Netupitant competitively binds to and blocks the activity of the human substance P/NK1 receptors in the central nervous system (CNS), thereby inhibiting NK1-receptor binding of the endogenous tachykinin neuropeptide substance P (SP), which may result in the prevention of chemotherapy-induced nausea and vomiting (CINV). Recent mechanistic studies using NG108-15 cells have shown that palonosetron and netupitant, an NK1 receptor antagonist currently in phase 3 clinical trials, exhibited synergistic effects when inhibiting the substance P response.The present studies showed that both netupitant and palonosetron-induced NK1 receptor internalization in NG108-15 cells and that when used together receptor internalization was additive.Netupitant was well tolerated with most treatment emergent adverse events (AEs) being mild.Palonosetron and netupitant, either alone or in combination, were effective in reducing the induced elevated pPKA levels during the delayed phase.