App note

Measuring a Therapeutic Antibody's Binding to Plasma Proteins

An antibody drug that sticks to an abundant blood protein has less of itself free to reach its target. SPR measures that binding, even when it is weak.

SystemP4SPR

ApplicationAntibody characterization, drug development

MoleculesTherapeutic IgG, peptide target, plasma protein

In short: Researchers at Brigham and Women's Hospital (Harvard Medical School) found that a biosimilar of lecanemab, an Alzheimer's disease antibody, binds fibrinogen in plasma. On the P4SPR, they measured both interactions on the same antibody surface: the target binding and a binding to fibrinogen about 1,000 times weaker.

The need

For small-molecule drugs, plasma protein binding is a routine check: a drug bound to albumin or another blood protein is not free to act, and the bound pool can change how long the drug lasts. For antibody drugs, the question is rarely asked. This study asked it for lecanemab, which is given by infusion every two weeks, and found a plasma partner: fibrinogen, one of the most abundant proteins in blood.

The binding is weak, which is exactly what makes it easy to miss. But fibrinogen is present at grams per litre, millions of times more than the antibody's target, so even a weak interaction can hold a meaningful share of the drug.

How it works on the P4SPR

The antibody is attached to an AffiCoat sensor by standard amine coupling. Its target, a short amyloid-β peptide, and purified fibrinogen are then injected over the same surface at a range of concentrations. Each one gives its own binding curve and its own affinity (KD), so the drug's intended and unintended partners are measured side by side, label-free (Figure 1).

Antibody on the sensor, binding its small peptide target tightly and the large fibrinogen protein weakly — Figure 1. One antibody surface, two partners: the small amyloid-β target binds tightly; fibrinogen, large and abundant, binds weakly and briefly.

What the study showed

	Result
Target binding (amyloid-β peptide)	Affinity in the same range as published for clinical lecanemab, which supports the biosimilar as a stand-in
Plasma partner	Fibrinogen, identified by fractionation and immunoblotting, then confirmed by SPR
Fibrinogen binding	About 1,000 times weaker than the target binding, in the range typical of plasma protein binding (Figure 2)

Affinity scale: target binding in the sub-micromolar range, fibrinogen binding about 1,000 times weaker — Figure 2. The two affinities on one scale. Both were measured on the same antibody surface.

Full method and data: Bellier et al., Annals of Clinical and Translational Neurology 2024 (reference below).

Why the P4SPR fits this question

What the lab needs	What the P4SPR brings
Confirm a suspected binding partner	Direct, label-free binding to the purified protein, after a hint from blots or fractionation
Strong and weak interactions in one setup	The same antibody surface measures a sub-micromolar target and a millimolar plasma partner
Large, sticky plasma proteins	AffiCoat low-fouling surface, so the signal comes from binding, not adsorption
Side-by-side comparison	Four channels read in turn: sample and reference on the same sensor
Running it in an academic lab	Benchtop and portable, no pump needed for affinity measurements

Beyond lecanemab

Any antibody drug or candidate can be checked the same way against the abundant proteins of blood: fibrinogen, albumin, immunoglobulins, complement. Finding a plasma partner early helps explain unexpected pharmacokinetics, and checking it is quicker than discovering it in the clinic. For screening many antibody candidates, see the screening platform.

What you need

Item	Source
P4SPR	Affinité Instruments
AffiCoat Sensor (SAMAC-01) and Amine Coupling Kit (AMIN-KIT)	Affinité Instruments
Antibody to immobilize	Your antibody or biosimilar
Target and candidate plasma proteins, purified	Commercial (here: amyloid-β peptide, human fibrinogen)
Running buffer; BSA for the plasma-protein samples	Standard reagents

Acknowledgements

Thank you to Jean-Pierre Bellier, Jasmeer Chhatwal and their colleagues for this work and for our continuing collaboration on lecanemab binding assays.

Illustration of the Affinité team with Jean-Pierre Bellier in his lab, with the SPR instrument on the bench — The Affinité team with Jean-Pierre Bellier at Harvard.

Reference

J.-P. Bellier et al., "Identification of fibrinogen as a plasma protein binding partner for lecanemab biosimilar IgG: implications for Alzheimer's disease therapy," Annals of Clinical and Translational Neurology, 2024. doi:10.1002/acn3.52227.

Illustrations by Affinité Instruments. Figure 2 places the two affinities reported in the reference above on one scale.