Pharmacology of antiallergic drugs

Antiallergic drugs, also known as antihistamines, are primarily used to alleviate symptoms of allergic reactions, such as sneezing, itching, and runny nose. Here’s an overview of their pharmacology:

Types of Antiallergic Drugs

1. Antihistamines:

   • First-Generation: Examples include diphenhydramine, chlorpheniramine, and brompheniramine. These drugs can cross the blood-brain barrier, often leading to sedation.
   • Second-Generation: Examples include cetirizine, loratadine, and fexofenadine. These are less likely to cause sedation and have a more favorable side effect profile.

2. Corticosteroids:

   • Used for their anti-inflammatory effects. Examples include prednisone and fluticasone. They help reduce inflammation and control allergic symptoms over a longer term.

3. Leukotriene Receptor Antagonists:

   • Drugs like montelukast block the action of leukotrienes, which are inflammatory mediators involved in allergic responses.

4. Mast Cell Stabilizers:

   • Cromolyn sodium prevents the release of histamine and other mediators from mast cells, helping to prevent allergic reactions.

5. Decongestants:

   • Drugs like pseudoephedrine and phenylephrine help relieve nasal congestion by constricting blood vessels in the nasal passages.

Mechanism of Action

• Antihistamines: They work by blocking H1 receptors, which prevents the action of histamine, a key mediator in allergic reactions.
• Corticosteroids: They inhibit the expression of various inflammatory cytokines and reduce the infiltration of inflammatory cells.
• Leukotriene Receptor Antagonists: They block the effects of leukotrienes, which are involved in bronchoconstriction and increased vascular permeability.
• Mast Cell Stabilizers: They stabilize mast cells and prevent the release of histamine and other inflammatory mediators.

Pharmacokinetics

• Absorption: Most antihistamines are well absorbed after oral administration. Second-generation antihistamines have longer half-lives, allowing for once-daily dosing.
• Distribution: First-generation antihistamines can cross the blood-brain barrier, leading to central nervous system effects.
• Metabolism: Many are metabolized in the liver, and their metabolites can also have pharmacological activity.
• Excretion: Primarily excreted via urine, with some variation depending on the specific drug.

Side Effects

• First-Generation Antihistamines: Sedation, dry mouth, dizziness, and blurred vision.
• Second-Generation Antihistamines: Generally better tolerated, but some may still cause mild sedation or headaches.
• Corticosteroids: Long-term use can lead to side effects such as weight gain, osteoporosis, and adrenal suppression.
• Leukotriene Receptor Antagonists: Can cause mood changes, abdominal pain, and headache.

Conclusion

Understanding the pharmacology of antiallergic drugs is crucial for effective treatment of allergic conditions. The choice of drug depends on the type and severity of the allergy, as well as patient-specific factors like comorbid conditions and potential side effects.