for TNBC (RRR 0.49 P = 0.013 and RRR 4.06 P < 0.001, respectively) but not for PRpos. On a single gene level, a lower incidence in CDH1 SNVs (RRR 0.09 P = 0.036) and a higher incidence of ESR1 SNVs (RRR 0.09 P = 0.036) was present in PRpos compared to PRneg. TNBC, on the other hand, was characterized by a lower prevalence of KRAS SNVs (RRR 0.09 P = 0.036) and PIK3CA SNVs (RRR 0.25 P < 0.001) and a higher prevalence of TP53 SNVs (RRR 0.09 P < 0.001). No ESR1mutations were detected in TNBC. Median OS was 15 months (mo) for TNBC, 21 mo for PRneg and 31 mo for PRpos (P < 0.001). The differential prognostic impact of ctDNA features on OS was then analyzed across the PRpos and PRneg subpopulations. ESR1 SNVs was prognostic for both subgroups in univariable analysis (HR 2.19 P < 0.001 and HR 1.96 P = 0.001 for PRpos and PRneg, respectively). In multivariable analysis corrected for lines of therapy, ESR1 SNVs retained its significance only in the PRpos subgroup (HR 1.41 P = 0.021) together with NF1 SNVs (HR 2.04 P = 0.034). On the other hand, TP53 SNVs had a significant impact on OS in the PRneg subgroup (HR 2.02 P = 0.014). Conclusions: Our study confirmed the prognostic impact of PR status in ER positive, HER2 negative MBC. Moreover, it suggested a differential profile in terms of ctDNA-detectable genomic features and their impact on survival, including well-established markers such as ESR1 SNVs, with a potential relevance for disease subtyping and future algorithms for drug development trials. Citation Format: Lorenzo Gerratana, Andrew Davis, Arielle Medford, Carolina Reduzzi, Katherine Clifton, Emily Podany, Whitney Hensing, Marko Velimirovic, Ami N. Shah, Lorenzo Foffano, Laura Munoz Arcos, Eleonora Nicolò, Charles S. Dai, Jennifer Keenan, Amir Behdad, Seth Wander, William Gradishar, Cynthia Ma, Fabio Puglisi, Aditya Bardia, Massimo Cristofanilli. Differential genomic profiling of progesterone receptor (PR) negative, estrogen receptor (ER) positive HER2-negative metastatic breast cancer (MBC) through circulating tumor DNA [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 PO4-15-04.

Abstract PO5-04-14: Quality of Life (QoL) among metastatic hormone receptor positive, HER2 negative breast cancer patients during first line systemic treatment

Abstract Background Current international guidelines recommend endocrine therapy combined with a CDK4/6 inhibitor as the preferred first line systemic treatment in patients diagnosed with metastatic hormone receptor positive (HR), HER2 negative breast cancer (BC). Randomized controlled trials that led to the approval of these agents in the first line disease setting are derived from controlled environments and selected patients. We aimed to assess real-world patients’ characteristics, safety and health related quality of life in a non-select cohort of patients being managed in a Brazilian private Cancer Center. Methods: We performed a cross sectional study at Beneficencia Portuguesa de Sao Paulo evaluating quality of life among patients with metastatic HR+/HER2 negative treated in the first line disease setting. Inclusion criteria included patients that received the combination of a CDK4/6i with any endocrine therapy (ET) between January 2018 and April 2023. Primary endpoint was quality of life assessed using the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire Core 30 (EORTC QLQ-C30) and EORTC QLQ-BR23, it is baseline, there is only one measurement for each patient. Results: We analyzed 72 patients who meet the inclusion criteria. Median age at diagnosis was 49 years old (ranging from 31 to 91). Approximately half of the population was premenopausal (51%). Palbociclib and abemaciclib were the most commonly used CDK4/6i, with 42 and 35% of patients, respectively. ECOG performance status was zero or one in almost the entire patient population analyzed (97%). 53% of the patients were diagnosed as de novo metastatic BC. Table 1 and 2 summarize the patients characteristics. Mean global health status was 75.4, with social and cognitive functioning scoring the highest (Mean 78.6 and 79.1, respectively). The most distressing symptom among the symptom scale was fatigue (Mean 27.2). Most symptoms and functioning scales did not meet the threshold for clinically meaningful differences between the different CDK4/6 inhibitors. Conclusion: Real-world data from our study is consistent with the historical expected health related quality of life among patients diagnosed with HR+/HER2 negative metastatic BC in the first line disease setting. Table. Citation Format: Débora Gagliato, Julio Antonio Araujo, Bianca Milena Verboski, Guilherme William Marcelino, Fabiana Nogueira Momberg, Gilmara Silveira da Silva, Rozana Mesquita Ciconelli, Antônio Buzaid. Quality of Life (QoL) among metastatic hormone receptor positive, HER2 negative breast cancer patients during first line systemic treatment [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 PO5-04-14.

Abstract PS17-06: A phase I trial of the PI3K inhibitor (PI3Ki) copanlisib and fulvestrant in combination with continuous or intermittent abemaciclib in patients with estrogen receptor-positive (ER+), HER2-negative (HER2-) metastatic breast cancer

Abstract Background: Activation of the phosphoinositide 3-kinase (PI3K) pathway is a prominent resistance mechanism to endocrine therapy and CDK4/6 inhibition (CDK4/6i) in ER+, HER2- breast cancer. There is reciprocal crosstalk between PI3K and cell cycle regulatory pathways. Triplet therapy that targets ER, CDK4/6, and PI3K prevents or delays disease progression in preclinical models of ER+, HER2- breast cancer. However, the toxicities of triplet therapy hinder its clinical development. Copanlisib (COP) is an intravenous (IV) PI3Ki with potent activity against α and δ isoforms. Intermittent high-dose PI3Ki is expected to be less toxic and more effective than continuous daily dosing. We set out to determine the recommended phase 2 dose (RP2D) for the COP/Fulvestrant (FUL)/Abemaciclib (ABE) combination. Methods: Eligible patients (pts) included men or women with ER+, HER2- metastatic breast cancer (MBC), with no more than 1 prior chemotherapy in the metastatic setting, and no limits on prior endocrine or targeted therapy. Prior FUL, CDK4/6i and PI3K/mTORi were allowed. Pts with HbA1c >8.5% were excluded. Pts received FUL 500 mg IM standard dosing and COP/ABE at assigned doses. Premenopausal women also received a GnRH agonist. Dose-limiting toxicity (DLT) was assessed during the first 28-day cycle. Adverse events (AEs) were graded according to NCI-CTCAE v5.0. Dose escalation/de-escalation was based on continual reassessment (CRM) and the RP2D was defined as the dose with the highest probability of the DLT rate within the acceptable 25~35%. Results: Between June 2020 and June 2023, 24 pts with ER+, HER2- MBC were enrolled. Table 1 shows the dose levels (DLs) and DLTs. The first 10 pts were enrolled to Part A on two consecutive DLs of COP (DL1: 45 mg IV D1 and D15, n=7; DL2: 45 mg IV D1, D8, and D15, n=3), with ABE fixed at 100 mg PO BID continuously. DL2a exceeded the maximum tolerated dose (MTD). DL1a was tolerable based on Cycle 1 AEs. However, during subsequent cycles, only 1 of the 7 pts in DL1a maintained the intended dose of ABE due to neutropenia, others reduced ABE to 50mg bid (n=5) or discontinued (n=1). This triggered a protocol amendment, adding Part B with ABE administered 5 days on and 2 days off (5-on/2-off) intermittent schedule. 14 pts were enrolled to Part B in 2 consecutive DLs of COP (DL1b: n=7; DL2b: n=7), the same as in Part A. ABE was fixed at 100 mg PO Bid 5-on/2-off. DL2b exceeded the MTD. DL1b was tolerable based on Cycle 1 AEs. In addition, in subsequent cycles, only 1 of 7 pts on DL1b required dose reduction of ABE to 50mg bid. RP2D is therefore defined as DL1b. Grade (G) 4 AEs were rare. The most common all cycle G3 AEs included transient hypertension (45%), rash (30%), anemia (25%), neutropenia (25%), and AST elevation (20%). As of 6/14/2023, 22 pts, with a median of 1 prior metastatic regimen (range 0-5), prior CDK4/6i in 18 (82%), liver mets in 8 (36%), and PIK3CA mutation in 8 (50%) of 16 pts with known mutation status, were evaluable for response. There were 5 partial responses and 3 stable diseases lasting for ≥24 weeks. Clinical benefit rate was 36.4% (8/22, 95%CI: 17.19% ~ 59.34%). Responses were observed in pts regardless of prior CDK4/6i, FUL, or PIK3CA mutation. Conclusion: COP/FUL/intermittent ABE can be safely administered with fair overall tolerability. Preliminary anti-tumor activity was observed, which will be further examined in the randomized phase II trial (NCT 03939897). Table 1 Dose-limiting Toxicities by Dose Level Citation Format: Cynthia Ma, Jingqin Luo, Jessica Moss, Maryann Kwa, Ritesh Parajuli, Katia Khoury, Emily Douglas, Ashley Frith, Caron Rigden, Foluso Ademuyiwa, Shana Thomas, Brittney Haas, Gerburg Wulf, Claire Dees, Rabih Said. A phase I trial of the PI3K inhibitor (PI3Ki) copanlisib and fulvestrant in combination with continuous or intermittent abemaciclib in patients with estrogen receptor-positive (ER+), 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 PS17-06.

Abstract PS15-02: A Phase Ⅰb Study of D-0502 as Monotherapy for Advanced or Metastatic ER-Positive and HER2-Negative Breast Cancer: Results from the Dose-Expansion Stage

Abstract Background: Endocrine therapy is the mainstay treatment for estrogen receptor (ER)-positive advanced breast cancer (BC). D-0502, an orally bioavailable selective estrogen receptor degrader (SERD), has previously demonstrated antitumor activity in various ER-positive (ER+) and human epidermal growth factor receptor 2 negative (HER2-) breast cancer cell lines and xenograft models. The phase Ⅰ open-label study (NCT03471663) evaluated the safety, tolerability, pharmacokinetics (PK) and efficacy of D-0502 as monotherapy and in combination with palbociclib in female patients with ER+ and HER2- breast cancer. The dose escalation part of D-0502 monotherapy (phase Ⅰa) has been presented before (SABCS 2021, PS11-26 Abstract #1348). Here, we present the results of D-0502 monotherapy from the dose-expansion stage in phase Ⅰb study. Methods: In the single-agent dose-expansion stage of phase Ⅰb study, patients (n=60) received D-0502 (400 mg QD) in 28-day cycles. Eligible patients included females with confirmed ER+, HER2- locally advanced, or metastatic BC; ECOG 0-1; measurable disease (RECIST v1.1); premenopausal or postmenopausal status; adequate hematologic, hepatic and renal functions. The primary objectives are to characterize the safety of D-0502. The secondary objectives are to evaluate the preliminary anti-tumor activity and the PK characteristic. Results: As of April 07, 2023, 60 female patients were enrolled and treated with 400mg single-agent D-0502. Median age was 57 (range: 32-82) years, 55.0% had an ECOG PS of 1. Disease progression resulting in treatment discontinuation occurred in 66.7% (40/60) of patients, and 6.7% (4/60) of patients discontinued treatment because of AEs. Treatment-related adverse events (TRAEs) were reported in 57 pts (95.0%), most of which were grade 1-2. No grade 4/5 TRAE has been reported. The incidence of serious adverse events was 6.7% (4/60). The most common (≥ 15%) TRAEs included vomiting, dizziness, nausea, increased aspartate aminotransferase and alanine aminotransferase, anaemia, decreased appetite, diarrhoea, and malaise. In the 51 efficacy evaluable patients, 8 patients had partial response (PR) and 27 had stable disease (SD); objective response rate (ORR) and disease control rate (DCR) were 15.7% (8/51) and 68.6% (35/51), respectively. The clinical benefit rate (CBR: CR + PR + SD ≥24 weeks) was 47.1% (24/51). The median PFS was 5.6 months (95% Cl: 2.0, 10.0). Conclusion: D-0502 monotherapy showed promising antitumor activity and tolerable toxicity in female patients with ER+ and HER2- locally advanced or metastatic BC. Currently this treatment is being evaluated in a phase Ⅲ study for patients with ER+ and HER2- locally advanced or metastatic BC in China. Citation Format: Jiayu Wang, Tao Sun, Qingyuan Zhang, Yanxia Shi, Xu Wang, Yiding Chen, Quchang Ouyang, Kunyan Li, Manojkumar Bupathi, w. Jeffery Edenfield, Andrea L.M. Silber, Hong Zong, Erika Hamilton, Dejan Juric, Kate Lathrop, Yihong Zhang, Kathryn Stazzone, Zhe Shi, Yaolin Wang, Ling Zhang, Binghe Xu. A Phase Ⅰb Study of D-0502 as Monotherapy for Advanced or Metastatic ER-Positive and HER2-Negative Breast Cancer: Results from the Dose-Expansion Stage [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 PS15-02.