PBT_Sessions 4_CO3

Anti-Allergy/Antiallergic Drugs

Overview of Allergies

  • Allergy: Sensitivity to a substance (allergen).

  • Prevalence:

    • 30-40% of the global population suffers from allergic diseases.

    • Estimated 300 million people have asthma (projected 400 million by 2025).

    • Allergic rhinitis affects 400 million annually, food allergies impact 200-250 million.

    • Approx. 250,000 avoidable asthma-related deaths per year.

General Principles of Prevention and Treatment

  1. Avoid contact with allergens.

  2. Specific desensitization via repeated introduction of small antigen doses.

  3. Nonspecific desensitization with immune depressants.

  4. Antiallergic drugs to stabilize mast cell membranes or block receptor interactions.

  5. Symptomatic treatment of existing allergic reactions.

Types of Antiallergic Drugs

  • Hypersensitivity Reaction Therapy:

    • Antiallergic Drugs:

      • Stabilize mast cell and basophil membranes to reduce mediator release.

      • Antihistamines block histamine receptors in tissues.

    • Tissue Damage Reduction Drugs:

      • Glucocorticosteroids.

    • Symptomatic Treatment Drugs:

      • Adrenaline, euphyllin.

Classes of Anti-Allergy Drugs

  • Antihistamine Drugs

  • Mast Cell Stabilizers

Antihistamines

  • Histamine:

    • Endogenous bioactive amine from mast cells (skin, GI, respiratory tract).

    • Physiological Actions:

      • Stimulant for gastric acid and pepsin secretion.

      • Functions as a neurotransmitter.

    • Pathophysiological Actions:

      • Mediator of hypersensitivity reactions, anaphylaxis, duodenal ulcers.

Histamine Receptors

  • H1 Receptors:

    • Coupled to Phospholipase C (Gq).

    • Effects: bronchoconstriction, vasodilation, increased vascular permeability, pain, and itching.

  • H2 Receptors:

    • Coupled to Adenylyl Cyclase (Gs).

    • Mediates gastric acid secretion and vasodilation.

  • H3 Receptors:

    • Involved in neurotransmitter release regulation.

  • H4 Receptors:

    • Primarily in mast cells, involved in chemotaxis.

Actions of Histamine

  • Vascular: Vasodilation, increased permeability leading to edema.

  • Heart: Affects conduction and cardiac output.

  • Lung: Bronchoconstriction and stimulation of sensory nerves.

  • Gastrointestinal System: Influences acid secretion, motility, and secretions.

  • Cutaneous Effects: Pain and itching from H1 activation.

Symptoms of Allergic Reactions

  • Mild: Erythema, urticaria, itching.

  • Moderate: Skin reactions, tachycardia, moderate hypotension, respiratory distress.

  • Severe/Anaphylactic: Severe hypotension, cardiac arrest, bronchospasm.

Types of Antihistamine Drugs

  • H1 Receptor Antagonists: Antiallergic.

  • H2 Receptor Antagonists: Reduce HCl secretion for treating ulcers.

Mechanism of Antihistamine Drugs

  • Block H1 receptors, reducing histamine effects on various tissues.

  • Antihistamines do not displace histamine but bind to receptors due to higher affinity of histamine for H1 receptors.

H1-Antagonists (Antihistamines)

  • First Generation:

    • Examples: Diphenhydramine, Doxylamine, Chlorpheniramine.

    • Characteristics: Sedative, muscarinic effects, potentiate CNS depressants, short action duration (15-20 min).

  • Indications: Allergic reactions, sedation, motion sickness, premedication.

  • Side Effects: CNS depression, GI disturbances, dry mucous membranes.

Second Generation H1-Antagonists (Non-sedating)

  • Examples: Loratadine, Cetirizine, Azelastine.

Mast Cell Stabilizers

  • Mechanism of Action: Block calcium channels, preventing degranulation of mast cells and histamine release.

  • Indications: Bronchial asthma, allergic rhinitis, allergic conjunctivitis; require regular use for effectiveness.

Allergenic Extracts

  • Composition: Sterile solutions from biological sources (pollen, molds, etc.).

  • Prepared as aqueous or glycerinated extracts.

  • Concentrations are measured in protein nitrogen units (PNU) or weight/volume ratios. Standardized in Allergy Units (AU).