What does the tissue-to-plasma partition coefficient (Kp) represent?

What does the tissue-to-plasma partition coefficient (Kp) represent?

• A. The ratio of tissue concentration to plasma concentration at equilibrium
• B. The ratio of plasma concentration to tissue concentration at equilibrium
• C. The rate constant for tissue binding
• D. The clearance rate from tissue

Answer: (A)

Kp represents how a drug distributes between tissue and plasma once equilibrium is reached. It is defined as the concentration in tissue divided by the concentration in plasma at steady state. Because it’s a ratio of concentrations, it’s dimensionless. If the tissue concentration is higher than plasma, Kp is greater than 1, indicating substantial tissue distribution or strong tissue binding. If the tissue concentration is lower, Kp is less than 1, showing limited distribution to that tissue. This concept helps explain tissue exposure and feeds into models of overall distribution in pharmacokinetics. It’s not a rate constant for binding, nor a clearance from tissue.

Kp represents how a drug distributes between tissue and plasma once equilibrium is reached. It is defined as the concentration in tissue divided by the concentration in plasma at steady state. Because it’s a ratio of concentrations, it’s dimensionless. If the tissue concentration is higher than plasma, Kp is greater than 1, indicating substantial tissue distribution or strong tissue binding. If the tissue concentration is lower, Kp is less than 1, showing limited distribution to that tissue. This concept helps explain tissue exposure and feeds into models of overall distribution in pharmacokinetics. It’s not a rate constant for binding, nor a clearance from tissue.