Palbociclib Renders Human Papilloma Virus-Negative Head and Neck Squamous Cell Carcinoma Vulnerable to the Senolytic Agent Navitoclax

We demonstrate that inhibition of cyclin-dependent kinases 4/6 (CDK4/6) leads to senescence in human papillomavirus (HPV)-negative (-) head and neck squamous cell carcinoma (HNSCC), but not in HPV-positive (+) HNSCC. The BCL-2 family inhibitor, navitoclax, has been shown to eliminate senescent cells effectively. We evaluated the efficacy of combining palbociclib and navitoclax in HPV HNSCC. Three HPV HNSCC cell lines (CAL27, HN31, and PCI15B) and three HPV HNSCC cell lines (UPCI-SCC-090, UPCI-SCC-154, and UM-SCC-47) were treated with palbociclib. Treatment drove reduced expression of phosphorylated Rb (p-Rb) and phenotypic evidence of senescence in all HPV cell lines, whereas HPV cell lines did not display a consistent response by Rb or p-Rb and did not exhibit morphologic changes of senescence in response to palbociclib. In addition, treatment of HPV cells with palbociclib increased both β-galactosidase protein expression and BCL-xL protein expression compared with untreated controls in HPV cells. Co-expression of β-galactosidase and BCL-xL occurred consistently, indicating elevated BCL-xL expression in senescent cells. Combining palbociclib with navitoclax led to decreased HPV HNSCC cell survival and led to increased apoptosis levels in HPV cell lines compared with each agent given alone. IMPLICATIONS: This work exploits a key genomic hallmark of HPV HNSCC ( disruption) using palbociclib to induce BCL-xL-dependent senescence, which subsequently causes the cancer cells to be vulnerable to the senolytic agent, navitoclax.