Clemastine (fumarate) (HS-592 (fumarate)) is a selective histamine H1 receptor antagonist with IC₅₀ of 3 nM. IC50 & Target: Histamine H1 Receptor^[1]. In Vitro: Clemastine (fumarate) (HS-592 (fumarate)) inhibits histamine induced rise in [Ca²⁺]i in HL-60 cells with an IC₅₀ of 3 nM as compared with that of chlorpheniramine or diphenhydramine with IC₅₀ values of 20 nM and 100 nM, respectively^[1]. Clemastine showed a first-pass reduction in the extent of absorption, with oral bioavailability calculated as 39.2 +/- 12.4%. Extravascular distribution of drug was suggested by the high volume of distribution (799 +/- 315 L) and low Cmax (0.577 +/- 0.252 ng/mL/mg) observed at 4.77 +/- 2.26 hours after administration, and by the biphasic decline in plasma concentration. The terminal elimination half-life (t1/2) of clemastine was 21.3 +/- 11.6 hours. Steady-state concentrations of clemastine were consistent with linear pharmacokinetic processes, and clearance was unaffected by age in the range studied, or by race^[2].