Therapy For Metastatic Breast Cancer Research Articles

Real world outcomes of sequential ADC therapy in metastatic breast cancer: Patients treated with sacituzumab govitecan and trastuzumab deruxtecan.

1085 Background: Sacituzumab govitecan (SG) and trastuzumab deruxtecan (TDXd) are antibody drug conjugates (ADCs) that are approved for the treatment of HER2 low metastatic breast cancer (MBC). Both carry payloads which inhibit topoisomerase 1, and it is unknown whether these ADCs retain clinical activity when used sequentially to each other. Here we present a retrospective study of patients treated with both SG and TDXd at our institution and report real world progression free survival (PFS), clinical factors associated with longer PFS, and whether ADC sequence impacts outcome. Methods: All patients with MBC treated at MSKCC who had received both SG and TDXd (in any order) were eligible for inclusion. Clinicopathological and demographic data were obtained from retrospective chart review. PFS was determined clinically and calculated using Kaplan-Meier survival analysis. Univariate and multivariate associations of survival were assessed using Cox proportional hazards models. Results: Eighty-five patients met eligibility per the criteria above: 54 with estrogen receptor positive (ER+) MBC and 31 with triple negative breast cancer (TNBC). 33 patients received SG followed by TDXd (14 ER+, 19 TNBC), and 52 received TDXd followed by SG (40 ER+, 12 TNBC). Median age at time of second ADC treatment (ADC2) was 59 among all patients, 62 among SG first, and 58 among TDXd first. This cohort was heavily pre-treated, with median 5 prior therapies at the time of first ADC (ADC1) and 7 at the time of ADC2. Both the SG first and TDXd first cohorts had median 1 line of intervening therapy between ADCs. In the SG first group, median ADC2 (TDXd) PFS was 3.5 months (95%CI 2.7-7.7), and in the TDXd first group median ADC2 (SG) was 2.8 months (95%CI 2.6-3.7). On multivariate analysis, neither ADC sequence (SG first vs. TDXd first, HR 1.30, 95%CI 0.71-2.37, p=0.4) nor ER status (TNBC vs. ER+, HR 1.21, 95%CI 0.63-2.32, p=0.6) showed statistically significant differences in ADC2 PFS between these groups. Instead, later treatment line at the time of ADC2 was significantly associated with shorter ADC2 PFS (continuous, HR 1.11, 95%CI 1.01-1.23, p=0.03). To separately evaluate differences in PFS for each ADC depending upon whether it was first or follow-up therapy, median ADC1 vs. ADC2 PFS is summarized by ADC (Table). Conclusions: In this heavily pre-treated patient cohort, the clinical activity of both SG and TDXd appear modest in the ADC2 setting. Further studies are needed to confirm these findings and to determine whether payload cross-resistance may undermine the benefit of sequential ADC therapy. [Table: see text]

A phase I/II trial of palbociclib, pembrolizumab, and endocrine therapy for patients with HR+/HER2- locally advanced or metastatic breast cancer (MBC): Clinical outcomes and stool microbial profiling.

1038 Background: Based on preclinical data highlighting synergy between CDK4/6 inhibitors and immune checkpoint inhibitors (ICIs), we conducted a phase I/II study of palbociclib, pembrolizumab, and endocrine therapy (ET) in pts with HR+/HER2- MBC. Given evidence suggesting a correlation between ICI response and stool microbiome composition in a wide array of cancers, we sought to determine if this biomarker could segregate responders in our cohort. Methods: Pts with stage IV HR+/HER2- MBC were enrolled and treated with palbociclib, pembrolizumab, and ET. Cohort 1 enrolled pts with stable disease on palbociclib + ET for at least 6 months. Cohort 2 and 3 enrolled pts in the first-line (1L) setting, with cohort 3 designed with a palbociclib + ET lead-in to evaluate the immune potentiating effect of palbociclib (not included in this report). The primary endpoint was objective response rate (ORR); secondary endpoints included progression-free survival (PFS), overall survival (OS), and toxicity. Correlative studies assessed the association between the gut microbiome and response using previously published methods (1). Results: Between March 2017 and May 2022, 40 pts were accrued, with 4, 19 and 17 pts in cohorts 1, 2 and 3, respectively. There were 39 females and 1 male, with a median age of 51 (range 39-75). The ORR in cohorts 1, 2 and 3 were 2/4 (50%), 11/19 (58%) and 13/17 (76%), respectively. Median PFS across cohorts 2 and 3 (reflecting 1L therapy) was 25 mo (95% CI 21- not reached), and at a median follow-up of 35 mo, OS was 76% (95% CI 0.60-0.96). The most common grade 3/4 adverse events across all cohorts included neutropenia (83%), leukopenia (56%), elevated alanine aminotransferase (13%), elevated aspartate aminotransferase (13%), and thrombocytopenia (10%). Gut microbial profiling (total N=24 pts; Arm 2 N=7, Arm 3 N=17) demonstrated multiple species higher in abundance in pts with a response (p < 0.05), with notable species belonging to the families Lachnospiraceae, Ruminococcaceae, and Rikenellaceae. Several metabolic pathways were enriched in responders (p-value < 0.05), including pathways related to fatty acid biosynthesis and the tricarboxylic acid (TCA) cycle. Conclusions: Pembrolizumab added to palbociclib + ET in the 1L setting demonstrated a higher response rate compared to palbociclib + ET alone in HR+/HER2- MBC. Specific gut profiles were associated with response, with similar taxa of bacteria associated with ICI response in other tumor types (2). Enriched metabolic pathways may generate microbiota-derived metabolites that have an immunomodulatory effect. These results suggest that the gut microbiome may be associated with response to ICI combination therapy in MBC, which warrants further evaluation. 1. Dizman et al.,Nature Med 2022. 2. Spencer et al Science 2021. Clinical trial information: NCT02778685 .

Use of liquid biomarker thymidine kinase activity (TKa) to predict outcome and progression in patients with metastatic breast cancer (MBC) in the GEICAM/2014-12 FLIPPER trial.

1028 Background: There is a need for more precise blood-based tumor biomarkers in MBC. TKa is a liquid proliferation biomarker providing prognostic, predictive and monitoring information in MBC. TKa can be measured in blood using the DiviTum TKa assay. Here we report the results of TKa in a trial of endocrine therapy (ET) fulvestrant (F) + the CDK4/6 inhibitor (CDK4/6i) Palbociclib (Pa) vs. F + Placebo (Pl), the GEICAM FLIPPER trial (NCT02690480), a multicenter, double-blind, randomized (1:1) study. F+Pa showed 1-year PFS rates of 83.5% vs. 71.9% (p=0.064) and better median (m)PFS (31.8 vs. 22.0 months, p=0.001) than F+Pl in first line treatment of HR+/HER2- MBC patients. F+Pa also significantly improved ORR and CBR. ET+CDK4/6i are currently the cornerstone of treatment for HR+/HER2- MBC. However, 10-30% of these patients display primary resistance to treatment. Early identification of patient subsets with different risk of progression during treatment with CDK4/6i may help tailor clinical management and disease monitoring. Methods: Sequential plasma was collected from 187 patients (pts) at baseline (BL) and at 12, 24, 36, 48 weeks (w) on treatment, and at progression. 910 samples were analyzed for TKa with the FDA approved and CE labelled DiviTum TKa assay (Biovica, Sweden) using DuA (DiviTum unit of Activity) as measuring unit. The cutoff for categorizing in high vs low is 250 DuA. The Kaplan-Meier method was used to estimate mPFS and mOS. Unadjusted and adjusted hazard ratio (HR) and odds ratio (OR) with 95% confidence interval (CI) were calculated with the Cox proportional hazards regression model and with logistic regression respectively, considering relevant confounders. Results: Low BL TKa predicted better PFS (30.5 months mPFS in low vs 19.8 in high, HR 1.62; 95% CI 1.11-2.35, p=0.011) and OS (HR 1.81; 95% CI 1.15-2.85, p=0.0099). Higher TKa at BL and at 12w was detected in pts that progressed before 12 months. Conversely, low BL TKa, maintained throughout week 12-48, were associated with objective response (HR 2.58; 95% CI 1.38-4.81, p=0.0029) and OS (HR 5.74; 95%CI 2.06-16, p=0.0008). At progression, pts on F+Pa tended to have higher TKa levels than F+Pl (246 vs. 138 DuA), reflecting faster growing tumors, not achieving statistical significance (p=0.0585). TKa monitoring and dynamics from BL and on treatment timepoints (week 12-48) show that the group of pts with low TK at BL that remain low during treatment is enriched with non-progressors and pts that progress after 12 months (p=0.0108). High TKa at BL predicted shorter OS in the F+Pa arm (HR 4.39; 95% CI 2.15-8.99, p<0.0001). Conclusions: Measuring blood TKa levels can monitor and predict efficacy of F+/-Pa at BL and during therapy in MBC. TKa dynamics before and during therapy are associated with PFS and OS. Future studies will provide additional knowledge on the utility of TKa in MBC. Clinical trial information: NCT02690480 .

Genomic testing and uptake of alteration-targeted therapies among patients with hormone-receptor positive (HR+) metastatic breast cancer (MBC).

e13084 Background: There is no recognized standard of care treatment after progression on first-line endocrine therapy plus CDK4/6 inhibitor in HR+ / HER2 negative MBC. Alteration-targeted therapies (ATT) have the potential to delay initiation of chemotherapy with more favorable toxicity, and perhaps improved efficacy. Next-generation sequencing (NGS) of tumor tissue or circulating tumor DNA (ctDNA) can identify actionable mutations for targeted therapy. In this study, we sought to assess the results of genomic testing and uptake of ATT among patients with HR+ / HER2-negative MBC. Methods: We studied a retrospective cohort of patients with MBC who received treatment in the Koontz Center for Advanced Breast Cancer (KABCC) at St. Luke’s Cancer Institute, a community-based health system in the Kansas City metro area. The analytic cohort included 125 patients with HR+ / HER2-negative MBC who received treatment after progression on first-line therapy. Genomic testing was performed as a standard of care from ctDNA or tumor-tissue using commercially available NGS assays. Targetable alterations were defined as those with FDA-approved or routinely used off-label treatment regimens in MBC ( PIK3CA, PTEN, AKT, ESR1, ERBB2, BRCA1, BRCA2, PALB2 mutations; TMB >10 or PDL1+). We identified patients who received any ATT and compared them with those who did not. Overall survival (OS) and time to initiation of chemotherapy (TTC) were compared between the two groups using Kaplan-Meier analyses and the log-rank test. Results: Ninety-nine patients (79.2%) had genomic testing and 49 (39.2%) were tested prior to second-line therapy. Of those tested, 84% had at least one targetable alteration. ESR1 mutations were most common, detected in 36% of patients, followed by PIK3CA (31.2%), PTEN (12%), BRCA1/2 (7.2%) and ERBB2 (5.6%). A total of 35 patients received ATT, and around half of those (17 patients) received ATT in the second line. Alpelisib was the most common ATT (62.9%), followed by elacestrant (20%) and neratinib (14.3%). There was no significant difference in OS or TTC between the two groups. Conclusions: Approximately 80% of patients with HR+ / HER2-negative MBC in our cohort underwent genomic testing after progression on first-line therapy, and the vast majority had at least one targetable alteration identified. However, only 35 patients received ATT with most receiving alpelisib. We observed no significant improvement in OS or TTC among those who received ATT, although the numbers were small. There have been two new targeted therapies approved in the past year and many novel agents are currently under investigation in clinical trials. Newer therapies with distinct targets, improved tolerability and efficacy have the potential to improve the uptake and clinical benefit of targeted therapy in MBC.

Efficacy of first-line dual oral pyrotinib plus capetabine therapy in HER2-positive metastatic breast cancer: A real-world retrospective study.

The combination of dual-targeted human epidermal growth factor receptor 2 (HER2) therapy and chemotherapy is the standard first-line regimen for recurrent/metastatic breast cancer (mBC). However, the toxicity of such combination therapy can lead to some patients being unable to tolerate adverse events or bear treatment costs. As a novel irreversible pan-ErbB receptor TKI (pyrotinib), can the dual oral administration of pyrotinib plus capetabine (PyroC) provide first-line survival benefits and serve as a more affordable treatment option? This real-world retrospective study included patients diagnosed with HER2-positive mBC who received PyroC as a first-line treatment at West China Hospital between May 2018 and July 2023. The survival data and toxicity profiles were reported in this study. A total of 64 patients received PyroC as first-line therapy. The median progression-free survival (PFS) was 19.6 months (95% CI 15.0-27.2), while overall survival (OS) has not yet been reached. Kaplan-Meier analysis indicated that age (≥60, p = 0.03) and metastasis sites (p = 0.004) were related to poor efficacy of PyroC, while there was no relationship between effectiveness and menstrual status, hormone receptor (HR) status or previous treatment with anti-HER2 therapy. Furthermore, the objective response rate (ORR) and disease control rate (DCR) were 79.7% and 98.4%, respectively. Of the patients, 78.1% reported treatment-related adverse events (TRAEs). The predominant adverse events were diarrhea (n = 46, 71.9%) and hand-foot syndrome (n = 10, 15.6%). The dual oral administration regimen (PyroC) has a promising ORR or PFS in HER2-positive mBC patients, with an acceptable safety profile and convenience.

LINC02535 + miR-30a-5p combination enhances proliferation and inhibits apoptosis in metastatic breast Cancer cells

Current clinical therapies for metastatic breast cancer (MBC) have limited therapeutic efficacy and induce significant systemic side effects, leading to poor patient compliance. To address this challenge, this investigation focuses on the design of LINC02535 + miR-30a-5p for treating breast cancer. In vitro cytotoxicity studies confirmed that LINC02535 + miR-30a-5p was more effective in 4 T1 cells, with reduced toxicity in NIH3T3 cells. Further verification of cellular morphology was achieved through various biochemical staining methods. Additionally, the antimetastatic attributes of LINC02535 + miR-30a-5p have been evaluated using both migration scratch and Transwell migration assays, which have collectively revealed excellent antimetastatic potential. The DNA fragmentation of the 4 T1 cells was assessed using a comet assay. LINC02535 + miR-30a-5p improved ROS levels and caused mitochondrial membrane potential alterations and DNA damage, which resulted in apoptosis. Therefore, we propose that LINC02535 + miR-30a-5p could be an alternative therapeutic strategy for breast cancer therapy.

CDK4/6 inhibitors: The Devil is in the Detail.

Update on the most recent clinical evidence on CDK4/6 inhibitors (CDK4/6i) in the treatment of hormone receptor (HR)-positive, human epidermal growth factor receptor (HER)2-negative breast cancer. Over the past decade, CDK4/6i have become part of the standard of care treatment of patients with both metastatic and high-risk early HR + /HER2- breast cancers. The three available CDK4/6i (palbociclib, ribociclib and abemaciclib) have been extensively studied in combination with endocrine therapy (ET) in metastatic breast cancer (mBC) with consistent prolongation of progression free survival; however, ribociclib has emerged as the preferred first line agent in mBC given overall survival benefit over endocrine monotherapy. In early BC, abemaciclib is the only currently approved agent while ribociclib has early positive clinical trial data. Toxicities and financial burden limit the use of CDK4/6i in all patients and resource-poor settings, and optimal timing of their use in mBC remains unclear. There is considerable evidence for the use of CDK4/6i in metastatic and early HR + /HER2- breast cancer, but knowledge gaps remain, and further research is necessary to better define their optimal use.

Abstract PO2-04-01: Predictors of exceptional response to first line HER2-targeted therapy for metastatic breast cancer

Abstract Background HER2-targeted therapies have transformed the trajectory of HER2-positive (HER2+) metastatic breast cancer (MBC), with a subgroup of responders remaining on first line (1L) therapy for many years (yrs). Despite these promising outcomes, the paradigm remains palliative, with some patients (pts) receiving therapy indefinitely. Opportunities to interrupt therapy are controversial and anecdotal in the setting of a limited understanding of long-term responders and lack of predictive biomarkers. Methods We identified prevalent pts with HER2+ MBC seen and consented at Dana-Farber Cancer Institute between 2010 and 2023, regardless of the original date of MBC diagnosis. Exceptional responders (ExRes) were pts without evidence of progressive disease (PD) 3 yrs from initiation of 1L therapy for MBC. Conventional responders (ConRes) were pts who experienced PD within 3 yrs of 1L treatment initiation. We compared clinicopathological characteristics and treatment patterns between ExRes and ConRes using Chi-square or Wilcoxon test. We analyzed median time to treatment switch due to PD (TTS-PD) – i.e., time from metastatic diagnosis to 1L treatment end due to PD - and overall survival, via the Kaplan-Meier method and with a landmark analysis at year 3 (Y3). Our primary aim was to identify predictors of exceptional response to 1L anti-HER2 therapy. Results Of 635 pts with HER2+ MBC, we identified 147 ExRes and 370 ConRes and excluded 118 pts due to follow up ≤ 3 yrs and no PD events. Median follow up was 7.1 yrs (IQR 5.5-11.0) for ExRes and 7.1 yrs (IQR 4.0-11.4) for ConRes. Median age at MBC diagnosis was 50.7 yrs (range 21.9-91.9) for ExRes and 49.8 yrs (26.9-82.3) for ConRes. ExRes presented more often with de novo MBC than ConRes (52.1 vs 30.6%, p< 0.0001). On metastatic samples, more ExRes than ConRes had HER2 3+ tumors by immunohistochemistry (IHC) (93.7 vs 81.0%, p=0.002), whereas the proportion of ER-positive disease was similar between the two groups (45.6 vs 53.4%, p=0.1). Most pts received (neo)adjuvant anti-HER2 agents in both groups (62.5 vs 72.7%, p=0.1). More ExRes than ConRes (55.8 vs 42.7% p=0.007) received 1L chemotherapy (CT) plus trastuzumab (H)/pertuzumab (P). Alternative 1L treatments were CT plus H (26.5 vs 25.7%), H +/- P +/- endocrine therapy (8.2 vs 10.5%), T-DM1 (2 vs 9.2%), tyrosine kinase inhibitors-based regimens (7.5 vs 10.8%). For pts with recurrent MBC, disease-free interval was longer for ExRes than ConRes (median 4.7 vs 3.4 yrs, p=0.01). Visceral involvement at MBC relapse was similar for ExRes and ConRes (78.2 vs 77.8%, p=0.9). Brain metastases at any timepoint were less frequent among ExRes than ConRes (42.9 vs 55.1%, p=0.01). Among ExRes, 70 (47.6%) pts experienced PD after Y3, with a median TTS-PD of 4.6 (4.1-5.1) yrs. In a landmark analysis at Y3, the 2-year TTS-PD was 68.5% (95% CI: 59.8%-75.7%) (5 yrs from treatment start), and 4-year overall survival was 86.5% (95% CI: 78.1%-91.9%) (7 yrs from treatment start). ConRes received a median of 5 (1-19) treatment regimens for MBC, with a median 1L TTS-PD of 12 (10.8-13) mo. A total of 89 (60.5%) ExRes and 214 (57.8%) ConRes underwent tumor sequencing. Data comparing genomic features between ExRes and ConRes will be presented. Conclusions In this prospective cohort, 28% of pts with HER2+ MBC had exceptional response to 1L anti-HER2 therapy. Consistent with prior data, ExRes present more frequently with de novo disease and HER2 IHC 3+ tumors. In our cohort, ExRes had a median follow-up of 7.1 years and 52.4% never experienced PD. Although a significant proportion of pts with HER2+ MBC derive long-term benefit from anti-HER2 therapy, the treatment paradigm remains palliative, and it is unknown whether biomarkers may reliably predict exceptional response and guide future treatment. Ongoing work will explore whether genomic features vary between ExRes and ConRes. Citation Format: Stefania Morganti, Tianyu Li, Katheryn Santos, Melissa Hughes, Nolan Priedigkeit, Yvonne Li, Marla Lipsyc-Sharf, Gregory Kirkner, Janet Files, Julie Kasparian, Elizabeth Grant, Gunjan Gupta, Giuseppe Curigliano, Sara Tolaney, Andrew Cherniack, Nabihah Tayob, Nancy Lin, Heather Parsons. Predictors of exceptional response to first line HER2-targeted therapy for 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 PO2-04-01.

Abstract PO4-06-04: Understanding metastasis mixed-treatment responses through genomic analyses

Abstract Background Early-stage and metastatic breast cancer (MBC) exhibit genomic heterogeneity, with inter-patient variability between primary tumors, intra-patient variability of primary tumors versus metastasis, and even between different metastatic sites within the same patient. Response to therapy in MBC can also be heterogeneous, termed "mixed response", with different degrees of responsiveness to the same drug(s) across metastatic sites within the same patient. Whether this treatment response variability is influenced by factors such as intrinsic characteristics of metastatic lesions and/or the microenvironment is unknown. Through genomic analysis, we aim to identify genomic features that may explain mixed clinical responses across metastatic sites, which will also provide valuable insights into treatment sensitivity and resistance. Methods Eligible patients with MBC were identified from the UNC Rapid Autopsy Program (RAP) with primary tumors and multiple metastases (≥ 2 different sites) with RNA sequencing data. An independent radiologist retrospectively reviewed imaging studies to identify patients with mixed radiographic responses to a defined line of therapy. The response to treatment was categorized for each metastasis, defined as a complete or partial response, as stabilization ≥ 6 months. Tumor measurements were performed for each patient at different time points. We performed supervised learning with linear regression incorporating "RNA sequencing-based gene expression signatures" as a fixed effect and "patient" as a random effect. We performed two different analyses: (1) Using all tumors and (2) using Basal-like tumors only. We finally selected significant signatures with a p-value >0.05. Results Ten patients with mixed response were identified, and 33 metastases were used for the analysis; 3 had de novo stage IV disease, 5 had triple-negative (TNBC), 4 HER2+, and 4 hormone receptor+ (HR+/HER2-) disease. Of 33 metastases, 6 were brain, 8 liver, 7 lung, and 12 “other”. Regarding intrinsic subtype, 15 metastatic tumors were Basal-like, 3 HER2-Enriched (HER2E), 6 Luminal A, and 9 Luminal B. In this dataset, 6 metastatic lesions showed response and 27 non-response. When analyzing all 33 metastases, KRAS high expression, KRAS amplicon, 16q23 amplicon, and apocrine features correlated with response independently of metastatic site. Higher T reg and CDKN2A gene expression levels were associated with non-response in metastasis. Within Basal-like subtype metastases only (N=15), CD8 T cells, PI3K pathway, KRAS gene and amplicon, 16q23 amplicon, and Basal-like-associated 5q11 loss correlated with response. However, T reg cells, higher levels of CDKN2A gene, and stem-cell-like features were associated with treatment non-response. Conclusions By examining matched metastases with discordant responsiveness to treatment within individual patients, this genomic analysis provides valuable insight into treatment sensitivity and resistance. Higher T reg cells and CDKN2A gene expression values correlate with non-response, while the KRAS gene, KRAS amplicon, and the 16q23 amplicon were associated with response. These results suggest that the molecular tumor and microenvironment features of metastases may contribute to sensitivity and resistance to therapy in MBC. Further validation and exploration are needed in larger multi-metastatic cohorts to fully understand the clinical implications of these findings. Citation Format: Susana Garcia-Recio, Paola Zagami, Amy Wheless, Kerry Thomas, Lisa Carey, Charles M Perou. Understanding metastasis mixed-treatment responses through genomic analyses [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 PO4-06-04.

Abstract PO4-14-10: Circulating Tumor DNA as a Biomarker for ADCs in Metastatic Breast Cancer

Abstract Background: Breast cancer is a complex disease characterized by heterogeneity, and the analysis of circulating tumor DNA (ctDNA) holds promise for capturing this heterogeneity. Recently, ctDNA testing has been employed in clinical practice to guide targeted therapies for metastatic breast cancer, particularly in cases of hormone receptor-positive disease with mutations in PIK3CA and ESR1. This approach becomes necessary when traditional tumor biopsies are inadequate for next-generation sequencing (NGS) testing or fail to capture the full extent of the cancer's heterogeneity. Moreover, the utilization of antibody-drug conjugates (ADC) in addressing tumor heterogeneity continues to expand. However, there has been no biomarker, outside of HER2 protein expression, to predict ADC response. In this project, we test the hypothesis that ctDNA can be utilized as a biomarker to predict response to novel therapies such as ADCs. Methods: We analyzed a subset of patients from the Dallas Metastatic Breast Cancer Study comprised of patients with metastatic breast cancer (n= 109) who underwent ctDNA testing using the Tempus xF liquid biopsy ctDNA sequencing panel. This panel detects 105-genes that are known oncogenic drivers and resistance mutations. The data was collected from a single academic medical center between the initial year of ctDNA collection in 2019 and 2023. Results: Among these patients, 85 (77.9%) had hormone receptor-positive disease, 13 (11.9%) had triple-negative breast cancer, and 11 (10.0%) had HER2-positive disease (including 3 (2.75%) patients with triple-positive disease). Only 22 of these patients had tissue biopsies that underwent NGS testing. Analysis of all patients revealed that the most common gene alteration was PIK3CA, which was identified in 70 patients. Specifically, the most frequently observed alteration was PIK3CA p.545K, found in 25 patients. Table 1 further details the frequency of each gene in this subset of patients. Among the patients, 34 out of 109 (31%) received an ADC, while only 8 out of 109 (7%) received immunotherapy. Patients with CDKN2A achieved the longest progression-free survival (PFS) on an ADC, with a median PFS of 7 months, whereas patients carrying mutations in TP53, PIK3R1, PIK3CA, PTEN, NF1, BRAF, RHOA, FGFR4, KRAS, TERT, GATA3, HNF1A, FB1, STOP had median PFSs of 2 months or less. Table 2 provides additional information on the response of patients with specific mutations to ADCs. Notably, only one patient received sequential ctDNA testing, and in that case, there was a gain of TP53 p.N239S following sacituzumab govitecan administration compared to the baseline ctDNA test, followed by a gain of PIK3CA p.P449_L455del after trastuzumab deruxtecan administration. Conclusions: As we observe changes in subtypes following treatment progression, researchers are actively seeking biomarkers to better characterize real-time changes in tumor biology that can predict response and resistance to treatments. Further studies are needed to identify additional genes present in ctDNA that can provide mechanistic insights into why certain patients respond favorably to ADCs while others do not. We are currently enrolling patients for a prospective study that aims to assess baseline ctDNA levels and monitor changes in variant allele frequency following treatment with ADCs and immunotherapy. Table 2 Median PFS on ADC by Gene Citation Format: Hannah Chang, Isabela Anawate, Alyssa Low, Shao-Po Huang, Julia Maues, Christine Hodgdon, Isaac Chan. Circulating Tumor DNA as a Biomarker for ADCs in 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 PO4-14-10.

Abstract PO2-23-12: Innovative Metastatic Breast Cancer Therapy, CBT300, Reverses Drug and Immune Resistance

Abstract For many patients with metastatic breast cancer (MBC), their disease is treatable but incurable. Although the rate of death from MBC has significantly decreased over the last 10 years due to early detection and new treatment options, MBC still accounts for over 40,000 deaths each year in the US that has been consistent for the past 30 years. Recently, it is estimated that 90% of those deaths were due to drug resistant recurrent disease. Nearly 45% of MBC deaths can be attributed to a subpopulation of patients with metastatic triple negative breast cancer (mTNBC). Despite MBC’s initial sensitivity to chemotherapy, it often recurs at greater than 40% in stage I-III patients and greater than 80% in stage IV patients which have a dismal 5-year survival of 12%. African American women have a 40% greater incidence of death from MBC despite a 4% lower risk of diagnosis. Unfortunately, current therapies do little to reduce drug resistance and death for recurrent drug resistant MBC. Currently, the only therapies approved by the FDA for recurrent TNBC patients are Keytruda plus chemotherapy and an Antibody Drug Conjugate (ADC) called Sacituzumab Govitecan. Keytruda (anti-PD-L1) is an immune checkpoint inhibitor(ICI) given with chemotherapy (nab-paclitaxel, paclitaxel, or gemcitabine plus carboplatin). The second recently approved therapy, Sacituzumab Govitecan (SG), is composed of an antibody to human trophoblast cell-surface antigen 2 (Trpo-2) coupled to SN-38 (topoisomerase I inhibitor). Although these results are very encouraging, the 80% of TNBC patients that either do not have high positive PD-L1 tumors, or don’t respond to these therapies and become drug resistant suggests that most patients with TNBC will have recurrent disease with little or no hope for survival. For contrast, our novel MBC therapy, CBT300, targets cell surface GRP78 that has been found in over 95% of MBC and in 93% of TNBC tumors. We can now show that inhibition of cell surface GRP78 can a) induce apoptosis of drug resistant TNBC cells in vitro and in vivo, b) eliminate drug resistance showing synergistic effects with chemotherapy in vitro and in vivo, c) decrease amount of chemotherapy in combination with CBT300 and d) reduce immune suppression. Recent publications show that GRP78 is found on many types of tumor cell surfaces but not on normal cell surfaces. In fact, tumor cell surface GRP78 (csGRP78) is important for many aspects of MBC development, including cell survival, proliferation, chemoresistance, angiogenesis, metastasis formation, immune suppression, and stem cell formation. Recently, it has been shown that increased cell surface GRP78 expression in TNBC patients was significantly associated with later stage, increased distant metastasis, increased aggressiveness, shorter disease-free survival, and decreased overall survival. In studies to help understand how cell surface GRP78 causes MBC progression and drug resistance, we discovered a novel GRP78 binding transmembrane protein on TNBC cells called Receptor Tyrosine Kinase Orphan Receptor-1 (ROR1). Using the GRP78 binding domain from ROR1 and a human Fc IgG1 domain, we created a biologic fusion protein that is a potent and cell surface specific GRP78 inhibitor, CBT300. We now show that CBT300’s elimination of cell surface GRP78 destabilizes and removes oncofetal proteins ROR1, Cripto-1, and checkpoint protein PD-L1 from tumor cell surfaces resulting in reversal of chemoresistance, reduction in immune suppression, inhibition of stem cell phenotype and increased tumor cell apoptosis. Since ROR1, Cripto-1 and PD-L1 are three of the major redundant pathways for tumor drug resistance, immune suppression and stem cell formation, the ability to inhibit all these pathways with CBT300, a single safe and very efficacious therapy, is very innovative and could be a significant advance for the treatment of MBC. Citation Format: Elizabeth Stolarik, Jorryn Zelek, Mariah Thigpen, Anita Davidson, Donald Davidson. Innovative Metastatic Breast Cancer Therapy, CBT300, Reverses Drug and Immune Resistance [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 PO2-23-12.

Abstract PO3-06-11: Undated analyses from a Phase Ib open-label study of Pucotenlimab (HX008) in combination with gemcitabine and cisplatin as first-line treatment of metastatic triple-negative breast cancer

Abstract Background: Gemcitabine plus cisplatin (GP) therapy war preferred in China as the first-line therapy for metastatic triple-negative breast cancer (mTNBC), with a median progression-free survival (PFS) of 7.73 months, and a median overall survival (OS) of 19.37 months [1]. Pucotenlimab (HX008) is a novel humanized IgG4 anti-PD-1 monoclonal antibody with an engineered Fc domain. Preliminary results from the phase 1b trial [2] have shown that HX008 combined with GP has a manageable safety profile and demonstrates promising antitumor activity in patients (pts) with mTNBC. Here, we present the updated results from the long-term survival analysis. Methods: Eligible subjects were those who were treatment-naïve mTNBC patients and had measurable lesion. Participants received HX008 at a dose of 3 mg/kg every 3 weeks in combination with chemotherapy (gemcitabine + cisplatin) for 6 cycles and maintained with HX008 until disease progression or unacceptable toxicity occurred. The primary endpoints included overall response rate (ORR) and safety, and the secondary endpoints included duration of response (DoR), PFS, and OS. Results: Between July 2019 and March 2020, a total of 31 pts were enrolled in this study. The median age of patients was 50 years (range 29–69). By the cut-off date (April 24th, 2023), the median follow-up period was 20.7 m (IQR: 12.2, 25.6). The confirmed ORR was consistent with the previously reported data and remains at 74.2% (95% CI: 55.4%, 88.1%). The median PFS was 9.0 months (95% CI, 7.3, 9.3). The median DoR was 7.3 months (95% CI, 5.0, 9.8). The OS was immature, with the result of 23.1 months (95% CI, 13.4, NE), while the 12-m, 24-m and 36-m OS rate was 77.4% (95%CI: 64.0%, 93.6%), 42.7% (95%CI: 28.0%, 65.3%), and 24.4% (95%CI: 11.3%, 52.7%), respectively. The most common grade 3 or 4 treatment-related adverse events included neutropenia (71.0%), leukopenia (45.2%), anemia (38.7%), and thrombocytopenia (32.3%). There was no treatment-related death and no new safety signals were identified. Conclusion: The combination therapy of HX008 plus GP demonstrates favorable clinical efficacy with a manageable safety profile in pts with mTNBC. The combination therapy led to a longer PFS and OS than GP therapy reported previously, regardless of PD-L1 status. The safety profile was consistent with the known toxic effects of each agent. In this updated analysis, no new safety signals were identified. Clinical trial information: NCT04750382

Abstract PO5-20-10: Treatment with CDK4/6 Inhibitor in a Patient with Metastatic Breast Cancer Causing Myelophthisis

Abstract Background: Symptomatic bone marrow metastasis causing myelophthisis is a rare manifestation of metastatic breast cancer, indicating a poor prognosis due to significant cytopenias. CDK4/6 inhibitors are first-line therapies for metastatic hormone-positive breast cancer but remain an unclear choice in breast cancer patients with significant cytopenias due to associated hematological adverse effects. We present a case of biopsy-proven hormone-positive breast cancer with bone marrow infiltration causing profound pancytopenia. CDK4/6 inhibitors were safely administered despite their cytopenic effects. Case Presentation: A 59-year-old female presented after an abnormal annual mammogram and biopsy showed calcifications stromal fibrosis, cyst formation, and usual ductal hyperplasia in the right upper outer quadrant. One month later, she presented to an outside hospital with gingival bleeding, epistaxis, progressive dyspnea on exertion, and generalized weakness. Labs showed severe anemia with a hemoglobin of 4.8 g/dL (baseline 12.1 g/dL), thrombocytopenia with platelets of 5 K/mm (baseline 215 K/mm), LDH of 879, reticulocyte count of 5%, and negative direct Coombs test. Peripheral smear showed no schistocytes, but showed nucleated RBCs, severely reduced platelets, and occasional giant platelets. The patient was treated for presumed ITP, and the presence of nucleated RBCs prompted bone marrow biopsy which confirmed metastatic invasive lobular carcinoma breast cancer (ER 90-95%, PR 90-95%, HER2 1+). She was discharged on letrozole 2.5 mg daily with close outpatient oncologic follow-up. One week later, she presented to our institution with persistent symptomatic anemia and thrombocytopenia. Inpatient systemic therapy was begun using dose-reduced Palbociclib (75mg) and letrozole with close CBC monitoring while also receiving packed red blood cells (goal >6.5) and platelet transfusions (goal >10). Repeat bone marrow biopsy showed extensive marrow fibrosis, but no significant hematopoietic elements. ADAMTS13 was 134%, ruling out TTP, and she was treated again for presumed ITP with dexamethasone and IVIG, Throughout the 4-week admission, her right breast mass decreased in size and she was discharged after completing 3 weeks of Palbociclib with biweekly outpatient transfusions. During follow-up, the patient’s hemoglobin and platelets slowly started to respond allowing her to become transfusion-independent and allowing dose escalation. Restaging scans 2 months after hospitalization showed an overall improvement of disease burden. The patient continued to do well for almost 18 months, however, later redeveloped visceral crisis and is responding to chemotherapy. Discussion: There is limited data on the management of breast cancer with bone marrow infiltration at presentation. A wide range of survival data in different case reports exists, ranging from 5 to 44 months. However, chemotherapy was used in these studies rather than CDK4/6 inhibitors. By far, cytopenias are the most concerning grade 3 and 4 side effect of Palbociclib. Data from the PALOMA-1 trial suggest that hematologic toxicities, specifically neutropenia, were different from those associated with cytotoxic chemotherapies and are more transient and reversible in nature. Studies have examined the in vitro mechanism of bone marrow suppression in Palbociclib versus cytotoxic chemotherapy, and its cytostatic effect on neutrophil precursors allows for hematological toxicities to be transient. With strong supportive care measures, patients in bone marrow visceral crisis can be safely treated with cytopenic CDK4/6 inhibitors. Citation Format: Nikita Dahake, Jordan Senchak, Jalil Nasibli, Jason Incorvati. Treatment with CDK4/6 Inhibitor in a Patient with Metastatic Breast Cancer Causing Myelophthisis [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 PO5-20-10.

Abstract RF02-02: BBO-10203, a first-in-class, orally bioavailable, selective covalent small molecule that inhibits RAS-driven PI3Kalpha activity without affecting glucose metabolism

Abstract PI3Kα is the most mutated kinase and the second most mutated oncogene in human cancer. Activation of PI3Ka can be achieved by receptor tyrosine kinases such as insulin receptor and insulin-like growth factor receptor 1 and/or by directly interacting with RAS family members. Previous elegant preclinical studies have established that RAS-driven PI3Ka activation is important in tumor cells but may not be involved in cell types controlling glucose metabolism. Alpelisib, a small molecule inhibitor of the kinase activity of PI3Ka, has been approved for the treatment of ER+ PIK3CA mutant breast cancer in combination with fulvestrant after endocrine therapy in advanced or metastatic breast cancer based on an improvement in PFS versus fulvestrant alone. Inhibition of PI3Ka activity by alpelisib in normal tissues resulted in a severe (G3/4) hyperglycemia rate of 37% with frequent dose interruptions and discontinuations. Additionally, preclinical studies have demonstrated that the dysregulation of glucose homeostasis resulting from PI3Ka kinase inhibition leads to hyperinsulinemia that increases pathway flux, rendering kinase inhibitors less effective. Here, we report on a novel covalent small molecule designed to inhibit RAS-mediated activation of the AKT pathway via PI3Ka without the resultant hyperglycemia associated with direct inhibition of PI3Ka kinase activity. BBO-10203 disrupts the physical interaction between RAS and PI3Ka in tumor cells resulting in potent signaling pathway inhibition. This agent selectively binds to PI3Ka and disrupts its interaction with K-,H-, and N-RAS with low single digit nanomolar potency (~5 nM). Breaking the interaction between these two oncogenes inhibits basal pAKT cellular levels (BT-474/KYSE-410 IC50: ~5 nM) in HER2 amplified (HER2amp) and wild-type or mutant PI3Ka cell lines. Even though BBO-10203 does not inhibit the kinase activity of PI3Ka, its effects on cancer cell signaling inhibition and transcriptional regulation highly resemble those of alpelisib. BBO-10203 displays excellent drug-like properties and oral bioavailability. Single dose treatment of KYSE-410 (HER2amp/KRASG12C) tumor bearing mice with increasing doses (1-100 mg/kg, PO) of BBO-10203 results in dose and time dependent inhibition of pAKT in vivo. Maximal inhibition (~80%) is achieved at 30 mg/kg and lasts for 24 hours. Repeated dose treatment of tumor bearing mice with BBO-10203 is well tolerated and results in significant efficacy in PIK3CA mutant as well as HER2amp human xenograft models. In the KYSE-410 xenograft model, BBO-10203 daily oral dosing of 30 mg/kg results in significant tumor regressions. Importantly, treatment with BBO-10203 does not affect insulin signaling in differentiated adipocytes in vitro, nor does it impact glucose metabolism in vivo at 3-times the maximal efficacious dose level in xenograft studies. In conclusion, we have identified a novel approach to inhibit the PI3Ka signaling pathway by blocking its interaction with, and activation by RAS. This approach can achieve strong pAKT inhibition in tumor cells without changes in glucose metabolism. Clinical investigation of BBO-10203 for the treatment of both ER+/PIK3CA mutant and HER2amp breast cancer is warranted. Citation Format: Pedro Beltran, Simanshu Dhirendra, Rui Xu, Ming Chen, Daniel Czyzyk, Sofia Donovan, Siyu Feng, Cindy Feng, Lijuan Fu, Felice Lightstone, Ken Lin, Anna Maciag, Dwight Nissley, Erin Riegler, Kerstin Sinkevicius, Andrew Stephen, James Stice, David Turner, Bin Wang, Keshi Wang, Yue Yang, Cathy Zhang, Frank McCormick, Eli Wallace. BBO-10203, a first-in-class, orally bioavailable, selective covalent small molecule that inhibits RAS-driven PI3Kalpha activity without affecting glucose metabolism [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 RF02-02.

Abstract PO1-14-07: Programme of mast cell subsets to potentiate breast cancer immunotherapy: from bed to bench to bed (the phase 2 platform RENAISSANCE trial)

Abstract Background: Immune checkpoint inhibitors (ICIs) have heralded a new era in breast cancer treatment; however, response rates remain limited, making precision immune-oncology a major unmet need. In addition to T cells, effective immune responses to ICIs rely on coordinated interactions between innate and adaptive immune cells. Mast cells are evolutionarily conserved, tissue-resident cells of importance to human health. Specific subsets of mast cells might be endowed with opposite roles in cancer treatment, yet the extent of mast cell heterogeneity and its clinical merit in immunotherapy remain undefined. Objective: We sought to comprehensively characterize mast cells in breast cancer, investigate their association with immunotherapy response with in-depth mechanistic insights, and identify actionable strategies to modulate mast cell functional states, thereby optimizing immunotherapy efficacy. Methods: We employed single-cell profiling on longitudinal breast cancer samples from three independent clinical trials (NCT04613674, NCT03197389 and GSE169246) to delineate mast cell heterogeneity in anti-PD-(L)1 therapy. By integrating multi-omic analyses, tissue characterization, preclinical experiments, transgenic mice, and high-throughput drug screening, we outlined the molecular features, underlying mechanisms, and clinical relevance of distinct mast cells to elicit ICI-responsive microenvironments. Subsequently, we launched RENAISSANCE (NCT05076682), a proof-of-concept, Bayesian adaptive, phase 2 platform trial, to evaluate the efficacy and safety of combining mast cell therapeutics with anti-PD-1 backbone therapy in metastatic triple-negative breast cancer (TNBC) patients who progressed after immunotherapy. The primary endpoint was the objective response rate (ORR) assessed using RECIST v1.1 criteria. Results: We identified a distinct population of mast cells termed antigen-presenting mast cells (APMCs), constituting approximately 30% of intratumoral mast cells and correlating with improved clinical benefit of anti-PD-(L)1 therapy in TNBC. APMCs displayed MHC-II and costimulatory molecules, and indicated the presence of tumor-reactive T cells and tertiary lymphoid structures. Using three immunocompetent mouse models, we confirmed the immunomodulatory capacity of APMCs in immunotherapy. Mechanistically, by employing Cpa3CreERT2Cd74fl/fl mice, we demonstrated that APMCs potentiate anti-PD-1 efficacy and antitumor T cell immunity through their antigen-presentation machinery. Interestingly, we identified cromolyn, an FDA-approved drug for allergy, as a potential therapeutic agent that elicited APMC-dependent CD8+ T cell cytotoxicity to synergize with anti-PD-1 therapy. Between February 2022 and March 2023, 10 patients with immunotherapy-refractory metastatic TNBC were enrolled to receive cromolyn plus camrelizumab backbone treatment. Given Bayesian predictive probability, this arm was “graduated” due to meeting the pre-specified efficacy boundary, with an ORR of 40.0% (4/10). The treatment was well tolerated with similar safety profiles of relevant drugs. Conclusions: Our findings provide crucial insights into the impact of mast cell heterogeneity on the clinical response to ICIs at a single-cell level, and pave the way for APMC-directed therapeutic interventions in cancer treatment. To our knowledge, this is the first prospective study in breast cancer of cromolyn plus anti-PD-1 backbone regimen after anti-PD-(L)1 immunotherapy failure, demonstrating significant antitumor activity and commendable tolerability. Consequently, we suggest a phase 3 randomized study to consolidate this finding, which might be an effective treatment in patients for whom there are few effective treatment options. Citation Format: Song-Yang Wu, Xi Jin, Yin Liu, Wen-Jia Zuo, Li Chen, Xiyu Liu, Lei Fan, Zhong-Hua Wang, Yan-Fei Liu, Yi-Zhou Jiang, Zhi-Ming Shao. Programme of mast cell subsets to potentiate breast cancer immunotherapy: from bed to bench to bed (the phase 2 platform RENAISSANCE 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 PO1-14-07.

Abstract PS08-04: Multicenter retrospective cohort study of the sequential use of the antibody-drug conjugates (ADCs) trastuzumab deruxtecan (T-DXd) and sacituzumab govitecan (SG) in patients with HER2-low metastatic breast cancer (MBC)

Abstract PS08-04: Multicenter retrospective cohort study of the sequential use of the antibody-drug conjugates (ADCs) trastuzumab deruxtecan (T-DXd) and sacituzumab govitecan (SG) in patients with HER2-low metastatic breast cancer (MBC)

Abstract PO1-17-09: Real-world clinical outcomes in US patients with brain metastases secondary to HR+/HER2- metastatic breast cancer treated with abemaciclib

Abstract Background: Abemaciclib has demonstrated pharmacologically relevant intracranial concentrations and antitumor activity in patients with brain metastases secondary to HR+/HER2- metastatic breast cancer (MBC). However, clinical data is limited, and further assessments are warranted. This retrospective study describes real-world outcomes following abemaciclib initiation in patients with brain metastases secondary to HR+/HER2- MBC. Methods: Data were accessed from the US nationwide Flatiron Health electronic health records-derived de-identified database from 01/01/2011 to 12/31/2021. This real-world study included adult patients with a diagnosis of HR+/HER2- MBC with brain metastases prior to abemaciclib initiation. Patient and treatment characteristics were summarized using descriptive statistics. Kaplan-Meier methods were used to assess clinical outcomes from abemaciclib initiation: time to treatment discontinuation (TTD), progression-free survival (rwPFS), overall survival (rwOS). Intracranial rw best responses were also reported. Results: This study included 82 patients (one male) diagnosed with MBC and brain metastases prior to abemaciclib initiation and the median follow-up was 12.8 months. At data cut-off, 17 (20.7%) patients remained on abemaciclib therapy. Twenty-three (28%) patients received abemaciclib as the first line of therapy for MBC. Of the 82 patients in the study, 68 (82.9%) patients used concomitant endocrine therapy. Prior to abemaciclib initiation, 25 (30.5%) patients received chemotherapy and 33 (40.2%) patients received a CDK4 and 6 inhibitor. Most patients (67.1%) initiated abemaciclib at a dose of 150 mg twice daily. The median time from diagnosis of brain metastasis to abemaciclib initiation was 2.1 months (interquartile range:1.0, 8.0); 16 (19.5%) did not undergo surgical resection or receive radiation to the brain during this time. Sixty-six (80.5%) patients received brain radiation of which 56.1% received stereotactic radiosurgery (SRS) and 47.0% received whole-brain radiation (WBRT) either alone or in combination with craniotomy/metastasectomy, intrathecal chemotherapy, or surgery. The median TTD for abemaciclib was 7.1 months (95% confidence interval [95% CI]: 4.6, 11.3); most common reasons for discontinuation were disease progression (39.0%) or adverse events (29.3%). The median rwPFS was 9.2 months (95% CI: 6.0, 11.6) and median rwOS was 20.8 months (95% CI: 13.9, 26.0). Of the patients where response data were available (n = 51), rw intracranial clinical benefit rate (CBR; complete response [CR]/partial response [PR]/stable disease [SD] ≥24 weeks of abemaciclib initiation) was 62.7% (Table 1). Conclusion: In this real-world study, most patients with brain metastases secondary to HR+/HER2- MBC who initiated abemaciclib treatment received SRS or WBRT prior to or concurrent with abemaciclib initiation. While the clinical outcomes, rwPFS and intracranial CBR were encouraging, these findings should be interpreted with caution due to the nature of the real-world study design. Table 1. Intracranial Real-World Best Responses Citation Format: Wambui Gathirua-Mwangi, Holly Martin, Dan He, Shen Zheng, Kristin Sheffield, Jincy John, Sarah Rybowski, Priscilla Brastianos. Real-world clinical outcomes in US patients with brain metastases secondary to HR+/HER2- metastatic breast cancer treated with abemaciclib [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 PO1-17-09.

Abstract PO1-04-03: Phase I/II Trial of Ibrutinib Plus Trastuzumab in HER2-Positive Metastatic Breast Cancer

Abstract Background: Ibrutinib is a first-in-class inhibitor of Bruton’s tyrosine kinase that has demonstrated potent inhibition of ErbB/HER family receptor tyrosine kinases in preclinical models. Ibrutinib also displays immunomodulatory effects, shifting the profile of the immune response from Th2-type to Th1-type cytokines (Dubovsky et al. 2013), resulting in enhanced antitumor immunity. As there is a need for novel HER2-targeted therapies following progression on established anti-HER2–directed therapies, and as combining multiple HER2-directed agents is more effective than single-agent therapy (Blackwell et al. 2012), this study explored the safety and efficacy of ibrutinib in combination with trastuzumab in patients (pts) with HER2-positive metastatic breast cancer (MBC) (NCT03379428). Trial design: Phase I was a dose-escalation study to define the recommended Phase II dose (RP2D) of ibrutinib plus trastuzumab. The primary objective of Phase II was to define the clinical benefit rate (CBR = complete response [CR] + partial response [PR] + stable disease [SD] ≥ 6 months) of ibrutinib plus trastuzumab; secondary objectives were to determine objective response rate (ORR = CR + PR), overall survival (OS), progression-free survival (PFS), and safety/tolerability. Pts had HER2-positive MBC with measurable disease per RECIST v1.1, disease progression on or within 6 months of completing ado-trastuzumab emtansine (T-DM1) therapy, and ≤4 (Phase I) or ≤5 (Phase II) prior chemotherapy regimens for MBC, with no limit on prior endocrine therapies. Results: 26 pts were treated in Phases I and II: median age, 65.1 y; 15.4% Asian, 15.4% Black, 46.2% Caucasian, 19.2% Hispanic, and 3.8% Other; 46.2% with ECOG performance status (PS) of 0, 42.3% with PS of 1, and 7.7% with PS of 2. Median prior lines of therapy for MBC was 2 (range: 0, 6); median prior lines of therapy in any setting was 4 (range: 1, 9). In any setting, 88.5% of pts had prior treatment (Tx) with trastuzumab, 80.8% had prior Tx with pertuzumab, and 100% had prior Tx with T-DM1. 0% of pts had brain, 38.5% had bone, 23.1% had liver, and 42.3% had lung metastases. All pts were HER2+ by local pathology, 61.5% positive by IHC and 42.3% by FISH; 12 pts had ER+ and/or PR+ and 14 pts had ER-/PR- cancers. The starting dose of ibrutinib for Phase I was 560 mg. Of the 3 pts enrolled at this dose level, all experienced grade 3 or higher adverse events (AEs), with 2 of the pts experiencing serious AEs (grade 4 alanine aminotransferase [ALT] increased and grade 5 respiratory failure). As such, 420 mg of ibrutinib was selected as the RP2D. At the time of data extraction, across Phase I/II, 16 pts (61.5%) experienced a Tx-related adverse event (TRAE). The most common (≥5%) TRAEs of any grade were bruising and rash (each 19.2%); fatigue and thrombocytopenia (each 15.4%); anemia, diarrhea, and edema (each 11.5%); and ALT increased, aspartate aminotransferase increased, blurred vision, ecchymoses, mucositis, nausea, pruritus, and vomiting (each 7.7%). A total of 5 pts (19.2%) experienced grade 3 or 4 TRAEs; no grade 5 TRAEs were reported. 26 pts were treated with ibrutinib and trastuzumab, and 26 pts were in the evaluable population. 1 pt had CR for an ORR of 3.8% (95% CIs: 0.1, 19.6); 9 pts had SD, and 4 pts had SD for ≥6 months for a CBR of 19.2% (95% CIs: 6.6, 39.4). Median OS was 27.1 months (range: 0.30, 27.1), and median PFS was 2.0 months (range: 0.03, 27.1). The PFS rate at 12 months was 34.6% (95% CIs: 15.6, 54.6). Conclusion: Ibrutinib plus trastuzumab had a manageable safety profile; however, the CBR did not reach the protocol-specified goal of 28%, and these results do not support further clinical investigation. An in-depth evaluation of the immune effects of ibrutinib in these pts is ongoing. Citation Format: Joyce O'Shaughnessy, Andrea Glidden, Tracy Locke, Amy Scales. Phase I/II Trial of Ibrutinib Plus Trastuzumab in HER2-Positive 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 PO1-04-03.

Abstract PO5-06-02: Phase II Trial of Combination of Atezolizumab, Cobimetinib and Eribulin (ACE) or Atezolizumab and Eribulin (AE) in patients with metastatic inflammatory breast cancer (IBC): Clinical data of both cohorts

Abstract Background: Metastatic IBC is an unmet clinical need due to the aggressiveness of this breast cancer subtype. Gene expression analysis of IBC samples post-neoadjuvant chemotherapy has revealed that dysregulation of immune pathways is common in tumors that do not have a complete pathologic response (pCR) to chemotherapy. The immune checkpoint PD-L1 is expressed in a third of IBC cases, and the MEK inhibitor, cobimetinib, has been shown to enhance the expression of PD-L1, the target of atezolizumab. Taken together, dual targeted therapy with atezolizumab and cobimetinib is hypothesized to be synergistic. Eribulin is a standard of care chemotherapy used in patients who have progressed on anthracycline-taxane based regimens. Materials and Methods: Patients received triple or double combination therapy depending on the timing of their enrollment. Patients with metastatic IBC who had measurable disease after at least 1 standard regimen were eligible. Any receptor subtype was allowed, and PD-L1 positivity was not required. The study began with a single cohort (n=17 patients) receiving all 3 agents with a lead-in phase to determine the MTD of cobimetinib in this setting. The study was later amended to include only atezolizumab and eribulin (cohort 2). Ten patients were treated in cohort 2. The study closed early due to lack of efficacy in cohort 2 and the changing landscape of metastatic breast cancer therapies. In cohort 1, atezolizumab was given every 2 weeks (840 mg), cobimetinib was given at 60/40/20 mg once daily, and eribulin was given at 1.4mg/m2 on day 1 and day 8 of 21-day cycles. In cohort 2, atezolizumab was given every 3 weeks (1200 mg) during the PD window and cycles 1-4, and then every 4 weeks (1680 mg) during maintenance, and eribulin was given at 1.4mg/m2 on day 1 and day 8 of 21-day cycles. Results: The MTD of cobimetinib was 40mg. In cohort 1, 17 patients were treated, 14 had at least 1 restaging evaluation. Seven patients had a partial response (50%) and 3 stable disease. The cohort was predominantly triple receptor-negative (n=13/17), with a median age of 51 yrs. The median PFS for patients in cohort 1 was 6.2 months (0-54.1). There were 2 exceptional responders, both with triple negative subtype, one of whom remains on therapy for >4.5 years without evidence of disease. The median overall survival was 29.6 months (2.9-133). The AE profile was largely expected, with 3 irAEs (1 grade 5 colitis, and 2 grade 2 pneumonitis), and one other patient with cardiac dysfunction requiring discontinuation of cobimetinib and atezolizumab. The MTD of cobimetinib was 40 mg. Cobimetinib-related toxicities were mild and largely gastrointestinal tract related. In cohort 2, only 1 patient experienced PR, and the median PFS was 3 months (1.2-6.2) and overall survival 8.3 months (1.8-19.9). One patient experienced autoimmune related hepatitis requiring discontinuation of protocol therapy. Due to excesive myelosuppression, dose of erublin was reduced to 1.1 mg per meter squared on day 1 and 8 and pegfilgastrim was added on day 9< Conclusions: The response profile of the triple combination demonstrated promise, and suggests that MEK inhibition may warrant further investigation as a strategy in metastatic IBC in a larger sample size. However, there does not appear to be synergy between atezolizumab and eribulin without the addition of targeted therapy. Citation Format: Angela Alexander, Hope Murphy, James Reuben, H. T. Carisa Le-Petross, Deanna Lane, Monica Huang, Savitri Krishnamurthy, Yun Gong, Dan Gombos, Nimisha Patel, Richard Allen, Suyu Liu, Anisha Patel, Andrew Futreal, Ignacio Wistuba, Rachel Layman, Naoto Ueno, Debu Tripathy, Bora Lim, Vicente Valero. Phase II Trial of Combination of Atezolizumab, Cobimetinib and Eribulin (ACE) or Atezolizumab and Eribulin (AE) in patients with metastatic inflammatory breast cancer (IBC): Clinical data of both cohorts [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 PO5-06-02.

Abstract PO5-10-02: Neighborhood deprivation index and time to initiation in patients with metastatic breast cancer

Abstract Background Cyclin-dependent kinase 4 and 6 inhibitors (CDKi) are first-line therapy for patients with estrogen-positive (ER+) metastatic breast cancer (MBC). The accessibility, usability, and outcomes according to demographic factors have not been well studied in real world practice. Methods Retrospective review of patients with ER+ MBC prescribed first line CDKi therapy at a large NCI-designated cancer center from January 2015 through December 2022. Patient records were abstracted for outcomes including 1) time from CDKi prescription to drug initiation (TTI), 3) time from CDKi initiation to progression, and 3) time from CDKi initiation to death/or 6/30/2022. Patient variables collected were: patient age (continuous), race (patient self-report – Black, White, Other), partner status (Married, Not married), Insurance type (Medicare, Medicaid, Private); Mean BMI; Number of comorbidities (5 categories). Neighborhood deprivation index (NDI) was determined from the patient’s address with higher scores indicating more deprivation. Descriptive, comparative, and correlational statistics were used. Results A total of 173 patients were included in the analysis. Mean age – 63.1, (SD 12.7, range 24-94); race – White n=157, Black n=16; marital status – Married n=102, Not married n=71; mean NDI – 61.0 (SD 23.7, range 6-100); mean BMI – 28.9 (SD 6.9, range 16-48); number of comorbidities – 0 n=35, 1-2 n= 35, 3-4 n = 55, 5-6 n = 20, 7+ n = 28; insurance type – Medicare n = 115, Medicaid n = 21, Private n = 37. TTI: the median TTI was 14 days. There were no statistically significant differences in TTI based on age, race, NDI, or other demographic variables. Time to progression: in the multivariate model, younger age at diagnosis and Medicaid insurance were associated with shorter time to progression. Overall survival: not married, younger age at diagnosis, and Medicaid insurance were associated with shorter overall survival. Discussion Demographic factors are associated with diverse outcomes of MBC therapy, not attributed to delay in CDKi treatment initiation. Personalization of support during MBC illness according to risk factors for worse outcomes may help to equalize incomes. Citation Format: Sneha Rajendran, Dianxu Ren, Marina Petruzzi, Margaret Rosenzweig. Neighborhood deprivation index and time to initiation in patients with 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 PO5-10-02.