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.