Abstract PO4-04-05: HER2 immunohistochemistry (IHC) expression in HER2-positive (HER2-pos) metastatic breast cancer (mBC): clinical and prognostic differences between IHC 2+ and IHC 3+ populations

Abstract Background: The exploration of heterogeneity in mBC has gained significant interest in optimizing therapeutic strategies. While the impact of differential HER2 IHC staining has been extensively studied in HER2-low neoplasms, limited data is available regarding IHC 2+ and IHC 3+ categories in HER2-pos mBC. The aim of our study is to compare, the clinical variables, metastatic patterns, and prognosis between patients classified as IHC 2+ and IHC 3+ in HER2-pos mBC. Methods: Pts with HER2 pos mBC, determined by HER2 IHC staining of 3+ or 2+ with amplification at the in situ hybridization assay, were selected from the GIM14 study database for analysis. Associations among variables were assessed by conducting logistic regression analyses. Additionally, prognostic factors for overall survival (OS) and time to treatment failure (TTF) were evaluated using both uni- and multi-variable Cox regression models. Results: A total of 766 HER2-pos mBC pts were included in the analysis. Among them, 302 (39%) had IHC score of 2+ and 464 (61%) had a IHC score of 3+. Central nervous system (CSN) metastases (mts) were observed in 68 (9%) pts (32% IHC 2+; 68% IHC 3+), lymph node (LN) mts in 330 (43%) pts (46% IHC 2+; 54% IHC 3+), lung (LU) mts in 182 (24%) pts (46% IHC 2+; 54% IHC 3+), liver (LI) mts in 227 (30%) pts (45% IHC 2+, 55% IHC 3+), skin (SK) mts in 68 (9%) pts (28% IHC 2+; 72% IHC 3+) and pleural (PL) mts in 40 (5%) pts (47% IHC 2+, 53% IHC 3+). In univariable (uni) analysis, IHC 3+ compared to IHC 2+ was significantly associated with SK mts (OR 1.76, P=0.043) and less likely to be associated with LI (OR 0.71, P=0.031), LU (OR 0.70, P =0.036), and LN (OR 0.63, P=0.002) mts. In multivariable (multi) analysis, LN mts maintained statistical significance (OR 0.62, P=0.004). In subgroup analysis, among IHC 3+ patients, factors associated with worse OS pts were neo Tx (HR 1.55, P=0.032), adjuvant radiotherapy (HR 1.34, P=0.042), neo ChT (HR 1.63, P=0.002), CNS mts (HR 1.98, P=0.002), and LI mts (HR 1.68, P 0.0001). In multi analysis, CNS mts (HR 1.73, P=0.028) and LI mts (HR 1.47, P=0.017) remained prognostic for IHC 3+ subgroup. The only factor associated with worse OS in IHC 2+ pts was LI mts (HR 1.67, P=0.005). Multi analysis in the overall HER2-pos population identified neo chemotherapy (ChT) (HR 1.49, P=0.009), CNS mts (HR 1.53, P=0.003) and PL mts (HR 1.50, P=0.036) as negative prognostic factors for OS, while PL mts (HR 1.74, P=0.048) were associated with a shorter time to treatment failure (TTF). Conclusions: Our exploratory data revealed that HER2-pos mBC with a IHC score of 3+ is less likely to be associated with LN mts and with visceral mts. The site of mts has prognostic significance, as CNS mts are associated with worse OS in pts with an IHC score of 3+ pts, while LI mts are associated with worse OS in both IHC 2+ and 3+ subgroups. However, the IHC score itself (2+ or 3+) does not have independent prognostic value. The relationship between HER2 IHC staining and treatment outcomes requires further investigation to better understand its potential impact on clinical practice. Citation Format: Arianna Dri, Eva Blondeaux, Claudia Bighin, Simona Gasparro, Stefania Russo, Lorenzo Foffano, Palma Pugliese, Andrea Fontana, Enrico Cortesi, Antonella Ferzi, Ferdinando Riccardi, Valentina Sini, Luca Boni, Alessandra Fabi, Filippo Montemurro, Michelino De Laurentiis, Grazia Arpino, Lucia Del Mastro, Lorenzo Gerratana, Fabio Puglisi. HER2 immunohistochemistry (IHC) expression in HER2-positive (HER2-pos) metastatic breast cancer (mBC): clinical and prognostic differences between IHC 2+ and IHC 3+ populations abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-04-05.