Therapeutic Drug Monitoring
Infliximab (IFX) is a chimeric antibody that targets the pro-inflammatory cytokine TNF-alpha. The introduction of infliximab has revolutionized the treatment of chronic inflammatory diseases like inflammatory bowel disease (IBD), rheumatoid arthritis (RA) and spondyloarthritis. It has been shown that infliximab can induce deep remission and improve the patient’s quality of life. Some patients do not respond to infliximab therapy upon induction (primary non-responders), while others lose response over time (secondary non-responders).
Secondary loss of response is often due to the development of anti-infliximab antibodies (ATI), because of the immunogenic character of the drug. ATI can develop in any patient undergoing infliximab therapy and are primarily neutralizing the activity of infliximab through immunocomplex formation. In addition, these immunocomplexes are rapidly cleared from the system. Analytically, they are responsible for subtherapeutic infliximab concentrations. Therefore, in the case of very low trough concentrations of infliximab (< 1 μg/mL), subsequent measurement of ATI may be helpful to determine the optimal treatment strategy.
Diagnostic Value
The diagnostic value of the Anti-Infliximab ELISA lies in its ability to stratify patients with subtherapeutic infliximab concentrations (< 1 μg/mL) in patients who need dose intensification or a drug (class) switch. Patients with low infliximab concentrations (< 1 μg/mL) and low ATI titers can benefit from infliximab dose intensification, as shown in several studies. However, the ATI titer of patients with low ATI titers undergoing a dose intensified treatment regimen must be adequally monitored. Patients that have high ATI titers are preferably switched to another drug, both within class or out of class.
Principle of the ATI ELISA
The apDia anti-Infliximab ELISA uses for the calibrator and the controls a highly specific monoclonal antibody – clone 10F9, developed at the KU Leuven - that only bridges infliximab (Remicade®).
Microtiter strips coated with infliximab (Remicade®) are incubated with calibrators, controls and diluted patient samples. During this incubation step ATI binds specifically to the infliximab on the solid phase. After removal of the unbound serum proteins by a washing procedure, the strips are incubated with biotin conjugated infliximab (Remicade®), binding directly to the antigen-antibody complex. After removal of the unbound biotin conjugate, the strips are incubated with peroxidase conjugated streptavidin. After removal of the unbound peroxidase conjugate, the strips are incubated with a chromogenic solution containing tetramethylbenzidin and hydrogen peroxide: a blue colour develops in proportion to the amount of immunocomplex bound to the wells of the strips. The enzymatic reaction is stopped by the addition of 0.5M H2SO4 and the absorbance values at 450 nm are determined. A standard curve is obtained by plotting the absorbance values versus the corresponding calibrator values. The concentration of ATI in patient samples is determined by interpolation from the calibration curve.