Combining Noxa-Inducing Drugs with ABT-263 to Efficiently Increase Cell Death in Head and Neck Squamous Cell Carcinoma (HNSCC)

Head and neck cancer is the sixth leading cancer worldwide. Head and neck squamous cell carcinoma (HNSCC) accounts for more than 90% of incident cases. Despite intense, multimodality treatment regimens for HNSCC including surgery, chemotherapy, and radiation, little progress has been made over the past 30 years in improving overall survival rates. Tumor cell death induced by both conventional and targeted chemotherapy is often mediated by the BCL-2 family-dependent mitochondrial apoptotic pathway. However, initiators of this apoptotic pathway, such as p53, are more than 50% of the time mutated or deleted in HNSCC rendering the disease refractory to treatment. To counter such resistance, direct therapeutic targeting of the BCL-2 family is conceptually appealing. For this purpose, we use three clinically-available drugs: cisplatin, fenretinide, and ABT-263 (navitoclax). Both cisplatin and fenretinide are known to induce a BH3-only pro-apoptotic protein, Noxa, which binds to and inactivates multi-domain anti-apoptotic protein MCL-1 and release from its interaction with multi-domain pro-apoptotic protein BAK, followed by the phosphorylation via CDK2 for the proteasome-mediated degradation. Activated BAK can now go through conformational change for the oligomerization at the outer membrane of the mitochondria to release cytochrome c into the cytosol and induce caspase-dependent apoptotic cell death. ABT-263 directly binds to multi-domain anti-apoptotic proteins, such as BCL-2 and BCL-XL, to inhibit their activity and leads to the activation of multi-domain pro-apoptotic protein BAX to induce apoptosis. We hypothesize that combining the Noxa-inducing drugs (cisplatin or fenretinide) along with ABT-263 can efficiently induce BAX and BAK activation and significantly increase cell death in HNSCC cells by simultaneously inhibiting the activity of MCL-1, BCL-2, and BCL-XL. Combination-induced treatments in four cell lines (HN8, HN30, HN31, and UMSCC1) tested led to significant increase in apoptotic cell death. Cisplatin and ABT-263 combined treatment is inducing the expression of Noxa and leading to increase in apoptosis in HN30, HN31, UMSCC1, but not HN8. Similarly, fenretinide and ABT-263 combined treatment is inducing the expression of Noxa in all four cell lines tested and is largely relying on expression of Noxa