Low HER2 Expression Research Articles

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 PO2-01-02: Clinicopathologic features and prognosis of breast cancer with low HER2 expression evaluated by fluorescence in situ hybridization

Abstract Background Although trastuzumab deruxtecan has been found to improve progression-free survival and overall survival of HER2-low breast cancer patients in DESTINY Breast04 trial, the characteristics and prognostic value of low HER2 expression remains to be elucidated. In this study, we retrospectively examined the clinicopathologic characteristics and prognosis of HER2-low expressing breast cancer. Methods A prospective database of 1026 clinical stage I to Ⅲ, HER2 negative breast cancer patients who underwent surgery from September 2012 to October 2022 at Teikyo University Hospital was analyzed. HER2 was evaluated by fluorescence in situ hybridization assay, and HER2-low and HER2-negative was defined as HER2/CEP17 ratio ≧1.0, and < 1.0, respectively. The chi-squared test was performed to estimate correlations between HER2-low status and clinicopathological factors. Distant-disease survival (DDFS) was calculated using the Kaplan-Meier curves and log-rank test. Multivariate logistic regression analyses were used to identify the independent prognostic factors. Results Median age, tumor size and ki67 of the entire 1026 patients was 56.0 years (range=23.0-93.0), 2.0 cm (range=0.3-15.0 cm), and 15.0 % (range=0.5-99.0 %), respectively. 155 (15.1 %) patients had lymph node metastasis. 904 (88.1 %) patients revealed hormone receptor positive (estrogen receptor [ER] positive; 904 patients, progesterone receptor [PgR] positive; 810 patients). Neoadjuvant and adjuvant chemotherapy was performed in 197 (19.2 %) and 160 (15.6 %) patients, respectively. Among all patients, 904 (88.1 %) patients had HER2-low tumors and 122 (11.9 %) patients had HER2-negative tumors. Positive rate of ER and PgR was significantly higher in HER2-low compared to HER2-negative patients (ER: 804 [88.9 %] patients vs 100 [82.0 %] patients; p=0.026 , PgR: 723 [80.0 %] patients vs 87 [71.3 %] patients; p=0.028). The median ki67 was significantly lower in HER2-low compared to HER2-negative patients (14.5 % vs 18.5 %, p=0.013). Pathologic complete response rates were not significantly different between HER2-low and HER2-negative patients (16.1 % [27/168 patients] vs 17.2 % [5/24 patients], p=0.528). With a median follow-up time of 39.7 months, DDFS was significantly better in HER2-low compared to HER2-negative patients (96.6 % vs 90.7 %, p=0.027). Multivariate logistic regression analyses showed that HER2-low expression was not independently related factors for DDFS. Conclusion These data suggested that HER2-low expression might not have significant association with prognosis despite significant correlation with ki67 and hormone receptor expression. Citation Format: Ayana Sato, Saki Naruse, Yuka Isono, Yuka Maeda, Miki Yamada, Akiko Matsumoto, Tatsuhiko Ikeda, Hiromitsu Jinno. Clinicopathologic features and prognosis of breast cancer with low HER2 expression evaluated by fluorescence in situ hybridization [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 PO2-01-02.

Abstract PO4-14-08: A gene expression-based classifier for HER2-low breast cancer

Abstract Background: The definition of HER2-low breast cancer from clinical trials of antibody-conjugated drugs (ADCs) relies on immunohistochemistry scoring (IHC). However, in daily-practice the accuracy of IHC is hindered by inter-observer variability in assessing HER2 non-overexpressing status. Here, we aimed to identify breast cancer primary tumors with low HER2 expression by leveraging gene expression profiling. Materials and methods: A discovery approach was applied to gene expression profile of internal INT1 (n= 125) and INT2 (n= 84) datasets. We identified specific differently expressed genes (DEGs) according to HER2 IHC categories 0, 1+, 2+ and 3+. Principal Component Analysis (PCA) was used to generate a HER2-low signature whose performance was confirmed in the independent INT3 (n= 95), and TCGA and GSE20194 publicly available datasets. The association between the HER2-low signature and HER2 IHC categories was evaluated by non-parametric Kruskal-Wallis (KW) test with post hoc pair wise comparisons (i.e. contrasts); the HER2-low signature discriminatory capability was assessed by estimating the area under the receiver operating characteristic (ROC) curve (AUC) with its corresponding 95% Confidence Interval (CI). Gene Ontology and KEGG analyses were performed to enrich the DEGs for functional information. Results: A 20-gene HER2-low signature, consisting of both up-regulated (n=11) and down-regulated (n=9) genes, was computed based on the DEGs according to HER2 IHC categories as follows: 11 genes specific for the 1+ category, 8 genes for the 2+ category, and 1 gene for the HER2-low category. HER2-low signature genes were significantly enriched with lipid and steroid metabolism pathways, peptidase regulation, and humoral immune response. The HER2-low signature levels demonstrated a significant association with HER2 IHC categories (KW p-value < 0.001) and were distributed in a bell-shaped pattern across IHC categories (low values 0 and 3+; high values 1+ and 2+), effectively distinguishing HER2-low from 0 (contrast p-value < 0.001) and 3+ (contrast p-value < 0.001). Notably, the signature levels were significantly higher in tumors scored with 1+ as compared to 0 (contrast p =0.002). The HER2-low signature association with IHC categories and discriminatory capability was confirmed in the independent INT3 and TCGA datasets with higher values in HER2-low compared to both 0 and 3+. The HER2-low signature achieved an AUC value of 0.72 (95%CI 0.62-0.83) in differentiating HER2 0/3+ from HER2 1+/2+ categories, which is worth noting in light of the individual ERBB2 mRNA AUC value of 0.48 (95%CI 0.34-0.62). Conclusions: The 20-gene HER2-low signature was generated by maximizing the differences in gene expression between tumors with different HER2 status according to IHC. In contrast to the previously published single ERBB2 gene expression assessment, our data presents compelling evidence for effectively distinguishing HER2-low tumors, including those scored as 1+ from HER2-0 tumors. Our signature holds potential in selecting novel candidates for ADC therapy. Citation Format: Serena Di Cosimo, Sara Pizzamiglio, Chiara Maura Ciniselli, Valeria Duroni, Vera Cappelletti, Loris de Cecco, Maria Carmen De Santis, Rosaria Orlandi, Marilena Iorio, Marco Silvestri, Giancarlo Pruneri, Paolo Verderio. A gene expression-based classifier for HER2-low 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 PO4-14-08.

Abstract PO3-22-05: Investigating HER2-low tumors from a locoregional perspective- a secondary analysis of a randomized radiotherapy trial

Abstract Background: With the introduction of antibody-drug conjugates directed against HER2, a new subgroup of tumors with low HER2 expression (1+ or 2+ without amplification) has emerged. HER2 signaling has been implicated in radioresistance in mechanistic studies and is an accepted biomarker of immunogenicity. The study aimed to contrast tumors with low HER2 expression versus zero HER2 expression from a prognostic and radiotherapy-predictive locoregional perspective. Methods: HER2 status was evaluated using immunohistochemistry and SISH in the randomized radiotherapy (RT) cohort, SweBCG91RT, by a board-certified pathologist. HER2-low status was defined as HER2 IHC 1+ or 2+ without amplification, HER2-zero as 0, and HER2-high as HER2 IHC 3+ or SISH amplified. Tumor-infiltrating lymphocytes were assessed on whole-tissue sections and categorized as low (< 10%) or high (≥ 10%). The endpoint was time to ipsilateral breast tumor recurrence (IBTR) as first event. Results: Out of 964 evaluated primary tumors, 433 (44.9%) were classified as HER2-low, 467 (48.4%) as HER2-zero and 64 (6.6%) as HER2-high. TILs levels did not differ between HER2-low and HER2-zero tumors (p=0.33). The hazard ratio for high TILs was 0.79 in HER2-zero tumor (CI 95% 0.43-1.45, p=0.44) and 0.96 in HER2-low tumor (CI 95% 0.56-1.66, p=0.90), and no interaction effect from TILs was observed (p=0.63). The benefit from RT was similar among HER2-low (HR 0.42, CI 95% 0.25-0.72, p=0.001) and HER2-zero tumors (HR 0.31, CI 95% 0.17-0.56, p< 0.001). Conclusions: HER2-low tumors do not differ from HER2-zero tumors regarding the benefit from postoperative RT. Unlike high HER2 expression, low expression is not associated with TILs infiltration. HER2-low tumors likely do not represent a biologically distinct subtype from a locoregional aspect. Citation Format: Per Karlsson, Anikó Kovács, Fredrika Killander, Emma Niméus, Erik Holmberg, Axel Stenmark Tullberg. Investigating HER2-low tumors from a locoregional perspective- a secondary analysis of a randomized radiotherapy trial [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-22-05.

Abstract PO4-03-10: HER2 Expression in Triple-Negative Breast Cancer: Clinicopathologic Features and Response to Neoadjuvant Chemotherapy

Abstract BACKGROUND: Half of the primary breast cancers reported as HER-2 negative have low HER-2 expression, defined as an immunohistochemical (IHC) score of 1+ or 2+, with negative in situ hybridization. Among these cases, 37% correspond to the triple-negative immunophenotype. The primary objective of this study was to evaluate HER-2 expression in patients with triple-negative breast cancer and compare the clinicopathologic features and response to neoadjuvant chemotherapy between HER-2 negative (0) and HER-2 low (1+/2+) subtypes. METHODS: We retrospectively analyzed 157 patients with early-stage triple-negative breast cancer who underwent neoadjuvant chemotherapy followed by surgery with curative intent and met the study's inclusion criteria. These patients were treated at the National Institute of Neoplastic Diseases between 2010 and 2021. HER-2 analysis was performed using immunohistochemistry (IHC), and in cases of HER-2 (2+) amplification, in situ hybridization was performed. RESULTS: Of all the included patients, 107 were classified in the HER-2 negative group (68%), and 50 were classified in the HER-2 low group (32%). Seventy-four percent of patients with HER-2 negative were in the age range of 40-64 years, compared to 68% in the HER-2 low group. A significant difference was found between the HER2-negative and HER2-low groups regarding age and Ki67. The median age of HER-2 low patients is significantly higher than the median age of HER-2 negative patients (52 years vs. 45 years, respectively). No significant association was found between HER-2 status and clinical stages or histological grade. However, the median Ki67 percentage of HER-2 negative patients is significantly higher than the median Ki67 of HER-2 low patients (70% vs. 50%, respectively (p=0.035)). Thirty-two percent of patients with HER-2 negative achieved a pathological complete response (pCR) compared to 18% of patients with HER-2 low. There were no differences in the pCR rate in the HER-2 1+ and 2+ subgroups. Among the patients included in both groups, who achieved pCR, no disease progression was observed at the time of analysis. CONCLUSIONS: Patients in the HER-2 negative group had a higher rate of complete pathological response to neoadjuvant chemotherapy compared to those in the HER-2 low group. No differences were observed in the pCR rate between HER-2 1+ and 2+ subgroups, suggesting that HER2 expression levels within the low range may not significantly impact treatment response. Table. Table. Citation Format: Zaida Morante, Iris Otoya, Natalia Valdivieso, Cindy Calle, Hugo Fuentes, Silvia Neciosup, Carlos Castañeda, Tatiana Vidaurre, Martin Falla, Valeria Colomo, Henry Gómez. HER2 Expression in Triple-Negative Breast Cancer: Clinicopathologic Features and Response to Neoadjuvant 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 PO4-03-10.

Abstract PO1-20-06: Phase 2 study of novel HER2-targeting, TLR7/8 immune-stimulating antibody conjugate (ISAC) BDC-1001 +/- pertuzumab in patients with HER2-positive metastatic breast cancer (MBC) previously treated with trastuzumab deruxtecan

Abstract Background: Therapies to effectively manage patients (pts) with metastatic HER2+ breast cancer have significantly improved over the years, but improvement is still needed in both efficacy and tolerability. BDC-1001 is an ISAC consisting of a trastuzumab biosimilar conjugated to a proprietary cell membrane impermeable TLR7/8 agonist via a non-cleavable linker, administered IV every 2 weeks. It is designed to trigger the innate immune system and generate a durable tumor-targeted adaptive immune response. Preclinical studies indicate that HER2-targeted ISACs elicit potent and durable immune-mediated antitumor efficacy, leading to complete tumor regression in a TLR- and Fc receptor-dependent manner [1]. Moreover, preclinical studies with a murine surrogate of BDC-1001 (trastuzumab-T785 ISAC) indicate improved anti-tumor activity with trastuzumab-T785 compared with trastuzumab/pertuzumab. In these preclinical studies, the combination of trastuzumab-T785 and pertuzumab demonstrated significantly enhanced efficacy in multiple HER2-expressing tumor models, including those with lower HER2 expression [1]. The addition of pertuzumab decreased the quantity of ISAC required for anti-tumor activity in the JIMT-1 HER2 IHC2+ model. Moreover, the combination of pertuzumab with the ISAC significantly increased the cytokine and chemokine concentrations in the tumor xenografts, compared with monotherapy or trastuzumab/pertuzumab, indicating enhanced myeloid activation in the tumor. These preclinical studies suggest that this combination may enhance the clinical activity of trastuzumab-based ISACs. The completed part of the BDC-1001 dose escalation trial (NCT04278144) demonstrated safety, PK and pharmacodynamic changes compatible with the ISAC mechanism of action, and a wide range of antitumor activity (incl. cases with breast cancer); resulting in a RP2D of 20 mg/kg q2w [2]. Two trials are underway, including a Phase 2 trial in other HER2+ malignancies, and a randomized Phase 2 trial with BDC-1001 alone and in combination with pertuzumab in patients with HER2-positive MBC previously treated with 2 or more prior anti-HER2 therapies, including trastuzumab deruxtecan. Methods: The Phase 2 multicenter, open-label, randomized study (BBI-20231001) is enrolling up to 66 pts with HER2-positive (ASCO/CAP guidelines 2018) MBC previously treated with 2 or more prior anti-HER2 therapies, including trastuzumab deruxtecan as one of the prior therapies. Pts must be 18 years or older, have measurable disease (RECIST v1.1), and have Eastern Cooperative Oncology Group performance status of 0 or 1. Pts will be administered BDC-1001 20 mg/kg every 2 weeks (IV q2w) and randomized 1:1 to receive BDC-1001 as a single agent or in combination with pertuzumab. The trial has a Simon 2-stage design within each arm. The primary objective is to determine the overall response rate per RECIST v1.1 of BDC-1001 alone and in combination with pertuzumab. Secondary objectives will evaluate safety, additional efficacy parameters, pharmacokinetics, and immunogenicity of BDC-1001 alone and in combination with pertuzumab. Exploratory objectives will include pharmacodynamic biomarkers in tumor tissue (baseline and on-treatment biopsies if feasible) and in peripheral blood to elucidate the mechanism of action and seek to identify biomarkers associated with BDC-1001 biological activity with or without pertuzumab. This study is expected to initiate accrual in 2H 2023. For additional information, please contact Bolt Biotherapeutics at 1-650-665-9295 or info@boltbio.com.

Abstract PO4-25-09: HER2-Low and Ultra-Low expression in young women with luminal breast cancer in México

Abstract Background and objective HER2-Low in breast cancer is a biomarker with new approaches for prognosis and medical treatment decisions. However, HER2-Ultra-Low is an area of novel research. Our objective was to describe the clinicopathologic characteristics and HER2 expression status of a group of Mexican young patients with luminal breast cancer. Methods One hundred and fifty-seven young patients (< 40 years) with luminal breast cancer diagnosed from 2014 to 2020 were included. Clinicopathologic features, hormone receptors, HER2 expression, and tumor infiltrating lymphocytes (TILs) were assessed. We categorized patients in three groups: HER2 negative (n=95, 60.5%), HER2-Low (n=33, 21.0%), and HER2-Ultra-Low (n=29, 18.5%). Descriptive statistics were used to report demographic, clinical and histopathological characteristics. Kaplan-Meier curves and log-rank or Mantel-Cox tests were used to determine survival. Statistical significance was established using a P value < 0.05. This analysis was performed using R (version 4.0.4) through R Studio interface (version 1.4.1106). Results The median of age at diagnosis was 36.1 years [range 32.8-38.8], with a median of follow-up of 5.08 years [range 3.58-6.52]. The most frequent histologic type was ductal infiltrating carcinoma (81%), and 49.4% presented grade two of differentiation. T2, N0 and clinical stage II were the most common (45.2%, 44.5, and 48.1%, respectively), and fourteen patients (9.0%) presented clinical stage IV. We found statistically significant differences in Ki67 and total TILs quantification between HER2 negative and HER2-Low. Progesterone receptor levels were higher for HER2 negative and Ultra-Low in comparison to HER2-Low. The total population had a 5-year overall survival of 84.9%. We found statistically significant differences in the median of survival between HER2 groups, with a shorter survival for HER2-Low. Conclusion To date, there are no studies that describe the clinical profile and OS of HER2-Low and Ultra-Low in young patients with luminal breast cancer in México. These preliminary findings could indicate the need for further investigation in prognosis, outcomes, and specific treatment in these patients. This study contributes to better understand the clinicopathologic profile of the HER2-Ultra-Low group and its differences with the HER2-Low group. Citation Format: Jesus Edgardo Hernandez Hernandez, César Octavio Lara Torres, Bryan Moreno Moran, Cynthia Villareal Garza, Fany Iris Porras Reyes, Víctor Manuel Pérez Sánchez, Alejandro Mohar. HER2-Low and Ultra-Low expression in young women with luminal breast cancer in México [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 PO4-25-09.

Abstract PS08-09: Impact of HER2 expression dynamics on the real-world activity of trastuzumab deruxtecan for metastatic breast cancer (RELIEVE)

Abstract Background: trastuzumab deruxtecan (T-DXd) is a standard treatment for patients (pts) with HER2-positive (HER2+) and HER2-low metastatic breast cancer (MBC). HER2-low expression has been shown to be a highly unstable biomarker, and no data exists on the activity of T-DXd among pts with changes in HER2 status over time. Furthermore, limited data exists on the performance of regimens administered after progression on T-DXd. Methods: We analyzed data for pts with MBC receiving T-DXd at Dana-Farber Cancer Institute between 7/2017 - 2/2023 and at Duke Cancer Center between 3/2020 - 4/2022. The disease was categorized HER2+ if IHC 3+ or ISH-amplified at any time point; HER2-negative cases were categorized as HER2-low (IHC 2+/ISH- or IHC 1+) or HER2-0 (IHC 0) based on the last biopsy before T-DXd; the HER2 status for the primary tumor and at first metastatic diagnosis were also collected. We determined time to next treatment (TTNT), overall survival (OS), toxicities with T-DXd, TTNT based on dynamic HER2 status and TTNT with post-T-DXd regimens. Genomic data was available for 58 pts using an in-house NGS assay on archival tumor samples. Results: A total of 191 pts were included in the analysis (126 HER2+, 44 HER2-low, 21 HER2-0). Median age at metastatic diagnosis was 50.9 (range 21 - 78), 26% had de-novo MBC and 38% had history of brain metastases before T-DXd. The proportion of hormone receptor (HR)-positive tumors based on the last biopsy prior to T-DXd was 53% in HER2+, 68% in HER2-low and 57% in HER2-0; pts within each cohort had received a median of 2 prior lines of chemotherapy before T-DXd (range 0 - 9). With a median follow-up of 10.4 months, median TTNT was 10.4 months for HER2+, 7.6 months for HER2-low and 3.7 months for HER2-0 MBC (p < 0.001). Switch in HER2 status between primary tumor and pre-T-DXd biopsy significantly impacted outcomes, with the shortest TTNT observed in pts switching from HER2-low to HER2-0 (TTNT 3.0 months) compared with 5.6 months if switching from HER2-0 to HER2-low, and 9.4 months if having stable HER2-low status (p=0.006). Conversely, HER2 status switch within the metastatic setting did not impact outcomes (Table 1). No difference in TTNT was seen regardless of HR status (negative vs positive, 7.4 vs 10.2 months, p=0.25) or number of prior chemotherapies (≤2 vs >2, 10.1 vs 8.8 months, p=0.50). Median OS is shown in Table 1. At last follow-up, 55 pts remain on T-DXd (28.8%), 108 progressed (56.5%), 28 stopped due to toxicity (14.7%). A total of 22 pts (11.5%) developed interstitial lung disease (ILD, 6.2% grade [G]1, 3.1% G2, 1.6% G3, 0.5% G4, no G5). ILD occurred after a median of 8 months from T-DXd initiation and resolved in 63.6% of the cases, with a median time to resolution of 4 months (2 months for grade 1 events). 5 pts (2.6%) developed cardiotoxicity. T-DXd was dose reduced in 61 pts (31.9%), most often for fatigue (14.7%), nausea/vomiting (9.9%) or hematological toxicity (6.3%). Among 105 pts receiving post-T-DXd treatments, TTNT was 4 months for HER2+, 3.1 months for HER2-low and 4.3 months for HER2-0 MBC (p=0.62). Pts with HER2-negative tumors and ERBB2 hemizygous deletions (6/58, 10.3 %) had a TTNT with T-DXd of 4.1 months, vs 7.6 months without ERBB2 deletions (p=0.41). Additional biomarker data will be presented at the meeting. Conclusions: T-DXd showed promising real-world activity in pretreated pts with MBC, with the longest TTNT observed among pts with HER2+ disease or with stable HER2-low disease over time. Real world TTNT is relatively short post-T-DXd and further studies evaluating resistance mechanisms and optimal treatments in this setting are needed. Table 1 – TTNT and OS among patients with metastatic breast cancer receiving T-DXd and post-T-DXd regimens according to HER2 status The activity of T-DXd significantly differed according to the HER2 status of the disease, with the longest TTNT observed among patients with HER2+ disease (at any timepoint in time) and among patients with stable HER2-low disease over time. Regimens administered immediately after T-DXd achieved a similar performance irrespective of the HER2 status of the disease, with a TTNT ranging between 3.1 and 4.3 months. Citation Format: Paolo Tarantino, Melissa Hughes, Ross Kusmick, Laura Alder, Alyssa Pereslete, Laura Noteware, Heather Moore, Amanda Van Swearingen, Tianyu Li, Hersh Gupta, Kalie Smith, Stefania Morganti, Janet Files, Kerry Sendrick, Simone Buck, Deborah Dillon, Rinath Jeselsohn, Yvonne Li, Andrew Cherniack, Aleix Prat, Nay Nwe Nyein Chan, David Rimm, Giuseppe Curigliano, Sarah Sammons, Carey Anders, Nancy Lin, Sara Tolaney. Impact of HER2 expression dynamics on the real-world activity of trastuzumab deruxtecan for metastatic breast cancer (RELIEVE) [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 PS08-09.

Abstract PO2-05-13: HER2 status presented an unstable switching from primary to recurrent breast cancer

Abstract Background: Breast cancer patients with HER2-2+ expression had heterogeneous characteristic. Treatment decision is of great importance after progression. Objective: We explored patients with HER2-2+ in matched primary or recurrent/metastatic tumor samples from January 2010 to June 2022 in our institute were included to evaluate the evolution from or to HER2-2+ expression. Meanwhile, with an emerging of novel entity as HER2-low expression breast cancer, we included this category when analysis as well. Results: In the cohort of a total of 159 breast cancer patients with HER2-2+ expression in either primary tumor or locoregional/distant metastases samples, HER2-2+ breast cancer accounted for 44.0% in primary tumor and 88.8% in recurrent diseases. There were 18.5% and 15.2% patients with HER2 gene amplification on ISH assay among primary and recurrent/metastatic HER2-2+ breast cancers, respectively. The overall rate of HER2 IHC discordance was 67.1%. Among primary HER2-2+ cases, 74.6% maintained in HER2-2+ when disease relapsed. Discordance was mostly driven by cases switching from HER2-2+ to HER2-1+ (64.7%). In HER2-2+ recurrent/metastatic cases, discordance was mostly driven by cases switching from HER2-0 to HER2-2+ (47.1%). The proportion of HER2 discordant cases got a higher statistically trend among HR-positive patients in compared with HR-negative patients (44.1% vs. 21.7%, p = 0.062). There were 35.0% in primary HR-positive/HER2-negative patients and 11.8% in primary triple-negative patients switching from HER2-0 to HER2-low when disease progressed, respectively. Meanwhile, HER2 discordance rate had a significant difference across different metastatic sites (p = 0.026). All contralateral breast and skin metastasis presented with discordant HER2 results. While, all central nervous system metastasis maintained the same HER2-low expression. HER2-positive phenotype in primary breast cancer samples loss their positivity in 42.1% cases. There was a total of 6.6% cases gained HER2 positivity in recurrent breast cancer samples. Conclusion: HER2 expression from primary to relapse breast cancer samples were highly unstable. Biopsy of recurrent sites was necessary to find novel treatment. Citation Format: Anjie Zhu, Hui-Ping Li. HER2 status presented an unstable switching from primary to recurrent breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-05-13.

Abstract PO2-16-10: Characterising HER2-low status in metastatic breast cancer: a real-world retrospective study of patients at a metropolitan cancer centre in Australia

Abstract Background: Traditionally, breast cancer subtyping has described patients as HER2-negative or HER2-positive, with the latter being those in whom HER2-directed therapies are utilised. A proportion of HER2-negative cancers can be classified as “HER2-low” as they express IHC 1+ or 2+ and are non-amplified on in-situ hybridisation (ISH). Until recently, our widely available HER2-targeted therapies have proven to be ineffective in this subtype. The DESTINY-BREAST04 trial (Modi et al, NEJM 2022) showed progression-free and overall survival efficacy of HER2-directed therapy using the antibody-drug conjugate (ADC) trastuzumab deruxtecan (T-DXd) in HER2-low disease, suggesting a need to identify this population for selected therapy. This study evaluated a real-world population of metastatic breast cancer (MBC) patients treated according to traditional breast cancer subtypes to determine the frequency of HER2-low histology, with a focus on selection of first-line therapy. Methods: This retrospective audit identified patients being treated for MBC at a single tertiary hospital (Austin Health in Melbourne, Australia) over a 3.5 year period. Patients were identified from an electronic database, with a date of last follow-up as August 2022. Medical records were interrogated for clinicopathological information including review of pathology reports for breast cancer subtypes. Traditional breast cancer subtypes were defined as (i) ER positive and HER2-negative (HER2 0 or 1+, or 2+ and non-amplified); (ii) HER2-positive (HER2 3+ and/or amplified) and (iii) TNBC (negative for ER and PgR, and HER2 0 or 1+, or 2+ and non-amplified). Selection and duration of first-line treatment for MBC was also recorded. Results: Between March 2018 and August 2022, 202 MBC patients were identified. Based on traditional subtypes, 126 (62%) were ER positive and HER2-negative, 46 (23%) were HER2-positive, 26 (13%) were TNBC and 4 (2%) were unknown. In the total population, 87 (43%) were classified as having HER2-low disease based on retrospective pathology reports. HER2-low represented 61% of ER positive, HER2-non amplified patients and 45% of TNBC patients. Metastatic specimen data was known for 172 cases and of these, 95 patients (55%) were re-biopsied on metastatic progression. This occurred more commonly with HER2-positive (81%) and ER positive HER2-negative (78%) primaries. The most common change in subtype was loss of ER, which occurred in 7 patients (3%). Changes in HER2 status occurred in 29 (14%) of patients, most commonly as either an increase from 1+ to 2+ or decrease from 1+ to 0. HER2-low status remained consistent for 67% of patients who underwent biopsy on recurrence. For patients with ER positive HER2-negative MBC, first-line combination therapy with a CDK4/6 inhibitor + AI was preferred (51%), followed by single agent endocrine therapy (30%) and chemotherapy (17%). The majority (84%) of HER2-positive patients received first-line dual HER2 therapy (trastuzumab and pertuzumab) with taxane chemotherapy. Half of TNBC patients received upfront chemotherapy, with physician preference being capecitabine (45%). There was no significant difference in PFS between the HER2-low vs. HER2-0 cohorts, for either ER positive or TNBC subtypes. No patients in this series were treated based on HER2-low definition due to no funded access to such therapies at the time. Conclusion: HER2-low has emerged as an identifier for patients who may respond to HER2-directed therapies using ADCs such as T-DXd. However, its clinical significance as a discrete clinical subtype remains uncertain. In this retrospective audit, HER2-low patients were identified in both ER positive and TNBC subtypes. Dynamic changes in HER2 staining occurred infrequently. Further evaluation is needed to address whether HER2-low expression provides any prognostic value and whether prospective reporting of HER2-low status may alter how these new HER2-directed therapies are used in future. Citation Format: Michelle Li, Belinda Yeo. Characterising HER2-low status in metastatic breast cancer: a real-world retrospective study of patients at a metropolitan cancer centre in Australia [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 PO2-16-10.

Abstract PO1-27-07: Is HER2-low a biologically distinct breast cancer (BC) subtype? Prognosis and pathological complete response rate after neoadjuvant treatment (NAT) in early breast cancer (BC) HER2 negative

Abstract Background: Overexpression of HER2 is a significant prognostic and predictive factor in early breast cancer. HER2 positive tumors are those scored by immunohistochemistry (IHC) 3+ or 2+ with amplification by in situ hybridization (ISH). These tumors may benefit from HER2-targeted treatment and additionally, in early HER2-positive BC, achieving a complete pathological response after neoadjuvant treatment improves patient survival. In recent years, new anti-HER2 antibody-drug conjugates have proven effective for HER2 low BC (defined as IHC score 1+ or 2+ not amplified) in the metastatic disease setting; however, in HER2-low early BC, studies testing novel anti-HER2 antibody-drug conjugates as neoadjuvant therapy (NAT) are ongoing. Currently, we don´t know exactly whether HER2 expression can influence the pathological response rate after NAT or disease-free survival (DFS) in these patients compared to HER2-zero (IHC 0). The aim of our study is to determine the impact on response rate to NAT and survival outcomes in early HER2-negative BC. Methods: We conducted a retrospective study in two hospitals in Andalucia, Spain. Patients with early HER2 negative BC treated with NAT from January 2015 to June 2022 were included. Patients were divided into HER2 low patients (IHC 1+ or 2+ not amplified) and HER2 0 (IHC 0). We collected clinical and pathological characteristics, pathological response rate using the Residual Cancer Burden (RCB) system where a complete pathological response (pCR) was defined as ypT0/ypTis and ypN0, and survival outcomes. The primary objective of the study was to analyze the pCR rate after NAT according to HER2 score, and secondary objectives were to assess disease-free survival and overall survival (DFS and OS rates). Results: 574 patients were included (100% women). 397 patients had hormone receptors (luminal BC) on these tumors, and 177 did not have them, i.e., triple-negative BC (TNBC). 225 patients were HER2-zero, and 312 patients were HER2-low (253/312 patients had luminal BC and 59/312 patients were TNBC). The pCR in HER2-low patients was 17.3% and 23.1% in HER2-zero patients (p=0.047). For luminal BC patients, the pCR was 16.7% in HER2-low tumors and 13.7% in HER2-zero tumors (p=0.29). Similarly, there was no difference in pCR rates in TNBC patients (45.8% in HER2-low and 53.8% in HER2-zero tumors, respectively, (p=0.958). The overall survival (OS) at 96 months was 88.7% (95% CI 85.6%-92%). Regarding survival outcomes, neither HER2 expression level was associated with higher 8-year DFS: 81.1% in HER2-low patients (95% CI 75.7%-86.8%) and 74.9% in HER2-zero patients (95% CI 62.3%-90.1%) p=0.64. There was no difference between histological subtypes at 8-years regarding DFS rate: 80.5% (95% CI 74.3%-87.2%) in luminal HER2-low patients and 76.3% (95% CI 60.8%-95.7%) in luminal HER2-zero patients, p=0.22; 83.8% in TNBC HER2-low (95% CI 74.5%-94.2%) and 77.5% in TNBC HER2-zero patients (95% CI 69.4%-86.6%) p=0.26. We also didn´t find significant differences in terms of OS. For HER2-low patients, the 8-year OS was 90.9% (95% CI 87%-94.9%) and for HER2-zero patients, it was 86.4% (95% CI 80.8%- 92.5%) p=0.46. There were also no differences in OS for different histological subgroups with an 8-year OS in luminal HER2-low breast cancer patients of 92.5% (95% CI 88%-96.7%) and 87% in HER2-zero (95% CI 78.9%-96.1%) p=0.91. In patients with triple-negative breast cancer phenotype, the OS was 84.2% for HER2-low (95% CI 74.5%-95.2%) and 85.3% for HER2-zero (95% CI 77.9%-93.4%) p=1. Conclusions: We found no difference in survival outcomes and response rate to conventional NAT between HER2-low and HER2-zero expression levels. Therefore, our data do not support the hypothesis of HER2-low as a biologically specific breast cancer subtype. Further research on this approach with HER2 antibody-drug conjugate schemes as an alternative to traditional NAT schemes is warranted. Citation Format: Ana Gil-Torralvo, Yeray Rodriguez, Alejandro Falcon, David Morales, Mónica Cejuela, Isabel Miras, Marta Amérigo, Manuel Ruíz - Borrego, Juan Bayo, Francisco Javier Salvador Bofill. Is HER2-low a biologically distinct breast cancer (BC) subtype? Prognosis and pathological complete response rate after neoadjuvant treatment (NAT) in early breast cancer (BC) HER2 negative [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 PO1-27-07.

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 PS11-06: Analysis of HER2 expression changes from breast primary to brain metastases including HER2 Low and impact on overall survival

Abstract Background: Previous studies have shown that breast cancer receptor subtype switching between matched primary and metastatic brain metastases (BrM) is common. HER2 expression at low levels, termed “HER2 Low”, has emerged as a new therapeutic biomarker for highly active antibody drug conjugates with potential intracranial activity. Trastuzumab deruxtecan is FDA approved for the treatment of HER2 Low and HER2 positive breast cancer with emerging evidence of CNS responses and clinical benefit in patients with BrM. This study aimed to investigate the relationship between HER2 expression including HER2-Low between matched breast cancer primary tumors and brain metastases. We also sought to evaluate the impact of low HER2 expression on overall survival in breast cancer brain metastases. Methods: We identified pts with MBC seen at least once at a single, NCI-designated center and had a diagnosis of BrM between 2003 and 2023. Of 1252 MBC pts with a diagnosis of BrM, 265 pts underwent resection/biopsy for BrM with available ER/PR/HER2 status. Only pts with available matched primary and BrM tissue were analyzed. HER2 expression was defined as: HER2+ (3+ or 2+/ISH amplified), HER2-Low (1+, 2+/ISH neg) or HER2-0 [by ASCO-CAP guidelines]. Estrogen receptor (ER) status was defined as ER≥1%. Multivariate overall survival (OS) analyses by Cox proportional hazard models were determined from time of BrM resection to death or last follow-up between HER2+, HER2-Low and HER2-0 patients controlling for ER and age. Results: 197 matched breast cancer primary and BrMs with available ER, PR, and HER2 testing were evaluated. Median age at BrM resection was 51 yrs. ER was positive in 48% (N=95) of BrMs. Of 265 resected BrMs, HER2 expression was found in 72%: 49.8% HER2+ (N=132), 22.2% HER2-Low (N=59), and 27.9% HER2-0 (N=74). HER2 expression by 197 primary tumors was: 57% HER2+ (N=112), 24% HER2-Low (N=48), 19% HER2 0 (N=37). Amongst 112 HER2+ primary tumors, 97% (N=109) had concordant HER2+ BrMs, 3 (2.6%) switched to HER2-Low, with 100% retaining some HER2 expression. Of 37 HER2-0 primaries, 35% (13/37) gained HER2-Low expression and 5.4% (2/37) gained HER2+ expression in the BrM. Amongst 48 HER2-Low primary tumors, half (52%) changed expression to either HER2+ (21%) or HER2-0 (31.2%) in the brain. 66 tumors had extracranial tissue biopsied prior to BrM resection available for HER2 analysis. Of 15 HER2-0 extracranial tumors, 47% gained HER2-Low and 13% gained HER2+ at time of BrMs. After adjusting for age and ER status at the time of BrMs, patients with HER2+ BrM had a statistically significant lower risk of death at time of follow up than those with HER2-Low BrM (HR=0.41, P=0.0006). The risk of death between patients with HER2-0 BrM did not differ from HER2-Low BrM after adjustment for ER and age (HR=0.96, p =0.9). Median survival from BrM resection was 12.4m, 16.1m, 47.6m for HER2-Low, HER2-0 and HER2+ respectively (p = .0004). Conclusions: HER2 expression amongst primary, extracranial and brain metastases is dynamic with frequent change. Over 40% of HER2-0 primary or extracranial tumors gain HER2 expression in the brain. These changes have therapeutic implications given HER2 targeting antibody drug conjugates with improved OS and emerging evidence of CNS activity, highlighting the need for better diagnostics, including those that do not require invasive tissue sampling, to determine HER2 status in the brain. Patients with HER2-Low and HER2-0 BrMs have inferior survival and are a clinical unmet need. Citation Format: Alyssa Pereslete, Melissa Hughes, Alyssa Patterson, Janet Files, Kyleen Nguyen, Lauren Buckley, Ashka Patel, Abigail Moore, Eric Winer, Tianyu Li, Sara Tolaney, Nancy Lin, Sarah Sammons. Analysis of HER2 expression changes from breast primary to brain metastases including HER2 Low and impact on overall survival [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 PS11-06.

Abstract PO1-02-07: HER2 Low Non-Metastatic Breast Cancer in Qatar: A Nationwide Retrospective Cohort Study to Evaluate the Response to Neoadjuvant Chemotherapy: A Real-World Analysis

Abstract Background: Breast Cancer is the most diagnosed cancer type worldwide and the second leading cause of cancer-related death among women. HER2-low breast cancer is a newly defined molecular subtype of breast cancer in which tumors exhibit a minimal overexpression of HER2 or no gene amplification. To illustrate, HER2-low tumors typically have an IHC score of 1+ or a score of 2+ with negative amplification. However, emerging evidence suggests that even a minimal amplification of HER2 may significantly impact the response to therapy and prognosis. Our study aims to unveil the impact of HER2-low expression on the response to anthracycline and taxane-based neoadjuvant chemotherapy (NACT) in comparison to the HER2-negative group in non-metastatic breast cancer. The primary objective was evaluating the response to NACT comparing the favorable response (complete and partial response) in each of the two groups of HER2-low and HER2-negative patients. Methods: This is a retrospective cohort study. All patients’ profiles with non-metastatic, HER2-Low, and HER2-negative breast cancers who received neo-adjuvant chemotherapy and underwent surgery between the 1st of January 2018 and the 30th of August 2022 were included. Patients' response was evaluated using surgical pathology reports to compare the two study groups (HER2-low and HER2-negative). Results: The total number of patients included was 262 patients, the majority were HER2-low 89% (233/262) vs. 11% (29/262) HER2-negative. A favorable response to NACT was shown in 71% (185/262) of all patients. The favorable response rate was similar in both groups, 70% (164/233) in the HER2-low group vs. 72% (21/29) in the HER2-negative group, OR: 1 (95% CI: 0.8-1), p-value 0.8. Similarly, the pathological complete response (pCR) rate was the same in both study groups at 14%, OR: 0.7 (95% CI: 0.2-3), p-value: 0.6. Interestingly, patients with hormone-positive tumors across both study groups had a higher response rate as compared to hormone-negative patients. However, statistical significance was shown in the HER2-low group only. In the HER2-low cohort, 73% of the patients (119/164) with hormone-positive tumors showed a favorable response OR: 0.8 (95% CI: 0.8- 1), p-value: 0.001. Comparatively, in the HER2-negative cohort, 52% of the patients (11/21) with hormone-positive tumors had a favorable response OR: 2 (95% CI: 1– 5), p-value: 0.05. Conclusion: Our results did not show any significant impact of HER2-low expression on neoadjuvant chemotherapy response. Nonetheless, a statistically significant difference in response was observed in the hormone-positive HER2-low breast cancer patients. Further studies are needed to evaluate the influence of hormonal expression on the response rate to NACT in the HER2-low patient population. Citation Format: Ahmed Kardousha, Wafaa Shehada, Ahmed Basha, Sahar Nasser, Mufid el Mistiri, Anas Hamad, Salha Al-Bader, Shereen Elazzazy. HER2 Low Non-Metastatic Breast Cancer in Qatar: A Nationwide Retrospective Cohort Study to Evaluate the Response to Neoadjuvant Chemotherapy: A Real-World Analysis [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 PO1-02-07.

Abstract PO3-02-07: HER2-low status in inflammatory breast cancer (IBC) is associated with hormone receptors positivity but not with pathological response to neoadjuvant chemotherapy or overall survival

Abstract Background: Half of HER2-negative breast cancers (BC) show HER2-low expression. The strong efficacy of recent anti-HER2 antibody-drug conjugates (ADC) in HER2-low tumors has risen the interest of HER2-low as a proper BC subtype. However, HER2-low non-inflammatory breast cancer (non-IBC) appears to have only marginal clinicopathological differences compared to HER2-0. Little is known about HER2-low clinical significance in IBC. Methods: Data from patients diagnosed with HER2-negative IBC between 1990 and 2021 were collected from the Institut Paoli-Calmettes European Comprehensive Cancer Center database. Two tumor groups were defined (HER2-low and HER2-0). HER2-low tumors were defined by HER2 IHC score of 1+ or 2+ with negative FISH, and HER2-0 by IHC score of 0+. Hormone Receptor (HR) status was determined according to the French guidelines (oestrogen and/or progesterone receptors by IHC with a 10% threshold for HR-positivity). Clinicopathological characteristics, pCR (defined as [ypT0/ypTis] and [pN0sn or ypN0]), and OS rates were compared between the two tumor groups. Results: The individual data of 256 IBC patients were analyzed. Thirty-seven percent of tumors were HER2-low and 63% were HER2-0. Among patients with stage III (n = 226) and stage IV (n = 30) IBC, 35% and 53% had HER2-low tumors, respectively. HR positivity, age over 50 years, SBR grade, and menopausal status were associated with HER2-low in univariate analysis, but only HR positivity remained significant in a binary logistic regression (OR: 1.95 [1.06-3.58]; p=3.15E-02). In stage III patients, the pCR rate after neoadjuvant chemotherapy was similar in HER2-low (24%) and HER2-0 (29%) patients (p=0.550). No difference was observed by analyzing pCR by HR status. In univariate analysis, overall survival was affected by the clinical axillary lymph node invasion the presence of metastasis, the pathological type and menopausal status, but not by HER2 status. Conclusion: In IBC, HER2-low is associated with HR positivity but has no effect on pathological response to chemotherapy and overall survival in our retrospective cohort, consistently with data available in non-IBC patients. Of 256 IBC patients, 37% were HER2-low. These results may be useful to design future clinical trials testing anti-HER2 ADCs in IBC. Citation Format: Alexandre de Nonneville, Pascal FINETTI, Laurys Boudin, Lucas Usclade, Patrice Viens, Frederic VIRET, Emilie Mamessier, Anthony Gonçalves, François BERTUCCI. HER2-low status in inflammatory breast cancer (IBC) is associated with hormone receptors positivity but not with pathological response to neoadjuvant chemotherapy or overall survival [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 PO3-02-07.

Abstract PO5-02-10: Favorable effect of HER2-low expression on prognosis for breast cancers after neoadjuvant chemotherapy

Abstract [Background] The clinicopathological characteristics, pathological complete response and survival outcomes of HER2-low, HER2-zero and HER2-positive breast cancers are yet to be elucidated in Chinese patients especially those administered neoadjuvant chemotherapy. [Methods] We retrospectively identified 259 patients with different HER2 statuses (zero, low and positive) who underwent surgery as planned after neoadjuvant chemotherapy from a prospectively maintained institutional cohort database in Renji Hospital, School of Medicine, Shanghai Jiao Tong University between 2014 and 2019. Overall survival (OS) denoted the time from surgery to death, irrespective of cause. Locoregional relapse-free survival (LRFS) was estimated from surgery to first occurrence of locoregional, ipsilateral relapse, or death, irrespective of cause. We defined pathologic complete response (pCR) as ypT0 ypN0 (absence of cancer in the breast and axillary nodes). The definition of ypT0/is ypN0 (absence of invasive cancer in the breast and axillary nodes, irrespective of ductal carcinoma in situ) was evaluated as an additional endpoint. [Results] A total of 89 (34.36%), 66 (25.48%) and 104 (40.16%) HER2-low, HER2-zero and HER2-positive breast cancers were enrolled in this analysis, respectively. The median follow-up interval was 5.74 years. Hormone receptor(HR) expression was numerically more common in HER2-low breast cancers than the other two subgroups (83.15% in HER2-low vs 72.73% in HER2-zero vs 77.88% in HER2-positive, P >0.05). The level of Ki-67 index was significantly lower in the HER2-low tumors than that of HER2-zero tumors (P=0.023). HER2-positive tumors had the highest proportion of histological grade III (P=0.005). The HER2-low breast cancers showed significantly better OS (P=0.0056) and LRFS (P=0.002) than HER2-zero counterparts. Subgroup analysis revealed the similar benefit in LRFS (P=0.018) rather than OS (P=0.055) for patients with HR positive/HER2-negative tumors. When the HER2-positive tumors was combined together for comparison, the OS (P=0.016) and LRFS (P=0.0062) also significantly differed among patients with HER2-low, HER2-zero, and HER2-positive tumors. Additionally, the HR negative patients had analogous OS outcome to the entire population according to different HER2 status (P=0.041). Multivariable Cox-regression analysis and external validation in the TCGA database as well as Kaplan-Meier plotter database supported that HER2-low tumors and HER2-positive tumors treated with HER2-directed therapy may have similar survival outcomes, while HER2-zero peers may have the worst outcome compared with the above two groups. There were no remarkable differences in pCR rates between HER2-low and HER2-zero tumors after neoadjuvant chemotherapy (ypT0ypN0 and ypT0/isypN0, P=0.452 and P=0.777, respectively). HER2-positive tumors had a significantly higher pCR rate, especially compared to HER2-low tumors in the entire population (both P< 0.001) as well as in the HR positive subgroup (both P< 0.0001). The pCR status affected survival outcomes for breast cancers with different HER2 status. As for those without pCR, the HER2-low subgroup showed a remarkably better performance compared with HER2-zero patients in terms of OS (P=0.018) and LRFS (P=0.005), while little difference was found for patients who achieved pCR. [Conclusion] Our study demonstrated that HER2-low breast cancer may be a different entity from HER2-zero or HER2-positive tumors, which provide illuminating evidence in view of patients receiving neoadjuvant treatment. Citation Format: Yingying Zhao, Wenjin Yin, Xinru Chen, Jingsong Lu. Favorable effect of HER2-low expression on prognosis for breast cancers after neoadjuvant 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 PO5-02-10.

Abstract PO3-01-05: Enhancing HER2 Evaluation: Correlation between APIS Breast Cancer Subtyping Kit and IHC/ISH for Accurate HER2 Quantification

Abstract Background: The quantification of HER2-low in breast cancer (BC) has garnered considerable interest due to emerging evidence of the efficacy of targeted therapies in this patient subgroup. Trastuzumab deruxtecan (T-Dxd), an antibody-drug conjugate targeted at HER2, has recently gained approval in the USA and Europe for treating HER2-low BC, which is currently defined as immunohistochemical (IHC) scores of 1+ or 2+ without HER2/ERBB2 in situ hybridization (ISH) amplification. HER2 quantification using IHC/ISH was primarily designed to identify tumors overexpressing HER2, rather than differentiate between HER2-low and the absence of expression. Consequently, the adequacy of these assays in accurately detecting HER2- low remains uncertain. Improving the accuracy of HER2 evaluation is crucial to prevent the incorrect selection of patients for T-Dxd treatment. There is a clear need to develop HER2 testing methods that are more precise, reliable, and capable of detecting HER2-low expression with greater sensitivity and reproducibility. This is particularly important as the minimum threshold of HER2 expression required for T-Dxd efficacy is still under investigation, with clinical data suggesting the potential activity of T-Dxd even in patients with IHC 0 HER2 score. Here, we evaluated the correlation between HER2 mRNA expression levels, detected by APIS Breast Cancer Subtyping Kit, and IHC HER2 classification. Methods: Formalin-fixed paraffin-embedded (FFPE) tumour tissue sections (n=642) obtained by core needle biopsy or resection underwent histological analysis in accordance with laboratory’s standard of care methods. Samples with a HER2 score of 2+ were referred to ISH to determine ERBB2 amplification. To evaluate the diagnostic performance, the concordance between HER2 IHC score, and APIS Breast Cancer Subtyping Kit RNA expression level was reported in terms of Overall Percent Agreement (OPA), Negative Percent Agreement (NPA), and Positive Percent Agreement (PPA), along with their corresponding 95% confidence intervals (CI). Results: HER2 expression detected by APIS Breast Cancer Subtyping Kit showed strong correlation with IHC/ISH, with an OPA of 94.2% (95% CI: 92.2-95.8), PPA of 89.2% (95% CI: 80.1-94.4) and NPA of 94.9% (95% CI: 92.8-96.4). The expression of HER2 was detected by APIS Breast Cancer Subtyping Kit in a subset of patients with 0 and 1+ IHC HER2 scores, suggesting its potential as a more sensitive and reliable method for detection of HER2 expression, and providing the opportunity to enhance HER2 stratification. Conclusions: APIS Breast Cancer Subtyping Kit can accurately detect HER2 expression and has the potential to further stratify the HER2-low patients. Further studies examining the relationship between HER2 expression and the response to anti-HER2 therapies could yield valuable insights into treatment administration and identify patients who could benefit from such therapies. Incorporating continuous quantification of HER2 could optimize patient outcomes. Citation Format: Anna Gasior, Joanna Gorniak, Sara Rollinson, Leanne Gough, Anne-Sophie Wegscheider, Axel Niendorf. Enhancing HER2 Evaluation: Correlation between APIS Breast Cancer Subtyping Kit and IHC/ISH for Accurate HER2 Quantification [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 PO3-01-05.

Abstract PO5-17-09: Event-free survival in HER2-low vs HER2-zero breast cancer patients submitted to neoadjuvant chemotherapy

Abstract Background: Breast cancer (BC) with low expression of HER2 (HER2-low) might constitute a group of tumors with unique clinical and biological characteristics. However, the prognostic impact of HER2-low status vs HER2-zero in terms of pathological complete response (pCR) and event-free survival (EFS) after neoadjuvant chemotherapy (NACT) remains controversial. The aim of this study was to evaluate pCR and EFS rates in HER2-low vs HER2-zero early BC patients treated with NACT. Methods: In this cross-sectional study, we analyzed 565 consecutive patients with HER2-low and HER2-zero early BC submitted to neoadjuvant therapy from 2017 to 2021 in a single academic center. Patients with biopsy-confirmed stage I-III BC, with HER2-low or HER2-zero status, submitted to locoregional therapy after NACT were included. Clinical and pathological characteristics, pathological response, treatment protocol and RFS were analyzed and considered for statistical analysis. HER2-low was defined by IHC 1+, or 2+ ISH non-amplified; HER2-zero was defined by IHC 0. Pathological response was determined locally, and pCR defined as the absence of residual invasive disease with or without ductal carcinoma in situ in the breast and axilla. EFS was defined as the time from diagnosis to distant recurrence, a second primary, or death from any cause, whichever occurred first. Results: 441 patients were included in the analysis, 236 (53.5%) patients with HER2-low tumors, and 205 (46.5%) with HER2-zero tumors. Overall, 279 (63.3%) were estrogen receptor (ER)-positive tumors, and 162 (36.5%) were ER-negative tumors. Among ER-positive tumors, 166 (59.5%) had HER2-low status, and 113 (40.5%) were HER2-zero. For the ER-negative subgroup, the corresponding rates were 70 (43.2%) and 92 (56.8%), respectively. Most patients had stage II-III BC (423/441; 96%), and 20.2% (89/441) had high nodal tumor burden. Many patients received anthracycline- and taxane-based NACT (425/441; 96.4%), and over 90% (148/162; 91.3%) of ER-negative patients received a platinum agent. The overall pCR rate were 18.1%. Among ER+ tumors, pCR rates were 6.4%, 6.0%, and 7.0% in the overall, HER2-low and HER2-zero tumors, respectively. In ER- tumors, the corresponding pCR rates were 38.3%, 35.7, and 40.2%, respectively. The differences in pCR rates between HER2-low and HER2-zero tumors were not statistically significant (ER-positive, p=0.724; ER-negative, p=0.559). After a median follow-up of 42 months, 90 EFS events occurred. In the HER2-low group there were 56 (23.7%) EFS events, and 34 (16.6%) EFS events in the HER2-zero (HR 1.19; 95% CI 0.77-1.82; p=0.427). Among 279 patients in the ER-positive group there were 48 events, 32 in the HER2-low subgroup and 16 in the HER2-zero group (HR 1.16; 95% CI 0.64-2.13; p=0.622). Among 162 ER-negative tumors there were 42 events, 24 in the HER2-low subgroup, and 24 in the HER2-zero subgroup (HR 1.56; 95% CI 0.84-2.88; p=0.158). NACT discontinuation occurred in 86 patients (ER-positive: 45, 16.1%; ER-negative: 41, 25.3%), mainly due to toxicity (anthracycline-taxane: 10.0%; platinum: 11.7%). Disease progression during NACT were reported in 6.8% (30/441) of patients. Conclusions: Our results show similar pCR rates and RFS among HER2-low and HER2-zero early BC patients after standard NACT, which are consistent with the current literature. Despite the predictive role of low HER2 expression in selecting patients for novel antibody-drug conjugates (ADCs) therapy, its prognostic value could not be confirmed. Citation Format: Guilherme Sartori, Susana Ramalho, Leonardo Roberto da Silva, Tomás Reinert, Mahira Lopes Da Rosa, Grazielle Morais Tavares, Vivian Vasconcelos, Higor Mantovani, Ana Elisa Ribeiro Da Silva Cabello, Guilherme Coelho, Jovana Mandelli, Facundo Zaffaroni, César Cabello, Carlos Barrios, Marcia Silveira Graudenz. Event-free survival in HER2-low vs HER2-zero breast cancer patients submitted to neoadjuvant 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 PO5-17-09.

Relationship between HER2-low status and efficacy of CDK4/6 inhibitors in advanced breast cancer: a real-world study.

Human epidermal growth factor receptor 2 (HER2)-low breast cancer (BC) is a new entity considered a biologically distinct subtype from HER2-zero BC. However, the importance of HER2 low expression on the activity of cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) remains unclear. We conducted a single-center retrospective study including hormone receptor-positive (HR +) /HER2- metastatic BC (mBC) patients treated with CDK4/6i plus endocrine treatment (ET) as first-line therapy. Clinical outcomes were analyzed according to HER2 expression. 258 women were analyzed with a median follow-up of 25.4months; 39.9% had HER2 low, and 60.1% had HER2 zero BC. Median progression-free survival (mPFS) in the HER2-low group was 27.6months compared with 44.3months in the HER2-zero group (p = 0.341). In patients receiving ribociclib, the mPFS in the HER2-low group was 24.2months compared with 53.1months in the HER2-zero group (multivariate-adjusted HR: 1.981, 95 Cl 1.094-3.586; p = 0.024). The survival probabilities at 24, 36 and 48months for the HER2 low and HER2 zero groups were 82%, 69%, 69% and 83%, 75% and 69%, respectively (p = 0.336). Objective response rate (p = 0.179) and disease control rate (p = 0.338) did not significantly differ between HER-2-low and HER-2-zero groups. The mPFS in the Her2-zero group was almost twice that of the Her2-low group, but the difference was not statistically significant. mPFS was significantly longer in the HER2-zero group compared to the HER2-low group in patients receiving ribociclib. More prospective studies are needed to understand the actual consequences of this biomarker.

Concordance between ER, PR, Ki67, and HER2-low expression in breast cancer by MammaTyper RT-qPCR and immunohistochemistry: implications for the practising pathologist.

There are limited data on the role of multigene tests and their correlation with immunohistochemistry (IHC), especially on core biopsy. MammaTyper is a quantitative conformite Europeeanne (CE) marked, National Institute for Health and Care excellence (NICE) approved, in in vitro diagnostic quantitative real-time polymerase chain reaction (RT-qPCR) test for assessment of mRNA expression of four biomarkers (ESR1, PGR, ERBB2, MKI67). We evaluated the concordance of MammaTyper with oestrogen receptor (ER), progesterone receptor(PR), HER2, and Ki67 by IHC on 133 core needle biopsies of breast cancer. HER2 was positive if IHC 3+ or 2+ and fluorescence in situ hybridization (FISH)-amplified. Global and hotspot Ki67 expression was analysed using a cutoff of ≥20% assessed manually and by digital image analysis. Agreements were expressed as overall percent agreement (OPA), positive percent agreement (PPA), negative percent agreement (NPA), and Cohen's kappa. RT-qPCR results of ESR1 were highly concordant with IHC with OPA of 94.7% using 1% cutoff and 91.7% when the low ER-positive category was included. The PPA and NPA between RT-qPCR and IHC for PR was 91.5% and 88.0%, respectively, when using the 1% cutoff. For ERBB2/HER2, the OPA was 95% and the PPA was 84.6%. 40 of 72 HER2 IHC score 0 tumours were classified as ERBB2 low. Best concordance between MKI67 by MammaTyper and Ki67 IHC was achieved using hotspot digital image analysis (OPA: 87.2%, PPA: 90.6%, NPA: 80%). RT-qPCR-based assessment of the mRNA expression of ESR1, PGR, ERBB2, and MKI67 showed high concordance with IHC, suggesting that the MammaTyper test on core needle biopsies represents a reliable, efficient, and reproducible alternative for breast cancer classification and refining HER2 low categorisation.