Tissue average binding and equilibrium distribution: an example with heparin in arterial tissues

Area of Science:

• Pharmacology
• Biomedical Engineering
• Drug Delivery

Background:

• Classical pharmacokinetic models fail to predict drug distribution with novel delivery systems due to insufficient detail on tissue binding.
• Accurate characterization of drug-tissue interactions is essential for optimizing drug delivery and therapeutic outcomes.

Purpose of the Study:

• To develop and validate a novel method for quantifying drug binding characteristics within arterial tissues.
• To measure heparin binding site density, dissociation constant, and fractional volume of distribution in porcine carotid arteries.

Main Methods:

• Tissue samples were incubated with varying concentrations of heparin to reach equilibrium.
• An equilibrium distribution curve was generated by plotting tissue-bound drug concentration against bulk phase concentration.
• Computational fitting of the curve to a tissue distribution model determined binding parameters.

Main Results:

• Heparin binding site density was measured as 4.2 µM (intact media), 2.5 µM (denuded media), and 2.2 nM (adventitia).
• Dissociation constants were 6.8 µM (intact media), 5.0 µM (denuded media), and 8.1 nM (adventitia).
• Fractional volumes of distribution were 0.61 (intact media), 0.70 (denuded media), and 0.87 (adventitia).

Conclusions:

• The developed technique accurately quantifies cumulative drug binding to all potential sites within tissues.
• These measurements provide essential parameters for detailed pharmacokinetic modeling of drug distribution.
• This method is vital for advancing the understanding and design of novel drug delivery systems.