Active Her-2/neu signal transduction in breast cancer –relevance of a ligand-independent activation mechanism and impact on the efficacy of trastuzumab-based treatment

Abstract

649 Background: Proteolytic shedding of the Her-2/neu extracellular domain (ECD) has been demonstrated to initiate receptor phosphorylation and thereby Her-2/neu activation in vitro. The aim of the present work was to investigate the clinical relevance of ECD cleavage for Her-2/neu activation and the clinical relevance of active intracellular Her-2/neu signalling reflected by tyrosine kinase phosphorylation in patients treated with trastuzumab-based therapy. Methods: Sera from 62 patients receiving trastuzumab-based treatment for Her-2/neu over-expressing metastatic breast cancer were analyzed for pre-treatment ECD levels by enzyme-linked immunosorbent assay (ELISA). The degree of Her-2/neu phosphorylation/activation of tumor specimens was assessed by immunohistochemistry using a Her-2/neu phosphorylation state specific antibody (PN2A). The Her-2/neu phosphorylation status was then correlated with the patients' ECD levels and clinical course of disease. Results: Serum ECD levels were significantly higher in 15 (24%) patients with tumours exhibiting activated Her-2/neu as compared to those without detectable Her-2/neu phosphorylation (median 148.2 ng/ml vs. 28.5 ng/ml, p=0.01). Whereas response rate only tended to be higher in patients with Her-2/neu-phosphorylated breast cancer (47% vs. 34%, p=0.197), both uni- an multivariate analyses revealed that the median progression-free survival under trastuzumab-based treatment was significantly longer in patients with Her-2/neu-phosphorylated breast cancer 11.7 (95% CI 5.2 to 18.3) as compared to the progression-free survival of 4.5 (95% CI 3.4 to 5.6, p=0.001) months observed in patients with tumours without phosphorylated Her-2/neu. Conclusions: These data strongly suggest that proteolytic cleavage of the ECD represents a biologically relevant ligand-independent mechanism of Her-2/neu activation in vivo. The influence of Her-2/neu activation status upon the efficacy of trastuzumab merits further investigation in larger prospective trials. No significant financial relationships to disclose.