Cells were then lysed and assessed for luciferase activity using the Luciferase Reporter Assay Kit (Promega) following the manufacturers instructions. Cell viability assays Cells were plated in 96-well plates and allowed to adhere overnight. This resistance could be reversed by STAT3 inhibition. Finally, HN5 cells with acquired resistance to the EGFR tyrosine kinase inhibitor, AG1478 displayed greater STAT3 activity than the HN5 control cell line. These AG1478-refractory HN5 cells were re-sensitized to AG1478, cetuximab and erlotinib when co-treated with a STAT3 inhibitor. Taken together, our current data indicates a key role of STAT3 activity in promoting resistance to anti-EGFR therapy and suggests that anti-EGFR therapy in combination with inhibitors that block STAT3 may provide therapeutic benefit for patients with mCRC and other EGFR driven tumor types. gene (present in 30C40% of mCRC), is currently the strongest predictive marker of resistance to EGFR-targeted therapy.12,21-27 Indeed, 90% of mCRC patients harboring mutations show no therapeutic benefit to cetuximab or panitumumab. Due to the lack of response, the American Society of Clinical Oncology and the European Medicines Agency have issued guidelines to screen patient biopsies for mutations prior to treatment,28,29 and subsequently only administer EBI-1051 EGFR-based brokers into patients with tumors expressing wild-type (wt) K-RAS.28,30 Disappointingly, however, wt expression does not predict successful response.21,22,26,31,32 B-RAF, PTEN, PI3-K, and N-RAS have all been identified as possible biomarkers to predict response to EGFR targeted therapy. Mutational analysis in these signaling molecules have revealed conflicting conclusions with some reports observing significant correlation with response to anti-EGFR treatment while others show no correlation.31-35 Signal transducer and activator of transcription 3 (STAT3) is a member of the STAT family of cytoplasmic transcription factors that are activated by many cytokine and growth factor receptors including the EGFR.36,37 Phosphorylated STAT3 transmits its signal from the EGFR to the nucleus where it initiates transcription of multiple cancer promoting genes such as SOCS3, SMAD7, and VEGF.36,38,39 Furthermore, STAT3 is constitutively active in many types of tumors including those in which anti-EGFR agents have been clinically approved (mCRC, HNSCC, NSCLC, and DKK1 pancreatic cancer).38,40,41 Importantly, STAT3 activation was recently identified as a potential predictive marker for resistance to anti-EGFR therapies in patients with mCRC and NSCLC.42,43 Inhibiting STAT3 in combination with anti-EGFR therapeutics have also revealed promising data pre-clinically, emphasizing the potential benefit of targeting of STAT3 to optimize anti-EGFR therapy in the clinic.44-47 Our present study utilizes a STAT3 transcriptional reporter to demonstrate that efficacy of anti-EGFR therapeutics correlates with their ability to inhibit STAT3 activation in culture and in animal xenograft studies. We also identify that reduced expression of the STAT3 phosphatase, protein tyrosine phosphatase delta (PTPRD), which is usually often reduced in expression in colon cancer, enhances STAT3 activity and subsequent STAT3-mediated resistance to anti-EGFR brokers in colon cancer. Results The efficacy of anti-EGFR brokers correlate with STAT3 activity As STAT3 activity has been shown to correlate with patient response rates to anti-EGFR therapy,42 we set out to test the hypothesis that this efficacy of anti-EGFR brokers is dependent EBI-1051 on its ability to inhibit STAT3 activity. To do this we stably transfected the STAT3 luciferase reporter construct, into 2 tumor cell lines with overexpressed EGFR, HN5, and A431. Parental HN5 and A431 EBI-1051 and transfected cells displayed comparable phospho-EGFR and phospho-STAT3 levels in both basal and EGF-stimulated conditions (Fig. S1A). As expected the HN5-APRE and A431-APRE cells clearly displayed enhanced STAT3 reporter activity compared with control cells when stimulated with EGF (Fig. S1B). We next examined the effect of anti-EGFR therapeutics, cetuximab, and erlotinib on STAT3 transcriptional activity in vitro. Both cetuximab and erlotinib dramatically reduced EGF-mediated STAT3 transcriptional activity in a dose dependant manner (Fig. S1C). We next examined whether comparable effects were seen in animal xenograft studies. We found that STAT3 transcriptional activity in A431-APRE xenografts was significantly reduced by a single dose of 0.25 or 0.5 mg 4 h post injection of cetuximab (Fig.?1A and B). This reduced STAT3 activity was still.