K. J. Kovatch1, C. Subramanian2, M. E. Prince1, J. Sanchez3, M. S. Cohen2 1University Of Michigan,Department Of Otolaryngology-Head And Neck Surgery,Ann Arbor, MI, USA 2University Of Michigan,Department Of General Surgery,Ann Arbor, MI, USA 3University Of Michigan,Medical School,Ann Arbor, MI, USA
Introduction: Despite advancements in adjuvant treatments for advanced head and neck squamous cell carcinoma (HNSCC), survival has remained relatively unchanged. Current standard of care for HNSCC involves a combination of surgery, chemotherapy, and radiation; however, drug resistance is high and often leads to recurrent or persistent disease. Heat Shock protein 90 (Hsp90) inhibitors have shown promise in the past in HNSCC, but were abandoned due to dose-limiting toxicity (DLT) as a monotherapy. Here, we study a novel, potent Hsp90 inhibitor (that does not elicit the heat shock response and its associated DLT) in combination therapy with current standard of care as a promising new treatment strategy for resistant HNSCC.
Methods: Cisplatin- and cetuximab-resistant cell lines were generated for two HNSCC oral cavity cell lines (UMSCC-103 and UMSCC-108) using co-culture and incremental dosing with each respective drug. Resistance was confirmed by measuring IC50 values using MTS assay. Combination index (CI) was performed with simultaneous treatment of Hsp90 inhibitor (KU757, 0.58-5.0 uM) and either cisplatin (5-20 uM) or cetuximab (3.5-5 uM) for 72 hours. Synergistic effect was defined as CI < 1 using the Chou-Talaylay method. Intrinsic radioresistance of cell lines was characterized using clonogenic assay with treatment dosing of 0-8 Gy.
Results: UMSCC-103 and 108 cell lines were co-cultured with up to 5uM cisplatin and 5uM cetuximab. IC50 values for UMSCC-103 and 108 increased 4- to 6-fold for cisplatin, confirming drug resistance. A modest resistance effect was seen for cetuximab, with 3-fold increase for UMSCC-108 and only marginal increase in UMSCC-103. Combination indices showed synergistic effects for all six cell lines (2 parent, 2 cisplatin-resistant, and 2 cetuximab-resistant) at the doses indicated in Table 1. These results were redemonstrated when cisplatin-resistant lines were treated with KU757 and cisplatin in series, suggesting a sensitizing effect of Hsp90 inhibition. Parent UMSCC-103 and UMSCC-108 cell lines showed radioresistance of 29% and 42% at 2 Gy, respectively. Clonogenic assay with KU757 pre-treatment showed radiosensitizing effect compared to control at 2-8 Gy.
Conclusion: Multidrug therapy inhibits multiple key regulatory pathways simultaneously, thereby lowering drug doses to improve toxicity profiles, and combat development of resistance. These results show for the first time ever synergistic treatment response of Hsp90 inhibition in combination with either cisplatin or cetuximab in both parent and drug-resistant cell lines. Further studies, including functional cellular studies and characterization of altered cellular pathways, are ongoing and may elucidate avenues to overcome mechanisms of resistance.
