Overexpression of HER2 is associated with resistance to standard therapies and, consequently, poor prognosis. The most successful of HER2-targeted therapies, and the only one approved by the FDA, uses trastuzumab (Herceptin) - a recombinant humanized anti-HER2 monoclonal antibody. Currently, alteration of gross tumor volume is used to assess the response to trastuzumab. However, this approach provides only a late indicator of response. The ex vivo assays of tissue samples are potentially valuable, but their procurement through biopsies is invasive and might be biased by tumor heterogeneity.

We studied the feasibility of PET imaging to quantify changes in HER2 expression and to predict the response to trastuzumab in breast cancer xenograft model using 18F-ZHER2-Affibody molecule as a tracer. We administered the tracer to mice bearing subcutaneous BT474 tumors, which display very high HER2 expression. The control and treatment groups consisted of 15-20 animals each. The tumor volume was in 100-250 mm3 range. The tumor size was monitored twice weekly using caliper measurements. Trastuzumab (50 mg/kg loading dose, 25 mg/kg maintenance dose) or saline (control) were given i.p. twice a week for a total of 5 doses. Animals were scanned before the treatment, 48 h and two weeks after the beginning of therapy. Immediately after the last scan, animals were euthanized and pieces of tumors were fixed and/or frozen for further analysis. The HER2 expression and activation of HER2 signaling pathways in the tumor samples were assessed by Western blot, ICH and ELISA.

Quantitative analysis of PET images indicated decrease of HER2 expression, associated with significant tumor regression (>30% volume reduction), in 12 of 20 treated tumors. However, only negligible changes in HER2 expression and stable disease were found in the remaining treated mice. No changes in HER2 expression and intensive growth of the tumors (increase more than 100% of tumor volume) were observed in the control group over the 2 weeks of therapy. These results were confirmed by ex vivo analysis showing dramatic decrease of total HER2 level in responding tumors. Despite the reduction of HER2 levels, there was persistent HER2 phosphorylation in all tumors. As previously shown, it might be due to activation of alternative HER receptors as a result of ADAM17-mediated autocrine ligand release. Tissue analysis showed a trend for trastuzumab-treated xenografts to have higher levels of ADAM17 and heregulin compared to the untreated mice. Other mechanisms that might account for the observed difference between the responders and the non-responders are currently investigated.

Our results suggest that monitoring of changes in HER2 expression after five doses of trastuzumab allows predicting the tumor response and identifying mice that would not fully benefit from the treatment.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5226.