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Membrane Permeability of Drugs

by B. Mahato

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Only the uncharged form of drugs can easily pass through the cell membrane. Most drugs are either weak acid or weak base. A weak acid is uncharged in its protonated form (AH), i.e. while it still has its proton. Once it looses its proton, it becomes negatively charged (A-). A weak base is uncharged in its unprotonated form (B), i.e. before it gains a proton. Once it is protonated, it becomes positively charged (BH+). Using Henderson-Hasselbalch equation, the proportion of the drug in uncharged form in a solution of a given pH can be predicted. Whichever drug has the highest proportion of the uncharged form will be the most permeable across a membrane.

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