Development of a competitive inhibition kinetic ELISA to determine the inhibition constant (Ki) of monoclonal antibodies

Antibody-antigen interactions are mediated by the same molecular recognition mechanisms as those of an enzyme and its substrate. On this basis, we developed a competitive inhibition kinetic ELISA to measure monoclonal antibody (mAb) inhibition constants. Serially diluted samples of ligand (mAb) and inhibitor (soluble antigen) were incubated to equilibrium in ELISA plates coated with a fixed concentration of antigen (receptor). Plates were washed, and bound mAb measured with antiglobulin-peroxidase. Initial velocity data of receptor-bound mAb at various ligand and inhibitor concentrations were analyzed with enzyme linear competitive inhibition methods by non-linear regression (NLR), linear transformations (Cornish-Bowden, Lineweaver-Burk, Hanes-Woolf, Dixon, Cortés [1/i0.5 vs. Vi/Vmax], Ascenzi [Ks/Vmax/Ks,0/Vmax vs. [I]]) and NLR IC50 plots, to derive mAb inhibition constants (Ki). We obtained similar mAb Ki and Kd values by ELISA and surface plasmon resonance, which confirmed the accuracy of the ELISA method. This competitive inhibition ELISA is a simple (it requires no labeling or prior knowledge of antibody concentration), sensitive (it detects Ki values in the low nanomolar range by conventional colorimetry), and reproducible method with which to calculate mAb inhibition constants.