Biochemical characterization of desloratadine, a potent antagonist of the human histamine H1 receptor

We have characterized desloratadine (5H-benzo[5,6]cyclohepta[1,2-b]pyridine, 8-chloro-6,11-dihydro-11-(4-piperidinylidene), CAS 100643-71-8) as a potent antagonist of the human histamine H-1 receptor. [H-3]Desloratadine bound to membranes expressing the recombinant human histamine H-1 receptor in Chinese hamster ovary cells (CHO-H-1) in a specific and saturable manner with a K-d of 1.1 +/- 0.2 nM, a B-max of 7.9 +/- 2.0 pmol/mg protein, and an association rate constant of 0.011 nM(-1) min(-1). The K-d calculated from the kinetic measurements was 1.5 nM. Dissociation of [H-3]desloratadine from the human histamine H, receptor was slow, with only 37% of the binding reversed at 6 h in the presence of 5 muM unlabeled desloratadine. Seventeen histamine H-1-receptor antagonists were evaluated in competition-binding studies. Desloratadine had a K-i of 0.9 +/- 0.1 nM in these competition studies. In CHO-H-1 cells, histamine stimulation resulted in a concentration-dependent increase in [Ca2+]; with an EC50 of 170 +/- 30 nM. After a 90-min preincubation with desloratadine, the histamine-stimulated increase in [Ca2+](i) was shifted to the right, with a depression of the maximal response at higher concentrations of antagonist. The apparent K-b value was 0.2 +/- 0.14 nM with a slope of 1.6 +/- 0.1. The slow dissociation from the receptor and noncompetitive antagonism suggests that desloratadine may be a pseudoirreversible antagonist of the human histamine H, receptor. The mechanism of desloratadine antagonism of the human histamine H, receptor may help to explain the high potency and 24-h duration of action observed in clinical studies. (C) 2002 Elsevier Science B.V. All rights reserved.