Probing Deeper: Using Free Light Chain Mass Spectrometry to Assess Amyloid Light-chain Amyloidosis Response

In amyloid light-chain (AL) amyloidosis, an underlying plasma cell or B-cell clone is responsible for the production of pathogenetic free light chains (FLCs) that aggregate and deposit as amyloid fibrils in multiple organs, leading to end-organ damage. Current therapies are directed at stopping the production of these toxic light chains by targeting the underlying clonal disorder. The central goal of treatment is to maximally reduce or, ideally, eliminate pathogenetic FLCs, as depth of hematologic response is correlated with organ response (OR) and overall survival (OS).^1,2  Conventionally, the measures of hematologic response are based on International Society of Amyloidosis (ISA) consensus criteria, which take into account the presence of monoclonal proteins in the serum and urine based on protein electrophoresis/immunofixation, as well as the FLC level or ratio based on nephelometric serum FLC assays.^3-5  A hematologic complete response (CR) or very good partial response (VGPR) has traditionally been considered the target to increase the likelihood of OR.

In comparison to patients with the more common plasma cell malignancy multiple myeloma, patients with AL amyloidosis typically present with a much lower bone marrow plasma cell burden, as well as concomitantly lower serological markers at the time of diagnosis. As novel anti-plasma cell therapies including chimeric antigen receptor T-cell (CAR-T) therapy and bispecific antibodies demonstrate increasing potency, the ability to detect residual disease, particularly proteotoxic amyloid–producing clones, becomes increasingly important.^6,7  While marrow-based minimal residual disease (MRD) testing has shown promise in AL amyloidosis, it is difficult to repeat this invasive procedure frequently over time.⁸  One of the main limitations of using serum FLC assays, however, is the inability to distinguish amyloidogenic monoclonal FLCs from polyclonal FLCs. More recently, mass spectrometry (MS) is being used as an ultrasensitive method of diagnosing and monitoring serum monoclonal proteins in plasma cell diseases.^9,10  Although highly sensitive in patients with intact immunoglobulins, the two commercially available MS assays (MASS-FIX and EXENT) assess total light chains without isolation of FLC, reducing sensitivity for detection of the underlying monoclonal FLC responsible for the disease process.^11,12  By using isotype-specific beads to select for kappa and lambda FLC, the FLC-selected proteins can be analyzed through matrix-assisted laser desorption/ionization time-of-flight MS in a technique known as FLC-MS.

Joshua Bomsztyk, MD, and colleagues used FLC-MS to assess hematologic responses from a prospective cohort of 487 AL amyloidosis patients treated at the U.K. National Health Service National Amyloidosis Centre with bortezomib-containing regimens without upfront daratumumab. These responses were correlated to OS and OR. Serum samples were analyzed at diagnosis and at six and 12 months after treatment initiation. Patients with evidence of a monoclonal spike by FLC-MS were determined to be FLC-MS positive, while those without were deemed FLC-MS negative. FLC-MS was compared to and combined with other conventional measures of hematologic response using serum FLC assays to determine impact on OS and OR rates.

The authors reported that at six and 12 months, 81 (17%) and 101 (21%) of the patients, respectively, were FLC-MS negative. At these same intervals, 162 (33%) and 164 (34%) of the patients, respectively, achieved CR and 165 (34%) and 169 (35%), respectively, achieved VGPR based on conventional ISA response criteria. Of those achieving ISA CR, only 45 (28%) and 64 (39%), respectively, were FLC-MS negative. Patients achieving FLC-MS negative status had significantly superior OS compared to those who were FLC-MS positive (median OS: not reached [NR] vs. 63 months, p≤0.001), including in the subgroup of patients with ISA CR (median OS: NR vs. 108 months, p=0.009). In multivariate analysis, attainment of FLC-MS negativity and cardiac stage were independent predictors of OS. At 12 months, OR was higher in patients with FLC-MS negative status versus those with FLC-MS positive status, including for cardiac responses (70% vs. 50%, p=0.015) and renal responses (37% vs. 23%, p=0.015). There was 100% concordance between light chain fibril type identified on tissue biopsy and by FLC-MS.

Strengths of the study include the relatively large sample size assessed and systematic assessment of a prospective cohort. Limitations of the study include the lack of comparison to commercially available mass spectrometry methods and bone marrow assessments such as MRD. At the time of writing, the FLC-MS assay is only available in the research setting and not yet available for clinical use.