Abstract A182: U3–1287 (AMG 888), a fully human anti-HER3 mAb, demonstrates in vitro and in vivo efficacy in the FaDu model of human squamous cell carcinoma of the head and neck (SCCHN).
Authors: Freeman Daniel J. , Ogbagabriel Selam , Bready James , Sun Ji-Rong , Radinsky Robert , Hettmann Thore 
Published: June 2012

Abstract
Background: Anti-EGFR mAbs have demonstrated clinical utility in SCCHN; however, most patients are resistant or acquire resistance to therapy. In preclinical models, activation of HER3 is a resistance mechanism to current HER inhibitors. Recent data (Wilson et al 2011) demonstrated that SCCHN cancer cell lines and primary tumors express both HER3 and heregulin, the ligand for HER3. This autocrine loop is one potential mechanism for the lack of sensitivity to anti-EGFR mAbs observed in SCCNH patients. Herein, we report in vitro and in vivo activity of U3–1287 (AMG 888) as a single agent and in combination with panitumumab, an anti-EGFR mAb, in the FaDu preclinical model of human SCCHN.
Methods: FaDu (SCCHN) cells were treated with U3–1287 (AMG 888), panitumumab, the combination or control mAbs for 4 hours. To determine in vitro efficacy on cell proliferation, FaDu cells were treated with 1 μg/mL U3–1287 (AMG 888), 1 μg/ml panitumumab, the combination or control mAbs in serum containing medium. After 72 hours, the growth of treated cells was measured by ATPLite assay. Levels of pHER3, pEGFR, pERK1/2 and pAKT were measured by Western blot analyzes to determine the inhibition of HER3 oncogenic signaling. To determine in vivo efficacy, mice bearing ∼200 mm3 FaDu xenografts were treated 2×/week with U3–1287 (AMG 888), panitumumab, the combination or control mAbs. Results: Treatment with either U3–1287 (AMG 888) or panitumumab resulted in inhibition of pHER3 and pAKT or pEGFR and pERK1/2, respectively. Treatment with the combination resulted in inhibition of pHER3, pEGFR, pAKT and pERK1/2. Although treatment with panitumumab or U3–1287 (AMG 888) alone significantly inhibited in vitro cell proliferation versus control mAb treated cells (57% (p&lt;0.001) and 38% (p&lt;0.005), respectively), greater inhibition was observed with the combination versus either single agent alone (75% (p&lt;0.01). A dose-dependent inhibition of FaDu xenograft growth was observed with panitumumab or U3–1287 (AMG 888) when used as single agents versus mAb controls (p&lt;0.02 and p&lt;0.01 versus mAb control, respectively). In the combination group (200 μg/ml U3–1287 (AMG 888) and 100 μg/ml panitumumab), treatment resulted in significant tumor inhibition and regressions in all tumors and versus either single agent alone (p&lt;0.0001). To test the durability of the regressions, FaDu xenografts were treated in a separate study with 200 μg/ml U3–1287 (AMG 888) and 100 μg/ml panitumumab again resulting in regressions in all tumors with 8/10 animals remaining tumor free 45 days post the final treatment. Western blot analyzes confirmed a greater inhibition of pHER3 and pAKT in tumors from the combination-treated group versus the single agent-treated groups.
Conclusions: U3–1287 (AMG 888) inhibits proximal and distal HER3 oncogenic signaling in FaDu SCCHN cells resulting in in vitro and in vivo growth inhibition. SCCHN xenografts were sensitive to U3–1287 (AMG 888) treatment as single agent or in combination with panitumumab. Combination treatment with U3–1287 (AMG 888) and panitumumab resulted durable regressions which was not observed with either single agent. These data together with the HER3/heregulin autocrine loop detected in patient samples (Wilson et al 2011) provides evidence for the potential clinical investigation of U3–1287 (AMG 888) in SCCHN.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A182.