WARNING: DIFFERENTIATION SYNDROME, CARDIAC CONDUCTION ABNORMALITIES AND ENCEPHALOPATHY INCLUDING WERNICKE’S Differentiation Syndrome: Patients with acute promyelocytic leukemia (APL) treated with arsenic trioxide have experienced differentiation syndrome, which may be life-threatening or fatal. Signs and symptoms may include unexplained fever, dyspnea, hypoxia, acute respiratory distress, pulmonary infiltrates, pleural or pericardial effusions, weight gain, peripheral edema, hypotension, renal insufficiency, hepatopathy, and multi-organ dysfunction, in the presence or absence of leukocytosis. If differentiation syndrome is suspected, immediately initiate high-dose corticosteroids and hemodynamic monitoring until resolution. Temporarily withhold arsenic trioxide injection [see Dosage and Administration (2.3), Warnings and Precautions (5.1)]. Cardiac Conduction Abnormalities: Arsenic trioxide can cause QTc interval prolongation, complete atrioventricular block and torsade de pointes, which can be fatal. Before administering arsenic trioxide injection, assess the QTc interval, correct electrolyte abnormalities, and consider discontinuing drugs known to prolong QTc interval. Do not administer arsenic trioxide to patients with a ventricular arrhythmia or prolonged QTc interval. Withhold arsenic trioxide injection until resolution and resume at reduced dose for QTc prolongation [see Dosage and Administration (2.3), Warnings and Precautions (5.2)]. Encephalopathy: Serious encephalopathy, including Wernicke’s, has occurred with arsenic trioxide. Wernicke’s is a neurologic emergency. Consider testing thiamine levels in patients at risk for thiamine deficiency. Administer parenteral thiamine in patients with or at risk for thiamine deficiency. Monitor patients for neurological symptoms and nutritional status while receiving arsenic trioxide. If Wernicke’s encephalopathy is suspected, immediately interrupt arsenic trioxide and initiate parenteral thiamine. Monitor until symptoms resolve or improve and thiamine levels normalize [see Warnings and Precautions (5.3)].