Question

Loratadine (proprietary name Claritin) is an antihistamine drug used to treat allergies, and marketed for its non-sedating properties. Loratadine is a tricyclic antihistamine, which selectively antagonizes peripheral histamine H1-receptors, which are G-protein coupled receptors. It is given orally and is well absorbed from the gastrointestinal tract. Loratadine has rapid and extensive first-pass hepatic metabolism; it is metabolized by isoenzymes of the cytochrome P450 system, including CYP3A4 and CYP2D6. Loratadine is almost totally bound to plasma proteins. Its metabolite, desloratadine (descarboethoxyloratadine), is also active, but binds to plasma proteins to a much lesser extent. Select the true statement(s):

a. until it’s eliminated, desloratadine will potentiate the effects of loratadine

b. loratadine will inhibit CYP3A4 in the presence of desloratadine

c. loratadine’s binding to the plasma proteins will induce CYP2D6

d. loratidine and desloratidine are expected to have similar Volumes of Distribution

e. loratidine and desloratidine are expected to have similar elimination half-lives

Answer 1

This medicine work through the metabolism of Loratadine by isoforms of cytochrome p450 genes and as result they produce desloratadine, as an active metabolite. Most of the plasma proteins occupied by Loratadine (up to 99%) followed by its metabolite desloratadine (up to 76%). Loradine is having half life of 8 hours, mean half life where as desloratadine is having mean half life of 27 h. Beside this they have variable distribution and they don't have Cyp3A4 inhibitory function as they don't show antagonistic interaction. Therefore, most probable statement would be “until it’s eliminated, desloratadine will potentiate the effects of loratadine” as rest of the statements are not appropriate at in this scenario.