Daratumumab (dara; brand name Darzalex®, Janssen Pharmaceuticals) is an IgG1 kappa monoclonal antibody targeting CD38. It is used to treat multiple myeloma, a malignant plasma cell disorder. Since its FDA-approval, case reports identifying a new small gamma fraction abnormality in serum protein electrophoresis (SPEP) and the identification of an IgG kappa on immunofixation (IFE) have been common. Dara is given in high enough concentrations that if a blood sample is drawn within a couple of days after the infusion, dara will show up as a band on SPEP/IFE. On agarose gel SPEP, its migration is very cathodal, at the end of the gamma fraction.
If the original multiple myeloma patient clone is of a different isotype, e.g., an IgA lambda, the finding of a new IgG kappa during follow-up should prompt the laboratory to report it as a potential therapeutic monoclonal antibody (t-mab), since the appearance of a second, new IgG kappa disease clone would be much rarer. The most common type of myeloma, however, is caused by monoclonal IgG kappa clones (approximately 35% of all myeloma cases). If the original clone migrates separately from dara, the laboratory may also infer that the second clone appearance is a t-mab and report that finding as such. The most challenging scenario would be when the endogenous IgG kappa and the t-mab co-migrate on SPEP. The review of patient histories, although labor-intensive, regains importance in the era of t-mabs.
Our practice in the laboratory is not to gate or quantitate a small IgG kappa that could potentially be a t-mab. However, I recently had a case where there was a 1g/dL IgG kappa M-spike on SPEP. The patient was on daratumumab. The clinicians were asking how much of that peak was daratumumab and how much of it was the endogenous IgG kappa clone. To answer that question one would use the DIRA Hydrashift assay, an immunofixation with an anti-dara antibody capable of shifting the dara out of the way so that you can quantitate the endogenous M-protein. The test is on its way to be FDA-approved. Future applications using mass spectrometry tests such as MASS-FIX may be able to address this as well, although not available just yet 1. Does quantitation really matter? The Janssen pharmacokinetics clinical trials show the peak for dara concentration is 0.1 g/dL. Even if the SPEP sample was drawn at peak after an infusion, the contribution of dara to the M-spike would be minimal except for very small M-spikes, and is unlikely to change management for most patients. In the example we had, 1 g/dL or 0.9 g/dL would mean the same thing for the patient: partial response, continue therapy.
In a recent meeting with our hematologists, we asked them how useful an assay to detect daratumumab would be. To our surprise, they were not hungry for the test. They said they know when it’s prescribed and they can interpret the finding of an IgG kappa in the gels without concerns. One may ask: what about complete response for clinical trials? This situation will require the DIRA assay and specialized laboratories will be hired to run the test for these studies. The samples from trials are unlikely to show up in regular clinical laboratories. For other t-mabs, specialized labs will have to develop unique assays when they interfere with SPEP, however a lot of them are not prescribed in a sufficiently high dose to interfere2. Would you offer a test for daratumumab interference on a routine basis in your laboratory? Not if that is applicable only for clinical trials, is our current position. The MASS-FIX as a replacement for IFE test can have a significant role in improving dara’s detection in the near future. In addition, the laboratory has an important role in troubleshooting and education about this potential interference and it is recommended to report it when observed.
- Mills JR, Kohlhagen MC, Dasari S, et al. Comprehensive Assessment of M-Proteins Using Nanobody Enrichment Coupled to MALDI-TOF Mass Spectrometry. Clin Chem. 2016;62:1334-1344.
- Willrich MA, Ladwig PM, Andreguetto BD, et al. Monoclonal antibody therapeutics as potential interferences on protein electrophoresis and immunofixation. Clin Chem Lab Med. 2016;54:1085-1093.