When you have headache , you take an analgesic OTC like ibuprofen to treat the pain but what is behind the scene !? the process of treating headache ?
The interaction between a chemical like ibuprofen and your body as a living system is the definition of pharmacology. The drug is a chemical substance that interacts with a living system. So,
Pharmacology : The study of the interaction between chemicals and living systems.
Drug : is a chemical substance that interact with a living system to give a response .
The major subdivisions of pharmacology :
1. Pharmacokinetics : that is what is your body do to the drug.
2. Pharmacodynamics : That is what the drug do to your body. ( D Mnemonics :biggrin: )
3. Pharmacotherapeutics : Related to the use of drug to diagnose or treat or prevent the disease.
4. Toxicology : The toxic or undesirable effects of the drug or a chemical substance.
Lets back to the headache story and using OTC ibuprofen to manage your headache :
According to the pharmackinetic definitions , what is happened to the ibuprofen when ingested to treat the headache ? What does your body do to the ibuprofen ?
- Absorption : it means rate and extent of the drug to move from site of administration.
- Distribution : when the drug leaves the blood stream and enters between the cells or the cells of the tissues.
- Biotransformation : The process which include modifying the drug through phase 1 and phase 2 to allow the elimination of the drug from the body. However some drugs “Prodrug” is activated through this process.
- Excretion : The process by which the drug is removed from your body.
Lets walk through each definition of Pharmacokinetics :w00t: :
1. Absorption :
As we defined it the rate and extent of the drug to move from the site of administration , so what are the sites of body administration ??
- Alimentary ( Oral , Buccal ,Sub-lingual & Rectal ).
- Parenteral ( Intravenous , Intramuscular , Subcutaneous & Intrathecal )
Some drugs has a high rate of absorption some other poorly absorbed , this determined by many factors like : ( Factors affecting the absorption of the drug )
- Route of administration type : for example Intravenous route is the most effective one (see this toon)
- Blood flow : As it increase , the absorption rate increases.
- Surface area : As it increase , the absorption rate increases too.
- Solubility of the drug : the hydrophilic part ( water soluble ) to lipophilic part ( lipid soluble ) ratio will determine the rate of absorption. This called Partition Coefficient
- Drug-Drug interactions : It depends on the types of drugs they may enhance or decrease or inhibit each others.
- PH : the acidity or alkalinity of the drug may affect its absorption.
After we consider all the previous factors , the portion or the fraction of the drug that reaches its site of action to exert a biological effect for ex: Analgesic effect is called bioavailability.
So if the WHOLE drug reaches to the site of action it means it has 100% bioavailabilty, the only route which give 100% bioavailability is the Intravenous route 😎 as 100% of the drug injected into the blood stream, other routes give less than 100% bioavailability.
Thers is a cases in which > than 90% of the drug doesn’t reach its site of action , thats means it is eliminated or inactivated before reaching its site of action , this called First Pass Metabolism.
First Pass metabolism : Biotransformation occurs to the drug before reaching its site of action , mainly occurs in the liver ex: Nitroglycerin.
2. Distribution :
The process by which the drug leaves the blood stream and enters the tissue. HOW ?
- Passive diffusion : the drug moves from area of high concentration to the area of low concentration.
- Active Transport : As it is active you must pay for it ! it is against the concentration gradient and the energy (Payment ) come from the dephosphorylation of ATP.
- Special carrier transport ( Helper 😆 ) :like passive diffusion but with a carrier like proteins.
So do all drugs distribute to the same extent ? or all organs allows drugs to enter to their cells ?
No , there are factors affecting drug distribution :
- Blood flow.
- Capillary permeability : it depends on the organ for example the brain has blood brain barrier which is tight and doesn’t allow big molecules to pass , but the junctions between the cells in liver is wide which allow big molecules to pass.
- Binding to plasma proteins : like albumin , the higher binding rate , the lower access to the cellular compartments.
- Drug structure : Small drug molecules with high lipophilic properties is likely to pass easily more than large molecules or water soluble molecules.
UPDATE : Part 2
To be continued …