Preclinical evaluation of ribociclib and its synergistic effect in combination with alpelisib in non-keratinizing nasopharyngeal carcinoma

Ribociclib is a specific cyclin dependent kinase (Cdk) 4/6 inhibitor that induces G(1) arrest by blocking the formation of cyclin D1-Cdk4/6 complex and inhibiting retinoblastoma (RB) phosphorylation. Cyclin D1 is overexpressed in over 90% of nasopharyngeal carcinoma (NPC) and CCND1 gene activation plays a critical role in NPC pathogenesis. This study evaluated the preclinical activities of ribociclib in NPC cell lines and patient derived xenograft (PDX) models. Over 95% cell growth inhibition was observed at 96 hours after ribociclib treatment. (IC50 concentrations: HK1 = 1.42 +/- 0.23 mu M; HK1-LMP1 = 2.18 +/- 0.70 mu M and C666-1 = 8.26 +/- 0.92 mu M). HK1 and C666-1 cells were chosen for analysis of ribociclib on kinase signaling, apoptosis and cell cycle. Treatment with ribociclib for 48 hours consistently showed a dose-dependent reduction in phosphorylated and total RB expression and G(1) cycle arrest was only observed. Combining ribociclib with the alpha-specific PI3K inhibitor alpelisib showed a synergistic effect in two NPC PDX models in nude mice. The co-treatment induced a significant reduction in tumor volume in both xeno-666 and xeno-2117 compared with ribociclib treatment alone and control (p < 0.01). In summary, ribociclib is active in NPC models and the effect on growth inhibition was augmented when combined with alpelisib. This study supports the clinical evaluation of ribociclib in NPC.