5x106 FaDu cells were injected intramuscularly into the left gastronecmius of immunodeficient Balb/c female mice. siRNA complexed with in vivo-jetPEI (N/P ratio of 8) were then injected IV or IP at day 1, 3, 5, 8, 10 and 12. Tumors were then measured.

PURPOSE: To investigate the efficacy of targeting polo-like kinase 1 (Plk1) combined with ionizing radiotherapy (RT) for head-and-neck squamous cell carcinoma (HNSCC). METHODS AND MATERIALS: Polo-like kinase 1 messenger ribonucleic acid (mRNA) was targeted by small interfering RNA (siRNA) transfection into the FaDu HNSCC cell line; reduction was confirmed using quantitative real-time polymerase chain reaction. The cellular effects were assessed using [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl) -2H-tetrazolium], clonogenic, flow cytometric, and caspase assays. In vivo efficacy of siPlk1 was evaluated using mouse xenograft models. RESULTS: Small interfering Plk1 significantly decreased Plk1 mRNA expression, while also increasing cyclin B1 and p21(Waf1/CIP1) mRNA levels after 24 h. This depletion resulted in a time-dependent increase in FaDu cytotoxicity, which was enhanced by the addition of RT. Flow cytometric and caspase assays demonstrated progressive apoptosis, DNA double-strand breaks (gamma-H2AX), G2/M arrest, and activation of caspases 3 and 7. Implantation of siPlk1-treated FaDu cells in severe combined immunodeficient mice delayed tumor formation, and systemic administration of siPlk1 inhibited tumor growth enhanced by RT. CONCLUSIONS: These data demonstrate the suitability of Plk1 as a potential therapeutic target for HNSCC, because Plk1 depletion resulted in significant cytotoxicity in vitro and abrogated tumor-forming potential in vivo. The effects of Plk1 depletion were enhanced with the addition of RT, indicating that Plk1 represents an important potential radiation sensitizer for HNSCC.