The mesylate salt of a centrally-acting neurokinin 1 (NK1) receptor antagonist with antidepressant and antiemetic activities. Casopitant competitively binds to and blocks the activity of the NK1 receptor, thereby inhibiting NK1-receptor binding of the endogenous tachykinin neuropeptide substance P (SP), which may result in antiemetic effects. SP is found in neurons of vagal afferent fibers innervating the brain-stem nucleus tractus solitarii and the area postrema, which contains the chemoreceptor trigger zone (CTZ), and may be elevated in response to chemotherapy. The NK1 receptor is a G-protein receptor coupled to the inositol phosphate signal-transduction pathway and is found in both the nucleus tractus solitarii and the area postrema.