H1 antagonists, also known as H1 blockers or antihistamines, are a class of drugs that inhibit the action of histamine at the H1 receptor sites.
They are widely used to treat various allergic conditions and are classified into three main categories:
1. First-Generation H-1 Antagonists
2.Second-Generation H-1 Antagonists
3. Mast Cell Stabilizers
Medical Uses of H-1 Antagonists
Allergic Rhinitis: Reduces symptoms like sneezing and nasal congestion.
Allergic Conjunctivitis: Alleviates itching and redness in the eyes.
Urticaria: Treats hives and skin rashes.
Angioedema: Reduces swelling beneath the skin.
Pruritus: Relieves itching from atopic dermatitis and insect bites.
Nausea and Vomiting: Some antihistamines have antiemetic properties.
Sedation: First-generation antihistamines are used as sleep aids due to their sedative effects.
Side Effects
Sedation and Drowsiness (especially with first-generation agents)
Blurred Vision
Dry Mouth
Constipation or Diarrhea
Nausea and Vomiting
Increased Appetite and Weight Gain
Anxiety or Insomnia
Dry Cough
Classification
First-Generation H1 Antagonists
Characteristics:
i. Sedative Effects: Easily cross the blood-brain barrier, causing drowsiness.
ii. Anticholinergic Activity: Block muscarinic acetylcholine receptors, leading to side effects like dry mouth and blurred vision.
iii. Affordable and Widely Available: Oldest class of antihistamines.
Mechanism of Action:
i. Histamine Blockade: Competitively inhibit histamine binding at H1 receptors.
ii. G-Protein Coupled Receptor Inhibition: Prevent activation of phospholipase C, reducing inositol phosphate (IP₃) and diacylglycerol (DAG) formation.
iii. Reduced Intracellular Calcium: Decreases the release of inflammatory mediators.
Functions:
i. Decrease Vascular Permeability: Reduces edema and swelling.
ii. Alleviate Pain and Headache: Minimizes discomfort associated with allergies.
iii. Prevent Hypotension: Stabilizes blood pressure by inhibiting vasodilation
Second-Generation H1 Antagonists
Characteristics:
i. Non-Sedative: Less likely to cross the blood-brain barrier, resulting in minimal sedation.
ii. Selective Action: More specific to peripheral H1 receptors.
iii. Hydrophilic Nature: Limits central nervous system penetration.
Mechanism of Action:
Similar to first-generation antihistamines but without significant central nervous system effects.
Functions:
i. Alleviate Allergy Symptoms: Effective in treating allergic rhinitis, conjunctivitis, and urticaria without causing drowsiness.
Mast Cell Stabilizers
Characteristics:
i. Prevent Histamine Release: Stabilize mast cell membranes, inhibiting degranulation.
ii. Anti-Inflammatory Activity: Reduce the release of other inflammatory mediators.
Mechanism of Action:
i Calcium Channel Blockade: Inhibit IgE-mediated calcium influx into mast cells.
ii. Prevent Degranulation: Without intracellular calcium, histamine vesicles cannot fuse with the cell membrane.