INTRODUCTION

Etanercept (trade name Enbrel®) is a protein drug used to treat autoimmune diseases by adsorbing tumor necrosis factor (TNF; a soluble inflammatory cytokine). Etanercept is a fusion protein produced through expression of recombinant DNA by linking the extracellular ligand-binding portion of TNFRSF1B to the Fc component of human immunoglobulin G1 (IgG1). It reduces the effect of naturally present TNF, functioning as a decoy receptor that binds to TNF.

Etanercept is indicated for the treatment of moderate to severe rheumatoid arthritis (RA), psoriatic arthritis, ankylosing spondylitis, and moderately to severely active polyarticular juvenile idiopathic arthritis. Serum concentration of ENBREL® may predict some clinical outcome during maintenance therapy.

PRINCIPLE OF THE ASSAY

This assay employs the sandwich enzyme immunoassay technique. Capture antibody is coated onto a 96 well microplate. Calibrator and test samples are pipetted into the appropriate wells. Etanercept present in biological matrices is bound by the immobilized anti-Etanercept antibody. After washing away any unbound substances, enzyme linked anti-Etanercept antibody is added to the wells. The plate is washed to remove any unbound antibody-enzyme reagent and a substrate solution is added to the wells for color development. The color development is proportional to the amount of Etanercept present in test samples. The color development is stopped and the intensity of the color is measured.

Etanercept prepared in human serum from 10,000ng/ml to 39.06ng/ml then diluted 1 in 50 in Assay Buffer (200ng/ml to 0.78ng/ml diluted). Typical range of standard curve recommended for use in assay is 2,500ng/ml to 78ng/ml (50ng/ml to 1.56ng/ml diluted).