Circulating tumour plasma cells in newly diagnosed multiple myeloma: Prevalence, clinical correlates, and phenotypic distinction  from bone marrow plasma cells in a south Indian cohort

Abstract INTRODUCTION Multiple myeloma (MM) is a haematological malignancy characterised by the clonal proliferation of plasma cells within the bone marrow. Traditionally, disease assessment has relied on bone marrow sampling and radiological imaging; however, the presence of circulating clonal plasma cells (CCPCs) in peripheral blood has emerged as a novel prognostic indicator, associated with aggressive disease biology and adverse outcomes. Although international studies have increasingly highlighted the significance of CCPCs, data on their prevalence and characteristics among South Asian MM patients remain scarce. This study aims to evaluate the frequency and phenotypic profile of CCPCs in newly diagnosed multiple myeloma (NDMM) patients at a tertiary care centre in India, and to assess their correlation with established clinical and biochemical risk parameters. METHODS Consecutive Patients newly diagnosed with multiple myeloma (NDMM) as per the IMWG criteria at Amrita Institute of Medical Sciences, who consented to the study (N=40) from August 2023 to June 2025, were included. CCPC was analysed by multiparametric flow cytometry from 2 mL of peripheral blood (PB) in EDTA tube before the initiation of treatment. The samples were processed within 24 hours by bulk lysis, surface/cytoplasmic staining and wash method. The cells were stained by Euro flow antibody panel - CD38 BV786, CD138 V450, CD45 V500c, CD19 PECy7, CD56 APC, CD27 PerCP Cy 5.5, CD81 APC H7, CD117 BV605, CD319 BV711 and kappa FITC, and Lambda PE. They were acquired and analysed either in FACS Canto (8 colour, two tube, 1.5 million events, LLOQ 0.003%) or Lyric (11 colour, single tube,4 million events, LLOQ 0.00125%). The clonality of circulating plasma cells was established by aberrant antigenic expression and light chain restriction. CCPCs were quantified both as a percentage of total CD45-positive viable events and as absolute counts per microliter. Associations between CCPC levels and patient characteristics (serum creatinine, calcium, β2-microglobulin, lactate dehydrogenase (LDH), presence of lytic lesions, Revised ISS (RISS), and mSMART 4.0) were assessed using Spearman's rank correlation. Median differences were assessed by the Wilcoxon signed-rank test. RESULTS In our cohort of 40 newly diagnosed multiple myeloma (NDMM) patients (median age 63.5 years; 20 males, 20 females), circulating clonal plasma cells (CCPCs) were identified in 97.5% of cases, with a median CCPC percentage of 0.689% (5.83 cells/μL). Notably, female patients had a significantly higher median CCPC% compared to males (0.3% vs 0.035%; p = 0.035). Although not statistically significant, higher CCPC% was observed among patients aged <65 years (0.23% vs 0.03%; p = 0.076), those with serum calcium >11 mg/dL (0.2% vs 0.1%; p = 0.086), and those with >4 lytic lesions (0.3% vs 0.03%; p = 0.086). CCPC% did not correlate with serum creatinine, LDH, involved free light chain levels, heavy chain type, or hemoglobin. However, moderate correlations were observed with bone marrow plasma cell percentage by morphology (r = 0.36, p = 0.020) and flow cytometry (r = 0.33, p = 0.04), β2-microglobulin (r = 0.399, p = 0.019), and R-ISS stage (r = 0.443, p = 0.007). A strong correlation was found with mSMART 4.0 risk category (r = 0.7, p < 0.001), with high-risk patients showing significantly elevated median CCPC% compared to standard-risk (0.47% vs 0.039%; p < 0.001). Phenotypic analysis revealed a significantly lower expression of CD138 on circulating plasma cells compared to bone marrow plasma cells (p = 0.03). CONCLUSION CCPCs were prevalent in a majority of Indian NDMM patients and correlated with advanced disease, as evident from its strong correlation with mSMART 4.0 risk stratification. Importantly, CCPCs demonstrated distinct phenotypic features compared to bone marrow plasma cells, including reduced CD138 expression, which may reflect biological heterogeneity and potential prognostic value. Given the limited data on CCPCs in South Asian populations, this study provides valuable insights into their prevalence and clinical relevance in newly diagnosed MM patients from India. Flow-based CCPC characterisation could be a practical, minimally invasive adjunct for disease assessment in resource-limited settings.