Abstract 3867: Dual blockade of HER2 in HER2-overexpressing tumor cells does not eliminate HER3 function completely: Clinical implications

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL There is mounting evidence that the most efficacious approach to treat HER2+ breast cancer is dual blockade with HER2 inhibitors such as the monoclonal antibody trastuzumab and the tyrosine kinase inhibitor lapatinib. We examined what effect dual blockade had on HER3 protein levels. The combination of lapatinib and trastuzumab treatment increased both cell-surface and total HER3 protein levels in BT474, SKBR3, and MDA453 cells. Treatment with the HER3 neutralizing antibody U3-1287 decreased HER3 levels starting at 1 h and continuing through 24 h in these cell lines. Moreover, adding U3-1287 to lapatinib and trastuzumab reversed the upregulation of cell surface and total HER3. Addition of the HER3 antibody to lapatinib and trastuzumab treatment improved the action of lapatinib and trastuzumab as assessed by the TUNEL apoptosis assay, three-dimensional matrigel growth and two-dimensional crystal violet growth assays. We next examined the effect of dual blockade of HER2 in combination with the HER3 antibody in cells with acquired resistance to trastuzumab. Treatment with U3-1287 blocked the upregulation of HER3 that followed therapy with lapatinib and trastuzumab. In mice bearing trastuzumab resistant xenografts, there was a reduction in tumor volume of mice treated with U3-1287 and trastuzumab compared to mice treated with single agent trastuzumab, U3-1287, or vehicle. We next hypothesized that inhibition of HER3 combined with dual HER2 blockade would result in decreased tumor recurrence in vivo. Mice bearing BT474 xenografts were treated with either lapatinib and trastuzumab (L+T) or lapatinib, trastuzumab and U3-1287 (L+T+U) for 3 weeks. After 3 weeks of treatment mice from both groups exhibited almost complete tumor regressions. At 7 weeks post-treatment 5 of 10 mice (50%) in the L+T group exhibited tumor re-growth (defined as a tumor larger than 200 mm3), whereas only 1 of 12 mice (8.3%) in the L+T+U group did so (p= 0.043, fisher's exact test). We are additionally performing studies in a transgenic mouse model with conditional ablation of HER3 to further examine if lapatinib and trastuzumab do not completely eliminate HER3 function. Finally we are comparing genetic inhibition of HER3 via stable inducible shRNA to pharmacological inhibition of HER3 to determine the most efficacious approach to inhibit HER3. In summary, a HER3 inhibitor prevents the upregulation of HER3 observed with lapatinib and trastuzumab treatment and sensitizes breast cancer cells to lapatinib and trastuzumab. Moreover, lapatinib and trastuzumab in combination with a HER3 inhibitor prevent tumor recurrence better than lapatinib and trastuzumab. These data suggest dual blockade of HER2 may only be partially effective in some instances due to maintenance of active HER3 Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3867. doi:1538-7445.AM2012-3867