Mechanism of Action of Proteasome Inhibitors and DeacetylaseInhibitors and the Biological Basis of Synergy in Multiple Myeloma
Teru Hideshima, Paul G. Richardson, and Kenneth C. Anderson
AbstractNovel agents, including the proteasome inhibitor bortezomib, have significantly improved the response
and survival of patients with multiple myeloma over the last decade. Despite these advances, many patients
relapse or do not benefit from the currently available therapies; thus, multiple myeloma remains an incurable
disease. Deacetylase inhibitors (DACi), including panobinostat and vorinostat, have recently emerged as
novel agents being evaluated in the treatment of multiplemyeloma. Deacetylases are a group of enzymeswith
effects on various intracellular proteins, including histones, transcription factors, and molecular chaperones.
Although DACi inhibit cell growth and induce apoptosis in multiple myeloma cells as a single agent,
synergistic activity has been observedwhen theywere used in combinationwith bortezomib. Themechanistic
basis of synergy is multifactorial and includes disruption of protein degradation and inhibition of the
interaction of multiple myeloma cells with the tumor microenvironment. This review summarizes recent
advancements in the understanding of the mechanism of action of proteasome inhibitors and DACi in
multiple myeloma and examines the biological basis of their synergistic effects. Data from the studies
summarized here have been used as the rationale for the implementation of phase II and III clinical trials of
DACi, alone and combined with bortezomib, in relapsed and refractory multiple myeloma. Mol Cancer Ther;
10(11); 2034–42. �2011 AACR.
Introduction to Multiple Myeloma
Epidemiology and treatmentMultiple myeloma is a plasma cell malignancy
predominantly localized in the bone marrow and char-acterized clinically by paraproteinemia (M-protein), de-structive bone disease, hypercalcemia, renal failure, andhematologic dysfunction. In 2010, it was estimated that20,180 new myeloma cases would be diagnosed in theUnited States alone, accounting for 1.3% of all newlydiagnosed cancer cases (1). Myeloma-related deathsaccounted for an estimated 1.9% of all cancer deaths,with an estimated 10,650 in 2010 (1).
Treatments for multiple myeloma have included corti-costeroids (e.g., dexamethasone and prednisone) andcytotoxic drugs (e.g., melphalan, vincristine, cyclophos-phamide, and doxorubicin; ref. 2). In the past decade,developments in the treatment of patients with multiplemyeloma have been substantial, including the U.S. Foodand Drug Administration (FDA) approval of 3 novel
agents: the immunomodulatory drugs thalidomide andlenalidomide and the proteasome inhibitor bortezomib.Randomized clinical trials with these agents have shownsignificant benefit in patient response and outcome (3–5).The most compelling evidence for the impact of thesetherapies is the remarkable improvement in the survivalof patients with multiple myeloma diagnosed since thedevelopment of these novel agents (6). However, a largeunmet need remains for patients with acquired or intrin-sic resistance to these therapies. A recent analysis showedthat patients who relapsed on and/or were refractory toprior bortezomib, thalidomide, or lenalidomide had pooroutcomes, with an overall survival of 6 months and anevent-free survival of 1 month (7). Therefore, despite theimportant developments in multiple myeloma treatment,the development of new agents to improve long-termoutcomes is needed, particularly in patients who derivelimited benefit from the currently available treatmentoptions.
Multiple myeloma disease biologyContinued research on the tumor microenvironment
has led to an increased understanding of the factors thataffect multiple myeloma cell growth and survival; thisunderstanding has been integral to the development ofnovel agents. Cell adhesion molecules and cytokines playa key role in tumor cell localization, invasion, and spreadof the disease (8). Within the bone marrow, adhesionmolecules facilitate the interaction of multiple myeloma
Authors' Affiliation: Jerome Lipper Myeloma Center, Department ofMedical Oncology, Dana-Farber Cancer Institute and Harvard MedicalSchool, Boston, Massachusetts
Corresponding Author: Teru Hideshima, Dana-Farber Cancer Institute,44 Binney St., Boston, MA 02115. Phone: 617-632-2144; Fax: 617-632-2140; E-mail: [email protected]
cells to both the bone marrow stromal cells (BMSC) andthe extracellular matrix (ECM; ref. 8). Binding of multiplemyeloma cells to BMSCs occurs, at least in part, throughbinding of very late antigen 4 (VLA4) to vascular celladhesion molecule 1 (VCAM1) and leukocyte function-associated antigen 1 (LFA1) to intracellular adhesionmolecule 1 (ICAM1; ref. 8). The interaction betweenmultiple myeloma cells and the ECM is mediated bythe binding of syndecan 1 (CD138) to collagen andVLA4 to fibronectin (8). Importantly, the interaction ofmultiple myeloma cells to BMSCs activates the transcrip-tion and secretion of interleukin-6 (IL-6), which facilitatesthe paracrine-mediated growth and survival of multiplemyeloma cells (8). IL-6 also downregulates the expressionof CD138, leading to the spread of cells into the blood-stream, which may lead to the development of plasmacell leukemia (8).Numerous cytokines, in addition to IL-6, play an im-
portant role in multiple myeloma cell proliferation, sur-vival, migration, and drug resistance in the tumormicroenvironment. Besides the direct effects of thesecytokines and/or chemokines onmultiple myeloma cells,several studies have examined the role of VEGF in neo-vascularization in the bone marrow microenvironmentand disease progression in multiple myeloma (9). Mye-loma cells secrete VEGF, which contributes to new bloodvessel formation in vitro (10). Moreover, VEGF-mediatedstimulation of microvascular endothelial cells resultsin increased secretion of IL-6, with continued multiplemyeloma cell growth (11, 12). In addition to its proangio-genic effects in the bone marrow, VEGF has been shownto directly induce tumor cell proliferation throughthe Raf/mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway,as well as tumor cell migration through a protein kinaseC–dependent pathway (12).TNF-a plays an important role in the biology of mul-
tiple myeloma cells within the tumor microenvironment.Although TNF-a has modest effects on multiple myelo-ma cell proliferation, it does lead to increased expressionof adhesion molecules, including ICAM1, on both mul-tiple myeloma cells and BMSCs. The cytokine also leadsto increased heterotypic adhesion, thereby triggeringNF-kB–dependent upregulation of transcription andsecretion of IL-6 and resulting in paracrine multiplemyeloma cell growth by BMSCs (13). Insulin-like growthfactor I (IGF-I) is another cytokine that supports multiplemyeloma cell proliferation and survival. IGF-I is presentin both the bonemarrowmicroenvironment and periphe-ral blood and stimulates multiple myeloma cell prolife-ration and survival (14), via activation of NF-kB and Akt,as well as increased expression of several antiapoptoticproteins, including FADD-like IL-1b–converting enzymeinhibitory protein, X-linked inhibitor of apoptosis, andsurvivin (14). Although IGF-I induces more prominentAkt activation than IL-6, it does not activate the Januskinase 2/STAT3 pathway, which is commonly activatedby gp130 family cytokines including IL-6.
The proliferation and survival of multiple myelomacells within the tumor microenvironment is, therefore,dependent on their interaction with the BMSC and theECM. These factors supporting multiple myeloma cellsurvival are complex and provide many mechanisms forthe development of resistance to commonly used agents.Novel strategies to target or disrupt these pathways areurgently needed.
Mechanistic Basis of Antimyeloma Activity ofProteasome Inhibitors in Multiple Myeloma
Proteasomesareabundantmultienzymecomplexes thatprovide themain pathway for degradation of intracellularproteins and contribute to the maintenance of proteinhomeostasis and clearance of misfolded and/or unfoldedand cytotoxic proteins (15). The 26S proteasome is a large2.4-MDa ATP-dependent proteolytic complex, located inboth the cytoplasm and nucleus. This proteasome consistsof a 20S core catalytic cylindrical complex capped at bothendsby19S regulatory subunits (15).Polyubiquitination isan essential event for proteins targeted for proteasomaldegradation (15). Proteins degraded by the proteasomeincludemediators of cell-cycle progression and apoptosis,such as the cyclins, caspases, B-cell lymphoma 2 (BCL2),and NF-kB activation (15).
It has been hypothesized that cancer cells are moredependent on the proteasome for clearance of abnormalor mutant proteins (15). In fact, several preclinical studieshave shown that malignant cells are more sensitive toproteasome inhibition than normal cells (16–20). The pro-teasome inhibitor bortezomib is a dipeptide boronic acidanalog that reversibly inhibits the chymotryptic activity ofthe 20S subunit of the proteasome (19). Bortezomib hasbeen shown to directly inhibit proliferation and induceapoptosis inmultiplemyeloma cell lines andpatient tumorcells resistant to conventional therapies (20). Furthermore,bortezomib showed enhanced anti–multiple myelomaactivity with dexamethasone and overcame resistance toapoptosis conferred by IL-6 or adhesion to BMSCs (20).Significant tumor growth inhibition and increased hostsurvival were also observed in vivo using a mouse–humanmultiple myeloma cell xenograft model (19).
Bortezomib showed remarkable clinical activity inpatients with multiple myeloma and was rapidlyapproved by the FDA in 2003 to treat relapsed andrefractory multiple myeloma. Clinical activity resultingin high response rates and increased progression-free andoverall survival has also been observed in multiple phaseIII clinical trials with bortezomib. Of note, bortezomib hasshown increased activity in combination with melphalanand prednisone compared with melphalan and predni-sone alone in newly diagnosed patients with multiplemyeloma (4), as well as increased activity as a singleagent compared with dexamethasone in patients withrelapsed or refractory disease (21).
The mechanism of action and target of bortezomib,leading to disruption of intracellular protein metabolism,
Proteasome and Deacetylase Inhibitors in Multiple Myeloma
www.aacrjournals.org Mol Cancer Ther; 10(11) November 2011 2035
are well characterized. The downstream biological effectsof proteasome inhibition are multifactorial, with directeffects on both multiple myeloma cells and the multiplemyeloma cell microenvironment, including inhibition ofcytokine secretion, suppression of adhesion moleculeexpression, and inhibition of angiogenesis. The initialrationale to use bortezomib in cancer was its inhibitoryeffect on NF-kB activity, thereby modulating transcrip-tion. Specifically, the NF-kB canonical pathway is regu-lated by inhibitor protein IkB, which blocks nucleartranslocation of the p50 (NF-kB1)/p65 (RelA) heterodi-mer. Importantly, IkB is a substrate of the proteasome,and proteasome inhibition by bortezomib can, therefore,lead to an increase in the cytoplasmic level of IkB, result-ing in a blockade of NF-kB translocation to the nucleusand DNA-binding activity (15). NF-kB has also beenidentified as a mediator of paracrine signaling betweenmultiple myeloma cells and BMSCs within the bonemarrow microenvironment. For example, NF-kB–depen-dent upregulation of IL-6 in BMSCs is induced by adhe-sion to multiple myeloma cells or via TNF-a secretion bymultiple myeloma cells (13, 22). TNF-a–induced upre-gulation of NF-kB leads to increased expression of theadhesion molecules ICAM1 and VCAM1 on multiplemyeloma cells and BMSCs, thus enhancing intercellularbinding (13). Bortezomib blocks the TNF-a–inducedupregulation of NF-kB, leading to decreased binding ofmultiple myeloma cells to BMSCs and related decreasedIL-6 secretion (13, 20). Of note, the specific IkB kinaseinhibitors PS-1145 and MLN120B also inhibit secretion ofIL-6 and adhesion of multiple myeloma cells and BMSCs;however, these agents only lead to partial inhibition ofmultiple myeloma cell growth (23). Therefore, bortezo-mib-triggered anti–multiple myeloma activities are notsolely mediated by NF-kB inhibition.
Bortezomib inhibits angiogenesis in the bone marrowmicroenvironment, which plays an important role in bothmultiple myeloma pathogenesis and disease progression.In preclinical models using multiple myeloma patient–derived endothelial cells, bortezomib inhibited cell pro-liferation, chemotaxis, adhesion, and capillary formation,thus supporting its angiogenic inhibitory activity in vivo(24). Bortezomib also inhibited the expression and secre-tion of several proangiogenic factors, including VEGF(24). In addition, VEGF-mediated migration of multiplemyeloma cells was inhibited by bortezomib (12).
Inhibition of the proteasome induces accumulation ofintracellular misfolded and/or unfolded proteins (20),which triggers the unfolded protein response (UPR)signaling pathway to protect cells against cellular stress(25, 26). Because multiple myeloma cells produce largeamounts of immunoglobulin, a functional UPR isrequired for their survival (25, 26). Treatment of multiplemyeloma cells with bortezomib leads to induction ofproapoptotic UPR components, including growth arrestand DNA damage–inducible gene 153 (26). Proteasomeinhibition also interferes with the stability of mRNAtranscripts of X box–binding protein 1, a downstream
transcription factor of IRE1a, regulating the UPR (25).Taken together, these data show the reliance of multiplemyeloma cells on the UPR for survival and identifydisruption of protein metabolism as a potential therapeu-tic target in multiple myeloma.
Mechanistic Basis of Antimyeloma Activity ofDeacetylase Inhibitors in Multiple Myeloma
Histone deacetylases (HDAC) have emerged as a rel-evant clinical target in multiple myeloma. HDACs andhistone acetyl transferases regulate the acetylation oftarget proteins (27). Specifically, HDACs remove acetylgroups from target proteins that regulate their activity(27). Eighteen HDACs have been identified in humansand divided into 4 classes based on their homology toyeast HDACs: class I (HDAC1, HDAC2, HDAC3, andHDAC8), class IIa (HDAC4, HDAC5, HDAC7, andHDAC9), class IIb (HDAC6 and HDAC10), class III (SIRTfamily), and class IV (HDAC11; ref. 27). The differentclasses of enzymes also differ in their subcellular local-ization, with class I HDACs found in the nucleus andclass II enzymes found in both nucleus and cytoplasm,and their intracellular targets (27). The name HDAC isbased on the identification of histone proteins as theinitial target of HDACs; however, a recent study in cancercell lines identified 3,600 acetylation sites on 1,750proteins associated with various intracellular functionsincluding gene expression, DNA replication and repair,cell-cycle progression, cytoskeletal reorganization, andprotein chaperone activity (28). Therefore, the term dea-cetylase (DAC) may be more appropriate when referringto these enzymes.
Although several deacetylase inhibitors (DACi) are invarious stages of clinical development, only 2 areapproved for treatment of cutaneous T-cell lymphoma,vorinostat (suberoylanilide hydroxamic acid), and romi-depsin (FK228 or FR901228; refs. 29, 30). DACi differ intheir structure and potency toward the HDAC enzymes.Romidepsin is a cyclic tetrapeptide with DAC inhibitoryactivity primarily toward class I HDACs (27). OtherDACi include the benzamide class, which includes moce-tinostat (MGCD103) and entinostat (MS-275) class I–spe-cific inhibitors (27). The hydroxamic acid–based DACi,vorinostat, panobinostat (LBH589), and belinostat(PXD101), are pan-DACi, with inhibitory activity againstclass I, II, and IV HDACs (Fig. 1; ref. 27). Panobinostat isamong the most potent DACi, with nanomolar DACinhibitory activity (31).
Preclinical studies showed that DACi have potentantimyeloma activity. The class I–specific DACi, romi-depsin, along with the pan-DACi dacinostat (LAQ824),vorinostat, and panobinostat, have been shown to inhibitproliferation and induce apoptosis in multiple myelomacell lines in vitro and in vivo in mouse xenograft models(32–36). In many of these preclinical studies, additive orsynergistic effects were observed when DACi were com-bined with other agents, including corticosteroids and
Hideshima et al.
Mol Cancer Ther; 10(11) November 2011 Molecular Cancer Therapeutics2036
proteasome inhibitors, establishing the rationale for clin-ical studies of DACi in combination with these agents(32–36).Clinical studies in patients with multiple myeloma
have shown limited single-agent activity of DACi (37,38). In a phase I trial of single-agent vorinostat (37, 38) in13 patients with relapsed and/or refractory multiplemyeloma, only 1 patient showed a minimal responseand 9 patients showed disease stabilization (37). In aphase II trial of single-agent romidepsin in 13 patientswith refractorymultiple myeloma, no objective responseswere observed; however, evidence of disease stabiliza-tion and resolution of disease-related symptoms wereseen (38). Overall, the activity of DACi as single agentshas been limited, and a clearer understanding of thebiological activity of these agents will help determinethe ideal combination therapies for clinical development.Because DACi act on many intracellular targets, the
biological basis of their antimyeloma activity is due to anumber of effects on multiple myeloma cells and theirinteraction with the tumor microenvironment. For exam-ple, LAQ824 induces upregulation of cyclin-dependentkinase inhibitor p21, leading to cell-cycle arrest followedby apoptosis through activation of caspases 8, 9, and 3
(33). Vorinostat also induced p21 expression, leading tocell-cycle arrest and apoptosis; however, no significantcaspase 8, 9, or 3 cleavage was observed, suggestingcaspase-independent apoptosis (39). Romidepsin hasbeen shown to induce multiple myeloma cell apoptosisthrough downregulation of the antiapoptotic proteinsBCL2, BCLXL, andmyeloid cell leukemia sequence 1 (35).
DACi also modulate interaction of multiple myelomacells with cellular components in the bone marrow mi-croenvironment. LAQ824 inhibited multiple myelomacell proliferation even in the presence of exogenousIL-6 or BMSC coculture (33). Vorinostat suppresses thestimulation of IL-6 secretion in BMSCs by multiplemyeloma cell adhesion, with no effect on BMSC viability(39). Vorinostat also suppresses autocrine IGF-I produc-tion, directly interrupting the IGF-I/IGF-IR/Akt signal-ing pathway critical for antiapoptosis and survival ofmultiple myeloma cells (40). DACi, therefore, can inducedirect multiple myeloma cell-cycle arrest and apoptosis,as well as disrupt signaling between multiple myelomacells and BMSCs.
Recent studies have shown that aggresomes representan alternative pathway for catabolism of misfoldedproteins and develop when production of misfolded
Figure 1. Molecular structures ofDACi and the proteasome inhibitorbortezomib.
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ubiquitinated proteins exceeds the capacity of protea-somes to degrade them (41). Misfolded proteins canform aggregates that are transported by microtubulesvia dynein motor complexes to the autophagosome,where they are degraded by lysosomes. HDAC6belongs to the class IIb HDACs and is broadly expressedin different types of cells. HDAC6 regulates acetylationof a-tubulin and facilitates the transport of the aggre-some to the lysosome (42). DACi that target HDAC6,such as the pan-DACi panobinostat and HDAC6-spe-cific inhibitor tubacin, lead to hyperacetylation of a-tubulin, disruption of the interaction between HDAC6and dynein, and resultant increases in ubiquitinatedproteins (41, 43). Therefore, inhibition of protein deg-radation through targeting of the aggresome by DACirepresents an attractive model for the treatment ofcancers such as multiple myeloma that are reliant onefficient protein metabolism.
Mechanisms of Synergy between ProteasomeInhibitors and Deacetylase Inhibitors in MultipleMyeloma
The molecular sequelae of proteasome inhibitors andDACi in multiple myeloma are associated with keypathways vital to the proliferation and survival ofmultiple myeloma cells. Although some common tar-gets and pathways are affected by each agent, some ofthe pathways targeted are complementary and mayunderlie the synergistic effects. In fact, synergisticantitumor activities between DACi and bortezomibhave been observed in several preclinical studies (36,41, 43–45). Either the pan-DACi vorinostat or panobino-stat with bortezomib have synergistic effects on inhibi-tion of cell growth and increasing apoptosis in multiplemyeloma cells (41, 44). Similar effects were observedwith the HDAC6-specific inhibitor tubacin combinedwith bortezomib, associated with a marked increase inpolyubiquitinated proteins (43). These effects wereobserved in both multiple myeloma cell lines and pri-mary tumor cells isolated from patients with multiplemyeloma. Importantly, the class I–specific inhibitorromidepsin and the pan-DACi panobinostat have alsoshown antitumor effects in vivo in human multiplemyeloma cell–mouse xenograft models (36, 45).
Despite the observation that synergistic effects wereobserved with bortezomib and a variety of DACi acrossseveral studies, the conclusions made about the biolog-ical basis of the synergy observed are varied. Thisvariation can be partially explained by the differentialpotency and targets of the various DACi tested in thesestudies. In addition, the pleiotropic effects that theseagents elicit in multiple myeloma cells, along withthe experimental design of the individual studies,may have led the investigators to focus on the mostrelevant biological effects observed. As the data frompreclinical studies have shown, numerous genes areaffected by bortezomib or DACi (39, 40, 46), and it is
therefore most likely that a combination of these effectsleads to the synergy observed between the 2 classes ofagents.
Themost well-characterizedmodel of synergy betweenproteasome inhibitors andDACi are the dual inhibition ofthe proteasome and aggresome pathways (Fig. 2; refs. 41,43). Targeting both the proteasome with bortezomib andthe aggresome with HDAC6 inhibitors in tumor cellsinduces greater accumulation of polyubiquitinatedproteins, resulting in increased cellular stress and apo-ptosis (41, 43). More specifically, proteasome inhibitiondrives the formation of aggresomes, which are dependenton the interaction of HDAC6 with tubulin and dyneincomplex (41). Moreover, the proteasome inhibitor (borte-zomib) and HDAC6 inhibitors (tubacin or panobinostat)lead to increased hyperacetylation of tubulin and gener-ation of polyubiquitinated proteins, thus increasing cel-lular stress response (i.e., c-JunN-terminal protein kinaseactivation) and leading to apoptosis, which is, in part,dependent on caspase activity (41–43).
Although disruption of protein degradation repre-sents a major contributor to the synergistic antitumoractivity observed between proteasome inhibitors andDACi, other studies have identified additional mechan-isms. For example, the combination of bortezomib andvorinostat results in enhanced cytochrome-c release,caspase and PARP cleavage, and inactivation ofNF-kB, followed by apoptosis (44). Conversely, antiox-idative agents, including N-acetyl-L-cysteine, blockthese effects (44).
In addition to the synergistic effects observed whenthese agents are combined, it is plausible that each agentaffects complementary pathways in multiple myelomacells, thereby leading to synergistic effects on growthinhibition and apoptosis. As summarized in the preced-ing sections, bortezomib and DACi both affect pathwaysassociated with the interaction of multiple myeloma cellsand the microenvironment, including cytokine signalingand cell adhesion (Fig. 3). In addition, overexpression ofproto-oncogenes and/or oncogenic genes is a commonmechanism of resistance in cancer, and a recent studyshowed that bortezomib specifically downregulates theexpression of class I HDACs, leading to histone hyper-acetylation (45). It was also noted that exogenous over-expression of HDAC1 caused resistance to bortezomibboth in vitro and in vivo, which was reversed by the class IDACi romidepsin (45). In addition, pan-DACi LAQ824has been shown to decrease the activity of the 20S protea-some, as determined by reduced proteasome chymotryp-sin-like activity (33). The ability of proteasome inhibitorsto downregulate HDACs, along with the observationthat DACi can decrease proteasome activity, may alsocontribute to the synergistic antitumor activities. Takentogether, proteasome inhibitors and DACi target severalrelevant mechanisms in multiple myeloma biology. Fur-ther research will uncover additional mechanisms thatcontribute to the synergistic antitumor activities andpotential avenues of resistance.
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The synergy between proteasome inhibitors and DACiis most likely dependent on a number of mechanismstargeting multiple myeloma cell biology. Multiple mye-loma cell proliferation, survival, and progression of dis-ease are dependent on the activation of key pathwayswithin the cell, as well as the interaction with elements inthe tumor microenvironment. One of the most compel-ling mechanisms underlying the synergy remains thedisruption of protein degradation by inhibition of theproteasome and aggresome. Because multiple myelomacells produce abundant amounts of immunoglobulin thatmust be properly folded or degraded, they are moredependent on efficient processing of proteins (41–43).This mechanism clearly contributes to synergy observedbetween the 2 agents; however, it is unlikely to be solelyresponsible for the synergy observed. Of note, a recent
report showed that romidepsin, a DACi with limitedHDAC6 inhibitory activity, enhanced the in vitro andin vivo activity of bortezomib in HDAC1-overexpressingmultiple myeloma cells, thus suggesting a role for theinteraction of these agents independent of the effects onprotein degradation (45). In addition, both proteasomeinhibitors and DACi decrease cytokine production andexpression of adhesion molecules, key factors supportingthe growth and survival of multiple myeloma cells (39,46). It is, therefore, most likely that a number of factorscontribute to the synergy between proteasome inhibitorsand DACi.
Although bortezomib clearly has proven activity as asingle agent and in combination therapy in patients withmultiple myeloma, initial trials with single-agent DACihave not led to significant clinical activity (4, 37, 38, 47).On the basis of preclinical data, combining DACi withproteasome inhibitors, such as bortezomib, represents an
Bortezomib
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Figure 2. Inhibitions of the proteasome and aggresome pathways by bortezomib and DACi. Unfolded and/or misfolded proteins are targeted by ubiquitinfor degradation by the proteasome and aggresome pathways. The proteasome inhibitor bortezomib leads to the accumulation of ubiquitin proteinaggregates. These aggregates are shuttled to the lysosome, where they are degraded via the aggresome pathway. Aggresome formation involves theshuttling of the protein aggregates along microtubules by dynein motor proteins. The interaction of the unfolded and/or misfolded protein complexesis facilitated by HDAC6. Conversely, DACi with inhibitory activity toward HDAC6 blocks this process. The combination of proteasome inhibitors andDACi leads to increased cellular stress and apoptosis.
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attractive strategy for the treatment of patients withmultiple myeloma. Preliminary data from phase I studiesevaluating the pan-DACi panobinostat or vorinostat incombination with bortezomib have shown responses inpatients who received bortezomib before study enroll-ment, including patients who failed to respond previous-ly to bortezomib (48, 49). These preliminary observationsare being evaluated further in phase II and III clinicaltrials. The 2 phase III trials will evaluate the role of DACias a strategy to increase treatment efficacy in patientswith relapsed multiple myeloma. The VANTAGE 088trial (NCT00773747) is evaluating the combination ofvorinostat plus bortezomib, and the PANORAMA 1 trial(NCT01023308) is evaluating the combination of panobi-nostat plus bortezomib and dexamethasone (50, 51). Both
trials are comparing the combination with bortezomibplus placebo. The results of these trials will help todetermine whether DACi can enhance the efficacy ofbortezomib in patients with relapsed multiple myeloma.Two single-arm phase II studies, VANTAGE 095(NCT00773838) and PANORAMA 2 (NCT01083602),are evaluating the combination of DACi, bortezomib,and dexamethasone in patients with relapsed and borte-zomib-refractory disease. The results from these trialswill determine if DACi can sensitize patients with borte-zomib-resistant multiple myeloma (50, 52). The results ofthese trials, along with further research on other novelDACi and proteasome inhibitors in development, willhelp guide the ideal setting and combination partners forthe treatment of patients with multiple myeloma.
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Figure 3. Bortezomib and DACi inhibit key pathways associated with multiple myeloma (MM) cell growth and survival. Growth and survival of multiplemyeloma cells are dependent on functioning intracellular pathways that drive proliferation of and interaction with the ECM and BMSC. The combinationof bortezomib and DACi leads to inactivation of NF-kB and multiple myeloma cell apoptosis. Both DACi and bortezomib suppress the production ofcytokines including IL-6 and IGF-I. Bortezomib also suppresses TNF-a, leading to inhibition of the interaction of multiple myeloma cells and BMSCs.Bortezomib has been shown to decrease the secretion of VEGF, leading to inhibition of angiogenesis. The cell-surface proteoglycan syndecan 1 isdownregulated by DACi, which affects the interaction of multiple myeloma cells with the ECM.
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Mol Cancer Ther; 10(11) November 2011 Molecular Cancer Therapeutics2040
T. Hideshima is a consultant for Acetylon Pharmaceuticals. P.G. Richard-son is a consultant and on advisory boards for Millennium and Celgene.K.C. Anderson is a consultant and on advisory board for Millennium,Celgene, and Novartis.
Acknowledgments
The authors thank William Fazzone for editorial assistance.
Grant Support
This study is supported by NIH SPORE IP50 CA-10070, PO-1 78378,and RO-1 CA-50947 grants; the LeBow Family Fund to Cure Myeloma(K.C. Anderson); and American Cancer Society Clinical Reward Profes-sorship (K.C. Anderson). Financial support for editorial assistance wasprovided by Novartis.
Received June 10, 2011; revised July 28, 2011; accepted July 28, 2011;published November 9, 2011.
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2011;10:2034-2042. Mol Cancer Ther Teru Hideshima, Paul G. Richardson and Kenneth C. Anderson Inhibitors and the Biological Basis of Synergy in Multiple MyelomaMechanism of Action of Proteasome Inhibitors and Deacetylase
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