A PK Study of 3 Dosages of Tolvaptan in Patients With Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

Inclusion Criteria: * Male or female subjects greater than or equal to 18 years of age or the age of legal consent. * Must have a BMI less than or equal to 32.0 kg/m2. * Subjects must have a diagnosis of SIADH prior to randomization. * Persistent euvolemic hyponatremia, evidenced by 3 serum sodium assessments of between 120 and 133 mmol/L, inclusive drawn locally as follows: one during the screening period, a second at check-in on Day -1, a third on Day 0 (12-24 hours prior to dosing), which will serve as the baseline value for efficacy endpoints * Subjects with relatively intact renal function, ie, estimated glomerular filtration rate using the CKD-EPI formula of greater than or equal to 60 mL/min/1.73m2. * Ability to provide written, informed consent prior to initiation of any trial related procedures, and ability, in the opinion of the PI, to comply with all the requirements of the trial. * Sexually active males who are practicing a highly effective method of birth control during the trial and for 30 days after the last dose of trial medication or who will remain abstinent during the trial and for 30 days after the last dose, or sexually active females of childbearing potential who are practicing a highly effective method of birth control during the trial and for 30 days after the last dose of trial medication or who will remain abstinent during the trial and for 30 days after the last dose, or female subjects of nonchildbearing potential (surgically sterile or postmenopa…