The Future of Proteasome Inhibitors in Relapsed/Refractory Multiple Myeloma

November 15, 2011

Bortezomib (Velcade), the first-in-class inhibitor of the proteasome,[1] or multicatalytic proteinase complex,[2] was originally found to be active against relapsed and relapsed/refractory multiple myeloma as a single agent in phase I through III clinical trials.[3-6

The ubiquitin-proteasome pathway was first validated as a target for cancer therapy with the demonstration of the activity of the boronic acid proteasome inhibitor (PI) bortezomib (Velcade) against relapsed and relapsed/refractory multiple myeloma. Another generation of PIs is now entering the clinical arena; this includes intravenous agents such as carfilzomib, CEP-18770, and marizomib, and oral drugs such as MLN9708 and ONX 0912. These novel agents will likely first be used for patients with disease that has either relapsed or been refractory to prior therapy (including bortezomib-based regimens) because of their ability to overcome drug resistance, or will be used in patients who are intolerant of, or are not candidates for bortezomib. Preclinical studies also suggest that PIs may act synergistically with other conventional and novel agents, or even with one another in rationally designed combination regimens. In addition, other inhibitors that selectively target only the immunoproteasome and not the constitutive proteasome, as well as agents that bind to noncatalytic proteasome subunits, are emerging as potential drug candidates. Taken together, it seems likely that we have only begun to appreciate the full potential of inhibition of the proteasome. This article extrapolates our current knowledge into an algorithm for the future use of these inhibitors against multiple myeloma.


Bortezomib (Velcade), the first-in-class inhibitor of the proteasome,[1] or multicatalytic proteinase complex,[2] was originally found to be active against relapsed and relapsed/refractory multiple myeloma as a single agent in phase I through III clinical trials.[3-6] Its benefits included a robust overall response rate (ORR) and complete response rate, strong response durability with improved time to progression compared with prior therapies, and superior overall survival compared with dexamethasone (even in clinically and cytogenetically high-risk patients), as well as a predictable and manageable toxicity profile. Combinations of bortezomib with other conventional and novel drugs extended the benefits for this patient population further, and a number of these regimens are now part of the standard of care.[7] This is especially true for bortezomib with pegylated liposomal doxorubicin (Doxil), as supported by phase III data.[8] Agents with activity against multiple myeloma in the relapsed and relapsed/refractory setting are often incorporated into front-line therapy, and this has certainly been true for bortezomib. In combination with either dexamethasone[9] or thalidomide and dexamethasone,[10] bortezomib is now part of the standard of care for induction therapy in transplant-eligible patients-and bortezomib with melphalan and prednisone[11] is a standard induction regimen for transplant-ineligible patients. Moreover, bortezomib's activity is being explored in many other combinations, such as with lenalidomide (Revlimid) and dexamethasone,[12,13] and in other settings, such as with melphalan as a pretransplant conditioning regimen,[14-17] after transplant as part of consolidation,[10,18] and as a component of maintenance approaches.[19-26] While the increasing use of bortezomib in early disease settings has been associated with substantial clinical benefits, it does raise the concern that further therapy targeting the proteasome in later lines may be less effective. Fortunately, retreatment with bortezomib in patients who have had an earlier response to therapy, in either the front-line or the relapsed and/or refractory setting, is one option that has been validated in a number of studies.[27-32] Even more encouraging is the development of emerging novel PIs for the relapsed and/or refractory setting that seem destined to play an important role in our armamentarium against myeloma. The future use of these emerging novel PIs in the relapsed and/or refractory setting is examined in this article.

Next-Generation Proteasome Inhibitors

Preclinical studies have validated a number of next-generation PIs in models of multiple myeloma, and the results have already been translated to the clinic. These next-generation PIs include agents that bind reversibly, such as the peptide boronates MLN9708[33,34] and CEP-18770.[35-37] In contrast, other PIs under study bind irreversibly, including the epoxy-ketones ONX 0912[38] and carfilzomib,[39,40], which was formerly known as PR-171, as well as the salinosporamide marizomib,[41-43] previously known as NPI-0052. All these drugs target at least the β5 catalytic subunit of the constitutively expressed 26S proteasome, as well as the corresponding β5i, or low molecular weight polypeptide (LMP)-7 subunit of the immunoproteasome; these two subunits (β5 and β5i) both contain chymotrypsin-like protease activity. In some cases, other constitutive proteasome and immunoproteasome subunits are bound as well, both in vitro and in vivo, although the contribution of these effects to their clinical antimyeloma activity has yet to be elucidated. An even larger number of PIs that have been validated in laboratory studies have not yet reached the clinic but either show some promise of doing so themselves or may serve as models for other agents that may become clinically useful. Attractive examples include agents that predominantly target proteasome catalytic sites other than β5, such as the β1 caspase-like[44] and β2 trypsin-like[45] activities. Other interesting inhibitors spare the constitutive proteasome while specifically targeting β subunits of the immunoproteasome, including either LMP-2[46-49] (eg, IPSI-001[48]), or LMP-7, (eg, ONX 0914 [formerly PR-924]).[50] Still other PIs seem to exert their effects on protein turnover by interacting with α subunits of the proteasome, which have been classically felt to be structural in nature and not catalytic, or at the interface between the α and β subunits. Examples of this group of PIs include the cathelin-like peptide PR-39,[51,52] 5-amino-8-hydroxyquinoline (5AHQ),[53] and chloroquine.[54] These may work in part by inducing allosteric changes in the structure of the proteasome, which could reduce or eliminate access by protein and peptide substrates to the proteolytically active β subunits.

Future Indications for Novel Proteasome Inhibitors

Next-generation PIs, both alone and in combination with other drugs, could play an important role in the treatment of patients with advanced multiple myeloma in the following prominent settings.

Patients with refractory myeloma

Patients with multiple myeloma that is both relapsed and refractory to their previous line of treatment, and especially those who have already received regimens containing bortezomib and lenalidomide, have a poor prognosis[55] and need new treatment approaches.[56] Since such patients make up a group for whom there is an unmet medical need, they represent an appropriate population for testing new drugs. Successful results in even a single-arm phase II study could lead to regulatory approvals.[57] Next-generation PIs in preclinical studies have been shown to have the ability to overcome drug resistance, including in some cases bortezomib resistance,[33,35-39,41-43] making them attractive candidates for this indication. Reports of initial phase I study data with carfilzomib,[58,59] marizomib,[60,61] and MLN9708[62] support the possibility that these agents will indeed be active in relapsed and refractory disease. Carfilzomib has advanced the furthest in clinical development, and as detailed earlier in this supplement, studies have now confirmed its substantial activity in this setting. In particular, the PX-171-003 phase IIb trial showed that 24% of 257 response-evaluable patients who had received a median of five prior lines of treatment and were refractory to their prior line of therapy, achieved at least a partial response with carfilzomib, while 36% had a minimal response or better.[63,64] Among patients whose disease was refractory to bortezomib in a previous line of therapy-which was the case for 65% of the population as a whole-up to 19% achieved a partial response or better, while 34% were able to obtain at least a minimal response. Response durability was also achieved, with durations of response of 7.4 months for those with a partial response or better, and 8.3 months for those with at least a minimal response.

The ability of one PI to overcome clinical resistance to another is not surprising, given that there are other examples, even in the myeloma field, of the successful sequential use of two different drugs that are nonetheless in the same class. Notably, patients who have been exposed to thalidomide (Thalomid) can respond to lenalidomide-based regimens even if their disease was previously refractory to the first-generation immunomodulatory agent.[65] Optimal use of all the currently available and future PIs would be significantly advanced by the development of a better understanding of the mechanisms of resistance to this class of drugs.

It seems clear based on preclinical studies that there are many pathways involved in de novo drug resistance that reduce the efficacy of bortezomib, and probably that of other PIs, in bortezomib-naive models. Prominent examples include inducible drug resistance mechanisms such as activation of heat shock proteins (HSPs) like HSP-27, HSP-70, and HSP-90[66-69]; stress response proteins like mitogen-activated protein kinase phosphatase[70-72]; and antiapoptotic signal transduction pathways such as protein kinase B/AKT, [67,73-75] and their downstream targets, including Bcl-2 family proteins.[76]

Secondary or acquired resistance is also an important phenomenon and exhibits its effects in patients who initially respond to bortezomib but who then do not benefit from retreatment, or in some cases even progress while on PI-based therapy. Laboratory models of drug-induced resistance suggest that overexpression of the β5 proteasome subunit,[77,78] or mutation of the β5 subunit to reduce bortezomib binding[77,79,80] may be involved. Other studies have implicated a role for multidrug-resistant proteins[81,82] that may act by promoting efflux of peptide-based drugs, like most PIs, from the myeloma cell. Whether any of these mechanisms are important in vivo, however, is not known at this time.

This leaves open the possibility that other changes in the myeloma cell itself, or in the local microenvironment or the host as a whole, could also contribute. Enhanced metabolism of bortezomib, which occurs through oxidative deboronation,[83,84] is one such potential mechanism. Another is induction of different proteases that may be able to substitute for some proteasome functions, such as tripeptidyl peptidase II, which has been observed in eukaryotes,[85] or tricorn protease,[86] which may normally work downstream of the proteasome.[87] Finally, mutations in the proteasome outside the β5 subunit could be important too, by activating other proteolytic activities, including the branched-chain amino acid–preferring and small neutral amino acid activities[2] that may substitute for the chymotrypsin-like activity primarily targeted by bortezomib. Such knowledge would have a strong impact on the appropriate therapeutic sequencing of the different PIs. For example, if studies of plasma cells from patients who relapsed after bortezomib therapy revealed enhanced deboronation activities, then epoxyketones like carfilzomib would be favored as the next line of therapy as opposed to other boronates, such as CEP-18770 or MLN9708. In contrast, if peptide efflux were responsible for resistance, then use of a non–peptide inhibitor such as marizomib would be favored-or one could use approaches that combined bortezomib or carfilzomib with an efflux pump inhibitor.

Patients with relapsed multiple myeloma

Agents that show activity in relapsed and refractory myeloma are typically rapidly moved into earlier disease settings, including relapsed myeloma, and then eventually newly diagnosed disease. Next-generation PIs are certainly following that path both as single agents[33,35,36,40] and in rationally designed combination regimens based on strong preclinical data showing their ability to cooperate with other antimyeloma agents.[37-39,41-43] Data are particularly well developed for carfilzomib, which as detailed earlier in this supplement, was able to induce a 53% ORR in a bortezomib-naive population of patients with relapsed myeloma who had a median of two prior lines of therapy, and who received 20 mg/m2 of carfilzomib during cycle 1, and 27 mg/m2 thereafter.[88] Carfilzomib was also able to be easily combined with lenalidomide and dexamethasone, as demonstrated in a phase Ib/II trial of this regimen for relapsed and/or refractory disease.[89] This study showed that full-dose carfilzomib was tolerated well, along with the standard lenalidomide and low-dose dexamethasone regimen.[90] Moreover, it achieved rapid responses and a 78% ORR,[89] leading to the initiation of a phase III trial comparing the three-drug regimen to lenalidomide and low-dose dexamethasone in patients with one to three prior lines of therapy.[91]


Ongoing Studies Incorporating Next-Generation Proteasome Inhibitors for Relapsed and/or Refractory Multiple Myelomaa

Bortezomib is now used in combination with a wide array of both conventional and novel agents against multiple myeloma, and it is likely that the same will become true for carfilzomib and other second-generation PIs. Indeed, a number of such trials are currently underway (Table). The regimens being studied include combinations with other standards of care like lenalidomide and dexamethasone, as well as combinations with novel drugs like the histone deacetylase inhibitor panobinostat and the kinesin spindle protein inhibitor ARRY-520. Still others are in the planning stages.

An especially interesting combination approach suggested by the different chemistries and targets of the various PIs is the possibility that two such agents could be combined to good effect. One such combination could include bortezomib, which can inhibit both the chymotrypsin-like activity and the post–glutamyl peptide hydrolyzing, or caspase-like activity,[92,93] together with marizomib, which inhibits the chymotrypsin-like and trypsin-like activities (and also the caspase-like activity, although to a lesser extent than this is inhibited by bortezomib).[41] Such a regimen has already been validated using both in vitro[41] and in vivo[42] models. Similarly, preclinical studies have shown that inhibitors that predominantly target proteasome catalytic sites other than β5, such as the β1 caspase-like[44] and β2 trypsin-like[45] activities, can sensitize to bortezomib.

It may also be possible to use bortezomib, which targets both the constitutive proteasome and the immunoproteasome, in combination with an immunoproteasome-specific inhibitor, although some data suggest that targeting both β5 and β5i is needed to optimally induce cell death.[48] It is also possible that additive effects could be achieved by combining bortezomib or carfilzomib with some of the agents that target the α proteasome subunits, such as clinically relevant analogs of 5AHQ[53] and chloroquine.[54] Validation of some of these approaches would still be needed, but one could envision randomized studies comparing, for example, bortezomib and dexamethasone to bortezomib with marizomib and dexamethasone-targeting either patients with previous bortezomib exposure, or PI-naive patients. Were the synergy that has been demonstrated preclinically to translate well to the clinic, it is possible that lower doses of each agent could be used, which would result in enhanced or at least similar antimyeloma activity with decreased treatment-emergent toxicities, such as peripheral neuropathy.

Myeloma patients who are intolerant of bortezomib

Early studies of single-agent bortezomib in relapsed and/or refractory myeloma[3-6] identified four major categories of toxicities. These included constitutional symptoms such as fatigue and nausea, cytopenias such as neutropenia and thrombocytopenia, gastrointestinal effects such as diarrhea and constipation, and peripheral neuropathy. After more than a decade of experience with bortezomib,[94] these remain the major notable toxicities, and of these, neuropathy, which was seen in 21% to 64% of patients treated in the phase III studies,[95-97] has most influenced dosing. A number of approaches can be used to limit the incidence and severity of bortezomib-induced peripheral neuropathy. Among these are the application of a dose-reduction algorithm,[98] the use of bortezomib in the context of combination regimens,[8] dosing of bortezomib once weekly,[99-103] and, most recently, administration of bortezomib as a subcutaneous injection.[104] Despite these options, grade 2–4 peripheral neuropathy remains a significant complication of bortezomib in both the newly diagnosed and the relapsed and/or refractory settings. For example, in a randomized study of bortezomib given either subcutaneously or intravenously with oral dexamethasone, the overall rate of neuropathy of any grade was 38% vs 53%, respectively, while the incidence of grade 3 or worse neuropathy was 6% vs 16%, respectively.[104]

Fortunately, for patients in whom bortezomib-induced peripheral neuropathy has previously developed, next-generation PIs seem to be emerging as an especially rational option. Presentations of data from phase I studies of MLN9708[62] and marizomib[60,61] have reported a low rate of neuropathy, although one could argue that this may be due to the relatively limited exposure of patients to these agents in early-phase clinical trials. More experience has been gained from studies of carfilzomib, both in terms of the number of patients treated and the duration of treatment. Phase I trials of single-agent carfilzomib showed relatively low levels of grade 3 and 4 peripheral neuropathy in all patients[58,59]; this has also been the case in the two completed phase II trials.[63,105,106] Indeed, a pooled safety analysis of all four studies revealed that only 20 patients (3.9%) experienced any grade of neuropathy, with just two (0.4%) having grade 3 symptoms.[107] Moreover, the presence of baseline peripheral neuropathy did not seem to have an impact on either the depth or durability of responses to carfilzomib, or on the tolerability of this agent.[108] One hypothesis that has been proposed to explain the different effects of bortezomib and carfilzomib is that both these drugs induce the mitochondrial high-temperature-requirement protein A2 (HtrA2)/Omi.[109] However, this serine protease, which has been implicated in neuronal cell survival, is inhibited by bortezomib but not by carfilzomib, possibly resulting in greater proapoptotic signaling in neurons of bortezomib-treated patients.

Trials targeting patients who previously could not tolerate bortezomib due to neuropathy-related adverse events will be needed in order for novel PIs to gain approval for this indication. These trials could include patients who were intolerant of bortezomib because of its other toxicities, including dermatologic effects such as epidermal necrolysis[110] and vasculitis,[111] pulmonary effects such as interstitial pneumonitis[112,113] and capillary leak syndrome,[114] hepatitis,[115] and cardiac toxicities,[116] among others. The low incidence of these effects would probably dictate that most patients eligible for such a study would indeed have neuropathy. Randomized studies in such patients comparing the novel inhibitor to bortezomib as a control may be difficult to perform for ethical reasons. However, it may be possible in the future to pursue such studies in bortezomib-naive patients selected specifically for their risk of neuropathy. A recent trial of bortezomib with melphalan and prednisone as initial induction therapy found that the presence of neuropathy at baseline was the best predictor of the development of treatment-emergent symptoms.[117]

If corroborated by other findings, this would suggest that patients with preexisting neuropathy could benefit more from regimens based on a next-generation PI such as carfilzomib. Moreover, molecular studies suggest that the use of gene-expression profiling and single-nucleotide polymorphism analyses of patients at baseline may be able to identify those who are at increased risk for bortezomib-induced peripheral neuropathy.[118,119] Such patients would also be candidates for studies testing nerve-sparing PIs such as carfilzomib.


During the first decade of experience with bortezomib, proteasome inhibition has been firmly established as part of the standard of care for patients with newly diagnosed and relapsed and/or refractory multiple myeloma. The next generation of PIs, including carfilzomib, CEP-18770, marizomib, MLN9708, and ONX 0912, which have novel pharmacologic, pharmacokinetic, and pharmacodynamic properties, have now entered the clinical arena; others will probably do so soon. Among the attractive molecular properties of some of these agents are their ability to inhibit the proteasome more specifically, their ability to bind more proteasome protease activities, their ability to target the immunoproteasome, and their ability to induce proteasome inhibition by binding to α subunits, which may induce allosteric changes in the proteasome itself. From a clinical perspective, these agents are attractive due to the possibility of oral dosing, the likelihood that they can overcome drug resistance (including to bortezomib), the ability to combine them easily with other antimyeloma agents, and the possibility of improving the safety profile with a lower incidence of peripheral neuropathy. Regulatory approvals in the relapsed and/or refractory setting will likely first be achieved for these agents as stand-alone approaches in patients whose myeloma has progressed during their most recent line of therapy. These agents will then be good candidates for use (either singly or in combination with standard or novel drugs, including other PIs) in patients with fewer lines of therapy who have relapsed after a previous treatment.


A Possible Future Therapeutic Decision Tree for Bortezomib-Refractory Patients

Randomized studies comparing bortezomib to next-generation agents will be needed to determine which drugs will be preferred in PI-naive patients; these studies may be able to identify subgroups who would benefit more from one agent or another. For patients who have previously received bortezomib, optimal use and sequencing of other PIs will be aided by an understanding of the mechanisms of de novo and acquired drug resistance in patient plasma cells, as well as the potential contributions to resistance from the marrow microenvironment and the macroenvironment (Figure). During the next decade, the validation of next-generation agents and their use in rationally designed combination regimens and sequences seem destined to further revolutionize the care of myeloma patients and bring us closer to a cure for this disease.

Acknowledgments:The author would like to thank Brian E. Szente, PhD, of Fishawack Communications for his editorial assistance with this manuscript. Editorial support was funded by Onyx Pharmaceuticals.

Financial Disclosure: Robert Z. Orlowski has served as a member of an advisory board for Onyx Pharmaceuticals, Inc.



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105. Siegel D, Wang L, Orlowski RZ, et al. PX-171-004, an ongoing open-label, phase II study of single-agent carfilzomib (CFZ) in patients with relapsed or refractory myeloma (MM); updated results from the bortezomib-treated cohort. 51st Annual Meeting of the American Society of Hematology; December 5-8, 2009; New Orleans, LA. Blood; 2009. Abstract 303.

106. Jagannath S, Vij R, Stewart K, et al. Final results of PX-171–003-A0, part 1 of an open-label, single-arm, phase II study of carfilzomib (CFZ) in patients (pts) with relapsed and refractory multiple myeloma (MM). 45th Annual Meeting of the American Society of Clinical Oncology; May 29-June 2, 2009; Orlando, FL. J Clin Oncol; 2009. Abstract 8504.

107. Singhal SB, Siegel DS, Martin T, et al. Pooled safety analysis from phase (ph) 1 and 2 studies of carfilzomib (CFZ) in patients with relapsed and/or refractory multiple myeloma (MM). 52nd Annual Meeting of the American Society of Hematology; December 4-7, 2010; Orlando, FL. Blood; 2010. Abstract 1954.

108. Martin T, Singhal SB, Vij R, et al. Baseline peripheral neuropathy does not impact the efficacy and tolerability of the novel proteasome inhibitor carfilzomib (CFZ): results of a subset analysis of a phase 2 trial in patients with relapsed and refractory multiple myeloma (R/R MM). 52nd Annual Meeting of the American Society of Hematology; December 4-7, 2010; Orlando, FL. Blood; 2010. Abstract 3031.

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118. Broyl A, Corthals SL, Jongen JL, et al. Mechanisms of peripheral neuropathy associated with bortezomib and vincristine in patients with newly diagnosed multiple myeloma: a prospective analysis of data from the HOVON-65/GMMG-HD4 trial. Lancet Oncol. 2010;11:1057-65.

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120. Papadopoulos KP, Siegel D, Singhal S, et al. Phase 1b evaluation of the safety and efficacy of a 30-minute IV infusion of carfilzomib in patients with relapsed and/or refractory multiple myeloma. 52nd Annual Meeting of the American Society of Hematology; December 4-7, 2010; Orlando, FL. Blood; 2010. Abstract 3024.

121. Hideshima T, Bradner JE, Wong J, et al. Small-molecule inhibition of proteasome and aggresome function induces synergistic antitumor activity in multiple myeloma. Proc Natl Acad Sci U S A. 2005;102:8567-72.

122. Catley L, Weisberg E, Kiziltepe T, et al. Aggresome induction by proteasome inhibitor bortezomib and alpha-tubulin hyperacetylation by tubulin deacetylase (TDAC) inhibitor LBH589 are synergistic in myeloma cells. Blood. 2006;108:3441-9.

123. Santo L, Hideshima T, Kung AL, et al. Selective inhibition of HDAC6 with a new prototype inhibitor (ACY-1215) overcomes bortezomib resistance in multiple myeloma (MM). 52nd Annual Meeting of the American Society of Hematology; December 4-7, 2010; Orlando, FL. Blood; 2010. Abstract 2997.

124. Chauhan D, Tian Z, Nicholson B, et al. Deubiquitylating enzyme USP-7, a novel therapeutic target in multiple myeloma. 51st Annual Meeting of the American Society of Hematology; December 5-8, 2009; New Orleans, LA. Blood; 2009. Abstract 610.