Aug 03, 2025
In-vitro plasma protein binding
- Nick Lynch1,2
- 1Curlew Research;
- 2ASAP Discovery Consortium
- ASAP Discovery
Protocol Citation: Nick Lynch 2025. In-vitro plasma protein binding. protocols.io https://dx.doi.org/10.17504/protocols.io.8epv5o866g1b/v1
License: This is an open access protocol distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Protocol status: Working
We use this protocol and it's working
Created: May 25, 2025
Last Modified: August 03, 2025
Protocol Integer ID: 218905
Keywords: ADME, DMPK, drug discovery, plasma protein, drug interaction, protein binding, accurate prediction of human pharmacokinetic parameter, human pharmacokinetic parameter, plasma protein, quantitative measurements essential for comprehensive drug development program, unbound drug fraction, protein level, binding study, drug development, equilibrium dialysis, administered drug, influences drug efficacy, protein, understanding drug behaviour, comprehensive drug development program, bound compound
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Abstract
In vitro plasma protein binding studies in various animal models (Human, rat, mouse, dog, fetal calf serum) are crucial for understanding drug behaviour and predicting human responses. These studies help determine how a drug interacts with plasma proteins, influencing its distribution, metabolism, and elimination.
Protein binding significantly influences drug efficacy and safety profiles. Highly bound compounds (>90%) exhibit greater sensitivity to changes in protein levels due to disease states, age, or co-administered drugs. These data support regulatory submissions and inform clinical trial design by enabling accurate prediction of human pharmacokinetic parameters from preclinical models.
The assay employs equilibrium dialysis, ultrafiltration, or ultracentrifugation methods to separate bound and unbound drug fractions, providing quantitative measurements essential for comprehensive drug development programs.
Troubleshooting
Safety warnings
Always wear appropriate PPE for this protocol
Refer to Material Safety Data Sheets for additional safety and handling information.
Summary
Plasma protein binding assay is performed in 48-well RED device
This protocol can be used with the following protein plasma:
- Rat
- Mouse
- Fetal calf serum
- Dog
- Human
Sample Preparation
The compounds are incubated at 5µM in a shaking incubator at 37ºC
The plasma is diluted to 50% with pH 7.4 phosphate buffer and warmed to 37ºC for 10min, test compound is then added to achieve 5µM
Assay
500μL of dialysis buffer is added to one side of the chamber of the RED device insert housed within the heated Teflon bock, and the incubation is initiated by the addition of 300μL of the test compound protein solution to the opposing chamber. The final solvent concentrations are 0.95% Methanol and 0.05% DMSO (99% plasma).
The samples are incubated for 4 hours to allow equilibrium to be reached
Equal volumes from the buffer and the plasma chambers (50μL) are transferred into separate wells of a deep well plate
50μL of 50% blank plasma are added to the buffer samples, and 50μL of buffer are added to the 50% plasma samples, followed by mixing (this ensures matrix matching of samples from both the free and bound fractions)
300μL of ice cold acetonitrile containing internal standard are added to precipitate the protein
The quenched samples are then centrifuged to precipitate the protein.
Analytical
The supernatants are analyzed by LC-MS/MS using generic analytical methods to quantify the concentration of test compound in each chamber.
Data Analysis
The fraction unbound is calculated by dividing the concentration in the buffer chamber by the concentration in the plasma chamber.
For each test compound injection,
response ratio = Test peak area / Internal standard peak area
From the standard curve, the response ratio is converted to concentration of test compound.
The binding value of test compound bound is calculated from,
Fraction Unbound = Concentration buffer chamber / Concentration plasma chamber
% Free = Fraction Unbound X 100
% Bound = 100% - % Free
The % Recovery is calculated via:
% Recovery = (Concentration buffer chamber + Concentration plasma chamber) * 100 / Total quantity incubated
Acknowledgements
Grateful to Concept Life Sciences for supplying the original protocol summary