HER2DX Genomic Assay in HER2-positive Early Breast Cancer Treated with Trastuzumab and Pertuzumab: A Correlative Analysis from PHERGain Phase II Trial.

To assess the predictive capability of HER2DX assay following (neo)adjuvant trastuzumab-pertuzumab (HP)-based therapy in HER2-positive (HER2+) early breast cancer (EBC). HER2DX was analyzed in baseline pre-treatment tumors from PHERGain trial. Patients with stage I-IIIA HER2+ EBC were randomized to group A (docetaxel, carboplatin, and HP [TCHP]) and group B (HP ± endocrine therapy). PET response was evaluated after 2 cycles. Group A received TCHP for 6 cycles regardless of PET response. Group B continued with HP ± endocrine therapy for 6 cycles (PET-responders) or with TCHP for 6 cycles (PET-non-responders). The primary objective was to associate HER2DX pCR-score with pathological complete response (pCR). The secondary objective was the association of HER2DX risk-score with 3-year invasive disease-free survival (iDFS). HER2DX was performed on 292 (82.0%) tumors. The overall pCR rate was 38.0%, with pCR rates of 56.4% in group A and 33.8% in group B. In multivariable analysis including treatment and clinicopathological factors, HER2DX pCR-score (continuous variable) significantly correlated with pCR (odds ratio [OR]=1.29, 95% confident interval [CI] 1.10-1.54, p<0.001). HER2DX-defined pCR-high, med, and low groups exhibited pCR rates of 50.4%, 35.8%, and 23.2%, respectively (pCR-high vs pCR-low OR=3.27, CI 1.54-7.09, p<0.001). In patients with residual disease, HER2DX high-risk group demonstrated numerically worse 3-year iDFS than the low-risk group (89.8% vs 100%; HR= 2.70, 95% CI 0.60-12.18, p=0.197). HER2DX predicts pCR in the context of neoadjuvant HP-based therapy, regardless of chemotherapy addition, and might identify patients at higher risk of recurrence among patients with residual disease.