Pharmacology basics and definitions – Easy [Part 1]

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 )
• Inhalation.
• Topical.
• Transdermal.
• Subcutaneous.

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 …