Rational design of proteasome inhibitors based on the structure of the endogenous inhibitor PI31/Fub1

Proteasome inhibitors are widely used anticancer drugs. The three clinically approved agents are modified small peptides that preferentially target one of the proteasome's three active sites (beta 5) at physiologic concentrations. In addition to these drugs, there is also an endogenous proteasome inhibitor, PI31/Fub1, that enters the proteasome's interior to simultaneously yet specifically inhibit all three active sites. Here, we have used PI31's evolutionarily optimized inhibitory mechanisms to develop a suite of potent and specific beta 2 inhibitors. The lead compound strongly inhibited growth of multiple myeloma cells as a standalone agent, indicating the compound's cell permeability and establishing beta 2 as a potential therapeutic target in multiple myeloma. The lead compound also showed strong synergy with the existing beta 5 inhibitor bortezomib; such combination therapies might help with existing challenges of resistance and severe side effects. These results represent an effective method for rational structure- guided development of proteasome inhibitors.