Abstract LB-B11: Assessment of HER3 status during lapatinib treatment in HER3-positive breast cancer using 89Zr-anti-HER3 mAb
Authors: Kol Arjan , Pool Martin , de Jong Steven , de Vries Elisabeth GE , Lub-de Hooge Marjolijn N. , Terwisscha van Scheltinga Anton GT 
Published: May 2016

Abstract
Treatment of human epidermal growth factor receptor 2 (HER2)-driven breast cancer with the HER-targeting tyrosine kinase inhibitor lapatinib can lead to a rapid compensatory increase in expression, signaling activity and relocalization of HER3 to the plasma membrane, which may attenuate the response to lapatinib. This might imply a potential role for a more dynamic assessment of HER3 tumor status using molecular imaging techniques, such as positron emission tomography (PET), instead of immunohistochemical HER3 staining on tumor biopsies. Here, we explored the feasibility of a dynamic assessment of HER3 status during lapatinib treatment in human breast cancer xenografts using zirconium-89 labeled anti-human HER3 monoclonal antibody (mAb) as a potential tracer for animal PET imaging.
The anti-human HER3 mouse mAb MAB3481 was used for all experiments. The effect of lapatinib treatment on HER3 expression and HER3 mAb internalization in human breast cancer cell lines SKBR3 and BT474 was determined using flow cytometry. Biodistribution was performed using 89Zr-anti-HER3 mAb in mice bearing BT474 or SKBR3 tumors. Mice received daily vehicle or a lapatinib dose of 25, 50 or 100 mg/kg orally. A tracer dose of 89Zr-anti-HER3 mAb combined with the aspecific tracer 111In-IgG was injected 3 days after treatment. Ex vivo organ distribution assessment of 89Zr-anti-HER3 mAb was performed 6 days after tracer injection. Ex vivo tumor analysis using western blotting, ELISA and immunohistochemistry were performed to measure HER3 levels.
In vitro, lapatinib treatment resulted in a ∼2-fold increase in membranous HER3 expression and HER3 internalization in SKBR3 and BT474 tumor cells. 89Zr-anti-HER3 mAb tumor uptake was significantly higher compared to 111In-IgG uptake in BT474 (P &lt; 0.01), demonstrating HER3 specific tumor uptake. SKBR3 xenografts did not show HER3 specific uptake, which was likely caused by the poor viability of the tumors. HER3 upregulation was observed in BT474 xenografts after lapatinib treatment for 9 days. The enhanced HER3 expression was related to a 76 ± 9% increase in 89Zr-anti-HER3 mAb tumor uptake at 25 mg/kg and 92 ± 27% at 50 mg/kg lapatinib. Lapatinib at the highest concentration (100 mg/kg) strongly inhibited tumor growth and did not increase tumor uptake of the tracer.
In conclusion, HER3-specific uptake of 89Zr-anti-HER3 mAb was shown in breast cancer xenografts. HER3 upregulation after lapatinib treatment was related to an enhanced 89Zr-anti-HER3 mAb uptake in these xenografts. These promising data warrant future dynamic assessment of HER3 status with 89Zr-anti-HER3 mAb PET imaging.
Citation Format: Arjan Kol, Martin Pool, Steven de Jong, Elisabeth GE de Vries, Marjolijn N. Lub-de Hooge, Anton GT Terwisscha van Scheltinga. Assessment of HER3 status during lapatinib treatment in HER3-positive breast cancer using 89Zr-anti-HER3 mAb. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr LB-B11.