Researchers Describe Use of 8-Color Flow Cytometry to Determine Level of Response to MM Therapy

A recent study described the use of multiparameter flow cytometry (MFC) to detect minimal residual disease (MRD), with an 8-color flow cytometric method with a 10⁵ sensitivity, using monoclonal antibodies in dried formulation.

Previously, standard 4- to 6-color methods were used, with a 10⁴ sensitivity, to detect MRD. MFC is one of the most common MRD detection methods; it has limited sensitivity but intermediate applicability. Currently, sensitivity can go as high as 10⁶. Another method is quantitative polymerase chain reaction (PCR), which has better sensitivity but is only applicable to 40% of patients with molecular alterations.

In this European study, 46 patients (aged 43-80 years; 28 male and 18 female) with MM were treated with bortezomib-based regimens and, when eligible, with autologous stem cell transplantation (ASCT). Response to therapy was assessed according to International Myeloma Working Group criteria.

MFC was carried out with an 8-color panel validated by the Euroflow Consortium, using a commercially available single 8-color tube in dried formulation. Almost 2 million events were acquired in order to obtain a 10⁵ sensitivity.

Sixteen patients (34.7%) achieved stringent complete remission (sCR). In this subset of patients, 14 fulfilled the criteria for flow MRD-negative status, whereas the remaining 2 achieved a flow MRD-positive status.

Three patients fulfilled the criteria of complete remission (CR), but not of sCR, because they did not achieve normalization of the serum free light chain ratio. In 1 case, MRD was detected. The remaining 2 patients were flow MRD-negative. The authors said that means that flow MRD-negative status showed a significant correlation with the sCR/CR status (16/19 patients; 84.2%).

Eight patients achieved very good partial response, and in all cases variable percentages of immunophenotyically abnormal plasma cells (ia-PCs) were detected. Variable percentages of ia-PCs were also found in all remaining poor responders (14 partial response and 5 minimal response). The researchers said that ia-PCs always displayed clonal restriction for the corresponding immunoglobulin light chain, while immunophenotypically normal PCs always showed polyclonal expression of cytoplasmic immunoglobulin light chain.

The researchers said their results show that the most useful application of highly sensitive MFC methods seems to be the evaluation of patients achieving at least CR status, if not sCR. In all three degrees of response, abnormal cells were always detected. They also noted that 8-color MFC with 10⁵ sensitivity assessed response in patients who had undergone first-line therapy, with or without ASCT.

Reference

Carulli G, Tarasco A, Sammuri P, et al. Assessment of response to therapy in multiple myeloma by multiparameter flow cytometry: usefulness of an eight-color single tube with monoclonal antibodies in dried formulation. Clin Ter. 2019;170(5):e352-356. doi: 10.7417/CT.2019.2159.