In PBPK modeling, what is the role of tissue-specific partition coefficients?

• A. They describe blood flow rates to tissues
• B. They describe the ratio of drug concentration in tissue to plasma at equilibrium and influence distribution
• C. They are constant across all tissues
• D. They are not used in distribution modeling

Answer: (B)

In PBPK modeling, tissue-specific partition coefficients capture how a drug distributes between tissue and plasma when equilibrium is reached. Each tissue has its own Kp, a ratio that reflects how much drug concentrates in the tissue compared with the plasma, at equilibrium. This value is shaped by the tissue’s composition—lipid content, water, and binding sites—as well as the drug’s properties, like lipophilicity and affinity for tissue components.

Because Kp sets the equilibrium distribution, it directly influences how much drug ends up in each tissue and, consequently, the overall pattern and extent of distribution over time. When you simulate how concentrations change across tissues, higher Kp means greater tissue accumulation and lower Kp means less, assuming plasma levels drive the distribution.

It’s important to note that partition coefficients are not about blood flow rates—that’s what perfusion governs for the rate of delivery. They are also not the same across all tissues; each tissue has its own Kp reflecting its unique environment and the drug’s properties. And they are used in distribution modeling to determine tissue concentrations, not to be dismissed as irrelevant.