Administration and Absorption of Drugs

1. Administration and Absorption of Drugs

2. 1. Rate of accumulation at its site of action 2. Concentration of the drug at the site of action 3. The duration of the drug’s contact at those sites Factors that effect the action of a drug

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

4. 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 Administration of drugs

5. 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 Routes of administration – Enteral routes

6. 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 Routes of administration – Parenteral routes

7. 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 Routes of administration – Parenteral routes

8. 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 Absorption of Drugs

9. 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. The bloodstream and drug movement

10. 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 Structure of capillaries

11. 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 Blood – brain barrier

12. 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 Blood-brain barrier

13. 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 Movement across semi-permeable membranes

14. 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 Movement across semi-permeable membranes

15. 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 Placental Barrier

16. 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 Solubility