C, Pancreatic cancers cells were xenografted in nude mice that were then treated with 2 or 10 mg/kg pertuzumab (P2 and P10) or sterile PBS twice/week. for pertuzumab efficacy in HER2low-expressing pancreatic cancer and HER3 expression might be a predictive biomarker of pertuzumab efficacy in such cancers. Further studies in clinical samples are required to confirm these findings and the interest of Rabbit polyclonal to AFF3 combining anti-HER2 and anti-HER3 therapeutic antibodies. Keywords:HER3, HER2, pertuzumab, pancreatic cancer == INTRODUCTION == Pancreatic cancer remains one of the most aggressive tumors. As at diagnosis 60 to 80% of patients already have locally advanced or metastatic disease [1], only palliative therapy is possible and the 5-12 months survival rate is lower than 5% [2]. In an effort to find new treatments, the molecular mechanisms involved in pancreatic cancer development are currently investigated and EGFR family members [3] have emerged as relevant and promising therapeutic targets. The significant, but modest, increase in AZD1152-HQPA (Barasertib) median survival obtained with the erlotinib/gemcitabine combination illustrates the potential and the challenges of anti-EGFR therapies in pancreatic cancer [4]. Moreover, a better selection of the patients who could benefit from these novel therapies could improve their efficacy and the prognosis, as exhibited for the anti-HER2 antibody trastuzumab in breast cancers. Indeed, trastuzumab blocks the growth of breast tumors that strongly express HER2 or withHER2gene amplification, but is not efficient in cancers with normal or low HER2 protein level. It is critical to highlight that trastuzumab therapeutic activity could have been underestimated by testing it in patients who had not been selected based on their HER2 status [5]. We have recently exhibited the interest of targeting EGFR/HER2 heterodimers in HER2low- expressing pancreatic cancer [6]. The combination of cetuximab and trastuzumab had a synergistic anti-tumor effect that was mainly due to the modification of the distribution of homo- and hetero-dimers, with disruption of EGFR/HER2 dimers and increased homodimer formation. To date the only HER2-targeting drug known to disturb ligand-activated HER2 dimerization is usually pertuzumab. This antibody binds to a different HER2 epitope than trastuzumab and thereby has a therapeutic effect in preclinical studies that does not strictly require HER2 protein overexpression AZD1152-HQPA (Barasertib) [8]. Nevertheless, pertuzumab was recently approved, in combination with trastuzumab and docetaxel, only for the treatment of HER2-positive (HER2high) metastatic breast cancer [9]. Moreover, pertuzumab inhibits neuregulin-stimulated cell growth mediated by HER2/HER3 dimers in breast malignancy, whereas trastuzumab is usually more efficient in the absence of neuregulin [8]. These two antibodies seem to regulate differently HER2 homo- and hetero-dimers and to have different ligand dependencies. HER3 is the main HER2 partner involved in pertuzumab response in ovarian cancer [10], but it could also influence pancreatic cancer tumorigenesis [11] because it is usually often overexpressed in pancreatic cancer and HER3 high expression correlates with advanced disease stages and lower overall survival [12,13]. Although HER3 has a very poor intracellular tyrosine kinase activity, its neuregulin-triggered transactivation by other members of the EGFR family induces direct phosphorylation of the six binding sites for the p85 regulatory subunit of PI3K, resulting in activation of the AKT signaling cascade [14]. Furthermore, HER3 is usually a key player in tumor cell resistance to EGFR- and HER2-targeted therapies, particularly in pancreatic cancer where it regulates EGFR signaling by modulating the response to erlotinib or cetuximab [13]. HER3-mediated AKT signaling participates also in gemcitabine resistance [15]. In HER2-amplified breast tumors, resistance to trastuzumab or to tyrosine kinase inhibitors has been associated with HER3 expression [15]. In this study, we investigated whether HER3 expression could affect the therapeutic response to pertuzumab in HER2lowpancreatic cancer by analyzingin vitroandin vivopertuzumab effects in HER2lowpancreatic cancer cell lines that express or not HER3. We show that this inhibitory effect of pertuzumab on cell viability and tumor progression AZD1152-HQPA (Barasertib) in pancreatic cancer xenografts correlates with HER3 protein expression and is neuregulin-dependent. Accordingly, HER3 knockdown led to resistance to pertuzumab therapy. We also found that HER3 mRNA level and cell surface expression were increased after pertuzumab treatment. Based on these results we tested the effects of combining pertuzumab with the anti-HER3 therapeutic antibody 9F7-F11 [17]. The combined treatment enhanced tumor growth inhibition in pancreatic cancer xenografts, suggesting that it might represent a new potential therapy for pancreatic cancers that co-express HER2 and HER3. Finally, we analyzed the co-expression of HER2 and HER3 by immunohistochemistry (IHC) in 45 pancreatic ductal adenocarcinomas (PDAC). == RESULTS == == HER3 expression and NRG11-induced proliferation in pancreatic cancer cells == The expression of HER family members and NRG11-induced proliferation was assessed in five pancreatic cancer cell lines that harborKRASmutations.