The proteasome
The proteasome is a large, hollow cylindrical multi-enzymatic complex that is present in both the cytoplasm and the nucleus of all eukaryotic cells. It is necessary for the degradation of intracellular proteins in eukaryotic cells whereas extracellular/transmembrane proteins are typically degraded by the aggresome/lysosomal pathway. 1 The proteins degraded by the former pathway are involved in signal transduction pathways that regulate cell growth and proliferation including: cell-cycle-regulatory proteins (cyclins A, B, D, and E; p21 and p27), the tumor suppressor p53, NF-κB, and adhesion molecules. 4
The formation of the 26S proteasome occurs in an ATP dependent fashion, when a 20S catalytic core is capped by a 19S regulatory subunit at both ends (see Figure 1A ). 1 The lysine residues of those proteins targeted for degradation are covalently modified with a polyubquitin protein chain, with each ubiquitin tag consisting of a 76 amino acid polypeptide. The ubiquitin chain is recognized by the lid-like structure of the 19S subunit and then removed. The target protein is then denatured in an energy dependent manner by the 6 ATPases at the base of the 19S subunit and threaded into the center of the 20S subunit. 2
As shown in Figure 1B , the 20S subunit is itself comprised of four rings, 2 α and 2 β subunits. Within the channel at the center, threonine residues of the indicated β units wield catalytic activity comparable to three enzymes: chymotrypsin (β5), trypsin (β2), and post-glutamyl peptide hydrolase (β1).
Preclinical development of bortezomib
In 1993, the company Myogenics was founded by Alfred Goldberg to decrease muscle wasting/cachexia by inhibiting the ubiquitin–proteasome pathway. A team of enzymologists created the first inhibitors of the proteasome: peptide aldehyde analogs of the proteasome’s chymotrypsin-like substrates. Chemists then created a dipeptide boronic acid analog that would eventually come to be known as bortezomib ( Figure 2 ). 3
When applied to the National Cancer Institute’s 60-cell line screen, bortezomib demonstrated potent growth inhibition against a broad range of tumor types. 5 Importantly, confirmation was also obtained that the intended biologic target was being inhibited. Additional studies with human myeloma cell lines and freshly isolated from myeloma patients confirmed that bortezomib not only inhibited tumor proliferation but also induced apoptosis and overcame drug resistance. 6
The growth inhibition of bortezomib was extended to the in vivo setting using a human plasmacytoma xenograft mouse model. Relative to controls, bortezomib treatment resulted in improved overall survival. 7 A fluorogenic pharamacodynamic assay was developed to measure the relative chymotryptic and tryptic activities of the proteasome in peripheral blood mononuclear cells. 8 This assay showed that bortezomib-mediated inhibition of the chymotrypsin-like activity of the 26S mammalian proteasome ( Figure 1B ) was dose-dependent and reversible, 7 thus helping guide dosing and optimize dose escalation in phase I studies.
Clinical development of bortezomib – relapsed/refractory multiple myeloma
In a phase I trial among patients with advanced hematological malignancies, bortezomib was noted to have activity in patients with refractory myeloma; among nine patients with multiple myeloma antitumor activity was noted in almost all patients including 1 patient achieving a complete response. 4 A subsequent, large, multicenter phase II trial involving 202 patients with relapsed, refractory myeloma yielded a 35% overall response rate which was comprised of a 4% complete remission (CR), 6% near CR, 18% partial remission (PR), and 7% minimal response (MR). 9 It was on the basis of this trial in large part, that bortezomib was approved by the United States Food and Drug Administration (FDA) in 2003, thus resulting in a remarkably short 8 years from drug discovery to FDA approval.
The phase III Assessment of Proteasome Inhibition for Extending Remissions (APEX) study compared bortezomib (1.3 mg/m 2 on days 1,4, 8, and 11 by intravenous push for eight 3-week cycles) to high dose dexamethasone (40 mg days 1–4, 9–12, and 17–20 orally for four 5-week cycles and then days 1–4 for five 4-week cycles) in 669 patients with relapsed multiple myeloma. The study was halted on interim analysis because bortezomib treatment resulted in higher response rates (38 vs 18%), longer time to progression (6.22 months vs 3.49 months), and improved overall survival. The median time to response was 43 days in both groups. 10 In an updated analysis, based on a median follow up of 22 months, the median overall survival was 29.8 vs 23.7 months ( P = 0.0272) despite a 62% crossover rate from dexamethasone to boretzomib. 11 As shown in Figure 3 , a comparison of the Grade 3/4 adverse events in each arm reveals that bortezomib treatment is associated with an increased incidence of thrombocytopenia, neutropenia, peripheral neuropathy, and diarrhea. 10 The thrombocytopenia and neuropathy are discussed in further detail below. Despite these toxicities, a prospective comparsion of health-related quality of life found improved outcomes with bortezomib. 12 Of note, subgroup analysis has also found no difference in safety or efficacy in patients with varying degrees of renal insufficiency. 13
Bortezomib therapy also appears to have beneficial effects on the bone. When alkaline phosphatase levels were compared with responders and nonresponders in the APEX study, the most powerful predictor of a response was a 25% increase in alkaline phosphatase at week 6 ( P < 0.0001) ( Figure 4 ). 14 Laboratory work has confirmed the ability of bortezomib to not only inhibit osteclast mediated bone destruction, but also directly induce bone formation. 15 , 16 Interestingly, as shown in Figure 4 , the increase in alkaline phosphatase was not observed on the dexamethasone arm, even in the responders. 14 This increase has also been recently found to be associated with improved time to progression. 17
As the safety and efficacy results for bortezomib monotherapy were accumulating, combination therapy was being explored in the preclinical setting. Hideshima et al found that the growth inhibitory effects of bortezomib and dexamethasone on a myeloma cell line were additive ( Figure 5A ). 6 Ma et al found that the addition of a noncytotoxic dose of bortezomib to chemotherapeutic agents could increase the sensitivity of chemoresistant myeloma cells by 100,000 to 1,000,000-fold without affecting normal hematopoietic cells ( Figure 5B ). 18
The largest published phase III clinical trial combining bortezomib with another chemotherapeutic agent randomized 646 myeloma patients with 2 or more lines of prior therapy to receive either the standard dose/schedule of bortezomib alone or with liposomal doxorubicin (PLD) on Day 4. The combination therapy was associated with a higher incidence of grade 3/4 events 80 vs 64% (due to myelosuppression, constitutional and gastrointestinal symptoms, and hand foot syndrome). There was also no significant difference in response rates. However, the time to progression (9.3 vs 6.5 months, P = 0.000004) and overall survival at 15 months (76% vs 65%, P = 0.03) both favored bortezomib with PLD. 19 This steroid sparing regimen is an excellent treatment option especially for those patients intolerant of steroids due to psychosis or brittle diabetes.
The proteasome inhibitor bortezomib has now been studied in combination with each of the three other classes of drugs with activity in myeloma: steroids, immunomodulatory agents (IMids), and conventional chemotherapeutics (anthracyclines and alkylating agents). For those phase I/II studies with 30 or more evaluable patients, summaries of the recent response data of doublet ( Table 1 ), triplet ( Table 2 ), and multiagent ( Table 3 ) permutations of the four classes of drugs in relapsed/refractory myeloma are shown in the indicated tables.
Bortezomib in previously untreated multiple myeloma
The only published phase III study of bortezomib in untreated myeloma is the Velcade as Initial Standard Therapy in Multiple Myeloma: Assessment with Melphalan Prednisone (VISTA) study. In this study, 682 nontransplant eligible patients with untreated myeloma were randomized to receive either melphalan and prednisone alone (MP) or with bortezomib (VMP) at the doses and schedule shown in Figure 6 .
Overall response rate for VMP was 71% vs 35% for MP with a very impressive CR rate of 30% vs 4% ( P < 0.001 for both comparisons). Of note, a 30% CR rate compares very favorably to the CR rates obtained for patients who receive high dose melphalan chemotherapy with autologous stem cell rescue (for which none of the patients in the VISTA study were eligible). With a median follow up of 16.3 months, the hazard ratio for overall survival for the VMP group was 0.61 ( P = 0.008). 34
All of the following efficacy outcomes were also significantly better for the VMP group relative to MP: median time to first response (1.4 vs 4.2 months), duration of response (20 vs 13 months), and treatment-free interval (17 months vs 9 months). The improved outcomes were seen in all subgroups, including age >75, creatinine clearance <60, and high risk cytogenetics (translocation (t) (4;14), t(14,16), or chromosome 17 deletion). 34 Of note, the lack of effect of high risk cytogenetics on efficacy with borezomib-based regimens has been a consistent finding across all front line studies. 34 – 37
Toxicities ≥ Grade 3 that were higher in the VMP arm included peripheral neuropathy (14 vs 0%), nausea/vomiting/diarrhea, fatigue/asthenia, and zoster. Herpes zoster was observed in 14% of VMP patients vs 4% in MP, but among patients receiving antiviral prophylaxis, the rate was 3%. 34
Of note, a recent analysis of varicella zoster virus (VZV) reactivation in the APEX study also found a significantly increased incidence in patients receiving bortezomib relative to dexamethasone 13% vs 5%, P = 0.0002. 38 Antiviral prophylaxis is therefore recommended for all myeloma patients receiving bortezomib based regimens. A variety of agents and regimens have been used including acyclovir 400 mg daily, valacyclovir 250 or 500 mg daily, famciclovir 500 mg daily. 39 , 40 Doses should be decreased in the setting of renal insufficiency as the metabolites of these drugs can accumulate and cause profound neuropsychiatric changes. 41 , 42 Finally, the varicella vaccine, which contains live attenuated virus, is not licensed for use in patients with neoplasms affecting the bone marrow or lymphatic systems. 43
Additional follow up data presented recently indicated that despite the fact that 43% of MP patients subsequently received bortezomib upon progression, intention to treat analysis still demonstrated increased overall survival for the VMP group. Moreover, there was no difference in response to IMiD-based second line treatments between the two groups. 44 The results of the VISTA study therefore demonstrate clearly improved efficacy with VMP without any adverse long term consequences of upfront bortezomib based regimens.
There are also several large phase III studies ongoing evaluating the use of bortezomib as induction therapy prior to stem cell transplantation (see Table 4 ). The Franchophone Myeloma Intergroup (IFM) 2005-01 study randomized 482 patients to receive either bortezomib-dexamethasone (Vel-Dex) or the traditional VAD. Of the 442 evaluable patients, the CR rates were 10% vs 3%, CR + near CR 19% vs 8%, and ≥ PR 83 vs 66% without any impairment in stem cell harvest. Moreover, the higher quality of responses persisted after the first melphalan 200 mg/m 2 followed by autologous stem cell rescue, with CR/near CR rates of 40 vs 22%, P = 0.0001. 36 Preliminary data from two other phase III studies comparing bortezomib in combination with doxorubicin and dexamethasone (PAD) to traditional VAD 37 and bortezomib, thalidomide, and dexamethasone (VTD) to TD 35 also found improved CR/nCR rates (23% vs 9%, P < 0.015 and 55% vs 32%, P < 0.001 respectively) after autologous stem cell transplantation.
These improvements in CR rates after transplant with bortezomib based induction therapies have clinical significance. Two large published phase III studies comparing single vs tandem autologous stem cell transplants in myeloma found that patients who did not achieve a CR/near CR after the first autologus stem cell transplant were the ones that could benefit from a second SCT. 48 , 49 Therefore, the higher CR rates being obtained with novel induction regimens may obviate the need for a second autologous transplant – with its attendant mortality, morbidity, and cost.
While the details of the various bortezomib based front line regimens are beyond the scope of this review, a summary of the responses noted to date are shown in Table 5 . With the understanding that response rates in single/few institution phase II studies are typically higher than those obtained in phase III multi-institutional settings, a regimen that stands out is bortezomib, lenalidomide, and dexamethasone (VRD). With 65 evaluable patients, the combination of bortezomib, lenalidomide, and dexamethasone resulted in a 100% response rate and a 38% CR/nCR rate. 50 A caveat of course, is that lenalidomide based induction regimens often result in inadequate stem cell harvests with granulocyte colony stimulating factor (GCSF) mobilization and therefore require cyclophosphamide or the recently FDA-approved CXCR inhibitor, plerixafor, to ensure adequate stem cell harvests.
Mechanism of action of bortezomib
While rational drug design and pharmacodynamic assays identified and confirmed the proteasome as the biologic target, without an understanding of the exact mechanism of action, the full therapeutic potential of proteasome inhibition cannot be realized. Research has focused on three possible themes that will be discussed below: the transcription factor NF-κB, the interaction of the pro-apoptotic factor NOXA and the c-myc oncogene, and finally, the transcription factor x-box binding protein 1 (XBP-1) and the unfolded protein response.
Initial focus was on the impact of bortezomib on NF-κB, which promotes tumor cell survival and proliferation. The inhibitor protein I-κB binds NF-κB in the cytoplasm, thereby rendering NF-κB inactive. A variety of cytokines and other cellular stimuli result in the phosphorylation and ubiquitination of I-κB by E3 ligase, thus targeting it for proteasome mediated degradation ( Figure 7 ). 2
Bortezomib, by blocking the latter process, results in increased availability of I-κB to inhibit NF-κB, resulting in the inhibition of tumor cell growth. Gene expression profiling studies in patients with myeloma who responded to bortezomib treatment also highlighted pathways such as NF-κB activity and cell adhesion, thereby confirming preclinical studies. 69
Additional work by Hideshima et al revealed that bortezomib activation seemed to be dependent on the activation of c-Jun NH2-terminal kinase (JNK) and subsequently caspases-8 and caspase-3 that elicit DNA damage and apoptosis. In parallel, bortezomib was noted to be associated with the up-regulation of p53. 70 While these inital studies shed some light on the mechanism of action, it is unclear if the changes observed in NF-κB and JNK are a cause or the result of the death process. Indeed, more recent studies suggest the antimyeloma activity of proteasome inhibition is actually p53 independent. 71
When myeloma cell lines are exposed to bortezomib, the proapoptotic factor NOXA is induced in a concentration dependent manner accompanied by the activation of caspases. NOXA is also induced by p53 and other transcriptional factors such as hypoxia-inducible factor 1 (HIF-1) and E2F-1, consistent with its involvement in the response to many types of cellular stress. Human NOXA contains one BH3 (Bcl-2 homology 3) domain, which has a high affinity for the antiapoptotic factor Mcl-1. Because Mcl-1 is a target for ubiquitination, proteasome inhibition increases levels of Mcl-1. The induction of NOXA is therefore essential to override high Mcl-1 levels and allow for the activation of the apoptotic machinery in response to bortezomib. 72 Also, NOXA’s interaction with anti-apoptotic members of the Bcl-2 family causes release of cytochrome c into the cytosol, leading to the activation of caspases and induction of apoptosis ( Figure 8 ). 73
Bortezomib induction of NOXA is also seen in melanoma and mantle cell lymphoma cell lines, with antisense NOXA oligonucleotide (but not control) resulting in a decrease in bortezomib induced apoptosis. 71 , 74 Of note, apoptosis/NOXA induction is not induced by conventional chemotherapeutic agents but is induced by other proteasome inhibitors (eg, MG132), suggesting a possible class specific effect. 73 , 75 To understand why NOXA is preferentially induced in tumor cells, the myriad transcription factors with consensus binding sites at the NOXA promoter were restricted to those that are conserved (as NOXA itself is) across mammalian species and also dysregulated by proteasome inhibition and tumorogenesis. The oncogene c-myc emerged as a candidate mediator of tumor specificity. Indeed, when c-myc levels were decreased by RNA interference, the tumor cell-specific induction of NOXA was abrogated. Exogenous c-myc also increased the sensitivity of nonmalignant cells to proteasome inhibition by bortezomib. 72
The interaction of NOXA and c-myc also provides a possible rationale for the encouraging clinical data noted thus far when histone deacetylase (HDAC) inhibitors are combined with bortezomib. The transcriptional activity of c-myc at the NOXA promoter can be favored by chromatin remodeling or modification proteins (including histone acetyl transferases, with acetylated histone H3 being a classical cofactor for myc). 72 HDAC inhibition is also thought to interfere with the targeting of ubiquinated proteins via the aggresome for eventual autophagy/degradation by the lysozome, an alternate pathway to proteasome-mediated degradation. 76
A third possible explanation for the specif icity of bortezomib for myeloma cells is based on the unfolded protein response (UPR). Plasma cells have highly developed rough endoplasmic reticulum (ER) and chaperone proteins that enable them to produce vast quantities of antibodies per second. If misfolded proteins accumulate in ER, the UPR signaling pathway is activated through its sensing mechanism IRE1α. 77 The IRE1 kinase, in turn, results in the removal of an intron from the transcription factor XBP1, resulting in a activated ie, spliced form XBP-1. 78 Interestingly XBP-1 is is highly expressed in plasma cells and is a prerequisite for transformation from antigen selected B cell to plasma cell.
Once the UPR is activated, the unfolded proteins are refolded by upregulation of the chaperone molecules or destroyed through cytosolic 26S proteasomes; otherwise, accumulation of unfolded protein results in apoptosis of the cell ( Figure 9 ). Proteasome inhibition triggers apoptosis by interfering with the UPR pathway, both at the sensing level as well as by preventing destruction of misfolded protein. 79
Pathophysiology and management of bortezomib toxicities
Thrombocytopenia
The thrombocytopenia associated with bortezomib therapy has been well characterized. The platelet count drops during Days 1 to 14 and then rapidly recovers to baseline level during Days 15 to 21 ( Figure 10 ). The mean reduction in relapsed/refractory patients is 60% and appears to be independent of the baseline platelet count, the concentration of the monoclonal protein, and bone marrow plasmacytosis. Murine studies demonstrated no cytotoxic effects on megakaryocytes, thus suggesting a mechanism distinct from traditional myelosuppressive chemotherapeutic agents. 80
Neuropathy
When the proteasome is inhibited, proteins accumulate in aggresomes at the periphery of cells and then track centrally via microtubules towards the microtubule-organizing center (MTOC). 81 When the distribution of microtubules between polymerized and soluble fractions was compared following the treatment of neuroblastoma and myeloma cells with five proteasome inhibitors, the polymerized fraction increased from 41% to 68% to approximately 55% to 99%, for up to 144 hours after the proteasome inhibitor was removed. Immunofluorescence studies did not reveal microtubule bundles seen with taxanes, suggesting microtubule stabilization occurred by a mechanism different than direct drug binding. 82 Animal models have also found significant mitochondrial and endoplasmic reticulum damage in dorsal root ganglia. 83 Other postulated mechanisms of bortezomib associated neuropathy include mitochondrial dysregulation of calcium homeostasis or dysregulation of growth factors important for neuron survival. 84
Clinically, it is important to note the baseline rate of neuropathy in patients with relapsed/refractory myeloma. In the phase II SUMMIT and CREST studies with bortezomib, 81% of patients had symptoms by FACT/GOG-Ntx questionnaire and 83% by neurologists’ examination. 85 This likely reflects not only the side effects of prior treatments, but also a manifestation of the disease itself. While the likelihood of developing severe peripheral neuropathy (PN) was more frequent in those patients with baseline neuropathy, the overall occurrence was independent of baseline neuropathy.
In the phase III APEX trial, of the 37% of patients who experienced peripheral neuropathy (PN), 9% had grade ≥ 3. The neuropathy was typically sensory, although 2% of patients did experience motor neuropathy. The neuropathy does appear to be dose related with PN typically occurring by cycle 5 and then reaching a plateau by cycle 8, associated with cumulative bortezomib doses of 26 and 42 mg/m 2 respectively. 86 Based on similar findings in previous studies, the APEX trial also incorporated dose-modification guidelines for PN (see Table 5 ).
Sixty-eight percent of patients in the APEX study who had dose modification for grade ≥ 2 PN experienced improvement or resolution to baseline in their symptoms at a median of 110 days without any compromise in efficacy. The development of neuropathy was independent of age, prior therapies (including thalidomide and vincristine), and glucose intolerance/diabetes. 86
A recent publication described a case series of five patients with myeloma who received bortezomib and then developed severe motor involvement. Electrophysiological evaluations showed demyelinating or mixed axonal-demyelinating neuropathy with prominent motor involvement. Cerebrospinal fluid showed albumin-cytological dissociation. Importantly, all four patients treated with either steroids or intravenous immunoglobulin had improved outcomes, suggesting a possible immunologic cause of this neuropathy. 88 Therefore, the development of motor neuropathy merits prompt neurological consultation.
Particularly in the setting of combination therapy, attenuation in the dosing schedule eg, weekly treatment, appears to be associated with significantly less neurotoxicity. For example, the incidence of grade 3 or higher neuropathy with VMP decreased from 14% to 2% with twice weekly vs weekly bortezomib with preliminary outcome data showing no loss in efficacy. 46 Interestingly, patients treated with the combination of the heat shock protein (HSP)-90 inhibitor tanesipmycin and bortezomib have not developed Grade 3 PN, suggesting a possible neuroprotective effect of this novel agent. 89 Of note, development/exacerbation of PN has also not been observed to date with the novel proteasome inhibitor carfilzomib, suggesting that this may not be a class specific effect. 90 , 91
Currently there is no proven effective prophylaxis for PN. A variety of agents are used for symptomatic relief of boretzomib associated PN including opioids, tricyclic antidepressants such as nortryptline, anticonvulsants such as gabapentin, serotonin-norepeinephrine reuptake inhibitors such as duloxetine, nonsteroidal anti-inflammatory agents, vitamins, and nutritional supplements such as α-lipoic acid, glutamine, and L-carnitine. 84 However, with recent data suggesting a possible decrease in the efficacy of bortezomib with concomitant vitamin C 92 and other supplements such as green tea 92 , neither the effectiveness in symptom palliation nor the absence of an interaction with bortezomib has been clearly established in randomized clinical trials.
The future of proteasome inhibition
A protein is first identified to be degraded by the polyubqiuitination of lysine residues. The process consists of sequential ubquitin activation, conjugation, and protein ligation – each catalyzed by E1, E2, and E3 enzymes ( Figure 11 ) – which creates the polyubiquitination chain. It appears that there is a family of small ubiquitin like modifiers such as Nedd8, SUMO, FAT10 and ISG15 that are also able to target proteins for degradation. 93 Each step of this process is therefore a putative therapeutic target. Efforts are underway to evaluate novel agents, with a Nedd8 activating enzyme inhibitor (MLN 4924) already in phase I clinical trials. 94
Based on the pharmacaphore that interacts with the proteasome’s active site, proteasome inhibitors can be divided into five classes: peptide aldehydes, peptide boronotes, peptide vinyl sulfones, peptide epoxyketones, and the only nonpeptide group – β lactone inhibitors ( Table 6 ). 2 The peptide aldehydes such as MG-132 are the first class to be studied and while cell permeable, they are not only rapidly oxidized and unstable, but also lack specificity with activity against nonproteasome enzymes such as serine and cysteine proteases.
The peptide boronates were derived by substitution of the aldehyde with boron to increase potency, selectivity, and stability. 2 Bortezomib is currently the only FDA approved proteasome inhibitor. Recently published preclinical data demonstrated activity comparable with bortezomib with another peptide boronate compound, CEP-18770, that is also water-soluble and orally bioavailable. 95 Bortezomib is also being used as a platform for phase I/II studies with numerous novel agents including an anti-IL6 antibody, heat shock protein inhibitors, and epigenetic modulators such as vorinostat or panobinostat. These novel agents may therefore shed light on mechanisms of bortezomib resistance. For example, in two different studies, three patients who were refractory to bortezomib had a response to bortezomib with the addition of a novel agent – either tanespimycin or vorinostat. 89 , 96
There have been some recent developments in the epoxyketone class of proteasome inhibitors. Epoxomicin is a natural compound initially isolated from an Actinomycete strain and found to have antimelanoma activity in preclinical models. 97 Carfilzomib (formerly PR-171; Proteolix ® ), is a tetrapeptide epoxyketone related to epoxomicin. There are two components of this agent, a peptide portion that binds to the substrate binding pocket(s) of the proteasome with high affinity and a epoxyketone pharmacophore that interacts with the catalytic amino-terminal threonine residue and irreversibly inhibits proteasome activity. Relative to bortezomib, carfilzomib more selectively inhibits the chymotrypsin-like activity of the proteasome with less cross-reactivity at the caspase-like and trypsin-like sites. At doses of 15 mg/m 2 or greater, there is > 80% proteasome inhibition in both red blood cells and peripheral blood mononuclear cells in humans. The ability to give this drug safely on consecutive days allows for sustained proteasome inhibition. 98 Preliminary data presented at the annual meeting of American Society of Hematology in 2008 from ongoing phase II studies indicate an overall response rate of greater than 50% and 26% in bortezomib-naïve 91 and bortezomib-exposed 90 patients with multiple myeloma, respectively. Cyclic thrombocytopenia was also noted but otherwise, the toxicity profile was different from bortezomib – increased creatinine and possible tumor lysis but no significant neuropathy. 90 , 91 , 99
The first member of the β lactone class of proteasome inhibition that received attention was derived from lactacystin, produced by Streptomyces . It was highly unstable intracellularly but was more specific than the peptide aldehydes. 2 Salinosporamide A (NPI-0052), a product of a marine actinomycete Salinispora tropica , has a bicyclic ring structure similar to lactacystin, but with various substitutions. 100 Preclinical studies have shown that unlike bortezomib, NPI-0052 inhibits all three protease activities of the proteasome. It is also orally bioactive, a more potent inducer of apoptosis in myeloma cells than bortezomib, and demonstrates activity in bortezomib resistant cell lines as well. 101 Preliminary reports from ongoing phase I studies in a variety of tumors indicate that the drug appears to be well tolerated. 102 – 104
The development of the first-in-class proteasome inhibitor bortezomib in multiple myeloma is a paradigm for the optimal interaction between the pharmaceutical industry, academic institutions, and patient advocacy groups. With ever increasing knowledge of the mechanism of action of this agent, the full therapeutic potential of this growing class of drugs can be realized.
Disclosure
Ajai Chari and Amitabha Mazumder have both received honoraria related to speakers’ bureau activities from Millennium Pharmaceuticals Inc., The Takeda Oncology Company. Sundar Jagannath has received honoraria related to Advisory Board Consultancy from Millennium Pharmaceuticals Inc., The Takeda Oncology Company.
References
1 Adams J The proteasome: a suitable antineoplastic target Nat Rev Cancer 2004 4 5 349 360 15122206 10.1038/nrc1361
2 Orlowski RZ Stinchcombe TE Mitchell BS Phase I trial of the proteasome inhibitor PS-341 in patients with refractory hematologic malignancies J Clin Oncol 2002 20 22 4420 4427 12431963 10.1200/JCO.2002.01.133
3 Adams J Potential for proteasome inhibition in the treatment of cancer Drug Discov Today 2003 8 7 307 315 12654543 10.1016/s1359-6446(03)02647-3
4 Sanchez-Serrano I Success in translational research: lessons from the development of bortezomib Nat Rev Drug Discov 2006 5 2 107 114 16518378 10.1038/nrd1959
5 Adams J Palombella VJ Sausville EA Proteasome inhibitors: a novel class of potent and effective antitumor agents Cancer Res 1999 59 11 2615 2622 10363983
6 Hideshima T Richardson P Chauhan D The proteasome inhibitor PS-341 inhibits growth, induces apoptosis, and overcomes drug resistance in human multiple myeloma cells Cancer Res 2001 61 7 3071 3076 11306489
7 LeBlanc R Catley LP Hideshima T Proteasome inhibitor PS-341 inhibits human myeloma cell growth in vivo and prolongs survival in a murine model Cancer Res 2002 62 17 4996 5000 12208752
8 Lightcap ES McCormack TA Pien CS Chau V Adams J Elliott PJ Proteasome inhibition measurements: clinical application Clin Chem 2000 46 5 673 683 10794750
9 Richardson PG Barlogie B Berenson J A phase 2 study of bortezomib in relapsed, refractory myeloma N Engl J Med 2003 348 26 2609 2617 12826635 10.1056/NEJMoa030288
10 Richardson PG Sonneveld P Schuster MW Bortezomib or high-dose dexamethasone for relapsed multiple myeloma N Engl J Med 2005 352 24 2487 2498 15958804 10.1056/NEJMoa043445
11 Richardson PG Sonneveld P Schuster M Extended follow-up of a phase 3 trial in relapsed multiple myeloma: final time-to-event results of the APEX trial Blood 2007 110 10 3557 3560 17690257 10.1182/blood-2006-08-036947
12 Lee SJ Richardson PG Sonneveld P Bortezomib is associated with better health-related quality of life than high-dose dexamethasone in patients with relapsed multiple myeloma: results from the APEX study Br J Haematol 2008 143 4 511 519 18986387 10.1111/j.1365-2141.2008.07378.x
13 San-Miguel JF Richardson PG Sonneveld P Efficacy and safety of bortezomib in patients with renal impairment: results from the APEX phase 3 study Leukemia 2008 22 4 842 849 18200040 10.1038/sj.leu.2405087
14 Zangari M Esseltine D Lee CK Response to bortezomib is associated to osteoblastic activation in patients with multiple myeloma Br J Haematol 2005 131 1 71 73 16173965 10.1111/j.1365-2141.2005.05733.x
15 Giuliani N Morandi F Tagliaferri S The proteasome inhibitor bortezomib affects osteoblast differentiation in vitro and in vivo in multiple myeloma patients Blood 2007 110 1 334 338 17371942 10.1182/blood-2006-11-059188
16 Terpos E Sezer O Croucher P Dimopoulos MA Myeloma bone disease and proteasome inhibition therapies Blood 2007 110 4 1098 1104 17494860 10.1182/blood-2007-03-067710
17 Zangari M Esseltine D Cavallo F Predictive value of alkaline phosphatase for response and time to progression in bortezomib-treated multiple myeloma patients Am J Hematol 2007 82 9 831 833 17546639 10.1002/ajh.20961
18 Ma MH Yang HH Parker K The proteasome inhibitor PS-341 markedly enhances sensitivity of multiple myeloma tumor cells to chemotherapeutic agents Clin Cancer Res 2003 9 3 1136 1144 12631619
19 Orlowski RZ Nagler A Sonneveld P Randomized phase III study of pegylated liposomal doxorubicin plus bortezomib compared with bortezomib alone in relapsed or refractory multiple myeloma: combination therapy improves time to progression J Clin Oncol 2007 25 25 3892 3901 17679727 10.1200/JCO.2006.10.5460
20 Berenson JR Yang HH Vescio RA Safety and efficacy of bortezomib and melphalan combination in patients with relapsed or refractory multiple myeloma: updated results of a phase 1/2 study after longer follow-up Ann Hematol 2008 87 8 623 631 18463870 10.1007/s00277-008-0501-0
21 Popat R Oakervee H Williams C Bortezomib, low-dose intravenous melphalan, and dexamethasone for patients with relapsed multiple myeloma Br J Haematol 2009 144 6 887 894 19183191 10.1111/j.1365-2141.2008.07572.x
22 Pineda-Roman M Zangari M van Rhee F VTD combination therapy with bortezomib-thalidomide-dexamethasone is highly effective in advanced and refractory multiple myeloma Leukemia 2008 22 7 1419 1427 18432260 10.1038/leu.2008.99 PMC3664925
23 Reece DE Piza G Trudel S A Phase I–II Trial of Bortezomib Plus Oral Cyclophosphamide and Prednisone for Relapsed/Refractory Multiple Myeloma ASH Annual Meeting Abstracts 2006 108 11 3536
24 Kropff M Bisping G Schuck E Bortezomib in combination with intermediate-dose dexamethasone and continuous low-dose oral cyclophosphamide for relapsed multiple myeloma Br J Haematol 2007 138 3 330 337 17614819 10.1111/j.1365-2141.2007.06656.x
25 Hajek R Zahradova L Gregora E The reduced intensity Cvd regimen: a good option with well balanced efficacy/toxicity ratio for elderly patients with poor status performance ASH Annual Meeting Abstracts 2008 11 16 112 11 3699
26 Lee SS Suh C Kim BS Bortezomib, doxorubicin and dexamethasone (PAD) combination therapy followed by thalidomide and dexamethasone (TD) as a salvage treatment for relapsed multiple myeloma (MM): preliminary analysis of efficacy and safety ASH Annual Meeting Abstracts 2007 110 11 2731
27 Palumbo A Gay F Bringhen S Bortezomib, doxorubicin and dexamethasone in advanced multiple myeloma Ann Oncol 2008 19 6 1160 1165 18326520 10.1093/annonc/mdn018
28 Richardson P Jagannath S Jakubowiak A Lenalidomide, bortezomib, and dexamethasone in patients with relapsed or relapsed/refractory multiple myeloma (MM): encouraging response rates and tolerability with correlation of outcome and adverse cytogenetics in a phase II study ASH Annual Meeting Abstracts 2008 112 11 1742
29 Poenisch W Bourgeois M Wang SY Bortezomib in combination with bendamustine and prednisone in the treatment of patients with refractory/relapsed multiple myeloma ASH Annual Meeting Abstracts 2007 110 11 2723
30 Palumbo A Ambrosini MT Benevolo G Bortezomib, melphalan, prednisone, and thalidomide for relapsed multiple myeloma Blood 2007 109 7 2767 2772 17148584 10.1182/blood-2006-08-042275
31 Terpos E Heath DJ Zervas K Dimopoulos MA The effect of bortezomib monotherapy and bortezomib-based regimens on bone metabolism in patients with relapsed/refractory multiple myeloma Paper presented at: Haematologica 2007
32 Ciolli S Leoni F Casini C Breschi C Santini V Bosi A The addition of liposomal doxorubicin to bortezomib, thalidomide and dexamethasone significantly improves clinical outcome of advanced multiple myeloma Br J Haematol 2008 141 6 814 819 18410447 10.1111/j.1365-2141.2008.07147.x
33 Kim YK Lee JJ Sohn SK Clinical Efficacy of VEL-CTD (bortezomib, cyclophosphamide, thalidomide, and dexamethasone) regimen in patients with relapsed or refractory multiple myeloma: a phase II study ASH Annual Meeting Abstracts 2008 112 11 3693
34 San Miguel JF Schlag R Khuageva NK Bortezomib plus melphalan and prednisone for initial treatment of multiple myeloma N Engl J Med 2008 359 9 906 917 18753647 10.1056/NEJMoa0801479
35 Cavo M Tacchetti P Patriarca F Superior complete response rate and progression-free survival after autologous transplantation with up-front velcade-thalidomide-dexamethasone compared with thalidomide-dexamethasone in newly diagnosed multiple myeloma ASH Annual Meeting Abstracts 2008 112 11 158
36 Harousseau J Updated results: Randomized phase III IFM trial comparing Vel/Dex with VAD induction in MM in pts ≤ 65 years Joint ASH/ASCO Symposium 2008 12
37 Sonneveld P van der Holt B Schmidt-Wolf IGH First analysis of HOVON-65/GMMG-HD4 randomized phase III trial comparing bortezomib, adriamycine, dexamethasone (pad) vs vad as induction treatment prior to high dose melphalan (HDM) in patients with newly diagnosed multiple myeloma (MM) ASH Annual Meeting Abstracts 2008 112 11 653
38 Chanan-Khan A Sonneveld P Schuster MW Analysis of herpes zoster events among bortezomib-treated patients in the phase III APEX study J Clin Oncol 2008 26 29 4784 4790 18711175 10.1200/JCO.2007.14.9641
39 Vickrey E Allen S Mehta J Singhal S Acyclovir to prevent reactivation of varicella zoster virus (herpes zoster) in multiple myeloma patients receiving bortezomib therapy Cancer 2009 115 1 229 232 19090004 10.1002/cncr.24006
40 Pour L Adam Z Buresova L Varicella-zoster virus prophylaxis with low-dose acyclovir in patients with multiple myeloma treated with bortezomib Clin Lymphoma Myeloma 2009 9 2 151 153 19406726 10.3816/CLM.2009.n.036
41 Asahi T Tsutsui M Wakasugi M Valacyclovir neurotoxicity: clinical experience and review of the literature Eur J Neurol 2009 16 4 457 460 19187258 10.1111/j.1468-1331.2008.02527.x
42 Hellden A Lycke J Vander T Svensson JO Odar-Cederlof I Stahle L The aciclovir metabolite CMMG is detectable in the CSF of subjects with neuropsychiatric symptoms during aciclovir and valaciclovir treatment J Antimicrob Chemother 2006 57 5 945 949 16540518 10.1093/jac/dkl067
43 Marin M Guris D Chaves SS Schmid S Seward JF Prevention of varicella: recommendations of the Advisory Committee on Immunization Practices (ACIP) MMWR Recomm Rep 2007 6 22 56 RR-4 1 40 17585291
44 Miguel JFS Schlag R Khuageva NK Updated follow-up and results of subsequent therapy in the phase III VISTA trial: bortezomib plus melphalan-prednisone versus melphalan-prednisone in newly diagnosed multiple myeloma ASH Annual Meeting Abstracts 12008 112 11 650
45 Mateos MV Oriol A Martinez J Bortezomib (Velcade)-melphalan-prednisone (VMP) versus velcade-thalidomide-prednisone (VTP) in elderly untreated multiple myeloma patients: which is the best partner for velcade: an alkylating or an immunomodulator agent? ASH Annual Meeting Abstracts 2008 112 11 651
46 Palumbo A Bringhen S Rossi D A prospective, randomized, phase III study of bortezomib, melphalan, prednisone and thalidomide (VMPT) versus bortezomib, melphalan and prednisone (VMP) in elderly newly diagnosed myeloma patients ASH Annual Meeting Abstracts 12008 112 11 652
47 Rosinol L Cibeira MT Martinez J Thalidomide/dexamethasone (TD) vs bortezomib(Velcade(R))/thalidomide/dexamethasone (VTD) vs VBMCP/VBAD/Velcade(R) as induction regimens prior autologous stem cell transplantation (ASCT) in younger patients with multiple myeloma (MM): first results of a prospective phase III PETHEMA/Gem trial ASH Annual Meeting Abstracts 2008 112 11 654
48 Attal M Harousseau JL Facon T Single versus double autologous stem-cell transplantation for multiple myeloma N Engl J Med 2003 349 26 2495 2502 14695409 10.1056/NEJMoa032290
49 Cavo M Tosi P Zamagni E Prospective, randomized study of single compared with double autologous stem-cell transplantation for multiple myeloma: Bologna 96 clinical study J Clin Oncol 2007 25 17 2434 2441 17485707 10.1200/JCO.2006.10.2509
50 Richardson P Lonial S Jakubowiak A Lenalidomide, bortezomib, and dexamethasone in patients with newly diagnosed multiple myeloma: encouraging efficacy in high risk Groups with updated results of a phase I/II study ASH Annual Meeting Abstracts 2008 112 11 92
51 Harousseau JL Attal M Leleu X Bortezomib plus dexamethasone as induction treatment prior to autologous stem cell transplantation in patients with newly diagnosed multiple myeloma: results of an IFM phase II study Haematologica 2006 91 11 1498 1505 17043025
52 Corso A Barbarano L Mangiacavalli S Bortezomib with HIG-dose dexamethasone as first Line therapy in patients with multiple myeloma candidates to high-dose therapy ASH Annual Meeting Abstracts 2007 110 11 3595
53 Rosinol L Oriol A Mateos MV Phase II PETHEMA trial of alternating bortezomib and dexamethasone as induction regimen before autologous stem-cell transplantation in younger patients with multiple myeloma: efficacy and clinical implications of tumor response kinetics J Clin Oncol 2007 25 28 4452 4458 17785704 10.1200/JCO.2007.12.3323
54 Jagannath S Durie BGM Wolf JL Long-term follow-up of patients treated with bortezomib alone and in combination with dexamethasone as frontline therapy for multiple myeloma ASH Annual Meeting Abstracts 2006 108 11 796
55 Orlowski RZ Peterson BL Sanford B Bortezomib and pegylated liposomal doxorubicin as induction therapy for adult patients with symptomatic multiple myeloma: Cancer and Leukemia Group B Study 10301 ASH Annual Meeting Abstracts 2006 108 11 797
56 Jakubowiak A Kendall T Al-Zoubi A Initial treatment with bortezomib (Velcade(R)), Doxil(R), and dexamethasone (VDD) is superior to thalidomide and dexamethasone (TD) as initial therapy prior to autologous stem cell transplantation (ASCT) for multiple myeloma (MM) ASH Annual Meeting Abstracts 2008 112 11 3713
57 Palumbo A Falco P Gay F Bortezomib-doxorubicin-dexamethasone as induction prior to reduced intensity autologous transplantation followed by lenalidomide as consolidation/maintenance in elderly untreated myeloma patients ASH Annual Meeting Abstracts 2008 112 11 159
58 Belch A Reece DE Bahlis NJ Bortezomib [VELCADETM], pegylated liposomal doxorubicin [DOXIL/CAELYX(R)] and dexamethasone in the treatment of previously untreated multiple myeloma patients: impact on quality-of-life ASH Annual Meeting Abstracts 2007 110 11 3618
59 Landau H Hassoun H Cohen A Sequential administration of bortezomib, liposomal doxorubicin and dexamethasone (BDD) followed by thalidomide and dexamethasone (td) results in rapid control of untreated high-risk multiple myeloma (MM) and improves depth of response ASH Annual Meeting Abstracts 2008 112 11 3712
60 Wang M Giralt S Delasalle K Handy B Alexanian R Bortezomib in combination with thalidomide-dexamethasone for previously untreated multiple myeloma Hematology 2007 12 3 235 239 17558699 10.1080/10245330701214236
61 Kaufman JL Gleason C Heffner L Lonial S Bortezomib, thalidomide, and dexamethasone as induction therapy for patients with symptomatic multiple myeloma ASH Annual Meeting Abstracts 2007 110 11 3605 10.1002/cncr.25143 20564642
62 Yoon SS Kim HJ Chung JS Sequential VAD (Vincristine, adriamycin, dexamethasone) and VTD (Bortezomib, thalidomide, dexamethasone) induction followed by high-dose therapy with autologous stem cell transplantation and maintenance treatment with bortezomib for newly diagnosed multiple myeloma: final analysis of phase II trial ASH Annual Meeting Abstracts 2008 112 11 3330
63 Berenson JR Yellin O Woytowitz D Bortezomib, ascorbic acid and melphalan (bam) therapy for patients (pts) with newly diagnosed multiple myeloma (MM): an effective and well-tolerated frontline regimen ASH Annual Meeting Abstracts 2007 110 11 3602 10.1111/j.1600-0609.2009.01244.x 19226361
64 Bensinger W Jagannath S Vescio R A phase II study of bortezomib (Velcade (R)), cyclophosphamide (Cytoxan(R)), thalidomide (Thalomid(R)) and dexamethasone as first-line therapy for multiple myeloma ASH Annual Meeting Abstracts 2008 112 11 94
65 Reeder CB Reece DE Kukreti V Cyclophosphamide, bortezomib and dexamethasone induction for newly diagnosed multiple myeloma: high response rates in a phase II clinical trial Leukemia 2009 10.1038/leu.2009.26 PMC2711213 19225538
66 Knop S Liebisch P Wandt H Bortezomib, intravenous cyclophosphamide and dexamethasone (VelCD) for previously untreated multiple myeloma: an interim analysis of the German DSMM XIa Trial ASH Annual Meeting Abstracts 2008 11 16 112 11 2776
67 Barlogie B Anaissie EJ Shaughnessy JD Jr Ninety percent sustained complete response (CR) rate projected 4 years after onset of CR in gene expression profiling (GEP)-defined low-risk multiple myeloma (MM) treated with total therapy 3 (TT3): basis for GEP-risk-adapted TT4 and TT5 ASH Annual Meeting Abstracts 2008 112 11 162
68 Barlogie B Anaissie E van Rhee F Incorporating bortezomib into upfront treatment for multiple myeloma: early results of total therapy 3 Br J Haematol 2007 138 2 176 185 17593024 10.1111/j.1365-2141.2007.06639.x
69 Mulligan G Mitsiades C Bryant B Gene expression profiling and correlation with outcome in clinical trials of the proteasome inhibitor bortezomib Blood 2007 109 8 3177 3188 17185464 10.1182/blood-2006-09-044974
70 Hideshima T Mitsiades C Akiyama M Molecular mechanisms mediating antimyeloma activity of proteasome inhibitor PS-341 Blood 2003 101 4 1530 1534 12393500 10.1182/blood-2002-08-2543
71 Qin JZ Ziffra J Stennett L Proteasome inhibitors trigger NOXA-mediated apoptosis in melanoma and myeloma cells Cancer Res 2005 65 14 6282 6293 16024630 10.1158/0008-5472.CAN-05-0676
72 Nikiforov MA Riblett M Tang WH Tumor cell-selective regulation of NOXA by c-MYC in response to proteasome inhibition Proc Natl Acad Sci U S A 2007 104 49 19488 19493 18042711 10.1073/pnas.0708380104 PMC2148316
73 Jullig M Zhang WV Ferreira A Stott NS MG132 induced apoptosis is associated with p53-independent induction of pro-apoptotic NOXA and transcriptional activity of beta-catenin Apoptosis 2006 11 4 627 641 16673057 10.1007/s10495-006-4990-9
74 Perez-Galan P Roue G Villamor N Montserrat E Campo E Colomer D The proteasome inhibitor bortezomib induces apoptosis in mantle-cell lymphoma through generation of ROS and Noxa activation independent of p53 status Blood 2006 107 1 257 264 16166592 10.1182/blood-2005-05-2091
75 Fernandez Y Verhaegen M Miller TP Differential regulation of NOXA in normal melanocytes and melanoma cells by proteasome inhibition: therapeutic implications Cancer Res 2005 65 14 6294 6304 16024631 10.1158/0008-5472.CAN-05-0686
76 Hideshima T Bradner JE Wong J Small-molecule inhibition of proteasome and aggresome function induces synergistic antitumor activity in multiple myeloma Proc Natl Acad Sci U S A 2005 102 24 8567 8572 15937109 10.1073/pnas.0503221102 PMC1150844
77 Davenport EL Moore HE Dunlop AS Heat shock protein inhibition is associated with activation of the unfolded protein response pathway in myeloma plasma cells Blood 2007 110 7 2641 2649 17525289 10.1182/blood-2006-11-053728
78 Lee AH Iwakoshi NN Anderson KC Glimcher LH Proteasome inhibitors disrupt the unfolded protein response in myeloma cells Proc Natl Acad Sci U S A 2003 100 17 9946 9951 12902539 10.1073/pnas.1334037100 PMC187896
79 Obeng EA Carlson LM Gutman DM Harrington WJ Jr Lee KP Boise LH Proteasome inhibitors induce a terminal unfolded protein response in multiple myeloma cells Blood 2006 107 12 4907 4916 16507771 10.1182/blood-2005-08-3531 PMC1895817
80 Lonial S Waller EK Richardson PG Risk factors and kinetics of thrombocytopenia associated with bortezomib for relapsed, refractory multiple myeloma Blood 2005 106 12 3777 3784 16099887 10.1182/blood-2005-03-1173 PMC1895114
81 Johnston JA Ward CL Kopito RR Aggresomes: a cellular response to misfolded proteins J Cell Biol 1998 143 7 1883 1898 9864362 10.1083/jcb.143.7.1883 PMC2175217
82 Poruchynsky MS Sackett DL Robey RW Ward Y Annunziata C Fojo T Proteasome inhibitors increase tubulin polymerization and stabilization in tissue culture cells: a possible mechanism contributing to peripheral neuropathy and cellular toxicity following proteasome inhibition Cell Cycle 2008 7 7 940 949 18414063 10.4161/cc.7.7.5625 PMC9416319
83 Cavaletti G Gilardini A Canta A Bortezomib-induced peripheral neurotoxicity: a neurophysiological and pathological study in the rat Exp Neurol 2007 204 1 317 325 17214983 10.1016/j.expneurol.2006.11.010
84 Argyriou AA Iconomou G Kalofonos HP Bortezomib-induced peripheral neuropathy in multiple myeloma: a comprehensive review of the literature Blood 2008 112 5 1593 1599 18574024 10.1182/blood-2008-04-149385
85 Richardson PG Briemberg H Jagannath S Frequency, characteristics, and reversibility of peripheral neuropathy during treatment of advanced multiple myeloma with bortezomib J Clin Oncol 2006 24 19 3113 3120 16754936 10.1200/JCO.2005.04.7779
86 Richardson PG Sonneveld P Schuster MW Reversibility of symptomatic peripheral neuropathy with bortezomib in the phase III APEX trial in relapsed multiple myeloma: impact of a dose-modification guideline Br J Haematol 2009 144 6 895 903 19170677 10.1111/j.1365-2141.2008.07573.x
87 Millennium Pharmaceuticals I VelcadeTM (bortezomib) for injection Prescribing information Cambridge, MA Millennium Pharmaceuticals, Inc 2005
88 Ravaglia S Corso A Piccolo G Immune-mediated neuropathies in myeloma patients treated with bortezomib Clin Neurophysiol 2008 119 11 2507 2512 18829381 10.1016/j.clinph.2008.08.007
89 Richardson PG Chanan-Khan A Lonial S Tanespimycin (T) + bortezomib (BZ) in multiple myeloma (MM): confirmation of the recommended dose using a novel formulation ASH Annual Meeting Abstracts 2007 110 11 1165
90 Jagannath S Vij R Stewart AK Initial results of PX-171-003, an open-label, single-arm, phase II study of carfilzomib (CFZ) in patients with relapsed and refractory multiple myeloma (MM) ASH Annual Meeting Abstracts 2008 112 11 864
91 Vij R Wang M Orlowski R Initial results of PX-171-004, an open-label, single-arm, phase II study of carfilzomib (CFZ) in patients with relapsed myeloma (MM) ASH Annual Meeting Abstracts 2008 112 11 865
92 Perrone G Hideshima T Ikeda H Ascorbic acid inhibits anti-tumor activity of bortezomib in vivo Leukemia 2009 10.1038/leu.2009.83 19369963
93 Yang Y Kitagaki J Wang H Hou DX Perantoni AO Targeting the ubiquitin-proteasome system for cancer therapy Cancer Sci 2009 100 1 24 28 19037995 10.1111/j.1349-7006.2008.01013.x PMC2643214
94 Soucy TA Smith PG Milhollen MA An inhibitor of NEDD8-activating enzyme as a new approach to treat cancer Nature 2009 458 7239 732 736 19360080 10.1038/nature07884
95 Piva R Ruggeri B Williams M CEP-18770: A novel, orally active proteasome inhibitor with a tumor-selective pharmacologic profile competitive with bortezomib Blood 2008 111 5 2765 2775 18057228 10.1182/blood-2007-07-100651
96 Weber D Badros AZ Jagannath S Vorinostat plus bortezomib for the treatment of relapsed/refractory multiple myeloma: early clinical experience ASH Annual Meeting Abstracts 2008 112 11 871
97 Hanada M Sugawara K Kaneta K Epoxomicin, a new antitumor agent of microbial origin J Antibiot (Tokyo) 1992 45 11 1746 1752 1468981 10.7164/antibiotics.45.1746
98 Demo SD Kirk CJ Aujay MA Antitumor activity of PR-171, a novel irreversible inhibitor of the proteasome Cancer Res 2007 67 13 6383 6391 17616698 10.1158/0008-5472.CAN-06-4086
99 Alsina M Trudel S Vallone M Molineaux C Kunkel L Goy A Phase 1 single agent antitumor activity of twice weekly consecutive day dosing of the proteasome inhibitor carfilzomib (PR-171) in hematologic malignancies ASH Annual Meeting Abstracts 2007 110 11 411
100 Feling RH Buchanan GO Mincer TJ Kauffman CA Jensen PR Fenical W Salinosporamide A: a highly cytotoxic proteasome inhibitor from a novel microbial source, a marine bacterium of the new genus salinospora Angew Chem Int Ed Engl 2003 42 3 355 357 12548698 10.1002/anie.200390115
101 Chauhan D Catley L Li G A novel orally active proteasome inhibitor induces apoptosis in multiple myeloma cells with mechanisms distinct from bortezomib Cancer Cell 2005 8 5 407 419 16286248 10.1016/j.ccr.2005.10.013
102 Hamlin PA Aghajanian C Hong D First-in-human phase 1 dose escalation study of NPI-0052, a novel proteasome inhibitor, in patients with lymphoma and solid tumor ASH Annual Meeting Abstracts 2008 112 11 4939
103 Price T Padrik P Townsend A Clinical trial of NPI-0052 (2nd generation proteasome inhibitor) in patients having advanced malignancies with expanded RP2D cohorts in lymphoma and CLL ASH Annual Meeting Abstracts 2008 112 11 4934
104 Richardson P Hofmeister CC Zimmerman TM Phase 1 clinical trial of NPI-0052, a novel proteasome inhibitor in patients with multiple myeloma ASH Annual Meeting Abstracts 2008 112 11 2770