Which drugs are more likely limited by permeability rather than perfusion?

Which drugs are more likely limited by permeability rather than perfusion?

• A. Polar molecules are perfusion-limited
• B. Lipophilic drugs are permeability-limited
• C. Polar and high molecular weight drugs are permeability-limited
• D. Low molecular weight lipophilic drugs are permeability-limited

Answer: (C)

In tissue distribution, there are two main rate-limiting steps: how fast blood delivers the drug to the tissue (perfusion) and how fast the drug can cross membranes to get into cells (permeability). If a drug crosses membranes easily, the distribution tends to track blood flow, so it’s perfusion-limited. If crossing membranes is slow, diffusion across membranes becomes the bottleneck, making distribution permeability-limited. Polarity and size are key: polar molecules don’t dissolve well in the lipid bilayer, and high molecular weight compounds face steric and solubility barriers that slow their passage. When either factor is present, the ability to permeate membranes controls how quickly the drug distributes into tissues. Therefore, drugs that are polar and/or have high molecular weight are more likely permeability-limited. In contrast, lipophilic, small drugs cross membranes readily and their distribution is typically limited by blood flow (perfusion).

In tissue distribution, there are two main rate-limiting steps: how fast blood delivers the drug to the tissue (perfusion) and how fast the drug can cross membranes to get into cells (permeability). If a drug crosses membranes easily, the distribution tends to track blood flow, so it’s perfusion-limited. If crossing membranes is slow, diffusion across membranes becomes the bottleneck, making distribution permeability-limited.

Polarity and size are key: polar molecules don’t dissolve well in the lipid bilayer, and high molecular weight compounds face steric and solubility barriers that slow their passage. When either factor is present, the ability to permeate membranes controls how quickly the drug distributes into tissues.

Therefore, drugs that are polar and/or have high molecular weight are more likely permeability-limited. In contrast, lipophilic, small drugs cross membranes readily and their distribution is typically limited by blood flow (perfusion).