Administration%20and%20Absorption%20of%20Drugs

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Administration and Absorption of Drugs
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Factors that effect the action of a drug

• 1. Rate of accumulation at its site of action
• 2. Concentration of the drug at the site of
  action
• 3. The duration of the drugs contact at those
  sites

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Pharmocokinetics

• Refers to the movement of chemicals in a
  biological system
• This includes
• Administration and absorption
• Distribution
• Binding properties
• Metabolic mechanisms
• Elimination from the system

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Administration of drugs

• To be effective a drug must enter the body and
  get to its site of action
• Best method of administration of a drug depends
  on the drug
• Most drugs dissolved in a fluid (saline) or
  contained in a mixture as in pills or capsules
  type of substance is important
• Injections tend to be fast acting, oral
  administration is usually slow acting and of a
  longer duration

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Routes of administration Enteral routes

• Administered through the alimentary canal oral
  or anal
• Common problem is that absorption rate can be
  highly variable
• Absorption of orally administered drugs is
  greatly influenced by stomach contents
• Hostile acid environment can interfere with
  absorption
• Other problems
• Some people cannot take pills
• Uncooperative patients may refuse or cheek them

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Routes of administration Parenteral routes

• Injection
• Intravenous drug injected into vein
• Advantages fastest way administer drug and
  dosage most accurate
• Disadvantages chance of infection if conditions
  not sterile and no way to retrieve the drug in
  case of allergic or toxic reaction
• Intramuscular drug injected into muscle mass
  slower but safer
• Subcutaneous injection under the skin or
  implanted under the skin
• Direct injection in the nervous system

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Routes of administration Parenteral routes

• Pulmonary routes inhalation into the lungs
• Topical routes placing drug on the surface
  usually a surface with a mucous membrane
• Sublingual placed under the tongue
• Intranasal powder or liquid absorbed through
  mucous membrane in the nose
• Skin skin patches or topical anesthesia

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Absorption of Drugs

• With the exception of drugs directly injected
  into the nervous system and topical anesthesia,
  drugs have to cross at least 2 membranes to
  reach its site of action
• At a minimum, it has to cross into the blood
  stream, out of the blood stream, and into the
  cell
• Walls of blood vessels and cell membranes are
  semi-permeable

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The bloodstream and drug movement

• How fast a drug gets into the bloodstream depends
  on route of administration
• Once in the blood, a drug has access to all types
  of tissue, muscle, fat skin, lungs, etc.

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Structure of capillaries

• Capillaries in most of the body are made up of
  cells with gaps in between the cells
• These gaps allow some substances, but not blood
  cells of large proteins, to move in and out of
  the capillaries
• Some drug molecules will bind to these large
  proteins reducing the amount of free molecules
  in the blood
• When these free molecules leave the blood stream,
  bound molecules become unbound to maintain a
  stable concentration

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Blood brain barrier

• In the brain, the cells are more tightly packed
  so molecules have to move through the cells
  themselves this makes up part of the
  blood-brain barrier
• Astrocytes make up the rest of the blood-brain
  barrier by sending out feet that help seal the
  capillary walls

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Blood-brain barrier

• Very few substances can cross this barrier
• Membranes of the capillary cells and astrocytes
  are made up of lipids
• Only lipid soluble substances can be absorbed
  into the cells and pass through them
• This barrier important because the brain has no
  immune system

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Movement across semi-permeable membranes

• Three types of fluid
• Fluid in the bloodstream
• Fluid outside the cells extra cellular fluid
• Fluid inside the cells intracellular fluid
• These fluids can move back and forth across
  membranes
• Example thirst
• Lack of fluid in blood
• Extra cellular fluid moves into bloodstream
• Intracellular fluid moves out of the cells
• Loss of fluid detected signal sent to drink

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Movement across semi-permeable membranes

• Filtration as fluid moves into a cell the
  membrane filters out large molecules which
  include many drug molecules
• Movement through diffusion follows chemical
  gradient chemicals flow from areas of high
  concentration to areas of low concentration
• If a chemical is highly concentrated outside a
  cell, and openings exist, it will move into the
  cell

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Placental Barrier

• A weak barrier between mother and fetus
• Large amounts of materials are exchanged between
  mother and fetus
• Most drugs easily cross the placental barrier
  almost every drug a mother takes the fetus takes

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Solubility

• Lipid soluble drugs drugs that can be dissolved
  in lipids fatty substances that make up cell
  membranes including the blood-brain barrier
• Water soluble drugs drugs that can dissolve in
  water they will not passively move through a
  cell membrane so they require active transport