Background: Breast cancer (BC) is one of the leading malignancies in incidence and mortality among women in the United States. BC is routinely assessed based on immunohistochemistry (IHC) and in situ hybridization (ISH) to classify patients (pts) as either HER2 positive (HER2+) (IHC3+, IHC2+ & ISH+) or HER2 negative (HER2-) (IHC0, IHC1+, IHC2+ & ISH-). The role of trastuzumab deruxtecan (T-DXd) in advanced BC pts with low HER2 expression (HER2-low) (IHC1+, IHC2+ & ISH-) is being evaluated in ongoing phase III clinical trials. This study aims to understand the prevalence, current treatment patterns and outcomes of HER2-low patients. Methods: This descriptive study analyzed data from a patient tumor registry in the Huntsman Cancer Institute and an electronic medical records database within University of Utah Health. Eligibility criteria included pts aged ≥18 years diagnosed with advanced BC (stages IIIB, IIIC or IV) between Jan 1, 2010 and Dec 31, 2019. HER2 status was ascertained from pathology records of IHC and ISH results until Mar 31, 2021. The highest IHC score and ISH+ over ISH- results were used in pts with multiple evaluations. Results: A total of 522 patients with advanced BC were identified from 2010-2019, of which 240 (46.0%) were HER2-low, 142 (27.2%) HER2+, 46 (8.8%) HER2 IHC 0, and 94 (18.0%) had unknown/incomplete HER2 results. Among the HER2-low patients, 198 (82.5%) were HR+, 40 (16.7%) were HR-, and 2 (0.8%) were unknown. The tumor characteristics and treatment utilization of HER2-low pts are summarized in Table 1. The median follow-up time among HER2-low pts with HR+ and HR- status was 28.3 months (mo) (95% CI: 24.8 – 32.6 mo) and 17.7 mo (95% CI: 15.6 – 34.2 mo), respectively. Median progression-free survival (PFS) for patients who received first-line therapy (1L) was 7.5 mo (95% CI: 3.4 – 17.0 mo) among HR+/HER2-low pts (n=188) and 4.5 mo (95% CI: 3.5 – 6.0 mo) among HR-/HER2-low pts (n=36). Median PFS of HER2-low pts who had received second-line therapy (2L) was 5.1 mo (95% CI: 2.1 – 14.2 mo) and 3.3 mo (95% CI: 1.7 – 7.5 mo) among patients with HR+ (n=78) and HR- (n=10) status, respectively. Conclusion: Approximately 1 in 2 advanced BC patients had test results meeting HER2-low criteria and most were HR+. Many patients with advanced HER2-low BC were treated with more than one line of systemic therapy. Decreased median PFS in later lines of treatment suggests the need for new treatment options to improve patient outcomes.
CLO22-090 Table 1 Tumor characteristics and treatment utilization among HER2-low advanced BC patients
N/A: Not applicable