Defining clinico-pathological characteristics of HER2 positive metastatic breast cancer (MBC) patients experiencing radiologic complete response (rCR) in a nationwide real-world cohort

rCR to first-line therapy is observed in approximately 6% of patients (pts) with HER2 positive (pos) MBC, but figures are often derived by small and/or dated cohorts. Aim of this study was to characterize HER2 pos MBC experiencing rCR in a modern nationwide cohort. Cases were extracted from the GIM14 study and classified according to best radiologic response at first-line of therapy assessed with RECIST 1.1 criteria and according to time-to-treatment-failure (TTF). rCR was defined as complete response (CR) with a TTF > 12 months. Association with the different variables was tested through logistic regression. Amongst all the pts included in GIM14, 783 had HER2 pos MBC. Data on radiologic response was available for 593 pts, of which 56 (9.44%) had had a rCR with TTF >12 months and 12 (3.14%) had a CR with TTF <12 months. Instead, the best response was a partial response for 223 pts (37.42%), stable disease for 163 pts (27.35%), and progression for 135 pts (22.65%); 436 pts (57.14%) had visceral metastases (mets) and 390 pts (50.19%) had only one site of distant mets. Taxanes were the main chemotherapy (CT) backbone (445 pts, 73.07%) and 353 pts (51.76%) received the combination with Trastuzumab-Pertuzumab. Median follow-up was 65.6 months. At multivariable analysis, hormone receptor status (p=0.0083), non-visceral mets (p=0.017), 1 metastatic site (p=0.044) and taxane/anthracyclines combination (p=0.0065) were significantly associated with rCR. HER2 score was significantly associated with rCR at univariable (p=0.029) but not at multivariable analysis (p=0.16). Type of anti-HER2 regimen and disease-free interval or de novo disease, were not associated with rCR. Rates of progression free pts at 2 and 5 years respectively were 71% and 42% for pts with rCR and 30% and 16% for pts without rCR. This study analysed a cohort HER2 pos MBC pts who experienced rCR to a first-line of therapy. As novel anti-HER2 agents are gaining momentum as increasingly effective treatment strategies, these results will pave the way to new clinical trials focused on liquid biopsy-based minimal residual disease to tailor treatment de-escalation.