LAUSR

1034 Background: Anti-HER2 therapy has improved survival in HER2+ MBC. Yet, large patient cohorts with no evidence of disease (NED) with long-term follow up are incompletely described in the literature. We evaluated the clinical characteristics of patients with HER2+ MBC and prolonged response to anti-HER2 therapy, exhibiting NED vs Residual disease (RES). Methods: Patients treated with chemotherapy plus trastuzumab (CT) from 2005-2013, or taxane plus trastuzumab-pertuzumab (TTP), or trastuzumab-emtansine (TDM1) from 2012-2016 for HER2+ MBC at Princess Margaret Cancer Centre in Toronto, Ontario or in Alberta, Canada were included. Duration on anti-HER2 therapy (without switch) was collected. Patients with median duration of response (MDR; months) 2x higher than phase II/III trials for each regimen (CT = 18.2; TTP = 40.4; TDM1 = 25.2) were included to select for prolonged response. Clinical features (ie: stage at diagnosis, survival, etc) and oncologist/radiologist reported best response were collected. Responses were grouped as NED (including sclerotic bone metastases) or RES. Clinical variables were evaluated by Chi-square and survival by Kaplan-Meier (log-rank). Results: 2403 patients (CT = 1830; TTP = 394; TDM1 = 179) were evaluated. After cut-off, 119 patients (5%) were included, with NED in 41% (49/119) and RES in 59% (70/119). More women aged < 50 vs ≥50 (p = 0.015) had NED (61%) than RES (39%). No breast surgery (curative or metastatic) showed higher (p = 0.015) rates of NED (49%) relative to RES (27%). More women having NED (92%) than RES (37%) were still alive (p < 0.0001) at data cut-off, with improved mPFS (years; p < 0.0001) for NED (11.2, 95%CI: 11.2-NR) vs RES (3.0, 2.7-3.6), and mOS (years; p < 0.0001) for NED (NR) vs RES (3.4, 95%CI: 2.8-4.2). Significance persisted with TDM1 patients excluded. Treatment type (p = 0.053) and number of organs with metastases (p = 0.067) were of borderline significance. Conclusions: Patients with NED have improved survival compared to RES. Younger age and avoiding breast surgery correlates with NED. Evaluation of genomic factors influencing NED are planned.