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Drug Absorption

Drug Absorption. Lesson 4. Organs & Tissues. Blood. T. Brain. A. Drug. CTZ. Absorption. Movement from administration site into circulatory system Complete when... concentration at target equals site of administration Controlled by membranes cell capillary walls

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Drug Absorption

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  1. Drug Absorption Lesson 4

  2. Organs & Tissues Blood T Brain A Drug CTZ

  3. Absorption • Movement from administration site into circulatory system • Complete when... • concentration at target equals • site of administration • Controlled by membranes • cell • capillary walls • blood-brain barrier • placental barrier ~

  4. Cell Membranes • Lipid bilayer • semipermeable • Fluid Mosaic Model • Phospholipids • Proteins ~

  5. Membrane Proteins OUTSIDE INSIDE

  6. Movement Across Membrane • Passive diffusion • thru un-gated channels • Carrier assisted transport • diffuse through gated channels • Active transport • requires energy ~

  7. Capillaries • Exchange of materials • b/n blood and cells • Capillary walls one cell thick • Pores • Spaces between cells • materials can move in/out ~

  8. Blood-Brain Barrier (BBB) • Barrier b/n circulatory system & brain • peripheral vs. CNS blood supply • Capillaries • cells in wall tightly packed • astrocytes • glia • wrap around capillaries • tight seal ~

  9. BBB: Function • Maintains stable brain environment • large fluctuations in periphery • Barrier • to poisons • Retains NTs & other chemicals • Regulates nutrient supplies • glucose levels • active transport ~

  10. Brain Capillary Typical Capillary Blood-Brain Barrier

  11. BBB: Development • Incomplete at birth • up to 2 yrs old • vulnerable during pregnancy • Weakening of BBB • trauma • infection • aging~

  12. Chemical Trigger Zone (CTZ) • Area Postrema in brainstem • BBB weaker • Substances can enter brain • safety mechanism • neurons monitor blood • rapid ­in blood concentration triggers vomiting • dopaminergic neurons ~

  13. Placental Membrane • Exchange nutrients & wastes w/ mother • Less selective than BBB • all lipid soluble substances • Fetus is vulnerable • incomplete BBB • lack enzymes for metabolism ~

  14. Placental Membrane • Teratogens • chemicals that cause birth defects • alcohol, nicotine, cocaine, etc. • Smoking  CO • reduced O2 levels  brain damage ~

  15. Solubility • Ability to dissolve in a medium • water-soluble • lipid-soluble • easily crosses membranes • also BBB • Molecular size • small absorbed easily ~

  16. Solubility: Ionization • Ionization decreases solubility • ion = charged particle • Lipid-soluble  little ionization • Water-soluble  ionizes easily • requires carrier assisted or active transport ~

  17. Solubility: Polarization • Polarity • positive & negative poles • H2O • Polar  hydrophilic • tend to ionize • will not cross membrane • Non-polar  hydrophobic • crosses membrane easily ~

  18. Water Ethanol H H H - + O C C OH H H H H Polar Non-polar

  19. Solubility: pH • pH scale: 0-14 • Drugs in solution can ionize • H2O  H+ and OH- • # of H+ in solution relative to OH- • High % H+ = acidic • Low % H+ = basic (alkaline) ~

  20. Water = neutral alkaline acidic GI = 1.2 Blood = 7.4 pH Scale 0 7.0 14

  21. Ion Trapping • Drug & pH important • Ionization decreases solubility • acidic drug in alkaline  ionize • alkaline drug in acidic  ionize • Per Os ? • acidic drug best ~

  22. Redistribution • Ion trapping: • Dynamic equilibrium maintained • Example: acidic drug in blood • 40% ionized to 60% un-ionized • Redistribution: as drug is metabolized • some ionized  un-ionized • ratio ionized to un-ionized retained • 4:6 ~

  23. Best Absorption • Lipid soluble • Small • Non-polar • Un-ionized • pH matches environment ~

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