Abstract #3998: ⁸⁹Zr-trastuzumab immunoPET visualizes HER2 downregulation induced by the HSP90 inhibitor NVP-AUY922 in human tumor xenograft

The 90 kDa Heat Shock Protein (HSP90) is a molecular chaperone protein which is involved in maturation and stability of many oncogenic client proteins. It serves as a target for anticancer drugs, since its inhibition affects multiple cancer-associated signaling pathways. NVP-AUY922 is a potent HSP90 inhibitor and is currently in clinical development. The Human Epidermal growth factor Receptor 2 (HER2) is overexpressed in ~25% of the breast cancers and is associated with aggressive tumor behavior. HER2 is also a client protein of HSP90, and therefore a potential biomarker for early evaluation of the molecular response to HSP90 targeted therapies in HER2 overexpressing tumors. For this purpose, we have used the HER2 antibody trastuzumab labeled with the positron emitting isotope ⁸⁹Zr to assess the alterations in HER2 expression after HSP90 targeted therapies with the non-invasive, high resolution, quantitative and sensitive immunoPET technique. Methods: Experiments were performed with NVP-AUY922 and the HER2 overexpressing human ovarian tumor SKOV-3 cell line. In vitro evaluation of the HER2 expression after NVP-AUY922 treatment was assessed by flow cytometry and in a cell binding assay with ⁸⁹Zr-trastuzumab. The in vivo experiments were performed in SKOV-3 xenograft bearing nude mice. NVP-AUY922 was administered intraperitoneally every other day in a dose of 50mg/kg. For ⁸⁹Zr-trastuzumab immunoPET imaging, animals were injected intravenously with 5 MBq ⁸⁹Zr-trastuzumab (100 \#956;g) 6 days before NVP-AUY922 treatment and after 3 doses of NVP-AUY922. MicroPET and microCT imaging was performed at 1, 3 and 6 days post injection of the tracer. Ex-vivo biodistribution was performed at day 6. Results: The SKOV-3 HER2 expression, assessed in vitro by flow cytometry, showed a 76±1.3% reduction after 24hr NVP-AUY922 treatment. The SKOV-3 ⁸⁹Zr-trastuzumab cellular binding was reduced by 75±9.3% after 24hr NVP-AUY922 treatment. Quantification of the PET images showed a reduction of 37% (p = 0.0143) in ⁸⁹Zr-trastuzumab tumor uptake after NVP-AUY922 treatment. PET results were confirmed by comparing ex-vivo biodistribution tumor uptake of NVP-AUY922 treated mice with control mice tumor uptake. Conclusions: HER2 immunoPET provides a tool to image and quantify the reduction in HER2 expression in the tumor following HSP90 inhibition non-invasively. Since the ⁸⁹Zr-trastuzumab PET tracer is already used clinically, this technique can easily be translated to determine the early molecular effects of HSP90 inhibitors in patients. Supported by a grant of the Dutch Cancer Society

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3998.