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Low blood flow or slow membrane passage can delay drug reaching the tissue.
AN INTERACTIVE PHARMACOLOGY JOURNEY
The free drug journey.
From an oral dose to a molecular target. Explore absorption, first-pass metabolism, clearance and the unbound drug that connects exposure to pharmacology.
9 CHAPTERSDistribution · binding · clearance3D anatomy · narrated pharmacologyScientific review preview
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Human Reference Atlas anatomy · credits & licencePDB molecular structures · illustrative pathways
When plasma levels don’t tell the whole story.
The animation follows a representative drug. What matters for binding is free drug at the target. A blood measurement may not tell you how much reaches that target.
Why can free drug in a cell differ from free drug in plasma?
The cell can pump it out
Efflux transporters can lower intracellular free drug by moving it back out of the cell.
The cell can metabolize it
Local enzymes can remove parent drug before it binds the target.
Uptake transport, pH differences and tissue heterogeneity can also matter. Free concentrations can approach equality at passive equilibrium when these influences do not oppose it. The principle remains: assess unbound exposure in the relevant target compartment.
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