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

OBJECTIVE: Multiparameter flow cytometry is considered the gold standard to evaluate minimal residual disease in multiple myeloma (MM) and patients in complete remission can achieve "Flow MRD-negative" status (i.e. immunophenotypically abnormal plasma cells not detectable). In the current study we report the usefulness of an eight-color flow cytometric method with a 10-5 sensitivity, using monoclonal antibodies in dried formulation. MATERIALS AND METHODS: Forty-six patients with MM were treated with bortezomib-based regimens and, when eligible, with autologous stem cell transplantation. Response to therapy was assessed according to the criteria validated by the International Myeloma Working Group. Multiparameter flow cytometry was carried out with an 8-color panel validated by the Euroflow Consortium. A commercially available single 8-color tube in dried formulation was used and almost 2,000,000 events were acquired, in order to obtain a 10-5 sensitivity. RESULT: Sixteen patients achieved stringent complete remission and another three patients achieved complete remission. In these groups of patients, the "Flow MRD-negative" status was achieved in sixteen cases. In patients who had a different degree of response (very good partial response, partial response, minimal response) immunophenotypically abnormal plasma cells were always detected. CONCLUSION: Using a single eight-color tube in dried formulation, and an acquisition strategy able to obtain a 10-5 sensitivity, not only is it possible to detect a deep response to modern therapy in patients who obtained at least complete remission, but it is also always possible to detect minimal residual disease in patients with either complete remission or stringent complete remission.