5553 Background: Lapatinib is a selective and potent dual, competitive inhibitor of EGFR and ErbB2. A high level of expression of EGFR is reported in SCCHN (ranging from 55% to 90%), which is a negative prognostic factor, whereas ErbB2 expression ranges between 40% to 60%. The rationale for EGFR inhibition in combination with fractionated radiotherapy is to enhance radiosensitivity and inhibit cellular proliferation, including accelerated repopulation, during treatment. Methods: Patients (pts) with locally advanced SCCHN were enrolled at escalating dose levels of lapatinib (500–1,500 mg/d) in combination with radiotherapy (66–70 Gy/6–7 weeks given 5 days a week in 2 Gy daily fractions) and intravenous cisplatin (100 mg/m2, days 1, 22 and 43 of radiotherapy). Each cohort was to include 3 pts, with expansion to 6 in the event of dose-limiting toxicity (DLT). Main eligibility criteria were confirmed SCCHN, excluding nasopharynx, stage III, IVa,b, and adequate organ function. Regular safety assessments were performed during therapy and follow-up period. Cardiac assessments using MUGA or ECHO scans were also performed. Results: Seventeen pts were treated (500 mg - 7; 1,000 mg - 7; 1,500 mg - 3; expanded cohort of 9 additional pts is currently recruiting). The most common side effects were Grade 1-3 dysphagia, mucositis and dermatitis, typically observed with cisplatin and radiotherapy in this population. Lapatinib-related adverse events were minor. In the 500mg cohort: 2 pts had diarrhea (Gr 1), 1 pt had tinnitus (Gr 1); in the 1,000mg cohort: 2 pts had nausea (Gr 2) and vomiting (Gr 2), 1 pt had transient elevated liver enzymes (Gr 3); in the 1,500mg cohort 2 pts had nausea (Gr 1) and 3 pts vomiting (1 pt Gr 1, and 2 pts Gr 2). One DLT was reported both at 500 mg and 1,000 mg dose levels; no further DLT were reported at the final 1,500 mg dose level. The optimally tolerated regimen was lapatinib 1,500 mg OD, given concurrently with conventional radiotherapy and cisplatin. Preliminary evidence of clinical activity was encouraging. Conclusions: The combination of lapatinib and concomitant chemoradiation is well tolerated, has demonstrated encouraging clinical activity in this population of locally advanced SCCHN patients, and warrants phase II studies in this disease. [Table: see text]