Isolation and profiling of single circulating tumor cells in myeloma: a new workflow for liquid biopsies.

Minimal residual disease (MRD) is a key prognostic marker for progression-free and overall survival in multiple myeloma (MM). Existing high sensitivity assays primarily focus on tumor burden assessment, rely on bone marrow sampling, and are limited in their ability to support frequent longitudinal disease monitoring. Here, we describe a proof-of-principle workflow for isolating morphologically preserved circulating tumor cells (CTCs) from peripheral blood (PB) using size-based filtration. Based on controlled spiking experiments with RPMI 8226 myeloma cells, we demonstrate an analytical limit of detection of approximately 1 tumor cell per 107 white blood cells. Isolated cells retain nuclear integrity and cytomorphology, allowing for downstream immuno­phenotyping, three-dimensional (3D) telomere fluorescence in situ hybridization (FISH), and single-cell telomere profiling, a known marker of genomic instability and disease progression in multiple myeloma. The proposed workflow demonstrated its feasibility for isolating, profiling, and analyzing plasma cells from PB of MM patients at different disease stages. It revealed distinct nuclear and telomeric features in MM CTCs compared with normal lymphocytes. The established technically robust liquid biopsy workflow enables 3D telomere profiling of MM CTCs that can be adopted for noninvasive MRD monitoring based on genomic instability rather than on the enumeration of MM plasma cells alone.