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

Novel Proteasome Inhibitors and Histone Deacetylase Inhibitors: Progress in Myeloma Therapeutics

The unfolded protein response is responsible for the detection of misfolded proteins and the coordination of their disposal and is necessary to maintain the cellular homoeostasis. Multiple myeloma cells secrete large amounts of immunoglobulins, proteins that need to be correctly folded by the chaperone system. If this process fails, the misfolded proteins have to be eliminated by the two main garbage-disposal systems of the cell: proteasome and aggresome. The blockade of either of these systems will result in accumulation of immunoglobulins and other toxic proteins in the cytoplasm and cell death. The simultaneous inhibition of the proteasome, by proteasome inhibitors (PIs) and the aggresome, by histone deacetylase inhibitors (HDACi) results in a synergistic increase in cytotoxicity in myeloma cell lines. This review provides an overview of mechanisms of action of second-generation PIs and HDACi in multiple myeloma (MM), the clinical results currently observed with these agents and assesses the potential therapeutic impact of the different agents in the two classes. The second-generation PIs offer benefits in terms of increased efficacy, reduced neurotoxicity as off-target effect and may overcome resistance to bortezomib because of their different chemical structure, mechanism of action and biological properties. HDACi with anti-myeloma activity in clinical development discussed in this review include vorinostat, panobinostat and selective HDAC6 inhibitor, ricolinostat.

Posttransplant maintenance therapy in multiple myeloma: the changing landscape

Transplant-eligible patients with multiple myeloma (MM) now have extended survival after diagnosis owing to effective modern treatment strategies that include new agents in induction therapy, autologous stem cell transplant (ASCT), consolidation therapy and posttransplant maintenance therapy. Standard of care for newly diagnosed, fit patients includes ASCT and, often nowadays, posttransplant maintenance. Several large studies have shown the efficacy of maintenance with thalidomide, lenalidomide and bortezomib in the treatment scheme of MM with regards to prolonging progression-free survival and, to a lesser degree, overall survival. Herein we discuss the data currently available to support the use of maintenance therapy in patients after ASCT as well as the newer available agents that may be a part of its changing landscape in the years to come.

Tyropeptins, proteasome inhibitors produced by Kitasatospora sp. MK993-dF2

Tyropeptins are new proteasome inhibitors isolated from the culture broth of Kitasatospora sp. MK993-dF2. Tyropeptins permeate cell membranes, inhibit intracellular proteasomes and reduce the degradation of ubiquitinated proteins in mammalian cells. We performed structure-based drug design and structure-activity relationship studies on tyropeptin derivatives to obtain valuable information of derivatives. Among the synthesized tyropeptin derivatives, some boronic acid derivatives exhibited potent antitumor effects against human multiple myeloma. In this review, we summarize the discovery of tyropeptins and the development of tyropeptin derivatives.

Targeting proteasomes in infectious organisms to combat disease

Proteasomes are multisubunit, energy-dependent, proteolytic complexes that play an essential role in intracellular protein turnover. They are present in eukaryotes, archaea, and in some actinobacteria species. Inhibition of proteasome activity has emerged as a powerful strategy for anticancer therapy and three drugs have been approved for treatment of multiple myeloma. These compounds react covalently with a threonine residue located in the active site of a proteasome subunit to block protein degradation. Proteasomes in pathogenic organisms such as Mycobacterium tuberculosis and Plasmodium falciparum also have a nucleophilic threonine residue in the proteasome active site and are therefore sensitive to these anticancer drugs. This review summarizes efforts to validate the proteasome in pathogenic organisms as a therapeutic target. We describe several strategies that have been used to develop inhibitors with increased potency and selectivity for the pathogen proteasome relative to the human proteasome. In addition, we highlight a cell-based chemical screening approach that identified a potent, allosteric inhibitor of proteasomes found in Leishmania and Trypanosoma species. Finally, we discuss the development of proteasome inhibitors as anti-infective agents.

Activity-based probes for the multicatalytic proteasome

Proteasomes are multisubunit protease complexes responsible for degrading most intracellular proteins. In addition to removing damaged proteins, they regulate many important cellular processes through the controlled degradation of transcription factors, cell cycle regulators, and enzymes. Eukaryotic proteasomes have three catalytic subunits, β1, β2, and β5, that each has different substrate specificities. Additionally, although we know that diverse cell types express proteasome variants with distinct activity and specificity profiles, the functions of these different pools of proteasomes are not fully understood. Covalent inhibitors of the protease activity of the proteasome have been developed as drugs for hematological malignancies and are currently under investigation for other diseases. Therefore, there is a need for tools that allow direct monitoring of proteasome activity in live cells and tissues. Activity-based probes have proven valuable for biochemical and cell biological studies of the role of individual proteasome subunits, and for evaluating the efficacy and selectivity of proteasome inhibitors. These probes react covalently with the protease active sites, and contain a reporter tag to identify the probe-labeled proteasome subunits. This review will describe the development of broad-spectrum and subunit-specific proteasome activity-based probes, and discuss how these probes have contributed to our understanding of proteasome biology, and to the development of proteasome inhibitors.

Emerging challenges in the design of selective substrates, inhibitors and activity-based probes for indistinguishable proteases

Proteases are enzymes that hydrolyze the peptide bond of peptide substrates and proteins. Despite significant progress in recent years, one of the greatest challenges in the design and testing of substrates, inhibitors and activity‐based probes for proteolytic enzymes is achieving specificity toward only one enzyme. This specificity is particularly important if the enzyme is present with other enzymes with a similar catalytic mechanism and substrate specificity but completely different functionality. The cross‐reactivity of substrates, inhibitors and activity‐based probes with other enzymes can significantly impair or even prevent investigations of a target protease. In this review, we describe important concepts and the latest challenges, focusing mainly on peptide‐based substrate specificity techniques used to distinguish individual enzymes within major protease families.

Design, synthesis and biological evaluation of novel non-peptide boronic acid derivatives as proteasome inhibitors

A novel series of non-peptide proteasome inhibitors bearing the 1, 4-naphthoquinone scaffold and boronic acid warhead was developed. In the biological evaluation on the chymotrypsin-like activity of human 20S proteasome, five compounds showed IC₅₀ values in the nanomolar range. Docking experiments into the yeast 20S proteasome rationalized their biological activities and allowed further optimization of this interesting class of inhibitors. Within the cellular proliferation inhibition assay and western blot analysis, compound 3e demonstrated excellent anti-proliferative activity against solid tumor cells and clear accumulation of ubiquitinated cellular proteins. Furthermore, in the microsomal stability assay compound 3e demonstrated much improved metabolic stability compared to bortezomib, emerging as a promising lead compound for further design of non-peptide proteasome inhibitors.

Synthesis and biological evaluation of terminal functionalized thiourea-containing dipeptides as antitumor agents

Terminal functionalized dipeptide derivatives containing the thiourea moiety were synthesized and evaluated for antitumor activity. Representative compoundI-11induced apoptosis by the ROS-dependent endoplasmic reticulum pathway in NCI-H460 cells.

Novel copper complexes as potential proteasome inhibitors for cancer treatment (Review)

The use of metal complexes in the pharmaceutical industry has recently increased and as a result, novel metal‑based complexes have initiated an interest as potential anticancer agents. Copper (Cu), which is an essential trace element in all living organisms, is important in maintaining the function of numerous proteins and enzymes. It has recently been demonstrated that Cu complexes may be used as tumor‑specific proteasome inhibitors and apoptosis inducers, by targeting the ubiquitin‑proteasome pathway (UPP). Cu complexes have demonstrated promising results in preclinical studies. The UPP is important in controlling the expression, activity and location of various proteins. Therefore, selective proteasome inhibition and apoptotic induction in cancer cells have been regarded as potential anticancer strategies. The present short review discusses recent progress in the development of Cu complexes, including clioquinol, dithiocarbamates and Schiff bases, as proteasome inhibitors for cancer treatment. A discussion of recent research regarding the understanding of metal inhibitors based on Cu and ligand platforms is presented.

New Peptidomimetic Boronates for Selective Inhibition of the Chymotrypsin-like Activity of the 26S Proteasome

Proteasome is a large proteinase complex that degrades proteins via its three catalytic activities. Among these activities, the "chymotrypsin-like" activity has emerged as the focus of drug discovery in cancer therapy. Here, we report new peptidomimetic boronates that are highly specific for the chymotrypsin-like catalytic activity of the proteasome. These new specific proteasome inhibitors were demonstrated to have higher in vitro potency and selective cytotoxicity for cancer cells compared to benchmark proteasome inhibitors: bortezomib and carfilzomib. In breast cancer cell lines, treatment with 1a or 2a induced accumulation of the high molecular weight polyubiqutinated proteins at similar levels observed for bortezomib and carfilzomib, indicating that cancer cell death caused by 1a/2a is chiefly due to proteasome inhibition.

Post-Transplantation Cyclophosphamide and Ixazomib Combination Rescues Mice Subjected to Experimental Graft-versus-Host Disease and Is Superior to Either Agent Alone

Lapses in the prevention of graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (HSCT) warrant novel approaches. Such approaches include, among others, the use of post-transplantation cyclophosphamide (PTC) and proteasome inhibitors. Although PTC alone consistently produces low rates of chronic GVHD, the incidence of acute GVHD remains significant. Inversely, prolonged post-transplantation administration of proteasome inhibitors carries a risk of paradoxical aggravation of GVHD. We examined whether the combination of cyclophosphamide and ixazomib addresses the limitations of each of these agents when used alone to prevent GVHD in mice subjected to allogeneic HSCT across MHC barriers. We chose ixazomib, an orally bioavailable proteasome inhibitor, because of its favorable physiochemical characteristics. The combination of cyclophosphamide and ixazomib improved overall survival of mice in comparison to an untreated control group and to groups receiving either cyclophosphamide alone or ixazomib alone. Furthermore, cyclophosphamide prevented the surge of IL-1β, GVHD aggravation, and sudden death associated with prolonged administration of ixazomib after HSCT. Finally, we demonstrated that although ixazomib was administered before cyclophosphamide, it did not impair the preferential depletion of proliferating as opposed to resting donor T cells. Our data suggest that the combination of cyclophosphamide and ixazomib for the prevention of GVHD after allogeneic HSCT is promising and merits further investigation in clinical trials.

The Role of Panobinostat Plus Bortezomib and Dexamethasone in Treating Relapsed or Relapsed and Refractory Multiple Myeloma: A European Perspective

Panobinostat is an oral pan-histone deacetylase inhibitor developed by Novartis. Panobinostat acts via epigenetic modification and inhibition of the aggresome pathway. In August 2015, the European Commission authorized panobinostat for use in combination with bortezomib and dexamethasone for the treatment of relapsed or relapsed and refractory multiple myeloma (MM) in patients who have received ≥2 prior regimens including bortezomib and an immunomodulatory drug. In January 2016, the National Institute for Health and Care Excellence recommended panobinostat for use in the same combination and patient population. The authorization and recommendation were based on results from the pivotal phase 3 PANORAMA 1 (NCT01023308) clinical trial, which demonstrated an improvement in median progression-free survival of 7.8 months for the three-drug combination compared with placebo plus bortezomib and dexamethasone in this patient population. This review will discuss the current treatment landscape for relapsed/refractory MM, the mechanism of action of panobinostat, clinical data supporting the European authorization, concerns about safety and strategies for mitigating toxicity, and how panobinostat fits into the current MM landscape in Europe.

FUNDING

Editorial support, funded by Novartis Pharmaceuticals.

Emodin potentiates the antiproliferative effect of interferon α/β by activation of JAK/STAT pathway signaling through inhibition of the 26S proteasome

The 26S proteasome is a negative regulator of type I interferon (IFN-α/β) signaling. Inhibition of the 26S proteasome by small molecules may be a new strategy to enhance the efficacy of type I IFNs and reduce their side effects. Using cell-based screening assay for new 26S proteasome inhibitors, we found that emodin, a natural anthraquinone, was a potent inhibitor of the human 26S proteasome. Emodin preferably inhibited the caspase-like and chymotrypsin-like activities of the human 26S proteasome and increased the ubiquitination of endogenous proteins in cells. Computational modeling showed that emodin exhibited an orientation/conformation favorable to nucleophilic attack in the active pocket of the β1, β2, and β5 subunits of the 26S proteasome. Emodin increased phosphorylation of STAT1, decreased phosphorylation of STAT3 and increased endogenous gene expression stimulated by IFN-α. Emodin inhibited IFN-α-stimulated ubiquitination and degradation of type I interferon receptor 1 (IFNAR1). Emodin also sensitized the antiproliferative effect of IFN-α in HeLa cervical carcinoma cells and reduced tumor growth in Huh7 hepatocellular carcinoma-bearing mice. These results suggest that emodin potentiates the antiproliferative effect of IFN-α by activation of JAK/STAT pathway signaling through inhibition of 26S proteasome-stimulated IFNAR1 degradation. Therefore, emodin warrants further investigation as a new means to enhance the efficacy of IFN-α/β.

Randomized phase 2 trial of ixazomib and dexamethasone in relapsed multiple myeloma not refractory to bortezomib

Proteasome inhibitors have become an integral part of myeloma therapy. Considerable efforts have gone into optimizing this therapeutic approach to obtain maximal proteasome inhibition with least toxicity. Ixazomib is the first oral proteasome inhibitor to enter the clinic and has been studied as a single agent as well as in various combinations. The current trial was designed to examine the efficacy and toxicity of combining 2 different doses of ixazomib (4 mg and 5.5 mg given weekly for 3 of 4 weeks) with 40 mg weekly of dexamethasone, in relapsed myeloma. Seventy patients were enrolled, 35 patients randomly assigned to each ixazomib dose. Overall, 30 (43%; 95% confidence interval, 31-55) of the patients achieved a confirmed partial response or better, with 31% achieving a response with 4 mg and 54% with 5.5 mg of ixazomib. The median event-free survival (EFS) for the entire study population was 8.4 months; 1-year overall survival was 96%. The EFS was 5.7 months for patients with prior bortezomib exposure and 11.0 months for bortezomib-naïve patients. A grade 3 or 4 adverse event considered at least possibly related to treatment was seen in 11 (32%) patients at 4 mg and in 21 (60%) at 5.5 mg. Dose reductions were more frequent with 5.5 mg dose. Overall, the ixazomib with dexamethasone has good efficacy in relapsed myeloma, is well-tolerated and with higher response rate at 5.5 mg, albeit with more toxicity. This study was registered at www.clinicaltrials.gov as #NCT01415882.

Enantioselective synthesis of adamantylalanine and carboranylalanine and their incorporation into the proteasome inhibitor bortezomib

The incorporation of adamantylalanine and carboranylalanine at the P2 site of bortezomib is well tolerated and provided potent cell permeable proteasome inhibitors with increased off-rates compared to bortezomib. Adamantylalanine and carboranylalanine were synthesized enantioselectively by an asymmetric Strecker reaction on Ellmans tert-butyl sulfinimines.

Proteasome inhibitors bortezomib and carfilzomib used for the treatment of multiple myeloma do not inhibit the serine protease HtrA2/Omi

The proteasome inhibitor bortezomib is associated with the development of peripheral neuropathy in patients, but the mechanism is not fully understood.

The proteasome inhibitor bortezomib is associated with the development of peripheral neuropathy in patients, but the mechanism by which bortezomib can induce peripheral neuropathy is not fully understood. One study suggested that off-target inhibition of proteases other than the proteasome, particularly HtraA2/Omi, may be the underlying mechanism of the neuropathy. The same study also concluded that carfilzomib, a second proteasome inhibitor that is associated with less peripheral neuropathy in patients than bortezomib, showed no inhibition of HtrA2/Omi. The goal of the work described here was to determine whether either proteasome inhibitors truly affected HtrA2/Omi activity. A variety of methods were used to test the effects of both bortezomib and carfilzomib on HtrA2/Omi activity that included in vitro recombinant enzyme assays, and studies with the human neuroblastoma SH-SY5Y cell line and HtrA2/Omi-knockout mouse embryonic fibroblasts. The compound ucf-101 was used to assess the effects of specific HtrA2/Omi inhibition. In contrast to previously published data, our results clearly demonstrated that neither bortezomib nor carfilzomib inhibited HtrA2/Omi activity in recombinant enzyme assays at concentrations up to 100 μM, while the specific inhibitor ucf-101 did inhibit the enzyme. The proteasome inhibitors did not inhibit HtrA2/Omi activity in either SH-SY5Y cells or mouse embryonic fibroblasts, as determined by expression of the HtrA2/Omi substrates eIF4G1 and UCH-L1. Based on our biochemical and cell-based assays, we conclude that neither bortezomib nor carfilzomib inhibited HtrA2/Omi activity. Therefore, it is unlikely that bortezomib associated peripheral neuropathy is a direct result of off-target inhibition of HtrA2/Omi.

Recent advances in understanding multiple myeloma

There have been major recent advancements in the understanding and management of multiple myeloma. Diagnostic criteria have been revised and former ultra-high-risk smoldering multiple myeloma is now considered multiple myeloma in need of treatment. Understanding clonal progression, evolution, and tides not only has helped elucidate the disease behavior but might help expand therapeutic choices in order to select appropriate treatment for patients. Unprecedented response rates with modern triplet induction therapies containing proteasome inhibitor and immunomodulators have made this approach standard for initial treatment. The US Food and Drug Administration approved four new drugs (two targeted antibodies and two oral agents) in 2015 in relapsed/refractory multiple myeloma and these drugs along with the other already-available drugs have now increased the choices of regimens. Even drugs without single-agent activity, such as panobinostat and elotuzumab, have an important role, especially in the proteasome inhibitor refractory setting. Recent studies done in the context of novel agent induction suggest that high-dose therapy followed by autologous transplant continues to improve response rates and progression-free survival, thus underscoring their role in transplant-eligible patients. Evolving paradigms in the treatment of multiple myeloma include newer promising immune approaches, such as adoptive cellular therapies, vaccines, or antibody-based immune manipulations. Though multiple myeloma is still considered incurable, it is clear that with the improved understanding of disease biology and clonal architecture of relapse combined with the availability of multi-targeted approaches, we are ever closer to a lasting cure or transformation into indolent and long-lasting disease courses or both.

NF-κB dysregulation in multiple myeloma

The nuclear factor-κB (NF-κB) transcription factor family plays critical roles in the pathophysiology of hematologic neoplasias, including multiple myeloma. The current review examines the roles that this transcription factor system plays in multiple myeloma cells and the nonmalignant accessory cells of the local microenvironment; as well as the evidence indicating that a large proportion of myeloma patients harbor genomic lesions which perturb diverse genes regulating the activity of NF-κB. This article also discusses the therapeutic targeting of the NF-κB pathway using proteasome inhibitors, a pharmacological class that has become a cornerstone in the therapeutic management of myeloma; and reviews some of the future challenges and opportunities for NF-κB-related research in myeloma.

Activation of Chymotrypsin-Like Activity of the Proteasome during Ischemia Induces Myocardial Dysfunction and Death

Inhibitors of the ubiquitin-proteasome system improve hemodynamic parameters and decrease the infarct size after ischemia reperfusion. The molecular basis of this protection is not fully understood since most available data report inhibition of the 26 proteasome after ischemia reperfusion. The decrease in cellular ATP levels during ischemia leads to the dissociation of the 26S proteasome into the 19S regulatory complex and the 20S catalytic core, which results in protein degradation independently of ubiquitination. There is scarce information on the activity of the 20S proteasome during cardiac ischemia. Accordingly, the aim of this work was to determine the effects of 30 minutes of ischemia, or 30 min of ischemia followed by 60 minutes of reperfusion on the three main peptidase activities of the 20S proteasome in Langendorff perfused rat hearts. We found that 30 min of ischemia produced a significant increase in the chymotrypsin-like activity of the proteasome, without changes in its caspase-like or trypsin-like activities. In contrast, all three activities were decreased upon reperfusion. Ixazomib, perfused before ischemia at a concentration that reduced the chymotrypsin-like activity to 50% of the control values, without affecting the other proteasomal activities, improved the hemodynamic parameters upon reperfusion and decreased the infarct size. Ixazomib also prevented the 50% reduction in RyR2 content observed after ischemia. The protection was lost, however, when simultaneous inhibition of chymotrypsin-like and caspase-like activities of the proteasome was achieved at higher concentration of ixazomib. Our results suggest that selective inhibition of chymotrypsin-like activity of the proteasome during ischemia preserves key proteins for cardiomyocyte function and exerts a positive impact on cardiac performance after reperfusion.

The next generation of novel therapies for the management of relapsed multiple myeloma

The advent of various novel therapies such as immunomodulators and proteasome inhibitors has transformed the treatment paradigm for patients with multiple myeloma (MM). As a result, the overall survival has improved dramatically over the last decade. Despite these advances, MM remains mostly incurable and most patients experience disease relapse after enjoying a period of disease control or remission. Fortunately, the scientific community continues to make strides in developing 'next-generation' therapies for the management of patients with relapsed MM. This review will summarize the efficacy of some of the newest therapeutic agents available for the treatment of patients with relapsed MM after their upfront treatment with the original novel agents such as thalidomide, lenalidomide and bortezomib.

Structure-Based Design of β5c Selective Inhibitors of Human Constitutive Proteasomes

This work reports the development of highly potent and selective inhibitors of the β5c catalytic activity of human constitutive proteasomes. The work describes the design principles, large hydrophobic P3 residue and small hydrophobic P1 residue, that led to the synthesis of a panel of peptide epoxyketones; their evaluation and the selection of the most promising compounds for further analyses. Structure-activity relationships detail how in a logical order the β1c/i, β2c/i, and β5i activities became resistant to inhibition as compounds were diversified stepwise. The most effective compounds were obtained as a mixture of cis- and trans-biscyclohexyl isomers, and enantioselective synthesis resolved this issue. Studies on yeast proteasome structures complexed with some of the compounds provide a rationale for the potency and specificity. Substitution of the N-terminus in the most potent compound for a more soluble equivalent led to a cell-permeable molecule that selectively and efficiently blocks β5c in cells expressing both constitutive proteasomes and immunoproteasomes.

Computational Approaches for the Discovery of Human Proteasome Inhibitors: An Overview

Proteasome emerged as an important target in recent pharmacological research due to its pivotal role in degrading proteins in the cytoplasm and nucleus of eukaryotic cells, regulating a wide variety of cellular pathways, including cell growth and proliferation, apoptosis, DNA repair, transcription, immune response, and signaling processes. The last two decades witnessed intensive efforts to discover 20S proteasome inhibitors with significant chemical diversity and efficacy. To date, the US FDA approved to market three proteasome inhibitors: bortezomib, carfilzomib, and ixazomib. However new, safer and more efficient drugs are still required. Computer-aided drug discovery has long being used in drug discovery campaigns targeting the human proteasome. The aim of this review is to illustrate selected in silico methods like homology modeling, molecular docking, pharmacophore modeling, virtual screening, and combined methods that have been used in proteasome inhibitors discovery. Applications of these methods to proteasome inhibitors discovery will also be presented and discussed to raise improvements in this particular field.

Next-generation proteasome inhibitor MLN9708 sensitizes breast cancer cells to doxorubicin-induced apoptosis

Doxorubicin (Dox), one of the most effective chemotherapy drug for cancer treatment, is limited by its severe side effects and chemoresistance. Dox induces DNA damage and leads to significant proteomic changes in the cancer cells, which makes the ubiquitin-proteasome system a potential target to enhance the efficacy of Dox therapy. The unsuccessful clinical trials of proteasome inhibitor PS-341 (bortezomib) in solid tumors led to the invention of MLN9708 (ixazomib), an orally bioavailable next-generation proteasome inhibitor with improved pharmacokinetic and pharmacodynamic features. In this preclinical study, we used eight human breast cancer cell lines, which represent the major molecular subtypes of breast cancer, to validate the cytotoxic effects of MLN9708, alone and in combination with Dox. We found that MLN9708 had cytotoxic effects, induced autophagy and MKP-1 expression, and enhanced Dox-induced apoptosis in these cell lines. MLN9708 also enhanced Dox-induced JNK and p38 phosphorylation and inhibited Dox-induced IκBα degradation. Our in vitro results suggest that MLN9708 has antitumor effects in breast cancer and can sensitize breast cancer cells to Dox treatment. This promising combination may be an effective and feasible therapeutic option for treating breast cancer and warrants clinical validation.

Oral Ixazomib, Lenalidomide, and Dexamethasone for Multiple Myeloma

BACKGROUND

Ixazomib is an oral proteasome inhibitor that is currently being studied for the treatment of multiple myeloma.

METHODS

In this double-blind, placebo-controlled, phase 3 trial, we randomly assigned 722 patients who had relapsed, refractory, or relapsed and refractory multiple myeloma to receive ixazomib plus lenalidomide-dexamethasone (ixazomib group) or placebo plus lenalidomide-dexamethasone (placebo group). The primary end point was progression-free survival.

RESULTS

Progression-free survival was significantly longer in the ixazomib group than in the placebo group at a median follow-up of 14.7 months (median progression-free survival, 20.6 months vs. 14.7 months; hazard ratio for disease progression or death in the ixazomib group, 0.74; P=0.01); a benefit with respect to progression-free survival was observed with the ixazomib regimen, as compared with the placebo regimen, in all prespecified patient subgroups, including in patients with high-risk cytogenetic abnormalities. The overall rates of response were 78% in the ixazomib group and 72% in the placebo group, and the corresponding rates of complete response plus very good partial response were 48% and 39%. The median time to response was 1.1 months in the ixazomib group and 1.9 months in the placebo group, and the corresponding median duration of response was 20.5 months and 15.0 months. At a median follow-up of approximately 23 months, the median overall survival has not been reached in either study group, and follow-up is ongoing. The rates of serious adverse events were similar in the two study groups (47% in the ixazomib group and 49% in the placebo group), as were the rates of death during the study period (4% and 6%, respectively); adverse events of at least grade 3 severity occurred in 74% and 69% of the patients, respectively. Thrombocytopenia of grade 3 and grade 4 severity occurred more frequently in the ixazomib group (12% and 7% of the patients, respectively) than in the placebo group (5% and 4% of the patients, respectively). Rash occurred more frequently in the ixazomib group than in the placebo group (36% vs. 23% of the patients), as did gastrointestinal adverse events, which were predominantly low grade. The incidence of peripheral neuropathy was 27% in the ixazomib group and 22% in the placebo group (grade 3 events occurred in 2% of the patients in each study group). Patient-reported quality of life was similar in the two study groups.

CONCLUSIONS

The addition of ixazomib to a regimen of lenalidomide and dexamethasone was associated with significantly longer progression-free survival; the additional toxic effects with this all-oral regimen were limited. (Funded by Millennium Pharmaceuticals; TOURMALINE-MM1 ClinicalTrials.gov number, NCT01564537.).

Exposure-safety-efficacy analysis of single-agent ixazomib, an oral proteasome inhibitor, in relapsed/refractory multiple myeloma: dose selection for a phase 3 maintenance study

Background Ixazomib is the first oral, small molecule proteasome inhibitor to reach phase 3 trials. The current analysis characterized the exposure-safety and exposure-efficacy relationships of ixazomib in patients with relapsed/refractory multiple myeloma (MM) with a purpose of recommending an approach to ixazomib dosing for maintenance therapy. Methods Logistic regression was used to investigate relationships between ixazomib plasma exposure (area under the curve/day; derived from individual apparent clearance values from a published population pharmacokinetic analysis) and safety/efficacy outcomes (hematologic [grade ≥ 3 vs ≤ 2] or non-hematologic [grade ≥ 2 vs ≤ 1] adverse events [AEs], and clinical benefit [≥stable disease vs progressive disease]) using phase 1 data in relapsed/refractory MM (NCT00963820; N = 44). Results Significant relationships to ixazomib exposure were observed for five AEs (neutropenia, thrombocytopenia, rash, fatigue, and diarrhea) and clinical benefit (p < 0.05). Dose–response relationships indicated a favorable benefit/risk ratio at 3 mg and 4 mg weekly, which are below the maximum tolerated dose of 5.5 mg. At 3 mg, the model predicted that: 37 % of patients will achieve clinical benefit; incidence of grade ≥ 3 neutropenia and thrombocytopenia will be 10 % and 23 %, respectively; and incidence of grade ≥ 2 rash, fatigue, and diarrhea will be 8 %, 19 %, and 19 %, respectively. Conclusions Based on the findings, patients in the phase 3 maintenance trial will initiate ixazomib at a once-weekly dose of 3 mg, increasing to 4 mg if acceptable tolerability after 4 cycles, to provide maximum clinical benefit balanced with adequate tolerability.

Oral proteasome inhibitor with strong preclinical efficacy in myeloma models

BACKGROUND

The proteasome is a validated anti-cancer target and various small-molecule inhibitors are currently in clinical development or on the market. However, adverse events and resistance associated with those proteasome inhibitors indicate the need for a new generation of drugs. Therefore, we focused on developing an oral proteasome inhibitor with improved efficacy and safety profiles.

METHOD

The in vitro inhibition of the 20S proteasome catalytic activities was determined in human multiple myeloma (MM) cellular lysates with fluorogenic peptide substrates specific for each catalytic subunit. Cell cytotoxicity was assessed with the ATP bioluminescence assay using human cell samples from tumor cell lines, MM patients or normal healthy donors. In mice bearing human MM xenografts, a single dose of LC53-0110 was administered orally, and concentration-time profiles of LC53-0110 and the 20S proteasome catalytic activities in plasma, blood, and tumor were determined. The efficacy of repeat-dose compound with regard to tumor growth inhibition in vivo was also evaluated in the same MM xenograft models.

RESULTS

LC53-0110 is far more specific for the chymotrypsin-like proteolytic (β5) site of the 20S proteasome as compared to bortezomib, carfilzomib, or ixazomib. LC53-0110 treatment showed accumulation of ubiquitinated proteins, inhibited cell viability with a low nM range potency in various tumor cell lines, and showed potent activity on CD138(+) cells isolated from MM patients who are resistant/refractory to current FDA-approved drug treatment. When a single dose was administered orally to tumor-bearing mice, LC53-0110 showed both greater maximum and sustained tumor proteasome inhibition as compared with ixazomib in MM xenograft models. The robust pharmacodynamic responses in tumor correlated with tumor growth regression. In addition, LC53-0151, an analog of LC53-0110, in combination with pomalidomide, a third-generation immunomodulatory drug, showed synergistic inhibition of tumor growth both in vitro and in the xenograft mouse model.

CONCLUSIONS

In view of the in vitro, in vivo, and ex vivo profiles, further investigation of additional LC compounds in preclinical studies is warranted for the nomination of a clinical development candidate.

The Effect of a High-Fat Meal on the Pharmacokinetics of Ixazomib, an Oral Proteasome Inhibitor, in Patients With Advanced Solid Tumors or Lymphoma

Ixazomib is the first oral proteasome inhibitor to be investigated in the clinic. This clinical study assessed whether the pharmacokinetics of ixazomib would be altered if administered after a high‐calorie, high‐fat meal. In a 2‐period, 2‐sequence, crossover study design, adult patients with advanced solid tumors or lymphoma received a 4‐mg oral dose of ixazomib as immediate‐release capsules on day 1 without food (fasted, administered following an overnight fast) or with food (fed, following consumption of a high‐calorie, high‐fat meal), followed by another dose on day 15 in the alternate food intake condition (fasted to fed or fed to fasted). Twenty‐four patients were enrolled; of these, 15 were included in the pharmacokinetic‐evaluable population. Administration of ixazomib after a high‐fat meal reduced both the rate and extent of absorption of ixazomib. Under fed conditions, the median time to peak plasma concentration (T_max) of ixazomib was delayed by approximately 3 hours compared with administration in the fasted state (1.02 hours vs 4.0 hours), and there was a 28% reduction in total systemic exposure (area under the curve, AUC) and a 69% reduction in peak plasma concentration (C_max). Together, the results support the administration of ixazomib on an empty stomach, at least 1 hour before or at least 2 hours after food. These recommendations are reflected in the United States Prescribing Information for ixazomib (clinicaltrials.gov identifier NCT01454076).

Proteasomal Inhibition by Ixazomib Induces CHK1 and MYC-Dependent Cell Death in T-cell and Hodgkin Lymphoma

Proteasome-regulated NF-κB has been shown to be important for cell survival in T-cell lymphoma and Hodgkin lymphoma models. Several new small-molecule proteasome inhibitors are under various stages of active preclinical and clinical development. We completed a comprehensive preclinical examination of the efficacy and associated biologic effects of a second-generation proteasome inhibitor, ixazomib, in T-cell lymphoma and Hodgkin lymphoma cells and in vivo SCID mouse models. We demonstrated that ixazomib induced potent cell death in all cell lines at clinically achievable concentrations. In addition, it significantly inhibited tumor growth and improved survival in T-cell lymphoma and Hodgkin lymphoma human lymphoma xenograft models. Through global transcriptome analyses, proteasomal inhibition showed conserved overlap in downregulation of cell cycle, chromatin modification, and DNA repair processes in ixazomib-sensitive lymphoma cells. The predicted activity for tumor suppressors and oncogenes, the impact on "hallmarks of cancer," and the analysis of key significant genes from global transcriptome analysis for ixazomib strongly favored tumor inhibition via downregulation of MYC and CHK1, its target genes. Furthermore, in ixazomib-treated lymphoma cells, we identified that CHK1 was involved in the regulation of MYC expression through chromatin modification involving histone H3 acetylation via chromatin immunoprecipitation. Finally, using pharmacologic and RNA silencing of CHK1 or the associated MYC-related mechanism, we demonstrated synergistic cell death in combination with antiproteasome therapy. Altogether, ixazomib significantly downregulates MYC and induces potent cell death in T-cell lymphoma and Hodgkin lymphoma, and we identified that combinatorial therapy with anti-CHK1 treatment represents a rational and novel therapeutic approach. Cancer Res; 76(11); 3319-31. ©2016 AACR.

Novel strategies to target the ubiquitin proteasome system in multiple myeloma

Multiple myeloma (MM) is a hematological malignancy characterized by the accumulation of plasma cells in the bone marrow (BM). The success of the proteasome inhibitor bortezomib in the treatment of MM highlights the importance of the ubiquitin proteasome system (UPS) in this particular cancer. Despite the prolonged survival of MM patients, a significant amount of patients relapse or become resistant to therapy. This underlines the importance of the development and investigation of novel targets to improve MM therapy. The UPS plays an important role in different cellular processes by targeted destruction of proteins. The ubiquitination process consists of enzymes that transfer ubiquitin to proteins targeting them for proteasomal degradation. An emerging and promising approach is to target more disease specific components of the UPS to reduce side effects and overcome resistance. In this review, we will focus on different components of the UPS such as the ubiquitin activating enzyme E1, the ubiquitin conjugating enzyme E2, the E3 ubiquitin ligases, the deubiquitinating enzymes (DUBs) and the proteasome. We will discuss their role in MM and the implications in drug discovery for the treatment of MM.

Switching from body surface area-based to fixed dosing for the investigational proteasome inhibitor ixazomib: a population pharmacokinetic analysis

AIMS

This population pharmacokinetic analysis of the investigational oral proteasome inhibitor ixazomib assessed the feasibility of switching from body surface area (BSA)-based to fixed dosing, and the impact of baseline covariates on ixazomib pharmacokinetics.

METHODS

Data were pooled from 226 adult patients with multiple myeloma, lymphoma or solid tumours in four phase 1 studies, in which ixazomib dosing (oral/intravenous, once/twice weekly) was based on BSA. Population pharmacokinetic modelling was undertaken using nonmem version 7.2.

RESULTS

Ixazomib pharmacokinetics were well described by a three compartment model with first order absorption and linear elimination. Ixazomib was absorbed rapidly (Ka 0.5 h(-1)), with dose- and time-independent pharmacokinetics. Estimated absolute bioavailability and clearance were 60% and 2l h(-1), respectively. Although a small effect of BSA (range 1.3-2.6 m(2)) was observed on the peripheral volume of distribution (V4), reducing the corresponding inter-individual variability by 12.9%, there was no relationship between BSA and ixazomib clearance (the parameter that dictates total systemic exposure following fixed dosing). Consistently, based on simulations (n = 1000), median AUCs (including interquartile range) were similar after BSA-based (2.23 mg m(-2)) and fixed (4 mg) oral dosing with no trend in simulated AUC vs. BSA for fixed dosing (P = 0.42). No other covariates, including creatinine clearance (22-213.7 ml min(-1)) and age (23-86 years), influenced ixazomib pharmacokinetics.

CONCLUSIONS

This analysis supports a switch from BSA-based to fixed dosing, without dose modification for mild/moderate renal impairment or age, in future adult studies of ixazomib, simplifying dosing guidance and clinical development.

Spotlight on ixazomib: potential in the treatment of multiple myeloma

Despite the significant therapeutic advances achieved with proteasome inhibitors (PIs) such as bortezomib and carfilzomib in prolonging the survival of patients with multiple myeloma, the development of drug resistance, peripheral neuropathy, and pharmacokinetic limitations continue to pose major challenges when using these compounds. Ixazomib is a second-generation PI with improved activity over other PIs. Unlike bortezomib and carfilzomib, which are administered by injection, ixazomib is the first oral PI approved by US Food and Drug Administration. This review discusses the biochemical properties, mechanisms of action, preclinical efficacy, and clinical trial results leading to the US Food and Drug Administration approval of ixazomib.

KRAS Genotype Correlates with Proteasome Inhibitor Ixazomib Activity in Preclinical In Vivo Models of Colon and Non-Small Cell Lung Cancer: Potential Role of Tumor Metabolism

In non-clinical studies, the proteasome inhibitor ixazomib inhibits cell growth in a broad panel of solid tumor cell lines in vitro. In contrast, antitumor activity in xenograft tumors is model-dependent, with some solid tumors showing no response to ixazomib. In this study we examined factors responsible for ixazomib sensitivity or resistance using mouse xenograft models. A survey of 14 non-small cell lung cancer (NSCLC) and 6 colon xenografts showed a striking relationship between ixazomib activity and KRAS genotype; tumors with wild-type (WT) KRAS were more sensitive to ixazomib than tumors harboring KRAS activating mutations. To confirm the association between KRAS genotype and ixazomib sensitivity, we used SW48 isogenic colon cancer cell lines. Either KRAS-G13D or KRAS-G12V mutations were introduced into KRAS-WT SW48 cells to generate cells that stably express activated KRAS. SW48 KRAS WT tumors, but neither SW48-KRAS-G13D tumors nor SW48-KRAS-G12V tumors, were sensitive to ixazomib in vivo. Since activated KRAS is known to be associated with metabolic reprogramming, we compared metabolite profiling of SW48-WT and SW48-KRAS-G13D tumors treated with or without ixazomib. Prior to treatment there were significant metabolic differences between SW48 WT and SW48-KRAS-G13D tumors, reflecting higher oxidative stress and glucose utilization in the KRAS-G13D tumors. Ixazomib treatment resulted in significant metabolic regulation, and some of these changes were specific to KRAS WT tumors. Depletion of free amino acid pools and activation of GCN2-eIF2α-pathways were observed both in tumor types. However, changes in lipid beta oxidation were observed in only the KRAS WT tumors. The non-clinical data presented here show a correlation between KRAS genotype and ixazomib sensitivity in NSCLC and colon xenografts and provide new evidence of regulation of key metabolic pathways by proteasome inhibition.

New pharmacotherapy options for multiple myeloma

INTRODUCTION

Novel agents and the availability of autologous stem-cell transplantation have revolutionized the treatment of patients with multiple myeloma. First-generation novel agents namely thalidomide, lenalidomide, and bortezomib have significantly improved response and survival of patients. Second-generation novel agents such as pomalidomide, carfilzomib, and monoclonal antibodies are being tested both in the newly diagnosed and relapse settings, and results are promising.

AREAS COVERED

In this review article, the main results derived from Phase III trials with thalidomide, lenalidomide, and bortezomib for the treatment of myeloma patients, both at diagnosis and at relapse, are summarized. Data about second-generation novel agents such as pomalidomide and carfilzomib are also reported. Newer effective drugs currently under investigation and the promising results with monoclonal antibodies are described.

EXPERT OPINION

The availability of new effective drugs has considerably increased the treatment options for myeloma patients. A sequential approach including induction, transplantation (when possible), consolidation, and maintenance is an optimal strategy to achieve disease control and prolong survival. Despite these improvements, the best combination, the optimal sequence, and the proper target of newer drugs need to be defined.

Selective Toxicity of Investigational Ixazomib for Human Leukemia Cells Expressing Mutant Cytoplasmic NPM1: Role of Reactive Oxygen Species

PURPOSE

This study was performed to determine whether the investigational proteasome inhibitor ixazomib demonstrated selective antineoplastic activity against acute myelogenous leukemia cells expressing a mutated nucleophosmin-1 gene and to gain a better understanding of its mechanisms of action.

EXPERIMENTAL DESIGN

The cytotoxic effects of ixazomib treatment were analyzed in human acute myelogenous leukemia (AML) cell lines and primary AML samples expressing wild-type or mutated NPM1 (NPMc(+)). The potential roles of oxidative stress in mediating cytotoxic activity were determined using flow cytometry, enzyme-based assays, and Western blots.

RESULTS

Apoptosis induced by ixazomib was abrogated by knockdown of NPM1/NPMc(+)expression using an inducible shRNA construct and enhanced by NPMc(+)overexpression. Cytotoxicity was associated with superoxide generation and was reduced by the addition of the antioxidant N-acetylcysteine. AML cells expressing NPMc(+)had significantly reduced levels of intracellular glutathione and NADPH associated with reduced antioxidant responses to drug treatment. Treatment of 3 patients with relapsed NPMc(+)AML resulted in an antileukemic effect in 1 patient as demonstrated by a marked reduction of leukemic blasts in the peripheral blood. Efficacy was associated with superoxide generation, reduced glutathione levels, and reduced mRNA and protein expression of antioxidant effectors in responding cells.

CONCLUSIONS

In this study, a direct association was observed between NPMc(+)expression in AML, reduced antioxidant responses, and enhanced sensitivity to an oral proteasome inhibitor that induces oxidative stress. These data suggest that intracellular determinants of antioxidant responses may be good predictors of therapeutic response to ixazomib.

Targeting ubiquitination for cancer therapies

Ubiquitination, the structured degradation and turnover of cellular proteins, is regulated by the ubiquitin-proteasome system (UPS). Most proteins that are critical for cellular regulations and functions are targets of the process. Ubiquitination is comprised of a sequence of three enzymatic steps, and aberrations in the pathway can lead to tumor development and progression as observed in many cancer types. Recent evidence indicates that targeting the UPS is effective for certain cancer treatment, but many more potential targets might have been previously overlooked. In this review, we will discuss the current state of small molecules that target various elements of ubiquitination. Special attention will be given to novel inhibitors of E3 ubiquitin ligases, especially those in the SCF family.

The therapeutic potential of microbial proteasome inhibitors

The proteasome influences cellular homeostasis through the degradation of regulatory proteins, many of which are also involved in disease pathogenesis. In particular, numerous regulatory proteins associated with tumor growth, such as cyclins, cyclin-dependent kinase inhibitors, tumor suppressors, and NF-κB inhibitors are degraded by the proteasome. Proteasome inhibitors can stabilize these regulatory proteins, resulting in the suppression of tumor development and the regulation of immune responses. Thus, proteasome inhibitors are promising candidate antitumor agents and immune-regulatory agents. Bortezomib is the first-in-class proteasome inhibitor approved for the treatment of multiple myeloma. Despite its high efficiency, however, a large proportion of patients do not attain sufficient clinical response due to toxicity and drug resistance. Therefore, the development of new proteasome inhibitors with improved pharmacological properties is needed. Natural products produced by microorganisms are a promising source of such compounds. This review provides an overview of proteasome inhibitors produced by microorganisms, with special focus on inhibitors isolated from actinomycetes.