Development of peptidomimetic boronates as proteasome inhibitors

Proteasome inhibition has emerged over the past decade as an effective therapeutic approach for the treatment of hematologic malignancies. It is a multicatalytic complex, whose proteolytic activity relies in three types of subunits: chymotrypsin-like (b5), trypsin-like (b2) and caspase-like (b1). Most important for the development of effective antitumor agents is the inhibition of the b5 subunits. In this context, the dipeptide boronate bortezomib (Velcade) represents the first proteasome inhibitor approved by the FDA and the lead compound in drug discovery. This paper describes the synthesis and biological evaluation of a series of conformationally constrained pseudopeptide boronates (1-3) structurally related to bortezomib. The synthesized compounds showed a promising inhibitory profile by blocking primarily the chymotrypsin-like activity of the proteasome with Ki values in submicromolar/micromolar range. These compounds also resulted quite selective since no significant inhibition was recorded in the test against bovine pancreatic a-chymotrypsin. The obtained results were rationalized by means of docking experiments based on a model of the crystal structure of bortezomib bound to the yeast 20S proteasome providing essential insights for further optimization of this class of inhibitors.