Antihistamines (FETT)

Cetirizine

Chlorphenamine

Ceterizine: second-generation H1 antihistamine

Chlorphen(ir)amine: first-generation H1 antihistamine

• Target: histamine H1 receptors

• Location of H1 receptors

  • Smooth muscles (especially bronchial and nasopharyngeal lining)

  • Vascular endothelial cell surfaces

  • Central nervous system

  • Heart

• Histamine effects on H1 receptors

  • ↑ Capillary dilation and permeability → hypotension and edema

  • ↑ Bronchiolar smooth muscle contraction (via IP3 and DAG release) → bronchoconstriction

  • ↑ Nasal and bronchial mucus production

  • ↑ Activation of peripheral nociceptive receptors → ↑ pain and pruritus

  • ↓ Conduction in AV node

• Competitive, reversible antagonism or inverse agonism of histamine H1 receptors (Gq protein-coupled receptor)

  • Inhibition of increased vascular permeability

  • Inhibition of allergic bronchial constriction

  • Central action: sedation (especially first-generation antihistamines due to crossing of blood-brain barrier)

• Sedation (significantly less pronounced in second-generation antihistamines due to their limited CNS activity)

• Anticholinergic effects, e.g., dry mouth and eyes, mydriasis, urinary retention, tachycardia, dizziness, tinnitus (mostly with first-generation antihistamines)

• Anti-α-adrenergic effects, e.g, postural hypotension (can lead to falls), weight gain

• Headaches

• Allergies

• Anaphylactic shock (e.g., clemastine, dimetindene)

• Insomnia: used as sedatives (sleep aid)

• Nausea: used as antiemetic agents (e.g., diphenhydramine in hyperemesis during pregnancy) because of their central effects

• Pruritus

• Motion sickness

• Allergies: used as antiallergic agent (e.g., in cases of rhinitis, conjunctivitis, hypersensitivity to drugs, urticaria)

• Pruritus

• Anaphylactic shock

• First-generation antihistamines

  • Benign prostatic hyperplasia

  • Angle-closure glaucoma

  • Pyloric stenosis

• Second-generation antihistamines: childhood and pregnancy