Automated Enumeration and Characterization of Circulating Multiple Myeloma Cells in Blood

Abstract 1825

Detection of circulating Multiple Myeloma cells (CMMC) by flow cytometry is an indicator of active disease. In addition, circulating plasma cells can be detected in earlier stages of disease, including MGUS and Smoldering Multiple Myeloma, and appear to correlate with prognosis. The capture and characterization of these circulating plasma cells from peripheral blood may provide novel biomarkers for the management of Multiple Myeloma patients, particularly in monitoring minimal residual disease and in progression from MGUS or Smoldering Multiple Myeloma to active disease. The enumeration and characterization of circulating tumor cells (CTC) in patients with metastatic breast, prostate or colorectal cancer using the CellSearch^® technology, has been shown to provide clinically relevant prognostic and predictive information. Here we describe the development of an automated assay for detecting circulating normal plasma cells (CPC) and multiple myeloma cells (CMMC) in blood using CellSearch. Assay results from Multiple Myeloma, MGUS patients, and from an aged matched control population are presented.

The CellTracks^® AutoPrep^® System and CellTracks Analyzer II^® systems were used to capture and enumerate CPC and CMMC. Magnetic particles conjugated to anti-CD138 are used to capture myeloma cells from 4.0mL of blood. Enriched cells are then stained with the nucleic acid dye DAPI and anti-CD38-Phycoerythrin (PE) antibody. Allophycocyanine (APC) conjugated anti-CD45 and anti-CD19 were used to exclude leukocytes and B-cells. In addition, FITC labeled anti-CD56 was added as a biomarker. The enriched and stained cells were transferred to a CellTracks^® cartridge and MagNest^® for magnetic mounting. The cartridge was scanned using the CellTracks Analyzer II^®. Individual images of cells were presented to the operator for review, and scored as CMMCs, based on fluorescence and cell morphology. In a model spike-in system the assay consistently recovered ∼60% of the cells from the Multiple Myeloma cell line H929 spiked into 4.0mL of blood from healthy donors. The assay was linear over the tested range of from 0 to 2000 spiked H929 cells (r² 0.98, slope 0.50, intercept 10). The assay was validated using blood from age matched healthy donors (n=22) and patients with Multiple Myeloma (n=66) and MGUS (n=7). In 4.0mL blood from normal donors, 0 CPC were detected in 12/22 (55%) and low numbers (1–6 CPC) were detected in 10/22 (45%) samples. Interestingly, one CD56 positive CPC (CMMC?) was found in a normal donor. CMMC in Multiple Myeloma patients ranged from 0 – 17,000 /4.0mL blood. One or more CMMC were detected in 91% of the patients, > 5 in 68%, > 10 in 58% and > 100 in 35%. Expression of CD56 was highly variable in the patient population. CMMC in MGUS patients ranged from 0 – 112 /4.0mL blood. One or more CMMC were detected in 6/7 of the patients, > 5 in 4/6, > 10 in 2/6 and > 100 in 1/6.

To further characterize CMMC, and differentiate CPC from CMMC, an interphase fluorescent in situ hybridization (FISH) assay was developed to be used with the capture and detection system described above. A four color FISH probe was used to simultaneously detect high risk mutations including two recurrent translocations of the IgH locus (t(4;14)(p16;q32) and t(14;16)(q32;q23)) as well as deletion of the TP53 locus (Δ17p13). The FISH assay was verified on cell lines H929, MM1s, and U266, which showed mutations at t(4;14), t(14;16) and Δ17p13, respectively. The FISH assay was tested on 9 CMMC patient samples and 8 samples yielded evaluable results. Two samples showed t(4;14)fusions, 3 patients showed aberrant FISH signal patterns indicating aneuploidy of chromosome 4 or 14 and the remaining patients showed normal FISH patterns.

Well controlled prospective clinical studies are needed to establish the prognostic and predictive value of the presence, and characteristics, of CMMC in multiple myeloma or MGUS. In addition, as with CTC, this automated CMMC assay should prove useful in evaluating the effectiveness of new treatments as well as the assessment of potential treatment targets on CMMC in this difficult disease.