Peptide and Peptide-Like Modulators of 20S Proteasome Enzymatic Activity in Cancer Cells

Exploration of novel four-membered-heterocycle constructed peptidyl proteasome inhibitors with improved metabolic stability for cancer treatment

Peptides have limitations as active pharmaceutical agents due to rapid hydrolysis by proteases and poor cell permeability. To overcome these limitations, a series of peptidyl proteasome inhibitors embedded with four-membered heterocycles were designed to enhance their metabolic stabilities. All synthesized compounds were screened for their inhibitory activities against human 20S proteasome, and 12 target compounds displayed potent efficacy with IC50 values lower than 20 nM. Additionally, these compounds exhibited strong anti-proliferative activities against multiple myeloma (MM) cell lines (MM1S: 72, IC50 = 4.86 ± 1.34 nM; RPMI-8226: 67, IC50 = 12.32 ± 1.44). Metabolic stability assessments of SGF, SIF, plasma and blood were conducted, and the representative compound 73 revealed long half-lives (Plasma: T1/2 = 533 min; Blood: T1/2 > 1000 min) and good proteasome inhibitory activity in vivo. These results suggest that compound 73 serve as a lead compound for the development of more novel proteasome inhibitors.

Characterization of a new series of non-covalent proteasome inhibitors with exquisite potency and selectivity for the 20S beta5-subunit

The mammalian 26S proteasome is a 2500 kDa multi-catalytic complex involved in intracellular protein degradation. We describe the synthesis and properties of a novel series of non-covalent di-peptide inhibitors of the proteasome used on a capped tri-peptide that was first identified by high-throughput screening of a library of approx. 350000 compounds for inhibitors of the ubiquitin–proteasome system in cells. We show that these compounds are entirely selective for the β5 (chymotrypsin-like) site over the β1 (caspase-like) and β2 (trypsin-like) sites of the 20S core particle of the proteasome, and over a panel of less closely related proteases. Compound optimization, guided by X-ray crystallography of the liganded 20S core particle, confirmed their non-covalent binding mode and provided a structural basis for their enhanced in vitro and cellular potencies. We demonstrate that such compounds show low nanomolar IC₅₀ values for the human 20S β5 site in vitro, and that pharmacological inhibition of this site in cells is sufficient to potently inhibit the degradation of a tetra-ubiquitin–luciferase reporter, activation of NFκB (nuclear factor κB) in response to TNF-α (tumour necrosis factor-α) and the proliferation of cancer cells. Finally, we identified capped di-peptides that show differential selectivity for the β5 site of the constitutively expressed proteasome and immunoproteasome in vitro and in B-cell lymphomas. Collectively, these studies describe the synthesis, activity and binding mode of a new series of non-covalent proteasome inhibitors with unprecedented potency and selectivity for the β5 site, and which can discriminate between the constitutive proteasome and immunoproteasome in vitro and in cells.

Novel beta-lactam derivatives: potent and selective inhibitors of the chymotrypsin-like activity of the human 20S proteasome

A series of beta-lactam derivatives has been designed and synthesized to inhibit the chymotrypsin-like activity of the human 20S proteasome. The most potent compounds of this new structural class of beta-subunit selective 20S proteasome inhibitors exhibit IC50 values in the low-nanomolar range and show good selectivity over the trypsin-like and post-glutamyl-peptide hydrolytic activities of the enzyme.