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 (þ) 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 HPVHNSCC cell lines (CAL27, HN31, and PCI15B) and three HPV^þ 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^þ 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 b-galactosidase protein expression and BCL-xL protein expression compared with untreated controls in HPV^- cells. Co-expression of b-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 HPVHNSCC (CDKN2A disruption) using palbociclib to induce BCL-xL–dependent senescence, which subsequently causes the cancer cells to be vulnerable to the senolytic agent, navitoclax.