Implications and outcomes of MRD‐negative multiple myeloma patients with immunofixation positivity (2019)

Assessment of disease activity and depth of response continues to be a moving target in multiple myeloma (MM). Where conventional complete response (CR) used to signify attainment of the deepest response to therapy, we now have tools including next generation sequencing (NGS) and multi-parametric flow cytometry (MFC) that have allowed for further disease stratification.1, 2 Assessment for minimal residual disease (MRD) has allowed for the detection of greater variability in depth of response, with multiple studies demonstrating inferior outcomes among those with residual tumor cells.3, 4 While evaluation of MRD has significantly advanced myeloma disease prognostication, its utility remains confined to evaluation of disease in the bone marrow (BM). We therefore rely on other markers of disease activity such as monoclonal protein in the serum and urine as well as imaging studies in providing a comprehensive assessment of response and disease state. We are occasionally confronted with conflicting response criteria parameters and in these cases, true disease state is accompanied with an element of uncertainty. With increased incorporation of MRD status in reporting response to therapy in the literature, it is imperative that we understand the strengths and limitations of its use for judicious result interpretation. This retrospective cohort study was designed to investigate the impact of residual serum monoclonal protein on outcomes of patients who are MRD negative by NGF and to assess whether this discordance results in greater variability in disease activity than originally assumed in these two groups. We retrospectively analyzed all myeloma patients who had NGF-based analysis of their BM from 2017-2018. Only patients who tested negative for MRD and were otherwise not found to be relapsing at the time of testing were included. Informed consent was obtained from all patients for review of their medical records. Plasma cells were detected by flow cytometry immunophenotyping as previously described.5 Briefly, 2-3 × 107 bone marrow cells were stained with antibodies to CD19, CD27, CD38, CD45, CD56, CD81, CD117, CD138, and cytoplasmic kappa and lambda. A minimum of 5 × 106 events were collected per tube with a target of 107 events, using FACS Canto II instrument (BD). Gating was performed using Infinicyt software. The study was considered adequate for minimum of 2 × 106 events collected, with the minimum sensitivity of 10−5 and maximum sensitivity of 2 × 10−6. Serum immunofixation was analyzed at the time of MRD analysis. The primary endpoints included 1 year probability of PD and TTP from date of testing, with censoring performed at the date of last contact. The Kaplan-Meier model was used to estimate 1 year probability of PD and median TTP among MRDneg/IFEneg and MRDneg /IFEpos groups and those whose IFE remained positive compared to patients who later became IFEneg from an initial IFEpos state. The Cox-proportional hazards model was used to compare MRDneg /IFEneg and MRDneg/IFEpos groups. A total of 289 patients were identified who had MRD with concomitant serum IFE assessments among which, 204 (71%) were MRDneg. Within the MRDneg group, 88 patients (43%) were IFEpos at the time of MRD assessment. Among the MRDneg/IFEpos group, 39 (44%) patients remained IFEpos on subsequent testing with a median follow-up of 5.6 months from initial MRD/IFE analysis and 39 (44%) patients became IFEneg after a median follow-up of 3.5 months with subsequent IFE testing unavailable in 10 patients. The median time for follow-up from MRD assessment was 5.7 months. The 1 year probability of PD in the MRDneg/IFEneg group was 20% compared to 41% in the MRDneg/IFEpos group (P < .01, Wilcoxon test) (Figure 1). When comparing those who remained IFEpos (44%) with those who became IFEneg (44%) upon subsequent testing, median TTP was not reached however there was a trend towards shorter TTP in the persistent IFE positive group (Figure 1). Among patients who remained IFEpos on subsequent testing, seven patients were found to have evidence of PD during follow-up while only four patients developed PD among those who later became IFEneg from an initial IFEpos state. While MRD assessment has advanced MM disease prognostication, it is important to acknowledge that its use is limited to assessment of disease activity in the BM. Furthermore, its utility is intended to assess deeper response in patients who meet criteria for CR as opposed to other disease states.3 We continue to rely on serum studies including immunofixation for CR evaluation and the significance of those who are MRDneg, yet have detectable monoclonal protein is not entirely clear. Our results demonstrate that persistent serum monoclonal protein in a state of MRD negativity predicts for shorter TTP. This likely reflects true, persistent disease that was not captured within the BM biopsy, and instead is reflected in the residual serum monoclonal protein.3 In those who later became IFEneg from an initial IFEpos state, we see less events of PD with a trend towards a longer TTP compared to patients with persistent immunofixation positivity. This finding may be explained by the prolonged half-life of M protein in those who eventually become IFEneg vs true disease persistence in those whose immunofixation status remains positive.6 Two of the persistent IFEpos patients were treated with daratumumab which may have interfered with the IFE result and these patients did not demonstrate evidence of PD during follow-up. One of the limitations of our study is the small sample size and limited follow-up time may not have allowed for a sufficient interval for assessment and development of PD. Secondly, comparison of current and original IFE isotype was not performed which may have differed in patients who received an autologous stem cell transplant. Our findings show in an MRDneg state there can be variance with regard to disease activity if serum studies are not suggestive of a CR, with this subset of patients being at higher risk for earlier disease relapse. Persistent monoclonal protein in the face of MRD negativity predicts for shorter TTP likely owing to persistent disease and incomplete BM capture. This supports the current strategy of assessing for MRD at the time of CR as opposed to other disease states to avoid unnecessary BM examinations and clarification of response to therapy. The Mayo Clinic Hematological Malignancies Program. S.K.K. and M.A.T. designed the study, collected and analyzed the data, and wrote the manuscript. D.J., S.V.R., A.D., M.Q.L., M.A.G., F.K.B, D.D., L.H., A.F., M.H., S.R.H, J.A.L., S.J.R., N.L., P.K., R.S.G., Y.L., W.I.G., T.K., R.W., and R.A.K. were involved in patient management, revised the manuscript, and approved the final version of the manuscript. The authors have nothing to disclose and there are no conflicts of interest to report.