Serum Free Light Chain Responses after High-Dose Intravenous Melphalan and Autologous Stem Cell Transplantation for AL (Primary) Amyloidosis.

AL amyloidosis is caused by a clonal plasma cell dyscrasia and characterized by widespread, progressive deposition of amyloid fibrils derived from monoclonal Ig light chains, leading to multisystem organ failure and death. Aggressive treatment of AL amyloidosis with high-dose melphalan followed by autologous stem cell transplant (HDM/SCT) can induce hematologic and clinical remissions and extend survival. Several approaches have been used to define hematologic responses of plasma cell dyscrasias underlying AL amyloidosis following HDM/SCT and other forms of treatment. The definition of a hematologic complete response (CR) that we have used requires that there be no evidence of a persistent monoclonal gammopathy by immunofixation electrophoresis (IFE) of serum and urine proteins, or of persistent plasmacytosis or plasma cell clonality in a bone marrow biopsy by immunohistochemistry. Others have defined hematologic response as a ≥ 50% reduction in free light chain (FLC) measurements. Hematologic responses by both criteria correlate with survival and clinical improvement following HDM/SCT. We have carried out a retrospective analysis of HDM/SCT treatment outcomes for patients with AL amyloidosis to determine the extent to which hematologic CR, by our standard criteria, correlates with FLC response. Serum free light chain concentrations (FLC) were measured by a sensitive nephelometric immunoassay in 67 patients with AL amyloidosis before and after treatment with HDM/SCT. After treatment with HDM/SCT, 27 patients (40%) achieved a CR by standard criteria. Of these 27 patients, 63% (n=17) demonstrated normalization of FLC levels and an improvement of ≥50% in FLC occurred in 100%. Of the 40 patients who did not achieve a CR, 25% (n=10) experienced normalization of FLC levels, and an improvement of ≥50% occurred in 78% (n=31), while only 5 patients (13%) experienced no significant change in FLC. The average improvement in FLC was 94% for patients who achieved a CR by standard criteria and 72% for those who did not (p=0.0001, t-test). Thus, HDM/SCT was found to induce improvements in FLC levels of ≥50% in the vast majority of AL amyloidosis patients treated with HDM/SCT (87%, or 58/67). These data indicate that a decrease in FLC of ≥50% is a substantially less stringent indicator of hematologic response than is CR, as defined by standard criteria. Nonetheless, these measures of hematologic response are complementary, since decreases in FLC can be detected earlier following treatment than changes in IFE and marrow studies required to determine CR.