Loratadine is a long-acting, second-generation, non-sedating tricyclic antihistamine (piperidine derivative) with selective antagonistic properties to peripheral histamine H1-receptors. Loratadine selectively inhibits H1-receptors primarily located on respiratory smooth muscle cells, vascular endothelial cells, the gastrointestinal tract, and immune cells. Unlike the first-generation antihistamines such as diphenhydramine, loratadine is a competitive histamine antagonist that does not cross the blood-brain barrier. Therefore, it does not affect the neurons of the central nervous system, thereby preventing daytime somnolence or sedation. Loratadine binds to H1-receptors in different cells and causes a decrease in vascular permeability (prevents edema and flushing), decreases smooth muscle tone (bronchodilation), and decreases the activation of the peripheral nociceptive receptors (decreases pain and pruritus). At high concentrations, second-generation antihistamines such as loratadine can inhibit histamine release from mast cells and basophils, thereby reducing ICAM-1 expression in epithelial cells and inhibiting type 1 hypersensitivity reactions (e.g., hay fever, urticaria, pruritus, edema).

Loratadine H1 Antihistamine Smooth Muscle Relaxation Pathway References