New approaches to management of multiple myeloma

Aponermin or placebo in combination with thalidomide and dexamethasone in the treatment of relapsed or refractory multiple myeloma (CPT-MM301): a randomised, double-blinded, placebo-controlled, phase 3 trial

BACKGROUND

Aponermin, a circularly permuted tumor necrosis factor-related apoptosis-inducing ligand, is a potential death receptor 4/5-targeted antitumour candidate. Previous phase 1/2 studies have demonstrated the efficacy of aponermin in patients with relapsed or refractory multiple myeloma (RRMM). To confirm the superiority of aponermin plus thalidomide and dexamethasone (aponermin group) over placebo plus thalidomide and dexamethasone (placebo group) in RRMM, a randomized, double-blinded, placebo controlled phase 3 trial was performed.

METHODS

Four hundred seventeen patients with RRMM who had previously received at least two regimens were randomly assigned (2:1) to receive aponermin, thalidomide, and dexamethasone or placebo, thalidomide, and dexamethasone. The primary endpoint was progression-free survival (PFS). Key secondary endpoints included overall survival (OS) and overall response rate (ORR).

RESULTS

A total of 415 patients received at least one dose of trial treatment (276 vs. 139). The median PFS was 5.5 months in the aponermin group and 3.1 months in the placebo group (hazard ratio, 0.62; 95% confidence interval [CI], 0.49-0.78; P < 0.001). The median OS was 22.4 months for the aponermin group and 16.4 months for the placebo group (hazard ratio, 0.70; 95% CI, 0.55-0.89; P = 0.003). Significantly higher rates of ORR (30.4% vs. 13.7%, P < 0.001) and very good partial response or better (14.1% vs. 2.2%, P < 0.0001) were achieved in the aponermin group than in the placebo group. Treatment with aponermin caused hepatotoxicity in some patients, as indicated by the elevated alanine transaminase, aspartate transaminase, or lactate dehydrogenase levels (52.2% vs. 24.5%, 51.1% vs. 19.4% and 44.9% vs. 21.6%, respectively), mostly grade 1/2, transient and reversible. The main grade 3/4 adverse events included neutropenia, pneumonia and hyperglycemia. The incidence of serious adverse events was similar between the two groups (40.6% vs. 37.4%). There was no evidence that aponermin leads to hematological toxicity, nephrotoxicity, cardiotoxicity, or secondary tumors.

CONCLUSIONS

Aponermin plus thalidomide and dexamethasone significantly improved PFS, OS and ORR with manageable side effects in RRMM patients who had received at least two prior therapies. These results support the use of aponermin, thalidomide, and dexamethasone as a treatment option for RRMM patients.

TRIAL REGISTRATION

The trial was registered at http://www.chictr.org.cn as ChiCTR-IPR-15006024, 17/11/2014.

Chronological Age and DNA Damage Accumulation in Blood Mononuclear Cells: A Linear Association in Healthy Humans after 50 Years of Age

Aging is characterized by the progressive deregulation of homeostatic mechanisms causing the accumulation of macromolecular damage, including DNA damage, progressive decline in organ function and chronic diseases. Since several features of the aging phenotype are closely related to defects in the DNA damage response (DDR) network, we have herein investigated the relationship between chronological age and DDR signals in peripheral blood mononuclear cells (PBMCs) from healthy individuals. DDR-associated parameters, including endogenous DNA damage (single-strand breaks and double-strand breaks (DSBs) measured by the alkaline comet assay (Olive Tail Moment (OTM); DSBs-only by γH2AX immunofluorescence staining), DSBs repair capacity, oxidative stress, and apurinic/apyrimidinic sites were evaluated in PBMCs of 243 individuals aged 18-75 years, free of any major comorbidity. While OTM values showed marginal correlation with age until 50 years (r_s = 0.41, p = 0.11), a linear relationship was observed after 50 years (r = 0.95, p < 0.001). Moreover, individuals older than 50 years showed increased endogenous DSBs levels (γH2Ax), higher oxidative stress, augmented apurinic/apyrimidinic sites and decreased DSBs repair capacity than those with age lower than 50 years (all p < 0.001). Results were reproduced when we examined men and women separately. Prospective studies confirming the value of DNA damage accumulation as a biomarker of aging, as well as the presence of a relevant agethreshold, are warranted.

The Combination of Panobinostat and Melphalan for the Treatment of Patients with Multiple Myeloma

Histone deacetylase inhibitors show synergy with several genotoxic drugs. Herein, we investigated the biological impact of the combined treatment of panobinostat and melphalan in multiple myeloma (MM). DNA damage response (DDR) parameters and the expression of DDR-associated genes were analyzed in bone marrow plasma cells (BMPCs) and peripheral blood mononuclear cells (PBMCs) from 26 newly diagnosed MM patients. PBMCs from 25 healthy controls (HC) were examined in parallel. Compared with the ex vivo melphalan-only treatment, combined treatment with panobinostat and melphalan significantly reduced the efficiency of nucleotide excision repair (NER) and double-strand-break repair (DSB/R), enhanced the accumulation of DNA lesions (monoadducts and DSBs), and increased the apoptosis rate only in patients’ BMPCs (all p < 0.001); marginal changes were observed in PBMCs from the same patients or HC. Accordingly, panobinostat pre-treatment decreased the expression levels of critical NER (DDB2, XPC) and DSB/R (MRE11A, PRKDC/DNAPKc, RAD50, XRCC6/Ku70) genes only in patients’ BMPCs; no significant changes were observed in PBMCs from patients or HC. Together, our findings demonstrate that panobinostat significantly increased the melphalan sensitivity of malignant BMPCs without increasing the melphalan sensitivity of PBMCs from the same patients, thus paving the way for combination therapies in MM with improved anti-myeloma efficacy and lower side effects.

Patient-Reported Factors in Treatment Satisfaction in Patients with Relapsed/Refractory Multiple Myeloma (RRMM)

This article identifies factors associated with patient‐reported satisfaction with multiple myeloma therapy and the treatment‐related time burden and indirect costs among patients with relapsed or refractory multiple myeloma and their caregivers. Improved understanding of these variables will inform treatment decisions across this complex treatment landscape.

Background.

Therapy choices in relapsed/refractory multiple myeloma (RRMM) should consider patient satisfaction with treatment, because it is associated with adherence to therapy, health outcomes, and medical safety. The primary objective of this pilot cross‐sectional observational study was to ascertain factors associated with patient‐reported treatment satisfaction in RRMM.

Patients and Methods.

Patients with a self‐reported diagnosis of RRMM recruited from PatientsLikeMe, MyelomaCrowd, and Facebook were administered an electronic survey that included questions on demographics and clinical history, treatment experience, economic burden, and standardized patient‐reported outcome measures, including the Treatment Satisfaction Questionnaire for Medication, Eastern Cooperative Oncology Group performance status (ECOG PS) measure, and Work Productivity and Activity Impairment Questionnaire: Specific Health Problem V2.0. Univariable and multivariable analyses were used to identify predictors of patient‐perceived treatment satisfaction.

Results.

One hundred sixty patients with RRMM participated in the study, with a median of two prior relapses and 66.3% reporting the most recent relapse within the last 12 months. ECOG PS ≥2 was associated with lower patient‐reported global satisfaction and perceived effectiveness of current treatment. In addition to shorter time spent receiving therapy, orally administered treatment was the strongest predictor of higher satisfaction with treatment convenience. For patients receiving an injectable drug‐containing regimen versus an all‐oral regimen, respectively, time spent receiving multiple myeloma‐directed therapy was higher (12.6 vs. 4.0 hours per month), and total monthly indirect costs were $1,033 and $241.

Conclusion.

Poor ECOG PS was linked to reduced treatment satisfaction and perceived effectiveness of current therapy, whereas an all‐oral regimen was associated with increased treatment convenience satisfaction.

Implications for Practice.

This study suggests that attributes including better Eastern Cooperative Oncology Group performance status, less time spent receiving treatment, and oral route of treatment administration lead to higher patient‐perceived satisfaction with relapsed/refractory multiple myeloma (RRMM) treatment. Oral route of administration was also associated with less time spent receiving treatment and reduced economic burden for patients. Increased attention to these factors in shared treatment decision making is warranted to help identify individual patient needs, preferences, and expectations for RRMM treatments, to resolve dissatisfaction issues, and to improve the experience of patients with RRMM.

Cost-effectiveness analysis of treating transplant-eligible multiple myeloma patients in Macedonia

Purpose

A decision-analytic model was developed to study the impact of induction regimens vincristine, adriamycin, dexamethasone (VAD); thalidomide, dexamethasone (TD); and bortezomib, dexamethasone (BorD), followed by autologous stem cell transplantation (ASCT) for treating multiple myeloma (MM) patients in Macedonia. Additionally, a cost-effectiveness analysis (CEA) of treatment sequences to predict health effects and costs of different treatment sequences was performed.

Methods

Model strategies were based on a previously published study for treating patients with MM in Macedonia. The data on disease progression and treatment effectiveness were obtained from the published reports of randomized clinical trials (GIMEMA M-B02005, IFM 2005-01). Utility parameters were extracted from the literature. To compare treatment combinations, a decision tree model was developed. Additionally, a cost analysis for one-time per-protocol costs was performed from a Macedonian national health care perspective. The incremental cost-effectiveness ratios (ICERs)/quality-adjusted life years (QALYs) gained for 1-, 10-, and 20-year time horizons were determined. Costs and health outcomes were discounted to evaluate the effects of time in the model.

Results

The one-time costs of BorD (EUR 5,656) were higher compared to VAD (EUR 303) and TD (EUR 329), increasing the overall costs for BorD. Thus, the BorD combination dominated in the baseline results (1 and 10 years) and the ICER for TD vs. VAD was EUR 7,564/QALY (20 years, undiscounted model). However, in the discounted 20-year model, BorD showed an ICER of EUR 138,747/QALY gained for BorD vs. TD.

Conclusion

The CEA performed indicated that considering 1-year time horizon costs, VAD may be a cost-effective alternative vs. TD or BorD. However, for the longer period (10 or 20 years) including the discounting of future costs and outcomes, the TD and BorD combinations showed higher health benefits in terms of QALYs and more cost-effective vs. VAD. These results should be considered as supportive evidence by decision-makers and providers when deciding on the most cost-effective induction treatment strategy prior to ASCT in MM patients.

Correlation Between Baseline 18F-FDG PET/CT Findings and CD38- and CD138-Expressing Myeloma Cells in Bone Marrow and Clinical Parameters in Patients with Multiple Myeloma

Objective

The aim of this study was to evaluate the relation between the rate of fluorine-18 (18F) fludeoxyglucose (FDG) uptake and CD38 and CD138 expression in myeloma cells in bone marrow and other clinical parameters in patients with multiple myeloma (MM).

Materials and Methods

Patients with the diagnosis of MM who underwent 18F-FDG positron emission tomography/computed tomography (PET/CT) for initial staging were evaluated retrospectively. We analyzed a total of 42 patients (43-83 years old, mean: 64.4±9.9). Hematological and biochemical tests including hemoglobin, hematocrit, C-reactive protein, β2-microglobulin, creatinine, albumin, calcium, lactate dehydrogenase, and erythrocyte sedimentation rate were recorded. In bone marrow samples, plasma cell ratio and CD38 and CD138 immunohistochemical staining were evaluated. On PET/CT images, mean standardized uptake values (SUV_mean) of the right anterior and posterior iliac crest and right proximal femora were calculated. The correlations between the average SUV_mean of bone marrow and CD38- and CD138-expressing myeloma cells and other parameters were analyzed by Spearman’s correlation test. Values of p<0.05 were considered statistically significant.

Results

Types of MM were IgG_K (45%), IgG_L (21%), IgA_K (7%), IgA_L (10%), and others (17%). Thirty-two (76%) patients were at stage III according to the Salmon-Durie staging system. There was a statistically significant positive correlation between bone marrow FDG uptake and percentage of plasma cells in bone marrow and CD38 and CD138 expression in plasma cells (r=0.403, r=0.339, and r=0.409) and β2-microglobulin and C-reactive protein levels (r=0.676, r=0.541). There was a negative correlation between bone marrow FDG uptake and hemoglobin and hematocrit values (r=-0.377 and r=-0.368). Other hematological parameters were not correlated with FDG uptake in bone marrow.

Conclusion

Increased FDG uptake is correlated with the percentage of CD38 and CD138 expression in plasma cells in bone marrow. In addition to initial staging, 18F-FDG PET/CT is useful in treatment planning and prognostic evaluation in MM patients.

Diagnosis of Plasma Cell Dyscrasias and Monitoring of Minimal Residual Disease by Multiparametric Flow Cytometry

Plasma cell dyscrasia (PCD) is a heterogeneous disease that has seen a tremendous change in outcomes due to improved therapies. Over the past few decades, multiparametric flow cytometry has played an important role in the detection and monitoring of PCDs. Flow cytometry is a high-sensitivity assay for early detection of minimal residual disease (MRD) that correlates well with progression-free survival and overall survival. Before flow cytometry can be effectively implemented in the clinical setting, sample preparation, panel configuration, analysis, and gating strategies must be optimized to ensure accurate results. Current consensus methods and reporting guidelines for MRD testing are discussed.

Curcumin ameliorates the in vitro efficacy of carfilzomib in human multiple myeloma U266 cells targeting p53 and NF-κB pathways

Multiple myeloma (MM) is a malignant B-cell neoplasm with accumulation of malignant plasma cells in bone marrow. Pharmacological therapy improves response frequency even if with various associated toxicities. Herein, we investigated if combination of curcumin with carfilzomib (CFZ) can induce a better cytotoxic effect on in vitro cultured U266 cells. Cell viability data showed that curcumin significantly ameliorates CFZ cytotoxic effect. Furthermore, curcumin alone did not affect proteasome at the tested dose, confirming the involvement of different mechanisms in the observed effects. U266 cells exposure to curcumin or CFZ increased reactive species (RS) levels, although their production did not appear further potentiated following drugs combination. Interestingly, NF-κB nuclear accumulation was reduced by treatment with CFZ or curcumin, and was more deeply decreased in cells treated with CFZ-curcumin combinations, very likely due to the different mechanisms through which they target NF-κB. Our results confirmed the induction of p53/p21 axis and G0/G1 cell cycle arrest in anticancer activities of both drugs, an effect more pronounced for the CFZ-curcumin tested combinations. Furthermore, curcumin addition enhanced CFZ proapoptotic effect. These findings evidence that curcumin can ameliorate CFZ efficacy, and lead us to hypothesize that this effect might be useful to optimize CFZ therapy in MM patients.

Targeting complete response with upfront bortezomib consolidation versus observation after the achievement of complete response following autologous transplantation for multiple myeloma (TUBA study)

Complete response (CR) after treatment for multiple myeloma is associated with superior progression-free survival (PFS). Multiple myeloma patients were prospectively recruited for induction treatment with bortezomib and dexamethasone (BD) followed by autologous hematopoietic cell transplantation (auto-HCT) between 2010 and 2012. If patients did not achieve CR after auto-HCT, BD consolidation therapy was added to target CR. After the BD induction phase (n = 46), greater than or equal to CR was achieved in 4 patients (8%). After auto-HCT (n = 34), greater than or equal to CR was achieved in 9 patients (20%) and very good partial response (VGPR) was achieved in 11 (24%). Of the 24 patients who received auto-HCT and whose response was less than CR, 21 received BD consolidation therapy for a median of 4 courses. Finally, the maximum response with or without BD consolidation was greater than or equal to CR in 19 (41%), VGPR in 7 (15%), and PR in 6 (13%). Through BD consolidation, CR was achieved in 8 of 11 patients with post-HCT VGPR and in 2 of 12 patients with post-HCT PR. In total, 4 year PFS and overall survival were 43 and 80%, respectively. After adjusting for clinical factors, there was no difference in PFS between CR patients after auto-HCT and BD consolidation, while patients with less than or equal to VGPR after consolidation had a significantly lower PFS. Patients with post-HCT CR showed good PFS, and targeting CR through BD consolidation could improve the CR rate. It would be worthwhile to prospectively compare the efficacy of consolidation only for patients who failed to achieve CR to a universal consolidation strategy.

DNA repair of myeloma plasma cells correlates with clinical outcome: the effect of the nonhomologous end-joining inhibitor SCR7

DNA repair activity of malignant cells seems to influence therapeutic outcome and patients' survival. Herein, we investigated the mechanistic basis for the link between DNA repair efficiency and response to antimyeloma therapy. Nucleotide excision repair (NER), interstrand cross-links repair (ICL/R), double-strand breaks repair (DSB/R), and chromatin structure were evaluated in multiple myeloma (MM) cell lines (melphalan-sensitive RPMI8226; melphalan-resistant LR5) and bone marrow plasma cells (BMPCs) from MM patients who responded (n = 17) or did not respond (n = 9) to subsequent melphalan therapy. The effect of DSB/R inhibition was also evaluated. Responders' BMPCs showed slower rates of NER and DSB/R (P <0022), similar rates of ICL/R, and more condensed chromatin structure compared with nonresponders. Moreover, apoptosis rates of BMPCs were inversely correlated with individual DNA repair efficiency and were higher in responders' cells compared with those of nonresponders (P = .0011). Similarly, RPMI8226 cells showed slower rates of NER and DSB/R, comparable rates of ICL/R, more condensed chromatin structure, and higher sensitivity than LR5 cells. Interestingly, cotreatment of BMPCs or cell lines with DSB/R inhibitors significantly reduced the rates of DSB/R and increased melphalan sensitivity of the cells, with the nonhomologous end-joining inhibitor SCR7 showing the strongest effect. Together, responders' BMPCs are characterized by lower efficiencies of NER and DSB/R mechanisms, resulting in higher accumulation of the extremely cytotoxic ICLs and DSBs lesions, which in turn triggers the induction of the apoptotic pathway. Moreover, the enhancement of melphalan cytotoxicity by DSB/R inhibition offers a promising strategy toward improvement of existing antimyeloma regimens.

Increased resistance to proteasome inhibitors in multiple myeloma mediated by cIAP2--implications for a combinatorial treatment

Despite the introduction of new treatment options for multiple myeloma (MM), a majority of patients relapse due to the development of resistance. Unraveling new mechanisms underlying resistance could lead to identification of possible targets for combinatorial treatment. Using TRAF3 deleted/mutated MM cell lines, we evaluated the role of the cellular inhibitor of apoptosis 2 (cIAP2) in drug resistance and uncovered the plausible mechanisms underlying this resistance and possible strategies to overcome this by combinatorial treatment. In MM, cIAP2 is part of the gene signature of aberrant NF-κB signaling and is heterogeneously expressed amongst MM patients. In cIAP2 overexpressing cells a decreased sensitivity to the proteasome inhibitors bortezomib, MG132 and carfilzomib was observed. Gene expression analysis revealed that 440 genes were differentially expressed due to cIAP2 overexpression. Importantly, the data imply that cIAPs are rational targets for combinatorial treatment in the population of MM with deleted/mutated TRAF3. Indeed, we found that treatment with the IAP inhibitor AT-406 enhanced the anti-MM effect of bortezomib in the investigated cell lines. Taken together, our results show that cIAP2 is an important factor mediating bortezomib resistance in MM cells harboring TRAF3 deletion/mutation and therefore should be considered as a target for combinatorial treatment.

Pooled analysis of pomalidomide for treating patients with multiple myeloma

BACKGROUND

Patients with refractory or relapsed multiple myeloma are considered to have a very poor prognosis, and new regimens are needed to improve this setting. Pomalidomide is a new immunomodulatory drug with high in vitro potency. Immunomodulatory drugs are hypothesized to act through multiple mechanisms. Here we performed a systemic analysis to evaluate pomalidomide-based chemotherapy (pomalidomide in combination with low-dose dexamethasone) as salvage treatment for patients with refractory and relapsed multiple myeloma.

METHODS

Clinical studies evaluating the efffectiveness of pomalidomide based regimens on response and safety for patients with refractory and relapsed multiple myeloma were identified using a predefined search strategy. Pooled response rate (RR) of treatment were calculated.

RESULTS

For pomalidomide based regimens, 4 clinical studies which including 291 patients with refractory and relapsed multiple myeloma were considered eligible for inclusion. Systemic analysis suggested that, in all patients, pooled RR was 41.2% (120/291). Major adverse effects were hematologic toxicity, including grade 1 or 2 anemia, leucopenia and thrombocytopenia with pomalidomide based treatment. No treatment related death occurred.

CONCLUSION

This pooled analysis suggests that pomalidomide in combination with low-dose dexamethasone is active with good tolerability in treating patients with refractory or relapsed multiple myeloma.

Synergistic Activity of Carfilzomib and Panobinostat in Multiple Myeloma Cells via Modulation of ROS Generation and ERK1/2

Relapse of disease and subsequent resistance to established therapies remain as major challenges in the treatment of multiple myeloma (MM). New therapeutic options are needed for these extensively pretreated patients. To explore an optimized combinational therapy, interactions between the irreversible proteasome inhibitor carfilzomib exhibiting a well-tolerated side-effect profile and histone deacetylase inhibitor (HDACi) panobinostat (LBH589) were examined in MM cells. Coadministration of carfilzomib and LBH589 led to a synergistic inhibition of proliferation in MM cells. Further studies showed that the combined treatment synergistically increased mitochondrial injury, caspase activation, and apoptosis in MM cells. Lethality of the carfilzomib/LBH589 combination was associated with the reactive oxygen species (ROS) generation and ERK1/2 inactivation. In addition, the free radical scavenger N-acetylcysteine (NAC) could block carfilzomib and LBH589-induced oxidative stress and the subsequent apoptosis. Together, these findings argue that the strategy of combining carfilzomib and LBH589 warrants attention in MM.

Tretatment approach of nontransplant patients with multiple myeloma

Multiple myeloma is still an incurable disease with pattern of regression and remission followed by multiple relapses raising from the residual myeloma cells surviving even in the patients who achieve complete clinical response to treatment. In recent years there is a huge improvement in treatment of patients with multiple myeloma. The milestones of these improvement are: autologous transplantation and high-dose melphalan, imunomodulating drugs (thalidomide, lenalidomide), proteosom inhibitors (bortesomib, carfilzomib). The most significant improvement in overall survival has been achieved in the patients younger than 65 years. So, the major challenge for hematologist is to translate this improvement in the elderly patients with multiple myeloma. Today, physicians are able to offer wider variety of treatment options for elderly patients with multiple myeloma. Therapeutic options should be tailored and personalized according to patient's characteristics by balancing efficacy and toxicity of each drug which is especially important for elderly patients. In the mode of sequencing treatment for elderly patients with multiple myeloma, our goal is to achieve and maintain maximal response while limiting treatment -related toxicities as much as possible. Second-generation novel agent, such as carfilzomib, pomalidomide, elotuzumab, bendamustine are currently being evaluated as an option to improve treatment outcome in elderly patients.

Arsenic trioxide-based therapy in relapsed/refractory multiple myeloma patients: a meta-analysis and systematic review

Multiple myeloma (MM) is a clonal malignancy characterized by the proliferation of malignant plasma cells in the bone marrow and the production of monoclonal immunoglobulin. Although some newly approved drugs (thalidomide, lenalidomide, and bortezomib) demonstrate significant benefit for MM patients with improved survival, all MM patients still relapse. Arsenic trioxide (ATO) is the most active single agent in acute promyelocytic leukemia, the antitumor activity of which is partly dependent on the production of reactive oxygen species. Due to its multifaceted effects observed on MM cell lines and primary myeloma cells, Phase I/II trials have been conducted in heavily pretreated patients with relapsed or refractory MM. Therapy regimens varied dramatically as to the dosage of ATO and monotherapy versus combination therapy with other agents available for the treatment of MM. Although ATO-based combination treatment was well tolerated by most patients, most trials found that ATO has limited effects on MM patients. However, since small numbers of patients were randomized to different treatment arms, trials have not been statistically powered to determine the differences in progression-free survival and overall survival among the experimental arms. Therefore, large Phase III studies of ATO-based randomized controlled trials will be needed to establish whether ATO has any potential beneficial effects in the clinical setting.

Myeloma cells resistance to NK cell lysis mainly involves an HLA class I-dependent mechanism

The anti-multiple myeloma (MM) potential of natural killer (NK) cells has been of rising interest in recent years. However, the molecular mechanism of NK cell cytotoxicity to myeloma cells remains unclear. In the present study, we investigated the expressions of human leukocyte antigen (HLA) class I and HLA-G in patient myeloma cells, and determined their relevance in patient tumor-cell susceptibility to NK cell cytotoxicity. Our results showed that patient myeloma cells (n = 12) were relatively resistant to NK-92 cell lysis, compared with myeloma cell lines (n = 7, P < 0.01). Gene expression profiling and flow cytometry analysis showed that both mRNA and protein of HLA class I were highly expressed in 12 patient myeloma cells. Interestingly, no or low HLA-G surface expression was detected, although multiple HLA-G transcripts were detected in these myeloma cells. NK cell function assay showed that down-regulating HLA class I expression on patient cells by acid treatment significantly increased the susceptibility of MM cells to NK-mediated lysis. Furthermore, we found that the blocking of membrane-bound HLA class I rather than HLA-G using antibodies on myeloma samples markedly increased their susceptibility to NK-mediated killing. These results demonstrated that the resistance of patient MM cells to NK lysis mainly involves an HLA class I-dependent mechanism, suggesting that HLA class I may be involved in protecting MM cells from NK-mediated attack and contribute to their immune escape in vivo.