In 2004, multiple myeloma was diagnosed in more than 15,000 people and accounted for approximately 11,000 deaths, representing 20% of fatalities due to hematologic malignancies in the United States.[1] Multiple myeloma is a hematologic B-cell malignancy associated with elevated serum and urine immunoglobulins, plasma cell infiltration of the bone marrow, soft-tissue plasmacytomas, and skeletal complications. Traditional therapies for multiple myeloma have included the combination of the alkylating agent melphalan(Drug information on melphalan) (Alkeran) with the steroid prednisone(Drug information on prednisone); combination chemotherapy with VAD (vincristine, doxorubicin(Drug information on doxorubicin) [Adriamycin], and dexamethasone(Drug information on dexamethasone)); high-dose chemotherapy with autologous stem cell transplantation (SCT); and nonmyeloablative or fully ablative allogeneic transplantation.[2,3] Although many patients respond to treatment, relapse is inevitable and median survival remains between 3 and 5 years[3,4]; hence, the need for new approaches is critical.[3] Multiple new agents for the treatment of multiple myeloma are under clinical examination, including the proteasome inhibitor bortezomib(Drug information on bortezomib) (Velcade), thalidomide(Drug information on thalidomide) (Thalomid) and its analogs such as lenalidomide (Revlimid), and arsenic trioxide (Trisenox), as well as a variety of other therapeutic strategies. This review briefly covers these emerging therapeutics and discusses their potential in myeloma therapy. It is important to note that clinical trials have applied different criteria to measure response to treatment. Commonly used response criteria and modified versions include those of the Southwest Oncology Group (SWOG),[5] the Eastern Cooperative Oncology Group (ECOG),[6] and the European Group for Blood and Marrow Transplantation (EBMT),[7] which all share certain outcome measures (eg, paraprotein reduction). However, the EBMT criteria define response more stringently and are now accepted as the most rigorous standard in drug development. Nevertheless, response rates from various agents should be compared with caution, because the criteria to measure response differ between trials. Bortezomib The proteasome, a multisubunit protease complex, plays an essential role in protein homeostasis in both normal and neoplastic cells. The proteasome is critical to regulated degradation of proteins involved in essential cellular functions, including protein turnover, cell adhesion, cell-cycle progression, antigen presentation, and inflammation.[8-10] Inhibition of proteasome function offers encouraging possibilities for the treatment of cancer, and various natural and synthetic molecules have been used to study proteasome inhibition in neoplastic cells.[11] However, only bortezomib (formerly known as PS-341), a boronic acid peptide derivative and potent (K_i = 0.6 nM), selective, reversible proteasome inhibitor,[12] has entered trials and been approved for clinical use. Mechanisms of Action
Several mechanisms by which bortezomib elicits antitumor effects in multiple myeloma have been identified. Inhibition of the proteasome with bortezomib has been shown to decrease myeloma cell proliferation through stabilization of the tumor suppressor p53 and the cyclin-dependent kinase inhibitors p21 and p27.[13,14] Bortezomib also promotes apoptosis via stabilization of the proapoptotic Bid and Bax proteins and inhibitor- κBα (IκBα), as well as activation of the c-Jun NH2-terminal kinase (JNK), as illustrated in Figure 1.[14-16] Stabilization of IκBα has been shown to result in inhibition of nuclear factor κB (NF-κB) activation and to prevent upregulation of the antiapoptotic Bcl-2, Bcl-xL, and XIAP proteins.[17,18] These effects produce an increase in caspase-9-mediated apoptosis.[19] Activated JNK also increases AP-1-mediated upregulation of Fas protein (CD95), which promotes caspase-8-mediated apoptosis.[ 19] Therefore, treatment with bortezomib may ultimately restore an apoptotic phenotype to myeloma cells through increased activation of both the intrinsic and extrinsic pathways. The antitumor activity of bortezomib also occurs in part through indirect effects on the bone marrow microenvironment (Figure 2). Bortezomib has been shown to inhibit the adhesion of myeloma cells with bone marrow stromal cells (BMSCs), thereby preventing the release of cytokines such as interleukin (IL)-6, vascular endothelial growth factor (VEGF), and insulin growth factor-1 (IGF-1).[13] Bortezomib- mediated inhibition of these cytokines, which promote proliferation, survival, and angiogenesis, has resulted in decreased viability of myeloma cells, as well as sensitization to antineoplastic agents.[20] This chemosensitization may also result from inhibition of NF-κB- mediated release of VEGF and IL-6 production by the BMSCs. NF-κB activity also has been associated with chemoresistance, and loss of NF-κB- mediated protection may result in chemosensitization.[21] Importantly, evidence has shown that normal cells are less sensitive to proteasome inhibition than neoplastic cells and that proteasome activity has recovered in normal tissues within 24 hours following treatment.[12,21] Relapsed, Refractory Multiple Myeloma
The results of a number of clinical studies of bortezomib in relapsed and/or refractory multiple myeloma are summarized in Table 1.[22-34] Phase I trials suggested that multiple myeloma is sensitive to proteasome inhibition and that a dosing schedule allowing for a 72-hour interval between individual doses and a drugfree rest period every third week has manageable toxicities.[35,36]

• SUMMIT-Based in part on the results of these studies, as well as highly supportive and provocative preclinical data in in vitro and in vivo models of multiple myeloma, SUMMIT,[22] a phase II trial, focused on patients with multiple myeloma that had both relapsed after prior therapy and was refractory to the last therapy. Patients (N = 202) received bortezomib, 1.3 mg/m² twice weekly for 2 weeks, with a 1-week rest between cycles. Responses were determined using modified EBMT criteria.[7] Complete response (CR) was defined as absence of paraprotein (undetectable by both electrophoresis and immunofixation), normal serum calcium concentration, and stable skeletal disease. To better define the range of responses classified as partial response (PR) by EBMT criteria, SUMMIT and CREST (described below) investigators included a new response category-near CR (nCR)- defined as 100% disappearance of paraprotein by electrophoresis but retention of positive immunofixation profile, with stable bone disease and a normal serum calcium concentration. Overall, 27% of the 193 evaluable patients demonstrated a major response (PR or better), with 4% achieving CR, an additional 6% achieving nCR, and 18% PR.[22] A landmark analysis demonstrated that patients who achieved a major response to bortezomib after two cycles survived significantly longer than those who did not (P = .007). Median survival with or without extended treatment was 17 months.[37] Data from an extension study with 63 patients showed that extended treatment with bortezomib for up to 32 cycles was associated with a manageable safety profile.[38] Bortezomib was effective in improving aspects of patient care such as improvement in global quality of life and reduction in disease symptoms. When patients who achieved major responses were analyzed apart from the remainder of the population, bortezomib treatment improved the overall quality of life and resulted in clinical benefits such as increased hemoglobin levels and platelet counts. Response rates were independent of number or type of prior therapies and other prognostic factors such as chromosome 13 deletion and β₂-microglobulin levels. Responses were, however, significantly associated with the percentage of plasma cells in the bone marrow and patient age. The most frequently reported toxicities of bortezomib were gastrointestinal, with nausea and diarrhea being most prevalent, along with fatigue, cyclical thrombocytopenia that recovered by the first day of the subsequent cycle, and peripheral neuropathy that resolved or improved in the majority of patients following discontinuation of treatment.
• CREST-The CREST study examined bortezomib in multiple myeloma patients who had relapsed after first-line therapy.[23] In this study of 54 patients with relapsed or refractory disease, two dosing schedules-bortezomib at 1.0 mg/m², and 1.3 mg/m², administered on days 1, 4, 8, and 11 with a 1-week rest each 3-week cycle-were assessed. The response rates (CR + PR) were 30% and 38% for patients who received bortezomib alone at 1.0 or 1.3 mg/m², respectively. The toxicities observed were similar to those reported in SUMMIT.
• APEX-APEX, an international, multicenter phase III study, compared bortezomib with high-dose dexamethasone in 669 patients with relapsed or refractory multiple myeloma.[24] The dexamethasone arm of the trial was stopped after a preplanned interim analysis revealed significant clinical benefits in survival and time to disease progression for patients receiving bortezomib. Dexamethasone patients were allowed to cross over to receive bortezomib in a companion study.
• Bortezomib/Dexamethasone- Preclinical evidence supports the concept that the effectiveness of bortezomib could be enhanced through combination with antineoplastic therapy. The combination of bortezomib with dexamethasone has resulted in at least additive anticancer effects in multiple myeloma cells.[13] Bortezomib has also markedly sensitized multiple myeloma cell lines to chemotherapeutic agents such as melphalan, mitoxantrone(Drug information on mitoxantrone) (Novantrone), and doxorubicin, and these combinations have yielded synergistic effects.[20,21] Based on these preclinical data in multiple myeloma models,[13] the addition of dexamethasone was permitted in both the SUMMIT and CREST phase II studies if patients presented with suboptimal responses to bortezomib monotherapy (progressive disease after two cycles or stable disease after the first four cycles). In SUMMIT, responses were observed in 13 (18%) of 74 evaluable patients who received dexamethasone with bortezomib after having demonstrated stable or progressive disease with single-agent bortezomib.[22] In CREST, the combination of bortezomib with dexamethasone also resulted in additional responses, with response rates (CR + PR) of 37% and 50% at the two dose levels.[23] Furthermore, responses to bortezomib plus dexamethasone were seen in patients previously documented as having refractory disease.[39] Results from a phase I study assessing combination therapy with pegylated liposomal doxorubicin (Doxil) in a variety of refractory hematologic malignancies, including multiple myeloma, have been reported. Patients (N = 42, including 24 with multiple myeloma) received doses of bortezomib ranging from 0.90 to 1.5 mg/m² on days 1, 4, 8, and 11, with liposomal doxorubicin (30 mg/m²) on day 4 of the 3-week cycle. In patients with multiple myeloma (n = 22 evaluable), this combination resulted in 5 CRs, 3 nCRs, and 8 PRs (72% CR or PR), while five patients had either a minor response or stable disease. Cycle 1 grade 3/4 doselimiting toxicities included diarrhea, constipation, hyponatremia, hypotension, confusion, neutropenia, and syncope. Clinically relevant (grade 3/4) toxicity related to drug treatment observed after cycle 1 included constipation, cytopenias, diarrhea, fatigue, impotence, palmar-plantar erythrodysesthesia, and peripheral neuropathy. More than 60% of patients who were resistant to or who relapsed from prior anthracycline therapy responded.[25]
• Bortezomib/Melphalan-Based on preclinical evidence of synergy between bortezomib and melphalan in multiple myeloma,[20,21] Berenson and colleagues examined combination bortezomib and melphalan in patients (N = 28) with relapsed or refractory multiple myeloma in a dose-escalation study.[26] In a preliminary report, bortezomib was given at an initial low dose of 0.7 to 1.0 mg/m² twice weekly for 2 weeks every 4 weeks, in conjunction with melphalan ranging from 0.025 to 0.25 mg/kg on days 1 to 4 every 4 weeks. Initial responses at this dose of bortezomib were encouraging, with over 40% of patients showing a clinical response (CR or PR). Grade 3 toxicities in this study were primarily hematologic. The majority of patients (> 75%) with baseline low-grade peripheral neuropathy (n = 8) exhibited stable neuropathy, but neuropathy in two patients worsened transiently.
• Bortezomib/Thalidomide-In an initial report of 56 multiple myeloma patients who had relapsed following autologous transplantation, treatment with a combination of bortezomib (1.0 or 1.3 mg/m²) and thalidomide (50 to 200 mg) with or without dexamethasone was examined.[28] Neither chromosomal abnormalities nor drug doses appeared to affect responses, although survival tended to be higher in patients without compared to those with chromosomal abnormalities. Grade 3/4 cumulative neurotoxicity was not seen in cycles 1 to 4. The preliminary results of a phase II study of bortezomib (1.3 mg/m²) in combination with thalidomide (100 mg) and liposomal doxorubicin (20 mg) suggested that this combination could induce major responses with manageable toxicities in patients with relapsed or refractory disease.[27]

Newly Diagnosed Multiple Myeloma
Bortezomib has also demonstrated activity as monotherapy or in combination regimens in the treatment of patients with newly diagnosed multiple myeloma in a number of preliminary reports (Table 1).[29-34] Although we have shown that single-agent bortezomib on the standard dose and schedule produced an overall response rate of 45% with manageable toxicities in newly diagnosed patients and less peripheral neuropathy compared with other studies,[29] clinical trials of bortezomib- based combinations in this setting appear more promising, with particularly high CR/nCR rates.[30-34] Jagannath et al found that bortezomib with dexamethasone (40 mg on the day of and after bortezomib administration) with less than PR after two cycles, or less than CR after four cycles, yielded CR, nCR, or PR in the majority of patients with newly diagnosed multiple myeloma.[30] The combination of bortezomib with dexamethasone also appeared to serve as an appropriate induction therapy prior to stem cell transplantation, because the combination was well tolerated and SCT was feasible.[31] Cavenagh et al conducted a trial of bortezomib, 1.3 mg/m², on days 1, 4, 8, and 11 for up to four 3-week cycles, with dexamethasone, 40 mg, on days 1 to 4, 8 to 11, and 15 to 18 of cycle 1 and on days 1 to 4 of subsequent cycles and doxorubicin, 0, 4.5, or 9.0 mg/m², on days 1 to 4 in the front-line setting.[32] In this study, over 20% of patients achieved CR, the vast majority had PR, and stem cell collection was feasible in all but one of 21 patients. The regimen of bortezomib, 1.0 to 1.3 mg/m² on days 1, 4, 8, 11, 22, 25, 29, and 32 of up to four 6-week cycles or on days 1, 8, 15, and 22 of up to five 5-week cycles, with melphalan, 9.0 mg/m², and prednisone, 60 mg/m², on days 1 to 4 also demonstrated impressive activity with manageable toxicity.[ 33] In this phase I/II study, no dose-limiting toxicities were observed with bortezomib treatment for up to 49 weeks. Preliminary data from another study also have demonstrated very encouraging results with the use of bortezomib, thalidomide, and dexamethasone[ 34] in the front-line setting. Thalidomide After its introduction in the 1950s as a sedative, thalidomide was linked to serious birth defects and was consequently removed from the marketplace in the 1960s.[3] It regained its position as a viable therapeutic agent when it was discovered to be efficacious in the treatment of erythema nodosum leprosum and has since been investigated in a variety of conditions.[ 40] Most importantly, although still considered experimental, thalidomide is now commonly used to treat multiple myeloma either as monotherapy or in combination with dexamethasone. Specifically, the efficacy and toxicity of thalidomide have been extensively studied in patients with relapsed, refractory, and newly diagnosed multiple myeloma. Mechanisms of Action
Although the antitumor mechanisms of thalidomide have not been fully defined, evidence suggests that induction of apoptosis, inhibition of cytokine production and angiogenesis, and immune modulation of T cells and natural killer cells are involved.[ 41-43] Thalidomide has been shown to promote apoptosis through the extrinsic pathway via activation of caspase-8 and inhibition of caspase inhibitor of apoptosis protein-2 (cIAP-2) (Figure 1).[43] The antimyeloma activity of thalidomide is also partially attributable to inhibition of the interaction of multiple myeloma cells with BMSCs (Figure 2). The reduced adhesion results in reduced secretion of cytokines, including IL-6, VEGF, and IGF-1, decreased cell viability and angiogenesis, and chemosensitization of multiple myeloma cells.[42] Inhibition of angiogenesis may be important, because microvessel density-a marker for angiogenesis- increases as multiple myeloma progresses, and a high level of angiogenesis is associated with decreased survival.[44] However, because paraprotein responses to thalidomide have not always correlated with differences in microvessel density, other mechanisms such as immunomodulation and effects of adhesion may be key.[45] Importantly, immune modulation by thalidomide appears to involve activation of phosphatidylinositol 3-kinase and increased IL-2 secretion of T lymphocytes as well as natural killer cell-mediated tumor cell lysis.[46]