Acquired resistance to proteasome inhibitors represents a considerable impediment to their effective clinical application. Carfilzomib and its orally bioavailable structural analog oprozomib are second-generation, highly-selective, proteasome inhibitors. However, the mechanisms of acquired resistance to carfilzomib and oprozomib are incompletely understood, and effective strategies for overcoming this resistance are needed. Here, we developed models of acquired resistance to carfilzomib in two head and neck squamous cell carcinoma cell lines, UMSCC-1 and Cal33, through gradual exposure to increasing drug concentrations. The resistant lines R-UMSCC-1 and R-Cal33 demonstrated 205- and 64-fold resistance, respectively, relative to the parental lines. Similarly, a high level of cross-resistance to oprozomib, as well as paclitaxel, was observed, whereas only moderate resistance to bortezomib (8- to 29-fold), and low level resistance to cisplatin (1.5- to 5-fold) was seen. Synergistic induction of apoptosis signaling and cell death, and inhibition of colony formation followed co-treatment of acquired resistance models with carfilzomib and the histone deacetylase inhibitor (HDACi) vorinostat. Synergism was also seen with other combinations, including oprozomib plus vorinostat, or carfilzomib plus the HDACi entinostat. Synergism was accompanied by upregulation of proapoptotic Bik, and suppression of Bik attenuated the synergy. The acquired resistance models also exhibited elevated levels of MDR-1/P-gp. Inhibition of MDR-1/P-gp with reversin 121 partially overcame carfilzomib resistance in R-UMSCC-1 and R-Cal33 cells. Collectively, these studies indicate that combining carfilzomib or oprozomib with HDAC or MDR-1/P-gp inhibitors may be a useful strategy for overcoming acquired resistance to these proteasome inhibitors.
Keywords: MDR-1/P-gp; acquired resistance; bortezomib; carfilzomib; entinostat; oprozomib; vorinostat.