P834: multiparametric flow cytometry for the detection of circulating tumor cells in patients with igm monoclonal gammopathies

Topic: 13. Myeloma and other monoclonal gammopathies - Biology & Translational Research Background: The development of “liquid biopsies” presents new opportunities for non-invasive monitoring of clonal heterogeneity. Optimal characterization of circulating tumor cells (CTCs) may represent a non-invasive method to capture relevant mutations present in plasma-cell clones, avoiding bone marrow (BM) biopsies that are painful and cannot be repeated multiple times during the course of therapy. Aims: Building upon these developments, we have performed multiparameter flow cytometry to detect clonal cells in the bone marrow and CTCs in the peripheral blood (PB) of IgM-monoclonal gammopathies. Methods: BM and PB samples were lysed following the Euroflow-proposed Pharmlyse protocol and cells obtained were stained with established phenotypic markers for the discrimination of the Waldenström macroglobulinemia (WM) clone. In specific, the clonal population in each sample was selected upon the positive expression of CD19, CD20, CD25, IgM, CD22 and negative expression of CD10 and CD5 (for the majority of the cases). Clonality of each aberrant population was verified by intracellular staining for kappa and lambda light chain restriction and a skewed k:l ratio. Results: A total of 108 matched BM and PB samples from 54 patients with IgM monoclonal gammpopathies were analyzed comprising 6 IgM-MGUS, 9 asymptomatic WM (aWM) and 30 symptomatic WM (sWM) patients. In addition, our study included 9 sequential samples of sWM patients before and 6 months after therapy with ibrutinib. The median age of the cohort was 78 years (range 45-88) of which 46% were males. Median BM infiltration was 60% (range 0-90%) and serum IgM levels were 1465 mg/dL (range 88-6730). Our analysis showed that CTCs could be detected in the majority of sWM patients (34/39, 87%), in 66% of aWM patients (6/9) but only in 17% IgM-MGUS cases (1/6). The median levels of CTCs were 0.3% (range 0.06%-9.1%) in aWM patients and 0.4% (range 0.03%-55.5%) in sWM patients. Among patients with detectable CTCs, there was a positive correlation with BM infiltration (p<0.05) and the IgM levels (p<0.05). Higher levels of CTCs at diagnosis were associated with worse IgM responses at a 3-month period post- therapy in sWM patients. At diagnosis, median CTC value of patients with stable disease (SD) was 4.33% (range 0%-15.3%), in patients with minimum response (MR).72% (range 0.04%-1.4%) while in patients who achieved partial response (PR) 0.4% (range 0%-2.9%). One patient with very good partial response (VGPR) at 3 months post therapy had no detectable CTCs at the time of diagnosis. In sequential samples, median CTC levels were 0.45% (0.25%-1.5%) before therapy with ibrutinib and 0.15% (0.1%-1.45%) at 6 months post therapy. In terms of the 6-month IgM response, after 6 months of therapy, 1 patient had SD and with persisting high CTC levels before and after therapy (1.5% vs 1.45% respectively), 3 patients showed PR with a significant CTCs reduction in all cases post therapy (median CTC levels: 0.4% vs. 0.2% respectively), whereas, among the 5 patients showing VGPR, 2 patients had undetectable CTCs post therapy (median CTC levels: 0.8% vs 0% respectively), 2 patients had significantly reduced CTCs post therapy (median CTC levels: 0.45% vs 0.16% respectively), and one patient had undetectable CTCs both before and after therapy. Analysis of additional patient samples is ongoing. Summary/Conclusion: In conclusion, our data suggest that evaluation of CTCs in PB is feasible in IgM monoclonal gammopathies and may efficiently reflect a patient’s BM tumor load for an invasive and repetitive assay of disease monitoring. Keywords: Monoclonal gammopathy, Waldenstrom’s macroglobulinemia, Peripheral blood