Clemastine (fumarate) (HS-592 (fumarate)) is a selective histamine H1 receptor antagonist with IC50 of 3 nM.

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].

References:
[1]. Seifert, R., et al., Histamine increases cytosolic Ca2+ in dibutyryl-cAMP-differentiated HL-60 cells via H1 receptors and is an incomplete secretagogue. Mol Pharmacol, 1992. 42(2): p. 227-34.
[2]. Schran, H.F., et al., The pharmacokinetics and bioavailability of clemastine and phenylpropanolamine in single-component and combination formulations. J Clin Pharmacol, 1996. 36(10): p. 911-22.