Abstract P1-04-01: High HER2/CEP17 ratio is associated with better treatment outcomes in advanced HER2-positive breast cancer treated with pertuzumab, trastuzumab, and docetaxel regardless of HER2 2+ or 3+ results

Abstract Background: Dual HER2 blockade with trastuzumab and pertuzumab combined with docetaxel (DHP) is the standard first-line treatment option for patients with HER2-positive metastatic breast cancer. However, HER2 positivity alone often fails to predict treatment outcome of HER2 targeted therapy. The magnitude of HER2 immunohistochemical staining (IHC) and HER2/CEP17 ratio from in situ hybridization (ISH) is a well-known predictive biomarker for HER2 targeted therapy. In the neo-adjuvant setting, patients with HER2 IHC 3+ have higher pathologic complete response (pCR) rate compared to HER2 IHC< 3+ with HER2 ISH positive. In addition, higher HER2/CEP17 ratio is associated with higher pCR rate in HER2-positive breast cancer patients treated with dual HER2 blockade regimens. However, there is no data whether there is an association between HER2/CEP17 ratio and treatment outcome of DHP regimen in patients with advanced of metastatic breast cancer especially among those with HER2 IHC 3+. Methods: We performed a retrospective cohort study with patients with locally advanced or metastatic HER2-positive breast cancer who were treated with first-line palliative DHP regimen between August 2008 and January 2021 at Seoul National University Hospital. In the clinical setting, HER2 IHC 3+ is defined as HER2 positive and no further ISH testing is required. Additional ISH was performed in patients with HER2 IHC 3+ without archival HER2 ISH results. The association between HER2/CEP17 ratio and treatment outcome was assessed. Results: In total, 165 patients were included with a median follow-up duration of 28.0 months. Among the 165 patients, 35 patients had archival ISH result and additional ISH was performed in 53 patients. The correlation between HER2/CEP17 ratio and treatment outcome was assessed in 88 patients. Cox proportional hazard analysis revealed that HER2/CEP17 ratio is correlated with PFS (HR 0.23, 95% CI 0.11-0.49, p < 0.001). When dichotomized by the median HER2/CEP17 ratio of 4.17, patients with higher HER2/CEP17 ratio had significantly longer PFS (37.5 vs. 17.4 months, p = 0.003) and numerically higher ORR (54.5% vs. 34.1%, p = 0.085). Multivariate analysis revealed that HER2/CEP17 ratio is an independent prognostic factor for PFS (HR 0.72, p = 0.001). Of 88 patients with ISH results, 25 had HER2 IHC 1+ or 2+ and 63 had HER2 IHC 3+. HER2/CEP17 ratio was associated with PFS in both HER2 IHC 1+/2+ patients (HR 0.12, 95% CI 0.02-0.88, p = 0.037) and HER2 IHC 3+ patients (HR 0.18, 95% CI 0.07-0.49, p = 0.001). Patients with higher HER2/CEP17 ratio had longer PFS in both HER2 IHC 1+/2+ patients (28.6 vs. 12.9 months, p = 0.003) and HER2 IHC 3+ patients (Not reached vs. 18.3 months, p = 0.005) when dichotomized by the median HER2/CEP17 of 2.95 and 4.75, respectively. Conclusion: This is the first study to report that higher HER2/CEP17 ratio is associated with longer PFS in HER2-positive advanced breast cancer patients treated with palliative first-line DHP. The strength of this study is that we identified prognostic role of ISH even in patients with HER2 IHC 3+. It would be helpful to perform ISH not only in patients whose HER2 IHC is ambiguous, but also in patients with HER2 IHC 3+ to make better prediction of treatment outcome. Table 1. Patient characteristics. Citation Format: Jeongmin Seo, Jiwon Koh, Dae-Won Lee, Han Suk Ryu, Kyung-Hun Lee, Tae-Yong Kim, Seock-Ah Im. High HER2/CEP17 ratio is associated with better treatment outcomes in advanced HER2-positive breast cancer treated with pertuzumab, trastuzumab, and docetaxel regardless of HER2 2+ or 3+ results [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-04-01.