FLC Assay and Multiparameter Flowcytometry Based Clonal Plasma Cell Measurement Are Independent but Complementary in Assessing the Treatment Response in Multiple Myeloma.

Although stringent complete response (sCR) defined by paraprotein negativity on immunofixation and serum free light chain (sFLC) ratio normalization are considered deeper responses in the IMWG criteria, recent report indicated that Multiparameter flow cytometry (MFC)-dased immunophenotypic response (IR) is a more relevant prognostic factor in MM patients. However, data on the prognostic impact of IR and sFLC ratio (sFLCκ/λ) normalization are still scarce. We investigated the prognostic impact of IR and sFLCκ/λ normalization in MM patients treated with novel agents.

A total of 124 consecutive patients (M:F=68:56; median age, 71 yr) were treated by chemotherapy regimens containing at least one novel agent (thalidomide, bortezomib, lenalidomide)from April 2005 to May 2012. Treatment responses were assessed using the IMWG criteria, and the best response to treatment during the clinical course was assessed by simultaneous serum immunofixation, sFLC measurements, and MFC analysis of bone marrow (BM) plasma cells. Normalization of sFLCκ/λ was defined 2 consecutive normal sFLCκ/λ apart from at least 4 weeks. MFC-defined minimal residual disease (MRD) was evaluated by single-tube 6-color MFC, CD45-CD38 gating strategy, and combination CD19, CD56, and cytoplasmic κ-λ analysis. Clonal plasma cell (PC) negativity by MFC (MFC-negative) was defined as <10⁻⁴ neoplastic PCs in BM samples on MFC. Overall survival (OS) and progression-free survival (PFS) were analyzed by the Kaplan–Meier (K-M) method and differences between curves were calculated by two-sided log-rank test. Univariate analysis was used to assess the impacts of factors on sFLCκ/λ normalization and MFC negativity (age, Durie–Salmon stage, ISS stage, LDH, hemoglobin, serum albumin, serum creatinine, FISH at diagnosis). The Cox regression proportional hazard model (stepwise regression) was used to explore the independent effects of these variables on PFS and OS.

At a median follow-up of 25.8 months, 3- and 5-year OS of all patients were 61.0% and 42.4%, respectively. CR was obtained in 25% (31/124), very good partial response (VGPR) in 33.5% (41/124), partial response (PR) in 30.5% (38/124), and stable disease or less (SD) in 11% (14/124). Normal sFLCκ/λ was achieved in 81% of CR, 56% of VGPR, 13% of PR, and 0% of SD or less response of patients. K-M estimated 3- and 5-year OS were 100% in CR patients; these were significantly better than in VGPR (75.8% and 43.2%, respectively) and PR patients (63% and 26.7.%, respectively). There were no significant differences in 3- or 5-year OS between VGPR and PR patients. Normal sFLCκ/λ and MFC negativity were achieved in 25 (81%) and 18 (58%) of 31 CR patients, respectively. Among 25 CR patients with normal sFLCκ/λ (stringent CR), 15 (60%) were MFC-negative and 10 (40%) were MFC-positive; three of 6 CR patients (50%)without normal sFLCκ/λ were MFC-positive. Twenty-three of 41 VGPR patients (56%) obtained normal sFLCκ/λ, while only 5 (12%) became MFC-negative; all 5 MFC-negative patients also obtained normal sFLCκ/λ. Among 52 patients with less than PR, only 5 (9.6%) obtained normal sFLCκ/λ and none achieved MFC negativity. Patients with MFC-negative CR showed significantly better PFS than patients with MFC-positive CR (p<0.05). Although patients in stringent CR with MFC-negative showed slightly better PFS compared to patients in stringent CR with MFC-positive, difference between the curves were not significant. Within the group of VGPR, PFS and OS were significantly longer in normal sFLCκ/λ patients than abnormal sFLCκ/λ(P<0.001). Univariate analysis showed that hemoglobin 10.0 g/dl>, age >70 yr, and abnormal LDH had negative prognostic impacts on attaining normal sFLCκ/λ, but none of these factors remained significant on multivariate analysis. Cox analysis showed that sFLCκ/λ normalization was an independent prognostic factor for longer PFS and OS in patients with CR, VGPR and PR (P=0.001).

This study confirmed that magnitude of CR and VGPR response defined by IMWG criteria was heterogeneous in terms of sFLCκ/λ normalization and MFC negativity. Although MFC and sFLC analysis frequently gave discrepant results among patients with CR and VGPR, both analyses appeared to give important complementary information for assessing the depth of CR and VGPR category.

No relevant conflicts of interest to declare.