Plasma Protein Binding of Challenging Compounds.

Area of Science:

• Pharmacology
• Drug Discovery
• Analytical Chemistry

Background:

• Accurate determination of fraction unbound (fu) is essential for drug candidate assessment.
• Current methods may struggle with highly bound compounds, potentially leading to misinterpretations.
• Equilibrium dialysis can be inaccurate for compounds with very low fu if equilibrium is not fully achieved.

Purpose of the Study:

• To address challenges in accurately measuring fraction unbound (fu) for drug candidates.
• To develop and validate improved methods for determining plasma protein binding (PPB) in challenging cases.
• To reassess the need for regulatory restrictions on reporting low fu values.

Main Methods:

• Analysis of over 2000 Pfizer drug discovery compounds using equilibrium dialysis.
• Application of a dilution method to accelerate equilibration for challenging compounds.
• Utilization of a presaturation method to improve accuracy for highly bound substances.

Main Results:

• Equilibrium dialysis showed no systematic bias in precision for low or high fu across a large compound set.
• Accuracy of PPB estimates for highly bound compounds can be poor with standard equilibrium dialysis.
• Improved dilution and presaturation methods accurately measured PPB for compounds with fu < 1%.

Conclusions:

• Accurate experimental fu values are critical for predicting drug-drug interaction potential and characterizing drug candidates.
• Enhanced methods provide reliable fu measurements for highly bound compounds.
• Current regulatory guidelines limiting the reporting of fu values below 1% may need reevaluation.