Why Proteasome Inhibitors Cannot ERADicate Multiple Myeloma

COMMENTARY

Multiple myeloma is a clonal proliferationofneoplastic plasma cellsthat can present clinically with hypercalcemia, renal insufficiency, anemia, bony lesions, bacterial infections, hyperviscosity, and amyloidosis. Worldwide, approximately 86,000 patients will be diagnosed each year with myeloma, which in many areas makes it the second most common hematologic malignancy,while about 63,000 patients die every year from disease-related complications (Becker, 2011). In the United States, due to an aging populace, it is anticipated that the number of cases of myeloma will grow by 57% between 2010 and 2030 (Smith et al., 2009), ranking myeloma behind only stomach and liver cancer in the rate of growth of new cases. These facts indicate that multiple myeloma is, and will continue to be, a significant source of morbidity and mortality. Fortunately, a number of advances over the past decade have dramatically improved patient outcomes. Among these are the advent of novel chemotherapeutics, including the immunomodulatory agents thalidomide, lenalidomide, and pomalidomide, andthe proteasome inhibitors (Moreau et al., 2012)bortezomib and carfilzomib. All of these drugs have garnered regulatory approvals and are used in patients with newly-diagnosed, relapsed, or relapsed/refractory disease, and have contributed to a doubling of the median overall survival in many populations. This has been fueled by a greater understanding of the pathobiology of myeloma, and by the development of more physiologically relevant pre-clinical models, allowing for greater success in translating laboratory concepts into patient-focused therapies. Indeed, a number of the most potent combination regimens in use have been rationally developed based on studies of the mechanisms of action of these drug classes (Mahindra et al., 2012).

Despite these significant advances, patients are currently treated on an empiric basis, typically with the regimens that have the highest response rates to which they have access, rather than based on the principles of precision medicine (Mendelsohn, 2013). The proteasome inhibitorsbortezomib and carfilzomib as single agents in the relapsed and refractory settings both have response rates of approximately 25%, indicating a role for innate, or primary resistance in modulating their activity. Response rates and, even more importantly, response durability can be improved by using them earlier in the disease process and by incorporating them into combination regimens. However, no clinically relevant biomarkers have been validated that would allow practitioners to prospectively identify patients with disease that is most likely to respond to proteasome inhibition, and the same is true for the immunomodulatory drugs. In addition, even in patients whose disease initially responds, resistance to proteasome inhibitor-based therapy emerges in many, indicating a role for acquired, or secondary resistance through incompletely understood mechanisms. As a result, some patients are exposed to the toxicities of these agents without a benefit, delaying their access to more effective options, and wasting precious healthcare resources. Clinically, these factors contribute to the natural history of the disease, which follows a course characterized by multiple relapses, and decreasing durations of benefit with each subsequent line of therapy.

The work of Tiedmemann and colleagues (Leung-Hagesteijn et al., 2013) in the current issue of Cancer Cell may, however,begin to change the current status quo. Early attempts to identify mechanisms of innate bortezomib resistance implicated a role for over-expression of proteasome genes, and especially of PSMD4(Shaughnessy et al., 2011). Later, studies of acquired resistance models suggested mutations of the β5 proteasome subunit targeted by bortezomib were involved. However, these β5 mutations were subsequently not detected in patient samples, as indicated in the introduction by Leung-Hagesteijn et al. More recently, induction of signaling through the insulin-like growth factor (IGF-1)/IGF-1 receptor pathway has been implicated in acquired resistance (Kuhn et al., 2012), and clinical trials to test this possibility are planned. Finally, up-regulation of a cluster of NRF2 oxidative stress response genes,including CHOP (DDIT3) was identified as a possible resistance signature in studies of murine myeloma models and clinically annotated gene expression profiling datasets (Stessman et al., 2013).

In contrast, Leung-Hagesteijn et al. provide a different perspectivestarting from kinome-wide RNA interference studies intended to identify modulators of proteasome inhibitor sensitivity. They report that suppression of inositol-requiring enzyme 1(IRE1) and its downstream effector,X-box binding protein 1 (XBP1), was associated with bortezomib resistance. This to some extent challenges a current dogma, since IRE1 and XBP1 make up one arm of the unfolded protein response (UPR). Indeed, UPR activation was felt to represent an adaptation to proteasome-assisted endoplasmic reticulum-associated degradation (ERAD), which causes ER stress in plasma cells due to their large loads of misfolded proteins. These IRE1^lo and XBP1^lo cells showed lower expression of plasma cell maturation markers, lower immunoglobulin synthesis, and lower levels of UPR activation, consistent with a lower stress level and proteasome load, which would make them resistant to proteasome inhibitors. Moreover, plasma cell maturation markers were enriched in samples from patients who responded to bortezomib-based therapy, and reduced in those with no response to bortezomib. Finally, analysis of patient samples revealed the presence of up to five tumor cell subpopulations, including B-cells, activated B-cells, pre-plasmablasts, plasmablasts, and plasma cells, which would be expected to have different sensitivities to bortezomib (Figure 1). Consistent with this possibility, myeloma B-cell and pre-plasmablast progenitors were found to survive proteasome inhibition, and be enriched in samples that were refractory to bortezomib.

These findings have a number of key implications for innate and acquired proteasome inhibitor resistance. First, they suggestthat the seeds for bortezomib resistance, and cross-resistance with other proteasome inhibitors, are already present in perhaps all patients and that this phenotype will inevitably emerge through the attrition of bortezomib-sensitive plasmablasts and plasma cells (Figure 1). Second, they support the possibility thatgene expression profiling or whole genome sequencing to identify the baseline proportion of these progenitor cells present in patients prior to initiation of therapy may predict the extent of benefit to be expected from bortezomib. Finally, they imply that potentially curative strategies will need to target both the committed plasma cells and the progenitor, or stem cell-like cells.

While these hypotheses are being tested, the current findings will also need to be validated in a wider array of primary samples, and in additional gene expression profilingdatasets. One question that needs to be answered is whether the same proteasome inhibitor-resistant progenitor cells survive therapy with bortezomib in combination with other agents. This would be especially of interest for the combination of bortezomib and lenalidomide, since the lattermay target clonogenic side-population cells(Jakubikova et al., 2011), and this regimen is one of our current standards of care against myeloma. Another important question is whether it would be possible to induce differentiation of progenitor cells to more committed plasma cells, such as through epigenetic therapies, to enhance or restore proteasome inhibitor sensitivity. Finally, it is likely that multiple mechanisms contribute to the development of the proteasome inhibitor-resistant phenotype. For example, while the current studies indicate that cells with a decreased proteasome load are resistant, it is also possible that changes that enhance plasma cell proteasome capacity would antagonize the beneficial effects of bortezomib and carfilzomib. Taken together, however, these findings do provide an important direction for future pre-clinical and clinical studies that will hopefully bring us to the point that we can apply precision medicine to the therapy of multiple myeloma.

This is a commentary on article Leung-Hagesteijn C, Erdmann N, Cheung G, Keats JJ, Stewart AK, Reece DE, Chung KC, Tiedemann RE. Xbp1s-negative tumor B cells and pre-plasmablasts mediate therapeutic proteasome inhibitor resistance in multiple myeloma. Cancer Cell. 2013;24(3):289-304.