Where Drugs Travel: Distribution & Plasma Protein Binding
Once a drug reaches the blood, the journey isn't over — it's just beginning. The bloodstream is a highway, but most of a drug never rides free: it hitches onto blood proteins like passengers strapped into seats, and only the few standing by the doors can actually step off and work. Understand who rides bound and who rides free, and you'll understand why one tiny new pill can suddenly make a patient on warfarin bleed.
A 70-year-old woman has taken the blood-thinner warfarin safely for years — her dose perfectly balanced. Then a doctor adds a new drug for an infection. Days later she arrives with bruises, a nosebleed that won't stop, and blood in her urine. Nobody changed her warfarin dose. So why is she suddenly over-anticoagulated? The answer isn't in how much warfarin she took — it's in how much of it was set FREE. To see it, we have to follow the drug after it leaves the gut and enters the river of blood.
What distribution actually means
Distribution is the drug's move from blood into tissues. After absorption, a drug is in the plasma. Distribution is how it then spreads from the bloodstream out into the interstitial fluid between cells and, for many drugs, inside the cells themselves. It decides which organs the drug reaches (its effect) and which it shouldn't (its side effects). Four things govern how fast and how far a drug distributes: blood flow to the organ, how leaky that organ's capillaries are, the drug's lipid solubility, and how tightly it binds — to blood proteins on one side and tissue components on the other.
Blood flow explains the timing. The richly-perfused organs — brain, heart, liver, kidneys — receive the drug within seconds to minutes. The poorly-perfused ones — fat, resting muscle, skin — fill up slowly over minutes to hours. So a drug's first wave hits the vital organs first, then slowly seeps into the bulkier, slower tissues. This two-speed delivery becomes the whole story of redistribution in Part 2.
The passengers: bound vs free
Here is the pivotal idea. In the blood, a drug exists in two states: bound to plasma proteins, or free (unbound) in the plasma water. The two are in constant equilibrium. But only the FREE drug can do anything — only free drug can leave the capillary, reach its receptor and act, be filtered by the kidney, or be metabolized by the liver. The bound fraction is pharmacologically silent: it is a floating reservoir, temporarily parked, releasing more free drug as the free portion is used up.
Two proteins do most of the binding. Albumin, the most abundant plasma protein, binds mainly acidic and neutral drugs (warfarin, phenytoin, aspirin, many others). Alpha-1-acid glycoprotein binds mainly basic drugs (like propranolol, lidocaine). Some drugs are extremely highly bound — warfarin is about 99% bound, meaning only ~1% is free and active at any moment. That single fact is the key to the bleeding woman.
The new drug was also highly protein-bound and competed for the same albumin seats, displacing warfarin. Even a small shift — from 1% free to 2% free — DOUBLES the active warfarin in her blood, tipping her into dangerous over-anticoagulation without a single change to her dose. (In reality the body often compensates by clearing the extra free drug, so pure displacement is usually transient — but with warfarin, where the new drug frequently ALSO blocks warfarin's metabolism, the effect is real and clinically feared.)
- Distribution = drug moving from blood into tissues and cells.
- Blood flow, capillary leakiness, lipid solubility, and binding govern it.
- Only FREE (unbound) drug acts, is metabolized, and is excreted.
- Albumin binds acidic/neutral drugs; α1-acid glycoprotein binds basic drugs.
- For a 99%-bound drug, a tiny change in binding can double the active free drug.
When the reservoir runs low: hypoalbuminemia
If the number of seats falls — in liver disease, kidney disease (nephrotic syndrome), malnutrition, burns, or old age — there is less albumin to bind drugs, so the free fraction of a highly-bound drug rises. The total measured drug level can look normal or even low, yet the patient shows toxicity, because the ACTIVE free portion is elevated. This is a classic trap with phenytoin: in a patient with low albumin, a 'normal' total phenytoin level can hide a toxic free level.
Measure free, not just total, when it matters. For highly-bound, narrow-window drugs like phenytoin in a low-albumin patient, order a FREE drug level or use a correction formula. Chasing the total level alone can make you overdose a patient whose free (active) drug is already high. Binding is invisible on a standard total assay.
- Thinking the bound fraction is active. It's an inert reservoir — only free drug works.
- Trusting a 'normal' total level in low albumin. The free (active) level may be toxic.
- Assuming displacement alone always causes toxicity. Often it's transient unless metabolism/excretion is also blocked.
- Ignoring that basic drugs bind α1-acid glycoprotein, not albumin — different rules.
Warfarin is ~99% protein-bound. A new drug displaces just 1% more. The active (free) warfarin roughly:
- Distribution spreads the drug from blood into tissues; perfusion sets the timing.
- Only free (unbound) drug is active, metabolized, and excreted; bound drug is a reservoir.
- Albumin binds acidic/neutral drugs; α1-acid glycoprotein binds basic drugs.
- For highly-bound, narrow-window drugs, low albumin or displacement can raise the free (active) level dangerously.
- Katzung BG. Basic & Clinical Pharmacology — Drug distribution & plasma protein binding.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Distribution & protein binding.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Drug distribution & body compartments.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Drug distribution & protein binding.
- Winter ME. Basic Clinical Pharmacokinetics — Protein binding & free drug concentration (phenytoin correction).
