Membrane Transport of Drugs

Membrane Transport of Drugs involves processes like passive diffusion, facilitated diffusion, active transport, and endocytosis for drug movement across cell membranes.

Membrane Transport of Drugs

• Understanding how drugs cross biological membranes is crucial to all processes of pharmacokinetics (ADME).
• Biological membranes are primarily lipid bilayers interspersed with proteins.

Several mechanisms allow drugs to traverse these membranes:

1. Passive Diffusion

   • Most common mechanism for many drugs.
   • Dependent on the concentration gradient (high → low).
   • Does not require energy.
   • Best suited for lipid-soluble (lipophilic) drugs that can easily pass through the hydrophobic core of the membrane.
   • Rate is influenced by: lipid solubility, degree of ionization (pKa), molecular size, thickness of the membrane, and the area of absorption.

2. Facilitated Diffusion

   • Utilizes a carrier protein or a specific transporter embedded in the membrane.
   • Moves drugs down the concentration gradient (no direct energy required).
   • Saturable and subject to competition (different substances may use the same transporter).

3. Active Transport

   • Also uses carrier proteins but can move drugs against their concentration gradient.
   • Requires energy (ATP) directly or indirectly.
   • Saturable and can be inhibited by substances competing for the same transporter.
   • Example: transport of levodopa across the blood-brain barrier via amino acid transporters.

4. Endocytosis and Exocytosis

   • Endocytosis: The cell membrane engulfs the drug particle (often larger molecules like protein drugs) forming a vesicle.
   • Exocytosis: The vesicle fuses with the cell membrane to release its contents outside the cell.
   • Examples: Vitamin B12 absorption in the gut involves receptor-mediated endocytosis.

5. Ion-Pair Transport

• Some highly ionized drugs can form neutral complexes (ion pairs) with endogenous substances that facilitate passive transport across membranes.

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