A Case of μ Heavy and λ Light Chain Amyloidosis in a Patient With Bi-Clonal (IgM κ and λ) Gammopathy Treated With Daratumumab.

Our case report is of an elderly male with a history of IgM κ lymphoplasmacytic lymphoma (LPL) presenting with generalized neuropathy and weakness. Due to his LPL history and worsening renal function, he underwent a renal biopsy revealing the presence of μ heavy and λ light chains, revealing a diagnosis of amyloidosis with unbound heavy & light chains (AHL), a rare type of amyloidosis. His bone marrow biopsy demonstrated κ light chain restriction by flow cytometry and amyloid deposition. The patient's serum had elevated free κ and λ light chains with a free light chain (FLC) ratio of 3.17. Serum immunofixation was positive for IgM κ and λ light chain clones. He completed six cycles of cyclophosphamide, bortezomib, dexamethasone, and rituximab (CyBorD+R), normalizing the FLC ratio. Still, he continued to present with persistently elevated M protein, IgM κ, and λ light chains on immunofixation. Thereafter, daratumumab, a human monoclonal antibody directed against CD38 expressed on myeloma cells was initiated, which led to a negative immunofixation study after two cycles accompanied by a reduction in protein excretion in the urine. The patient achieved a complete hematological response with daratumumab. To date, our case is the only reported μ heavy and λ light chain amyloidosis patient with bi-clonal (IgM κ and λ) gammopathy to be successfully treated with daratumumab.