The hydrochloride salt of palonosetron, a carbazole derivative and a selective serotonin receptor antagonist with antiemetic activity. Palonosetron competitively blocks the action of serotonin at 5-hydroxytryptamine type 3 (5-HT3) receptors located on vagal afferents in the chemoreceptor trigger zone (CTZ), resulting in suppression of chemotherapy-induced nausea and vomiting. The CTZ is located in the area postrema on the dorsal surface of the medulla oblongata at the caudal end of the fourth ventricle and outside the blood-brain barrier (BBB).