Theraputic Monoclonal Antibody Interference In Immunofixation Electrophoresis

Abstract 4996

The application of monoclonal antibodies represents an increasingly powerful weapon in the arsenal against hematologic diseases. With the advent of new therapies come previously unrecognized laboratory interference that if not properly identified could have clinical repercussions. We encountered two patients, neither of whom had a history of plasma cell dyscrasia, who underwent treatment with rituximab for immune thrombocytopenic purpura (ITP) and were discovered to have a monoclonal IgG kappa band on serum protein electrophoresis (SPEP) followed by immunofixation (IFX). Both patients had severe, steroid-resistant ITP and received rituximab on an empiric basis, at a weekly dose of 375 mg/m² IV. In Patient #1, an evaluation for secondary causes of thrombocytopenia included SPEP and IFX. The IFX yielded a faint, atypical IgG kappa band that migrated to the far cathodal zone of the gel. No band was seen on SPEP alone. Since the patient was treated with rituximab the day before these studies were performed, we considered the possibility that the IgG kappa band was due to circulating rituximab. Rituximab is a chimeric antibody composed of human IgG1 kappa chain constant regions and heavy- and light-chain variable regions from a murine anti-CD20 antibody. The elimination half-life of this therapeutic antibody increases with subsequent doses and is approximately 60 hours for the first dose and 174 hours for the fourth dose (range 26 to 442 hours). An aliquot of rituximab was obtained from pharmacy and analyzed by SPEP/IFX, showing an IgG kappa monoclonal protein band identical to the M-protein in the patient's serum. Repeat assessment of serum from this patient obtained immediately prior to her fourth treatment with rituximab demonstrated that the IgG kappa paraprotein was less intense on IFX compared to the first IFX. Several hours after the fourth rituximab infusion was completed, a more intense band corresponding to the IgG kappa paraprotein was detected again on IFX. In Patient #2, who also received rituximab for steroid-resistant ITP, a similar circulating rituximab-related protein was detected on IFX performed one hour after his fourth rituximab infusion. IFX performed on a sample obtained immediately before treatment did not demonstrate a paraprotein, and testing one week after treatment showed that the band had resolved. We believe that awareness of the presence of a faint monoclonal protein by SPEP/IFX following recent administration of rituximab is important to avoid unnecessary further evaluation for a pathologic monoclonal gammopathy. Alternatively, if suspicion exists, SPEP/IFX should be conducted prior to the initiation of treatment with rituximab or after the treatment course is completed in order to avoid uncertainty. Furthermore, it is important to be cognizant of this finding in patients on maintenance rituximab for an indolent lymphoma and an associated paraprotein who may undergo interval monitoring of the monoclonal gammopathy. It is possible that a similar phenomenon may be seen with other therapeutic monoclonal antibodies with a dosing regimen and half-life that is similar to rituximab, such as cetuximab, a monoclonal antibody directed against the epidermal growth factor receptor. Finally, the presence of a characteristic band on immunofixation may allow for the qualitative assessment of the presence of a therapeutic monoclonal antibody in the serum. Further studies are necessary to determine the sensitivity and specificity of such a laboratory test.