Epinastine hydrochloride (WAL-801CL HCl) is an antihistamine and mast cell stabilizer that is used in eye drops to treat allergic conjunctivitis.

Epinastine shows a high affinity to H1-receptors in receptor binding studies in the guinea pig ileum. [1] Epinastine is able to displace specific [3H]NC-5Z binding at low concentrations in the locust nervous tissue. Epinastine binds to the honey bees neuronal octopamine receptor with Ki of 1.1 nM. Epinastine antagonises octopamine-induced cAMP formation in the insect brain. [2] Epinastine causes an inhibition of histamine release from rat peritoneal mast cells induced by both antigen-antibody reaction and compound 48/80. Epinastine is similarly effective in inhibiting compound 48/80-induced histamine release not only from isolated rat peritoneal mast cells but also from rat mesenterial pieces. Epinastine is effective in inhibiting not only Ca2+ uptake into lung mast cells in actively sensitized guinea pigs but also Ca2+ release from the intracellular Ca store of rat peritoneal mast cells exposed to both compound 48/80 and substance P. [3] Epinastine shows a dose- and time-dependent suppressive effect on IL-8, one of the chemokines for eosinophils, released from eosinophils isolated from atopic diseases. [4]

Epinastine inhibits histamine-induced reactions in the skin or the lung of rats, dogs and guinea pigs. [1]