Immunotherapies have the potential to impact our understanding of the immune system and treat a diverse array of conditions, from chronic autoimmune diseases to different types of cancer. Therapeutic monoclonal antibodies (t-mAbs) are part of this revolution, and more than 70 of them are already Food and Drug Administration (FDA)-approved.

Antibodies are not just immunoassay reagents anymore—they also are present in clinical serum samples from more and more patients each day.

Clinical laboratories will have many roles as t-mAbs expand: developing new assays to differentiate a t-mAb from an endogenous, disease-causing clone or a potential test interference; monitoring t-mAb therapeutic ranges for better patient outcomes; and assessing for the loss of response to therapy that is associated with formation of autoantibodies against the t-mAb (Figure 1) (1).

The expanding use of t-mAbs presents an opportunity for clinical laboratory professionals. No matter the area in which laboratorians work, they soon will encounter a t-mAb case and have to deal with it: perhaps as a potential interference, an unsuspected immunofixation electrophoresis (IFE) immunoglobulin G (IgG) kappa, or perhaps as part of therapeutic drug monitoring (TDM) and immunogenicity assessments. It is our responsibility to think of t-mAbs as interferences when troubleshooting immunoassays, and to develop ways for labs to cope with these new entities.

This article examines a few scenarios clinical laboratories may face that open a new, versa-tile role for mass spectrometry (MS) in this field. A companion article explores specific strategies for MS testing of t-mAbs.

Differentiating a Disease-causing Clone From a t-mAb

Serum protein electrophoresis (SPE) and IFE are essential in the diagnostic work-up of suspected monoclonal gammopathies. The finding of a monoclonal protein, even if small, usually triggers additional costly and invasive investigations, such as biopsies of bone marrow, kidney, or heart. With the approval of two IgG kappa t-mAbs for treatment of myeloma (daratumumab and elotuzumab) and others in the pipeline, the visualization of an iatrogenic IgG kappa monoclonal protein has become a common finding in high-volume immunology laboratories (2, 3).

In the absence of clinical history or medication records, an incidental finding of IgG kappa may suggest a monoclonal gammopathy of undetermined significance (MGUS), the most prevalent pre-malignant monoclonal gammopathy.

When monitoring individuals with a known endogenous monoclonal protein, the finding of a t-mAb may be misinterpreted for the original disease clone. This can interfere with the interpretation of a complete response or suggest a new emerging clone. To alert the ordering physician and save healthcare resources, laboratories should consider including a comment in the IFE interpretative report suggesting the new finding could be a t-mAb. In this scenario, confirming the presence of a t-mAb requires specialized testing.

Pre-analytical specimen treatments are available that use anti-idiotypic antibodies against the t-mAb that change its electrophoretic migration on SPE and IFE (4). MS-based methods also have broad potential to deal with this problem, primarily identifying the accurate mass of the t-mAb (5-7).

In transfusion medicine, however, laboratories need to identify t-mAbs for another reason. The drug daratumumab, an anti-CD38 t-mAb, results in panreactivity of antibody screening as-says for routine compatibility testing. The panreactivity interference complicates and delays the selection of compatible red blood cells for transfusion (8).

Laboratories had to come up with a protocol using a reducing agent (dithiothreitol) to mitigate this interference in order to report accurate test results. Patient histories and medication records have regained importance in the era of t-mAbs, and cross-talk among clinicians and laboratorians is needed more than ever.

A clinical laboratory may also deal with questions from clinicians confused by unexpected laboratory results in patients on t-mAbs. This may warrant interpretation as well as troubleshooting. Versatile laboratory techniques are required for troubleshooting in a timely manner.

Therapeutic Drug Monitoring and Immunogenicity

In autoimmune disorders such as inflammatory bowel disease (IBD), the introduction of t-mAbs a decade ago that target tumor necrosis factor alpha (TNF-α) has significantly improved the quality of life for patients by delaying the onset of abdominal surgeries.

Therapy with mAbs is expensive and more than one-third of patients on TNF-α inhibitors show no response to induction therapy (primary non-responders). Moreover, in up to 50% of the responders, therapy becomes ineffective over time (secondary non-responders). The loss of response to therapy is the primary indication for TDM.

The loss of therapeutic response may be associated with immunogenicity, an immune response with the development of autoantibodies against the t-mAb. Anti-drug-antibodies (ADAs) can hamper therapy efficacy when they bind to the antigen-binding fraction of the t-mAb and neutralize its effect, leading to faster clearance. For this reason, clinical laboratories commonly assess TDM and immunogenicity together in a reflex format after determining t-mAb concentrations.

Concentrations of TNF inhibitors above certain thresholds result in improved responses to therapy and better outcomes when compared to empirical trial and error, and thus, success in keeping patients on t-mAbs for as long as possible (9). However, if ADAs are found in the set-ting of partial response or loss of response and low trough concentrations of the t-mAb, a change in therapeutic regimen is required—either by adding an immunomodulator to suppress the immune response, increasing the dose, or switching to a different t-mAb (9, 10).

Outside of IBD, comprehensive studies are not widely available showing the link between serum t-mAb concentrations and the appearance of ADAs with loss of response and outcomes. But these studies may gain importance in the near future as t-mAbs in other fields such as oncology are used for longer periods of time and response rates vary. The recommendations for patient management based on TDM are mostly available for TNF inhibitors, but the strategy is likely to continue to grow and be translated to other areas as more studies are performed.

How to Tackle the Field of t-mAbs

Developing immunoassays can be time-consuming and labor intensive since specific reagents and ADAs need to be generated for each new t-mAb, which requires a minimum of 2–3 months for production. Even with advances in MS, immunoassay platforms will not become obsolete and are necessary for the immunogenicity assays often associated with the TDM of t-mAbs.

Many methods are available to measure t-mAbs and ADAs: enzyme-linked immunosorbent assays, electrochemiluminescence immunoassays, reporter gene assays, and size exclusion liquid chromatography (11), but MS has turned into an essential tool in the era of proteomics and t-mAbs.

In such a fast-paced field with a dozen new t-mAbs approved by FDA in 2018 alone, a triple quadrupole, time-of-flight, or high resolution accurate mass instrument can be used to identify or quantify the t-mAb without the need for analyte-specific reagents (besides standards, of course). MS aids in keeping down costs and in performing proof-of-concept/troubleshooting experiments, allowing for many different assay variations with relatively fast test development and visualization of eventual interferences.

The future holds more applications of t-mAbs and evaluation of ADAs for clinical laboratories. Developing innovative assays is one of the things we do best, and guiding test utilization is a strength of clinical laboratories, along with strong oversight of assays and close collaboration with prescribers reviewing patients’ outcomes. Certainly, t-mAbs are not going away, they are here to stay—bridging important gaps for patient care and opening new roads for us to pave.

Maria Alice V. Willrich, PhD, DABCC, FADLM, is an assistant professor of laboratory medicine and pathology and medicine in the College of Medicine of Mayo Clinic in Rochester, Minnesota. +Email: [email protected]


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