**4. TRAIL treatment of haematological malignancies**

It has been shown that TRAIL induces growth arrest and apoptosis in cancer cells independently of wtp53 function, Bcl-2 and Bcl-XL (Walczak & Krammer, 2000) and MDR gene expression (Snell et al., 1997). Thus, TRAIL may offer an alternative or complementary approach to conventional anticancer therapy. Unlike other members of the TNF superfamily, such as CD95L and TNF that are precluded from use in systemic anticancer therapy due to their severe toxic side effects (Tartaglia and Goeddel, 1992), TRAIL is effective in selectively killing both *in vitro* and *in vivo* a vast array of tumour cells from lung, breast, kidney, colon, prostate, thyroid and skin cancers (Walczak & Krammer, 2000; Papenfuss et al., 2008), without causing significant organ toxicity and inflammation *in vivo*.

Fig. 1. Schematic representation of key mechanisms involved in TRAIL resistance of haematological malignancies. Activation of TRAIL-Rs can trigger both death and survival pathways, depending on the cell system and environmental conditions. TRAIL-R1 and TRAIL-R2 can lead to apoptotic cell death through the recruitment of FADD and the following cleavage of caspase-8 and –10. Both DRs together with the "decoy" TRAIL-R4 are also involved in the priming of survival genes through the activation of a) NF-B and JNK

MAPK/ERK1/2 pathways, by means of still unclear mechanisms (highlighted with a question mark). Other mechanisms leading to TRAIL resistance include different caspase or

It has been shown that TRAIL induces growth arrest and apoptosis in cancer cells independently of wtp53 function, Bcl-2 and Bcl-XL (Walczak & Krammer, 2000) and MDR gene expression (Snell et al., 1997). Thus, TRAIL may offer an alternative or complementary approach to conventional anticancer therapy. Unlike other members of the TNF superfamily, such as CD95L and TNF that are precluded from use in systemic anticancer therapy due to their severe toxic side effects (Tartaglia and Goeddel, 1992), TRAIL is effective in selectively killing both *in vitro* and *in vivo* a vast array of tumour cells from lung, breast, kidney, colon, prostate, thyroid and skin cancers (Walczak & Krammer, 2000; Papenfuss et al., 2008), without causing significant organ toxicity and inflammation *in vivo*.

pathways triggered by the engagement of TRAF2 and RIP; b) PI3K/Akt and

**4. TRAIL treatment of haematological malignancies** 

PI3K physiological inhibitors.

Moreover, TRAIL exerts a variable cytotoxic activity on haematological malignancies (Snell et al., 1997) and synergistically cooperates with: i) chemotherapeutic drugs, such as etoposide, campthotecin-11, doxorubicin, 5-fluorouracil, taxol (Sabatini et al., 2004; Henson et al., 2008); and ii) ionizing radiation (Chinnaiyan et al., 2000; Di Pietro et al., 2001), causing substantial regression or complete ablation of solid (colon and mammary) cancers in animal models. Besides acting as a tumour suppressor *in vivo* in primary tumours, TRAIL could play a substantial role in suppressing tumour metastasis. In fact, it has been observed that this cytokine may partially limit the formation of hepatic metastases of a variety of mouse tumours (Seki et al., 2003). A study performed in TRAIL -/- null mice demonstrated that the incidence of spontaneous lymphoid malignancies was increased by 25% in comparison with control animals (Zerafa et al., 2005), suggesting a crucial role of TRAIL in the immunesurveillance against lymphoid malignancies. Although it is not established whether TRAIL causes liver toxicity in humans (Jo et al., 2000; Lawrence et al., 2001), pre-clinical studies performed in mice and non-human primates indicated that rTRAIL protein promotes potent apoptosis-inducing activity against tumour cells without a relevant systemic toxicity (Walczak et al., 1999). Phase I and phase II clinical trials in patients with advanced solid tumours or non-Hodgkin lymphoma (NHL) appeared to go in the same direction, indicating that both rTRAIL and TRAs are safe and well tolerated (Koschny et al., 2007; Tolcher et al., 2007). Therefore, TRAIL ligand and TRAs are strong candidates for an effective but tolerable treatment of solid cancers, either used alone or in combination with radio-chemotherapy.

#### **4.1 Acute myeloid leukaemia (AML)**

The cytotoxic activity of TRAIL has been evaluated in haematological diseases by different groups of investigators, including our research group (Secchiero & Zauli, 2008; Sancilio et al., 2008; Impicciatore et al., 2010). Overall the activity of TRAIL as a single treatment in acute and chronic leukaemia is poor. Unlike the poor outcome of TRAIL treatment in primary AML blasts, continuous cell lines derived from AML display a pronounced sensitivity to the apoptotic action of TRAIL (Snell et al., 1997; Secchiero et al., 2004b). Moreover, when TRAIL is used in combination with chemotherapeutic agents (fludarabine, cytosine arabinoside or daunorubicin) additive or super-additive apoptotic effects are obtained, due to the ability of these agents to activate apical caspase-8/-10 (Jones et al., 2003). In line with these findings, other authors demonstrated that triterpenoids, natural and synthetic compounds with demonstrated anti-tumour activity, induced a substantial increase in cell death in both B-CLL and AML blasts, by inducing a concentrationdependent decrease in the levels of FLIP protein (Suh et al., 2003; Pedersen et al., 2002). A recent report has related the poor response of AML to the simultaneous expression of death and decoy receptors (Inukai et al., 2006), whereas co-expression of death receptors with the decoy receptor TRAIL-R3 resulted in significant shortened overall survival of AML patients (Chamuleau et al., 2011). Another weak point in leukaemia treatment is represented by p53 gene deletions or mutations, that usually occur in less than 15% of AML cases. To augment the poor response of AML to TRAIL cytotoxicity, Secchiero et al. (2007) have recently adopted the strategy to combine rTRAIL with Nutlin-3, a potent non-genotoxic activator of the p53 pathway (Impicciatore et al., 2010). In this investigation Nutlin-3 synergized with TRAIL in inducing apoptosis both in AML cell lines and primary M4-type and M5-type AML blasts, but not in mutp53 AML cells, suggesting that the combined treatment of Nutlin-3 plus TRAIL might offer a novel therapeutic strategy for wtp53 AML cells.

Signalling Pathways Leading to TRAIL Resistance 211

B-CLL cells. As a consequence, the possibility of sensitizing B-CLL cells to TRAIL-mediated cytotoxicity could reside in the modulation of c-FLIP levels or in the up-regulation of DR surface expression. In consistence with this hypothesis, the combination of TRAIL with anti-CD95 ligand has proved effective in inducing apoptosis of CD40-activated B-CLL cells (Dicker et al., 2005). A more recent study, aimed at evaluating molecular mechanisms of TRAIL resistance of B-CLL, identified a different TRAIL sensitivity of Zap-70low and Zap-70high B-CLL subsets, proposing this negative prognostic marker as responsible to redirect TRAIL signallling from pro-apoptotic to pro-inflammatory pathway (Richardson et al.,

The potential therapeutic use of TRAIL has been also explored in the therapy of refractory diffuse large B-cell lymphoma (DLBCL) (Cillessen et al., 2006), cutaneous T-cell lymphoma (CTCL) (Braun et al., 2007), mantle B cell lymphoma (MCL) (Roue et al., 2007) and plasmacytoid dendritic cell (PDC) leukaemia (Blum et al., 2006), which shows a particularly aggressive clinical course. In particular, Cillessen et al. (2006) have shown that 12 out of a total of 22 DLBCL samples, including 7 clinically chemotherapy-refractory lymphomas, were sensitive to TRAIL-mediated apoptosis. TRAIL cytotoxic effects were also detected in CD4+CD56+ PDC leukaemia (Blum et al., 2006), as well as in the majority of MCL cell lines and primary cultures investigated by Roue et al. (2007), whose research group used TRAIL in combination with the IB kinase inhibitor BMS-345541 to overcome resistance of MCL samples. In a recent review (Sancilio et al., 2008), we suggested the possibility to combine two biologically active and well-tolerated agents with different mechanisms of action, such as rituximab and agonist MoAbs against DRs, as an attractive treatment strategy for patients affected with B-cell lymphoma. The *in vivo* mechanisms through which rituximab mediates its effects have not been fully elucidated, though ADCC (antibody-dependent cellular cytotocxicity*)*, CMC (complement-mediated cytotoxicity) and apoptosis have been suggested and supported by several studies (Bonavida, 2007). By contrast, a number of *in vitro* experimental evidences have been obtained in B-NHL cell lines as a model system (Bonavida, 2007). The findings here described demonstrate that rituximab treatment is able to modulate different signalling pathways, like p38-MAPK, Raf-1/MEK/ERK1/2 and NF- B, leading to the down-regulation of Bcl-2/Bcl-XL gene products, known players of the intrinsic apoptotic pathway. Through this mechanism, chemo-sensitization of drug-resistant

B-NHL cell lines to various drug-induced apoptosis could be achieved.

A number of studies from several groups of investigators have allowed clearly establishing that myeloma is the most susceptible haematological malignancy to rTRAIL used as a single agent (Secchiero et al., 2004b). In particular, Gazitt (1999) demonstrated for the first time that TRAIL induces substantial apoptosis in freshly isolated, flow-sorted myeloma cells obtained from different MM patients. Subsequently, the same group of investigators (Liu et al., 2003) demonstrated that TRAIL is a potent inducer of apoptosis, independent of Bcl-2. Moreover, consistently with the potential role of NF-B and Akt pathways in counteracting apoptosis induction by either chemotherapy or TRAIL, the cell permeable nuclear factor NF-B inhibitor SN50 sensitized TRAIL-resistant MM cells to TRAIL cytotoxicity (Mitsiades et al., 2002) and the Akt inhibitor IL-6-Hydroxymethyl-chiro-inositol 2-(R)-2-O-methyl-3-Ooctadecylcarbonate-induced cell death of both Dex- and Doxo-sensitive and -resistant cell

2006).

**4.5 Lymphoma** 

**4.6 Multiple myeloma (MM)** 

#### **4.2 Acute lymphoblastic leukaemia (ALL)**

Clodi et al. (2000) demonstrated that TRAIL has a modest activity in primary ALL since it killed a maximum of 29% of precursor-B-cell blasts within 18 hours treatment against the 75% of the sensitive Jurkat cell line. Childhood T-ALL is frequently accompanied by hyperleukocytosis at disease presentation, suggesting that T-ALL tends to acquire mechanisms for escaping immune surveillance of the hosts that promote its rapid clonal expansion. Clinically, dramatic advances have been made in the treatment of childhood T-ALL. However, despite the use of intensive risk-adapted chemotherapy, treatment failure occurs in approximately 25% of patients (Goldberg et al., 2003). Since the prognosis of relapsed T-ALL remains dismal, the development of a new therapeutic modality is urgently required. In a recent report on T-ALL cell lines and primary samples of childhood T-ALL the failure of anti-leukaemic activity of soluble rTRAIL was linked to the low cell surface expression levels of TRAIL-R1 and TRAIL-R2, which could not be modified by the demethylating agent 5-aza2'deoxycytidine (Akahanea et al., 2010).

### **4.3 Chronic myeloid leukaemia (CML)**

Only few studies have investigated the effects of rTRAIL on CML blasts. An interesting study of Tanaka et al. (2007) demonstrated increased levels of serum TRAIL and TRAIL mRNA in neutrophils of CML patients during IFN therapy, suggesting a novel antineoplastic role of neutrophils mediated by the expression/release of TRAIL. Since neutrophils, unlike activated lymphocytes, display a low susceptibility to TRAIL cytotoxicity (Meurette et al., 2006), these findings are of particular value. Other studies have shown that TRAIL, used as a single agent, significantly reduces the number of myeloid colonies and clusters from patients affected with CML and myelodysplastic syndromes (MDS) (Zang et al., 2001; Uno et al., 2003), while normal human stem cells treated with high doses of TRAIL maintain a repopulating potential when transplanted into NOD/SCID mice (Zang et al., 2001). Moreover, it was recently demonstrated that the loss of Bcr-Abl in imatinib-resistant CML cells leads to the down-regulation of c-FLIP and the subsequent increase in TRAIL sensitivity, suggesting that TRAIL could be an effective strategy for the treatment of imatinib-resistant CML with loss of Bcr-Abl (Park et al., 2009).

#### **4.4 B-type chronic lymphocytic leukaemia (B-CLL)**

A pressing need for the identification of novel therapeutic approaches regards B-CLL disease. It is known in fact that B-CLL patients may have initial clinical responses to alkylating agents, such as chlorambucil, or adenosine analogs, such as fludarabine, but they ultimately become refractory to therapy. Preliminary studies, carried out on cell lines and a modest number of primary samples, have shown a low cytotoxic activity of TRAIL on lowgrade B-CLL (MacFarlane et al., 2002). Collectively, low-grade B-cell malignancies constitute one of the most common form of potentially lethal cancer in Europe and North America, with B-CLL representing the most prevalent of these disorders (Reed et al., 2002). B-CLL is characterized by the accumulation of mature non-proliferating B cells defective in apoptotic mechanisms and resistant to anticancer therapy. A number of molecular defects and biologic features have been identified in this pathology. Olsson et al. (2001) revealed a higher constitutive expression of the long form of FLIP (FLIP-L) in B-CLL as compared to normal tonsillar B cells. MacFarlane et al. (2002) demonstrated that resistance to TRAIL was upstream of caspase-8 activation, since little or no caspase-8 was processed in TRAIL-treated B-CLL cells. As a consequence, the possibility of sensitizing B-CLL cells to TRAIL-mediated cytotoxicity could reside in the modulation of c-FLIP levels or in the up-regulation of DR surface expression. In consistence with this hypothesis, the combination of TRAIL with anti-CD95 ligand has proved effective in inducing apoptosis of CD40-activated B-CLL cells (Dicker et al., 2005). A more recent study, aimed at evaluating molecular mechanisms of TRAIL resistance of B-CLL, identified a different TRAIL sensitivity of Zap-70low and Zap-70high B-CLL subsets, proposing this negative prognostic marker as responsible to redirect TRAIL signallling from pro-apoptotic to pro-inflammatory pathway (Richardson et al., 2006).

#### **4.5 Lymphoma**

210 Advances in Cancer Therapy

Clodi et al. (2000) demonstrated that TRAIL has a modest activity in primary ALL since it killed a maximum of 29% of precursor-B-cell blasts within 18 hours treatment against the 75% of the sensitive Jurkat cell line. Childhood T-ALL is frequently accompanied by hyperleukocytosis at disease presentation, suggesting that T-ALL tends to acquire mechanisms for escaping immune surveillance of the hosts that promote its rapid clonal expansion. Clinically, dramatic advances have been made in the treatment of childhood T-ALL. However, despite the use of intensive risk-adapted chemotherapy, treatment failure occurs in approximately 25% of patients (Goldberg et al., 2003). Since the prognosis of relapsed T-ALL remains dismal, the development of a new therapeutic modality is urgently required. In a recent report on T-ALL cell lines and primary samples of childhood T-ALL the failure of anti-leukaemic activity of soluble rTRAIL was linked to the low cell surface expression levels of TRAIL-R1 and TRAIL-R2, which could not be modified by the

Only few studies have investigated the effects of rTRAIL on CML blasts. An interesting study of Tanaka et al. (2007) demonstrated increased levels of serum TRAIL and TRAIL mRNA in neutrophils of CML patients during IFN therapy, suggesting a novel antineoplastic role of neutrophils mediated by the expression/release of TRAIL. Since neutrophils, unlike activated lymphocytes, display a low susceptibility to TRAIL cytotoxicity (Meurette et al., 2006), these findings are of particular value. Other studies have shown that TRAIL, used as a single agent, significantly reduces the number of myeloid colonies and clusters from patients affected with CML and myelodysplastic syndromes (MDS) (Zang et al., 2001; Uno et al., 2003), while normal human stem cells treated with high doses of TRAIL maintain a repopulating potential when transplanted into NOD/SCID mice (Zang et al., 2001). Moreover, it was recently demonstrated that the loss of Bcr-Abl in imatinib-resistant CML cells leads to the down-regulation of c-FLIP and the subsequent increase in TRAIL sensitivity, suggesting that TRAIL could be an effective strategy for the

A pressing need for the identification of novel therapeutic approaches regards B-CLL disease. It is known in fact that B-CLL patients may have initial clinical responses to alkylating agents, such as chlorambucil, or adenosine analogs, such as fludarabine, but they ultimately become refractory to therapy. Preliminary studies, carried out on cell lines and a modest number of primary samples, have shown a low cytotoxic activity of TRAIL on lowgrade B-CLL (MacFarlane et al., 2002). Collectively, low-grade B-cell malignancies constitute one of the most common form of potentially lethal cancer in Europe and North America, with B-CLL representing the most prevalent of these disorders (Reed et al., 2002). B-CLL is characterized by the accumulation of mature non-proliferating B cells defective in apoptotic mechanisms and resistant to anticancer therapy. A number of molecular defects and biologic features have been identified in this pathology. Olsson et al. (2001) revealed a higher constitutive expression of the long form of FLIP (FLIP-L) in B-CLL as compared to normal tonsillar B cells. MacFarlane et al. (2002) demonstrated that resistance to TRAIL was upstream of caspase-8 activation, since little or no caspase-8 was processed in TRAIL-treated

**4.2 Acute lymphoblastic leukaemia (ALL)** 

**4.3 Chronic myeloid leukaemia (CML)** 

demethylating agent 5-aza2'deoxycytidine (Akahanea et al., 2010).

treatment of imatinib-resistant CML with loss of Bcr-Abl (Park et al., 2009).

**4.4 B-type chronic lymphocytic leukaemia (B-CLL)** 

The potential therapeutic use of TRAIL has been also explored in the therapy of refractory diffuse large B-cell lymphoma (DLBCL) (Cillessen et al., 2006), cutaneous T-cell lymphoma (CTCL) (Braun et al., 2007), mantle B cell lymphoma (MCL) (Roue et al., 2007) and plasmacytoid dendritic cell (PDC) leukaemia (Blum et al., 2006), which shows a particularly aggressive clinical course. In particular, Cillessen et al. (2006) have shown that 12 out of a total of 22 DLBCL samples, including 7 clinically chemotherapy-refractory lymphomas, were sensitive to TRAIL-mediated apoptosis. TRAIL cytotoxic effects were also detected in CD4+CD56+ PDC leukaemia (Blum et al., 2006), as well as in the majority of MCL cell lines and primary cultures investigated by Roue et al. (2007), whose research group used TRAIL in combination with the IB kinase inhibitor BMS-345541 to overcome resistance of MCL samples. In a recent review (Sancilio et al., 2008), we suggested the possibility to combine two biologically active and well-tolerated agents with different mechanisms of action, such as rituximab and agonist MoAbs against DRs, as an attractive treatment strategy for patients affected with B-cell lymphoma. The *in vivo* mechanisms through which rituximab mediates its effects have not been fully elucidated, though ADCC (antibody-dependent cellular cytotocxicity*)*, CMC (complement-mediated cytotoxicity) and apoptosis have been suggested and supported by several studies (Bonavida, 2007). By contrast, a number of *in vitro* experimental evidences have been obtained in B-NHL cell lines as a model system (Bonavida, 2007). The findings here described demonstrate that rituximab treatment is able to modulate different signalling pathways, like p38-MAPK, Raf-1/MEK/ERK1/2 and NF- B, leading to the down-regulation of Bcl-2/Bcl-XL gene products, known players of the intrinsic apoptotic pathway. Through this mechanism, chemo-sensitization of drug-resistant B-NHL cell lines to various drug-induced apoptosis could be achieved.

#### **4.6 Multiple myeloma (MM)**

A number of studies from several groups of investigators have allowed clearly establishing that myeloma is the most susceptible haematological malignancy to rTRAIL used as a single agent (Secchiero et al., 2004b). In particular, Gazitt (1999) demonstrated for the first time that TRAIL induces substantial apoptosis in freshly isolated, flow-sorted myeloma cells obtained from different MM patients. Subsequently, the same group of investigators (Liu et al., 2003) demonstrated that TRAIL is a potent inducer of apoptosis, independent of Bcl-2. Moreover, consistently with the potential role of NF-B and Akt pathways in counteracting apoptosis induction by either chemotherapy or TRAIL, the cell permeable nuclear factor NF-B inhibitor SN50 sensitized TRAIL-resistant MM cells to TRAIL cytotoxicity (Mitsiades et al., 2002) and the Akt inhibitor IL-6-Hydroxymethyl-chiro-inositol 2-(R)-2-O-methyl-3-Ooctadecylcarbonate-induced cell death of both Dex- and Doxo-sensitive and -resistant cell

Signalling Pathways Leading to TRAIL Resistance 213

Shukla, 2010). The clinical use of TRAs is a very promising and innovative approach to increase selectivity and reduce undesired toxicity of cancer treatments in comparison with modern anticancer drugs (protein kinase inhibitors or MoAb agonists for growth receptors) (Russo et al., 2010). These compounds were generated to selectively bind and activate their respective DRs without affecting decoy receptors or OPG. DRs engagement, using recombinant death ligands or agonistic antibodies, leads to the activation of both extrinsic and intrinsic apoptosis pathways, while, generally, chemotherapy or radiotherapy triggers the mitochondrial/intrinsic pathway (Fig. 2). Therefore, the conventional therapeutic approach could be implemented by DR-induced apoptosis when DRs are expressed and functional on tumour cells. As already mentioned, although soluble rTRAIL as well as TRAs are not completely free from toxicity, both reagents elicit a significant lower hepatotoxicity when administered systemically compared to CD95 receptor agonists (Lawrence et al., 2001). Besides the advantage of an improved specificity and a lower toxicity of TRAs over TRAIL ligand, pharmacokinetic studies performed in primates and humans have shown that these agents have a longer half-life (around 15 days) than soluble TRAIL (30 min) that makes them easier to dose and administer (Duiker et al., 2006). Preclinical studies performed *in vitro* in cultured human cell lines and *in vivo* in murine xenograft cancer models (Cretney et al., 2007) showed favourable results when TRAs were used as single agents and enhanced cytotoxicity when they were combined with chemotherapy or radiotherapy (Marini et al., 2006). In particular, HGS-ETR1 (anti-TRAIL-R1, mapatumumab) as well as HGS-ETR2 (anti-TRAIL-R2, lexatumumab) was able to induce apoptosis in primary and cultured lymphoma cells increasing cell death when associated either with conventional chemotherapy (doxorubicin) or novel drugs like proteasome inhibitors (bortezomib) (Georgakis et al., 2005). As well, multiple solid tumours including lung, colon and renal carcinoma were found responsive to TRAs treatment used alone or in combination with chemotherapy (Pukac et al., 2005). To date, the fully humanized MoAbs HGS-ETR1, HGS-ETR2 and HGS-TR2J (anti-TRAIL-R2) (all three from Human Genome Sciences, Rockville, MD) are used in ongoing trials for the treatment of advanced solid tumours, lymphoma or MM (Mahmood & Shukla, 2010). A number of excellent reviews on different therapeutic approaches to specifically target TRAIL and DR pathways have been recently published (Ashkenazi et al.,

2008; Papenfuss et al., 2008; Mahmood & Shukla, 2010; Russo et al., 2010).

Of particular interest is the current use of rTRAIL and TRAs for the treatment of B cell malignancies (Mahmood & Shuka, 2010). As shown by other authors, the DR pathway is intact and functional in various types of cancers, including B-cell lymphomas (Snell et al., 1997; Georgakis et al., 2005). B-cell sensitivity to TRAs is a fundamental requirement for therapeutic efficacy, since TRAIL-R1 and TRAIL–R2 mutations, observed in NHL as well as in other human tumours (Lee et al., 2001), make neoplastic B cells insensitive to TRAIL and, presumably, to agonistic antibodies mimicking its action. TRAIL-R1 and TRAIL-R2 map to human chromosome 8p21-22, a site of frequent allelic loss in tumours. This led to the hypothesis that, as potential tumour suppressors, TRAIL-Rs may also harbour somatic mutations in human tumours. The most frequent mutations identified so far concern TRAIL-R2 and affect the intracellular domain of the receptor, i.e. the FADD-binding domain, and, as a consequence, its capability of inducing apoptosis (Bin et al., 2007). Although still poor is the knowledge of how TRAIL-Rs mutations affect signalling events, it is predictable that a patient displaying a TRAIL-R2 mutation would not benefit from treatment with either rTRAIL or an anti-TRAIL-R2 antibody but from treatment with mapatumumab or a

clones. Interestingly, also thalidomide, which holds great promise as a new anti-neoplastic agent for the treatment of refractory MM, triggers activation of caspase-8 and downregulates NF-B activity and c-FLIP (Mitsiades et al, 2002). These studies form the basis for clinical trials of these agents, alone and coupled with conventional and novel therapies, to improve outcome in MM. It is worth underlining that while the potential therapeutic use of rTRAIL or TRAs in most myeloid and lymphoid malignancies is still to be evaluated, rTRAIL appears to be a very promising candidate for the therapy of MM, either alone or in combination with valproic acid, a histone deacetylase inhibitor, arsenic trioxide, IFN, or with the low-molecular-weight Smac mimetic LBW242 (Secchiero & Zauli, 2008). Moreover, the use of specific anti-TRAIL-R1 or anti-TRAIL-R2 agonistic antibodies, more than the treatment with TRAIL itself (Locklin et al., 2007), has proved an effective strategy to counteract OPG-mediated effects and increase TRAIL-induced apoptosis of MM cells (Secchiero & Zauli, 2008). Of note, other preclinical studies aimed at targeting the RANK/RANKL/OPG pathway have paved the way to clinical experimentation likely to lead to new therapeutic approaches (Buckle et al., 2010).
