Pharmacodynamics

Involves how the drugs act on target cells to alter cellular function.
A. Receptor and non-receptor mechanisms: 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.
• 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 thesynthesis 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.