Abstract Background The impact of HER2-targeted therapy alone followed by the addition of chemotherapy at disease progression (PD) versus upfront combination was investigated by the SAKK 22/99 trial. The aim of this exploratory analysis of the SAKK 22/99 trial was to characterize the specific subset of patients deriving long-term benefit from trastuzumab monotherapy alone and to identify potential predictive factors of long-term response. Methods This is an unplanned post-hoc analysis of patients randomized to Arm A (trastuzumab monotherapy). Patients were divided in two groups: patients with durable clinical benefit from trastuzumab monotherapy and short-term responders without durable clinical benefit from trastuzumab monotherapy Univariate and multivariate analyses of clinical characteristics correlating with response duration was performed. Results Eighty six patients were randomized in arm A, 24 patients (28%) were long-term responders and 62 (72%) were short-term responders with a 5y-overall survival (OS) of 54% (95% CI 31–72) and of 18% (95%CI 10–30), respectively. Absence of ER expression, absence of PgR expression and presence of visceral disease emerged as possible negative predictive factors for durable clinical benefit. Conclusion Durable clinical benefit can be achieved with trastuzumab monotherapy in a subgroup of HER2-positive patients with advanced disease and it is predictive for longer OS. Further investigations of predictive biomarkers are necessary to better characterize this subgroup of patients and develop further de-escalating strategies. Trial registration NCT00004935; first posted 27.01.2003, retrospectively registered.
