Mechanisms of resistance to the main classes of novel agents for multiple myeloma .
The number of novel therapies for the treatment of multiple myeloma (MM) is rapidly increasing with proteasome inhibitors, immunomodulatory agents and monoclonal antibodies being the most well-known therapeutic classes whilst histone deacetylase inhibitors, selective inhibitors of nuclear export and CAR-T cells amongst others also being actively investigated. However, in parallel with the development and application of these novel myeloma therapies is the emergence of novel mechanisms of resistance, many of which remain elusive, particularly for more recently developed agents. Whilst resistance mechanisms have been best studied for proteasome inhibitors, particularly Bortezomib, class effects do not universally apply to all proteasome inhibitors, and within-class differences in efficacy, toxicity and resistance mechanisms have been observed. Immunomodulatory agents share the common cellular target cereblon and thus resistance patterns relate to cereblon expression and its pathway components. However, the cell surface antigens to which monoclonal antibodies are directed means these agents frequently exhibit unique within-class differences in clinical efficacy and resistance patterns. Despite the progressive biological elucidation of resistance mechanisms to these novel therapies, attempts to specifically exploit these processes lag considerably behind and until such approaches become available, resistance to these therapies will remain a concern.
- novel therapy
- drug resistance
- proteasome inhibitor
- immunomodulatory agent
- monoclonal antibody
There has recently been an explosion of novel agents for the treatment of MM that have dramatically improved overall response rates (ORR), progression-free survival (PFS) and overall survival (OS) by targeting the malignant plasma cell and bone marrow microenvironment in unique ways. The main classes of novel agents are proteasome inhibitors, immunomodulatory agents and monoclonal antibodies, however several other classes of novel agents are emerging, including histone deacetylase inhibitors, BH3 mimetics, checkpoint inhibitors and selective inhibitors of nuclear export, as are alternative approaches, such as chimeric antigen receptor T-cells (CAR-T) with MM cell specificity. Whilst CAR-T technology in MM remains in pre-clinical and early clinical trial stages of development, this immunological approach is rapidly gaining momentum with several groups developing CAR-T cells for therapeutic use . Despite these therapeutic advances, many MM patients develop disease relapse suggesting the development of drug resistance whilst some are primary refractory. In this chapter, for the three major classes of novel agents, we present a discussion on known biological mechanisms of resistance together with clinical trial efforts, if any, to overcome these. Of all therapeutic classes of novel agents, mechanisms of resistance to proteasome inhibitors have been studied in greatest detail and are the focus of this chapter.
2. Proteasome inhibitors
Plasma cells secrete immunoglobulin in response to infection and a range of other stimuli which requires folding in the endoplasmic reticulum (ER) lumen prior to secretion from the cell, resulting in a degree of ER stress due to misfolded protein . ER stress is heightened in MM due to the high, sustained production of monoclonal immunoglobulin and a build-up of misfolded protein within the ER lumen. This ER stress activates three ER membrane stress sensors, protein kinase RNA-like ER kinase (PERK), inositol-requiring enzyme 1 (IRE1) and activating transcription factor 6 (ATF6) in a homeostatic process termed the Unfolded Protein Response (UPR) . Activation of the UPR results in a global reduction in protein translation and the upregulation of ER chaperones and folding machinery to cope with the misfolded protein load, thereby rectifying the high ER stress levels that initiated the process. However, high sustained levels of ER stress can overwhelm the corrective capacity of the UPR which turns from a pro-survival, homeostatic mechanism to one that commits the MM cell to apoptosis. By inhibiting the 26S proteasome and preventing the degradation of misfolded proteins, proteasome inhibitors induce ER stress and a terminal UPR . However, there are other mechanisms through which these agents exert their activity. Indeed, proteasome inhibitors are able to modulate a diverse array of cell signalling pathways whilst rendering the microenvironment less supportive of MM cell growth . Perhaps due to the significant clinical impact the first-in-class proteasome inhibitor Bortezomib has made, resistance mechanisms to this agent have been studied in greatest detail compared to other proteasome inhibitors (Table 1 and Figure 1A).
|Resistance type||Resistance mediator(s)||Resistance mechanism|
|Mutations||Impaired ability of proteasome inhibitors to bind to the catalytically active |
|Aberrant expression of ubiquitin-proteasome pathway components||β5 and other proteasome subunits||Increased or decreased numbers of binding sites for proteasome inhibitors, altering their ability to inhibit proteolysis|
|Activation of the aggresome-autophagy pathway||HDAC6 and autophagic machinery||Sequestration of toxic proteins in aggresomes and their removal by autophagy-mediated degradation|
|Heat shock protein induction||Grp78, Hsp90 and other family members including Hsp70 and Hsp8||Increased protein chaperoning resulting in greater ability to deal with misfolded and other toxic proteins|
|Drug efflux activity||P-glycoprotein and other ATP-binding cassette (ABC) superfamily members||Cellular efflux of proteasome inhibitors thereby reducing their ability to interact with proteasome subunits and other intracellular processes|
|Antioxidant response pathway induction||Over-expression of nuclear factor, erythroid 2 like 2 (||Assists proteasome assembly by inducing expression of proteasome maturation protein (POMP)|
|Plasma cell differentiation||Reduced expression of Xbp1||Correlates with reduced immunoglobulin synthesis and ER stress/proteasome load therefore reduced sensitivity to proteasome inhibitors|
|Bone marrow microenvironment||Adhesion molecules on MM cells and bone marrow stromal cells (e.g. CD138, CD44, VCAM-1, LFA-1, MUC-1, ICAM-1 etc.)||Microenvironmental protection from proteasome inhibitors and other anti-MM therapies by increased MM cell migration, homing and adhesion to the bone marrow and activation of survival and proliferative intracellular signalling pathways|
|Survival signalling pathways||IL-6, VEGF, HGF, c-MET, NF-κB, PI3K/AKT, IGF-1/IGF-1R, tight junction protein 1 (TJP1) and EGFR/JAK/STAT signalling||Proliferation and cell survival signalling reducing the efficacy of proteasome inhibitors. Increased angiogenesis and MM cell migration. Induction of EGFR/JAK/STAT signalling associated with increased expression of proteasome subunits|
|Cereblon pathway abnormalities||Reduced cereblon expression||Less available target for IMiD binding|
|Cereblon and other pathway component mutations||Reduced ability for IMiDs to bind to cereblon and other pathway components|
|Ras/Raf pathway activation||KRAS G12D and BRAF V600E mutations||Ras/Raf pathway activating mutations result in MM cell proliferation and resistance to IMiDs|
|Adhesion to bone marrow stroma||CD44 (Wnt/β-catenin signalling)||Greater adhesion to bone marrow stromal cells protecting MM cells from IMiDs|
|Target antigen expression||Reduced expression of CD38, SLAM7 and other cell surface proteins||Less available target for mAb binding through various mechanisms including trogocytosis|
|Resistance to complement-dependent cytotoxicity (CDC)||Increased expression of CD46, CD55 and CD59||Reduced ability for mAbs to activate CDC|
|Soluble antigen||Extracellular CD38 and SLAM7||Extracellular binding of mAbs to target antigen resulting in reduced mAb binding to cell surface antigen|
|Development of neutralising antibodies||Anti-mAb antibodies||Host derived anti-mAb antibodies neutralise therapeutic mAbs before reaching their cellular targets|
2.1. The ubiquitin-proteasome pathway
The ubiquitination and proteasome degradation pathway is a multistep enzymatic cascade in eukaryotes through which the cell removes excess and misfolded proteins and regulates cellular processes including cell proliferation and survival . The process involves the conjugation of ubiquitin via a lysine residue at position 48. Proteins tagged with lysine 48-linked chains of ubiquitin are marked for degradation in the proteasome enzyme complex [5, 6]. Eukaryotic cells contain the 26S proteasome which consists of a 20S core particle that is bound to two 19S regulatory particles [7, 8]. The 19S regulatory particle is responsible for substrate recognition, deubiquitination, unfolding and translocation into the 20S core particle which contains the active sites that hydrolyze substrate peptide bonds . The 20S core particle is composed of four rings that are composed of seven α (α1–α7) subunits or seven β subunits (β1–β7), that are stacked in a specific order (α7β7β7α7). These rings generate three interconnected chambers: two outer chambers that are formed by the adjacent α and β rings and a catalytic chamber that is formed by the two adjacent β rings. Only the β1, β2 and β5 subunits are catalytically active proteases [10, 11]. Near the β subunit’s active site lies a substrate specificity pocket which binds to 10 amino acid stretches in the substrate that flank the peptide bond that is cleaved and thereby determines the cleaving preferences of each β subunit [12, 13]. In particular, the β1 subunit has caspase-like activity (cleaving after acidic residues), β2 exhibits trypsin-like activity (cleaving after basic residues), and β5 has chymotrypsin-like activity (cleaving after hydrophobic residues) [14, 15].
Proteins that are targeted for proteasomal degradation must cross the 19S regulatory subunit in order to reach the proteolytic 20S core where they are degraded into peptides that vary from 3 to 25 amino acids in length [16, 17]. Each substrate is cleaved in multiple locations without release of partially hydrolyzed substrates from the core particle and the mechanism of degradation is conserved for all catalytically active β subunits [16, 18]. In eukaryotes, the 20S core particle components can change in response to biological stimuli. For example, stimulation of cells with interferon gamma induces the expression of all three catalytically active β subunits. These subunits, along with a unique 11S regulatory particle, form a complex called the immunoproteasome which is involved in generating peptides for presentation to major histocompatibility complex class I molecules, but also has classic proteolytic activity [19, 20, 21]. Increased expression of the immunoproteasome complex has been reported in MM, where it may represent the predominant form of the proteasome [22, 23, 24, 25]. It is also noteworthy that relapsed MM may be associated with lower levels of the immunoproteasome and increased levels of the constitutive proteasome .
2.2. Proteasome inhibitors used to treat myeloma
Proteasome inhibitors are potent anti-MM agents which inhibit one or more proteolytic subunits of the 20S proteasome. Their efficacy is attributed to a number of factors including inhibition of NF-κB signalling, although this has recently come under question, induction of ER stress with the activation of a terminal UPR, and modification of the bone marrow microenvironment, amongst others [26, 27]. Several generations of proteasome inhibitors have been developed with Bortezomib, Carfilzomib and Ixazomib approved for clinical use in a number of countries. The proteasome inhibitors differ in their relative selectivity for β catalytic subunits, and half-life and reversibility of β subunit inhibition, that translates into differential anti-MM efficacy and toxicity profiles . Thus, the individual proteasome inhibitors demonstrate significant within-class pharmacokinetic and pharmacodynamic variation and resistance to one proteasome inhibitor does not necessarily suggest resistance to another.
The first-in-class proteasome inhibitor Bortezomib (N-acyl-pseudo dipeptidyl boronic acid) is a dipeptide that binds
2.2.1. Later-generation proteasome inhibitors
There are ongoing attempts to expand and improve the repertoire of proteasome inhibitors. Marizomib
2.3. Mechanisms of resistance to proteasome inhibitors
2.3.1. Mutations and aberrant expression of ubiquitin-proteasome pathway components
22.214.171.124. Pre-clinical/clinical findings
Several point mutations in proteasome subunits that render them insensitive to Bortezomib inhibition have been identified. A single point mutation in the Bortezomib binding pocket of the β5 subunit (
Significantly increased protein expression of the β5 subunit and only modest increases in β1 and β2 subunits were observed in Bortezomib-resistant THP1 cells which were reversible upon withdrawal of Bortezomib from cell cultures . Over-expression of β subunits has also been detected in some MM cell lines, as well as those of some other haematologic malignancies, however, studies in MM suggest that the induction of these proteins is at most modest with minimal contribution to resistance . Furthermore, free β5 subunits are catalytically inactive by themselves and cannot generally bind proteasome inhibitors unless assembled into functional proteasomes . The expression levels of tight junction protein 1 (TJP1/ZO-1) were shown to be strongly associated with Bortezomib sensitivity with the downstream mechanism being suppression of EGFR signalling, which decreased the levels of proteasome subunit synthesis in at STAT3-dependent manner . High TJP1 expression in patient MM cells was associated with a significantly higher chance of responding to Bortezomib and a longer duration of response .
2.3.2. Activation of the aggresome-autophagy pathway
126.96.36.199. Pre-clinical/clinical findings
Cytosolic small protein aggregates form when misfolded proteins accumulate, which are then transported towards the microtubule organising centre into a structure called the aggresome. Acetylation of α-tubulin, which is reversed by histone deacetylase 6 (HDAC6), modulates the structure and function of the microtubule, thus playing a pivotal role in the movement of misfolded protein aggregates to the aggresome . Cells that lack HDAC6 were found to be defective in the removal of protein aggregates and are not able to form large aggresomes . Autophagy is predominantly a pro-survival homeostatic process whereby double-membrane vesicles known as autophagosomes sequester cytosolic proteins, including aggresomes, followed by fusion with lysosomes for degradation. Thus, misfolded proteins can be degraded via the ubiquitin-proteasome and/or aggresome-autophagy pathways and simultaneous blockade of both by combining Bortezomib and the HDAC inhibitor Panobinostat, respectively, showed synergistic anti-MM activity in pre-clinical models . By inhibiting the proteasome, Bortezomib results in an increase in aggresome formation and also induction of autophagy, the latter a likely compensatory mechanism to eliminate misfolded proteins and other substrates of the ubiquitin-proteasome system which could be involved in resistance to proteasome inhibitors . Thus, clinical studies combining a proteasome inhibitor with HDAC and/or autophagy inhibition have a sound biological basis for overcoming resistance to proteasome inhibitors.
188.8.131.52. Clinical studies to circumvent resistance
A large phase III study demonstrated a superior PFS when Panobinostat was combined with Bortezomib and Dexamethasone over Bortezomib and Dexamethasone alone in relapsed/refractory MM patients, leading FDA approval of Panobinostat in 2015 . Despite this, no differences in OS or ORR were evident although the proportion of patients achieving a complete response (CR) was higher with Panobinostat. Given the activity of Carfilzomib in Bortezomib-resistant MM, early clinical studies are ongoing examining the combination of Panobinostat and Carfilzomib in relapsed/refractory MM and are expected to yield favourable results (NCT01496118). With regard to autophagy, a phase II trial evaluating the combination of Bortezomib and the autophagy inhibitor Chloroquine in patients with relapsed and/or refractory MM, supported by the finding of synergistic MM cell death in the pre-clinical setting, showed a clinical benefit rate of 40%, further cementing the role of the aggresome-autophagy pathway in proteasome inhibitor-resistant MM .
2.3.3. Heat shock protein induction
184.108.40.206. Pre-clinical/clinical findings
The heat shock response is part of the cell repair machinery that maintains homeostasis under stressful conditions such as infection, inflammation, starvation, hypoxia, and exposure to toxins, which is carried out by heat shock proteins (HSPs) . HSPs assist in protein folding and preventing undesirable protein aggregation . Blockade of proteasome-mediated protein degradation leads to the induction of HSPs and related chaperones, which have been shown to confer resistance to proteasome inhibitors . Two well characterised HSPs in this setting are Grp78 (
Grp78 resides in the ER lumen where it is bound to the luminal domains of the three ER stress protein sensors, ATF6, PERK and IRE1 . Upon accumulation of misfolded proteins in the ER, Grp78 (1) detaches from ATF6, PERK and IRE1 enabling activation of the homeostatic UPR and (2) chaperones the misfolded proteins for degradation by the 20S proteasome . In MM, Grp78 was reported to play a role in resistance to proteasome inhibitors, and MM cells surviving proteasome inhibitor treatment showed increased Grp78 expression, which further increased with progressive disease . However, this was not corroborated by others who could not demonstrate any significant differences in Grp78 expression in bone marrow plasma cells obtained from patients with MGUS, newly-diagnosed MM or relapsed/refractory MM . Inhibition of Grp78 can induce MM cell death and pharmacological inhibition of Grp78 with Metformin, genetic ablation or mutational inactivation followed by Bortezomib treatment led to the accumulation of aggresomes, impaired autophagy and enhancement of the anti-MM effects of Bortezomib .
Hsp90 expression also increases with the accumulation of misfolded proteins in the ER lumen and has been investigated as a potential target to enhance the efficacy of Bortezomib . Hsp90 was found to stabilise Grp78 at the post-transcriptional level, and treatment of Bortezomib-resistant mantle cell lymphoma cells with the Hsp90 inhibitor IPI-504 together with Grp78 knockdown led to synergistic cell death when combined with Bortezomib . Other HSPs have also been shown to confer resistance to Bortezomib, including Hsp70 and small heat shock protein B8 (Hsp8) in MM and Hsp27 in lymphoma [61, 62].
220.127.116.11. Clinical studies to circumvent resistance
No advanced clinical trials employing Grp78 modulation in MM patients have been undertaken, although a study using an anti-Grp78 monoclonal antibody induced a PR in a heavily pre-treated patient when combined with Bortezomib and Lenalidomide . Whilst early clinical trials have identified safe dose ranges for Hsp90 inhibitors, which have been tested either alone or in combination with Bortezomib and Dexamethasone in relapsed/refractory MM, results have been disappointing and to date no agents have progressed beyond the phase I/II stage .
2.3.4. Drug efflux
18.104.22.168. Pre-clinical/clinical findings
The efflux of drugs by members of the ATP-Binding Cassette (ABC) superfamily is a well-established mechanism by which tumours are able to acquire therapeutic resistance . Whilst the multi-drug efflux transporter MDR1/P-glycoprotein (P-pg/ABCB1) has been shown to correlate with MM relapse and drug resistance [66, 67], its role in Bortezomib resistance has been controversial and it is generally thought that Bortezomib is a poor substrate . P-gp was rarely detected in newly diagnosed MM patients , however, over-expression was associated with disease relapse and drug resistance, specifically to Vincristine, Doxorubicin, Etoposide and glucocorticoids [66, 67, 69]. Carfilzomib, on the other hand, is a
2.3.5. Antioxidant response pathway induction
22.214.171.124. Pre-clinical/clinical findings
Elevated levels of antioxidant-related pathway genes have been associated with drug resistance in other tumours, including resistance to Bortezomib in patients with mantle cell lymphoma . Bortezomib resistance-related gene expression signatures revealed enrichment for Nuclear Factor, Erythroid 2 Like 2 (
2.3.6. Plasma cell differentiation
126.96.36.199. Pre-clinical/clinical findings
The transcription factor Xbp-1, a downstream component of the IRE1 arm of the UPR, is required for the differentiation of B-cells into plasma cells and more recently has been shown to be associated with Bortezomib sensitivity [76, 77]. Patient-derived bone marrow MM cells can be subdivided into populations based on their expression of Xbp-1, with plasma cells expressing low or absent Xbp-1 enriched in the bone marrow of patients who have relapsed after Bortezomib therapy or who have progressive disease . These low or absent Xbp-1 expressing plasma cells were less differentiated with lower levels of immunoglobulin synthesis, reduced ER stress and less proteasome load. Conversely, at MM diagnosis, the majority of bone marrow plasma cells expressed higher Xbp-1 levels, conferring sensitivity to Bortezomib, although subpopulations of plasma cells with lower levels could be detected . It is hypothesised that these subpopulations of plasma cells with low Xbp-1 expression are responsible for eventual relapse after induction therapy . Interestingly, these findings would suggest that patients who are resistant to proteasome inhibitors should have non-secretory MM, however, only a small minority of these patients have this disease phenotype. To date, the degree of plasma cell differentiation has not been considered in clinical trials.
2.3.7. Bone marrow microenvironment and survival signalling pathways
188.8.131.52. Pre-clinical/clinical findings
The bone marrow microenvironment (BMME) includes (1) the non-cellular compartment formed by extracellular matrix (ECM) proteins (laminin, fibronectin and collagen) and soluble factors (cytokines, chemokines and growth factors) and (2) a cellular compartment comprising haemopoietic cells and non-haemopoietic cells (fibroblasts, osteoblasts, osteoclasts, endothelial cells, endothelial progenitor cells, pericytes, mesenchymal stem cells and mesenchymal stromal cells) which support MM cell survival and growth . The interaction between ECM proteins and bone marrow stromal cells (BMSCs) with MM cells plays a crucial role in MM pathogenesis and drug resistance by secreting growth factors, cytokines and extracellular vesicles (exosomes) and by the expression of adhesion proteins .
Various soluble factors have been shown to confer resistance to Bortezomib and other therapeutic agents in MM. IL-6 enhances vascular endothelial growth factor (VEGF) secretion promoting angiogenesis which plays a role in MM cell migration . Whilst Bortezomib can inhibit IL-6 and VEGF production, secretion of IL-6 by stromal cells and MM cells leads to Bortezomib resistance . Hepatocyte growth factor (HGF) is upregulated during MM progression, enhancing the expression of its receptor, c-MET . This signalling pathway is constitutively activated in MM cells and endothelial cells from patients with relapsed/refractory MM and mediates drug resistance . Accordingly, an inhibitory effect on endothelial cells obtained from patients refractory to Bortezomib or Lenalidomide was demonstrated using the c-MET inhibitor SU11274 alone or in combination with Bortezomib or Lenalidomide, resulting in downregulation of angiogenic activity .
Constitutive activation of pro-survival signalling pathways (e.g. NF-κB and AKT) has been reported to reduce the sensitivity of MM cells to Bortezomib . Insulin-like growth factor (IGF-1) is produced by plasma cells and is present in the BM microenvironment, where it promotes proliferation and drug resistance in MM cells through activation of MAPK and PI3K/AKT signalling cascades . Over-expression of IGF-1/IGF-1R pathway components has been shown to be a potential mechanism for resistance to proteasome inhibitors with blockade of downstream IGF-1 effectors able to resensitise MM cell lines to Bortezomib . Studies evaluating compounds that affect the IGF-1/IGF-1R interaction are ongoing with OSI-906, a small molecule inhibitor of IGF-1R, able to resensitise MM cells to Bortezomib . A downstream target of IGF-1, AKT, increases in expression in response to proteasome inhibitors in pre-clinical MM studies and an early phase clinical trial suggests that AKT inhibition might overcome resistance to Bortezomib . As previously discussed, reduced expression of tight junction protein 1 (TJP1/ZO-1) and downstream activation of EGFR signalling are strongly correlated with Bortezomib resistance .
Interactions between MM cells and the BM stroma and/or ECM components provide a mechanism whereby MM cells are protected from the cytotoxic effects of anti-MM therapies. Such interactions include those mediated by adhesion molecules of the integrin family, Syndecan-1 (CD138), CD44, vascular cell adhesion molecule-1 (VCAM-1), lymphocyte function-associated antigen-1 (LFA-1), Mucin-1 antigen (MUC-1) and intercellular adhesion molecule-1 (ICAM-1) . The adhesion of MM cells to stromal cells triggers IL-6 secretion, NF-κB activation in stromal cells and activation of signalling pathways that result in MM cell survival and proliferation . Such effects are seen with integrin β7 which increases MM cell adhesion, migration and homing into bone marrow and reduces Melphalan and Bortezomib-induced apoptosis . Similar MM-promoting effects have been reported for the stromal cell-derived factor (SDF-1)/CXCR4 axis, however, clinical translation has not ensued .
Other important mechanisms of BMME-induced drug resistance are emerging. BMSCs can modulate certain miRNAs in MM cells . The expression of miR-27a is associated with Bortezomib resistance in MM patients  whilst suppression of miR-15a and -16 by BMSCs was shown to be responsible for the protection of MM cells from Bortezomib-induced apoptosis . miR-29 acts as a tumour suppressor miRNA and is downregulated in patient MM cells and in MM cell lines with acquired resistance to Bortezomib, Carfilzomib and Ixazomib . Finally, exosomes mediate local cell-cell signalling by transferring mRNAs, miRNAs and proteins. It has been shown that exosomes derived from BMSCs inhibited Bortezomib-induced cell death to protect MM cells from apoptosis .
184.108.40.206. Clinical studies to circumvent resistance
In a phase II study, the anti-IL-6 antibody Siltuximab was administered with Dexamethasone to patients with relapsed and/or refractory MM . Although no responses to Siltuximab alone were observed, the addition of Dexamethasone resulted in ORR, PFS and OS of 23%, 3.7 months and 20.4 months, respectively. Despite these findings, this strategy has not progressed further. The c-MET inhibitor Tivantinib was examined as a single agent in a phase II study in relapsed/refractory MM patients . Overall, 36% of patients showed stable disease as their best response with the authors concluding that Tivantinib did not show promise for unselected relapsed/refractory MM patients, however, the fact that a significant proportion did show disease stability suggests combining c-MET inhibition with other anti-MM therapy could be explored. There are a small number of phase I studies employing a monoclonal anti-IGF-1R antibody alone or in combination with Bortezomib in relapsed/refractory MM, however, the authors of one study conclude that due to low response rates, even in combination with Bortezomib, further development is not justified . Note should be made that patient recruitment into this study was not performed based on evaluation of IGF-1R expression on patient MM cells. No small molecule inhibitors of IGF-1R have so far been tested clinically. A phase I clinical trial in relapsed/refractory MM patients suggests that AKT inhibition with Afuresertib might overcome resistance to Bortezomib . In this study, the ORR was 8.8%, however, despite these potentially promising results in heavily pre-treated patients, more advanced clinical trials have not been undertaken.
3. Immunomodulatory agents
The immunomodulatory drugs (IMiDs), Thalidomide, Lenalidomide and Pomalidomide have also made a major impact in the management of MM. Despite a checkered history in the 1950s and 1960s due to teratogenicity, Thalidomide has high anti-MM activity and has been incorporated into many treatment regimens. The second generation IMiD Lenalidomide and third generation IMiD Pomalidomide represent sequential improvements in efficacy and toxicity profiles with demonstrable activity in patients who have developed resistance to an earlier generation IMiD . With regard to Lenalidomide, the MM-009  and MM-010  phase III trials demonstrated the superiority of Lenalidomide and Dexamethasone over Dexamethasone in relapsed/refractory MM patients whilst the pivotal MM-003 study  demonstrated the efficacy of Pomalidomide and Dexamethasone in MM patients who were refractory to both Bortezomib and Lenalidomide.
The anti-MM effects of IMiDs are related to their binding to the E3 ubiquitin ligase cereblon (CRBN) and subsequent ubiquitination and degradation of two B-cell transcription factors, Ikaros (IKZF1) and Aiolos (IKZF3) . A landmark study identified CRBN as a primary target in Thalidomide teratogenicity, further demonstrating that Thalidomide binds to CRBN, disrupting the function of the E3 ubiquitin ligase complex, ultimately leading to the downregulation of fibroblast growth factor genes and the teratogenic effects associated with Thalidomide . Subsequently, it was shown that the anti-MM efficacy of IMiDs is directly related to CRBN expression.
3.1. Mechanisms of resistance to immunomodulatory agents
3.1.1. Pre-clinical/clinical findings
Resistance mechanisms to IMiDs have been elucidated to a far lesser extent than have those for proteasome inhibitors (Table 1 and Figure 1B) and mostly hinge on the presence of functional CRBN in MM cells . MM patients exposed to and thought to be resistant to Lenalidomide had lower CRBN levels compared to paired samples before and after therapy . Subsequently, it was shown that high expression of CRBN is associated with a favourable response to Thalidomide and Lenalidomide in newly-diagnosed MM patients [102, 103] and no IMiD response occurred in patients with very low CRBN levels . Moreover, in MM patients refractory to Pomalidomide, CRBN levels predicted for differences in PFS (3 versus 8.9 months) and OS (9.1 versus 27.2 months) when comparing patients in the lowest CRBN expression quartile versus those with higher expression . Notably, as CRBN expression decreases in MM patients who develop resistance to Lenalidomide therapy, this does not affect sensitivity to Bortezomib, Melphalan and Dexamethasone [101, 105]. Low levels of the CRBN binding protein IKZF1 and high levels of another CRBN binding protein Karyopherin Subunit Alpha 2 (KPNA2) also correlated with lack of response to Pomalidomide and/or OS . Specifically, patients with low IKZF1 expression had a median OS of 7.3 months compared with 27.2 months in those with higher IKZF1 expression which was also correlated with a similar pattern of PFS (4.9 vs. 7.3 months) .
In relapsed/refractory MM patients, the majority (88%) of whom were refractory to an IMiD, an increased prevalence of mutations in the Ras pathway genes KRAS, NRAS and/or BRAF (72%), as well as TP53 (26%), CRBN (12%) and CRBN pathway genes (10%) were observed . Notably, all CRBN-mutated patients and 91% of the CRBN pathway-mutated patients were unresponsive to IMiD based treatment. Moreover, three patients with CRBN mutations at the time of IMiD resistance did not possess these genetic aberrations at the time of IMiD sensitivity. Importantly, the introduction of these mutations in MM cells conferred Lenalidomide resistance
3.1.2. Clinical studies to circumvent resistance
Whilst the CRBN pathway has been shown to be pivotal in IMiD responsiveness, no clinical studies have eventuated that make use of this important biology as a strategy to overcome resistance to IMiDs and many questions remain such as how much functional CRBN is actually required to maintain IMiD sensitivity. Despite the controversies surrounding CRBN, activating mutations in Ras pathway components, such as KRAS G12D and BRAF V600E, could potentially be targeted with existing compounds in MM patients harbouring these mutations . Such studies have not yet been conducted, although two patients with BRAF V600E positive relapsed/refractory MM achieved significant reductions in tumour burden when treated with the BRAF inhibitor Vemurafenib whilst a patient with highly resistant and rapidly progressive MM also harbouring the BRAF V600E mutation achieved a rapid and sustained response with dual BRAF and MEK inhibition .
4. Monoclonal antibodies
Binding of monoclonal antibody (mAb) to its target antigen on MM cells has been shown to induce cell death through several mechanisms including antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), antibody-dependent cellular phagocytosis (ADCP), induction of apoptosis through FcγR-mediated crosslinking of tumour-bound antibodies and modulation of target antigen enzymatic activity after antibody binding . Three mAbs, Daratumumab, Elotuzumab and Pembrolizumab have advanced to phase III clinical trials with Daratumumab the most successful of these.
CD38 is variably expressed on haemopoietic and some non-haemopoietic cells with surface expression depending on the differentiation and activation status of the cell. High cell surface expression occurs on benign and malignant plasma cells  with the fully-humanised anti-CD38 mAb Daratumumab demonstrating impressive outcomes when combined with Bortezomib (CASTOR) or Lenalidomide (POLLUX) in the relapsed/refractory MM setting [112, 113]. Other CD38 mAbs, such as Isatuximab (chimeric) and MOR202 (fully human), with differing biological activities from Daratumumab are currently being evaluated in clinical trials (Isatuximab: NCT03275285, NCT03319667, NCT02990338; MOR202: NCT01421186). Elotuzumab binds to signalling lymphocytic activation molecule family member 7 (SLAM7) reducing MM cell binding to bone marrow stroma and activating ADCC . Interestingly, whilst no responses to Elotuzumab as a single agent were observed, the addition of Elotuzumab to Lenalidomide and Dexamethasone in relapsed/refractory MM patients (ELOQUENT-2 trial) resulted in improvements in ORR and PFS, and Elotuzumab is currently the subject of ongoing clinical trials (NCT01891643, NCT02495922, NCT01335399) . Pembrolizumab targets the programmed death 1 (PD-1)/programmed cell death ligand 1 (PD-L1) pathway, a critical initiator of immune activation, playing a role in mediating tolerance . However, two phase III trials KEYNOTE-183 and KEYNOTE-185 have recently been suspended by the US Food and Drug Administration due to more deaths being observed in the Pembrolizumab arms and further information on the use of Pembrolizumab in MM is pending.
4.1. Mechanisms of resistance to monoclonal antibodies
4.1.1. Pre-clinical/clinical findings
The relatively recent addition of mAbs to MM pharmacotherapy means there is a paucity of studies examining resistance mechanisms although these are now being explored with their increasing clinical use (Table 1 and Figure 1C). Examination of CD38 expression on MM cells in 102 patients treated with Daratumumab monotherapy has been insightful . With regard to the effect of Daratumumab on residual bone marrow plasma cells, two important points were clear from this analysis. Firstly, CD38 cell surface expression on plasma cells is highest before Daratumumab treatment and is significantly decreased during treatment. At the time of progressive disease, plasma cells isolated from the bone marrow of these patients exhibited low expression of CD38 suggesting Daratumumab therapy would be less effective, a finding corroborated previously . Secondly, pre-treatment CD38 expression on the surface of MM cells was higher in patients who achieved at least a PR compared to those who did not. Recently, it was shown that Daratumumab-CD38 complexes and accompanying cell membrane are actively transferred from MM cells to monocytes and granulocytes in a process called trogocytosis that was also associated with reduced MM cell surface expression of CD49d, CD56 and CD138 . However, Daratumumab-induced reductions in CD38 expression on MM cells occur in patients with deep and durable responses suggesting reductions in CD38 alone are not responsible for Daratumumab resistance . Cell surface expression of the complement-inhibitory proteins, CD46, CD55 and CD59, was not associated with clinical response but significantly increased only at the time of disease progression. Furthermore, all-
In addition to the cell surface expression of target antigens on MM cells, several other potential mechanisms of resistance to mAbs may be at play. Soluble forms of CD38  and SLAM7  may affect the efficacy of Daratumumab and Elotuzumab, respectively. Another potential mechanism of resistance is the development of neutralising antibodies to the therapeutic antibody. This phenomenon was noted in 39% of patients treated with single agent Elotuzumab resulting in more pronounced effects on serum Elotuzumab concentrations . Furthermore, in the ELOQUENT-2 trial, 15% of patients developed anti-Elotuzumab antibodies on at least one occasion , however, antibodies directed against Daratumumab have to this day not been detected. Other factors that may contribute to the clinical efficacy of mAb therapy include the frequency and activity of effector immune cells , Fcγ receptor polymorphisms  and even KIR and HLA genotypes .
4.1.2. Clinical studies to circumvent resistance
Whilst the mechanisms of resistance to mAbs are being elucidated, clinical studies specifically designed to overcome these biological processes are largely lacking with the exception of an ongoing phase I/II trial of Daratumumab in combination with all-
5. Other factors potentially influencing resistance to myeloma therapies
5.1. Cytogenetics, mutation patterns and clonal evolution
Cytogenetic abnormalities in MM are broadly divided into copy number changes or translocations, most commonly involving the immunoglobulin heavy chain gene . Various cytogenetic abnormalities were shown to be associated with the likelihood of durable responses to therapy but they do not directly explain mechanisms of drug resistance or disease progression . High risk genetic features frequently result in the dysregulation of transcription factors or tumour suppressors and include t(4;14), t(14;16), t(16;20), del(17p) and copy number changes of chromosome 1, which are used for stratifying MM patients in clinical trials and are now becoming important in guiding therapy in routine practice . For example, the EMN02/HO95 study demonstrated the benefit of double autologous stem cell transplantation in patient with high-risk genetics, essentially negating the adverse prognosis of high genetic risk MM . Similarly, the addition of Bortezomib to induction regimens in patients receiving HDM/ASCT may partially overcome cytogenetically defined poor risk . On the other hand, patients with trisomies may respond particularly well to lenalidomide based protocols . Mutational events such as those involving p53 are associated with particularly poor PFS, however, the significant heterogeneity of point mutational events elucidated in whole exome sequencing studies means generalisations of such molecular changes are not possible .
The development of whole exome sequencing and copy number profiling was combined with cytogenetics in a landmark paper by a consortium of European and American groups . This elegant paper demonstrated that the majority of MM patients had multiple sub-clones present at the time of diagnosis and that within sub-clones there could be differing mutational events potentially driving behaviour . When serial MM samples were analysed, diverse patterns of clonal evolution were detected. In some cases, simple clonal selection could be observed following a linear pattern of clonal evolution . Differential clonal responses could explain the clinical observation that a MM patient may respond to a treatment initially, lose this response, respond to another treatment and at the time of subsequent relapse respond again to the initial therapy . Branching evolution was also observed in some progressing patients . During disease evolution differing processes may contribute to the mutational repertoire and the relative contributions may vary over time in the same patient resulting in mutational heterogeneity, frequently with very few recurrent genes .
5.2. The myeloma stem cell
Identification of the multiple myeloma stem cell (MMSC) has been a challenge predominantly because an agreed phenotype with MM propagating potential has not been definitively established, in part due to differences in experimental techniques and assays. The dominant viewpoint is that clonotypic CD138− cells represent MMSCs, however, some researchers have also shown that clonotypic CD138+ plasma cells have properties of cancer stem cells such as self-renewal, tumour-initiating potential and drug resistance [133, 134]. Controversy also exists as to whether the MMSC derives from a clonotypic B cell (CD19+CD138−) or clonotypic non-B cell (CD19−CD138+/−). Clonotypic B cells were found to be resistant to a range of anti-MM therapies including Bortezomib and Lenalidomide and possessed a high drug efflux capacity . However, clonotypic non-B cells have also been shown in many studies to result in robust MM reconstitution in the absence of a CD19+ population . To shed some light on this dichotomy with respect to clonotypic non-B cells, there appears to be an interconversion between undifferentiated pre-plasma cells (CD19−CD138−) and differentiated plasma cells (CD19−CD138+) thus representing reversible, bi-directional phenotypic and functional states that share MMSC activity . Furthermore, the pre-plasma cells were found to be more quiescent, primarily located at extramedullary sites, and up to 300-fold more drug resistant to agents including Bortezomib . These informative findings imply phenotypic and functional plasticity between undifferentiated and differentiated clonotypic plasma cells which could explain why differentiated MM plasma cells possess clonogenic capacity and also reconciles inconsistencies surrounding the MMSC phenotype.
Several factors have been attributed to the MMSC that confer drug resistance. (1) Side population (SP) MM cells, which possess stem-like properties, show stronger activity of several ABC transporters when compared to main population (MP) cells . (2) High levels of aldehyde dehydrogenase (ALDH) have been demonstrated in CD138− plasma cells compared to their CD138+ counterparts rendering the CD138− population more resistant to certain chemotherapeutic agents which result in the generation of toxic aldehyde intermediates that are metabolised by ALDH1 . In one study, forced expression of member A1 of the ALDH1 family of proteins resulted in resistance to Bortezomib . (3) Increased expression of Bcl-2 family members in MMSCs expressing the retinoid acid receptor alpha 2 (RARα2) endowed these cells with increased drug resistance , and more recently, increased expression of Bruton’s tyrosine kinase (BTK) in MMSCs also induced drug resistance . (4) CD19−CD138+ plasma cells and CD19−CD138− pre-plasma cells harbour MMSC activity but exhibit differential resistance to treatment since pre-plasma cells are more quiescent than plasma cells, shown by a lower proportion of these cells in S phase of the cell cycle . Finally, (5) the Wingless (Wnt), Hedgehog and Notch signalling pathways are all highly active in MMSCs and may be responsible for maintaining stem cell properties, propagating MM and promoting therapeutic resistance together with a supportive and protective BMME .
Continued improvements in the efficacy and toxicity profiles of an ever-expanding number of novel MM therapies are challenging the current paradigm of high-dose therapy and autologous stem cell transplantation for newly-diagnosed MM. However, despite these advances, resistance to novel agents has been observed and will continue to be observed, requiring innovative ways to circumvent this problem. Changing therapy from one novel agent containing treatment regimen to a different one upon MM progression or relapse is reasonable, however, there is often little scientific basis for choosing the sequence of such regimens and the era of precision medicine for MM patients remains distant. Moreover, the inability to tailor treatment regimens for an individual patient based on the biology of their MM due to government mandated prescribing restrictions likely contributes to inadequate responses and drug resistance.
In addition to those discussed, there are other potential mechanisms through which resistance to novel therapies in MM may occur, such as the role miRNAs play in promoting MM, and this list is likely to increase. However, despite the varied resistance mechanisms reported to date, the survival of patients with MM continues to improve. Whilst genetic profiling has established a so-called high-risk group of MM patients, these genetic changes do not specifically explain why resistance to a particular novel agent develops. Thus, in this Chapter, an exposition of specific biological aberrations that have been linked to drug resistance has been presented.
We would like to thank the staff of the Haematology Department at Flinders Medical Centre, South Australia, for their ongoing support and encouragement to produce this Chapter.
Conflict of interest
The authors have no conflicts of interest to declare.
Mikkilineni L, Kochenderfer JN. Chimeric antigen receptor t-cell therapies for multiple myeloma. Blood. 2017; 130:2594-2602. DOI: 10.1182/blood-2017-06-793869
Vincenz L, Jager R, O'Dwyer M, Samali A. Endoplasmic reticulum stress and the unfolded protein response: Targeting the achilles heel of multiple myeloma. Molecular Cancer Therapeutics. 2013; 12:831-843. DOI: 10.1158/1535-7163.MCT-12-0782
Accardi F, Toscani D, Bolzoni M, Dalla Palma B, Aversa F, Giuliani N. Mechanism of action of bortezomib and the new proteasome inhibitors on myeloma cells and the bone microenvironment: Impact on myeloma-induced alterations of bone remodeling. BioMed Research International. 2015; 2015:172458. DOI: 10.1155/2015/172458
Streich FC Jr, Lima CD. Structural and functional insights to ubiquitin-like protein conjugation. Annual Review of Biophysics. 2014; 43:357-379. DOI: 10.1146/annurev-biophys-051013-022958
Walden H, Podgorski MS, Schulman BA. Insights into the ubiquitin transfer cascade from the structure of the activating enzyme for nedd8. Nature. 2003; 422:330-334. DOI: 10.1038/nature01456
Ciechanover A. Proteolysis: From the lysosome to ubiquitin and the proteasome. Nature Reviews. Molecular Cell Biology. 2005; 6:79-87. DOI: 10.1038/nrm1552
Coux O, Tanaka K, Goldberg AL. Structure and functions of the 20s and 26s proteasomes. Annual Review of Biochemistry. 1996; 65:801-847. DOI: 10.1146/annurev.bi.65.070196.004101
Baumeister W, Walz J, Zuhl F, Seemuller E. The proteasome: Paradigm of a self-compartmentalizing protease. Cell. 1998; 92:367-380
Glickman MH, Rubin DM, Coux O, Wefes I, Pfeifer G, Cjeka Z, et al. A subcomplex of the proteasome regulatory particle required for ubiquitin-conjugate degradation and related to the cop9-signalosome and eif3. Cell. 1998; 94:615-623
Heinemeyer W, Fischer M, Krimmer T, Stachon U, Wolf DH. The active sites of the eukaryotic 20 s proteasome and their involvement in subunit precursor processing. The Journal of Biological Chemistry. 1997; 272:25200-25209
Chen P, Hochstrasser M. Autocatalytic subunit processing couples active site formation in the 20s proteasome to completion of assembly. Cell. 1996; 86:961-972
Ditzel L, Huber R, Mann K, Heinemeyer W, Wolf DH, Groll M. Conformational constraints for protein self-cleavage in the proteasome. Journal of Molecular Biology. 1998; 279:1187-1191. DOI: 10.1006/jmbi.1998.1818
Nussbaum AK, Dick TP, Keilholz W, Schirle M, Stevanovic S, Dietz K, et al. Cleavage motifs of the yeast 20s proteasome beta subunits deduced from digests of enolase 1. Proceedings of the National Academy of Sciences of the United States of America. 1998; 95:12504-12509
Arendt CS, Hochstrasser M. Identification of the yeast 20s proteasome catalytic centers and subunit interactions required for active-site formation. Proceedings of the National Academy of Sciences of the United States of America. 1997; 94:7156-7161
Dick TP, Nussbaum AK, Deeg M, Heinemeyer W, Groll M, Schirle M, et al. Contribution of proteasomal beta-subunits to the cleavage of peptide substrates analyzed with yeast mutants. The Journal of Biological Chemistry. 1998; 273:25637-25646
Akopian TN, Kisselev AF, Goldberg AL. Processive degradation of proteins and other catalytic properties of the proteasome from Thermoplasma acidophilum. The Journal of Biological Chemistry. 1997; 272:1791-1798
Kisselev AF, Akopian TN, Goldberg AL. Range of sizes of peptide products generated during degradation of different proteins by archaeal proteasomes. The Journal of Biological Chemistry. 1998; 273:1982-1989
Groll M, Heinemeyer W, Jager S, Ullrich T, Bochtler M, Wolf DH, et al. The catalytic sites of 20s proteasomes and their role in subunit maturation: A mutational and crystallographic study. Proceedings of the National Academy of Sciences of the United States of America. 1999; 96:10976-10983
Glynne R, Powis SH, Beck S, Kelly A, Kerr LA, Trowsdale J. A proteasome-related gene between the two abc transporter loci in the class ii region of the human mhc. Nature. 1991; 353:357-360. DOI: 10.1038/353357a0
Martinez CK, Monaco JJ. Homology of proteasome subunits to a major histocompatibility complex-linked lmp gene. Nature. 1991; 353:664-667. DOI: 10.1038/353664a0
Nandi D, Jiang H, Monaco JJ. Identification of mecl-1 (lmp-10) as the third IFN-gamma-inducible proteasome subunit. Journal of Immunology. 1996; 156:2361-2364
Altun M, Galardy PJ, Shringarpure R, Hideshima T, LeBlanc R, Anderson KC, et al. Effects of ps-341 on the activity and composition of proteasomes in multiple myeloma cells. Cancer Research. 2005; 65:7896-7901. DOI: 10.1158/0008-5472.CAN-05-0506
Busse A, Kraus M, Na IK, Rietz A, Scheibenbogen C, Driessen C, et al. Sensitivity of tumor cells to proteasome inhibitors is associated with expression levels and composition of proteasome subunits. Cancer. 2008; 112:659-670. DOI: 10.1002/cncr.23224
Kuhn DJ, Hunsucker SA, Chen Q, Voorhees PM, Orlowski M, Orlowski RZ. Targeted inhibition of the immunoproteasome is a potent strategy against models of multiple myeloma that overcomes resistance to conventional drugs and nonspecific proteasome inhibitors. Blood. 2009; 113:4667-4676. DOI: 10.1182/blood-2008-07-171637
Parlati F, Lee SJ, Aujay M, Suzuki E, Levitsky K, Lorens JB, et al. Carfilzomib can induce tumor cell death through selective inhibition of the chymotrypsin-like activity of the proteasome. Blood. 2009; 114:3439-3447. DOI: 10.1182/blood-2009-05-223677
Kubiczkova L, Pour L, Sedlarikova L, Hajek R, Sevcikova S. Proteasome inhibitors—Molecular basis and current perspectives in multiple myeloma. Journal of Cellular and Molecular Medicine. 2014; 18:947-961. DOI: 10.1111/jcmm.12279
Hideshima T, Ikeda H, Chauhan D, Okawa Y, Raje N, Podar K, et al. Bortezomib induces canonical nuclear factor-kappab activation in multiple myeloma cells. Blood. 2009; 114:1046-1052. DOI: 10.1182/blood-2009-01-199604
Kuhn DJ, Chen Q, Voorhees PM, Strader JS, Shenk KD, Sun CM, et al. Potent activity of carfilzomib, a novel, irreversible inhibitor of the ubiquitin-proteasome pathway, against preclinical models of multiple myeloma. Blood. 2007; 110:3281-3290. DOI: 10.1182/blood-2007-01-065888
Stewart AK, Rajkumar SV, Dimopoulos MA, Masszi T, Spicka I, Oriol A, et al. Carfilzomib, lenalidomide, and dexamethasone for relapsed multiple myeloma. The New England Journal of Medicine. 2015; 372:142-152. DOI: 10.1056/NEJMoa1411321
Moreau P, Masszi T, Grzasko N, Bahlis NJ, Hansson M, Pour L, et al. Oral ixazomib, lenalidomide, and dexamethasone for multiple myeloma. The New England Journal of Medicine. 2016; 374:1621-1634. DOI: 10.1056/NEJMoa1516282
Bonnet A, Moreau P. Safety of ixazomib for the treatment of multiple myeloma. Expert Opinion on Drug Safety. 2017; 16:973-980. DOI: 10.1080/14740338.2017.1344212
Chauhan D, Catley L, Li G, Podar K, Hideshima T, Velankar M, et al. A novel orally active proteasome inhibitor induces apoptosis in multiple myeloma cells with mechanisms distinct from bortezomib. Cancer Cell. 2005; 8:407-419. DOI: 10.1016/j.ccr.2005.10.013
Richardson PG, Zimmerman TM, Hofmeister CC, Talpaz M, Chanan-Khan AA, Kaufman JL, et al. Phase 1 study of marizomib in relapsed or relapsed and refractory multiple myeloma: Npi-0052-101 part 1. Blood. 2016; 127:2693-2700. DOI: 10.1182/blood-2015-12-686378
Spencer A, Harrison S, Zonder J, Badros A, Laubach J, Bergin K, et al. A phase 1 clinical trial evaluating marizomib, pomalidomide and low-dose dexamethasone in relapsed and refractory multiple myeloma (npi-0052-107): Final study results. British Journal of Haematology. 2018; 180:41-51. DOI: 10.1111/bjh.14987
Badros A, Singh Z, Dhakal B, Kwok Y, MacLaren A, Richardson P, et al. Marizomib for central nervous system-multiple myeloma. British Journal of Haematology. 2017; 177:221-225. DOI: 10.1111/bjh.14498
Chauhan D, Singh AV, Aujay M, Kirk CJ, Bandi M, Ciccarelli B, et al. A novel orally active proteasome inhibitor onx 0912 triggers in vitro and in vivo cytotoxicity in multiple myeloma. Blood. 2010; 116:4906-4915. DOI: 10.1182/blood-2010-04-276626
Oerlemans R, Franke NE, Assaraf YG, Cloos J, van Zantwijk I, Berkers CR, et al. Molecular basis of bortezomib resistance: Proteasome subunit beta5 (psmb5) gene mutation and overexpression of psmb5 protein. Blood. 2008; 112:2489-2499. DOI: 10.1182/blood-2007-08-104950
Lu S, Chen Z, Yang J, Chen L, Gong S, Zhou H, et al. Overexpression of the psmb5 gene contributes to bortezomib resistance in t-lymphoblastic lymphoma/leukemia cells derived from jurkat line. Experimental Hematology. 2008; 36:1278-1284. DOI: 10.1016/j.exphem.2008.04.013
Lu S, Yang J, Song X, Gong S, Zhou H, Guo L, et al. Point mutation of the proteasome beta5 subunit gene is an important mechanism of bortezomib resistance in bortezomib-selected variants of jurkat t cell lymphoblastic lymphoma/leukemia line. The Journal of Pharmacology and Experimental Therapeutics. 2008; 326:423-431. DOI: 10.1124/jpet.108.138131
Balsas P, Galan-Malo P, Marzo I, Naval J. Bortezomib resistance in a myeloma cell line is associated to psmbeta5 overexpression and polyploidy. Leukemia Research. 2012; 36:212-218. DOI: 10.1016/j.leukres.2011.09.011
Ri M, Iida S, Nakashima T, Miyazaki H, Mori F, Ito A, et al. Bortezomib-resistant myeloma cell lines: A role for mutated psmb5 in preventing the accumulation of unfolded proteins and fatal ER stress. Leukemia. 2010; 24:1506-1512. DOI: 10.1038/leu.2010.137
Chapman MA, Lawrence MS, Keats JJ, Cibulskis K, Sougnez C, Schinzel AC, et al. Initial genome sequencing and analysis of multiple myeloma. Nature. 2011; 471:467-472. DOI: 10.1038/nature09837
Lichter DI, Danaee H, Pickard MD, Tayber O, Sintchak M, Shi HL, et al. Sequence analysis of beta-subunit genes of the 20s proteasome in patients with relapsed multiple myeloma treated with bortezomib or dexamethasone. Blood. 2012; 120:4513-4516. DOI: 10.1182/blood-2012-05-426924
Barrio S, Stühmer T, Teufel E, Barrio-Garcia C, Chatterjee M, Schreder M, et al. Parallel evolution of multiple psmb5mutations in a myeloma patient treated with bortezomib. Blood. 2016; 128:3282-3282
Fuchs D, Berges C, Opelz G, Daniel V, Naujokat C. Increased expression and altered subunit composition of proteasomes induced by continuous proteasome inhibition establish apoptosis resistance and hyperproliferation of Burkitt lymphoma cells. Journal of Cellular Biochemistry. 2008; 103:270-283. DOI: 10.1002/jcb.21405
Garcia-Mata R, Bebok Z, Sorscher EJ, Sztul ES. Characterization and dynamics of aggresome formation by a cytosolic GFP-chimera. The Journal of Cell Biology. 1999; 146:1239-1254
Zhang XD, Baladandayuthapani V, Lin H, Mulligan G, Li B, Esseltine DW, et al. Tight junction protein 1 modulates proteasome capacity and proteasome inhibitor sensitivity in multiple myeloma via egfr/jak1/stat3 signaling. Cancer Cell. 2016; 29:639-652. DOI: 10.1016/j.ccell.2016.03.026
Kawaguchi Y, Kovacs JJ, McLaurin A, Vance JM, Ito A, Yao TP. The deacetylase hdac6 regulates aggresome formation and cell viability in response to misfolded protein stress. Cell. 2003; 115:727-738
McConkey DJ, White M, Yan W. Hdac inhibitor modulation of proteotoxicity as a therapeutic approach in cancer. Advances in Cancer Research. 2012; 116:131-163. DOI: 10.1016/B978-0-12-394387-3.00004-5
Hideshima T, Bradner JE, Wong J, Chauhan D, Richardson P, Schreiber SL, et al. Small-molecule inhibition of proteasome and aggresome function induces synergistic antitumor activity in multiple myeloma. Proceedings of the National Academy of Sciences of the United States of America. 2005; 102:8567-8572. DOI: 10.1073/pnas.0503221102
Catley L, Weisberg E, Kiziltepe T, Tai YT, Hideshima T, Neri P, 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; 108:3441-3449. DOI: 10.1182/blood-2006-04-016055
San-Miguel JF, Hungria VT, Yoon SS, Beksac M, Dimopoulos MA, Elghandour A, et al. Panobinostat plus bortezomib and dexamethasone versus placebo plus bortezomib and dexamethasone in patients with relapsed or relapsed and refractory multiple myeloma: A multicentre, randomised, double-blind phase 3 trial. The Lancet Oncology. 2014; 15:1195-1206. DOI: 10.1016/S1470-2045(14)70440-1
Montanari F, Lu M, Marcus S, Saran A, Malankar A, Mazumder A. A phase ii trial of chloroquine in combination with bortezomib and cyclophosphamide in patients with relapsed and refractory multiple myeloma. Blood. 2014; 124:5775
Zhang L, Fok JH, Davies FE. Heat shock proteins in multiple myeloma. Oncotarget. 2014; 5:1132-1148. DOI: 10.18632/oncotarget.1584
Mitsiades N, Mitsiades CS, Poulaki V, Chauhan D, Fanourakis G, Gu X, et al. Molecular sequelae of proteasome inhibition in human multiple myeloma cells. Proceedings of the National Academy of Sciences of the United States of America. 2002; 99:14374-14379. DOI: 10.1073/pnas.202445099
Adomako A, Calvo V, Biran N, Osman K, Chari A, Paton JC, et al. Identification of markers that functionally define a quiescent multiple myeloma cell sub-population surviving bortezomib treatment. BMC Cancer. 2015; 15:444. DOI: 10.1186/s12885-015-1460-1
Steiner N, Borjan B, Hajek R, Johrer K, Gobel G, Willenbacher W, et al. Expression and release of glucose-regulated protein-78 (grp78) in multiple myeloma. Oncotarget. 2017; 8:56243-56254. DOI: 10.18632/oncotarget.17353
Abdel Malek MA, Jagannathan S, Malek E, Sayed DM, Elgammal SA, Abd El-Azeem HG, et al. Molecular chaperone grp78 enhances aggresome delivery to autophagosomes to promote drug resistance in multiple myeloma. Oncotarget. 2015; 6:3098-3110. DOI: 10.18632/oncotarget.3075
Mitsiades CS, Mitsiades NS, McMullan CJ, Poulaki V, Kung AL, Davies FE, et al. Antimyeloma activity of heat shock protein-90 inhibition. Blood. 2006; 107:1092-1100. DOI: 10.1182/blood-2005-03-1158
Roue G, Perez-Galan P, Mozos A, Lopez-Guerra M, Xargay-Torrent S, Rosich L, et al. The hsp90 inhibitor ipi-504 overcomes bortezomib resistance in mantle cell lymphoma in vitro and in vivo by down-regulation of the prosurvival ER chaperone bip/grp78. Blood. 2011; 117:1270-1279. DOI: 10.1182/blood-2010-04-278853
Chauhan D, Li G, Shringarpure R, Podar K, Ohtake Y, Hideshima T, et al. Blockade of hsp27 overcomes bortezomib/proteasome inhibitor ps-341 resistance in lymphoma cells. Cancer Research. 2003; 63:6174-6177
Hamouda MA, Belhacene N, Puissant A, Colosetti P, Robert G, Jacquel A, et al. The small heat shock protein b8 (hspb8) confers resistance to bortezomib by promoting autophagic removal of misfolded proteins in multiple myeloma cells. Oncotarget. 2014; 5:6252-6266. DOI: 10.18632/oncotarget.2193
Rasche L, Menoret E, Dubljevic V, Menu E, Vanderkerken K, Lapa C, et al. A grp78-directed monoclonal antibody recaptures response in refractory multiple myeloma with extramedullary involvement. Clinical Cancer Research. 2016; 22:4341-4349. DOI: 10.1158/1078-0432.CCR-15-3111
Cavenagh J, Oakervee H, Baetiong-Caguioa P, Davies F, Gharibo M, Rabin N, et al. A phase i/ii study of kw-2478, an hsp90 inhibitor, in combination with bortezomib in patients with relapsed/refractory multiple myeloma. British Journal of Cancer. 2017; 117:1295-1302. DOI: 10.1038/bjc.2017.302
Nigam SK. What do drug transporters really do? Nature Reviews. Drug Discovery. 2015; 14:29-44. DOI: 10.1038/nrd4461
Cornelissen JJ, Sonneveld P, Schoester M, Raaijmakers HG, Nieuwenhuis HK, Dekker AW, et al. Mdr-1 expression and response to vincristine, doxorubicin, and dexamethasone chemotherapy in multiple myeloma refractory to alkylating agents. Journal of Clinical Oncology. 1994; 12:115-119. DOI: 10.1200/JCO.19220.127.116.11
Grogan TM, Spier CM, Salmon SE, Matzner M, Rybski J, Weinstein RS, et al. P-glycoprotein expression in human plasma cell myeloma: Correlation with prior chemotherapy. Blood. 1993; 81:490-495
Lu S, Wang J. The resistance mechanisms of proteasome inhibitor bortezomib. Biomarker Research. 2013; 1:13. DOI: 10.1186/2050-7771-1-13
Dalton WS. Detection of multidrug resistance gene expression in multiple myeloma. Leukemia. 1997; 11:1166-1169
Verbrugge SE, Assaraf YG, Dijkmans BA, Scheffer GL, Al M, den Uyl D, et al. Inactivating psmb5 mutations and p-glycoprotein (multidrug resistance-associated protein/atp-binding cassette b1) mediate resistance to proteasome inhibitors: Ex vivo efficacy of (immuno)proteasome inhibitors in mononuclear blood cells from patients with rheumatoid arthritis. The Journal of Pharmacology and Experimental Therapeutics. 2012; 341:174-182. DOI: 10.1124/jpet.111.187542
Hawley TS, Riz I, Yang W, Wakabayashi Y, Depalma L, Chang YT, et al. Identification of an abcb1 (p-glycoprotein)-positive carfilzomib-resistant myeloma subpopulation by the pluripotent stem cell fluorescent dye cdy1. American Journal of Hematology. 2013; 88:265-272. DOI: 10.1002/ajh.23387
Besse A, Stolze SC, Rasche L, Weinhold N, Morgan GJ, Kraus M, et al. Carfilzomib resistance due to abcb1/mdr1 overexpression is overcome by nelfinavir and lopinavir in multiple myeloma. Leukemia. 2017; 32:391-401. DOI: 10.1038/leu.2017.212
Weniger MA, Rizzatti EG, Perez-Galan P, Liu D, Wang Q, Munson PJ, et al. Treatment-induced oxidative stress and cellular antioxidant capacity determine response to bortezomib in mantle cell lymphoma. Clinical Cancer Research. 2011; 17:5101-5112. DOI: 10.1158/1078-0432.CCR-10-3367
Stessman HA, Baughn LB, Sarver A, Xia T, Deshpande R, Mansoor A, et al. Profiling bortezomib resistance identifies secondary therapies in a mouse myeloma model. Molecular Cancer Therapeutics. 2013; 12:1140-1150. DOI: 10.1158/1535-7163.MCT-12-1151
Li B, Fu J, Chen P, Ge X, Li Y, Kuiatse I, et al. The nuclear factor (erythroid-derived 2)-like 2 and proteasome maturation protein axis mediate bortezomib resistance in multiple myeloma. The Journal of Biological Chemistry. 2015; 290:29854-29868. DOI: 10.1074/jbc.M115.664953
Leung-Hagesteijn C, Erdmann N, Cheung G, Keats JJ, Stewart AK, Reece DE, et al. Xbp1s-negative tumor b cells and pre-plasmablasts mediate therapeutic proteasome inhibitor resistance in multiple myeloma. Cancer Cell. 2013; 24:289-304. DOI: 10.1016/j.ccr.2013.08.009
Perez-Galan P, Mora-Jensen H, Weniger MA, Shaffer AL 3rd, Rizzatti EG, Chapman CM, et al. Bortezomib resistance in mantle cell lymphoma is associated with plasmacytic differentiation. Blood. 2011; 117:542-552. DOI: 10.1182/blood-2010-02-269514
Manier S, Sacco A, Leleu X, Ghobrial IM, Roccaro AM. Bone marrow microenvironment in multiple myeloma progression. Journal of Biomedicine & Biotechnology. 2012; 2012:157496. DOI: 10.1155/2012/157496
Vacca A, Ria R, Ribatti D, Semeraro F, Djonov V, Di Raimondo F, et al. A paracrine loop in the vascular endothelial growth factor pathway triggers tumor angiogenesis and growth in multiple myeloma. Haematologica. 2003; 88:176-185
Hao M, Zhang L, An G, Sui W, Yu Z, Zou D, et al. Suppressing mirna-15a/-16 expression by interleukin-6 enhances drug-resistance in myeloma cells. Journal of Hematology & Oncology. 2011; 4:37. DOI: 10.1186/1756-8722-4-37
Zhang YW, Su Y, Volpert OV, Vande Woude GF. Hepatocyte growth factor/scatter factor mediates angiogenesis through positive vegf and negative thrombospondin 1 regulation. Proceedings of the National Academy of Sciences of the United States of America. 2003; 100:12718-12723. DOI: 10.1073/pnas.2135113100
Moschetta M, Basile A, Ferrucci A, Frassanito MA, Rao L, Ria R, et al. Novel targeting of phospho-cmet overcomes drug resistance and induces antitumor activity in multiple myeloma. Clinical Cancer Research. 2013; 19:4371-4382. DOI: 10.1158/1078-0432.CCR-13-0039
Ferrucci A, Moschetta M, Frassanito MA, Berardi S, Catacchio I, Ria R, et al. A hgf/cmet autocrine loop is operative in multiple myeloma bone marrow endothelial cells and may represent a novel therapeutic target. Clinical Cancer Research. 2014; 20:5796-5807. DOI: 10.1158/1078-0432.CCR-14-0847
Markovina S, Callander NS, O'Connor SL, Kim J, Werndli JE, Raschko M, et al. Bortezomib-resistant nuclear factor-kappab activity in multiple myeloma cells. Molecular Cancer Research. 2008; 6:1356-1364. DOI: 10.1158/1541-7786.MCR-08-0108
Podar K, Chauhan D, Anderson KC. Bone marrow microenvironment and the identification of new targets for myeloma therapy. Leukemia. 2009; 23:10-24. DOI: 10.1038/leu.2008.259
Kuhn DJ, Berkova Z, Jones RJ, Woessner R, Bjorklund CC, Ma W, et al. Targeting the insulin-like growth factor-1 receptor to overcome bortezomib resistance in preclinical models of multiple myeloma. Blood. 2012; 120:3260-3270. DOI: 10.1182/blood-2011-10-386789
Spencer A, Yoon SS, Harrison SJ, Morris SR, Smith DA, Brigandi RA, et al. The novel akt inhibitor afuresertib shows favorable safety, pharmacokinetics, and clinical activity in multiple myeloma. Blood. 2014; 124:2190-2195. DOI: 10.1182/blood-2014-03-559963
Di Marzo L, Desantis V, Solimando AG, Ruggieri S, Annese T, Nico B, et al. Microenvironment drug resistance in multiple myeloma: Emerging new players. Oncotarget. 2016; 7:60698-60711. DOI: 10.18632/oncotarget.10849
Neri P, Ren L, Azab AK, Brentnall M, Gratton K, Klimowicz AC, et al. Integrin beta7-mediated regulation of multiple myeloma cell adhesion, migration, and invasion. Blood. 2011; 117:6202-6213. DOI: 10.1182/blood-2010-06-292243
Alsayed Y, Ngo H, Runnels J, Leleu X, Singha UK, Pitsillides CM, et al. Mechanisms of regulation of cxcr4/sdf-1 (cxcl12)-dependent migration and homing in multiple myeloma. Blood. 2007; 109:2708-2717. DOI: 10.1182/blood-2006-07-035857
Abdi J, Jian H, Chang H. Role of micro-RNAs in drug resistance of multiple myeloma. Oncotarget. 2016; 7:60723-60735. DOI: 10.18632/oncotarget.11032
Wang J, Hendrix A, Hernot S, Lemaire M, De Bruyne E, Van Valckenborgh E, et al. Bone marrow stromal cell-derived exosomes as communicators in drug resistance in multiple myeloma cells. Blood. 2014; 124:555-566. DOI: 10.1182/blood-2014-03-562439
Voorhees PM, Manges RF, Sonneveld P, Jagannath S, Somlo G, Krishnan A, et al. A phase 2 multicentre study of siltuximab, an anti-interleukin-6 monoclonal antibody, in patients with relapsed or refractory multiple myeloma. British Journal of Haematology. 2013; 161:357-366. DOI: 10.1111/bjh.12266
Baljevic M, Zaman S, Baladandayuthapani V, Lin YH, de Partovi CM, Berkova Z, et al. Phase ii study of the c-met inhibitor tivantinib (arq 197) in patients with relapsed or relapsed/refractory multiple myeloma. Annals of Hematology. 2017; 96:977-985. DOI: 10.1007/s00277-017-2980-3
Moreau P, Cavallo F, Leleu X, Hulin C, Amiot M, Descamps G, et al. Phase i study of the anti-insulin-like growth factor 1 receptor (igf-1r) monoclonal antibody, ave1642, as single agent and in combination with bortezomib in patients with relapsed multiple myeloma. Leukemia. 2011; 25:872-874. DOI: 10.1038/leu.2011.4
Holstein SA, McCarthy PL. Immunomodulatory drugs in multiple myeloma: Mechanisms of action and clinical experience. Drugs. 2017; 77:505-520. DOI: 10.1007/s40265-017-0689-1
Weber DM, Chen C, Niesvizky R, Wang M, Belch A, Stadtmauer EA, et al. Lenalidomide plus dexamethasone for relapsed multiple myeloma in north America. The New England Journal of Medicine. 2007; 357:2133-2142. DOI: 10.1056/NEJMoa070596
Dimopoulos M, Spencer A, Attal M, Prince HM, Harousseau JL, Dmoszynska A, et al. Lenalidomide plus dexamethasone for relapsed or refractory multiple myeloma. The New England Journal of Medicine. 2007; 357:2123-2132. DOI: 10.1056/NEJMoa070594
San Miguel J, Weisel K, Moreau P, Lacy M, Song K, Delforge M, et al. Pomalidomide plus low-dose dexamethasone versus high-dose dexamethasone alone for patients with relapsed and refractory multiple myeloma (mm-003): A randomised, open-label, phase 3 trial. The Lancet Oncology. 2013; 14:1055-1066. DOI: 10.1016/S1470-2045(13)70380-2
Ito T, Ando H, Suzuki T, Ogura T, Hotta K, Imamura Y, et al. Identification of a primary target of thalidomide teratogenicity. Science. 2010; 327:1345-1350. DOI: 10.1126/science.1177319
Zhu YX, Braggio E, Shi CX, Bruins LA, Schmidt JE, Van Wier S, et al. Cereblon expression is required for the antimyeloma activity of lenalidomide and pomalidomide. Blood. 2011; 118:4771-4779. DOI: 10.1182/blood-2011-05-356063
Bedewy AM, El-Maghraby SM. Do baseline cereblon gene expression and il-6 receptor expression determine the response to thalidomide-dexamethasone treatment in multiple myeloma patients? European Journal of Haematology. 2014; 92:13-18. DOI: 10.1111/ejh.12207
Heintel D, Rocci A, Ludwig H, Bolomsky A, Caltagirone S, Schreder M, et al. High expression of cereblon (crbn) is associated with improved clinical response in patients with multiple myeloma treated with lenalidomide and dexamethasone. British Journal of Haematology. 2013; 161:695-700. DOI: 10.1111/bjh.12338
Schuster SR, Kortuem KM, Zhu YX, Braggio E, Shi CX, Bruins LA, et al. The clinical significance of cereblon expression in multiple myeloma. Leukemia Research. 2014; 38:23-28. DOI: 10.1016/j.leukres.2013.08.015
Lopez-Girona A, Mendy D, Ito T, Miller K, Gandhi AK, Kang J, et al. Cereblon is a direct protein target for immunomodulatory and antiproliferative activities of lenalidomide and pomalidomide. Leukemia. 2012; 26:2326-2335. DOI: 10.1038/leu.2012.119
Zhu YX, Braggio E, Shi CX, Kortuem KM, Bruins LA, Schmidt JE, et al. Identification of cereblon-binding proteins and relationship with response and survival after imids in multiple myeloma. Blood. 2014; 124:536-545. DOI: 10.1182/blood-2014-02-557819
Kortum KM, Mai EK, Hanafiah NH, Shi CX, Zhu YX, Bruins L, et al. Targeted sequencing of refractory myeloma reveals a high incidence of mutations in crbn and ras pathway genes. Blood. 2016; 128:1226-1233. DOI: 10.1182/blood-2016-02-698092
Bjorklund CC, Baladandayuthapani V, Lin HY, Jones RJ, Kuiatse I, Wang H, et al. Evidence of a role for cd44 and cell adhesion in mediating resistance to lenalidomide in multiple myeloma: Therapeutic implications. Leukemia. 2014; 28:373-383. DOI: 10.1038/leu.2013.174
Xu J, Pfarr N, Endris V, Mai EK, Md Hanafiah NH, Lehners N, et al. Molecular signaling in multiple myeloma: Association of ras/raf mutations and mek/erk pathway activation. Oncogene. 2017; 6:e337. DOI: 10.1038/oncsis.2017.36
Mey UJM, Renner C, von Moos R. Vemurafenib in combination with cobimetinib in relapsed and refractory extramedullary multiple myeloma harboring the braf v600e mutation. Hematological Oncology. 2017; 35:890-893. DOI: 10.1002/hon.2353
van de Donk NW, Janmaat ML, Mutis T, Lammerts van Bueren JJ, Ahmadi T, Sasser AK, et al. Monoclonal antibodies targeting cd38 in hematological malignancies and beyond. Immunological Reviews. 2016; 270:95-112. DOI: 10.1111/imr.12389
Dimopoulos MA, Oriol A, Nahi H, San-Miguel J, Bahlis NJ, Usmani SZ, et al. Daratumumab, lenalidomide, and dexamethasone for multiple myeloma. The New England Journal of Medicine. 2016; 375:1319-1331. DOI: 10.1056/NEJMoa1607751
Palumbo A, Chanan-Khan A, Weisel K, Nooka AK, Masszi T, Beksac M, et al. Daratumumab, bortezomib, and dexamethasone for multiple myeloma. The New England Journal of Medicine. 2016; 375:754-766. DOI: 10.1056/NEJMoa1606038
Hsi ED, Steinle R, Balasa B, Szmania S, Draksharapu A, Shum BP, et al. Cs1, a potential new therapeutic antibody target for the treatment of multiple myeloma. Clinical Cancer Research. 2008; 14:2775-2784. DOI: 10.1158/1078-0432.CCR-07-4246
Lonial S, Dimopoulos M, Palumbo A, White D, Grosicki S, Spicka I, et al. Elotuzumab therapy for relapsed or refractory multiple myeloma. The New England Journal of Medicine. 2015; 373:621-631. DOI: 10.1056/NEJMoa1505654
Zhang T, Wang S, Lin T, Xie J, Zhao L, Liang Z, et al. Systematic review and meta-analysis of the efficacy and safety of novel monoclonal antibodies for treatment of relapsed/refractory multiple myeloma. Oncotarget. 2017; 8:34001-34017. DOI: 10.18632/oncotarget.16987
Nijhof IS, Casneuf T, van Velzen J, van Kessel B, Axel AE, Syed K, et al. Cd38 expression and complement inhibitors affect response and resistance to daratumumab therapy in myeloma. Blood. 2016; 128:959-970. DOI: 10.1182/blood-2016-03-703439
Krejcik J, Frerichs KA, Nijhof IS, van Kessel B, van Velzen JF, Bloem AC, et al. Monocytes and granulocytes reduce cd38 expression levels on myeloma cells in patients treated with daratumumab. Clinical Cancer Research. 2017; 23:7498-7511. DOI: 10.1158/1078-0432.CCR-17-2027
Funaro A, Horenstein AL, Calosso L, Morra M, Tarocco RP, Franco L, et al. Identification and characterization of an active soluble form of human cd38 in normal and pathological fluids. International Immunology. 1996; 8:1643-1650
Tai YT, Dillon M, Song W, Leiba M, Li XF, Burger P, et al. Anti-cs1 humanized monoclonal antibody huluc63 inhibits myeloma cell adhesion and induces antibody-dependent cellular cytotoxicity in the bone marrow milieu. Blood. 2008; 112:1329-1337. DOI: 10.1182/blood-2007-08-107292
Zonder JA, Mohrbacher AF, Singhal S, van Rhee F, Bensinger WI, Ding H, et al. A phase 1, multicenter, open-label, dose escalation study of elotuzumab in patients with advanced multiple myeloma. Blood. 2012; 120:552-559. DOI: 10.1182/blood-2011-06-360552
Nijhof IS, Groen RW, Lokhorst HM, van Kessel B, Bloem AC, van Velzen J, et al. Upregulation of cd38 expression on multiple myeloma cells by all-trans retinoic acid improves the efficacy of daratumumab. Leukemia. 2015; 29:2039-2049. DOI: 10.1038/leu.2015.123
van de Donk NW, Kamps S, Mutis T, Lokhorst HM. Monoclonal antibody-based therapy as a new treatment strategy in multiple myeloma. Leukemia. 2012; 26:199-213. DOI: 10.1038/leu.2011.214
Marra J, Du J, Hwang J, Wolf JL, Martin TG, Venstrom JM. Kir and hla genotypes influence clinical outcome in multiple myeloma patients treated with sar650984 (anti-cd38) in combination with lenalidomide and dexamethasone. Blood. 2014; 124:2126
Kuehl WM, Bergsagel PL. Multiple myeloma: Evolving genetic events and host interactions. Nature Reviews. Cancer. 2002; 2:175-187. DOI: 10.1038/nrc746
Rajan AM, Rajkumar SV. Interpretation of cytogenetic results in multiple myeloma for clinical practice. Blood Cancer Journal. 2015; 5:e365. DOI: 10.1038/bcj.2015.92
Cavo M, Petrucci MT, Di Raimondo F, Zamagni E, Gamberi B, Crippa C, et al. Upfront single versus double autologous stem cell transplantation for newly diagnosed multiple myeloma: An intergroup, multicenter, phase iii study of the European myeloma network (emn02/ho95 mm trial). Blood. 2016; 128:991-991
Sonneveld P, Schmidt-Wolf IG, van der Holt B, El Jarari L, Bertsch U, Salwender H, et al. Bortezomib induction and maintenance treatment in patients with newly diagnosed multiple myeloma: Results of the randomized phase iii hovon-65/ gmmg-hd4 trial. Journal of Clinical Oncology. 2012; 30:2946-2955. DOI: 10.1200/JCO.2011.39.6820
Rajkumar SV, Kapoor P, Ketterling RP, Lacy MQ, Gertz MA, Buadi F, et al. Impact of fish abnormalities on response to lenalidomide in patients with multiple myeloma. Blood. 2013; 122:3210-3210
Bolli N, Avet-Loiseau H, Wedge DC, Van Loo P, Alexandrov LB, Martincorena I, et al. Heterogeneity of genomic evolution and mutational profiles in multiple myeloma. Nature Communications. 2014; 5:2997. DOI: 10.1038/ncomms3997
Brioli A, Melchor L, Cavo M, Morgan GJ. The impact of intra-clonal heterogeneity on the treatment of multiple myeloma. British Journal of Haematology. 2014; 165:441-454. DOI: 10.1111/bjh.12805
Paino T, Paiva B, Sayagues JM, Mota I, Carvalheiro T, Corchete LA, et al. Phenotypic identification of subclones in multiple myeloma with different chemoresistant, cytogenetic and clonogenic potential. Leukemia. 2015; 29:1186-1194. DOI: 10.1038/leu.2014.321
Kim D, Park CY, Medeiros BC, Weissman IL. Cd19-cd45 low/− cd38 high/cd138+ plasma cells enrich for human tumorigenic myeloma cells. Leukemia. 2012; 26:2530-2537. DOI: 10.1038/leu.2012.140
Matsui W, Huff CA, Wang Q, Malehorn MT, Barber J, Tanhehco Y, et al. Characterization of clonogenic multiple myeloma cells. Blood. 2004; 103:2332-2336. DOI: 10.1182/blood-2003-09-3064
Matsui W, Wang Q, Barber JP, Brennan S, Smith BD, Borrello I, et al. Clonogenic multiple myeloma progenitors, stem cell properties, and drug resistance. Cancer Research. 2008; 68:190-197. DOI: 10.1158/0008-5472.CAN-07-3096
Hosen N, Matsuoka Y, Kishida S, Nakata J, Mizutani Y, Hasegawa K, et al. Cd138-negative clonogenic cells are plasma cells but not b cells in some multiple myeloma patients. Leukemia. 2012; 26:2135-2141. DOI: 10.1038/leu.2012.80
Chaidos A, Barnes CP, Cowan G, May PC, Melo V, Hatjiharissi E, et al. Clinical drug resistance linked to interconvertible phenotypic and functional states of tumor-propagating cells in multiple myeloma. Blood. 2013; 121:318-328. DOI: 10.1182/blood-2012-06-436220
Jakubikova J, Adamia S, Kost-Alimova M, Klippel S, Cervi D, Daley JF, et al. Lenalidomide targets clonogenic side population in multiple myeloma: Pathophysiologic and clinical implications. Blood. 2011; 117:4409-4419. DOI: 10.1182/blood-2010-02-267344
Yang Y, Zhou W, Xia J, Gu Z, Wendlandt E, Zhan X, et al. Nek2 mediates aldh1a1-dependent drug resistance in multiple myeloma. Oncotarget. 2014; 5:11986-11997. DOI: 10.18632/oncotarget.2388
Yang Y, Shi J, Tolomelli G, Xu H, Xia J, Wang H, et al. Raralpha2 expression confers myeloma stem cell features. Blood. 2013; 122:1437-1447. DOI: 10.1182/blood-2013-02-482919
Yang Y, Shi J, Gu Z, Salama ME, Das S, Wendlandt E, et al. Bruton tyrosine kinase is a therapeutic target in stem-like cells from multiple myeloma. Cancer Research. 2015; 75:594-604. DOI: 10.1158/0008-5472.CAN-14-2362
Nishikawa SI, Osawa M. Niche for normal and cancer stem cells. Ernst Schering Foundation Symposium Proceedings. 2006; 5:1-12
Wallington-Beddoe CT, Kuss BJ, Sobieraj-Teague M, Pitson SM. Resistance to proteasome inhibitors and other targeted therapies in myeloma. British Journal of Haematology. 2018. DOI: 10.1111/bjh.15210