Progression free survival of myeloma patients who become IFE-negative correlates with the detection of residual monoclonal free light chain (FLC) by mass spectrometry

Abstract

Deeper responses are associated with improved survival in patients being treated for myeloma. However, the sensitivity of the current blood-based assays is limited. Historical studies suggested that normalisation of the serum free light chain (FLC) ratio in patients who were negative by immunofixation electrophoresis (IFE) was associated with improved outcomes. However, recently this has been called into question. Mass spectrometry (MS)-based FLC assessments may offer a superior methodology for the detection of monoclonal FLC due to greater sensitivity. To test this hypothesis, all available samples from patients who were IFE negative after treatment with carfilzomib and lenalidomide-based induction and autologous stem cell transplantation (ASCT) in the Myeloma XI trial underwent FLC-MS testing. FLC-MS response assessments from post-induction, day+100 post-ASCT and six months post-maintenance randomisation were compared to serum FLC assay results. Almost 40% of patients had discordant results and 28.7% of patients with a normal FLC ratio had residual monoclonal FLC detectable by FLC-MS. FLC-MS positivity was associated with reduced progression-free survival (PFS) but an abnormal FLC ratio was not. This study demonstrates that FLC-MS provides a superior methodology for the detection of residual monoclonal FLC with FLC-MS positivity identifying IFE-negative patients who are at higher risk of early progression.

Fig. 1. CONSORT diagram.

CONSORT diagram showing the flow of patients included in this study.

Fig. 2. Workflow and example mass spectra showing how FLC-MS is used to identify and track monoclonal FLC across serial samples.

The workflow for analysis of a serum sample by FLC-MS is shown in (A). B shows example mass spectra of how FLC-MS is used to track monoclonal FLC across serial samples. An example mass spectrum from a polyclonal sample run against free kappa is shown in the top mass spectrum. At presentation monoclonal kappa FLC with an m/z of 11781 for the doubly charged light chain were identified. Persistent residual monoclonal kappa FLC are detectable by FLC-MS (peak at m/z 11785 for the doubly charged light chain) at the end of induction chemotherapy. There is also an oligoclonal peak within the postinduction spectrum with a completely distinct m/z to the monoclonal light chain. At the postmaintenance time point there are no residual monoclonal kappa FLC detectable by FLC-MS. There is a very small abnormality in the kappa FLC spectrum (m/z 11713 for the doubly charged light chain) which is likely due to a very small oligoclonal peak.

Fig. 3. Agreement between serum FLC ratio and FLC-MS assessments.

Agreement between serum FLC ratio and FLC-MS assessments at the end of induction chemotherapy (A), day+100 post-ASCT (B) and 6 months post maintenance randomisation are shown in (C).

Fig. 4. Example mass spectra from patients with discrepant serum FLC ratio and FLC-MS assessments.

A shows example mass spectra from a patient with a normal serum FLC ratio at day+100 post-ASCT but persistent positivity by MS is shown in. B shows example mass spectra from a patient with an abnormal serum FLC ratio at day+100 post-ASCT but no residual monoclonal FLC were identified by FLC-MS. The large oligoclonal peak with a different m/z was present and was the likely the cause of the abnormal serum FLC ratio observed in this sample.

Fig. 5. Agreement between serum FLC, FLC-MS and bone marrow MRD assessments.

Agreement between bone marrow MRD and FLC-MS assessments in patients at the end of induction chemotherapy (A), day+100 post ASCT (B) and postmaintenance randomisation (C). The agreement between bone marrow MRD and serum FLC assessments at postinduction, day+100 post-ASCT and post maintenance randomisation are shown in (D–F).

Fig. 6. PFS according FLC status at the end of induction chemotherapy, day + 100 post-ASCT and 6 months post maintenance randomisation.

A–C Show the PFS for IFE-negative patients with a normal versus abnormal serum FLC ratio at the end of induction chemotherapy, day+100 post-ASCT and six months post maintenance randomisation. D–F show the PFS for IFE negative with patients with a normal FLC ratio (classified as a normal serum FLC ratio or an abnormal serum FLC ratio due to suppression of the uninvolved FLC) versus an abnormal FLC ratio at the end of induction chemotherapy, day+100 post-ASCT and six months post maintenance randomisation. G–I show the PFS for IFE-negative patients according to FLC-MS status at the end of induction chemotherapy, day+100 post-ASCT and six months postmaintenance randomisation.