Pharmacology Concepts: Receptors, Agents, and Drug Action

Posted on Jul 21, 2025 in Medicine

Partition Coefficient

The partition coefficient is the ratio of concentrations of a compound between two immiscible liquids, such as octanol and water, at equilibrium.

Bioisosteres

Bioisosteres are molecules or functional groups that have similar shapes and electronic properties, often producing similar biological effects. They can be used to modify the structure of a lead compound to improve its potency, selectivity, or pharmacokinetic properties.

Classification of Bioisosteres

  • Classical Bioisosteres
  • Non-Classical Bioisosteres

Phenylephrine

C9H13NO2.

Mechanism of Action (MoA)

Phenylephrine is a selective α1-adrenergic receptor agonist, causing:

  • Vasoconstriction: Constriction of blood vessels, increasing blood pressure.
  • Smooth muscle contraction: Contraction of smooth muscle in various tissues.

Uses: Decongestant, Hypotension treatment.

Cholinolytic Agents

Cholinolytic agents, also known as anticholinergics or parasympatholytics, are drugs that block the action of acetylcholine (ACh) at muscarinic or nicotinic receptors.

Mechanism of Action (MoA)

  • Muscarinic receptor antagonism
  • Nicotinic receptor antagonism

Examples

  • Atropine
  • Scopolamine
  • Ipratropium

Neostigmine

Structure

Neostigmine is a synthetic quaternary ammonium compound with a chemical formula of C12H19N2O2.

Mechanism of Action (MoA)

Neostigmine is a reversible cholinesterase inhibitor, specifically inhibiting acetylcholinesterase (AChE). This inhibition:

  • Increases acetylcholine levels.
  • Enhances cholinergic transmission in the nervous system.

Therapeutic Uses

  • Myasthenia gravis treatment
  • Reversal of neuromuscular blocking agents

Adrenergic Receptors

Adrenergic receptors are a class of G protein-coupled receptors that respond to the neurotransmitters norepinephrine (noradrenaline) and epinephrine (adrenaline).

Classification

  • Alpha (α) Adrenergic Receptors:
    • α1 receptors: Involved in smooth muscle contraction, vasoconstriction.

Alpha Adrenergic Antagonists

Alpha adrenergic antagonists, also known as alpha blockers, are medications that block the action of alpha adrenergic receptors.

Examples

  • Prazosin
  • Phentolamine
  • Tamsulosin

Cholinesterase Reactivators

Cholinesterase reactivators are compounds that restore the activity of acetylcholinesterase (AChE) after it has been inhibited by organophosphorus compounds (e.g., nerve agents, pesticides).

Examples

  • Pralidoxime (2-PAM): Reactivates AChE, used to treat organophosphate poisoning.

Cholinergic Receptors

Cholinergic receptors are proteins that respond to the neurotransmitter acetylcholine (ACh). There are two main types:

Classification

  • Muscarinic receptors
  • Nicotinic receptors

Non-Narcotic Anti-Inflammatory Agents

Non-narcotic anti-inflammatory agents, also known as nonsteroidal anti-inflammatory drugs (NSAIDs), are medications that reduce inflammation and pain without the use of opioids.

Mechanism of Action (MoA)

  • Cyclooxygenase (COX) inhibition: Blocks the enzyme COX, reducing the production of prostaglandins, which are pro-inflammatory mediators.

Examples: Ibuprofen, Celecoxib, etc.

Chlorpromazine Hydrochloride

C17H19ClN2S.

Mechanism of Action (MoA)

Dopamine receptor antagonism: Blocks dopamine D2 receptors in the brain, reducing dopamine activity and alleviating psychotic symptoms.

Ionization and Drug Action

Ionization affects a drug’s:

  • Absorption: Ionized drugs are less likely to cross lipid membranes, reducing absorption.
  • Distribution: Ionization influences a drug’s ability to cross biological barriers.
  • Action: Ionized drugs may interact differently with receptors or enzymes.