Abstract Introduction: Oncogenic activation of Kras is a key event in the development of pancreatic ductal adenocarcinoma (PDAC), leading to initiation and progression of pancreatic intraepithelial neoplasia (PanIN), the preneoplastic precursor to PDAC. Endogenous mouse models with activating KrasG12D mutations recapitulate this process very well. Epidermal growth factor receptor (Egfr) is known to be elevated in PDAC and is to date the only approved target for targeted therapy, although the exact mechanism of action remains unknown. In this study we thus investigated the effect of conditional Egfr ablation in a genetically engineered mouse (GEM) model of PDAC. Experimental design: Using a Cre/lox approach, we deleted Egfr from Ptf1a-positive pancreatic progenitor cells in different GEMs of PDAC and in mice with caerulein-induced acute pancreatitis. Acinar epithelial explants of mutant mice were used to investigate the role of Egfr in vitro. In vivo therapy-studies in GEM were performed using different inhibitors of Egfr. Results: Unexpectedly, KrasG12D;EgfrΔ/Δ mice revealed complete absence of PanIN lesions and showed only rare fields of ductal-like lesions at 6 months of age. In a mouse model of induced acute pancreatitis KrasG12D;EgfrΔ/Δ mice did not develop PanINs as did KrasG12D control mice. In a highly aggressive KrasG12D;p53Δ/Δ GEM model for PDAC, tumor initiation and progression rate was reduced although not completely blocked in EGFR-ablated mice. In vivo, early therapy/chemoprevention of KrasG12D;p53Δ/Δ mice with either cetuximab, an antibody directed against the Egfr ligand-binding domain, or erlotinib, a small molecule tyrosine kinase domain inhibitor, displayed significantly reduced development of acino-ductal metaplasia (ADM) in cetuximab-treated mice and to a even higher extend in erlotinib-treated animals, indicating a ligand-independent role of Egfr in ADM- and PanIN-induction. To closer investigate the underlying mechanisms in vitro, we isolated acinar epithelial explants of EgfrΔ/Δ and KrasG12D; EgfrΔ/Δ mice and corresponding controls. Acinar epithelial explants of EgfrΔ/Δ and KrasG12D;EgfrΔ/Δ did not undergo ADM in opposite to KrasG12D or Egf-stimulated wildtype acinar-epithelial explants. Closer examination revealed that initiating events like the expression of the progenitor cell marker nestin in acinar explants of KrasG12D;EgfrΔ/Δ mice and the expression of pStat3 in acinar tissue of KrasG12D; EgfrΔ/Δ mice were still detectable, similar to KrasG12D controls, but progression to ductal-like structures was blocked. Protein-expression levels of pStat3 and pErk were reduced in KrasG12D;EgfrΔ/Δ mice although active GTP-bound Kras levels were not altered in KrasG12D;EgfrΔ/Δ mice. Additionally, we performed immunofluorescence and confocal analysis of Egfr and Kras in acinar-epithelial explants of KrasG12D and KrasG12D;EgfrΔ/Δ mice, which did not reveal Egfrdependent differences in localization of Kras. Conclusions: These data provide strong evidence that Egfr seems to be essential for induction of ADM and PanIN development in a Kras-independent manner that needs to be further investigated. Even additional stimuli such as caerulein-induced inflammation did not lead to the development of precursor lesions suggesting an early requirement of Egfr in the carcinogenic process. This abstract is also presented as Poster A44. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr PR11.