Lust, John A.

Abstract Background: Thalidomide is an active anti-myeloma agent with single agent activity in all stages of the disease. When combined with dexamethasone disease response is seen in nearly two-thirds of patients with newly diagnosed MM. Though initially evaluated for treatment because of its anti-angiogenic properties and the finding of increased bone marrow angiogenesis in MM, it has become clear that thalidomide exerts its anti-myeloma activity through several different mechanisms. While the mechanism of action of dexamethasone has been studied to a great extent in the setting of myeloma, the molecular basis for the activity of this combination is largely unknown. Methods: Myeloma cells from patients with newly diagnosed MM who were enrolled in a phase III trial (ECOG E1A00) comparing thalidomide and dexamethasone combination (T+D) with dexamethasone alone (D) were studied by gene expression profiling. Samples from diagnosis were compared to those at four months following therapy to study the effect of treatment. Four patients who achieved a partial response with T+D were studied and in addition compared to four patients achieving a partial response with D alone. Plasma cells from bone marrow aspirates were separated by magnetic bead selection of CD138 positive cells and studied using Affymetrix HG-U133A chips using standard methodology. The arrays were analyzed using Genespring 7.2 software following GCRMA normalization and genes with differential expression between the two datasets were examined. Differentially expressed genes were further analyzed using Ingenuity Pathways Analysis program Results: 549 genes were two fold or more differentially expressed between the pretreatment and post treatment samples in the T+D group, of which 275 genes were mapped to different networks using the Ingenuity program. Examination of the canonical pathways demonstrated several pathways that were altered as a result of the therapy. The most significant pathways included those related to immune function including antigen presentation (multiple HLA class II antigens), and PRG2 (NK cell activator). Several mediators of B cell signaling including CD45, PKC (α ,β), SHP2, and Erk1/2 were up regulated following treatment. Increase in the expression of several adhesion molecules including β integrins, fibronectin, VCAM1, L selectin, and LFA3 were noted. Up regulation of IL-6 signaling pathway including gp130, JAK2 and IL8 were noted following therapy. Striking increase in the gene expression for CXCR4 and its ligand SDF-1α was noted as well. 129 genes were differentially regulated by dexamethasone alone, of which 67 were mapped to different networks. The pathways influenced by the dexamethasone alone differed significantly from that of the combination. Conclusion: This exploratory analysis offers an interesting insight into the potential mechanisms of action of the T+D combination in MM. The up regulation of immune response related genes are in accordance with some of the mechanisms described for thalidomide. Also, the differences observed between the patients receiving T+D vs. D allows for assessment of the effects of thalidomide. These findings need confirmation in a larger study.

Abstract Background: Although microvessel density increases significantly from monoclonal gammopathy of undetermined significance (MGUS) to multiple myeloma (MM), we have been unable to demonstrate an increase in expression of VEGF by isolated plasma cells between MGUS and MM (add your Blood ref). When MM cell lines are grown in co-culture with marrow derived stromal cells there is upregulation of VEGF and IL6. This effect is thought to be mediated primarily by the stromal cells, although plasma cells can also secrete these cytokines to form an autocrine pathway. We examined if there is differential expression of VEGF when plasma cells derived from various stages of plasma cell disorders (MGUS, smoldering multiple myeloma (SMM), and MM) are studied in co-culture with stromal cells. Methods: Plasma cells were obtained from fresh marrow aspirates from patients with MGUS, SMM or MM. Normal control plasma cells were obtained at the time of orthopedic surgeries. Plasma cells were isolated from whole marrow using a magnetic bead separation technique with antibodies against CD138. The marrow stromal cells were obtained from an established stromal cell line from a patient with MGUS. Plasma cells were then grown in co-culture with stromal cells, with control wells having either only stromal cells or only plasma cells, with RPMI medium containing 10% fetal bovine serum. The supernatant was removed after 48 hour cultures, spun down to remove cells and frozen at −70C. VEGF and IL6 levels were determined using ELISA kits (R&D, Minneapolis, MN). Results: A total of 71 samples were studied including normals (10), MGUS (8), SMM (15) and MM (38). The median VEGF levels with plasma cells alone were 30.2 pg/mL (range; 16–132), with stromal cells alone was 737 pg/ml (316–1826) and with the co-culture was 1057 pg/mL (306–4267), representing a median increase of 41% (−48% to 331%) with co-culture. There was no significant difference between the disease groups in terms of the baseline VEGF secretion by plasma cells or the increase in VEGF with coculture (Figure). The median IL-6 levels with plasma cells alone were 22.3 pg/mL (range; 4–276), with stromal cells alone was 2457 pg/ml (906–5359) and with the co-culture was 5982 pg/mL (301–129,000), representing a median increase of 99% (−67% to 3362%) with co-culture. There was no significant difference between the disease groups in terms of the baseline VEGF secretion by plasma cells or the increase in VEGF with coculture (Figure). Conclusions: In this study we clearly demonstrate the upregulation of VEGF and IL-6 when primary cells are grown in contact with marrow derived stromal cells validating previous observations with cell lines, and once again highlighting the importance of these cytokines and the marrow microenvironment. However, since there is no significant difference between the various groups studied, higher absolute levels of VEGF and IL-6 in MM may be solely related to the effect of an increased tumor burden on marrow stromal cells. However, this does not exclude differences between disease stages in terms of the marrow stromal cells, which we are currently studying. Figure Figure

Abstract Background: While the therapeutic armamentarium for plasma cell disorders have expanded recently with introduction of thalidomide, lenalidomide and bortezomib, these diseases are still incurable and more agents are needed. We have previously demonstrated expression of CD 52 on plasma cells in MM, MGUS and AL amyloidosis. Based on this data we conducted a Phase II Study of Campath (Alemtuzumab; Monoclonal Anti-CD52 Antibody) in Primary Systemic Amyloidosis and Relapsed or Refractory Multiple Myeloma. Methods: The study was an IRB approved, Phase II protocol, with a two stage design and interim analysis at 14 patients, to assess the efficacy and toxicity of Alemtuzumab. The regimen involved a dose escalation during week one (Day I 3 mg SQ; Day II 10 mg SQ; Day III 30 mg SQ; then 30 mg SQ three times a week for a planned total of 17 additional weeks). All patients received concurrent Co-Trimoxazole, Fluconazole and Valacyclovir prophylaxis. The planned accrual was 32 patients. Results: A total of 14 patients were accrued, 8 patients with MM and 6 patients with AL. There were 8 females and 6 males in the study. The median age was 62 years (range: 50–78) years. The patients were heavily pretreated with the median number of treatments being 4. There were 5 grade 3–4 toxicities (CMV viremia (2 pts), skin rash (2 pts), and febrile neutropenia (1 pt)). Seven patients had evidence of disease progression and were removed from study. One patient completed the treatments with a mixed response. One patient had a sudden death while on study. The median duration of therapy was 6 weeks (range 3 weeks to 17 weeks). The accrual was stopped after 14 patients when the interim analysis showed a lack of anti-tumor efficacy and significant toxicity. Conclusion: This phase II study in patients with relapsed / refractory myeloma or amyloidosis demonstrated no evidence of clinical activity with use of Campath. In addition it was also associated with significant toxicity in this population. We do not recommend use of campath as a primary treatment modality in these patients. The lack of efficacy despite demonstrated expression of CD52 on clonal plasma cells once again highlights the need for combining treatments with multiple mechanisms in order to improve the response and outcome in these patients. Whether it can synergize with other therapies at lower doses without significant toxicity remains unknown.

Abstract Background: We reported a high response rate with the combination of lenalidomide plus dexamethasone (Rev/Dex) as initial therapy for newly diagnosed multiple myeloma (Blood2005;106:4050–3). We now present new data on time to progression (TTP), progression free survival (PFS), and overall survival (OS) from this phase II trial, and also include updated response data. Patients and Methods: 34 patients (23 male and 11 female) were enrolled. Lenalidomide was given orally 25 mg daily on days 1–21 of a 28-day cycle. Dexamethasone was given orally at a dose of 40 mg daily on days 1–4, 9–12, 17–20 of each cycle. Patients were allowed to go off treatment after 4 cycles of therapy to pursue autologous stem cell transplantation (SCT), but treatment beyond four cycles was permitted at the physician’s discretion. For patients continuing therapy beyond 4 months, the dose of dexamethasone was reduced to 40 mg on days 1–4 of each cycle. Response was assessed by modified EBMT/International Myeloma Working Group Uniform Response criteria. All patients received aspirin (81 mg or 325 mg daily) as prophylaxis against DVT. Results: The median age was 64 years (range, 32–78). All patients were evaluable for response and toxicity. Median follow up is 21 months. Thirteen patients proceeded to SCT following initial therapy with Rev/Dex and were censored at that time point for purposes of calculation of response, TTP and PFS. Patients who discontinued therapy to proceed to SCT received a median of 4 cycles of therapy (range, 4–13), while those staying on Rev/Dex (n=21) received a median of 19 cycles of therapy (range, 2–30). Thirty-one of 34 patients (91%) achieved an objective response to therapy; including 6 patients (18%) achieving a complete response (CR) and 13 patients (38%) achieving very good partial response for a CR+VGPR rate of 56%. The CR+VGPR rate among the 21 patients staying on Rev/Dex as primary therapy without SCT was 67% (CR 24%, VGPR 43%). Median TTP, PFS, and OS have not been reached (Figure). By Kaplan-Meier method, the estimated 2 year progression rate was 18%. The 2-year PFS rate and OS rate were 74% and 91%, respectively. Fifty-five percent of patients experienced grade 3 or higher non-hematologic toxicity at any point during therapy, most commonly fatigue (21%), neutropenia (21%), anxiety (6%), pneumonitis (6%), muscle weakness (6%), and rash (6%). Two patients died on study: one attributed to infection unrelated to therapy, the patient had stopped all therapy for over a month before the fatal infection occurred; the other death was due to infection felt possibly related to therapy. One patient developed a pulmonary embolism (grade 4 toxicity), but recovered with therapy; no other patient developed deep vein thrombosis or pulmonary embolism. Conclusion: Rev/Dex is highly active and well tolerated in the treatment of newly diagnosed multiple myeloma with a high CR+VGPR rate of 56% for the trial, and 67% among the subset of patients receiving this regimen as primary therapy. Responses are durable with a low progression rate at 2 years.

Abstract Background: Patients with SMM/IMM are at high risk for progression to active MM and are appropriate candidates for chemoprevention trials. High IL-1beta levels are a useful surrogate marker for progression from SMM/IMM to active myeloma. Methods: We carried out a Phase II clinical trial of IL-1Ra (Anakinra) in patients with SMM/IMM to determine the biologic activity, progression-free rate, and toxicity of IL-1Ra. Since IL-1beta induces paracrine IL-6, we hypothesized that IL-1Ra would inhibit IL-6 production and myeloma cell growth. Patients that had ≥ 10% bone marrow plasma cells and/or an IgG or IgA M-spike ≥ 3 g/dL and did not require immediate chemotherapy were eligible. Patients received 100 mg of Anakinra (IL-1Ra) SQ qd for a total duration of 6 months. Non-progressors were allowed to continue on therapy with IL-1Ra until they converted to active myeloma. IL-1beta levels were measured in a bioassay with a read out of IL-1 inducible IL-6 production. CRP levels served as a surrogate for IL-6 production. Results: Thirty-six patients are included for analysis based on intent to treat at diagnosis. At baseline, twenty-nine patients had elevated functional IL-1beta levels consistent with progressive disease and 12 patients had radiologic evidence of disease on bone survey. The median TTP for the entire group was 1 year and 2 patients exhibited a minor response based on M-protein reduction. A proportional hazards analysis was performed and the variables examined were: % bone marrow plasma cells, plasma cell labeling index (PCLI), circulating plasma cells, albumin, M-protein level, IgA subtype, CRP, CRP % reduction ≥ one-third, beta-2 microglobulin, creatinine, calcium, hemoglobin, urine total protein, presence of lytic bone disease, and IL-1 level. Univariate Cox model results showed that % BMPC, PCLI, circulating plasma cells, CRP % reduction, presence of lytic disease, and IL-1 level were all statistically significant (p < .05) variables. However, only the CRP % reduction from baseline remained significant in the multivariate analysis (p < 0.01). The median TTP for patients without (n=18) and with (n=18) a decrease in CRP was 6 months and > 2 years, respectively (p < .0001). Toxicities included injection site reactions during the first month of therapy and asymptomatic neutropenia necessitating a reduction in therapy to every other day. Five patients in whom the CRP decreased and the baseline PCLI was > 0 also demonstrated a parallel decrease in the PCLI. Several of the patients with progressive disease on IL-1Ra alone could later be rescued with the addition of low dose dexamethasone (20 mg once a week) suggesting that the endogenous IL-1beta levels were too high to inhibit with IL-1Ra alone. Conclusion: In summary, IL-1Ra can prolong the TTP to active myeloma in responsive SMM/IMM patients through inhibition of IL-6 production (decrease in CRP) and myeloma cell growth. Figure Figure

Abstract Background: Primary systemic amyloidosis (AL) is an incurable plasma cell disorder. For selected patients high dose chemotherapy with peripheral blood stem cell support is effective, but even in those patients, only 50% derive organ responses. Other patients are too sick to undergo that therapy. Thalidomide has limited utility in this disease, largely because of its toxicity profile in patients with AL. Lenalidomide, Celgene’s lead clinical compound in a new group of drugs called IMiDs®, is highly active in patients with multiple myeloma, especially in conjunction with dexamethasone. We sought to determine the toxicity and efficacy of lenalidomide in patients with AL. Methods: Patients with symptomatic AL who had a measurable plasma cell disorder (defined as serum M-spike ≥ g/dL urine M-spike >200 mg/24 hours; or involved immunoglobulin free light chain (FLC) ≥10 mg/dL and an abnormal FLC ratio) and adequate organ reserve defined as a creatinine ≤3 mg/dL, absolute neutrophil count ≥1000, platelet count ≥75000, and a hemoglobin ≥8 g/dL were eligible. Patients were started on lenalidomide 25 mg/day for 21 days followed by a 7 day rest (1 cycle). Dose modifications were made based on toxicity. If there was evidence of progression before 3 months or no evidence of hematologic response after 3 cycles, dexamethasone 40 mg p.o. days 1–4 and 15–18 was added. Results: Twenty-three patients were enrolled between 10/28/04 and 4/7/05; 14 were previously treated. Patient characteristics are shown in Table. Organ involvement was as follows: cardiac (61%); renal (70%), hepatic (22%); nerve (13%). Nine patients withdrew from study before completing 3 months of therapy. The reasons were: cancel (1); death (4); adverse events (2); and progression (2). At the time of our preliminary analysis in July 2005, an additional 3 patients have withdrawn from study: death (1) and patient refusal (2). Of the 5 patients who died, 4 had severe cardiac involvement and at least 3 organs involved by amyloid. The median follow-up for the eleven patients remaining on study is 6.2 months. Of the 12 patients who have crossed the 3 month treatment landmark, there are 7 hematologic partial responses (4 confirmed and 3 unconfirmed), two renal responses and one liver response. Of these same 12 patients, all but one has had dexamethasone added to their treatment program. The most common grade 3–4 adverse advents at least possibly attributable to lenalidomide were neutropenia (43%), thrombocytopenia (26%), rash (17%), dyspnea (9%), fatigue (9%), and edema (4%). Expansion of the trial is planned to accrue an additional 10 evaluable patients. Conclusions: Early results suggest that lenalidomide ± dexamethasone has activity in patients with primary systemic amyloidosis. Supported in part by the Caliguiri Fund for Amyloidosis Research, Robert A. Kyle Hematologic Malignancies Program, and Celgene. Patient Characteristics Median Range Age 64 44–88 Major Organs 2 0–3 Involved FLC, mg/dl 23 4.1–278 Serum alb, g/dl 2.8 1.2–3.7 Urine protein, g/24 hrs 3.7 0.02–14.3 Creatinine, mg/dl 1.3 0.7–2.6 Alkaline phos IU/l (nml <115) 99 57–1729 NY Heart Class 1 1–3 Troponin T, mcg/l 0.03 0.01–0.55 NT-proBNP, ng/l 1419 109–42844 LV Septum, mm 13 9–24 LV EF, % 63 22–72

Abstract Background: Multiple myeloma (MM) is a plasma cell malignancy that is incurable with current approaches. The median survival for patients with MM is around four years and a significant proportion of patients experience a course characterized by multiple relapses treated with different therapies. The median survival for patients relapsing after the initial therapy is nearly 18 months and successive treatment strategies result in decreasing response durations, likely reflecting acquired drug resistance. In order to better understand the biological changes associated with advanced, relapsed, refractory MM, we compared gene expression profiles (GEP) of malignant plasma cells isolated from patients with relapsed refractory MM and compared them to plasma cells from patients with newly diagnosed MM. Methods: In order to obtain two relatively homogenous group of patients, we compared samples from 44 patient with newly diagnosed MM enrolled in the ECOG E1A00 clinical trial (comparing thalidomide and dexamethasone to dexamethasone alone) to 44 patients with relapsed refractory MM enrolled in a phase II trial of Velcade (SUMMIT), where most patients had four or more previous relapses. Plasma cells from bone marrow aspirates were separated by magnetic bead selection of CD138 positive cells and studied using Affymetrix HG-U133A chips using standard methodology. The arrays were analyzed using Genespring 7.2 software following GCRMA normalization and genes with differential expression between the two datasets were examined. Differentially expressed genes were further analyzed using Ingenuity Pathways Analysis program. Results: A total of 864 genes were identified which were at least two fold and significantly different between the newly diagnosed and relapsed patients. Using Ingenuity software, 437 of these genes were mapped to different biological networks. Examination of the canonical pathways demonstrated several important cellular pathways differentially regulated between the two groups. Several important mediators of the cytokines, receptors and respective signaling pathways appear to be down regulated in the relapsed group and included IGF-1, HGF, SDF-1 alpha, gp130 and importantly the MEK/ERK pathway. Additionally expression of adhesion molecules such as VCAM1 and PECAM was decreased in the relapsed group compared to newly diagnosed pts. There appear to increased tissue hypoxia in the relapsed marrow as indicated by up regulation of HIF-1 alpha as well increased levels of Placental growth factor. Myeloma cells from relapsed disease were characterized by decreased expression of mcl1, FLIP1, and bcl-xL and increased caspase 8 relative to newly diagnosed group. Also seen was decreased expression of the glucocorticoid and interferon receptors in the relapsed setting. Conclusion: Comparison of the GEP between MM cells from newly diagnosed and relapsed pts demonstrates important differences that have potential biological relevance. The plasma cell in the relapsed setting appears to be more independent of the tumor microenvironment. Additionally, differential expression of some of the genes provides clues to mechanisms of drug resistance commonly observed in the relapsed pts. We are in the process of validating some of the key findings from these analyses.

Abstract Background: Multiple myeloma (MM) is an incurable hematological malignancy characterized by the expansion of a plasma cell clone that localizes to the bone marrow. Stromal cells residing in the bone marrow respond to signals from MM cells and other cell types by producing cytokines and other proteins that stimulate tumor cell growth, survival, adhesion, migration, and drug resistance. We have examined the proteins produced by stromal cells in response to stimulation by bone marrow from patients diagnosed with monoclonal gammopathy of undetermined significance (MGUS), smoldering multiple myeloma (SMM), and MM. Furthermore, we have begun analyzing the properties of one of these proteins, the pro-angiogenic chemokine IL-8, in MM. Methods: Bone marrow aspirates from patients with MGUS (n=3), SMM (n=7), and MM (n=6) were cultured for 48 hours, and the culture supernatants were incubated with stromal cells for an additional 48 hours. Protein levels were analyzed using antibody array and ELISA. Microvessel density (MVD) was determined as a measure of angiogenesis in patient bone marrow samples using CD34 staining. Flow cytometry analysis of MM cell lines and patient bone marrow samples was performed using monoclonal antibodies against IL-8 receptors CXCR1 and CXCR2. Results and Conclusion: We observed a significant increase in stromal cell IL-8 production stimulated by bone marrow cells from patients with active myeloma and a subset of SMM patients (16.67 ± 9.82 ng/ml) in comparison to bone marrow of patients with MGUS and all other SMM patients (0.55 ± 0.17 ng/ml; P=0.0004). Use of an IL-1 inhibitor and recombinant IL-1β demonstrated that IL-8 production was dependent upon IL-1β signaling. Increased BM microvessel density correlated with stimulation of stromal cell IL-8 production (P=0.0005). Furthermore, the majority of MM cell lines (7/9) and MM patient plasma cells were found to express IL-8 receptors CXCR1 and CXCR2. In addition to its function as a pro-angiogenic factor, IL-8 may directly influence MM cells through its CXCR1 and CXCR2 receptors. We conclude that stromal cell IL-8 production parallels MM disease activity, is IL-1β induced, correlates with bone marrow angiogenesis, and may influence MM disease via impact upon both the microenvironment and tumor cells.

Abstract Background: Primary systemic amyloidosis (AL) is an incurable plasma cell disorder. Lenalidomide, especially in conjunction with dexamethasone, has been shown to be highly active in patients with multiple myeloma. Methods: We studied the toxicity and efficacy of lenalidomide in patients with symptomatic AL. Patients received single agent lenalidomide. If progression by 3 months or no evidence of hematologic response after 3 cycles, dexamethasone was added. Originally, twenty-three patients (Cohort 1) were enrolled according to study design. Because of a significant early drop out rate and notable activity of the regimen, the trial was modified to include an additional 15 patients (Cohort 2). Baseline characteristics and adverse events are available for all enrolled patients, but at the time of this writing, response data are available for Cohort 1 patients due to short follow-up of Cohort 2, but will be updated by the time of the meeting. Results: Median age was 64 years, with 69% male. Twenty-three were previously treated. Organ involvement was cardiac (67%), renal (64%), hepatic (17%), nerve (17%). Thirty-three, twenty-two, and forty-four percent of patients were cardiac biomarker stage 1, 2, and, 3 respectively. Of the 37 patients, one was a cancel, and 6 have not yet made it through 3 months of protocol treatment and event monitoring. The respective median follow-ups for Cohorts 1 and 2 are 17 and 3.4 months. Of the remaining, 30 patients, within the first 3 cycles of therapy fifteen patients discontinued treatment: 7 early deaths and 8 adverse events or other causes. Three additional patients died 0.5 to 2 months after stopping treatment. The best predictor for early withdrawal and/or death was baseline NT-proBNP and cardiac biomarker staging system (cut-offs for serum troponin T <0.035 ng/ml and NT-proBNP <332 pg/ml--Stage I neither above cut-off; Stage III, both above cut-off; and Stage II, one above cut-off). Figure1 Figure1. Figure 2 Figure 2. Of the twenty 22 patients assessed for response ten patients responded to treatment for an overall response rate of 45%, including 23% organ responders. Among the patients with organ responses, there were four renal responses, two cardiac responses and two liver responses. All but one of the responders had dexamethasone added to their treatment program. The most common grade 3−4 adverse events at least possibly attributable to lenalidomide were neutropenia (38%), thrombocytopenia (21%), fatigue (15%), and rash (12%). Conclusions: Lenalidomide and dexamethasone has significant activity in patients with AL with an overall response rate of 45%. Baseline NT−proBNP may be an effective eligibility screening strategy for subsequent trials.

Abstract Background: The serum free light chain (FLC) assay is increasingly used to monitor patients (pts) with oligo-secretory or non-secretory multiple myeloma (MM) and pts with primary amyloidosis lacking measurable monoclonal protein in the serum or urine. Criteria to use this assay to assess response to therapy have recently been proposed (Rajkumar SV, Kyle RA. Best Pr Clin Haematol2005;18:585–601) but have not been validated. The goal of this study was to validate the response criteria for the FLC assay in a prospective trial of lenalidomide plus dexamethasone in newly diagnosed MM. Methods: 34 pts were enrolled in the trial; 27 pts who had serial FLC assessments were studied. FLC estimation was carried out using the serum FLC assay (FreeliteH, The Binding Site Limited, UK) performed on a Dade-Behring Nephelometer. Pts with κ /λ FLC ratio <0.26 were defined as having monoclonal λ FLC and those with ratios >1.65 as having a monoclonal κ FLC. The monoclonal light chain isotype was considered the “involved” FLC isotype, and the opposite light chain type as the “uninvolved” FLC type. Partial response (PR) required an abnormal baseline FLC ratio and any one of the two following criteria: 1) a 50% decrease in the level of the involved FLC plus a 50% decrease (or normalization) in the ratio of involved/uninvolved FLC or 2) 50% decrease in the difference between involved and uninvolved FLC levels. Complete response (CR) required normalization of FLC ratio and negative serum and urine immunofixation. Response at 4 months or earlier by the Bladé criteria was compared to FLC response criteria from the same evaluation. Results: Three pts had normal FLC levels and ratio and were not included in the analysis. 23 of the remaining 24 achieved a PR or better by Bladé criteria. A 50% decrease in the level of the involved FLC plus a 50% decrease (or normalization) in the ratio of involved/uninvolved FLC correctly classified 20 of the 22 responding pts (sensitivity 91%); 2 pts could not be evaluated since baseline FLC ratio could not be calculated. On the other hand, a 50% decrease in the mathematical difference between involved and uninvolved FLC levels correctly classified all 24 responding pts (sensitivity 100%). The one non-responding pt by Blade criteria was correctly classified by both FLC criteria. All pts were correctly classified by both criteria when only those with a baseline “involved” FLC level of at least ≥10mg/dL (≥100mg/L) were considered (15 pts). Conclusions: This study demonstrates that the serum FLC assay can be used to assess response to therapy. A 50% decrease in the difference between “involved” and “uninvolved” FLC levels will suffice as FLC criteria for PR, eliminating the need for the alternative criteria based on the involved FLC level and the ratio. We recommend that this FLC response criteria be used only in pts not having measurable levels of serum and /or urine M protein.The FLC response criteria will now enable most patients with oligo-secretory and non-secretory MM to enter trials for which they are currently ineligible due to “lack of measurable serum or urine M protein.” This study is limited by the lack of adequate non-responders to calculate specificity and lack pts who progressed to validate progression criteria and needs further validation.