Plasma Protein Binding (PPB) Assays
Measure the Pharmacologically Active Fraction of Your Drug
Only the unbound (free) fraction of a drug is generally available to distribute into tissues, interact with pharmacological targets, undergo metabolism, and be eliminated. Plasma protein binding (PPB) studies determine the fraction unbound (fu) in plasma, providing essential data for pharmacokinetic interpretation, dose selection, IVIVE, PBPK modeling, and drug-drug interaction assessment.
At WuXi Biology and Testing, our integrated DMPK platform offers reliable plasma protein binding assays across multiple species and therapeutic modalities to support drug discovery and development from lead optimization through IND-enabling studies.
In Vitro ADME Studies Annually
In Vivo Studies Annually
Successful IND Filings
Why Is Plasma Protein Binding Important?
Plasma protein binding influences the fraction of unbound drug available for tissue distribution, target engagement, metabolism, and elimination. Understanding the fraction unbound (fu) early in development helps researchers interpret pharmacokinetic data, compare compounds, and make more informed development decisions.
PPB data are commonly used to:
- Determine the fraction unbound (fu)
- Interpret pharmacokinetic exposure
- Predict tissue distribution
- Support IVIVE and PBPK modeling
- Evaluate species differences
- Support dose selection
- Support drug-drug interaction assessment
For many compounds, plasma protein binding is a critical parameter for translating in vitro findings into expected in vivo behavior.
Choosing the Right Plasma Protein Binding Method
No single plasma protein binding method is appropriate for every compound. The optimal approach depends on compound properties, therapeutic modality, and study objectives.
Equilibrium Dialysis
The most widely accepted approach for measuring plasma protein binding.
Best for:
- Small molecules
- Routine PPB determination
- Cross-species comparison
- IVIVE
Ultrafiltration
Rapid and efficient, particularly when non-specific binding is minimal.
Best for:
- High-throughput screening
- Early discovery
- Suitable compounds with low membrane binding
Ultracentrifugation
Separates free drug without membranes, making it useful for compounds susceptible to membrane interactions.
Best for:
- Highly lipophilic compounds
- Compounds with significant membrane adsorption
- Special applications
Electrophoretic Mobility Shift Assay (EMSA)
Developed for oligonucleotide therapeutics, EMSA avoids some of the limitations associated with conventional membrane-based PPB assays and provides a robust approach for evaluating protein binding of ASOs, siRNA, and other oligonucleotide modalities.
Best for:
- ASOs
- siRNA
- Oligonucleotide therapeutics
- Emerging modalities
Integrated Plasma Protein Binding Solutions
Plasma protein binding should not be interpreted in isolation.
Our integrated workflow combines PPB with:
- Metabolic stability
- CYP inhibition and induction
- Permeability
- Solubility
- Metabolite identification
- PK studies
- PK/PD modeling
Together, these data provide a more complete understanding of drug disposition and support informed decision-making throughout drug development.
Explore Solutions for Your Program
Explore solutions by development phase, drug modality, therapeutic area, or integrated discovery needs.
You May Also Be Interested In
Get a Quote
Ready to plan your next study? Share your program needs with our team for a tailored estimate.
Frequently Asked Questions
Plasma protein binding measures the proportion of a drug that binds to plasma proteins such as albumin and α1-acid glycoprotein. The remaining unbound fraction (fu) is generally considered available for tissue distribution, target engagement, metabolism, and elimination.
The fraction unbound is widely used to interpret pharmacokinetic data, estimate tissue exposure, support IVIVE and PBPK modeling, and compare compounds during lead optimization.
Albumin is the major binding protein for many acidic and neutral drugs, while α1-acid glycoprotein primarily binds basic compounds. Lipoproteins and other plasma proteins may also contribute depending on the compound.
Equilibrium dialysis is generally considered the reference method for plasma protein binding measurements because it minimizes many experimental artifacts. Ultrafiltration provides faster results but may be affected by non-specific membrane binding for some compounds.
Not necessarily. Many successful drugs exhibit high plasma protein binding. The pharmacological effect depends on maintaining sufficient unbound drug concentrations at the site of action rather than on the percentage of protein binding alone.
Yes. Species differences in plasma protein binding can influence pharmacokinetic interpretation and are commonly evaluated during preclinical development to support translational studies.