Activating alterations in PIK3CA, the gene coding for the catalytic subunit of phosphoinositide-3-kinase (PI3K), are prevalent in head and neck squamous cell carcinoma (HNSCC) and thought to be one of the main drivers of these tumors. However, early clinical trials on PI3K inhibitors (PI3Ki) have been disappointing due to the limited durability of the activity of these drugs. To investigate the resistance mechanisms to PI3Ki and attempt to overcome them, we conducted a molecular-based study using both HNSCC cell lines and patient-derived xenografts (PDXs). We sought to simulate and dissect the molecular pathways that come into play in PIK3CA-altered HNSCC treated with isoform-specific PI3Ki (BYL719, GDC0032). In vitro assays of cell viability and protein expression indicate that activation of the mTOR and cyclin D1 pathways is associated with resistance to PI3Ki. Specifically, in BYL719-resistant cells, BYL719 treatment did not induce pS6 and pRB inhibition as detected in BYL719-sensitive cells. By combining PI3Ki with either mammalian target of rapamycin complex 1 (mTORC1) or cyclin D1 kinase (CDK) 4/6 specific inhibitors (RAD001 and abemaciclib, respectively), we were able to overcome the acquired resistance. Furthermore, we found that PI3Ki and CDK 4/6 inhibitors have a synergistic anti-tumor effect when combined in human papillomavirus (HPV)-negative/PIK3CA-WT tumors. These findings provide a rationale for combining PI3Ki and CDK 4/6 inhibitors to enhance anti-tumor efficacy in HNSCC patients.

Background: Radiation-induced salivary gland (SG) hypofunction is mediated via microvascular dysfunction and radical oxygen species. N-acetylcysteine amide (NACA) has shown antioxidant properties with low toxicity. We explored NACA’s potential as a radioprotector of SG function. Methods: Bovine aortic endothelial cells (BAECs) were treated with NACA before irradiation with a single 10 Gy dose. Apoptosis was assessed by bis-benzimide staining and quantified via fluorescence microscopy. In vivo, NACA was administered to mice prior to a single 15 Gy head and neck irradiation. Eight weeks post-irradiation, saliva production was measured using pilocarpine stimulation; lysozyme levels were analyzed by ELISA. SGs were collected for immunohistochemistry. Results: BAEC apoptosis was substantially lower in NACA-treated cells vs. radiation-only (10% vs. 23%). In vivo, mice lost significant weight and developed severe hair loss eight weeks post-irradiation—attenuated by NACA pretreatment. Saliva production was reduced by 72% post-radiation, with a corresponding drop in lysozyme. NACA increased salivary flow by 42% and prevented lysozyme reduction. Post-radiation decline in microvessel density was also prevented by NACA. Conclusions: These outcomes suggest NACA may serve as a radioprotector of SG function in patients undergoing radiotherapy for head and neck cancer.