Tafasitamab interference in immunofixation electrophoresis

To the Editor,

The development of therapeutic monoclonal antibodies has transformed treatment of malignancies and autoimmune diseases. However, the presence of therapeutic monoclonal antibodies in patient specimens can interfere with clinical laboratory tests, including serum protein electrophoresis (SPEP) and immunofixation, potentially leading to misinterpretation of results.

We identified a small immunoglobulin G (IgG) kappa clonal band by immunofixation in a serum specimen from a patient who had no history of prior protein electrophoresis testing in our laboratory (Figure 1). Review of the electronic medical record revealed that this patient had a history of kappa monoclonal free light chain multiple myeloma diagnosed seven years prior, and achieved a stringent complete response with treatment including autologous stem cell transplantation and lenalidomide. Serum free light chain testing performed concurrently with immunofixation was consistent with the continued absence of a monoclonal kappa free light chain, with kappa free light chains measured at 0.005 g/L (0.5 mg/dL), lambda free light chains measured at 0.006 g/L (0.6 mg/dL), and a kappa/lambda free light chain ratio of 0.9. However, while receiving maintenance lenalidomide, the patient developed a second hematologic malignancy, characterized by flow cytometry as B lymphoblastic leukemia/lymphoma. At the time of specimen collection, the patient was undergoing treatment of this second malignancy with a regimen that included tafasitamab. Tafasitamab is a humanized IgG kappa therapeutic monoclonal antibody targeting CD19, which is present on the surface of surface of B lymphocytes. The patient had received multiple doses of tafasitamab prior to immunofixation, with the most recent administration being only 4 days prior to the specimen draw. Pharmacokinetic studies described the mean maximum concentration of tafasitamab at steady-state as 0.5 g/L (500 μg/mL) [1], consistent with the size of the observed band in the patient’s specimen.

Figure 1:

The IgG kappa clonal band in the patient’s serum comigrates with the IgG kappa therapeutic monoclonal antibody tafasitamab. Serum gel electrophoresis immunofixation for a normal control sample (left), the patient’s sample (middle), and the patient’s sample adjacent to 0.5 g/L tafasitamab (right). For each gel, serum electrophoresis (E) is shown along with immunofixation for IgG heavy chain (G), IgA heavy chain (A), IgM heavy chain (M), kappa light chain (κ), and lambda light chain (λ). The control sample was applied at a dilution of 1:6 for IgG and 1:3 for the other immunofixation lanes. The patient’s samples were applied at a dilution of 1:3 for IgG and undiluted for the other immunofixation lanes. The tafasitamab was applied undiluted in both lanes.

We therefore performed immunofixation of tafasitamab (MedChemExpress) diluted to 0.5 g/L and observed an IgG kappa band with identical electrophoretic migration to the band found in the patient’s’ specimen (Figure 1). This result strongly suggested that the clonal band in the patient’s specimen represented interference from tafasitamab and not a new endogenous monoclonal component.

Detection of therapeutic monoclonal antibodies such as tafasitamab can mimic monoclonal gammopathies, posing a diagnostic conundrum for clinicians and laboratory professionals. This highlights the importance of correlating laboratory findings with clinical history. Although tafasitamab is a therapy for B cell lymphoma and not multiple myeloma, for which immunofixation monitoring is indicated, patients treated for multiple myeloma with lenalidomide may be at an increased risk of subsequently developing a second hematologic malignancy, including B lineage disease [2], [3], [4]. Increased age is also a risk factor for lymphoma, and as these patients continue to be monitored for recurrence of myeloma by immunofixation, therapeutic monoclonal antibodies such as tafasitamab that are used for the treatment of lymphoma may become more common in tertiary centers that use such novel therapies. In addition to serum protein immunofixation, tafasitamab has also been described to interfere with flow cytometric assessment for residual disease, causing the potential to mimic the presence of residual lymphoma cells, leading to false conclusion of treatment failure [5].

As our case illustrates, obtaining a complete medication history is crucial to avoid misinterpretation and unnecessary additional workup of patients undergoing therapy with monoclonal antibodies. Fortunately, many therapeutic monoclonal antibodies do not achieve serum concentrations high enough to be detected by serum protein immunofixation, although the list of potentially detectable drugs is growing. Awareness of the possibility of this interference and the migration characteristics of therapeutic monoclonal antibodies can help to prevent misdiagnosis.