A 56-year-old lady, who is a known case of lung cancer, presents with vomiting, headache and seizures. Investigations reveal serum osmolarity of 265 mosm/L (normal is 285-295 mosm/L) and a serum sodium level of 125 mEq/L (normal value is 136-152 mEq/L). Urine osmolarity is greater than 100 mOsm/L She has normal water intake. Which of the following drugs is useful for this patient?