18 Investigating the Pattern of Detection of Therapeutic Monoclonal Antibodies Elotuzumab and Daratumumab by Routine Serum Protein Electrophoresis and Immunofixation in Patients with Myeloma

Recently approved therapeutic monoclonal antibodies (t-mAbs) for myeloma, daratumumab (anti-CD38, IgG K), and elotuzumab (anti-SLAMF7, IgG K) may confound follow-up serum protein electrophoresis (SPEP) and immunofixation (IF) results in treated patients. We investigated the frequency and timing of detection of these t-mAbs during routine SPEP and IF ordered on myeloma patients undergoing therapy.

From pharmacy chart review, we identified 19 occurrences of t-mAb therapy (nine daratumumab and 10 elotuzumab) in 16 patients over one year, with the number of doses ranging from 3 to >20 (ongoing). Retrospective review of SPEP and IF ordered prior to, during, and after t-mAb infusion was performed, with positive t-mAb detection defined as a new peak on SPEP and/or new IgG K band on IF, with migration pattern consistent with t-mAb and distinguishable from the myeloma M-protein. Migration patterns of t-mAbs were confirmed by spiking pooled normal sera with individual drugs.

In six occurrences of therapy, the t-mAb could not be differentiated from persistent myeloma M-protein. Of the remaining 13 occurrences, daratumumab was detected in 6/6 (100%) and elotuzumab in 5/6 (83%), both at concentrations up to 0.1 g/dL. Follow-up SPE or IF occurred 7–49 days after initiation of therapy, and t-mAb was detected on first follow-up SPE/IF in 9/11 occurrences. Earliest t-mAb detection was seven days after start of therapy, with persistence throughout the full duration of therapy in all patients. T-mAb was detectable at least 14 days after cessation of therapy in patients with follow-up.

T-mAbs are detected by routine SPEP/IF in 92% of treated patients and appear at least as soon as seven days after initial dose. T-mAbs persist throughout the duration of therapy, warranting increased monitoring by the clinical lab and suspicion whenever a new monoclonal band is detected by SPE/IF in myeloma patients.