Clemastine Fumarate is a selective histamine H1 receptor antagonist with IC50 of 3 nM.
Target: Histamine H1 Receptor
Clemastine 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].

Alcaftadine

Alcaftadine(R89674) is a H1 histamine receptor antagonist, which is used to prevent eye irritation brought on by allergic conjunctivitis.
Target: H1 Histamine Receptor
Alcaftadine is a broad-spectrum antihistamine displaying a high affinity for histamine H1 and H2 receptors and a lower affinity for H4 receptors. alcaftadine was more effective than placebo and at least as effective as olopatadine 0.01% in preventing ocular itching at 15 minutes and at 16 hours after administration. Alcaftadine 0.025% ophthalmic solution has been approved by the U.S. Food and Drug Administration for prevention of itching associated with allergic conjunctivitis in patients over 2 years of age [1]. Alcaftadine is a safe and effective option for the prevention of ocular itching associated with allergic conjunctivitis, is dosed once daily, and is competitively priced among prescription medications for allergic conjunctivitis [2].