Incorporation of Non-naturalAmino Acids ImprovesCell Permeability and Potency of Specific Inhibitors of ProteasomeTrypsin-like Sites.

Proteasomes degrade the majority of proteins in mammaliancellsby a concerted action of three distinct pairs of active sites. Thechymotrypsin-like sites are targets of antimyeloma agents bortezomiband carfilzomib. Inhibitors of the trypsin-like site sensitize multiplemyeloma cells to these agents. Here we describe systematic effortto develop inhibitors with improved potency and cell permeability,yielding azido-Phe-Leu-Leu-4-aminomethyl-Phe-methyl vinyl sulfone(4a, LU-102), and a fluorescent activity-based probefor this site. X-ray structures of 4aand related inhibitorscomplexed with yeast proteasomes revealed the structural basis forspecificity. Nontoxic to myeloma cells when used as a single agent, 4asensitized them to bortezomib and carfilzomib. This sensitizingeffect was much stronger than the synergistic effects of histone acetylaseinhibitors or additive effects of doxorubicin and dexamethasone, raisingthe possibility that combinations of inhibitors of the trypsin-likesite with bortezomib or carfilzomib would have stronger antineoplasticactivity than combinations currently used clinically. [ABSTRACT FROM AUTHOR]