once a week (on Days 20, 27 and 34). than H1975. Therefore, combination therapy which uses erlotinib can be considered effective if epidermal growth factor receptor phosphorylation is usually inhibited by erlotinib, even in erlotinib-resistant tumor xenograft models. Our results suggest that the continuous inhibition of epidermal growth factor receptor phosphorylation by erlotinib after progressive disease enhances the antitumor activity of chemotherapy. Keywords:erlotinib, docetaxel, irinotecan, resistance, epidermal growth factor receptor, non-small cell lung cancer, pancreatic cancer, progressive disease == Introduction == The epidermal growth factor receptor (EGFR) is usually a transmembrane glycoprotein with an extracellular EGF-binding domain name and an intracellular domain name possessing intrinsic tyrosine Flurizan kinase activity (1,2). Ligand binding activates the receptor’s tyrosine kinase, initiating cascades of intracellular signaling such as those via the Ras protein (3). High levels of EGFR expression have been reported in a wide range of human malignancies (46) Rabbit Polyclonal to SFRS7 and enhanced expression of EGFR has previously been shown in non-small cell lung cancer (NSCLC) (7). Since it was reported that EGFR overexpression is usually a factor of poor prognosis (8,9), treatments targeting EGFR would be expected to show survival benefits. Erlotinib (Tarceva) is an oral, small molecule tyrosine kinase inhibitor that reversibly binds to the intracellular domain name of EGFR. This blocks autophosphorylation of EGFR with subsequent inhibition of the downstream signaling pathways which promote cell proliferation. Erlotinib is used for metastatic NSCLC and pancreatic cancer in many countries. Clinical results have exhibited that erlotinib monotherapy or combination therapy with gemcitabine showed a Flurizan survival benefit for NSCLC or pancreatic cancer, respectively (10,11). However, most of these patients developed progressive disease (PD) during such therapies and it is usually considered best to switch to chemomonotherapy after developing PD. It is reported that this major mechanisms of erlotinib resistance are gatekeeper mutation (T790M) of EGFR andc-Metamplification (12,13) in tumor cells. On the other hand, it is reported that this tumor cells express active EGFR even after acquiring resistance to erlotinib (13,14). Considering that EGFR overexpression is usually a factor of poor prognosis, discontinuing erlotinib treatment after PD has developed may be an inappropriate option and combining erlotinib with the next stage of chemotherapy may be an appropriate therapy. We have previously reported that this combination of docetaxel with erlotinib showed a synergistic effect in NSCLC cell linesin vivoirrespective of EGFR or K-RAS mutation status (15). Therefore, we investigated the antitumor effect of combination therapies Flurizan of erlotinib with various chemotherapeutic brokers docetaxel, irinotecan and gemcitabine, using erlotinib-resistant Flurizan tumor cell xenografts as well as anin vivoerlotinib PD xenograft model, to show the clinical relevance of continuing erlotinib treatment after development of PD. == Materials and methods == == Chemicals == Erlotinib was provided by F. Hoffman-La Roche (Basel, Switzerland) as a fine powder and was dissolved in distilled water made up of 6% (w/v) Captisol (CyDex Pharmaceuticals, KS, USA) and diluted with saline forin vivoexperiments. Erlotinib was dissolved in DMSO forin vitroexperiments. Docetaxel was synthesized by Kanto Chemical Co., Inc. (Tokyo, Japan) as a fine powder and was dissolved in saline containing 2.5% (v/v) polysorbate 80 (Sigma-Aldrich Co., USA) and 2.5% (v/v) ethanol forin vivoexperiments. Irinotecan was purchased from Daiichi Sankyo Pharmaceutical Co., Ltd. (Tokyo, Japan) as an aqueous answer and diluted with saline. == Animals == Male 5-week-old BALB-nu/nu mice (CAnN.Cg-Foxn1