Peripheral circulating myeloma cells accurately identify bone marrow MRD positivity in multiple myeloma patients receiving CAR T or autologous transplant

Abstract Depth of response assessed by minimal residual disease (MRD) assays is an important prognostic factor in multiple myeloma (MM). Bone marrow MRD assessment using next-generation sequencing (NGS, ClonoSEQ) is considered the gold standard and is widely used in clinical trial settings. However, bone marrow sampling is invasive, subject to patchy infiltration leading to sampling bias, and typically performed only once a year. Therefore, this approach can miss MRD conversion from negative to positive for nearly 12 months, delaying detection of early relapse and undermining potential timely intervention. Recent studies in the T cell engager setting suggest that circulating multiple myeloma cells (CMMCs) in peripheral blood, defined as CD138+CD38+CD19−CD45− malignant plasma cells, reflect disease burden more dynamically than paraprotein kinetics, clearing rapidly in responders and re-emerging with relapse (Dhakal et al. Blood 144 (2024): 4687). CMMCs that can be enriched, enumerated, and molecularly characterized, offering a minimally invasive and real-time biomarker of systemic disease. Whether CMMC detection correlates with bone marrow MRD in MM patients receiving cellular therapies has not been systematically evaluated. Methods We conducted a study of 51 MM patients undergoing either chimeric antigen receptor (CAR) T- cell therapy or autologous stem cell transplant (ASCT). Peripheral blood and bone marrow samples were collected prospectively. MRD was assessed on bone marrow aspirates using ClonoSEQ with a sensitivity threshold of 10⁻6. Peripheral blood (4 mL) was collected in CellRescue™ Preservative Tubes and processed within 96 hours using the CELLSEARCH® platform. CMMCs were isolated with anti-CD138 ferrofluid, stained with CD38-PE, CD19/CD45-APC, and DAPI, and enumerated by automated fluorescence microscopy. CMMC positivity was defined as 2 cells/4 mL. Concordance, sensitivity, specificity, and Cohen's kappa were calculated. Spearman correlation assessed the relationship between CMMC counts and MRD burden. Results The median age at sampling was 66 years old (range: 45–77). The cohort consisted of 57% male (n=29) and 43% female (n=22) patients. The racial distribution included 71% White (n=36), 16% Black (n=8), 6% Asian (n=3), 2.0% Native American (n=1), and 6% unknown (n=3). Myeloma subtypes IgG Kappa (27.5%) and IgG Lambda (25.5%) being most common. High-risk cytogenetic features such as del(17p), t(4;14), and gain(1q) were present at the time of diagnosis among 40% of patients. The Revised International Staging System (R-ISS) distribution included 31% with stage I, 35% with stage II, and 25% with stage III disease. At the time of sampling, 64% of patients were receiving daratumumab -based triplet or quadruple therapy. Response assessment showed that 53% of patients were in very good partial response (VGPR) or better. Peripheral blood counts at sampling were within normal range in the majority of patients. The median time from diagnosis to sampling was 13.9 months. CMMC positivity (2 cells/4 mL) was observed in 28 of 51 patients (55%). Among these, 25 were also MRD positive in the bone marrow, yielding a sensitivity of 89.3% and specificity of 91.3% for CMMCs in detecting MRD as defined by NGS (i.e, as gold standard). The overall concordance between CMMC detection and bone marrow MRD was 90.2%, with a Cohen's kappa coefficient of 0.79, indicating substantial agreement. Spearman correlation analysis demonstrated a strong positive correlation between CMMC counts and quantitative MRD levels in the bone marrow (ρ = 0.72, p 0.001). When stratified by therapy, CMMC performance remained robust, with comparable sensitivity in both the CAR T (88.2%) and ASCT (90.9%) cohorts. Conclusions In this 51 patient prospective study of MM patients undergoing CAR T or ASCT, CMMC enumeration using CELLSEARCH accurately identified MRD-positive bone marrow. These findings support the utility of CMMCs as a minimally invasive, real-time biomarker for systemic disease in multiple myeloma. Incorporating CMMC monitoring may allow for more frequent surveillance, early detection of MRD conversion, and improved risk-adapted strategies in the era of highly effective cellular therapies.