Tolvaptan (Otsuka Pharmaceutical Co, Ltd, Tokyo, Japan)
is an oral arginine-vasopressin (AVP) type 2 receptor antagonist
with an aquaretic effect.12–15 Tolvaptan increases urine volume
without increasing electrolyte excretion into urine (ie, aquaresis).14
Schrier et al have reported that tolvaptan increased serum
sodium levels consistently in patients with hyponatremia.16
Tolvaptan is indicated for patients with hyponatremia and syndrome
of inappropriate antidiuretic hormone (SIADH) in the
USA, and for SIADH in the EU.17 In Japan, tolvaptan has been
approved for “volume overload in heart failure patients when
adequate response was not obtained with other diuretics (eg,
loop diuretics)” since 2010. The approval was based on the results
from the QUalification of Efficacy and Safety in the study
of Tolvaptan in cardiac edema study (QUEST) study, in which
Matsuzaki et al reported that tolvaptan showed efficacy in HF