Investigation into the interference of the monoclonal antibody daratumumab on the free light chain assay

Objectives Recently monoclonal antibody therapy has been introduced in the treatment of multiple Myeloma (MM). One such efficacious therapy is the anti-CD38 monoclonal antibody, daratumumab (Dara). Since it is an Ig-G-kappa it can interfere with both the serum protein electrophoresis and immunofixation electrophoresis (IFE). The free light chain (FLC) assay is also useful in the diagnosis and therapeutic monitoring of MM. Hence we tested the effect of Dara on the FLC assay. Methods 30 serum samples from patients with known IgG-kappa (n = 20) and non-IgG-kappa M –proteins (n = 10) were spiked with Dara at a final concentration of 1.0 mg/mL and the FLC performed on samples. On a further 20 samples we performed IFE to determine the migration of Dara. Results On IFE, Dara migrated in the same area of the gamma zone. In the 30 samples in which we assayed FLC there was no significant differences in levels of kappa, lambda and the ratio of kappa to lambda between untreated and Dara–spiked samples. Conclusion Whilst Dara can interfere with the IFE to determine clinical responses the FLC assay can be useful in patients who have abnormal FLC ratios prior to Dara therapy to determine responses especially in IgG–kappa Myeloma.