Introduction: Bevacizumab (anti-VEGF) and cetuximab (anti- EGFR) are approved for the treatment of cancer. However, in advanced colorectal cancer, the combination of bevacizumab and cetuximab did not improve survival (Tol et al. NEJM 2009). Up to now, the reason for the lack of activity of the combination therapy remains unclear. Previously, we have shown that bevacizumab reduces the vascular density of xenografts and consequently limits the delivery of cetuximab to tumors. Thereby, it can potentially limit the therapeutic efficacy of cetuximab. Alternatively, cetuximab treatment could alter VEGF expression, limiting tumor targeting by bevacizumab. The aim of this study was to determine the effect of cetuximab treatment on VEGF expression and targeting of bevacizumab to the tumor.

Material and methods: Mice with subcutaneously implanted SUM149 xenografts, which express EGFR and VEGF, were treated intraperitoneally with 1 mg/kg cetuximab (twice a week). Before start of treatment and after 7, 14, and 21 days, the uptake of radiolabeled bevacizumab in the tumor was measured by immunoSPECT/CT. Three days prior to scanning, mice received 15 MBq of 111In-bevacizumab. After scanning, tumors were dissected for ex vivo biodistribution and immunohistochemical analysis of VEGF and CD34 expression.

Results: After cetuximab treatment, tumor targeting of bevacizumab was significantly reduced on immunoSPECT/CT compared to untreated tumors. Tumor uptake measured ex vivo was 28.9 ± 4.0 %ID/g for untreated tumors, compared to 18.2 ± 2.8 %ID/g for tumors treated 21 days with cetuximab (p = 0.009). Immunohistochemical analysis showed that the vascular density was unaltered while VEGF expression decreased during cetuximab treatment.

Conclusion: Cetuximab treatment reduced VEGF expression and targeting of radiolabeled bevacizumab to the tumor. This could at least partly explain why the combination of bevacizumab and cetuximab does not result in improved therapeutic efficacy.

Citation Format: Sandra Heskamp, Otto C. Boerman, Janneke D.M. Molkenboer-Kuenen, Wim J.G. Oyen, Winette T.A. van der Graaf, Hanneke W.M. van Laarhoven. Cetuximab treatment reduces VEGF expression and targeting of radiolabeled bevacizumab in breast cancer xenografts. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 866. doi:10.1158/1538-7445.AM2013-866