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, or multicatalytic proteinase complex, 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. This is especially true for bortezomib with pegylated liposomal doxorubicin (Doxil), as supported by phase III data. 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 or thalidomide and dexamethasone, bortezomib is now part of the standard of care for induction therapy in transplant-eligible patients—and bortezomib with melphalan and prednisone 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 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 and β2 trypsin-like 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), or LMP-7, (eg, ONX 0914 [formerly PR-924]). 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), and chloroquine. 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 and need new treatment approaches. 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. 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 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. 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.
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, or tricorn protease, which may normally work downstream of the proteasome. 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 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.
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