Measuring Kinetics in Serum

Biomolecular interaction kinetics remain a cornerstone of both fundamental research and applied drug development. Yet for decades, the tools available for kinetic analysis have required researchers to accept a fundamental compromise: precision or physiological relevance, but not both.

The introduction of in-solution kinetic analysis directly in serum removes that compromise. By allowing immobilization-free kinetic measurements in native biological matrices, this capability marks a turning point in how scientists—whether in academia or industry—can approach molecular interactions.

In-solution Characterization of Biomolecular Interaction Kinetics under Native Conditions

In their recently published paper entitled ''In-solution Characterization of Biomolecular Interaction Kinetics under Native Conditions'' Phillip Willmer and his colleagues  demonstrate how FIDA can quantify kinetic interactions in undiluted human serum, enabling measurements that were previously impractical due to background interference or matrix effects. The authors applied this to the interleukin-2/interleukin-2 receptor system, obtaining kinetic data consistent with previous buffer-based values but reflecting matrix-specific effects such as structural rearrangements. A label-free measurement of a small molecule interaction further illustrates the flexibility of the platform. These findings validate FIDA as a robust, matrix-tolerant biophysical tool that expands the scope of kinetic analysis across academic and pharmaceutical workflows—without the need for assay adaptation or sample purification.

What are the consequences?

Scientific Enablement

Conventional methods for measuring binding kinetics—such as surface plasmon resonance (SPR) or biolayer interferometry (BLI)—rely on surface immobilization, purified reagents, and defined buffer systems. These techniques have delivered detailed kinetic parameters, but typically in conditions far removed from the biological context in which interactions actually occur.

The ability to perform kinetic measurements directly in human serum provides several key scientific advantages:

• Physiological fidelity: Interaction data reflect the true binding behavior in a native, complex environment.
• Endogenous competition and matrix effects: Researchers can now observe how native proteins or small molecules affect affinity and off-rates.
• Unmodified molecules: No need for labeling or tagging, which can alter molecular behavior.

This enables quantitative, grounded studies of molecular interactions as they occur in biological systems—a long-standing goal that is now accessible with FIDA.

Advancing the Academic Frontier

For academic researchers, this capability significantly broadens the types of scientific questions that can be addressed:

• How do specific proteins compete with a therapeutic antibody or ligand?
• What is the true affinity of a cytokine for its receptor in serum?
• How do disease-related changes in plasma/serum composition affect molecular interactions?

Previously, such questions were often inferred through indirect assays or approximated with surrogate systems. Now, direct measurement is possible. The result is greater experimental resolution, increased confidence in conclusions, and the opportunity to study kinetics in a biologically meaningful context, not just a technically convenient one.

This shift moves the field from abstract approximations of interaction behavior toward quantitative biophysics under real-world conditions.

A New Workflow for Pharmaceutical Development

In drug discovery and development, the practical impact is equally significant. Serum is a common hurdle in pharmacology: it introduces variability, background, and interference that are often difficult to quantify. In-solution kinetics in serum offers a more streamlined, information-rich workflow:

• Early-stage evaluation: Determine whether a compound binds its target with high affinity and acceptable off-rates under physiologically relevant conditions.
• Selectivity and specificity: Identify unwanted interactions with serum components that may affect therapeutic index or biodistribution.
• Lead optimization: Rank compounds not only by affinity in buffer, but by performance in serum—reducing late-stage surprises.

By providing precise, quantitative kinetic parameters directly in serum, this approach bridges the gap between in vitro assay results and in vivo pharmacodynamics. It replaces inference with direct measurement and enables faster, more confident decisions at every stage of the pipeline.

Moving from Assay to Insight

Kinetic measurements in serum do more than add technical flexibility—they reframe how molecular interactions are studied, interpreted, and applied.

• For researchers, this is an opportunity to ask more complex, relevant questions and obtain answers that hold true beyond the test tube.
• For developers, it is a chance to bring kinetic insight into environments that more accurately reflect therapeutic reality.

This is not simply an incremental improvement in assay conditions. It is a transition toward biophysical analysis under biological conditions, a shift from simplified systems to systems that matter.

As the usage of FIDA for in-solution kinetics in serum and plasma becomes more widespread, the field can move forward—with fewer assumptions, fewer artifacts, and a clearer view of molecular behavior where it counts.

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