Low circulating tumor cell levels correlate with favorable outcomes and distinct biological features in multiple myeloma

<title>Abstract</title> Background There is growing interest in multiple myeloma (MM) circulating tumor cells (CTCs), but its rareness in peripheral blood (PB) and inconsistency in cutoffs question their clinical utility. Herein, we aimed at defining the optimal CTC cutoff using a highly sensitive approach. Moreover, we analyzed matched bone marrow (BM) and PB samples in order to highlight inconsistencies between the two sites and identify microenvironmental profiles associating with enhanced CTC dissemination. Methods Next-generation flow cytometry was performed for the detection of myeloma cells in matched BM and PB samples from 550 newly-diagnosed MM patients. Various multivariable regression models were applied for the detection of the optimal CTC clinical cutoff (median follow-up: 41 months). BM microenvironment was characterized by deep phenotyping using both CyTOF and various multiparametric flow cytometry panels. Results CTCs were detected in 90% of patients (median 0.01%, range: 0.0002% − 12.6%) and increased levels associated with adverse features. Unexpected correlations were observed between high CTC percentages and a diffused MRI pattern, a distinct BM composition characterized by altered B-cell differentiation together with an expansion of effector cells and tumor-associated macrophages, as well as a greater phenotypic dissimilarity between BM and PB tumor cells. Progression-free survival (PFS) and overall survival (OS) gradually worsened with each logarithmic increment of CTCs. Conversely, patients without CTCs showed unprecedented outcomes (5-year PFS and OS: 83% and 97%, respectively). A cutoff of 0.02% CTCs was independent of the ISS, LDH and cytogenetics in a multivariate analysis of risk factors for PFS and proved clinically relevant for both TE and TI patients. The 0.02% CTC cutoff synergized with the MGUS-like phenotype and the R-ISS for improved risk stratification. MRD-negativity was less frequent if CTCs were ≥ 0.02% at diagnosis, but whenever achieved, the poor prognosis of these patients was abrogated. Conclusions This study shows the clinical utility of CTC assessment in MM and provides evidence towards a consensus cutoff for risk stratification. The liquid biopsy is more informative of the entire tumor burden than a single-spot BM aspiration; hence, CTC analysis may serve as the new hallmark for the real-time evaluation of a patient’s disease status.