Evaluation of the proteasome inhibitor MLN9708 in preclinical models of human cancer

Novel agents derived from the currently approved treatments for MM: novel proteasome inhibitors and novel IMIDs

INTRODUCTION

Several novel proteasome inhibitors (PIs) and immunomodulatory agents (IMIDs) with similar, but not exactly the same, mechanisms of action than their predecessors have been developed in the last years with three different aims: to increase the efficacy; to overcome the resistance and to exhibit a better toxicity profile.

AREAS COVERED

This review summarizes the mechanism of action of novel PIs (carfilzomib, ONX-0912, MLN-9708, marizomib and CEP-18770) and IMIDs (pomalidomide), stressing the similarities and differences with their parental drugs. It also reviews their most updated clinical results. A search of the recent literature in published papers and abstracts from the most important oncology scientific meetings (ASCO and ASH) has been performed.

EXPERT OPINION

Novel PIs and IMIDs show clinical activity as single agents and in combination with dexamethasone, with similar or even higher efficacy than their predecessors; moreover, they may even overcome resistance to their parental drugs, indicating that there are some differences in their mechanisms of action and resistance. The investigation of these mechanisms of resistance and ways to overcome it would allow the optimization of the sequential use of these agents, and the design of novel therapeutic strategies and more appropriate scientifically based combinations.

Proteasome inhibitors: an expanding army attacking a unique target

Proteasomes are large, multisubunit proteolytic complexes presenting multiple targets for therapeutic intervention. The 26S proteasome consists of a 20S proteolytic core and one or two 19S regulatory particles. The 20S core contains three types of active sites. Many structurally diverse inhibitors of these active sites, both natural product and synthetic, have been discovered in the last two decades. One, bortezomib, is used clinically for treatment of multiple myeloma, mantle cell lymphoma, and acute allograft rejection. Five more recently developed proteasome inhibitors are in trials for treatment of myeloma and other cancers. Proteasome inhibitors also have activity in animal models of autoimmune and inflammatory diseases, reperfusion injury, promote bone and hair growth, and can potentially be used as anti-infectives. In addition, inhibitors of ATPases and deubiquitinases of 19S regulatory particles have been discovered in the last decade.

DangER: protein ovERload. Targeting protein degradation to treat myeloma

Myeloma is a malignancy of the antibody-producing plasma cells and, as such, these cells synthesize large quantities of unfolded or misfolded immunoglobulin. The build-up of excess protein triggers a number of downstream signal transduction cascades, including endoplasmic reticulum stress and autophagy. As a result, myeloma cells are uniquely reliant on these and other protein handling pathways for their survival. Strategies aimed at targeting this vulnerability have proved successful with the proteasome inhibitor, bortezomib, already licensed for clinical use. In addition to the proteasome, various other points within the protein handling pathways are also the subject of drug discovery projects, with some already progressing into clinical trials. These include compounds directed against heat shock proteins, the unfolded protein response and pathways both upstream and downstream of the proteasome. More recently, the role of autophagy has been recognized in myeloma. In this review, we discuss the various pathways used by myeloma cells for survival, with particular emphasis on the emerging role and conundrum of autophagy, as well as highlighting pre-clinical research on novel inhibitors targeting protein handling pathways.

Proteasome inhibition and combination therapy for non-Hodgkin's lymphoma: from bench to bedside

Although patients with B-cell non-Hodgkin's lymphoma (NHL) usually respond to initial conventional chemotherapy, they often relapse and mortality has continued to increase over the last three decades in spite of salvage therapy or high dose therapy and stem cell transplantation. Outcomes vary by subtype, but there continues to be a need for novel options that can help overcome chemotherapy resistance, offer new options as consolidation or maintenance therapy postinduction, and offer potentially less toxic combinations, especially in the elderly population. The bulk of these emerging novel agents for cancer treatment target important biological cellular processes. Bortezomib is the first in the class of proteasome inhibitors (PIs), which target the critical process of intracellular protein degradation or recycling and editing through the proteasome. Bortezomib is approved for the treatment of relapsed or refractory mantle cell lymphoma. The mechanisms of proteasome inhibition are very complex by nature (because they affect many pathways) and not fully understood. However, mechanisms of action shared by bortezomib and investigational PIs such as carfilzomib, marizomib, ONX-0912, and MLN9708 are distinct from those of other NHL treatments, making them attractive options for combination therapy. Preclinical evidence suggests that the PIs have additive and/or synergistic activity with a large number of agents both in vitro and in vivo, from cytotoxics to new biologicals, supporting a growing number of combination studies currently underway in NHL patients, as reviewed in this article. The results of these studies will help our understanding about how to best integrate proteasome inhibition in the management of NHL and continue to improve patient outcomes.

Management of treatment-emergent peripheral neuropathy in multiple myeloma

Peripheral neuropathy (PN) is one of the most important complications of multiple myeloma (MM) treatment. PN can be caused by MM itself, either by the effects of the monoclonal protein or in the form of radiculopathy from direct compression, and particularly by certain therapies, including bortezomib, thalidomide, vinca alkaloids and cisplatin. Clinical evaluation has shown that up to 20% of MM patients have PN at diagnosis and as many as 75% may experience treatment-emergent PN during therapy. The incidence, symptoms, reversibility, predisposing factors and etiology of treatment-emergent PN vary among MM therapies, with PN incidence also affected by the dose, schedule and combinations of potentially neurotoxic agents. Effective management of treatment-emergent PN is critical to minimize the incidence and severity of this complication, while maintaining therapeutic efficacy. Herein, the state of knowledge regarding treatment-emergent PN in MM patients and current management practices are outlined, and recommendations regarding optimal strategies for PN management during MM treatment are provided. These strategies include early and regular monitoring with neurological evaluation, with dose modification and treatment discontinuation as indicated. Areas requiring further research include the development of MM-specific, patient-focused assessment tools, pharmacogenomic analysis of patient DNA, and trials to assess the efficacy of pharmacological interventions.

Molecular targeted approaches in mantle cell lymphoma

Mantle cell lymphoma (MCL) is a malignancy of mature B cells characterized by the translocation t(11;14) that leads to aberrant expression of cyclin D1. Response to first-line chemotherapy is good, but most patients relapse, resulting in a median survival of 5 to 7 years. The important PI3K/AKT/mTOR pathway can be targeted with small molecules. mTOR inhibitors have clinical activity and temsirolimus has been approved in Europe. Second-generation mTOR inhibitors and the PI3K inhibitor CAL-101 offer additional means to target the pathway. Promising results with the BTK inhibitor PCI-32765 suggest that B-cell receptor signaling could play a role. For unknown reasons, MCL appears to be particularly sensitive to disruption of protein homeostasis. The proteasome inhibitor bortezomib achieves responses in up to 50% of relapsed patients. Much work has been done in elucidating the mechanism of its cytotoxicity, its incorporation into combination therapies, and the development of second-generation proteasome inhibitors. Deacetylase and HSP90 inhibitors are also promising classes of drugs that can synergize with proteasome inhibitors. Finally, BH3 mimetics are emerging as tools to sensitize tumor cells to chemotherapy. Participation in clinical trials offers patients an immediate chance to benefit from these advances and is essential to maintain the momentum of progress.

Proteasome inhibitors

In May 2003, the US Food and Drug Administration (FDA) granted accelerated approval for the use of the first-in-class proteasome inhibitor bortezomib as a third-line therapy in multiple myeloma, and the European Union followed suit a year later. Bortezomib has subsequently been approved for multiple myeloma as a second-line treatment on its own and as a first-line therapy in combination with an alkylating agent and a corticosteroid. Furthermore, bortezomib has also been approved as a second-line therapy for mantle cell lymphoma. In this chapter, the focus is on the current clinical research on bortezomib, its adverse effects, and the resistance of multiple myeloma patients to bortezomib-based therapy. The various applications of bortezomib in different diseases and recent advances in the development of a new generation of inhibitors that target the proteasome or other parts of the ubiquitin-proteasome system are also reviewed.

The 26S proteasome complex: an attractive target for cancer therapy

The 26S proteasome complex engages in an ATP-dependent proteolytic degradation of a variety of oncoproteins, transcription factors, cell cycle specific cyclins, cyclin-dependent kinase inhibitors, ornithine decarboxylase, and other key regulatory cellular proteins. Thus, the proteasome regulates either directly or indirectly many important cellular processes. Altered regulation of these cellular events is linked to the development of cancer. Therefore, the proteasome has become an attractive target for the treatment of numerous cancers. Several proteasome inhibitors that target the proteolytic active sites of the 26S proteasome complex have been developed and tested for anti-tumor activities. These proteasome inhibitors have displayed impressive anti-tumor functions by inducing apoptosis in different tumor types. Further, the proteasome inhibitors have been shown to induce cell cycle arrest, and inhibit angiogenesis, cell-cell adhesion, cell migration, immune and inflammatory responses, and DNA repair response. A number of proteasome inhibitors are now in clinical trials to treat multiple myeloma and solid tumors. Many other proteasome inhibitors with different efficiencies are being developed and tested for anti-tumor activities. Several proteasome inhibitors currently in clinical trials have shown significantly improved anti-tumor activities when combined with other drugs such as histone deacetylase (HDAC) inhibitors, Akt (protein kinase B) inhibitors, DNA damaging agents, Hsp90 (heat shock protein 90) inhibitors, and lenalidomide. The proteasome inhibitor bortezomib is now in the clinic to treat multiple myeloma and mantle cell lymphoma. Here, we discuss the 26S proteasome complex in carcinogenesis and different proteasome inhibitors with their potential therapeutic applications in treatment of numerous cancers.

Regulation of the nucleocytoplasmic trafficking of viral and cellular proteins by ubiquitin and small ubiquitin-related modifiers

Nucleocytoplasmic trafficking of many cellular proteins is regulated by nuclear import/export signals as well as post-translational modifications such as covalent conjugation of ubiquitin and small ubiquitin-related modifiers (SUMOs). Ubiquitination and SUMOylation are rapid and reversible ways to modulate the intracellular localisation and function of substrate proteins. These pathways have been co-opted by some viruses, which depend on the host cell machinery to transport their proteins in and out of the nucleus. In this review, we will summarise our current knowledge on the ubiquitin/SUMO-regulated nuclear/subnuclear trafficking of cellular proteins and describe examples of viral exploitation of these pathways.

Discovery of a potent and highly β1 specific proteasome inhibitor from a focused library of urea-containing peptide vinyl sulfones and peptide epoxyketones

Syringolins, a class of natural products, potently and selectively inhibit the proteasome and show promising antitumour activity. To gain insight in the mode of action of syringolins, the ureido structural element present in syringolins is incorporated in oligopeptide vinyl sulfones and peptide epoxyketones yielding a focused library of potent new proteasome inhibitors. The distance of the ureido linkage with respect to the electrophilic trap strongly influences subunit selectivity within the proteasome. Compounds 13 and 15 are β5 selective and their potency exceeds that of syringolin A. In contrast, 5 may well be the most potent β1 selective compound active in living cells reported to date.

New molecular targets in mantle cell lymphoma

Mantle cell lymphoma (MCL) is a malignancy of mature B cells characterized by aberrant expression of cyclin D1 due to the translocation t(11;14). Epigenomic and genomic lesions in pathways regulating B-cell activation, cell cycle progression, protein homeostasis, DNA damage response, cell proliferation and apoptosis contribute to its pathogenesis. While patients typically respond to first-line chemotherapy, relapse is the rule resulting in a median survival of 5-7 years. The PI3K/AKT/mTOR appears as a key pathway in the pathogenesis and can be targeted with small molecules. Most experience is with mTOR inhibitors of the rapamycin class. Second-generation mTOR inhibitors and the PI3K inhibitor CAL-101 are novel options to more effectively target this pathway. Bruton's tyrosine kinase inhibition by PCI-32765 has promising activity and indicates immunoreceptor signaling as a novel therapeutic target. Up to 50% of relapsed patients respond to the proteasome inhibitor bortezomib suggesting that MCL may be particularly sensitive to disruption of protein homeostasis and/or induction of oxidative stress. Recent work has focused on elucidating the mechanism of bortezomib-induced cytotoxicity and the development of second-generation proteasome inhibitors. DNA hypomethylating agents and histone deacetylase inhibitors effect epigenetic de-repression of aberrantly silenced genes. These epigenetic pharmaceuticals and HSP90 inhibitors can synergize with proteasome inhibitors. Finally, BH3 mimetics are emerging as tools to sensitize tumor cells to chemotherapy. Participation in clinical trials offers patients a chance to benefit from these advances and is essential to maintain the momentum of progress. Innovative trial designs may be needed to expedite the clinical development of these targeted agents.

Molecular pathways: targeting proteasomal protein degradation in cancer

With the approval by the U.S. Food and Drug Administration of bortezomib for the treatment of multiple myeloma and mantle cell lymphoma, the proteasome was clinically validated as a target in oncology. The proteasome is part of a complex cellular pathway that controls the specificity and rate of degradation of the majority of proteins in the cell. The search for additional drug targets in the proteasomal pathway is ongoing. In parallel, the next generation of proteasome inhibitors, exhibiting some properties distinct from that of bortezomib, are currently being studied in clinical trials. The key question will be whether these distinctions can improve upon the clinical efficacy and safety standards established by bortezomib and refine our understanding of the mechanism by which proteasome inhibitors are effective in the treatment of cancer.

TRIM proteins and cancer

Emerging clinical evidence shows that the deregulation of ubiquitin-mediated degradation of oncogene products or tumour suppressors is likely to be involved in the aetiology of carcinomas and leukaemias. Recent studies have indicated that some members of the tripartite motif (TRIM) proteins (one of the subfamilies of the RING type E3 ubiquitin ligases) function as important regulators for carcinogenesis. This Review focuses on TRIM proteins that are involved in tumour development and progression.

Specific cell-permeable inhibitor of proteasome trypsin-like sites selectively sensitizes myeloma cells to bortezomib and carfilzomib

Proteasomes degrade the majority of proteins in mammalian cells, are involved in the regulation of multiple physiological functions, and are established targets of anticancer drugs. The proteasome has three types of active sites. Chymotrypsin-like sites are the most important for protein breakdown and have long been considered the only suitable targets for antineoplastic drugs; however, our recent work demonstrated that inhibitors of caspase-like sites sensitize malignant cells to inhibitors of the chymotrypsin-like sites. Here, we describe the development of specific cell-permeable inhibitors and an activity-based probe of the trypsin-like sites. These compounds selectively sensitize multiple myeloma cells to inhibitors of the chymotrypsin-like sites, including antimyeloma agents bortezomib and carfilzomib. Thus, trypsin-like sites are cotargets for anticancers drugs. Together with inhibitors of chymotrypsin- and caspase-like sites developed earlier, we provide the scientific community with a complete set of tools to separately modulate proteasome active sites in living cells.

Antitumor activity of the investigational proteasome inhibitor MLN9708 in mouse models of B-cell and plasma cell malignancies

PURPOSE

The clinical success of the first-in-class proteasome inhibitor bortezomib (VELCADE) has validated the proteasome as a therapeutic target for treating human cancers. MLN9708 is an investigational proteasome inhibitor that, compared with bortezomib, has improved pharmacokinetics, pharmacodynamics, and antitumor activity in preclinical studies. Here, we focused on evaluating the in vivo activity of MLN2238 (the biologically active form of MLN9708) in a variety of mouse models of hematologic malignancies, including tumor xenograft models derived from a human lymphoma cell line and primary human lymphoma tissue, and genetically engineered mouse (GEM) models of plasma cell malignancies (PCM).

EXPERIMENTAL DESIGN

Both cell line-derived OCI-Ly10 and primary human lymphoma-derived PHTX22L xenograft models of diffuse large B-cell lymphoma were used to evaluate the pharmacodynamics and antitumor effects of MLN2238 and bortezomib. The iMyc(Cα)/Bcl-X(L) GEM model was used to assess their effects on de novo PCM and overall survival. The newly developed DP54-Luc-disseminated model of iMyc(Cα)/Bcl-X(L) was used to determine antitumor activity and effects on osteolytic bone disease.

RESULTS

MLN2238 has an improved pharmacodynamic profile and antitumor activity compared with bortezomib in both OCI-Ly10 and PHTX22L models. Although both MLN2238 and bortezomib prolonged overall survival, reduced splenomegaly, and attenuated IgG2a levels in the iMyc(Cα)/Bcl-X(L) GEM model, only MLN2238 alleviated osteolytic bone disease in the DP54-Luc model.

CONCLUSIONS

Our results clearly showed the antitumor activity of MLN2238 in a variety of mouse models of B-cell lymphoma and PCM, supporting its clinical development. MLN9708 is being evaluated in multiple phase I and I/II trials.

Novel therapies in MM: from the aspect of preclinical studies

During the last decade, thalidomide, lenalidomide, and bortezomib have been approved by the US Food and Drug Administration for the treatment of MM; however, MM remains incurable. The development and progression of multiple myeloma (MM) is a complex multi-step process involving genetic abnormalities in tumor cells at both early and late stages. Moreover, soluble factors and cell-cell contact within the tumor bone marrow (BM) microenvironment promotes MM cell growth, survival, and drug resistance. A number of novel agents targeting both tumor cells and growth factors in the BM milieu have been developed. Currently they are under evaluation in preclinical studies, as single agents and/or in combination, to improve outcome of MM patients.

Experimental approaches in the treatment of multiple myeloma

Myeloma therapy has undergone significant advances in recent years resulting in a marked improvement in survival. Knowledge of the active pathways involved in myeloma pathogenesis has led to the discovery of novel agents and greatly expanded the potential armamentarium available for treatment. This better understanding of the disease and resistance mechanisms has resulted in new agent classes that are being evaluated in preclinical and early clinical studies. In addition, dosing for existing agents is being optimized, and they are being given in new combinations. In this article, we review experimental agents that are showing promise in multiple myeloma treatment. New biological agents in clinical trials hold the promise of efficacy through novel mechanisms of action, with a significant reduction of dose-limiting toxicities compared with classic cytotoxic chemotherapeutics. Second-generation proteasome inhibitors and immunomodulatory agents are furthest along in clinical development, and histone deacetylase inhibitors, heat shock protein 90 inhibitors, Akt inhibitors and monoclonal antibodies are some of the other agents entering later-phase clinical trials. We also review developments in targeting the myeloma stem cell as an exciting new treatment direction.

Marizomib, a proteasome inhibitor for all seasons: preclinical profile and a framework for clinical trials

The proteasome has emerged as an important clinically relevant target for the treatment of hematologic malignancies. Since the Food and Drug Administration approved the first-in-class proteasome inhibitor bortezomib (Velcade) for the treatment of relapsed/refractory multiple myeloma (MM) and mantle cell lymphoma, it has become clear that new inhibitors are needed that have a better therapeutic ratio, can overcome inherent and acquired bortezomib resistance and exhibit broader anti-cancer activities. Marizomib (NPI-0052; salinosporamide A) is a structurally and pharmacologically unique β-lactone-γ-lactam proteasome inhibitor that may fulfill these unmet needs. The potent and sustained inhibition of all three proteolytic activities of the proteasome by marizomib has inspired extensive preclinical evaluation in a variety of hematologic and solid tumor models, where it is efficacious as a single agent and in combination with biologics, chemotherapeutics and targeted therapeutic agents. Specifically, marizomib has been evaluated in models for multiple myeloma, mantle cell lymphoma, Waldenstrom's macroglobulinemia, chronic and acute lymphocytic leukemia, as well as glioma, colorectal and pancreatic cancer models, and has exhibited synergistic activities in tumor models in combination with bortezomib, the immunomodulatory agent lenalidomide (Revlimid), and various histone deacetylase inhibitors. These and other studies provided the framework for ongoing clinical trials in patients with MM, lymphomas, leukemias and solid tumors, including those who have failed bortezomib treatment, as well as in patients with diagnoses where other proteasome inhibitors have not demonstrated significant efficacy. This review captures the remarkable translational studies and contributions from many collaborators that have advanced marizomib from seabed to bench to bedside.

In vitro and in vivo selective antitumor activity of a novel orally bioavailable proteasome inhibitor MLN9708 against multiple myeloma cells

PURPOSE

The success of bortezomib therapy for treatment of multiple myeloma (MM) led to the development of structurally and pharmacologically distinct novel proteasome inhibitors. In the present study, we evaluated the efficacy of one such novel orally bioactive proteasome inhibitor MLN9708/MLN2238 in MM using well-established in vitro and in vivo models.

EXPERIMENTAL DESIGN

MM cell lines, primary patient cells, and the human MM xenograft animal model were used to study the antitumor activity of MN2238.

RESULTS

Treatment of MM cells with MLN2238 predominantly inhibits chymotrypsin-like activity of the proteasome and induces accumulation of ubiquitinated proteins. MLN2238 inhibits growth and induces apoptosis in MM cells resistant to conventional and bortezomib therapies without affecting the viability of normal cells. In animal tumor model studies, MLN2238 is well tolerated and inhibits tumor growth with significantly reduced tumor recurrence. A head-to-head analysis of MLN2238 versus bortezomib showed a significantly longer survival time in mice treated with MLN2238 than mice receiving bortezomib. Immununostaining of MM tumors from MLN2238-treated mice showed growth inhibition, apoptosis, and a decrease in associated angiogenesis. Mechanistic studies showed that MLN2238-triggered apoptosis is associated with activation of caspase-3, caspase-8, and caspase-9; increase in p53, p21, NOXA, PUMA, and E2F; induction of endoplasmic reticulum (ER) stress response proteins Bip, phospho-eIF2-α, and CHOP; and inhibition of nuclear factor kappa B. Finally, combining MLN2238 with lenalidomide, histone deacetylase inhibitor suberoylanilide hydroxamic acid, or dexamethasone triggers synergistic anti-MM activity.

CONCLUSION

Our preclinical study supports clinical evaluation of MLN9708, alone or in combination, as a potential MM therapy.

Chemical and biological evaluation of dipeptidyl boronic acid proteasome inhibitors for use in prodrugs and pro-soft drugs targeting solid tumors

Bortezomib, a dipeptidyl boronic acid and potent inhibitor of the 26S proteasome, is remarkably effective against multiple myeloma (MM) but not against solid tumors. Dose-limiting adverse effects from "on target" inhibition of the proteasome in normal cells and tissues appear to be a key obstacle. Achieving efficacy against solid tumors therefore is likely to require making the inhibitor more selective for tumor tissue over normal tissues. The simplest strategy that might provide such tissue specificity would be to employ a tumor specific protease to release an inhibitor from a larger, noninhibitory structure. However, such release would necessarily generate an inhibitor with a free N-terminal amino group, raising a key question: Can short peptide boronic acids with N-terminal amino groups have the requisite properties to serve as warheads in prodrugs? Here we show that dipeptides of boroLeu, the smallest plausible candidates for the task, can indeed be sufficiently potent, cell-penetrating, cytotoxic, and stable to degradation by cellular peptidases to serve in this capacity.

Mantle cell lymphoma

MCL is a well-characterized clinically aggressive lymphoma with a poor prognosis. Recent research findings have slightly improved the outcome of this neoplasm. The addition of rituximab to conventional chemotherapy has increased overall response rates, but it does not improve overall survival with respect to chemotherapy alone. The use of intensive frontline therapies including rituximab and consolidated by ASCT ameliorates response rate and prolongs progression-free survival, but any impact on survival remains to be proven. Furthermore, the optimal timing, cytoreductive regimen and conditioning regimen, and the clinical implications of achieving a disease remission even at molecular level remain to be elucidated. The development of targeted therapies as the consequence of better dissection of pathogenetic pathways in MCL might improve the outcome of conventional chemotherapy in most patients and spare the toxicity of intense therapy in a minority of MCL patients characterized by a relatively indolent disease. Patients not eligible for intensive regimens, such as hyperC-VAD, may be considered for less demanding therapies, such as the combination of rituximab either with CHOP or with purine analogues, or bendamustine. Allogeneic SCT can be an effective option for relapsed disease in patients who are fit enough and have a compatible donor. Maintenance rituximab may be considered after response to immunochemotherapy for relapsed disease, although there are currently no data to recommend this approach as the first-line strategy. As the optimal approach to the management of MCL is still evolving, it is critical that these patients be enrolled in clinical trials to identify better treatment options.

Proteasome inhibitors in cancer therapy

The ubiquitin proteasome pathway plays a critical role in regulating many processes in the cell which are important for tumour cell growth and survival. Inhibition of proteasome function has emerged as a powerful strategy for anti-cancer therapy. Clinical validation of the proteasome as a therapeutic target was achieved with bortezomib and has prompted the development of a second generation of proteasome inhibitors with improved pharmacological properties. This review summarises the main mechanisms of action of proteasome inhibitors in cancer, the development of proteasome inhibitors as therapeutic agents and the properties and progress of next generation proteasome inhibitors in the clinic.

Novel proteasome inhibitors to overcome bortezomib resistance

The proteasome is an intracellular enzyme complex that degrades ubiquitin-tagged proteins and thereby regulates protein levels within the cell. Given this important role in maintaining cellular homeostasis, it is perhaps somewhat surprising that proteasome inhibitors have a therapeutic window. Proteasome inhibitors have demonstrated clinical efficacy in the treatment of multiple myeloma and mantle cell lymphoma and are under evaluation for the treatment of other malignancies. Bortezomib is the first and only Food and Drug Administration-approved proteasome inhibitor that inhibits this enzyme complex in a reversible fashion. Although bortezomib improves clinical outcomes when used as a single agent, most patients do not respond to this drug and those who do respond almost uniformly relapse. As such, efforts are underway to develop proteasome inhibitors that act through mechanisms distinct from that of bortezomib. Specifically, inhibitors that bind the active site of the proteasome and inhibit the complex irreversibly have been developed and are in advanced clinical trials. Inhibitors that act on sites of the proteasome outside of the catalytic center have also been identified and are in preclinical development. In this review, we discuss the structure and function of the proteasome. We then focus on the molecular biology, chemistry, and the preclinical and clinical efficacy of novel proteasome inhibitors as strategies to inhibit this target and overcome some forms of bortezomib resistance.

Prevention of muscle disuse atrophy by MG132 proteasome inhibitor

INTRODUCTION

Our goal was to determine whether in vivo administration of the proteasome inhibitor MG132 can prevent muscle atrophy caused by hindlimb unloading (HU).

METHODS

Twenty-seven NMRI mice were assigned to a weight-bearing control, a 6-day HU, or a HU+MG132 (1 mg/kg/48 h) treatment group.

RESULTS

Gastrocnemius wasting was significantly less in HU+MG132 mice (-6.7 ± 2.0%) compared with HU animals (-12.6 ± 1.1%, P = 0.011). HU was also associated with an increased expression of MuRF-1 (P = 0.006), MAFbx (P = 0.001), and USP28 (P = 0.027) mRNA, whereas Nedd4, E3α, USP19, and UBP45 mRNA did not change significantly. Increases in MuRF-1, MAFbx, and USP28 mRNA were largely repressed after MG132 administration. β5 proteasome activity tended to increase in HU (+16.7 ± 6.1%, P = 0.086). Neither β1 and β2 proteasome activities nor ubiquitin-conjugated proteins were changed by HU.

CONCLUSIONS

Our results indicate that in vivo administration of MG132 partially prevents muscle atrophy associated with disuse and highlight an unexpected regulation of MG132 proteasome inhibitor on ubiquitin-ligases.

Preclinical evaluation of the antitumor activity of bortezomib in combination with vitamin C or with epigallocatechin gallate, a component of green tea

Purpose

To investigate whether clinically relevant levels of epigallocatechin gallate (EGCG, a component of green tea) or vitamin C (ascorbic acid) could antagonize bortezomib antitumor activity in CWR22 human prostate xenograft tumors.

Methods

The pharmacokinetics (PK) of EGCG and ascorbic acid were determined in immunocompromised mice and compared with concentrations measured in human PK studies of dietary supplements. Antitumor activity of bortezomib in combination with EGCG or ascorbic acid was determined using several dosing regimens to evaluate different target plasma concentrations of EGCG and ascorbic acid.

Results

Bortezomib dosed twice-weekly at 0.8 mg/kg IV demonstrated tumor growth inhibition (TGI) of 53.9–58.9%. However, when combined with EGCG such that the plasma concentrations of EGCG were >200 μM at the time of bortezomib dosing, all antitumor activity was abrogated (TGI = −17.7%). A lower concentration of EGCG (11–16 μM), which is severalfold higher than measured clinically in humans taking EGCG supplements (0.6–3 μM), was not antagonistic to bortezomib (TGI 63.5%). Pharmacodynamic studies of proteasome inhibition reflected these findings. Ascorbic acid (40 and 500 mg/kg PO daily) was evaluated under a similar study design and did not antagonize bortezomib antitumor activity (TGI 57.2 and 72.2%).

Conclusions

No antagonism of bortezomib is seen in preclinical in vivo experiments, where EGCG or ascorbic acid plasma concentrations are commensurate with dietary or supplemental intake. The data suggest that patients receiving bortezomib treatment do not need to avoid normal dietary consumption of green tea, vitamin C-containing foods, or EGCG or vitamin C dietary supplements.

Electronic supplementary material

The online version of this article (doi:10.1007/s00280-011-1591-2) contains supplementary material, which is available to authorized users.

Threonine peptidases as drug targets against malaria

INTRODUCTION

Malaria is caused by the intracellular parasite Plasmodium falciparum. Although numerous therapies are available to fight the disease, the number of pharmacophores is small, and constant development of novel therapies, especially with new targets, is desirable to fight developing resistance against presently prescribed drugs.

AREAS COVERED

This review discusses research on plasmodial threonine peptidases along with recent advances in proteasome inhibitor development.

EXPERT OPINION

While PfHslV is an attractive drug target in malaria, more investigation is required to clarify its functional role in the parasite. More efforts should also be invested in assessing the plasmodial proteasome as a drug target. The few papers investigating the effect of proteasome inhibitors on different stages of the life cycle point towards important roles not only during asexual, but also in hepatic and sexual stages, in humans and the mosquito. If this holds true, this is a key argument to further develop proteasome inhibitors for use against malaria.

Ubiquitin-like protein conjugation and the ubiquitin-proteasome system as drug targets

Ubiquitin is a highly conserved 76 amino-acid protein that covalently attaches to protein substrates targeted for degradation by the 26S proteasome. The coordinated effort of a series of enzymes, including an activating enzyme (E1), a conjugating enzyme (E2) and a ligase (E3), uses ATP to ultimately form an isopeptide bond between ubiquitin and a substrate.

Another class of enzymes called deubiquitylating enzymes (DUBs) deconstruct these linkages and also have an essential role in ubiquitin function. In addition, ubiquitin-like proteins (UBLs), including NEDD8, SUMO and ISG15, share a characteristic three-dimensional fold with ubiquitin but have their own dedicated enzyme cascades and distinct (although sometimes overlapping) biological functions.

The ubiquitin–proteasome system (UPS) and UBL conjugation pathways have important roles in various human diseases, including numerous types of cancer, cardiovascular disease, viral diseases and neurodegenerative disorders. The proteasome inhibitor bortezomib (Velcade; Millennium Pharmaceuticals) is the first clinically validated drug to target the UPS and is approved for the treatment of multiple myeloma. This suggests that other diseases may conceivably be targeted by modulating components of the UPS and UBL conjugation pathways using small-molecule inhibitors.

A significant hurdle to identifying drug-like inhibitors of enzyme targets within the UPS and UBL conjugation pathways is the limited understanding of the molecular mechanisms and biological consequences of UBL conjugation.

Here, we provide an overview of the enzyme classes in the UPS and UBL pathways that are potential therapeutic targets, and highlight considerations that are important for drug discovery. We also discuss the progress in the development of small-molecule inhibitors, and review developments in understanding of the role of the components of the UPS and the UBL pathways in disease and their potential for therapeutic intervention.

The ubiquitin–proteasome system (UPS) and ubiquitin-like protein (UBL) conjugation pathways are integral to cellular protein homeostasis, and their functional importance in various diseases, including cancer, cardiovascular disease and neurodegenerative disorders, is now beginning to emerge. Brownell and colleagues review developments in understanding of the role of the components of the UPS and the UBL pathways in disease and their potential for therapeutic intervention.

The ubiquitin–proteasome system (UPS) and ubiquitin-like protein (UBL) conjugation pathways are integral to cellular protein homeostasis. The growing recognition of the fundamental importance of these pathways to normal cell function and in disease has prompted an in-depth search for small-molecule inhibitors that selectively block the function of these pathways. However, our limited understanding of the molecular mechanisms and biological consequences of UBL conjugation is a significant hurdle to identifying drug-like inhibitors of enzyme targets within these pathways. Here, we highlight recent advances in understanding the role of some of these enzymes and how these new insights may be the key to developing novel therapeutics for diseases including immuno-inflammatory disorders, cancer, infectious diseases, cardiovascular disease and neurodegenerative disorders.

Therapeutically targeting the SUMOylation, Ubiquitination and Proteasome pathways as a novel anticancer strategy

The ubiquitin (Ub)+proteasome proteolytic pathway is responsible for the selective degradation of the majority of nuclear and cytosolic proteins. The proteasome is a high molecular weight multicatalytic protease that serves as the catalytic core of the complex Ub-dependent protein degradation pathway and is an exciting new target for the development of novel anticancer therapies. Inhibition of the proteasome, and consequently Ub-dependent proteolysis, with the small molecule pharmacologic agent bortezomib led to approval by the US Food and Drug Administration (FDA) for the treatment of multiple myeloma (MM) that has subsequently been extended to other hematologic malignancies. Inhibition of the proteasome results in the intracellular accumulation of many ubiquitinated proteins that control essential cellular functions such as cellular growth and apoptosis. The accumulation of high molecular weight Ub~protein conjugates eventually triggers apoptosis, with tumor cells more susceptible to proteasome inhibition than non-malignant cells. The defined mechanism of action for proteasome inhibitors has not been completely characterized, not all patients respond to proteasome inhibitor-based therapy, and inevitably patients develop resistance to proteasome inhibitors. Further investigation of the Ub+proteasome system (UPS) is needed to develop more effective inhibitors, to develop agents that overcome bortezomib resistance and to avoid adverse effects such as neuropathy. Furthermore, there are newly uncovered pathways, e.g., the SUMOylation and NEDDylation pathways, which similarly attach Ub-like proteins (ULPs) to protein substrates. The functional consequence of these modifications is only beginning to emerge, but these pathways have been linked to tumorigenesis and may similarly provide therapeutic targets. The immunoproteasome is a specialized form of the proteasome that produces peptides that are presented at the cell surface as major histocompatibility complex (MHC) class I antigens. Proteasome inhibitors decrease the presentation of antigenic peptides to reduce tumor cell recognition by cytotoxic T cells (CTLs) but unexpectedly increase tumor cell recognition by natural killer (NK) cells. Inhibitors of the UPS are validated, cytotoxic agents that may be further exploited in immunotherapy since they modulate tumor cell recognition by effectors of the immune system. Targeting the UPS, SUMOylation and NEDDylation pathways offers great promise in the treatment of hematologic and solid malignancies.

Mantle cell lymphoma: biology, pathogenesis, and the molecular basis of treatment in the genomic era

Mantle cell lymphoma (MCL) is a B-cell non-Hodgkin lymphoma of which at least a subset arises from antigen-experienced B cells. However, what role antigen stimulation plays in its pathogenesis remains ill defined. The genetic hallmark is the chromosomal translocation t(11;14) resulting in aberrant expression of cyclin D1. Secondary genetic events increase the oncogenic potential of cyclin D1 and frequently inactivate DNA damage response pathways. In combination these changes drive cell-cycle progression and give rise to pronounced genetic instability. Several signaling pathways contribute to MCL pathogenesis, including the often constitutively activated PI3K/AKT/mTOR pathway, which promotes tumor proliferation and survival. WNT, Hedgehog, and NF-κB pathways also appear to be important. Although MCL typically responds to frontline chemotherapy, it remains incurable with standard approaches. Proteasome inhibitors (bortezomib), mTOR inhibitors (temsirolimus), and immunomodulatory drugs (lenalidomide) have recently been added to the treatment options in MCL. The molecular basis for the antitumor activity of these agents is an area of intense study that hopefully will lead to further improvements in the near future. Given its unique biology, relative rarity, and the difficulty in achieving long-lasting remissions with conventional approaches, patients with MCL should be encouraged to participate in clinical trials.

Nature of pharmacophore influences active site specificity of proteasome inhibitors

Proteasomes degrade most proteins in mammalian cells and are established targets of anti-cancer drugs. The majority of proteasome inhibitors are composed of short peptides with an electrophilic functionality (pharmacophore) at the C terminus. All eukaryotic proteasomes have three types of active sites as follows: chymotrypsin-like, trypsin-like, and caspase-like. It is widely believed that active site specificity of inhibitors is determined primarily by the peptide sequence and not the pharmacophore. Here, we report that active site specificity of inhibitors can also be tuned by the chemical nature of the pharmacophore. Specifically, replacement of the epoxyketone by vinyl sulfone moieties further improves the selectivity of β5-specific inhibitors NC-005, YU-101, and PR-171 (carfilzomib). This increase in specificity is likely the basis of the decreased cytotoxicity of vinyl sulfone-based inhibitors to HeLa cells as compared with that of epoxyketone-based 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.

Combination therapy of bortezomib with novel targeted agents: an emerging treatment strategy

Clinical trials evaluating combinations of targeted agents with bortezomib, the first-in-class proteasome inhibitor, have been initiated, with the objective of enhancing its single agent activity in hematologic malignancies (myeloma, mantle cell lymphoma), as well as expanding its efficacy in solid tumors. In most cases, preclinical studies have provided a supportive rationale for designing these doublet combination studies. Novel, small molecule-targeted agents being investigated with bortezomib in clinical trials include protein deacetylase inhibitors, kinase inhibitors, farnesyltransferase inhibitors, heat-shock protein 90 inhibitors, pan-Bcl-2 family inhibitors, and other classes of targeted inhibitors. Preliminary clinical data, available from a number of ongoing trials, suggest that most of these combinations are well tolerated and some have promising clinical efficacy that will require subsequent confirmation. Translational studies, conducted as part of the trials, may provide important insights into the putative mechanism of action delineated by preclinical studies of the combinations. The emergence of novel proteasome inhibitors may also expand the opportunities for optimizing these combination therapies. There is potential for an increasingly broad clinical trials program to investigate this therapeutic approach in a range of tumor types, as well as to consider additional agents in sequence or in combination.

Upcoming diagnostic and therapeutic developments in classical Hodgkin's lymphoma

With improvements in therapy, increasing dose intensity, early recognition of toxicity, and enhanced supportive care, current outcomes in patients with classical Hodgkin's lymphoma (HL) may be superior to disease-free and overall survival (OS) predicted by existing prognostic models, including the International Prognostic Score (IPS). The addition of biologic markers to recognized clinical prognostic factors, including those of the IPS, may to improve patient risk stratification and guide therapy in the future. However, the identification of these markers has been problematic due to the lack of large, confirmatory prospective trials, reproducibility and feasibility of the assays, and failure to improve upon already recognized clinical risk factors. One biomarker in particular, CD68, present on tumor infiltrating macrophages and detectable by immunohistochemical staining, is significantly associated with both shortened progression-free and disease-specific survivals in patients with HL at diagnosis and at relapse. In addition, less than 5% CD68+ cells correlates with a 100% disease-specific survival in patients with early-stage HL. CD68 represents just one of the many prognostic markers that could eventually be used to risk-stratify therapy. In addition, biologic markers may not only serve as prognostic markers, but also as therapeutic targets in HL. This review examines current data using the IPS to determine patient outcome, discuss several potential biologic prognostic markers, and summarize new therapies that are currently in clinical development in HL.