General Principles Of Pharmacology:Passage of drug into the CNS
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The largest pores are extremely permeable, allowing the passage of all but the largest of proteins. In contrast, the pores in
muscle allow only very small amounts of albumin to pass. Drugs can pass into the CNS through the capillary circulation (through
the blood-brain barrier) or via the cerebrospinal fluid (through the choroid plexus). In both cases, the barriers are similar to
those encountered for the entry of a drug into the cytoplasm of a typical cell. Thus, except when a drug has a specific uptake
mechanism, only small, lipid-soluble drugs are able to enter the CNS.
Inflammation of the epithelial layer may increase its permeability and allow charged and, therefore, normally impermeant drugs (for example, penicillin and streptomycin) to attain therapeutic concentrations in meningitis.
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