Abstract P3-06-36: Assessment of phosphorylated HER2 protein as predictive biomarker to stratify anti-HER2 treatment in HER2 non-amplified patients – A translational study in the GeparQuattro and GeparQuinto trials
Authors: Avril Stefanie , von Minckwitz Gunter , Gündisch Sibylle , Gade Stephan , Malinowsky Katharina , Fasching Peter A , Karn Thomas , Hartmann Arndt , Untch Michael , Denkert Carsten , Becker Karl-Friedrich , Loibl Sibylle 
Published: May 2015

Abstract
Aims: There is evidence that the benefit of anti-HER2 treatment in combination with chemotherapy may not be limited to patients with HER2 amplification. This study tested if phosphorylated HER2 (pHER2) and co-activation of HER2 downstream targets are predictive of response to anti-HER2 treatment in HER2 non-amplified patients.
Methods: Patients initially classified as HER2 positive (IHC, FISH) by local testing from the GeparQuattro and GeparQuinto trials received neoadjuvant anti-HER2 treatment (trastuzumab or lapatinib) in combination with anthracycline-taxane-based chemotherapy. However, on a central pathology review 98 out of 1060 patients were considered HER2 non-amplified. In order to assess the potential benefit from anti-HER2 treatment in these patients the levels of pHER2 and downstream targets including pHER3, EGFR, AKT, PI3K, ERK, PTEN, S6RP and their phosphorylated forms were quantitatively assessed using reverse-phase protein arrays. Histopathological complete response (pCR; ypT0/is) and disease free and overall survival served as reference standard. Optimal cutpoints for each protein were calculated to achieve maximum separation between pCR and non-pCR using ROC analysis.
Results: 25 (26%) patients achieved pCR. The level of pHER2 was not significantly different between groups of histopathologic responders and non-responders. S6RP and pS6RP were the only proteins significantly associated with pCR (p=0.01 and 0.03, respectively) with a higher pretherapeutic expression in patients who subsequently achieved pCR. In contrast, low expression of S6RP and pS6RP were associated with longer disease free (p&lt;0.01) and overall survival (p&lt;0.05). Expression of S6RP and pS6RP did not correlate with that of other HER2 signaling proteins, whereas all remaining proteins were positively correlated with each other.
Conclusions: Expression of S6 ribosomal protein (S6RP), a downstream target of S6 kinase, and its phosphorylated form pS6RP are significantly associated with short and long-term outcome following anti-HER2 treatment in HER2 non-amplified breast cancer patients. In contrast, activation status of the HER2 pathway reflected by pHER2 and other downstream targets was not predictive of response, and showed no significant correlation with S6RP expression. These results suggest that S6RP and pS6RP may be novel predictive biomarkers for response to anti-HER2 treatment in non-amplified patients, possibly through a mechanism independent of global HER2 pathway activation.
Citation Format: Stefanie Avril, Gunter von Minckwitz, Sibylle Gündisch, Stephan Gade, Katharina Malinowsky, Peter A Fasching, Thomas Karn, Arndt Hartmann, Michael Untch, Carsten Denkert, Karl-Friedrich Becker, Sibylle Loibl. Assessment of phosphorylated HER2 protein as predictive biomarker to stratify anti-HER2 treatment in HER2 non-amplified patients – A translational study in the GeparQuattro and GeparQuinto trials [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P3-06-36.