Unveiling the Traits of HER2-Low Breast Cancer: A Comparative Analysis of IHC1+ vs IHC2+/ISH-Negative Subgroups – Insights from a 3-Year Cohort Study

Jorge Correia^1*Catarina Pulido¹Joana Albuquerque¹Gil Prazeres¹Inês Margarido¹Mariana Câmara¹Rita Neto¹Gonçalo Fernandes^1,2João Godinho¹Mónica Nave¹Francisco Mascarenhas¹Isabel Estudante¹Paulina Lopes¹Ana Catarino¹José Luís Passos-Coelho¹

• ¹Hospital da Luz Lisboa, Lisbon, Portugal
• ²Universidade Catolica Portuguesa, Lisbon, Portugal

Background:Half of all breast cancer (BC) cases fall into the HER2-low category, defined as immunohistochemistry (IHC)1+ or IHC2+ in situ hybridization negative(ISH-). Two-thirds of these cases are IHC1+, while one-third is IHC2+/ISH-. New anti-HER2 antibody-drug conjugates(ADCs) have emerged as treatment options for metastatic or unresectable HER2-low BC patients. However, the heterogeneity between IHC1+ and IHC2+/ISH- subgroups and the clinical implications of varying HER2-low expression remain unclear. Objectives:This study aimed to compare demographic and clinicopathological differences between IHC1+ and IHC2+/ISH- subgroups and evaluate their response to neoadjuvant chemotherapy(NACT) in a cohort of patients with HER2-low BC. Methods:All consecutive patients diagnosed with HER2-low invasive BC between 2018 and 2020 at our institution were included in this retrospective cohort study. Clinicopathological characteristics were compared between IHC1+ and IHC2+/ISH- subgroups. Pathologic complete response(pCR) rates were assessed in patients undergoing NACT, and a multivariable logistic regression model was used to identify factors associated with pCR. Results: A total of 222 patients were included, evenly divided between IHC1+(n=105, 47%) and IHC2+/ISH-(n=117, 53%) tumors, with no significant differences in baseline characteristics. Both subgroups predominantly comprised female patients (99% IHC1+ vs.98% IHC2+/ISH-), postmenopausal (55% vs.58%), with early-stage BC (94% vs.98%) and estrogen receptor (ER)-positive tumors (90% vs.90%). Around two-thirds had grade 2 tumors (63% vs.64%), and the median Ki-67 index was 20% in both subgroups. Most BC were classified as luminal B-like (56% vs.58%), followed by luminal A-like (35% vs.34%), and TNBC (9% vs.8%). Among the 43 patients with HER2-low BC who received NACT, 36% of IHC1+ patients achieved pCR, compared to only 5% in the IHC2+/ISH- subgroup (p=0.021). Multivariable analysis revealed that IHC2+/ISH- status (vs. IHC1+) was significantly associated with lower odds of pCR (OR=0.07, 95% CI: 0.00–0.51, p = 0.025), while higher baseline Ki-67 and ER-negative status showed non-significant trends toward higher pCR rates after adjustment for other variables. Conclusion: Despite similar clinicopathological features, IHC2+/ISH- status was independently associated with lower pCR rates compared to IHC1+. These findings suggest that HER2-low subgroups may influence response to NACT and should be considered in multivariable prediction models, potentially informing stratified treatment approaches in the era of anti-HER2 ADCs.