Abstract 3527: Assessing signal modulation in preclinical models and tumor tissue from patients enrolled in a Phase 1 study of the HER3/EGFR dual action antibody, MEHD7945A (MEHD), as an exploratory pharmacodynamic (PD) analysis.
Authors: Elicia M. Penuel, Luciana Burton, Howard Stern, Lackner Mark, Andrea Pirzkall, Lukas Amler, Gabriele Schaefer
Journal: Cancer Research (2013)

Abstract Background Aberrant activation of HER family members including EGFR results in unregulated proliferative signals involved in tumorigenesis. HER3 plays a unique role as both a dimerization-dependent allosteric catalyst and critical activator of PI3 kinase signaling (PI3K). MEHD is a dual-action antibody that blocks ligand binding to EGFR and HER3 to abrogate signaling downstream from these two receptors. Early clinical evidence shows PD modulation and biological activity. The two major signaling pathways activated by ligand binding to these receptors are the RAS/MAPK and PI3K/AKT pathways. Phosphorylation of these critical signaling proteins can be monitored to evaluate inhibition of ligand driven receptor activity. Methods Phosphorylation of signaling proteins in the RAS/MAPK and PI3K/AKT pathways was measured by Western blot analysis and reverse-phase protein arrays (RPPA) in MEHD-sensitive cell lines and xenograft models. Paired tumor biopsies from patients in the Phase 1 MEHD study were collected pre and post MEHD dosing. Half of each sample was formalin-fixed and paraffin-embedded for immunohistochemistry (IHC) using pS6 (S235/236), pPRAS40 (T246), and pMAPK (T202/Y204); the other half was frozen and embedded in Optimum Cutting Temperature compound (OCT) for analysis of multiple signaling pathway endpoints by RPPA. Results Inhibition of EGFR and/or HER3 activated signaling by MEHD was confirmed in sensitive cell lines and xenograft models by western blot analysis for key analytes including pMAPK and pS6. Broader analysis of signaling in the RAS/MAPK and PI3K/AKT pathways by RPPA demonstrated additional markers that were inhibited by MEHD. Signaling from EGFR driven models was blocked by EGFR inhibition alone but signaling in models in which both EGFR and HER3 were contributing factors, as assessed by tumor growth inhibition, were not inhibited by EGFR inhibition alone. However, MEHD was equally effective at blocking signaling in both models. Analytes that were inhibited by MEHD in these pre-clinical models were measured by RPPA and IHC in paired biopsy specimens from the Ph1 MEHD study as a PD assessment. Thirty-two paired biopsies were obtained. 20/32 had sufficient tissue and tumor for assessment and 11/20 showed decreased phosphorylation of pERK, pPRAS40, or pS6 by IHC and/or RPPA. Conclusions Preclinical evaluation of RAS/MAPK and PI3K/AKT signaling in MEHD-sensitive cell lines and xenograft models identified pERK, pPRAS40, and pS6 as measurable indicators of MEHD target engagement and biological activity. Evaluation of these markers in patient tumor biopsies demonstrated that signaling pathways consistent with HER3 and/or EGFR inhibition are downregulated in tumors, suggesting that MEHD is biologically active in patients (see Juric et al., AACR abstract). Citation Format: Elicia M. Penuel, Luciana Burton, Howard Stern, Lackner Mark, Andrea Pirzkall, Lukas Amler, Gabriele Schaefer. Assessing signal modulation in preclinical models and tumor tissue from patients enrolled in a Phase 1 study of the HER3/EGFR dual action antibody, MEHD7945A (MEHD), as an exploratory pharmacodynamic (PD) analysis. [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 3527. doi:10.1158/1538-7445.AM2013-3527