Abstract 5303: Serial HER2-PET imaging with 89Zr-trastuzumab-F(ab’)2 of lapatinib and 17AAG treated mice bearing SKBR3 xenografts

Abstract

Abstract Background: Understanding molecular responses involved in human epidermal growth factor receptor-2 (HER2) targeted drugs may improve treatment of HER2 positive breast cancer. In vivo assessment of effects on HER2 expression is of value in this setting. The EGFR/HER2 tyrosine kinase inhibitor lapatinib inhibits HER2 signaling and stabilizes HER2 at the cell membrane, while the heat shock protein-90 (HSP90) inhibitor 17AAG degrades HER2/EGFR. We therefore evaluated the effect on HER2 dynamics in vivo with 89Zr-trastuzumab-F(ab’)2 HER2-PET imaging in human tumor bearing mice treated with lapatinib and 17AAG. Materials and methods: Effects of lapatinib and 17AAG on EGFR and HER2 membrane expression were evaluated in vitro in the human breast cancer cell line SKBR3 using flow cytometry. In vivo effects on HER2 expression were evaluated with 89Zr-trastuzumab-F(ab’)2 μPET imaging in SKBR3 xenograft bearing mice. 89Zr-trastuzumab-F(ab’)2 (5 MBq; 20 μg) was injected two times with a 7 day interval and μPET imaging was performed 24 hours after tracer injection. Lapatinib was dosed 100 mg/kg/day orally for 6 days, starting 1 day after the first μPET scan. 17AAG was dosed 3×50 mg intraperitoneal in the 24 hours prior to the second tracer injection. A non-treated group of mice served as control and for validation of 89Zr-trastuzumab-F(ab’)2 μPET imaging. Imaging data were compared with ex vivo biodistribution analysis, immunohistochemistry, and with an in vitro HER2 internalization experiment. Results: On flow cytometry, lapatinib induced upregulation of EGFR (+70%; P = 0.0011), but not HER2 (+19%; P = 0.1275). 17AAG treatment lowered both EGFR (−41%; P = 0.016) and HER2 (−76%; P = 0.022) expression. This EGFR/HER2 reduction by 17AAG was inhibited by lapatinib. In vivo, 89Zr-trastuzumab-F(ab’)2 tumor uptake on the second μPET scan was lowered by lapatinib (−32%; P = 0.00004), 17AAG (−34%; P = 0.0022) and the lapatinib plus 17AAG combination (−53%; P = 0.011). 89Zr-trastuzumab-F(ab’)2 tumor uptake in mice treated with lapatinib plus 17AAG was lower than with either lapatinib or 17AAG alone (P = 0.0043 and P = 0.0022, respectively). In vitro, 89Zr-trastuzumab-F(ab’)2 internalization was reduced by lapatinib (−25%; P = 0.0022), explaining the lower 89Zr-trastuzumab-F(ab’)2 tumor uptake after lapatinib. Conclusion: 89Zr-trastuzumab-F(ab’)2 μPET visualized treatment induced impact on HER2 expression and -dynamics. Lapatinib inhibited HER2 internalization, and thereby decreased 89Zr-trastuzumab-F(ab’)2 tumor uptake. HSP90 inhibition lowered HER2 membrane expression and thereby 89Zr-trastuzumab-F(ab’)2 tumor uptake. This knowledge on HER2 dynamics ameliorates understanding of efficacy of several combinations of HER2 targeted agents. Supported by grant 2007-3739 of the Dutch Cancer Society. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5303. doi:10.1158/1538-7445.AM2011-5303