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

Role of the RANK/RANKL Pathway in Multiple Myeloma

Receptor activator of nuclear factor-kappa B (RANK) and its ligand, RANKL, are expressed in a variety of tissues throughout the body; their primary role is in the regulation of bone remodeling and development of the immune system. Consistent with these functions, evidence exists for a role of RANK/RANKL in all stages of tumorigenesis, from cell proliferation and carcinogenesis to epithelial-mesenchymal transition to neoangiogenesis and intravasation to metastasis to bone resorption and tumor growth in bone. Results from current studies also point to a role of RANK/RANKL signaling in patients with multiple myeloma, who have increased serum levels of soluble RANKL and an imbalance in RANKL and osteoprotegerin. Current therapies for patients with multiple myeloma demonstrate that RANKL may be released by tumor cells or osteoprogenitor cells. This article will review currently available evidence supporting a role for RANK/RANKL signaling in tumorigenesis, with a focus on patients with multiple myeloma.

Validation of Babesia proteasome as a drug target

Babesiosis is a tick-transmitted zoonosis caused by apicomplexan parasites of the genus Babesia. Treatment of this emerging malaria-related disease has relied on antimalarial drugs and antibiotics. The proteasome of Plasmodium, the causative agent of malaria, has recently been validated as a target for anti-malarial drug development and therefore, in this study, we investigated the effect of epoxyketone (carfilzomib, ONX-0914 and epoxomicin) and boronic acid (bortezomib and ixazomib) proteasome inhibitors on the growth and survival of Babesia. Testing the compounds against Babesia divergens ex vivo revealed suppressive effects on parasite growth with activity that was higher than the cytotoxic effects on a non-transformed mouse macrophage cell line. Furthermore, we showed that the most-effective compound, carfilzomib, significantly reduces parasite multiplication in a Babesia microti infected mouse model without noticeable adverse effects. In addition, treatment with carfilzomib lead to an ex vivo and in vivo decrease in proteasome activity and accumulation of polyubiquitinated proteins compared to untreated control. Overall, our results demonstrate that the Babesia proteasome is a valid target for drug development and warrants the design of potent and selective B. divergens proteasome inhibitors for the treatment of babesiosis.

Next-generation proteasome inhibitors for cancer therapy

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

Ixazomib in the management of relapsed multiple myeloma

The development of proteasome inhibitors contributed to the dramatic life expectancy improvement observed in myeloma patients over the past decades. Ixazomib is a boron-containing selective and reversible proteasome inhibitor that demonstrated antimyeloma activity with excellent safety profile. Ixazomib is the first orally available proteasome inhibitor approved in combination with lenalidomide and dexamethasone for the treatment of myeloma patients who received at least one prior therapy. The present review addresses the current knowledge regarding the clinical use of ixazomib in relapsed myeloma patients.

Phase 1/2 trial of ixazomib, cyclophosphamide and dexamethasone in patients with previously untreated symptomatic multiple myeloma

Ixazomib is the first oral proteasome inhibitor to enter the clinic. Given the efficacy of bortezomib in combination with cyclophosphamide and dexamethasone, we studied the combination of ixazomib, cyclophosphamide and dexamethasone (ICd) in newly diagnosed multiple myeloma (NDMM) and patients with measurable disease, irrespective of transplant eligibility, were enrolled. The phase 1 was to determine the maximum tolerated dose (MTD) of cyclophosphamide in the combination. Patients received ixazomib 4 mg (days 1, 8, 15), dexamethasone 40 mg (days 1, 8, 15, 22), and cyclophosphamide 300 or 400 mg/m² days 1, 8, 15, 22; cycles were 28 days. We enrolled 51 patients, 10 in phase 1 and 41 patients in phase 2. The median age was 64.5 years (range: 41–88); 29% had high or intermediate risk FISH. The MTD was 400 mg/m² of cyclophosphamide weekly. The best confirmed response in all 48 patients included ≥ partial response in 77%, including ≥ VGPR in 35%; 3 patients had a sCR. The response rate for all 48 evaluable patients at 4-cycles was 71%; the median time to response was 1.9 months. Common adverse events included cytopenias, fatigue and GI intolerance. ICd is a convenient, all oral combination that is well tolerated and effective in NDMM.

Proteasome Inhibitors in Waldenström Macroglobulinemia

Waldenström macroglobulinemia (WM) remains an incurable B-cell lymphoproliferative disorder, yet therapy is only considered for patients with symptomatic disease. Primary therapy options for WM include combinations based on anti-CD20 monoclonal antibodies, mainly rituximab. However, proteasome inhibitors have become an important part of WM therapy both as primary therapy and as salvage option. Bortezomib is the proteasome inhibitor most studied and with extensive clinical experience, but new proteasome inhibitors (carfilzomib, ixazomib, oprozomib), with different toxicity profiles, routes of administration, and probably with preserved or improved activity, have become available and may also find their way into WM therapy.

Design, synthesis, in vitro and in vivo evaluation, and structure-activity relationship (SAR) discussion of novel dipeptidyl boronic acid proteasome inhibitors as orally available anti-cancer agents for the treatment of multiple myeloma and mechanism studies

A series of novel dipeptidyl boronic acid inhibitors of 20S proteasome were designed and synthesized. Aliphatic groups at R¹ position were designed for the first time to fully understand the SAR (structure-activity relationship). Among the screened compounds, novel inhibitor 5c inhibited the CT-L (chymotrypsin-like) activity with IC₅₀ of 8.21 nM and the MM (multiple myeloma) cells RPMI8226, U266B and ARH77 proliferations with the IC₅₀ of 8.99, 6.75 and 9.10 nM, respectively, which showed similar in vitro activities compared with the compound MLN2238 (biologically active form of marketed MLN9708). To investigate the oral availability, compound 5c was esterified to its prodrug 6a with the enzymatic IC₅₀ of 6.74 nM and RPMI8226, U266B and ARH77 cell proliferations IC₅₀ of 2.59, 4.32 and 3.68 nM, respectively. Furthermore, prodrug 6a exhibited good pharmacokinetic properties with oral bioavailability of 24.9%, similar with MLN9708 (27.8%). Moreover, compound 6a showed good microsomal stabilities and displayed stronger in vivo anticancer efficacy than MLN9708 in the human ARH77 xenograft mouse model. Finally, cell cycle results showed that compound 6a had a significant inhibitory effect on CT-L and inhibited cell cycle progression at the G2M stage.

Population Pharmacokinetic Analysis of Ixazomib, an Oral Proteasome Inhibitor, Including Data from the Phase III TOURMALINE-MM1 Study to Inform Labelling

Ixazomib is an oral proteasome inhibitor, approved in USA, Canada, Australia and Europe in combination with lenalidomide and dexamethasone, for the treatment of patients with multiple myeloma who have received at least one prior therapy. We report a population pharmacokinetic model-based analysis for ixazomib that was pivotal in describing the clinical pharmacokinetics of ixazomib, to inform product labelling. Plasma concentration–time data were collected from 755 patients who received oral or intravenous ixazomib in once- or twice-weekly schedules in ten trials, including the global phase III TOURMALINE-MM1 study. Data were analysed using nonlinear mixed-effects modelling (NONMEM software version 7.2, ICON Development Solutions, Hanover, MD, USA). Ixazomib plasma concentrations from intravenous and oral studies were described by a three-compartment model with linear distribution and elimination kinetics, including first-order linear absorption with a lag time describing the oral dose data. Body surface area on the volume of the second peripheral compartment was the only covariate included in the final model. None of the additional covariates tested including body surface area (1.2–2.7 m²), sex, age (23–91 years), race, mild/moderate renal impairment and mild hepatic impairment were found to impact systemic clearance, suggesting that no dose adjustment is required based on these covariates. The geometric mean terminal disposition phase half-life was 9.5 days, steady-state volume of distribution was 543 L and systemic clearance was 1.86 L/h. The absolute bioavailability of an oral dose was estimated to be 58%.

Ixazomib: A Review in Relapsed and/or Refractory Multiple Myeloma

The oral proteasome inhibitor ixazomib (Ninlaro^®) is approved in the USA, EU and Japan in combination with lenalidomide and dexamethasone, for the treatment of patients with multiple myeloma (MM) who have received at least one prior therapy. In adults with relapsed and/or refractory MM who had received one to three prior therapies, progression-free survival (PFS) was significantly prolonged in patients who received the ixazomib- versus placebo-based triple therapy in the pivotal, global TOURMALINE-MM1 trial and its regional expansion (China continuation study). A significantly longer time to progression and favourable hazard ratios for PFS were observed across all prespecified subgroups, including patients with high cytogenetic risk. Overall response was achieved in a significantly higher proportion of patients receiving ixazomib- than placebo-based treatment. Ixazomib had a manageable tolerability profile in patients with MM. Ixazomib is the first orally-administered proteasome inhibitor approved for patients with MM, and in combination with lenalidomide and dexamethasone represents an important new option for use in patients with relapsed and/or refractory MM who have previously received at least one prior therapy.

Preclinical comparison of proteasome and ubiquitin E1 enzyme inhibitors in cutaneous squamous cell carcinoma: the identification of mechanisms of differential sensitivity

Proteasome inhibitors have distinct properties and the biochemical consequences of suppressing ubiquitin E1 enzymes and the proteasome differ. We compared the effects of the proteasome inhibitors bortezomib, ixazomib and carfilzomib and the ubiquitin E1 enzyme inhibitor MLN7243/TAK-243 on cell viability and cell death in normal keratinocytes and cutaneous squamous cell carcinoma (cSCC) cell lines. The effects of both a pulse of treatment and more extended incubation were investigated. This is relevant to directly-delivered therapy (topical treatment/intratumoral injection) where the time of exposure can be controlled and a short exposure may better reflect systemically-delivered inhibitor pharmacokinetics. These agents can selectively kill cSCC cells but there are variations in the pattern of cSCC cell line sensitivity/resistance. Variations in the responses to proteasome inhibitors are associated with differences in the specificity of the inhibitors for the three proteolytic activities of the proteasome. There is greater selectivity for killing cSCC cells compared to normal keratinocytes with a pulse of proteasome inhibitor treatment than with a more extended exposure. We provide evidence that c-MYC-dependent NOXA upregulation confers susceptibility to a short incubation with proteasome inhibitors by priming cSCC cells for rapid BAK-dependent death. We observed that bortezomib-resistant cSCC cells can be sensitive to MLN7243-induced death. Low expression of the ubiquitin E1 UBA1/UBE1 participates in conferring susceptibility to MLN7243 by increasing sensitivity to MLN7243-mediated attenuation of ubiquitination. This study supports further investigation of the potential of proteasome and ubiquitin E1 inhibition for cSCC therapy. Direct delivery of inhibitors could facilitate adequate exposure of skin cancers.

[Pharmacological characteristics and clinical study results of the oral proteasome inhibitor ixazomib (NINLARO® capsules; 2.3 mg, 3 mg, and 4 mg)]

Ixazomib (Ninlaro^® capsule) is an oral small molecule 20S proteasome inhibitor created by Millennium Pharmaceuticals, Inc (Takeda Oncology Company). Ubiquitin proteasome system is a major regulatory system for maintaining protein homeostasis, and an important mechanism for degrading proteins, such as those involved in proliferation regulation, cell cycle regulation and apoptosis, in cells. Ixazomib selectively and reversibly binds to the β5 subunit of the 20S proteasome, inhibits its chymotrypsin-like activity, and thereby accumulates ubiquitinated proteins. It induces ER stress and apoptosis of myeloma cells. The phase 3, randomized, double-blind, multicenter global study (TOURMALINE-MM1) in patients with relapsed and/or refractory multiple myeloma, who have received 1 to 3 prior lines of therapy, showed that addition of ixazomib to lenalidomide-dexamethasone (ixazomib-Rd) demonstrated significant improvement in progression-free survival (hazard ratio = 0.742, P = 0.012) versus placebo-Rd (20.6 vs. 14.7 months in the median) (data cut-off as of October 30, 2014). Ixazomib has been approved by the United States Food and Drug Administration in November 2015, and the European Medicines Agency in November 2016 for the treatment of multiple myeloma (MM) patients who have received at least one prior therapy. In Japan, ixazomib was approved for the treatment of relapsed and/or refractory MM in March, 2017. It is expected to demonstrate that the oral proteasome inhibitor ixazomib is an effective and convenient treatment option in clinical practice.

Proteasome-associated deubiquitinases and cancer

Maintenance of protein homeostasis is a crucial process for the normal functioning of the cell. The regulated degradation of proteins is primarily facilitated by the ubiquitin proteasome system (UPS), a system of selective tagging of proteins with ubiquitin followed by proteasome-mediated proteolysis. The UPS is highly dynamic consisting of both ubiquitination and deubiquitination steps that modulate protein stabilization and degradation. Deregulation of protein stability is a common feature in the development and progression of numerous cancer types. Simultaneously, the elevated protein synthesis rate of cancer cells and consequential accumulation of misfolded proteins drives UPS addiction, thus sensitizing them to UPS inhibitors. This sensitivity along with the potential of stabilizing pro-apoptotic signaling pathways makes the proteasome an attractive clinical target for the development of novel therapies. Targeting of the catalytic 20S subunit of the proteasome is already a clinically validated strategy in multiple myeloma and other cancers. Spurred on by this success, promising novel inhibitors of the UPS have entered development, targeting the 20S as well as regulatory 19S subunit and inhibitors of deubiquitinating and ubiquitin ligase enzymes. In this review, we outline the manner in which deregulation of the UPS can cause cancer to develop, current clinical application of proteasome inhibitors, and the (pre-)clinical development of novel inhibitors of the UPS.

Molecular Mechanisms of the Cardiotoxicity of the Proteasomal-Targeted Drugs Bortezomib and Carfilzomib

Bortezomib and carfilzomib are anticancer drugs that target the proteasome. However, these agents have been shown to exhibit some specific cardiac toxicities by as yet unknown mechanisms. Bortezomib and carfilzomib are also being used clinically in combination with doxorubicin, which is also cardiotoxic. A primary neonatal rat myocyte model was used to study these cardiotoxic mechanisms. Exposure to submicromolar concentrations of bortezomib and carfilzomib resulted in significant myocyte damage and induced apoptosis. Both bortezomib and carfilzomib inhibited the chymotrypsin-like proteasomal activity of myocyte lysate in the low nanomolar concentration range and exhibited time-dependent inhibition kinetics. The high sensitivity of myocytes, which were determined to contain high specific levels of chymotrypsin-like proteasomal activity, to the damaging effects of bortezomib and carfilzomib was likely due to the inhibition of proteasomal-dependent ongoing sarcomeric protein turnover. A brief preexposure of myocytes to non-toxic nanomolar concentrations of bortezomib or carfilzomib greatly increased doxorubicin-mediated damage, which suggests that the combination of doxorubicin with either bortezomib or carfilzomib may produce more than additive cardiotoxicity. The doxorubicin cardioprotective agent dexrazoxane partially protected myocytes from doxorubicin plus bortezomib or carfilzomib treatment, in spite of the fact that bortezomib and carfilzomib inhibited the dexrazoxane-induced decreases in topoisomerase IIβ protein levels in myocytes. These latter results suggest that the doxorubicin cardioprotective effects of dexrazoxane and the doxorubicin-mediated cardiotoxicity were not exclusively due to targeting of topoisomerase IIβ.

The proteasome and proteasome inhibitors in multiple myeloma

Proteasome inhibitors are one of the most important classes of agents to have emerged for the treatment of multiple myeloma in the past two decades, and now form one of the backbones of treatment. Three agents in this class have been approved by the United States Food and Drug Administration-the first-in-class compound bortezomib, the second-generation agent carfilzomib, and the first oral proteasome inhibitor, ixazomib. The success of this class of agents is due to the exquisite sensitivity of myeloma cells to the inhibition of the 26S proteasome, which plays a critical role in the pathogenesis and proliferation of the disease. Proteasome inhibition results in multiple downstream effects, including the inhibition of NF-κB signaling, the accumulation of misfolded and unfolded proteins, resulting in endoplasmic reticulum stress and leading to the unfolded protein response, the downregulation of growth factor receptors, suppression of adhesion molecule expression, and inhibition of angiogenesis; resistance to proteasome inhibition may arise through cellular responses mediating these downstream effects. These multiple biologic consequences of proteasome inhibition result in synergistic or additive activity with other chemotherapeutic and targeted agents for myeloma, and proteasome inhibitor-based combination regimens have become established as a cornerstone of therapy throughout the myeloma treatment algorithm, incorporating agents from the other key classes of antimyeloma agents, including the immunomodulatory drugs, monoclonal antibodies, and histone deacetylase inhibitors. This review gives an overview of the critical role of the proteasome in myeloma and the characteristics of the different proteasome inhibitors and provides a comprehensive summary of key clinical efficacy and safety data with the currently approved proteasome inhibitors.

Proteasome inhibitors for the treatment of multiple myeloma

INTRODUCTION

Multiple Myeloma (MM) management is rapidly evolving, with a spectrum of novel treatments that have changed our approach to the therapy. Proteasome inhibitors (PIs) have revolutionized the scenario of both relapsed/refractory and newly diagnosed patients. The efficacy of bortezomib, the first PI approved, followed by carfilzomib and, the oral ixazomib, have been tested in several trials as single agents or in combination.

AREAS COVERED

In this review, the authors summarize mechanism of action, efficacy and safety of proteasome inhibitors in MM and focus on data derived from clinical trials, analyzing adverse events and their relative management.

EXPERT OPINION

The authors believe that, currently, the best course of action in the treatment of MM is to use PIs in combination with immunomodulatory drugs (IMiDs) and/or with monoclonal antibodies for all patients. However, based on the patient-specific characteristics, it is important to avoid inappropriate discontinuation by knowing the single side effects of every agent in order to balance their efficacy and safety.

The combination of MLN2238 (ixazomib) with interferon-alpha results in enhanced cell death in melanoma

The ubiquitin-proteasome signaling pathway is critical for cell cycle regulation and neoplastic growth. Proteasome inhibition can activate apoptotic pathways. Bortezomib, a selective proteasome inhibitor, has anti-melanoma activity. MLN2238 (ixazomib), an oral proteasome inhibitor, has improved pharmacotherapeutic parameters compared to bortezomib. Interferon-alpha (IFN-α), an immune boosting agent, is FDA-approved for treatment of melanoma. In this study in vitro and in vivo evaluation of the antitumor potential of ixazomib and combination treatments with ixazomib and IFN-α were performed. Apoptosis induced by ixazomib was first observed at 12 hours and was maximal at 48 hours with similar levels of cell death compared to bortezomib. IFN-α alone had little effect on cell viability in vitro. However, the combination of ixazomib with IFN-α significantly enhanced ixazomib's ability to induce apoptotic cell death in BRAF V600E mutant and BRAF wild-type human melanoma tumor cells. The combination of ixazomib and IFN-α also enhanced inhibition of cell proliferation in BRAF V600E mutant melanoma tumor cells; however, this was not seen in BRAF wild-type cells. Ixazomib-induced apoptosis was associated with processing of the pro-apoptotic proteins procaspase-3, -7, -8, and -9, and cleavage of poly-ADP-ribose polymerase (PARP). In an in vivo xenograft model of human melanoma, combination treatment with IFN-α-2b and ixazomib demonstrated a significant reduction in tumor volume when compared to vehicle (p = 0.005) and single therapy ixazomib (p = 0.017) and IFN-α-2b (p = 0.036). These pre-clinical results support further evaluation of combination treatment with ixazomib and IFN-α for the treatment of advanced BRAF V600E mutant melanoma.

Phase I/II trial of the oral regimen ixazomib, pomalidomide, and dexamethasone in relapsed/refractory multiple myeloma

In this phase I/II trial, a triplet regimen of ixazomib (Ixa: 3 or 4 mg), pomalidomide (Pom: 4 mg), and dexamethasone (Dex: 40 mg) was administered to 32 lenalidomide-refractory multiple myeloma (MM) patients; 31 were evaluable for response and toxicity. At dose level 1 (DL1, 3 mg Ixa), 1/3 patients experienced grade 3 fatigue, grade 3 lung infection, grade 4 neutropenia, and grade 4 thrombocytopenia; all were considered dose-limiting. Per 3 + 3 phase I design, an additional three patients were enrolled to DL1, with no further dose-limiting toxicity (DLT). At dose level 2 (DL2, 4 mg Ixa), 1/3 patients had dose-limiting febrile neutropenia, neutropenia, and thrombocytopenia (grade 4 each). DL2 was expanded to enroll three additional patients with no further DLT, establishing the recommended phase II dose (RP2D). In phase II, 19 additional patients were treated at RP2D. With a median follow-up of 11.9 months, 48% achieved  ≥ partial response (PR), with 5 patients (20%) achieving very good partial response (VGPR) and 76% experiencing ≥ stable disease. The most common adverse events (≥grade 2) were anemia, neutropenia, thrombocytopenia, and infections. Peripheral neuropathy was infrequent. In summary, Ixa/Pom/Dex is a well-tolerated and effective oral combination therapy for patients with relapsed/refractory MM.

Targeting the neddylation pathway in cells as a potential therapeutic approach for diseases

The ubiquitin-proteasome system (UPS) is an important system that regulates the balance of intracellular proteins, and it is involved in the regulation of multiple vital biological processes. The approval of bortezomib for relapsed and refractory multiple myeloma has proven that agents targeting the UPS have the potential to be effective treatment strategies for diseases. Among of all of the components of the UPS, cullin-RING ligases (CRLs) are the focus of research. CRLs are the largest family of ubiquitin E3 ligases and they play a critical role in substrate binding. CRL activity is modulated by many pathways in which neddylation modification is the essential process for cullin activation. Thus, targeting the neddylation pathway of cullins could indirectly affect CRL activity, thereby interfering with substrate protein ubiquitination. In addition to cullin proteins, there are some other target proteins of neddylation, such as p53, mouse double minute 2, and epidermal growth factor receptor. For target proteins, neddylation modification mainly causes functions changes, not degradation. In addition, the level of neddylation is also closely related to disease development and prognosis. In this review, we summarize the research on some target proteins and active target agents of neddylation pathways, and explore the role of neddylation in disease therapy. We came to the conclusion that conducting research on neddylation may be a potential approach to discover some novel targets and agents that could be effective without serious side effects.

The Proteasome and Myeloma-Associated Bone Disease

Bone disease is the hallmark of multiple myeloma (MM), a hematological malignancy characterized by osteolytic lesions due to a severe uncoupled and unbalanced bone remodeling with pronounced osteoblast suppression. Bone metastasis is also a frequent complication of solid tumors including advanced breast or prostate cancer. In the past years, the ubiquitin-proteasome pathway has been proved critical in regulating the balance between bone formation and bone resorption. Proteasome inhibitors (PIs) are a new class of drugs, currently used in the treatment of MM, that affect both tumor cells and bone microenvironment. Particularly, PIs stimulate osteoblast differentiation by human mesenchymal stromal cells and increase bone regeneration in mice. Interestingly, in vitro data indicate that PIs block MM-induced osteoblast and osteocyte cell death by targeting both apoptosis and autophagy. The preclinical data are supported by the following effects observed in MM patients treated with PIs: increase of bone alkaline phosphatase levels, normalization of the markers of bone turnover, and reduction of the skeletal-related events. Moreover, the histomorphometric data indicate that the treatment with bortezomib stimulates osteoblast formation and maintains osteocyte viability in MM patients. This review updates the evidence on the effects of PIs on bone remodeling and on cancer-induced bone disease while focusing on MM bone disease.

Preclinical evaluation of antitumor activity of the proteasome inhibitor MLN2238 (ixazomib) in hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is one of the common malignancies and is an increasingly important cause of cancer death worldwide. Surgery, chemotherapy, and radiation therapy extend the 5-year survival limit in HCC patients by only 6%. Therefore, there is a need to develop new therapeutic approaches for the treatment of this disease. The orally bioavailable proteasome inhibitor MLN2238 (ixazomib) has been demonstrated to have anticancer activity. In the present study, we investigated the preclinical therapeutic efficacy of MLN2238 in HCC cells through in vitro and in vivo models, and examined its molecular mechanisms of action. MLN2238 inhibited cell viability in human HCC cells HepG2, Hep3B, and SNU475 in a time- and dose-dependent manner. Flow cytometry analysis demonstrated that MLN2238 induced G2/M cell cycle arrest and cellular apoptosis in HCC cells. Cell cycle arrest was associated with increased expression levels of p21 and p27. MLN2238-induced apoptosis was confirmed by caspase-3/7 activation, PARP cleavage and caspase-dependent β-catenin degradation. In addition, MLN2238 activated ER stress genes in HCC cells and increased the expression of the stress-inducible gene nuclear protein-1. Furthermore, MLN2238 treatment induced upregulation of myeloid cell leukemia-1 (Mcl-1) protein, and Mcl-1 knockdown sensitized HCC cells to MLN2238 treatment, suggesting the contribution of Mcl-1 expression to MLN2238 resistance. This result was also confirmed using the novel Mcl-1 small molecule inhibitor A1210477. Association of A1210477 and MLN2238 determined synergistic antitumor effects in HCC cells. Finally, in vivo orally administered MLN2238 suppressed tumor growth of Hep3B cells in xenograft models in nude mice. In conclusion, our results offer hope for a new therapeutic opportunity in the treatment of HCC patients.

Mantle cell lymphoma in the era of precision medicine-diagnosis, biomarkers and therapeutic agents

Despite advances in the development of clinical agents for treating Mantle Cell Lymphoma (MCL), treatment of MCL remains a challenge due to complexity and frequent relapse associated with MCL. The incorporation of conventional and novel diagnostic approaches such as genomic sequencing have helped improve understanding of the pathogenesis of MCL, and have led to development of specific agents targeting signaling pathways that have recently been shown to be involved in MCL. In this review, we first provide a general overview of MCL and then discuss about the role of biomarkers in the pathogenesis, diagnosis, prognosis, and treatment for MCL. We attempt to discuss major biomarkers for MCL and highlight published and ongoing clinical trials in an effort to evaluate the dominant signaling pathways as drugable targets for treating MCL so as to determine the potential combination of drugs for both untreated and relapse/refractory cases. Our analysis indicates that incorporation of biomarkers is crucial for patient stratification and improve diagnosis and predictability of disease outcome thus help us in designing future precision therapies. The evidence indicates that a combination of conventional chemotherapeutic agents and novel drugs designed to target specific dysregulated signaling pathways can provide the effective therapeutic options for both untreated and relapse/refractory MCL.

Progress curve analysis of the kinetics of slow-binding anticancer drug inhibitors of the 20S proteasome

Bortezomib, carfilzomib, ixazomib, oprozomib, and delanzomib are anticancer drugs that target the proteasomal system. Carfilzomib and oprozomib are epoxyketones that form an irreversible covalent bond with the 20S proteasome, whereas bortezomib, ixazomib, and delanzomib are boronic acids that form slowly reversible adducts. The binding kinetics of some of these drugs have either not been well characterized, or have been studied under a variety of different conditions. Utilizing a fluorogenic substrate the kinetics of the slow-binding inhibition of the chymotrypsin-like proteasomal activity of human 20S proteasome was determined under a standard set of conditions in order to compare the kinetic and equilibrium properties of these drugs. Progress curve analysis was used to obtain second order "on" and first-order "off" rate constants, and equilibrium- and kinetically-determined inhibitor dissociation constants. Oprozomib inhibited the 20S proteasome with a second-order binding "on" rate constant that was 60-fold slower than for ixazomib, the fastest binding drug. Delanzomib dissociated from its complex with the 20S proteasome with a half-time that was more than 20-fold slower than for ixazomib, the fastest dissociating drug. The differences in the binding and the dissociation of these drugs may, in part, explain some of their pharmacological and toxicological properties.

Improving the efficacy of proteasome inhibitors in the treatment of renal cell carcinoma by combination with the human immunodeficiency virus (HIV)-protease inhibitors lopinavir or nelfinavir

OBJECTIVES

To assess the potential of second-generation proteasome inhibition by carfilzomib and its combination with the human immunodeficiency virus (HIV) protease inhibitors (HIV-PIs) lopinavir and nelfinavir in vitro for improved treatment of clear cell renal cell cancer (ccRCC).

MATERIALS AND METHODS

Cytotoxicity, reactive oxygen species (ROS) production, and unfolded protein response (UPR) activation of proteasome inhibitors, HIV-PIs, and their combination were assessed in three cell lines and primary cells derived from three ccRCC tumours by MTS assay, flow cytometry, quantitative reverse transcriptase-polymerase chain reaction and western blot, respectively. Proteasome activity was determined by activity based probes. Flow cytometry was used to assess apoptosis by annexin V/propidium iodide assay and ATP-binding cassette sub-family B member 1 (ABCB1) activity by MitoTracker™ Green FM efflux assay (Thermo Fisher Scientific, MA, USA).

RESULTS

Lopinavir and nelfinavir significantly increased the cytotoxic effect of carfilzomib in all cell lines and primary cells. ABCB1 efflux pump inhibition, induction of ROS production, and UPR pre-activation by lopinavir were identified as underlying mechanisms of this strong synergistic effect. Combined treatment led to unresolved protein stress, increased activation of pro-apoptotic UPR pathway, and a significant increase in apoptosis.

CONCLUSION

The combination of the proteasome inhibitor carfilzomib and the HIV-PIs lopinavir and nelfinavir has a strong synergistic cytotoxic activity against ccRCCin vitro at therapeutically relevant drug concentrations. This effect is most likely explained by synergistic UPR triggering and ABCB1-modulation caused by HIV-PIs. Our findings suggest that combined treatment of second-generation proteasome inhibitors and HIV-PIs should be investigated in patients with metastatic RCC within a clinical trial.

Small-Molecule Screens: A Gateway to Cancer Therapeutic Agents with Case Studies of Food and Drug Administration-Approved Drugs

High-throughput screening (HTS) of small-molecule libraries accelerates the discovery of chemical leads to serve as starting points for probe or therapeutic development. With this approach, thousands of unique small molecules, representing a diverse chemical space, can be rapidly evaluated by biologically and physiologically relevant assays. The origins of numerous United States Food and Drug Administration-approved cancer drugs are linked to HTS, which emphasizes the value in this methodology. The National Institutes of Health Molecular Libraries Program made HTS accessible to the public sector, enabling the development of chemical probes and drug-repurposing initiatives. In this work, the impact of HTS in the field of oncology is considered among both private and public sectors. Examples are given for the discovery and development of approved cancer drugs. The importance of target validation is discussed, and common assay approaches for screening are reviewed. A rigorous examination of the PubChem database demonstrates that public screening centers are contributing to early-stage drug discovery in oncology by focusing on new targets and developing chemical probes. Several case studies highlight the value of different screening strategies and the potential for drug repurposing.

The evaluation of the anti-cancer activity of ixazomib on Caco2 colon solid tumor cells, comparison with bortezomib

Proteasome inhibition has recently emerged as a clinically effective anticancer therapeutic approach. The first proteasome inhibitor, bortezomib (Velcade, PS-341), and new proteasome inhibitors including ixazomib have become more important in the development of targeted cancer therapies. Under physiological conditions, MLN9708 (ixazomib citrate), the stable citrate ester drug substance, hydrolyzes rapidly to MLN2238 (ixazomib), the biologically active boronic acid. It is a second-generation proteasome inhibitor, similar to the well-known proteasome inhibitor bortezomib, which is currently being investigated in phase 3 trials as a treatment for multiple Myeloma. Despite the proven efficacy of these drugs in hematologic malignancies, clinical activity is limited to solid tumors such as colon adenocarcinoma. This study is the first to investigate and compare the antiproliferative and apoptotic effects of MLN2238 and bortezomib on human colon adenocarcinoma Caco2 cells. The antiproliferative effects of MLN2238 and bortezomib were determined using WST-1; apoptotic effects of this drug were determined by caspase-3 and a mitochondrial membrane potential (JC-1) activity assay. Expression levels associated with proteasome inhibition and apoptosis of NF-κB and c-myc mRNA were evaluated by RT-PCR. At 24 and 48 h, MLN2238 showed significant time- and concentration-dependent antiproliferative and apoptotic effects on Caco2 cells. Depending on increasing mitochondrial depolarization and caspase-3 activation, MLN2238 induced apoptosis at level similar to that of bortezomib. In addition, MLN2238 downregulated NF-κB and c-myc mRNA expression levels. For the first time, MLN2238 was shown to induce antiproliferative and apoptotic effects on human colon adenocarcinoma cells that are comparable with those of bortezomib; these in vitro data in Caco2 cells support the development of MLN2238 for colon cancer.

Multicomponent mapping of boron chemotypes furnishes selective enzyme inhibitors

Heteroatom-rich organoboron compounds have attracted attention as modulators of enzyme function. Driven by the unmet need to develop chemoselective access to boron chemotypes, we report herein the synthesis of α- and β-aminocyano(MIDA)boronates from borylated carbonyl compounds. Activity-based protein profiling of the resulting β-aminoboronic acids furnishes selective and cell-active inhibitors of the (ox)lipid-metabolizing enzyme α/β-hydrolase domain 3 (ABHD3). The most potent compound displays nanomolar in vitro and in situ IC50 values and fully inhibits ABHD3 activity in human cells with no detectable cross-reactivity against other serine hydrolases. These findings demonstrate that synthetic methods that enhance the heteroatom diversity of boron-containing molecules within a limited set of scaffolds accelerate the discovery of chemical probes of human enzymes.

A phase I study to assess the mass balance, excretion, and pharmacokinetics of [14C]-ixazomib, an oral proteasome inhibitor, in patients with advanced solid tumors

This two-part, phase I study evaluated the mass balance, excretion, pharmacokinetics (PK), and safety of ixazomib in patients with advanced solid tumors. In Part A of the study, patients received a single 4.1 mg oral solution dose of [¹⁴C]-ixazomib containing ~500 nCi total radioactivity (TRA), followed by non-radiolabeled ixazomib (4 mg capsule) on days 14 and 21 of the 35-day PK cycle. Patients were confined to the clinic for the first 168 h post dose and returned for 24 h overnight clinic visits on days 14, 21, 28, and 35. Blood, urine, and fecal samples were collected during Part A to assess the mass balance (by accelerator mass spectrometry), excretion, and PK of ixazomib. During Part B of the study, patients received non-radiolabeled ixazomib (4 mg capsules) on days 1, 8, and 15 of 28-day cycles. After oral administration, ixazomib was rapidly absorbed with a median plasma T_max of 0.5 h and represented 70% of total drug-related material in plasma. The mean total recovery of administered TRA was 83.9%; 62.1% in urine and 21.8% in feces. Only 3.23% of the administered dose was recovered in urine as unchanged drug up to 168 h post dose, suggesting that most of the TRA in urine was attributable to metabolites. All patients experienced a treatment-emergent adverse event, which most commonly involved the gastrointestinal system. These findings suggest that ixazomib is extensively metabolized, with urine representing the predominant route of excretion of drug-related material.Trial ID: ClinicalTrials.gov # NCT01953783.

Immunoproteasome-selective and non-selective inhibitors: A promising approach for the treatment of multiple myeloma

The ubiquitin-proteasome system (UPS) is the major non-lysosomal proteolytic system for the degradation of abnormal or damaged proteins no longer required. The proteasome is involved in degradation of numerous proteins which regulate the cell cycle, indicating a role in controlling cell proliferation and maintaining cell survival. Defects in the UPS can lead to anarchic cell proliferation and to tumor development. For these reasons UPS inhibition has become a significant new strategy for drug development in cancer treatment. In addition to the constitutive proteasome, which is expressed in all cells and tissues, higher organisms such as vertebrates possess two immune-type proteasomes, the thymoproteasome and the immunoproteasome. The thymoproteasome is specifically expressed by thymic cortical epithelial cells and has a role in positive selection of CD8+ T cells, whereas the immunoproteasome is predominantly expressed in monocytes and lymphocytes and is responsible for the generation of antigenic peptides for cell-mediated immunity. Recent studies demonstrated that the immunoproteasome has a preservative role during oxidative stress and is up-regulated in a number of pathological disorders including cancer, inflammatory and autoimmune diseases. As a consequence, immunoproteasome-selective inhibitors are currently the focus of anticancer drug design. At present, the commercially available proteasome inhibitors bortezomib and carfilzomib which have been validated in multiple myeloma and other model systems, appear to target both the constitutive and immunoproteasomes, indiscriminately. This lack of specificity may, in part, explain some of the side effects of these agents, such as peripheral neuropathy and gastrointestinal effects, which may be due to targeting of the constitutive proteasome in these tissues. In contrast, by selectively inhibiting the immunoproteasome, it may be possible to maintain the antimyeloma and antilymphoma efficacy while reducing these toxicities, thereby increasing the therapeutic index. This review article will be focused on the discussion of the most promising immunoproteasome specific inhibitors which have been developed in recent years. Particular attention will be devoted to the description of their mechanism of action, their structure-activity relationship, and their potential application in therapy.

Disulfiram is a slow-binding partial noncompetitive inhibitor of 20S proteasome activity

The alcohol abuse drug disulfiram has also been shown to exhibit potent cell growth inhibitory and anticancer activity. While a number of cellular and animal studies have suggested that disulfiram exhibits its anticancer activity through interaction with the proteasome, direct evidence for inhibition of proteasome activity is lacking. In this study we show that disulfiram potently inhibits the chymotrypsin-like activity of purified human 20S proteasome at low micromolar pharmacological concentrations. The enzyme progress curves displayed characteristics of a slow-binding reaction, similar to that observed for the FDA-approved proteasomal-targeted anticancer drugs bortezomib and carfilzomib. The apparent second order rate constant for reaction with 20s proteasome that was derived from an analysis of the progress curves was about 250-fold smaller than for bortezomib and carfilzomib. The concentration dependence of the enzyme kinetics was consistent with partial noncompetitive inhibition, whereby the putative disulfiram-proteasome adduct retains, partial but decreased enzyme activity. Disulfiram, which is known to have a high affinity for protein thiols, likely reacted with a non-critical cysteine residue, and not at the proteasome substrate binding site.

Synthesis and biological evaluation of the natural product komaroviquinone and related compounds aiming at a potential therapeutic lead compound for high-risk multiple myeloma

Alternatives of treatments for multiple myeloma (MM) have become increasingly available with the advent of new drugs such as proteasome inhibitors, thalidomide derivatives, histone deacetylase inhibitors, and antibody drugs. However, high-risk MM cases that are refractory to novel drugs remain, and further optimization of chemotherapeutics is urgently needed. We had achieved asymmetric total synthesis of komaroviquinone, which is a natural product from the plant Dracocephalum komarovi. Similar to several leading antitumor agents that have been developed from natural compounds, we describe the antitumor activity and cytotoxicity of komaroviquinone and related compounds in bone marrow cells. Our data suggested that komaroviquinone-related agents have potential as starting compounds for anticancer drug development.

The Proteasome in Modern Drug Discovery: Second Life of a Highly Valuable Drug Target

As the central figure of the cellular protein degradation machinery, the proteasome is critical for cell survival. Having been extensively targeted for inhibition, the constitutive proteasome has proven its role as a highly valuable drug target. However, recent advances in the protein homeostasis field suggest that additional chapters can be added to this successful story. For example, selective immunoproteasome inhibition promises high clinical efficacy for autoimmune disorders and inflammation, and proteasome inhibitors might serve as novel therapeutics for malaria or other microorganisms. Furthermore, utilizing the destructive force of the proteasome for selective degradation of essential drivers of human disorders has opened up a new and exciting area of drug discovery. Thus, the field of proteasome drug discovery still holds exciting questions to be answered and does not simply end with inhibiting the constitutive proteasome.

New developments in the management of relapsed/refractory multiple myeloma - the role of ixazomib

Ixazomib is the first oral proteasome inhibitor to be approved, in combination with lenalidomide and dexamethasone, for the treatment of patients with multiple myeloma who have received at least one prior therapy. Approval was on the basis of results from the phase 3, double-blind, placebo-controlled TOURMALINE-MM1 study, which demonstrated a 35% improvement in progression-free survival with the all-oral combination of ixazomib plus lenalidomide-dexamethasone versus lenalidomide-dexamethasone alone (median: 20.6 vs 14.7 months; hazard ratio: 0.74, p=0.012; median follow-up 14.7 months). The addition of ixazomib to the lenalidomide-dexamethasone regimen was associated with limited additional toxicity and had no adverse impact on patient-reported quality of life. Common grade ≥3 adverse events with ixazomib include gastrointestinal adverse events, rash, and thrombocytopenia. Here, we review the efficacy, safety, pharmacokinetics, and patient-reported quality of life data seen with ixazomib, and discuss the role of this oral agent in the treatment of patients with relapsed/refractory multiple myeloma, including in patients with high-risk cytogenetic abnormalities and those with multiple prior therapies.

Preclinical efficacy and biological effects of the oral proteasome inhibitor ixazomib in diffuse large B-cell lymphoma

Despite advances in deciphering the molecular pathogenesis of diffuse large B-cell lymphoma (DLBCL), patients with relapsed/refractory disease, particularly those with adverse genetic features (e.g., mutated p53 or double hit lymphoma (DHL)) have very poor prognoses, and effective therapies are lacking. In this study we examined the preclinical efficacy and associated biological effects of the first oral proteasome inhibitor, ixazomib, in DLBCL in vitro and in vivo models. We demonstrated that ixazomib exhibited anti-tumor activities in 28 representative DLBCL cell lines, 10 primary DLBCL samples, and a DHL xenotransplant mouse model, at clinically achievable drug concentrations. Ixazomib sensitivity in DLBCL cells is correlated with immunoproteasomal activity; stimulating lymphoma cells with interferon gamma induced immunoproteasome activity and sensitized these cells to ixazomib. In addition, we showed that ixazomib induces apoptosis and the DNA damage response pathway, through activation of the checkpoint kinase 2 (CHK2). Hence, pharmacological inhibition of CHK2 enhances the anti-tumor activity of ixazomib in DLBCL cells. Our results indicate that ixazomib is an effective proteasome inhibitor active in DLBCL, including DHL, and its combination with a CHK2 inhibitor offers a potentially more robust therapeutic regimen for treatment-resistant DLBCL.

A Phase 1 Study to Assess the Relative Bioavailability of Two Capsule Formulations of Ixazomib, an Oral Proteasome Inhibitor, in Patients With Advanced Solid Tumors or Lymphoma

The oral proteasome inhibitor ixazomib is approved in multiple countries in combination with lenalidomide and dexamethasone for the treatment of patients with multiple myeloma who have received at least 1 prior therapy. Two oral capsule formulations of ixazomib have been used during clinical development. This randomized, 2-period, 2-sequence crossover study (Clinicaltrials.gov identifier NCT01454076) assessed the relative bioavailability of capsule B in reference to capsule A in adult patients with advanced solid tumors or lymphoma. The study was conducted in 2 parts. In cycle 1 (pharmacokinetic cycle), patients received a 4-mg dose of ixazomib as capsule A or capsule B on day 1, followed by a 4-mg dose of the alternate capsule formulation on day 15. Pharmacokinetic samples were collected over 216 hours postdose. After the pharmacokinetic cycle, patients could continue in the study and receive ixazomib (capsule B only) on days 1, 8, and 15 of each 28-day cycle. Twenty patients were enrolled; of these, 14 were included in the pharmacokinetic-evaluable population. Systemic exposures of ixazomib were similar after administration of capsule A or capsule B. The geometric least-squares mean ratios (capsule B versus capsule A) were 1.16 for C_max (90% confidence interval [CI], 0.84-1.61) and 1.04 for AUC_0-216 (90%CI, 0.91-1.18). The most frequently reported grade 3 drug-related adverse events were fatigue (15%) and nausea (10%); there were no grade 4 drug-related adverse events. These results support the combined analysis of data from studies that used either formulation of ixazomib during development.

Safety of ixazomib for the treatment of multiple myeloma

INTRODUCTION

Despite a major positive impact of proteasome inhibitors (PI), such as bortezomib and carfilzomib, on the survival of patients with multiple myeloma (MM) over the last few years, their use in clinical practice is limited by the development of drug resistance, significant side-effects or constraining administration schedules. Ixazomib is the first, and for now the only, oral PI, which was approved by the US Food and Drug Administration in 2015 and by the European Medicines Agency in 2016. Areas covered: In this review, we provide an overview of the preclinical and early-phase studies of ixazomib used as single-agent and in combination. Furthermore, we discuss the results of a recently published pivotal trial, which evaluated the safety profile and clinical benefit of the combination of ixazomib, lenalidomide and dexamethasone versus lenalidomide and dexamethasone alone in 722 patients with relapsed/refractory MM. Expert opinion: Ixazomib combines the comfort of oral administration, substantial clinical efficacy and a good safety profile with manageable side-effects, which mainly comprise low-grade hematological, digestive or cutaneous events, and the agent will therefore play an active part in long-term treatment strategies, both as single agent and as part of combination regimens. Ongoing phase III trials are currently defining its place in first-line, maintenance and relapse settings.

Human 20S proteasome activity towards fluorogenic peptides of various chain lengths

The proteasome is a multicatalytic protease responsible for the degradation of misfolded proteins. We have synthesized fluorogenic substrates in which the peptide chain was systematically elongated from two to six amino acids and evaluated the effect of peptide length on all three catalytic activities of human 20S proteasome. In the cases of five- and six-membered peptides, we have also synthesized libraries of fluorogenic substrates. Kinetic analysis revealed that six-amino-acid substrates are significantly better for chymotrypsin-like and caspase-like activity than shorter peptidic substrates. In the case of trypsin-like activity, a five-amino-acid substrate was optimal.

Current and developing synthetic pharmacotherapy for treating relapsed/refractory multiple myeloma

INTRODUCTION

The introduction of novel agents has significantly improved multiple myeloma (MM) patient outcome during the last two decades. MM received the most drug approvals for any one malignancy during this time period, both in the United States as well as in Europe. Areas covered: Proteasome inhibitors, immunomodulatory drugs, and monoclonal antibodies are prototype drug classes, which target both specific MM cell functions, as well as the tumor supportive bone marrow microenvironment, and represent current cornerstones of MM therapy. Importantly, the unprecedented extent and frequency of durable responses, in relapsed/refractory multiple myeloma (RRMM), in particular, is predominantly based on the combinatorial use of these agents with conventional chemotherapeutics or representatives of other drug classes. This article will summarize past landmark discoveries in MM that led to the dramatic progress of today's clinical practice. Moreover, developing strategies will be discussed that are likely to yet improve patient outcome even further. Expert opinion: Despite significant therapeutic advancements, MM remains an incurable disease. With several novel agents in the preclinical and early clinical pipeline, among those novel CD38 and BCMA mAbs, immune checkpoint inhibitors, as well as ricolinostat, selinexor, venetoclax, CAR-T cells, and vaccines, further advances in MM patient outcome are expected in the near future.

The potential of ixazomib, a second-generation proteasome inhibitor, in the treatment of multiple myeloma

The therapeutic armamentarium for multiple myeloma has recently benefited from the addition of several new agents (including second-generation proteasome inhibitors, monoclonal antibodies and histone deacetylase inhibitors). This review will focus on ixazomib, an orally bioavailable second-generation proteasome inhibitor. Specifically, we will review the preclinical data, clinical trial experience, potential indications as well as unanswered questions pertaining to this new agent in multiple myeloma.

Boron-based small molecules in disease detection and treatment (2013–2016)

Recent years have seen tremendous development in the design and synthesis of boron-based compounds as potential therapeutics and for detection applications. The present review highlights the most recent development of these boron-based small molecules, covering clinically used ixazomib, tavaborole, crisaborole and other molecules from 2013 to 2016.

A phase 1/2 study of the oral proteasome inhibitor ixazomib in relapsed or refractory AL amyloidosis

This phase 1/2 study assessed the safety, tolerability, and preliminary efficacy of the oral proteasome inhibitor (PI) ixazomib in patients with relapsed/refractory immunoglobulin light chain (AL) amyloidosis. Ixazomib was administered to adult patients with relapsed/refractory AL amyloidosis after 1 or more prior lines of therapy (including bortezomib) on days 1, 8, and 15 of 28-day cycles, for up to 12 cycles. Patients with less than partial response after 3 cycles received oral dexamethasone (40 mg, days 1-4) from cycle 4. A 3+3 dose-escalation phase was followed by 2 expansion cohorts (PI-naive and PI-exposed patients) at the maximum tolerated dose (MTD). Twenty-seven patients were enrolled: 11 during dose escalation (6 at 4.0 mg and 5 at 5.5 mg) and 16 during dose expansion (4.0 mg). Three patients experienced dose-limiting toxicities: 1 at 4.0 mg and 2 at 5.5 mg; the MTD was determined as 4.0 mg. Most common adverse events (AEs) included nausea, skin and subcutaneous tissue disorders (SSTD), diarrhea, and fatigue; grade 3 or higher AEs included dyspnea, fatigue, and SSTD. Overall, the hematologic response rate was 52% in patients treated at the MTD (n = 21). Organ responses were seen in 56% of patients (5 cardiac, 5 renal). Median hematologic progression-free survival was 14.8 months; 1-year progression-free and overall survival rates were 60% and 85%, respectively (median follow-up, 16.9 months). Weekly oral ixazomib appears to be active in patients with relapsed/refractory AL amyloidosis, with a generally manageable safety profile. The study was registered at clinicaltrials.gov as #NCT01318902 A phase 3 study is ongoing (#NCT01659658).

Ixazomib enhances parathyroid hormone-induced β-catenin/T-cell factor signaling by dissociating β-catenin from the parathyroid hormone receptor

The proteasome inhibitor ixazomib (Izb) dissociates β-catenin from the PTH receptor to enhance PTH stimulation of β-catenin/TCF signaling through the cAMP/PKA signaling pathway. These findings provide a rationale for the use of Izb as an adjunct in the treatment of osteoporosis with PTH.

The anabolic action of PTH in bone is mostly mediated by cAMP/PKA and Wnt-independent activation of β-catenin/T-cell factor (TCF) signaling. β-Catenin switches the PTH receptor (PTHR) signaling from cAMP/PKA to PLC/PKC activation by binding to the PTHR. Ixazomib (Izb) was recently approved as the first orally administered proteasome inhibitor for the treatment of multiple myeloma; it acts in part by inhibition of pathological bone destruction. Proteasome inhibitors were reported to stabilize β-catenin by the ubiquitin-proteasome pathway. However, how Izb affects PTHR activation to regulate β-catenin/TCF signaling is poorly understood. In the present study, using CRISPR/Cas9 genome-editing technology, we show that Izb reverses β-catenin–mediated PTHR signaling switch and enhances PTH-induced cAMP generation and cAMP response element–luciferase activity in osteoblasts. Izb increases active forms of β-catenin and promotes β-catenin translocation, thereby dissociating β-catenin from the PTHR at the plasma membrane. Furthermore, Izb facilitates PTH-stimulated GSK3β phosphorylation and β-catenin phosphorylation. Thus Izb enhances PTH stimulation of β-catenin/TCF signaling via cAMP-dependent activation, and this effect is due to its separating β-catenin from the PTHR. These findings provide evidence that Izb may be used to improve the therapeutic efficacy of PTH for the treatment of osteoporosis and other resorptive bone diseases.