General Principles of Pharmacology

The nature of drugs:

In general, a drug may be any substance that bring about a change in biologic function through its chemical actions. In most cases the drug molecule interacts in two ways that is agonist(activator) and antagonist(inhibitor) with a specific molecule within the biologic system that plays a regulatory role. This molecule is called receptor. 

• Agonist;  drug bind to and activate the receptor in some fashion which directly or indirectly brings about the effect.

• Antagonist;  drug that inhibits the receptor activity directly or indirectly 

In a very small number of cases drugs are chemical antagonist i.e interact directly with other drugs.
To interact chemically with its receptor a drug molecule must have appropriate size, electrical charge, shape and atomic composition. Drug transportation from the site of administration and the time which drug take to excrete from body are also considered.

A . Physical nature of drugs; Drugs may be solid at room temperature e.g aspirin or liquid e.g ethanol or gaseous e.g nitrous oxide.
Number of useful or dangerous drugs are inorganic elements e.g lithium ion and heavy metals. Many organic drugs are weak acids or weak bases. 

B . Drug size; The molecular size of drugs varies from very small to very large e.g Lithium ion MW 7, alteplase MW 59,050. However , most drugs have molecular weights between 100 and 1000. Drugs much larger than MW 1000 do not diffuse readily between body compartments.

C . Drug reactivity and drug receptor bond; Drugs interact with receptors by three ways i.e by covalent bonding, electrostatic bonding and hydrophobic bonding.

• Covalent bonds between drug and receptor are very strong and irreversible under biologic conditions.
• Electrostatic bonds are common than covalent bonding in drug-receptor interactions. They are weaker than covalent bonding. They may be a strong hydrogen bond or weak induced dipole dipole interactions such as van der Waals forces.
• Hydrophobic bonds are usually quiet weak and practically important because drugs tat binds to weaker bonds are more selective than the drugs that bind strongly. This is because weak bonds requires a very precise fit of a drug to its receptor if an interaction is to occur

D . Drug shape; The drug shape is complementary to that of its receptor site in the same way that a key is complementary to its lock. Furthermore the phenomenon of stereo-isomerism is so common in biology that half of useful drugs are chiral molecules i.e they can exist as an enantiomeric pairs. In most of the cases one of these enantiomers are much more potent than its mirror image enantiomer. For e.g Ketamine is an intravenous anesthetics. The (+) enantiomer is more potent and less toxic than (-) enantiomer.

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