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Pharmacology: Pharmacodynamics: Antihistamines (loratadine) diminish or abolish the main actions of histamine in the body by competitive, reversible blockade of histamine receptor sites on tissues. Histamine H1-receptors are responsible for vasodilatation, increased capillary permeability, flare and itch reactions in the skin, and to some extent for contraction of smooth muscle in the bronchi and gastrointestinal tract.
Loratadine is a long-acting tricyclic antihistamine with selective peripheral histamine H1-receptor antagonistic activity. Studies have shown that neither loratadine nor its metabolites readily crosses the blood-brain barrier. It has higher affinity for the peripheral rather than the central nervous system H1-receptors. Hence, less problem of daytime sedation for those who have to drive or operate machinery.
Pharmacokinetics: It is rapidly absorbed from the gastrointestinal tract after oral administration. The peak plasma concentration is being attained at about one hour. Loratadine undergoes extensive metabolism. Descarboethoxyloratadine, which is the major metabolite has potent histamine H1-receptor blocking activity. Reported half-lives for loratadine and descarboethoxyloratadine are about 12 and 18 hours, respectively.
Loratadine is about 98% bound to plasma proteins while its major metabolite, descarboethoxyloratadine is less extensively bound. Loratadine and its metabolite have been detected in breast milk, but do not appear to cross the blood-brain barrier to a significant extent. Loratadine and its metabolites are excreted in the urine and faeces.
