Changes in pH differently affect the binding properties of histamine H1 receptor antagonists

We investigated the effect of acidic pH, a condition that can be encountered during inflammation accompanying allergic reaction, on the binding properties of histamine H1 receptor antagonists, including levocetirizine ((2-{4-[(R)-(4-chlorophenyl)(phenyl)methyl]piperazin-1-yl}ethoxy)acetic acid; Xyzal®), fexofenadine (rac-2-[4-[1-Hydroxy-4-[4-(hydroxydiphenylmethyl) piperidin-1-yl]butyl]phenyl]-2-methylpropionic acid hydrochloride; Allegra®) and desloratadine (8-Chloro-6,11-dihydro-11-(4-piperidylidene)-5H-benzo[5,6]cyclohepta[1,2-b]pyridine; Clarinex®). Lowering the pH from 7.4 to 5.8 decreased the affinity of [3H]mepyramine for histamine H1 receptors from 1.7 to 7.5 nM while the opposite was observed with [3H]levocetirizine, whose affinity increased from 4.1 to 1.5 nM. Competition curves with [3H]mepyramine indicated that decreasing the pH from 7.4 to 5.8 led to a 2- to 5-fold increase in the affinity of fexofenadine and levocetirizine, no change in affinity for desloratadine and a 5- to 10-fold decrease in affinity for mepyramine and histamine. Kinetic experiments showed that the increase in affinity of levocetirizine and, to a lesser extent, fexofenadine were totally attributable to a lower dissociation rate at acidic pH (t1/2 increasing from 77 to 266 min and from 71 to 135 min, respectively). Although the affinity of desloratadine remained unchanged, lowering the pH caused a decrease in its dissociation rate (t1/2 of 50 and 256 min at pH 7.5 and 5.8, respectively) accompanied by a concomitant 3.5-fold decrease in its association rate constant. The loss of affinity of mepyramine at acidic pH was driven by a decrease in its association rate constant. Interaction between the carboxylic moiety of levocetirizine and Lys191 is responsible for its slow dissociation rate from the receptor. We found that the magnitude of the pH effect on the dissociation rate of levocetirizine was maintained after mutating Lys191 into alanine, suggesting that a tighter interaction of levocetirizine with Lys191 at lower pH is not the cause of its even slower dissociation rate from the receptor. Although these changes may seem limited in amplitude, we show that they may have substantial effects on receptor occupancy in vivo.