The Utility of the Follow-up with Serum Free Light Chain After Autologous Stem Cell Transplantation in Multiple Myeloma Patients with Measurable M-Protein.
Authors: Sohn Byeong Seok , Kim Eun Kyoung , Yoon Dok Hyun , Kang Myoung Joo , Choi Dae Ro , Ahn Heui June , Cho Yoo Jin , Kim Kyu-pyo , Lee Dae Ho , Kim Sang We , Suh Cheolwon , Lee Jung Shin 
Published: October 2019

Abstract
Abstract 4878

Introduction
According to international uniform response criteria for multiple myeloma suggested in 2006, the response assessment for patients with oligo- and non-secretory multiple myeloma (MM) can be evaluated by the serum free light chain (FLC) assay. Although the FLC response criteria are not applicable in MM patients with measurable disease, there were several reports suggesting that serial measurement of serum FLC may detect relapse earlier than protein electrophoresis studies. We, therefore, investigated the preceding changes in serial serum FLC assay until progressive disease was confirmed by the international uniform response criteria in post-ASCT patients with measurable disease.

Patients and Method
We included patients from the AMC MM transplant registry, who met the following (1) undertook ASCT for measurable disease (2) showed, at least, two serial response assessment of stable disease or complete response before progression or relapse by serum or urine M-protein, (3) had periodic serum FLC assay simultaneously tested with serum and/or urine protein electrophoresis at each response assessment. Progressive disease (PD) was defined by increase of ≥ 25% from baseline in serum M-protein (the absolute increase must be ≥ 0.5mg/dL) and/or urine M-component (the absolute increase must be ' 200mg/24h) according to international uniform response criteria. In this investigation, significant increase in the difference between involved and uninvolved FLC (dFLC) and in the involved FLC (iFLC) was defined by increase of ≥ 25% from baseline. The positive predictive value of three cutoff levels for absolute increase, 10mg/L, 20mg/L, 100mg/L, were evaluated for both dFLC and iFLC provided serum FLC ratio was abnormal. Each patient was followed up with 1-3 month intervals according to the protocol for MM patients after ASCT.

Result
A total 29 patients of 138 patients in the AMC MM transplant registry satisfied above criteria.
When the cut-off level for absolute increase was defined as 100mg/L, the significant increase of iFLC in 12 patients (41%) and dFLC in 11 patients (38%) preceded or accompanied with the time of progressive disease observed by M-protein. The median value of preceding time was 2 month (range -5 - 0). When the cut-off level was defined as 20mg/L, the sustained significant increase of iFLC in 21 patients (72%) and dFLC in 17 patients (59%) preceded or accompanied with the time of progressive disease with median of 2 month (range -9 - 0) and 2 month (range, -5 – 0), respectively. At the cut-off level of 10mg/L, the sustained significant increase of iFLC in 23 patients (79%) and dFLC in 21 patients (72%) preceded or accompanied with the time of progressive disease observed by M-protein. The median of preceding time was 2 month (range -11 - 0) and 1 month (range, -11 - 0), respectively. Twenty-eight dFLC values were observed as negative values out of a total 123 data from 29 patients. Of these values, 12 were below normal iFLC concentration, 14 within normal range of iFLC (kappa 8.5 - 23.7 mg/L, lambda 9.5 - 23.5 mg/L), and 2 above normal iFLC concentration.

Conclusion
In this study, about 70% of patients showed sustained significant increase of iFLC that preceded or accompanied the time of progressive disease observed by M-protein by a median of 2 months at a cut-off absolute increase of 20mg/L. Although there is a subtle difference in prediction rates according to defined cut-off levels, serial follow up of iFLC and sustained increase by 25% during follow-up seems to have a utility in the prediction of progression after ASCT. In addition, interpretations of dFLC may be difficult as it is frequently observed as negative value in post-ASCT MM patients. Therefore, the serial and sustained increase of iFLC may be useful in lower iFLC concentrations. However, there should be more validation with large patients' population.

Disclosures
No relevant conflicts of interest to declare.