Proteasome composition in immune cells implies special immune­cell­specific immunoproteasome function.

Immunoproteasomes are a special class of proteasomes, which can be induced with IFN-γ in an inflammatory environment. In recent years, it became evident that certain immune cell types constitutively express high levels of immunoproteasomes. However, information regarding the basal expression of proteolytically active immunoproteasome subunits in different types of immune cells is still rare. Hence, we quantified standard proteasome subunits (ß1c, ß2c, ß5c) and immunoproteasome subunits (LMP2, MECL-1, LMP7) in the major murine (CD4+ T cells, CD8+ T cells, CD19+ B cells, CD11c+ dendritic cells, CD49d+ natural killer cells, Ly-6G+ neutrophils) and human immune cell (CD4+ T cells, CD8+ T cells, CD19+ B cells, CD1c+CD141+ myeloid dendritic cells, CD56+ natural killer cells, granulocytes) subsets. The different human immune cell types were isolated from peripheral blood and the murine immune cell subsets from spleen. We found that proteasomes of most immune cell subsets mainly consist of immunoproteasome subunits. Our data will serve as a reference and guideline for immunoproteasome expression and imply a special role of immunoproteasomes in immune cells.

Global signal peptide profiling reveals principles of selective Sec61 inhibition.

Cotransins target the Sec61 translocon and inhibit the biogenesis of an undefined subset of secretory and membrane proteins. Remarkably, cotransin inhibition depends on the unique signal peptide (SP) of each Sec61 client, which is required for cotranslational translocation into the endoplasmic reticulum. It remains unknown how an SP's amino acid sequence and biophysical properties confer sensitivity to structurally distinct cotransins. Here we describe a fluorescence-based, pooled-cell screening platform to interrogate nearly all human SPs in parallel. We profiled two cotransins with distinct effects on cancer cells and discovered a small subset of SPs, including the oncoprotein human epidermal growth factor receptor 3 (HER3), with increased sensitivity to the more selective cotransin, KZR-9873. By comparing divergent mouse and human orthologs, we unveiled a position-dependent effect of arginine on SP sensitivity. Our multiplexed profiling platform reveals how cotransins can exploit subtle sequence differences to achieve SP discrimination.

Signal peptide mimicry primes Sec61 for client-selective inhibition.

Preventing the biogenesis of disease-relevant proteins is an attractive therapeutic strategy, but attempts to target essential protein biogenesis factors have been hampered by excessive toxicity. Here we describe KZR-8445, a cyclic depsipeptide that targets the Sec61 translocon and selectively disrupts secretory and membrane protein biogenesis in a signal peptide-dependent manner. KZR-8445 potently inhibits the secretion of pro-inflammatory cytokines in primary immune cells and is highly efficacious in a mouse model of rheumatoid arthritis. A cryogenic electron microscopy structure reveals that KZR-8445 occupies the fully opened Se61 lateral gate and blocks access to the lumenal plug domain. KZR-8445 binding stabilizes the lateral gate helices in a manner that traps select signal peptides in the Sec61 channel and prevents their movement into the lipid bilayer. Our results establish a framework for the structure-guided discovery of novel therapeutics that selectively modulate Sec61-mediated protein biogenesis.

Development of subunit selective substrates for Trichomonas vaginalis proteasome.

The protozoan parasite, Trichomonas vaginalis (Tv) causes trichomoniasis, the most common, non-viral, sexually transmitted infection in the world. Only two closely related drugs are approved for its treatment. The accelerating emergence of resistance to these drugs and lack of alternative treatment options poses an increasing threat to public health. There is an urgent need for novel effective anti-parasitic compounds. The proteasome is a critical enzyme for T. vaginalis survival and was validated as a drug target to treat trichomoniasis. However, to develop potent inhibitors of the T. vaginalis proteasome, it is essential that we understand which subunits should be targeted. Previously, we identified two fluorogenic substrates that were cleaved by T. vaginalis proteasome, however after isolating the enzyme complex and performing an in-depth substrate specificity study, we have now designed three fluorogenic reporter substrates that are each specific for one catalytic subunit. We screened a library of peptide epoxyketone inhibitors against the live parasite and evaluated which subunits are targeted by the top hits. Together we show that targeting of the ß5 subunit of T. vaginalis is sufficient to kill the parasite, however, targeting of ß5 plus either ß1 or ß2 results in improved potency.

Zetomipzomib (KZR-616) attenuates lupus in mice via modulation of innate and adaptive immune responses.

Zetomipzomib (KZR-616) is a selective inhibitor of the immunoproteasome currently undergoing clinical investigation in autoimmune disorders. Here, we characterized KZR-616 in vitro and in vivo using multiplexed cytokine analysis, lymphocyte activation and differentiation, and differential gene expression analysis. KZR-616 blocked production of >30 pro-inflammatory cytokines in human peripheral blood mononuclear cells (PBMCs), polarization of T helper (Th) cells, and formation of plasmablasts. In the NZB/W F1 mouse model of lupus nephritis (LN), KZR-616 treatment resulted in complete resolution of proteinuria that was maintained at least 8 weeks after the cessation of dosing and was mediated in part by alterations in T and B cell activation, including reduced numbers of short and long-lived plasma cells. Gene expression analysis of human PBMCs and tissues from diseased mice revealed a consistent and broad response focused on inhibition of T, B, and plasma cell function and the Type I interferon pathway and promotion of hematopoietic cell lineages and tissue remodeling. In healthy volunteers, KZR-616 administration resulted in selective inhibition of the immunoproteasome and blockade of cytokine production following ex vivo stimulation. These data support the ongoing development of KZR-616 in autoimmune disorders such as systemic lupus erythematosus (SLE)/LN.

Co-inhibition of immunoproteasome subunits LMP2 and LMP7 enables prevention of transplant arteriosclerosis.

AIMS: The loss of vascular wall cells in allotransplanted arteries is the initial event leading to transplant arteriosclerosis (TA) and ensuing loss of allograft function. Pharmacological agents able to prevent TA are currently lacking. We previously showed that selective inhibition of the immunoproteasome prevented the chronic rejection of renal allografts. However, the role and mechanisms of selective inhibition of a single immunoproteasome subunit to prevent immune-mediated vascular allograft rejection and TA is not clear. METHODS AND RESULTS: The effect and potential mechanism of combined or individual inhibition of peptidolytically active immunoproteasome LMP7 (ß5i) and LMP2 (ß1i) subunits on immune rejection-mediated TA was investigated using the epoxyketone inhibitor ONX 0914, and the recently developed LMP7-selective inhibitor KZR-329 and LMP2-selective inhibitor KZR-504 in a rat aorta transplantation model. We find that co-inhibition of LMP7 and LMP2 in allogeneic recipients significantly suppressed T-cell activation and function by expressing inhibitory surface markers and then activating inhibitory signals. Moreover, co-inhibition of LMP7 and LMP2 substantially reduced the number of immunoglobulin G-secreting cells and plasma cells and production of alloantibodies through activating the unfolded protein response and incapacitating the survival niche of plasma cells in the bone marrow. Consequentially, the accumulation of inflammatory cytokines, complement, and antibodies is reduced and the apoptosis of vascular wall cells decreased in aortic allografts via LMP7 and LMP2 co-inhibition with ONX 0914 treatment or combined KZR-329 and KZR-504 treatment. However, neither individual inhibition of LMP7 by KZR-329 nor individual inhibition of LMP2 by KZR-504 showed suppression of immune rejection and TA. CONCLUSIONS: We define a critical role of LMP7 and LMP2 in TA and strongly propose co-inhibition of both immunoproteasome subunits as promising therapeutic approach to suppress TA and allograft rejection.

Effective therapy of polymyositis in mice via selective inhibition of the immunoproteasome.

Polymyositis (PM) is a chronic autoimmune inflammatory myopathy resulting in muscle weakness. The limited approved therapies and their poor efficacy contribute to its comorbidity. We investigated the therapeutic use of ONX 0914 and KZR-616, selective inhibitors of the immunoproteasome, in C protein-induced myositis (CIM), a mouse model of PM that closely resembles the human disease. Diseased mice (day 13 postimmunization) were treated with 10 mg/kg ONX 0914, KZR-616, or vehicle on alternate days until day 28. Endpoints included muscle strength assessed by a grip strength meter, serum creatine kinase activity, histology, and immunohistochemistry analysis. Treatment with ONX 0914 or KZR-616 prevented the loss of grip strength in mice after CIM induction, while vehicle-treated animals displayed progressive muscle weakness. Immunoproteasome inhibition lowered PM-associated leukocyte infiltration of the muscle and prevented increased serum creatine kinase levels. LMP7-deficient mice were resistant to CIM induction, as they showed no alterations in grip strength or creatine kinase (CK) levels or muscular alterations. In conclusion, selective inhibition of the immunoproteasome displays therapeutic efficacy in a preclinical mouse model of PM with suppression of muscle inflammation and preservation of muscle strength. Positive results from this study support the rationale for using KZR-616 in clinical studies.

Inhibition of the Sec61 translocon overcomes cytokine-induced glucocorticoid resistance in T-cell acute lymphoblastic leukaemia.

Glucocorticoid (GC) resistance is a poor prognostic factor in T-cell acute lymphoblastic leukaemia (T-ALL). Interleukin-7 (IL-7) mediates GC resistance via GC-induced upregulation of IL-7 receptor (IL-7R) expression, leading to increased pro-survival signalling. IL-7R reaches the cell surface via the secretory pathway, so we hypothesized that inhibiting the translocation of IL-7R into the secretory pathway would overcome GC resistance. Sec61 is an endoplasmic reticulum (ER) channel that is required for insertion of polypeptides into the ER. Here, we demonstrate that KZR-445, a novel inhibitor of Sec61, potently attenuates the dexamethasone (DEX)-induced increase in cell surface IL-7R and overcomes IL-7-induced DEX resistance.

Proteasome Inhibitors and Their Pharmacokinetics, Pharmacodynamics, and Metabolism.

The proteasome is responsible for mediating intracellular protein degradation and regulating cellular function with impact on tumor and immune effector cell biology. The proteasome is found predominantly in two forms, the constitutive proteasome and the immunoproteasome. It has been validated as a therapeutic drug target through regulatory approval with 2 distinct chemical classes of small molecular inhibitors (boronic acid derivatives and peptide epoxyketones), including 3 compounds, bortezomib (VELCADE), carfilzomib (KYPROLIS), and ixazomib (NINLARO), for use in the treatment of the plasma cell neoplasm, multiple myeloma. Additionally, a selective inhibitor of immunoproteasome (KZR-616) is being developed for the treatment of autoimmune diseases. Here, we compare and contrast the pharmacokinetics (PK), pharmacodynamics (PD), and metabolism of these 2 classes of compounds in preclinical models and clinical studies. The distinct metabolism of peptide epoxyketones, which is primarily mediated by microsomal epoxide hydrolase, is highlighted and postulated as a favorable property for the development of this class of compound in chronic conditions.

Discovery and Early Clinical Development of Selective Immunoproteasome Inhibitors.

Inhibitors of the proteolytic activity of the 20S proteasome have transformed the treatment of multiple B-cell malignancies. These agents have also been employed with success in the treatment of patients with autoimmune diseases and immune-mediated disorders. However, new agents are needed to fully unlock the potential of proteasome inhibitors as immunomodulatory drugs. The discovery that selective inhibitors of the immunoproteasome possess broad anti-inflammatory activity in preclinical models has led to the progression of multiple compounds to clinical trials. This review focuses on the anti-inflammatory potential of immunoproteasome inhibition and the early development of KZR-616, the first selective inhibitor of the immunoproteasome to reach clinical testing.

Immunoproteasome inhibition induces plasma cell apoptosis and preserves kidney allografts by activating the unfolded protein response and suppressing plasma cell survival factors.

Chronic antibody-mediated rejection is the leading cause of allograft dysfunction and loss after kidney transplantation, and current immunosuppressive regimens fail to target the plasma cells that produce alloantibodies. We previously showed that treatment with the immunoproteasome inhibitor ONX 0914 prevented the expansion of plasma cells and prevented chronic allograft nephropathy and organ failure after kidney transplantation in rats, but the mechanism has remained elusive. In the current study, we confirmed a long-term reduction in alloantibody production and improvements in allograft histology in rats treated with ONX 0914 or with the broad-spectrum proteasome inhibitor bortezomib. Plasma cells from allotransplanted rats expressed immunoproteasomes at high levels. Immunoproteasome inhibition with ONX 0914 led to ubiquitin-conjugate accumulation, activation of the unfolded protein response, and induction of apoptosis in plasma cells. In addition, ONX 0914 suppressed the expression of adhesion molecules (VLA-4 and LFA-1), plasma cell survival factors (APRIL and IL-6), and IFN-γ-inducible chemokines in bone marrow, while the APRIL receptor BCMA, the IL-6 receptor, and the chemokine receptors CXCR4 and CXCR3 were down-regulated on plasma cells. Taken together, immunoproteasome inhibition blocked alloantibody production by inducing apoptosis of plasma cells through activating the unfolded protein response and suppressing plasma cell survival factors in the bone marrow.

Diagnostic accuracy of intraoral photographic orthodontic records.

OBJECTIVE: The aims of this study were (1) to investigate how relevant intraoral photographs are to contemporary orthodontic diagnosis and (2) to assess orthodontists' ability to accurately diagnose angle classification and dental midlines using standardized intraoral photographs. METHODS: Study participants were orthodontists who completed a survey regarding photography protocols and their use of intraoral photographs for diagnosis. Each participant was randomized to complete 1 visual diagnostic task regarding either angle classification or midlines. Accuracy was compared across groups and camera angulations. RESULTS: In all, 80% of 192 respondents reported using photographs and clinic notes to plan orthodontic treatment; 50% also included dental casts. For the angle task, accuracy judging molar and canine classification was 79.9% and 51.3%, respectively with ideal standardized photographs. As camera angulation deviated, accuracy decreased significantly (P < 0.0001). For the midline task, accuracy judging the direction of deviation decreased with a small camera angulation change yet increased with a large change (P < 0.001). CONCLUSIONS: When using ideal intraoral photographs alone to diagnose angle classification and midline relationships, accuracy is not likely to be greater than 80%. As camera angulation becomes less ideal, by 15 degrees when judging angle classification or 4 degrees when judging midlines, accuracy is likely to significantly decrease. CLINICAL SIGNIFICANCE: For the clinician who wants to have the most accurate and complete records, our results suggest that intra-oral photos alone may not be adequate when it comes to judging occlusal relationships such as angle classification and esthetic parameters like midlines. When using ideal intraoral photographs to diagnose angle classification and midline relationships, accuracy is not likely to be greater than 80%. As camera angulation becomes less ideal, by as little as 15 degrees when judging angle classification or 4 degrees when judging midlines, accuracy is likely to decrease significantly. Understanding these limitations will allow clinicians to improve both their clinical photography technique and their diagnostic skills.

Co-inhibition of immunoproteasome subunits LMP2 and LMP7 is required to block autoimmunity.

Cells of hematopoietic origin express high levels of the immunoproteasome, a cytokine-inducible proteasome variant comprising the proteolytic subunits LMP2 (ß1i), MECL-1 (ß2i), and LMP7 (ß5i). Targeting the immunoproteasome in pre-clinical models of autoimmune diseases with the epoxyketone inhibitor ONX 0914 has proven to be effective. ONX 0914 was previously described as a selective LMP7 inhibitor. Here, we show that PRN1126, developed as an exclusively LMP7-specific inhibitor, has limited effects on IL-6 secretion, experimental colitis, and experimental autoimmune encephalomyelitis (EAE). We demonstrate that prolonged exposure of cells with ONX 0914 leads to inhibition of both LMP7 and LMP2. Co-inhibition of LMP7 and LMP2 with PRN1126 and LMP2 inhibitors LU-001i or ML604440 impairs MHC class I cell surface expression, IL-6 secretion, and differentiation of naïve T helper cells to T helper 17 cells, and strongly ameliorates disease in experimental colitis and EAE. Hence, co-inhibition of LMP2 and LMP7 appears to be synergistic and advantageous for the treatment of autoimmune diseases.

Immunoproteasome inhibition prevents chronic antibody-mediated allograft rejection in renal transplantation.

Chronic antibody-mediated rejection is the major cause of fading allograft function and loss after renal transplantation. Currently, pharmacological agents for the suppression of chronic antibody-mediated rejection are lacking. Non-selective proteasome inhibitors suppress antibody-mediated allograft rejection. However, extensive adverse side effects of these inhibitors severely limit their application. In contrast, immunoproteasome inhibition is effective in preclinical models of autoimmune diseases and was applied over weeks without obvious adverse side effects. ONX 0914, an immunoproteasome subunit LMP7 (ß5i)-selective inhibitor, impeded the chronic rejection of kidneys transplanted from Fischer to allogeneic Lewis rats. ONX 0914 inhibited immunoproteasome induction both in immune organs and renal allografts. Selective immunoproteasome inhibition reduced the numbers of B and plasma cells, and suppressed donor-specific alloantibody production. The infiltration of T cells, B cells and macrophages as well as interferon-γ, interleukin-17, IgG and complement deposition were reduced in renal allografts of ONX 0914-treated recipients. Chronic nephropathy was ameliorated and renal allograft function preserved, enabling long-term survival of recipients. Thus, our studies define a critical role of the immunoproteasome in chronic kidney allograft rejection and suggest immunoproteasome inhibition as a promising therapeutic approach to suppress chronic antibody-mediated rejection.

Clinical activity of carfilzomib correlates with inhibition of multiple proteasome subunits: application of a novel pharmacodynamic assay.

While proteasome inhibition is a validated therapeutic approach for multiple myeloma (MM), inhibition of individual constitutive proteasome (c20S) and immunoproteasome (i20S) subunits has not been fully explored owing to a lack of effective tools. We utilized the novel proteasome constitutive/immunoproteasome subunit enzyme-linked immunosorbent (ProCISE) assay to quantify proteasome subunit occupancy in samples from five phase I/II and II trials before and after treatment with the proteasome inhibitor carfilzomib. Following the first carfilzomib dose (15-56 mg/m(2) ), dose-dependent inhibition of c20S and i20S chymotrypsin-like active sites was observed [whole blood: ≥67%; peripheral blood mononuclear cells (PBMCs): ≥75%]. A similar inhibition profile was observed in bone marrow-derived CD138(+) tumour cells. Carfilzomib-induced proteasome inhibition was durable, with minimal recovery in PBMCs after 24 h but near-complete recovery between cycles. Importantly, the ProCISE assay can be used to quantify occupancy of individual c20S and i20S subunits. We observed a relationship between MM patient response (n = 29), carfilzomib dose and occupancy of multiple i20S subunits, where greater occupancy was associated with an increased likelihood of achieving a clinical response at higher doses. ProCISE represents a new tool for measuring proteasome inhibitor activity in clinical trials and relating drug action to patient outcomes.

Inhibition and deficiency of the immunoproteasome subunit LMP7 attenuates LCMV-induced meningitis.

In addition to antigen processing, immunoproteasomes were recently shown to exert functions influencing cytokine production by monocytes and T cells, T-helper cell differentiation, and T-cell survival. Moreover, selective inhibition of the immunoproteasome subunit LMP7 ameliorated symptoms of autoimmune diseases including CD4(+) T-cell mediated EAE. In this study, we show that LMP7 also plays a crucial role in the pathogenesis of lymphocytic choriomeningitis virus (LCMV)-induced meningitis mediated by CTLs. Mice lacking functional LMP7 display delayed and reduced clinical signs of disease accompanied by a strongly decreased inflammatory infiltration into the brain. Interestingly, we found that selective inhibition and genetic deficiency of LMP7 affect the pathogenesis of LCMV-induced meningitis in a distinct manner. Our findings support the important role of LMP7 in inflammatory disorders and suggest immunoproteasome inhibition as a novel strategy against inflammation-induced neuropathology in the CNS.

Inhibition of the Immunoproteasome Subunit LMP7 with ONX 0914 Ameliorates Graft-versus-Host Disease in an MHC-Matched Minor Histocompatibility Antigen-Disparate Murine Model.

In the current study we evaluated the effects of immunoproteasome inhibition using ONX 0914 (formerly PR-957) to ameliorate graft-versus-host disease (GVHD). ONX 0914, an LMP7-selective epoxyketone inhibitor of the immunoproteasome, has been shown to reduce cytokine production in activated monocytes and T cells and attenuate disease progression in mouse models of rheumatoid arthritis, colitis, systemic lupus erythematosus, and, more recently, encephalomyelitis. Inhibition of LMP7 with ONX 0914 in the B10.BR→CBA MHC-matched/minor histocompatibility antigen (miHA)-disparate murine blood and marrow transplant (BMT) model caused a modest but significant improvement in the survival of mice experiencing GVHD. Concomitant with these results, in vitro mixed lymphocyte cultures revealed that stimulator splenocytes, but not responder T cells, treated with ONX 0914 resulted in decreased IFN-γ production by allogeneic T cells in both MHC-disparate (B10.BR anti-B6) and miHA-mismatched (B10.BR anti-CBA) settings. In addition, a reduction in the expression of the MHC class I-restricted SIINFEKL peptide was observed in splenocytes from transgenic C57BL/6-Tg(CAG-OVA)916Jen/J mice exposed to ONX 0914. Taken together, these data support that LMP7 inhibition in the context of BMT modulates allogeneic responses by decreasing endogenous miHA presentation and that the consequential reduction in allogeneic stimulation and cytokine production reduces GVHD development.

Assessment of cytokine-modulated proteasome activity.

This chapter presents two methods for assessment of proteasome function. The first is a modification of the standard fluorogenic peptide cleavage assay which takes into account the effect of ATP on proteasome activity. This method is described in both its macro and high throughput micro-assay forms. The second is the Proteasome Constitutive Immuno-Subunit (active site) ELISA or ProCISE method. ProCISE is a modification of active site directed probe analysis and allows for convenient differentiation between active constitutive and immuno-subunits. While the utility of measuring proteasome activity and its relationship to cytokine action and inflammation are clear, the assessment and interpretation is not always straightforward. Therefore, we also discuss the pitfalls of the standard fluorogenic assay, particularly in the interpretation of results obtained, and the advantages of the newer, ProCISE assay.

Carfilzomib and oprozomib synergize with histone deacetylase inhibitors in head and neck squamous cell carcinoma models of acquired resistance to proteasome inhibitors.

Acquired resistance to proteasome inhibitors represents a considerable impediment to their effective clinical application. Carfilzomib and its orally bioavailable structural analog oprozomib are second-generation, highly-selective, proteasome inhibitors. However, the mechanisms of acquired resistance to carfilzomib and oprozomib are incompletely understood, and effective strategies for overcoming this resistance are needed. Here, we developed models of acquired resistance to carfilzomib in two head and neck squamous cell carcinoma cell lines, UMSCC-1 and Cal33, through gradual exposure to increasing drug concentrations. The resistant lines R-UMSCC-1 and R-Cal33 demonstrated 205- and 64-fold resistance, respectively, relative to the parental lines. Similarly, a high level of cross-resistance to oprozomib, as well as paclitaxel, was observed, whereas only moderate resistance to bortezomib (8- to 29-fold), and low level resistance to cisplatin (1.5- to 5-fold) was seen. Synergistic induction of apoptosis signaling and cell death, and inhibition of colony formation followed co-treatment of acquired resistance models with carfilzomib and the histone deacetylase inhibitor (HDACi) vorinostat. Synergism was also seen with other combinations, including oprozomib plus vorinostat, or carfilzomib plus the HDACi entinostat. Synergism was accompanied by upregulation of proapoptotic Bik, and suppression of Bik attenuated the synergy. The acquired resistance models also exhibited elevated levels of MDR-1/P-gp. Inhibition of MDR-1/P-gp with reversin 121 partially overcame carfilzomib resistance in R-UMSCC-1 and R-Cal33 cells. Collectively, these studies indicate that combining carfilzomib or oprozomib with HDAC or MDR-1/P-gp inhibitors may be a useful strategy for overcoming acquired resistance to these proteasome inhibitors.

Anti-leukemic activity and mechanisms underlying resistance to the novel immunoproteasome inhibitor PR-924.

PR-924 is a novel prototypic immunoproteasome inhibitor bearing markedly enhanced specificity for the ß5i immunoproteasome subunit, compared to the classical proteasome inhibitor bortezomib. Here, we assessed the growth inhibitory potential of PR-924 in three human hematologic malignancy cell lines (CCRF-CEM, THP1, and 8226) and their bortezomib-resistant sublines. Parental cells displayed equal sensitivity to PR-924 (IC50: 1.5-2.8 µM), whereas their bortezomib-resistant tumor lines displayed a 10-12 fold cross-resistance to PR-924. However, PR-924 cross-resistance factors for bortezomib-resistant sublines were markedly lower compared to the resistance factors to bortezomib. Proteasome inhibition experiments confirmed that PR-924 specifically inhibited ß5i activity, even far below concentrations that exerted anti-proliferative activity. We further determined whether PR-924 activity might be compromised by acquisition of drug resistance phenomena. Indeed, CEM cells rendered stepwise resistant to 20 µM PR-924 (CEM/PR20) displayed 13-fold PR-924-resistance and 10-fold cross-resistance to bortezomib. CEM/PR20 cells were devoid of mutations in the PSMB8 gene (encoding ß5i), but acquired Met45Ile mutation in the PSMB5 gene (encoding constitutive ß5), consistent with ß5 mutations observed in bortezomib-resistant cells. Furthermore, compared to parental CEM cells, CEM/PR20 cells exhibited 2.5-fold upregulation of constitutive proteasome subunit expression, whereas immunoproteasome subunit expression was 2-fold decreased. In conclusion, PR-924 displayed potent anti-leukemic activity including toward bortezomib-resistant leukemia cells. Despite the specificity of PR-924 to the ß5i immunoproteasome subunit, its anti-leukemic effect required concentrations that blocked both ß5 and ß5i subunits. This is underscored by the emergence of mutations in PSMB5 rather than in PSMB8.