Dose-dense adjuvant chemotherapy in patients with HER2-low early breast cancer: An exploratory analysis of the GIM2 trial.

532 Background: Dose-dense (DD) adjuvant chemotherapy represents the standard treatment for patients with node-positive early-stage HER2-negative breast cancer, but its role in HER2-low disease is unknown. In this exploratory analysis of the Gruppo Italiano Mammella 2 (GIM2) trial, we investigated efficacy of DD chemotherapy among patients affected by HER2-negative breast cancer according to HER2 immunohistochemistry (IHC) score (low vs. zero). Methods: Patients with node-positive early breast cancer were randomly assigned to receive either DD or standard schedule anthracycline- and taxane-based chemotherapy. HER2 status was assessed locally. Tumours with HER2 score 0 were classified as HER2-zero, while those with a HER2 score 1+ or 2+ without fluorescence in situ hybridization (FISH) amplification were defined as HER2-low. Some tumours were classified as HER2-negative with unknown IHC score. Survival outcomes were compared between patients with HER2-zero vs HER2-low tumours. Results: Among 2003 patients enrolled in the GIM2 trial, 1243 subjects were eligible for this analysis. Median age was 52 years (interquartile range IQR 44-59) and 604 patients (48.6%) were pre-menopausal. Hormone receptor status was positive in 87.9% of cancers. 475 tumours were classified as HER2-zero (38.2%), 446 (35.9%) were defined HER2-low and 322 (25.9%) were HER2-negative with unknown IHC score. No significant differences were found in terms of baseline characteristics between these three subgroups. At a median follow-up of 14.9 years (IQR 8.4-16.2), no significant interaction was observed between treatment effect and HER2 status in invasive disease-free survival (iDFS) (p for interaction=0.46) nor in overall survival (OS) (p for interaction=0.43). Comparing patients with HER2-zero breast cancer with those affected by HER2-low tumours, no significant differences in iDFS were observed (15yr iDFS were 61% vs. 54%, respectively, p=0.25), nor OS differences (15yr OS were 75% vs. 72%, respectively, p=0.19). When investigating DD schedule efficacy among patients with HER2-zero tumours, hazard ratio (HR) for iDFS was 0.64 (95% confidence interval CI 0.46-0.87) and HR for OS was 0.57 (95%CI 0.38-0.88). Similarly, within patients affected by HER2-low breast cancer, HR was 0.83 (95%CI 0.62-1.11) for iDFS and 0.83 (95%CI 0.57-1.22) for OS. These results were consistent with the findings of efficacy of DD chemotherapy in the overall population included in this analysis (HR for iDFS 0.73 95%CI 0.61-0.89; HR for OS 0.69 95%CI 0.54-0.88). Conclusions: In this exploratory analysis of a large phase III adjuvant trial, HER2 status (i.e. HER2-zero vs HER2low) did not show prognostic value. In patients with high-risk early breast cancer, the benefit of DD chemotherapy was observed irrespective of HER2 status. Clinical trial information: NCT00433420 .