Background: Vandetanib (V) is an oral inhibitor of VEGFR, EGFR and RET signaling. Methods: Eligible patients (pts) with previously untreated, unresected HNSCC received once-daily V (starting dose 100 mg) for 14 days (d) followed by V + RT (2.2 Gy/d, 5 d/wk; total 66 Gy) for 6 wks (regimen 1) or V + RT (2 Gy/d, 5 d/wk; total 70 Gy) + cisplatin (30 mg/m² weekly) for 7 wks (regimen 2). The primary objective was to determine the safety, tolerability and maximum tolerated dose (MTD) of V. We used multianalyte biomarker profiling (Luminex Corp) and ELISA to assess 57 plasma markers before and after 14 d of V and at completion of RT (d50), and EGFR, p16, E-cadherin and vimentin immunohistochemistry in tumor tissue. Clinical results were correlated with biomarkers. Results: Of 33 pts (median age 53 yrs; 28 male; all M0) who received treatment, 30 completed therapy (regimen 1, n=12; regimen 2, n=18). In regimen 1, there were no DLTs in the 100 mg cohort and 1 DLT in the 200 mg cohort (each n=6). In regimen 2, there were no DLTs in the 100 mg cohort and 3 DLTs in the 200 mg cohort (each n=6); 100 mg was defined as the MTD and the cohort expanded to 15 pts. The most common grade ≥3 AEs were dysphagia (27%), stomatitis (24%) and mucosal inflammation (21%); 4 pts discontinued V due to AEs. All 29 pts evaluable for efficacy (RECIST) achieved a complete response. At data cut-off, 11/29 had completed 2-yr follow-up without recurrence and 6 had recurrence. Steady-state exposure to V was unaffected by RT/cisplatin. Blood biomarkers were analyzed in 31 (baseline), 30 (d15) and 26 (d50) pts; 27 pts had tissue analyses. High levels of baseline plasma HGF, IL-8, MIF and eotaxin were associated with shorter progression-free survival (PFS; P<0.05). Significant increases in six plasma factors (HGF, OPN, IL-6, soluble IL-2R, IP-10 and MCP-1) were observed after RT, but not V alone. Tumor p16 expression was associated with superior PFS (P=0.01). Conclusions: The combination of V with RT ± cisplatin is tolerable with encouraging efficacy. Baseline HGF, IL-8, MIF and eotaxin are potentially useful markers of adverse outcome. Inflammatory and hypoxia-regulated cytokines appear to be upregulated after RT.