Assay Development Guidance

Every discovery program needs a reliable way to measure binding. The three most common platforms—ELISA, surface plasmon resonance (SPR), and bio-layer interferometry (BLI)—each have different strengths and failure modes, and choosing the wrong one for your stage of discovery can waste months of effort.

ELISA is the workhorse of primary screening. It's cheap, high-throughput, and accessible to any lab. But it gives you a single endpoint measurement—no kinetics, no information about on-rate vs. off-rate, and plenty of opportunities for false positives from nonspecific binding, avidity effects, or aggregated antigen. A good ELISA protocol includes proper blocking optimization, antigen coating density titration, and negative controls against irrelevant targets and empty wells. Skip these steps and your hit list will be contaminated with sticky binders that look great on plate but fail in every downstream assay.

SPR (Biacore, Carterra) and BLI (Octet) provide real-time kinetic data: association rate (k_on), dissociation rate (k_off), and equilibrium dissociation constant (K_D). SPR is the gold standard for affinity measurement in regulatory filings. BLI offers higher throughput and simpler fluidics but can be less sensitive for very slow off-rates. Both require careful attention to surface chemistry, regeneration conditions, and mass transport effects. The most common pitfall is over-immobilizing antigen on the sensor surface, which creates avidity artifacts that make bivalent molecules appear to have picomolar affinity when their true monovalent K_D is in the nanomolar range.

Binding is necessary but not sufficient. Your molecule needs to do something—block a receptor-ligand interaction, agonize a pathway, recruit effector cells, or neutralize a toxin. Functional assays bridge the gap between binding data and biological relevance, and they should be established early enough in the program to influence lead selection, not just validate it after the fact.

Cell-based assays add another layer of complexity and relevance. Reporter gene assays (luciferase, GFP) provide quantitative readouts of pathway activation or inhibition. ADCC and CDC assays measure effector function. Internalization assays confirm that your molecule reaches the right cellular compartment for antibody-drug conjugate or targeted degradation applications. Each of these requires its own optimization cycle: cell line selection, seeding density, incubation times, and controls. The investment is substantial, but functional data is what separates a lead candidate from a binding curiosity.

Assay artifacts are the silent killers of discovery programs. Aggregated antigen inflates apparent affinity. Polyreactive binders pass primary screens but fail specificity panels. Batch-to-batch variation in antigen preparation introduces noise that obscures real SAR trends. The defense is systematic: validate your antigen by SEC and thermal shift before every screening campaign, include positive and negative controls on every plate, run orthogonal assays to confirm primary screen hits, and never trust a single data point from a single assay format. Rigor at the assay level is what makes everything downstream—lead optimization, developability assessment, and candidate selection—possible.