Most of the drugs act by interacting with a cellular component called receptor. Some drugs act through simple physical or chemical
reactions without interacting with any receptor.
• Receptors are protein molecules present either on the cell surface or with in the cell e.g. adrenergic receptors, cholinoceptors, insulin receptors, etc.
• The endogenous neurotransmitters, hormones, autacoids and most of the drugs
produce their effects by binding with their specific receptors.
• Aluminium hydroxide and magnesium trisilicate, which are used in the treatment of peptic ulcer disease act by non-receptor mechanism by neutralizing the gastric acid.
Many drugs are similar to or have similar chemical groups to the naturally occurring chemical and have the ability to bind onto a receptor where one of two things can happen- either the receptor will respond or it will be blocked.
A drug, which is able to fit onto a receptor, is said to have affinity for that receptor. Efficacy is the ability of a drug to produce an effect at a receptor. An agonist has both an affinity and efficacy whereas antagonist has affinity but not efficacy or intrinsic activity. When a drug is able to stimulate a receptor, it is known as an agonist and therefore mimics the endogenous transmitter.
When the drug blocks a receptor, it is known as antagonist and therefore blocks the action of the endogenous transmitter (i.e. it will prevent the natural chemical from acting on the receptor).
However, as most drug binding is reversible, there will be competition between the drug and the natural stimulus to the receptor.
The forces that attract the drug to its receptor are termed chemical bonds and they are (a) hydrogen bond (b) ionic bond (c) covalent bond (d) Vander waals force. Covalent bond is the strongest bond and the drug-receptor complex is usually irreversible.
K1 K3
DR Biological effect
D+R K2
Where D = Drug, R= receptor DR= Drug receptor complex (affinity)
K1 = association constant
K2 = dissociation constant
K3 = intrinsic activity
When first messengers like neurotransmitters, hormones, autacoids and most of drugs bind with their specific receptors, the drug receptor complex is formed which subsequently causes the synthesis and release of another intracellular regulatory molecule termed as second messengers e.g. cyclic AMP, calcium, cyclic GMP, inositol triphosphate (IP3), diacylglycerol and calmodulin which in turn produce subcellular or molecular mechanism of drug action.
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