Discovery of PI-1840, a novel noncovalent and rapidly reversible proteasome inhibitor with anti-tumor activity

Histone deacetylase (HDAC) inhibitor specificity determinants are preserved in a class of dual HDAC/non-covalent proteasome inhibitors

Many disease states require multiple drugs to inhibit multiple targets for their effective treatment/management, i.e. a drug cocktail regimen, or "polypharmacy". Polypharmacology, in contrast, is the development of single agents that can inhibit multiple targets. Each strategy is associated with advantages and disadvantages. Motivated by promising clinical trial data for the treatment of multiple myeloma with the combination of the HDAC6 inhibitor ricolinostat and the proteasome inhibitor bortezomib, we herein describe a focused family of dual HDAC/non-covalent proteasome inhibitors, and explore the impact of linker and zinc-binding group identities on HDAC1/6 isozyme selectivity. In general, previously reported specificity determinants of monovalent HDAC1/6 inhibitors were preserved in our dual HDAC/proteasome inhibitors.

Design, synthesis, and biological evaluation of some novel naphthoquinone-glycine/β-alanine anilide derivatives as noncovalent proteasome inhibitors

A series of novel noncovalent glycine/β-alanine anilide derivatives possessing 2-chloronaphthoquinone structure as a pharmacophoric unit were designed, synthesized, and evaluated for their antiproliferative and antiproteasomal activities against MCF-7 cell line, in vitro. According to biological activity results, all the target compounds showed antiproliferative activity in the range of IC₅₀  = 7.10 ± 0.10-41.08 ± 0.14 μM and most of them exhibited inhibitory efficacy with varying ratios against the three catalytic subunits (β1, β2, and β5) presenting caspase-like (C-L), trypsin-like (T-L) and chymotrypsin-like (ChT-L) activities of proteasome. The antiproteasomal activity evaluations revealed that compounds preferentially inhibited the β5 subunit compared with β1 and β2 subunits of the proteasome. Among the compounds, compounds 7 and 9 showed the highest antiproliferative activity with an IC₅₀ value of 7.10 ± 0.10 and 7.43 ± 0.25 μM, respectively. Additionally, compound 7 displayed comparable potency to PI-083 lead compound in terms of β5 antiproteasomal activity with an inhibition percentage of 34.67 at 10 μM. This compound showed an IC₅₀ value of 32.30 ± 0.45 μM against β5 subunit. Furthermore, molecular modeling studies of the most active compound 7 revealed key interactions with β5 subunit. The results suggest that this class of compounds may be beneficial for the development of new potent proteasome inhibitors.

Proteasome Inhibitors and Their Potential Applicability in Osteosarcoma Treatment

Simple Summary

Bone cancer has seen minimal benefits in therapeutic options in the past 30 years. Proteasome inhibitors present a new avenue of research for the treatment of bone cancer. Proteasome inhibitors impair the function of the proteasome, a structure within the cell that removes unwanted and misfolded proteins. Bone cancer cells heavily rely on the proteasome to properly function and survive. Impairing the proteasome function can have detrimental consequences and lead to cell death. This review provides a thorough summary of the in vitro, in vivo, and clinical research that has explored proteasome inhibitors for the treatment of bone cancer.

Abstract

Osteosarcoma (OS) is the most common type of bone cancer, with ~30% of patients developing secondary/metastatic tumors. The molecular complexity of tumor metastasis and the lack of effective therapies for OS has cultivated interest in exploiting the proteasome as a molecular target for anti-cancer therapy. As our understanding towards the behavior of malignant cells expands, it is evident that cancerous cells display a greater reliance on the proteasome to maintain homeostasis and sustain efficient biological activities. This led to the development and approval of first- and second-generation proteasome inhibitors (PIs), which have improved outcomes for patients with multiple myeloma and mantle cell lymphoma. Researchers have since postulated the therapeutic potential of PIs for the treatment of OS. As such, this review aims to summarize the biological effects and latest findings from clinical trials investigating PI-based treatments for OS. Integrating PIs into current treatment regimens may better outcomes for patients diagnosed with OS.

Nitazoxanide and related thiazolides induce cell death in cancer cells by targeting the 20S proteasome with novel binding modes

Nitazoxanide and related thiazolides are a novel class of anti-infectious agents against protozoan parasites, bacteria and viruses. In recent years, it is demonstrated thatthiazolides also can induced cell cycle arrest and apoptotic cell death in cancer cells. Due to their fast proliferating nature, cancer cells highly depend on the proteasome system to remove aberrant proteins. Many of these aberrant proteins are regulators of cell cycle progression and apoptosis, such as the cyclins, BCL2 family members and nuclear factor of κB (NF-κB). Here, we demonstrate at both molecular and cellular levels that the 20S proteasome is a direct target of NTZ and other thiazolides. By concurrently inhibiting the multiple catalytic subunits of 20S proteasome, NTZ promotes cell cycle arrest and triggers cell death in colon cancer cells, either directly or a sensitizer to other anti-tumor agents, especially doxorubicin. We further show that the binding mode of NTZ in the β5 subunit of the 20S proteasome is different from that of bortezomib and other existing proteasome inhibitors. These findings provide new insights in the design of novel small molecular proteasome inhibitors as anti-tumor agents suitable for solid tumor treatment in an oral dosing form.

Design, synthesis and biological evaluation of triaryl compounds as novel 20S proteasome inhibitors

Thirty novel triaryl compounds were designed and synthesized based on the known proteasome inhibitor PI-1840. Most of them showed significant inhibition against the β5c subunit of human 20S proteasome, and five of them exhibited IC₅₀ values at the sub-micromolar level, which were comparable to or even more potent than PI-1840. The most active two (1c and 1d) showed IC₅₀ values of 0.12 and 0.18 μM against the β5c subunit, respectively, while they displayed no obvious inhibition against the β2c, β1c and β5i subunits. Molecular docking provided informative clues for the subunit selectivity. The potent and subunit selective proteasome inhibitors identified herein represent new chemical templates for further molecular optimization.

Design and synthesis of tripeptidyl furylketones as selective inhibitors against the β5 subunit of human 20S proteasome

A series of tripeptidic proteasome inhibitors with furylketone as C-terminus were designed and synthesized. Biochemical evaluations against β1, β2 and β5 subunits revealed that they acted selectively on β5 subunit with IC₅₀s against chymotrypsin-like (CT-L) activity in micromolar range. LC-MS/MS analysis of the ligand-20S proteasome mixture showed that the most potent compound 11m (IC₅₀ = 0.18 μM) made no covalent modification on 20S proteasome. However, it was identified acting in a slowly reversible manner in wash-out assay and the reversibility was much lower than that of MG132, suggesting the possibility of these tripeptidic furylketones forming reversible covalent bonds with 20S proteasome. Several compounds were selected for anti-proliferative assay towards multiple cancer cell lines, and compound 11m displayed comparable potency to positive control (MG132) in all cell lines tested. Furthermore, the pharmacokinetic (PK) data in rats indicated 11m behaved similarly (C_max, 2007 μg/L; AUC_0-t, 680 μg/L·h; V_ss, 0.66 L/kg) to the clinical used agent carfilzomib. All these data suggest 11m is a good lead compound to be developed to novel anti-tumor agent.

Non‑covalent proteasome inhibitor PI‑1840 induces apoptosis and autophagy in osteosarcoma cells

Osteosarcoma (OS) is the predominant form of primary bone malignancy in children and adolescents. Although the combination of chemotherapy and modified surgical therapy leads to marked improvements in the survival rate, the therapeutic outcomes remain unsatisfactory. Therefore, the identification of novel drugs with higher efficacy and fewer side‑effects is urgently required. Proteasome inhibitors have been approved by the Food and Drug Administration (FDA) for the treatment of certain cancers, although none of them are directed against OS. Non‑covalent proteasome inhibitors, such as PI‑1840, are superior to covalent ones in numerous respects in view of their chemical structure; however, to date, no studies have been published on the effects of non‑covalent proteasome inhibitors on OS cells. In the present study, the antineoplastic effects of PI‑1840 were systematically evaluated in the OS cell lines, MG‑63 and U2‑OS. Cell viability and morphological changes were assessed by Cell Counting Kit‑8 (CCK‑8) and live/dead assays. The cell cycle was analyzed using flow cytometry (FCM) and western blot analysis (assessing the levels of the proteins p21, p27, and the tyrosine kinase, WEE1). The extent of cell apoptosis and autophagy were assessed by FCM, western blot analysis [of the apoptosis‑associated proteins, microtubule‑associated protein 1 light chain 3 α (LC3) and Beclin1], and mRFP‑GFP‑LC3 adenovirus transfection assay. Transwell and wound healing assays, and western blot analysis of the matrix metalloproteinases (MMPs)2 and 9 were performed to preliminarily evaluate the migration and invasion capability of the cells. In the present study, our results revealed that PI‑1840 inhibited the proliferation of OS cells and induced apoptosis, partly due to attenuation of the nuclear factor‑κB (NF‑κB) pathway. In addition, PI‑1840‑induced autophagy was detected, and inhibiting the autophagy of the OS cells led to an increase in the survival rate of the U2‑OS cells rather than of the MG‑63 cells. Furthermore, PI‑1840 attenuated the migration and invasion capabilities of the OS cells. In conclusion, the present study revealed PI‑1840 to be a promising drug for the treatment of OS.

Another look at phenolic compounds in cancer therapy the effect of polyphenols on ubiquitin-proteasome system

Inhibitors of the ubiquitin-proteasome system (UPS) have been the object of research interests for many years because of their potential as anti-cancer agents. Research in this field is aimed at improving the specificity and safety of known proteasome inhibitors. Unfortunately, in vitro conditions do not reflect the processes taking place in the human body. Recent reports indicate that the components of human plasma affect the course of many signaling pathways, proteasome activity and the effectiveness of synthetic cytostatic drugs. Therefore, it is believed that the key issue is to determine the effects of components of the human diet, including effects of chemically active polyphenols on the ubiquitin-proteasome system activity in both physiological and pathological (cancerous) states. The following article summarizes the current knowledge on the direct and indirect synergistic and antagonistic effects between polyphenolic compounds present in the human diet and the efficiency of protein degradation via the UPS.

Design, synthesis, and biological evaluation of novel phenol ether derivatives as non-covalent proteasome inhibitors

A series of novel phenol ether derivatives were designed, synthesized, and evaluated as non-covalent proteasome inhibitors. Most compounds exhibited moderate to excellent proteasome inhibitory activity. In particular, compound 18x proved to be the most potent compound (chymotrypsin-like: IC₅₀ = 49 nM), exhibiting a 2-fold higher potency compared to the reported PI-1840. Besides, compound 18x exhibited excellent metabolic stability and selective anti-proliferative activity against solid cancer cell lines including HepG2 and HGC27, providing incentive for the further development as a potential anticancer agent against solid cancers.

Pharmacophore-based virtual screening for identifying β5 subunit inhibitor of 20S proteasome

Proteasomal system plays an important role in maintaining cell homeostatis. Overexpression of proteasomes leads to several major diseases, such as cancer and autoimmune disorder. The β5 subunit of proteasome is a crucial active site in proteolysis, and targeting proteasome β5 subunit is essential for proteasome inhibition. In the present study, a pharmacophore-based virtual screening and molecular docking were employed to identify ligands as inhibitors of β5 subunit of proteasome. The pharmacophore features were built with one hydrogen bond donor, two hydrogen bond acceptors, and one hydrophobic feature using native ligand of proteasome (HU10), which was then used to screen ZINC database using ZINCPharmer. The retrieved virtual hits were subjected to molecular docking analysis using iDock. The best six hits were subjected to molecular dynamics (MD) simulation and each complex was stable during 40 ns MD simulation as indicated by root-mean-square-deviation (RMSD) and root-mean-square-fluctuation (RMSF) values. The current study identifies 5 best hits having better binding potentials than HU10 as predicted by molecular mechanics Poisson-Boltzmann Surface Area (MM-PBSA) method, i.e. Lig1540/ZINC33356240, Lig1546/ZINC33356235, Lig1522/ZINC20854878, Lig980/ZINC12391945, and Lig1119/ZINC19865241, which can be used in the development of new proteasome inhibitors.

Next-generation proteasome inhibitors for cancer therapy

Over 2 decades ago, the proteasome was considered a risky or even untenable therapeutic target. Today, proteasome inhibitors are a mainstay in the treatment of multiple myeloma (MM) and have sales in excess of 3 billion US dollars annually. More importantly, the availability of proteasome inhibitors has greatly improved the survival and quality of life for patients with MM. Despite the remarkable success of proteasome inhibitor therapies to date, the potential for improvement remains, and the development and optimal use of proteasome inhibitors as anticancer agents continues to be an active area of research. In this review, we briefly discuss the features and limitations of the 3 proteasome inhibitor drugs currently used in the clinic and provide an update on current efforts to develop next-generation proteasome inhibitors with the potential to overcome the limitations of existing proteasome inhibitor drugs.

Cadmium pyrithione suppresses tumor growth in vitro and in vivo through inhibition of proteasomal deubiquitinase

The ubiquitin-proteasome system (UPS) is indispensable to the protein quality control in eukaryotic cells. Due to the remarkable clinical success of using proteasome inhibitors for clinical treatment of multiple myeloma, it is anticipated that targeting the UPS upstream of the proteasome step be an effective strategy for cancer therapy. Deubiquitinases (DUB) are proteases that remove ubiquitin from target proteins and therefore regulate multiple cellular processes including some signaling pathways altered in cancer cells. Thus, targeting DUB is a promising strategy for cancer drug discovery. Previously, we have reported that metal complexes, such as copper and gold complexes, can disrupt the UPS via suppressing the activity of 19S proteasome-associated DUBs and/or of the 20S proteasomes, thereby inducing cancer cell death. In this study, we found that cadmium pyrithione (CdPT) treatment led to remarkable accumulation of ubiquitinated proteins in cultured cancer cells and primary leukemia cells. CdPT potently inhibited the activity of proteasomal DUBs (USP14 and UCHL5), but slightly inhibited 20S proteasome activity. The anti-cancer activity of CdPT was associated with triggering apoptosis via caspase activation. Moreover, treatment with CdPT inhibited proteasome function and repressed tumor growth in animal xenograft models. Our results show that cadmium-containing complex CdPT may function as a novel proteasomal DUB inhibitor and suggest appealing prospects for cancer treatment.

Reversal of hyperactive Wnt signaling-dependent adipocyte defects by peptide boronic acids

Deregulated Wnt signaling and altered lipid metabolism have been linked to obesity, diabetes, and various cancers, highlighting the importance of identifying inhibitors that can modulate Wnt signaling and aberrant lipid metabolism. We have established a Drosophila model with hyperactivated Wnt signaling caused by partial loss of axin, a key component of the Wnt cascade. The Axin mutant larvae are transparent and have severe adipocyte defects caused by up-regulation of β-catenin transcriptional activities. We demonstrate pharmacologic mitigation of these phenotypes in Axin mutants by identifying bortezomib and additional peptide boronic acids. We show that the suppressive effect of peptide boronic acids on hyperactive Wnt signaling is dependent on α-catenin; the rescue effect is completely abolished with the depletion of α-catenin in adipocytes. These results indicate that rather than targeting the canonical Wnt signaling pathway directly, pharmacologic modulation of β-catenin activity through α-catenin is a potentially attractive approach to attenuating Wnt signaling in vivo.

Differential global structural changes in the core particle of yeast and mouse proteasome induced by ligand binding

Two clusters of configurations of the main proteolytic subunit β5 were identified by principal component analysis of crystal structures of the yeast proteasome core particle (yCP). The apo-cluster encompasses unliganded species and complexes with nonpeptidic ligands, and the pep-cluster comprises complexes with peptidic ligands. The murine constitutive CP structures conform to the yeast system, with the apo-form settled in the apo-cluster and the PR-957 (a peptidic ligand) complex in the pep-cluster. In striking contrast, the murine immune CP classifies into the pep-cluster in both the apo and the PR-957-liganded species. The two clusters differ essentially by multiple small structural changes and a domain motion enabling enclosure of the peptidic ligand and formation of specific hydrogen bonds in the pep-cluster. The immune CP species is in optimal peptide binding configuration also in its apo form. This favors productive ligand binding and may help to explain the generally increased functional activity of the immunoproteasome. Molecular dynamics simulations of the representative murine species are consistent with the experimentally observed configurations. A comparison of all 28 subunits of the unliganded species with the peptidic liganded forms demonstrates a greatly enhanced plasticity of β5 and suggests specific signaling pathways to other subunits.