Proteasome Inhibitor-based Combination Strategies
In the last decade proteasome inhibitors have proven to be an important platform for myeloma therapies owing to their effects on plasma cell apoptosis. Combination strategies using proteasome inhibitors based on preclinical data warrant further clinical study. The earliest data presented in 2002 involve the combination of the heat shock protein (HSP) 90 inhibitor (geldanamycin analog, 17-allylamino-17-demethoxy-geldanamycin [17-AAG]) with PS-341 (bortezomib). This work stems from the observation of Mitsiades et al. upon demonstration that exposure to bortezomib induces an increase in HSP transcripts (hsp90, hsp70 and hsp40 families; hsp28; hsp apg-1; and mitochondrial hsp75) reflecting a compensatory stress response with upregulation of hsp27, hsp70 and hsp90 and other markers of cellular stress. Furthermore, in vitro and in vivo work has demonstrated that the combination of an HSP-90 inhibitor with bortezomib resulted in sensitizing myeloma cells to bortezomib-mediated apoptosis and induce tumor regression in mouse models. The same concept was tested in a Phase I clinical trial by Richardson and colleagues combining tanespimycin with bortezomib. This study showed an overall response rate of 27%. Of note, the patient cohort also included patients who were bortezomib-resistant, suggesting reversal of bortezomib resistance. In addition, other HSP inhibitors are in development, and given the preclinical data suggesting that overexpression of HSPs are found broadly in cancer cells, the concept of combinations of agents using an HSP inhibitor as part of the therapy is being tested in multiple different tumors, including myeloma.[11–14]
Additional data is emerging combining bortezomib with histone deacetylase inhibitors (HDACi; vorinostat, panibinostat, romidepsin). In preclinical studies, these combinations have demonstrated remarkable synergy. This mechanism is hypothesized to be related to inhibition of HDAC6, which is essential for an alternative pathway of protein catabolism: the aggresome/autophagy pathway. Upon exposure to proteasome inhibition, the alternative pathway (aggresome pathway) is activated, and protein catabolism occurs via this pathway. The combination of bortezomib and HDAC6-specific agent, tubacin, was able to inhibit the proliferation of myeloma cells in vitro resulting in preclinical synergy, which is now being evaluated in clinical practice. Kikuchi et al. observed enhanced bortezomib-induced apoptosis, as well as amplified histone hyperacetylation with knockdown of HDAC1, whereas HDAC1 over-expression repressed both activities, resulting in bortezomib resistance in myeloma cells. Administration of the HDACi romidepsin restored sensitivity to bortezomib in HDAC1-overexpressing cells, both in vitro and in vivo. Additional data from David et al. suggest that in addition to blocking autophagy, aggresome trafficking can also be blocked using agents such as tipifarnib, which interfere with movement of aggresomes, but do not directly block autophagy.[18,19] Recently, Santo et al. reported at ASH 2011 that selective HDAC6 inhibition via ACY-1215, alone and in combination with bortezomib, inhibits osteoclastogenesis, enhances osteoblastogenesis and inhibits multiple myeloma (MM) cell growth in the xenograft mouse model, suggesting synergistic cytotoxicity due to simultaneous inhibition of the proteasome and aggresome pathways. Badros and colleagues evaluated the combination of the HDACi vorinostat with bortezomib in a Phase I trial, demonstrating significant responses. Among patients who were deemed bortezomib-resistant, the overall response rate was 30%.[21,22] Similar results combining vorinostat with bortezomib were reported in another Phase I trial by Weber et al. at ASH 2007, showing one partial remission and one minimal response among three bortezomib-resistant patients. A trial by Harrison et al. reported promising results of 60% PR and 8% CR using romidepsin (depsipeptide) with bortezomib. Further trials combining vorinostat with bortezomib are being tested. The most recent results evaluating this combination were reported at ASH 2011 from the Vantage 095 and Vantage 088 trials. Vantage 095 is a Phase IIb trial that demonstrated ORR of 18% in relapsed myeloma patients who were refractory to bortezomib and ImiD-based therapies. Among these heavily pretreated patients the median duration of response was 6.3 months, suggesting that this combination offers new hope for double-refractory myeloma patients. Vantage 088, a randomized, Phase III double-blind study of vorinostat or placebo in combination with bortezomib in patients with myeloma evaluated 637 patients enrolled from 174 centers in 33 countries. The ORR for vorinostat plus bortezomib versus placebo plus bortezomib was 54% vs 41% (p < 0.0001); clinical benefit response was 71% versus 53% (p < 0.0001), respectively, and the hazard ratio (HR) for PFS was 0.774 (95% CI: 0.64–0.94; p = 0.01) favoring the vorinostat arm, suggesting that this combination may provide a new treatment option for patients with relapsed and refractory MM. It is important to realize that the short PFS advantage for the combination is likely a consequence of toxicity, and newer dose and schedules are being tested to try and optimize efficacy and toxicity.
Combining cytotoxic therapy with this regimen in relapsed/refractory patients has been shown to result in improved ORR but survival statistics are less clear. Data, as summarized in Figure 1, demonstrate that single-agent bortezomib[26,27] is superior to high-dose dexamethasone alone. In the Phase III studies, combining liposomal doxorubicin (PLD) with bortezomib increased PFS from 6.5 months for bortezomib alone to 9.3 months with combination. Along similar lines, the Phase III trial comparing the efficacy and safety of triplet combination VTD versus thalidomide–dexamethasone (TD) in similar patient populations relapsing after ASCT demonstrated a PFS of 19.5 months for the VTD arm (vs 13.8 months for TD arm), the longest PFS reported in a Phase III trial in relapsed myeloma patients.[29,30] This signifies that combination therapy not only improves the response rates but also prolongs survival. In a Phase I trial reported at ASH 2011, a combination of vorinostat, pegylated liposomal doxorubicin and bortezomib was reported to show >50% ORR in bortezomib-resistant disease. Recently, San Miguel and colleagues have reported on the activity of panobinostat with bortezomib and in their Phase I/II experience, demonstrated significant activity for the combination approach. The overall response was reported in 36 out of 47 (76%) of patients and 10 out of 15 (66%) of bortezomib-refractory patients.[32,33] In the PANORAMA 2 trial combining panobinostat with bortezomib in patients with relapsed and bortezomib-refractory myeloma patients, ORR was 29%. The most striking thing about these combinations are the fact that there is little single-agent activity for either HSP-90 inhibition or HDACi, and that the real effect is through the combination with proteasome inhibition.
Response rates across trials for relapsed myeloma evaluating single-agent regimens, doublet regimens and triplet regimens, by novel agents. C: Cytoxan; D: Dexamethasone; PR: Partial remission; R: Lenalidomide; T: Thalidomide; V: Bortezomib; VGPR: Very good partial remission.
Response rates across trials with induction therapies for newly diagnosed myeloma evaluating single-agent regimens, doublet regimens and triplet regimens, by novel agents.
A: Doxorubicin; C: Cytoxan; CR: Complete remission; CyBorD: Cyclophosphamide, bortezomib and dexamethasone; D: Dexamethasone; nCR: Near complete remission; PR: Partial remission; R: Lenalidomide; RVDD: Lenalidomide, bortezomib, pegylated liposomal doxorubicin, and dexamethasone; T: Thalidomide; V: Bortezomib; VGPR: Very good partial remission; vtD: Reduced doses of bortezomib and thalidomide plus dexamethasone.
Taking a somewhat different tack, Chen-Kiang and colleagues have been working extensively with the cyclin-dependent kinase inhibitor, PD 0332991. This is an agent that induces prolonged G1 arrest via inhibition of cyclin-dependent kinase 4/6 with subsequent enhanced sensitization and synchronization of cells to the actions of cytotoxic agents and bortezomib. In vitro and in vivo models have suggested synergy when PD 0332991 is combined with bortezomib, and this too is being tested in a Phase I clinical trial.
In newly diagnosed transplant-ineligible patients, the VISTA trial evaluated the efficacy of combining proteasome inhibitors with alkylating agents: bortezomib, melphalan and prednisone (VMP) and was compared with MP. CR and VGPR rates were 33 versus 4% and 8 versus 4%, respectively. Achieving a CR was associated with significantly longer time to progression, time to next therapy and OS. Final updated OS analysis after 5-year of follow-up was presented at ASH 2011. Of note, of the 237 patients in the MP arm, 145 patients received subsequent bortezomib. OS was 56.4 versus 43.1 months for patients in the VMP arm versus MP arm (HR: 0.695; p = 0.0004), respectively. OS was 45.4 months (HR: 0.714; p = 0.0029) for patients who received bortezomib salvage therapy and those that have not received second-line therapy. Thesedata suggest the combination of bortezomib with MP resulted in prolonging OS relative to sequencing MP followed by bortezomib at relapse.
In the same patient population, the efficacy of combining proteasome inhibitors with alkylating agents and an IMiD were compared for outcomes: bortezomib, melphalan, prednisone and thalidomide (VMPT), which was followed by maintenance with bortezomib and thalidomide (VT) and was compared with VMP. VMPT–VT (38% CR rates) was superior to VMP (24% CR rates) in terms of CR and PFS, but without OS benefit. The risk of increased thromboembolism and cardiac toxicity in the VMPT–VT regimen clearly shows that toxicity from quadruplet combinations could trounce the benefits of the potential antimyeloma effect.
Acquired resistance of myeloma cells to bortezomib is inevitable either by modulating the cell-signal pathways, altering the proteasome complex or escalating alternate ways of protein degradation. Various novel proteasome inhibitors have shown promising activity even in bortezomib-resistant cell lines.
MLN9708 is a reversible proteasome inhibitor and an oral boronate analog of bortezomib. It hydrolyzes to MLN2238, which is the active form. Preclinically, MLN9708 demonstrated significant antimyeloma activity. In the Phase I trials in relapsed/refractory (R/R) myeloma, biweekly and weekly dose-escalation cohorts to establish maximum tolerated dose (MTD) were well tolerated and rendered disease stabilization for 12.9 months and 9.5 months in 61 and 45% patients, respectively. MLN9708 in combination with lenalidomide and dexamethasone demonstrated ORR of 100% and VGPR or better of 60%; a highly efficacious, oral triplet regimen as induction therapy for newly diagnosed MM favoring combination therapies.
CEP-18770 is another oral reversible proteasome inhibitor; given its potent anti-tumor activity in myeloma models,[46,47] a Phase I/II study in patients with R/R MM is currently being evaluated. Combination of CEP-18770 with lenalidomide and dexamethasone was efficacious in the xenograft models, establishing a rationale for this combination in a Phase I/II study for R/R MM patients.
NPI-0052 is another oral proteasome inhibitor with demonstrated efficacy in MM tumor models resistant to bortezomib. A Phase I trial of NPI-0052 in patients with R/R MM showed ORR of 28% with durable stable disease of >6 months. Preclinical studies of NPI-0052 in combination with lenalidomide triggered in vitro and in vivo synergistic cytotoxicity in MM and translated to tumor growth and prolonged survival in animal models. Other Phase I trials of NPI-0052 in patients with R/R MM alone or in combination are ongoing. NPI-0052 in combination with vorinostat is currently being tested in patients with non-small-cell lung cancer, pancreatic cancer, melanoma and lymphoma.
ONX 0912 is another oral irreversible tripeptide epoxyketone proteasome inhibitor shown to induce apoptosis in MM cells resistant to bortezomib in vitro. Anti-MM activity was enhanced upon combining ONX 0912 with bortezomib, lenalidomide and dexamethasone or a pan-HDACi. A Phase I study of ONX 0912 in patients with advanced refractory or recurrent solid tumors showed an acceptable safety profile and levels of proteasome inhibition greater than 80%. Further combination strategies are promising.
Carfilzomib is an epoxyketone; it is structurally dissimilar to bortezomib, inhibits proliferation and induces apoptosis in bortezomib-resistant MM cell lines. Relative to bortezomib, carfilzomib demonstrated increased apoptosis in the cell line models of MM through both the intrinsic (caspase-9) and extrinsic pathways (caspase-8 and caspase-3). Owing to the encouraging safety profile and clinical efficacy of carfilzomib in the Phase I studies PX-171-001 and PX-171-002, Phase IIstudies PX-171-003-A0 and PX-171-004 were conducted in patients with R/R MM. The response rates in both of these studies at a 'stepped-up' dose of 20–27 mg/m2 maximized the clinical benefit of single-agent carfilzomib. Carfilzomib induced higher ORR in bortezomib-naive and resistant patients. Carfilzomib in combination with lenalidomide and dexamethasone (CRd) in patients with R/R MM in PX-171-006 study resulted in ORR of 78% (CR/sCR 18%, VGPR 22%, PR 38%, minimal response 2%, stable disease 8%). Prolonged administration of the regimen with manageable toxicities was possible (14–23 months). On the basis of these results, a large Phase III trial called 'ASPIRE' comparing the combination of CRd versus lenalidomide and low-dose dexamethasone (PX-171-009) is underway. In the front-line setting with CRd, the ORR was 94% with ≥VGPR rates of 65% and sCR/CR/nCR rates of 53%. Carfilzomib was also efficacious in combination with thalidomide and dexamethasone (Carthadex) as induction and consolidation therapy in transplant-eligible patients with MM. This combination was well tolerated with grade ≤2 peripheral neuropathy in 24% patients and resulted in ORR of 84%; 16% patients achieved CR/sCR. Other dose-finding studies of carfilzomib in combination with IMiDs (lenalidomide and pomalidomide), HDACi (panobinostat and vornisostat), combination with cytotoxic therapies (PLD, cytoxan) are currently ongoing based on the preclinical rationale providing the insight that we can use the principle of synergism by combination of these agents to achieve increased destruction of tumor and attain deeper responses in order to achieve prolonged PFS and OS.
Expert Rev Hematol. 2012;5(5):533-545. © 2012 Expert Reviews Ltd.