5523 Background: Lymph node metastasis frequently occurs in SCCHN and is generally associated with poor prognosis. The molecular mechanisms regulating tumor cell dissemination and thus the potential targets for novel treatment strategies are still largely unresolved. In this study we tested whether circulating tumor cells (CTCs) are associated with lymph node metastasis in SCCHN. We further established a method to characterize CTCs for the expression of potential drug targets and started with their characterization for EGFR expression. Methods: After informed patient consent, 7.5 ml blood was collected into heparinized tubes. After lysis of erythrocytes samples were enriched for CTCs by depletion of CD45+ blood leukocytes. Samples were stained with fluorescence-labeled antibodies to EpCAM, pan-cytokeratin, EGFR and CD45 or the relevant isotype control antibodies. Absolute numbers of EpCAM+Cytokeratin+CD45- CTCs and their EGFR expression profile were determined by flow cytometry. Results: Currently, a total of 40 SCCHN patients mainly with advanced disease (76% T3-4 stage; 70% N1 or beyond), but all without clinically detectable distant metastases, have been enrolled in this study with the most frequent tumor localization being the oropharynx (65%). Two or more CTCs per 3.75 ml of blood were detected in 40 percent of SCCHN patients (mean ± SD: 3.8 ± 2.6; range: 2-10) whereas none were found in peripheral blood samples from normal control subjects (n=10). The frequency of CTCs depended on the extent of lymph node metastasis with significantly lower CTC numbers in cases with none or only one affected lymph node (p=.01) but did not correlate with T stage (p=.69). In 47% of the cases with detectable CTCs these cells expressed EGFR. Conclusions: The presence of CTCs correlated with lymph node metastasis in SCCHN. The frequency of patients with detectable CTCs was within the range of the reported frequency of patients who will also develop distant metastases. Further studies will clarify whether the presence of circulating tumor cells may be a predictive factor for subsequent hematogenous metastases. In addition, EGFR was identified as one of the potential targets in this rare cell population. Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Merck Serono Merck Serono Merck Serono