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PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

❑ Furthermore, certain anatomic barriers inhibit distribution, a concept referred to

as permeability-limited distribution. This situation occurs for polar drugs

diffusing across tightly join lipoidal membranes.

❑ It is also influenced by:

 the oil/water partition coefficient and

 degree of ionization of a drug.

❑ For example, the blood-brain barrier limits the amount of drug entering the CNS

from the bloodstream. This limitation is especially great for:

✓ Highly ionized drugs and

✓ For those with large molecular weights.

❑ After a drug begins to distribute to tissue, the concentration in tissue increases

until it reaches an equilibrium: at which the amounts of drug entering and leaving

the tissue are the same.

❑ The drug concentration in a tissue at equilibrium depends on:

❑ The plasma drug concentration and

❑ The rate at which drug distributes into that tissue.

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