Quantitation of monoclonal antibody by capture ELISA based on initial enzyme activity rate

Description of a non-competitive ELISA based on time course analysis of ligand binding at saturation, and a direct method for calculating the affinity of monoclonal antibodies

We present a time-course saturation ELISA for measuring the equilibrium constant of the monoclonal antibody (mAb) SIM 28 against horse radish peroxidase (HRP). The curves of HRP binding to a series of fixed mAb dilutions were plotted to completion, and the Kt (= Ks) value (time to occupy 50 % of the mAb paratopes) was determined for each mAb dilution and HRP concentration. Analysis of the kinetic mechanism of the reaction by Lineweaver-Burk and Hanes plots showed that the slope and y-intercept were affected, indicating that mAb ligand saturation follows non-competitive inhibition kinetics in this assay format. In this kinetics, the inhibition constant Ki (= Kd) is the time required to double the slope or halve the Vmax of the Lineweaver-Burk plot. The Kt values of the time courses were doubled (2 x Kt) and normalized by dividing by the total reaction time to obtain a unitless factor which, when multiplied by the concentration of HRP, gives the Ki. The affinity constant of mAb SIM 28 was determined from ELISA data (n = 16) by three methods: i) doubling of Kt, ii) Beatty equation (Kaff = (n-1)/2 (n [HRP']t - [HRP]t), and iii) SPR (Biacore) analysis. The calculated affinities (mean ± 95 % confidence limits) were i) 4.6 ± 0.67 × 10-9 M, ii) Kaff = 0.23 ± 0.03 × 109 M-1 (Kd = 4.8 ± 0.81 × 10-9 M), and iii) 4.3 ± 0.57 × 10-9 M, respectively. The similar results obtained with the three different techniques indicate that this time-course saturation ELISA, combined with the double Kt method, is a repeatable and direct approach to mAb affinity determination.

One-Step Quantification of anti-Covid-19 Antibodies via Dual Affinity Ratiometric Quenching Assays

The global pandemic caused by acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has affected millions of people and paralyzed healthcare systems worldwide. Developing rapid and accurate tests to detect and quantify anti-SARS-CoV-2 antibodies in complex fluids is critical to (i) track and address the spread of SARS-CoV-2 variants with different virulence and (ii) support the industrial manufacturing and clinical administration of anti-SARS-CoV-2 therapeutic antibodies. Conventional immunoassays, such as lateral flow, ELISA, and surface plasmon resonance (SPR), are either qualitative or, when quantitative, are laborious and expensive and suffer from high variability. Responding to these challenges, this study evaluates the performance of the Dual-Affinity Ratiometric Quenching (DARQ) assay for the quantification of anti-SARS-CoV-2 antibodies in bioprocess harvests and intermediate fractions (i.e., a Chinese hamster ovary (CHO) cell culture supernatant and a purified eluate) and human fluids (i.e., saliva and plasma). Monoclonal antibodies targeting the SARS-CoV-2 nucleocapsid as well as the spike protein of the delta and omicron variants are adopted as model analytes. Additionally, conjugate pads loaded with dried protein were studied as an at-line quantification method that can be used in clinical or manufacturing laboratories. Our results indicate that the DARQ assay is a highly reproducible (coefficient of variation ∼0.5-3%) and rapid (<10 min) test, whose sensitivity (∼0.23-2.5 ng/mL), limit of detection (23-250 ng/mL), and dynamic range (70-1300 ng/mL) are independent of sample complexity, thus representing a valuable tool for monitoring anti-SARS-CoV-2 antibodies.

Monoclonal antibody on-rate constant determined from time-course data of ligand binding by capture ELISA: Evaluation of eight data analysis methods

We describe an ELISA method with which to determine monoclonal antibody (mAb) on-rate constants (k₊₁) based on time-course data of ligand (L) binding to plate-bound mAb. The assay was performed in pseudo-first order kinetic conditions ([L] > > [mAb]) and at various starting ligand concentrations. Time-course initial velocity was analyzed by several methods to derive the pseudo-first order (k_obs) and second order (k₊₁) association rate constants of the antibody; the methods included i) an exponential first order rate equation, ii) reaction half-time from the Michaelis-Menten relationship, iii) the V_max/K_m tangent of the time-course curve, iv) Boeker's extrapolated-v_o method, v-vi) modified Hanes-Woolf and Lineweaver-Burk linear plots, vii) a LOS plot, and viii) initial velocity gradient. Due to k₊₁ value dispersion associated with the methods of analysis, the on-rate constant of mAb SIM 253-19 anti-cholera toxin was estimated as an average value of 1.79 ± 0.11 × 10⁶ M^-1 s^-1, 95% CL (1.68-1.89) and 5.8 (%CV [coefficient of variation]), which is similar to the k₊₁ obtained by surface plasmon resonance, 1.60 ± 0.17 × 10⁶ M^-1 s ^-1 (mean ± half range). This kinetic ELISA is a sensitive, quantitative method by which to determine antibody association rate constants.