Pharmacodynamics

Pharmacodynamics

• Pharmacodynamics refers to what the drug does to the body – including the biochemical and physiological effects of drugs and their mechanisms of action.

Principles of Drug Action

1. Stimulation

   • Increases the activity of a specific organ or system.
   • Example: Adrenaline stimulates β1-receptors to increase heart rate and contractility.

2. Depression

   • Decreases the activity of a body system or function.
   • Example: Barbiturates depress the CNS leading to sedation or anesthesia.

3. Irritation

   • Causes mild inflammation or discomfort that leads to a therapeutic response.
   • Example: Capsaicin cream causes irritation that helps relieve pain by depleting substance P.

4. Replacement

   • Supplies deficient substances in the body.
   • Example: Insulin in diabetes, thyroxine in hypothyroidism.

5. Cytotoxic Action

   • Drugs that selectively kill specific cells, often used in cancer treatment.
   • Example: Chemotherapeutic agents like cyclophosphamide destroy malignant cells.

Mechanisms of Drug Action

Drugs exert their effects by interacting with specific targets:

1. Enzymes

   • Drugs may inhibit or activate
   • Inhibitors: Aspirin irreversibly inhibits COX enzymes.
   • Activators: Some drugs enhance enzyme activity (less common).
   • Example: Neostigmine inhibits acetylcholinesterase, increasing acetylcholine.

2. Ion Channels

   • Drugs can block or open ion channels to affect ion flow across membranes.
   • Example: Lidocaine blocks sodium channels, preventing nerve conduction (local anesthesia).

3. Transporters (Carriers)

   • Drugs can inhibit transporters involved in the movement of substances across membranes.
   • Example: SSRIs like fluoxetine inhibit serotonin reuptake transporters.

4. Receptors

   • The most common drug targets.
   • Drugs may act as:
     • Agonists: Activate receptors (e.g., Salbutamol on β2 receptors).
     • Antagonists: Block receptors (e.g., Propranolol on β receptors).
     • Partial agonists: Partially stimulate receptors (e.g., Buprenorphine).
     • Inverse agonists: Reduce basal receptor activity (e.g., Rimonabant on CB1).

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