2826 - Combining Plk1 Inhibition and Radiation to Target Head and Neck Cancer

J. Korns¹, M. Lehn², T. Wise-Draper³, and V. Takiar^4,5; ¹Department of Radiation Oncology, University of Cincinnati College fof Medicine, Cincinnati, OH, ²Department of Radiation Oncology, University of Cincinnati, Cincinnati, OH, ³Division of Hematology Oncology, University of Cincinnati, Cincinnati, OH, ⁴Department of Radiation Oncology, University of Cincinnati College of Medicine, Cincinnati, OH, ⁵Cincinnati VA Medical Center, Cincinnati, OH

Purpose/Objective(s): Current frontline therapies for head and neck squamous cell carcinoma (HNSCC) patients are insufficient to improve survival outcomes. The purpose of this study was to identify a novel combinatorial therapy that improves radiation efficacy in HNSCC patients. Polo-like kinase 1 (Plk1) is known to be overexpressed in many cancers including HNSCC. High Plk1 expression in HNSCC patients is correlated with worse survival (TCGA, p<0.5). We hypothesized the combination of Plk1 inhibition and radiation would reduce HNSCC tumor growth compared to either single agent alone. Materials/

Methods: Analysis of the Cancer Genome Atlas (TCGA) was done to determine overall survival outcomes of HNSCC patients. Plk1 inhibition was induced using the highly specific Plk1 small molecule inhibitor, onvansertib (currently in phase I/II clinical trials). HPV- (HN5, Cal27) and HPV+ (UMSCC47, UDSCC2) HNSCC cell lines were used to assess cell viability and spheroid growth after Plk1 inhibition. To determine the functionality of onvansertib in HNSCC cells, cell cycle arrest was assessed by flow cytometry. Immunoblots were performed to evaluate pPlk1 and total Plk1 expression after onvansertib treatment. RNA-seq analysis was performed after treatment with both onvansertib and radiation in vitro. Colony forming ability of HNSCC cells was assessed after Plk1 inhibition and radiation. To determine the efficacy of Plk1 inhibition and radiation in vivo, Cal27 or UMSCC47 cells were injected in nude mice using a flank xenograft model.
Results: Plk1 inhibition reduced cell viability and spheroid growth of HPV- HNSCC cells more than HPV+ (n=3). Onvansertib also reduced Plk1 phosphorylation in HNSCC cells (n=2). When Plk1 inhibition was combined with radiation in vitro, colony formation was reduced (p<0.001, n=3) and G2/M stalling was increased (p<0.01, n=3) in HPV- HNSCC cells compared to either single agent alone. In vivo, the combination of Plk1 inhibition and radiation reduced Cal27 tumor growth (p<0.01) but not UMSCC47 tumor growth compared to either treatment alone. In addition, RNA-seq analysis revealed that Plk1 inhibition reduced expression of matrix metalloproteinases (MMPs) in HPV- HNSCC cells. MMPs have been identified to play a major role in HNSCC tumor progression and epithelial to mesenchymal transition (EMT). We also identified the EMT gene signature to be reduced after Plk1 inhibition (p<0.0001). Radiation alone significantly increased MMP expression. However, Plk1 inhibition mitigated the induction of MMP expression when combined with radiation.
Conclusion: HPV- HNSCC patients are typically more resistant than HPV+ HNSCC patients to current frontline chemotherapy and radiation. Our observations indicate the combination of Plk1 inhibition and radiation to be a promising therapy to improve survival outcomes for HPV- HNSCC patients.