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Abstract Background The use of therapeutic monoclonal antibodies (t-mAbs) in the treatment of multiple myeloma (MM) has created challenges in the clinical laboratory. Co-migration of t-mAbs (predominantly IgGK) and the endogenous M-protein during serum protein electrophoresis can result in inability to distinguish between a patient’s M protein and the t-mAb. This can hinder accurate assessment of therapeutic responses or may lead to extensive workup with immunofixation electrophoresis and gel-shift assays for accurate confirmation of a complete response, or to exclude the emergence of a new clone. There is a need for an analytically specific routine method to identify t-mAb interference. MALDI-TOF mass spectrometry is an especially promising tool, as due to its ability to identify M proteins’ molecular mass, it has the potential to recognize any t-mAbs with known amino acid composition. We have used the EXENT® Immunoglobulin Isotypes (GAM) Assay for the EXENT Analyzer (The Binding Site, part of Thermo Fisher Scientific) to assess the reproducibility of daratumumab (anti-CD38, IgGK) detection in an analytical study, and assessed the EXENT System’s ability to detect daratumumab in a cohort of MM patients. Methods The expected mass to charge ratio (m/z) of the +2-charge state of daratumumab’s kappa light chain was calculated based on published information. Daratumumab was spiked into normal human serum at 1.0 g/L, and this material was tested in 5 replicates a day over 5 days. In addition, 109 samples from 34 MM patients receiving daratumumab, carfilzomib, lenalidomide and dexamethasone (Dara-KRd) treatment were also tested. Samples were obtained after 14 or 28 days following daratumumab administration in various treatment cycles, ranging from cycle 4 to end-of-treatment. Results The expected m/z for the +2-charge state of daratumumab’s kappa light chain was calculated as 11,691.01 to 11,693.1. In the reproducibility experiment, the median m/z was 11691.3, within the expected range. The difference between the lowest and highest m/z was 1.3 across the 25 replicates. Daratumumab was detected in 103 out of 109 samples (97.3%): 51/57 (89.5%) in IgGK samples, and in all IgGL (23) and IgA (29) samples. In two IgGK patients, where daratumumab was not detected in 2 out of 4 samples and in 1 out of 3 samples, respectively, the patients’ own M proteins were in the proximity (24.8 and 27.1 m/z) of daratumumab’s m/z. In the other three cases where daratumumab was not modeled by the algorithm, the daratumumab peak overlapped with the endogenous M-protein's adduct peak (estimated m/z 11,688) in one sample; it was masked by the shoulder of a large (8.42 g/L) IgGK M-protein in another sample; and no obvious reason was identified in the third sample. There was no difference in the detection rate between 14 days and 28 days post administration. Conclusions The EXENT Immunoglobulin Isotypes (GAM) Assay has successfully identified the presence of daratumumab in approximately 90% of IgGK patient samples in addition to all other (non-IgGK) MM samples. These results demonstrate the potential of the EXENT Immunoglobulin Isotypes (GAM) Assay for recognizing known t-mAb interference.
Lakos et al. (Tue,) studied this question.