Tolvaptan is a selective, competitive arginine vasopressin receptor 2 antagonist with an IC50 of 1.28μM for the inhibition of AVP-induced platelet aggregation. IC50 value: 1.28 uM (inhibition of AVP-induced platelet aggregation) Target: vasopressin receptor 2 Tolvaptan (OPC-41061) is a selective, competitive arginine vasopressin receptor 2 antagonist with an IC50 of 1.28μM for the inhibition of AVP-induced platelet aggregation. Tolvaptan (OPC-41061) is used to treat hyponatremia (low blood sodium levels) associated with congestive heart failure, cirrhosis, and the syndrome of inappropriate antidiuretic hormone (SIADH). Tolvaptan (OPC-41061) is also in fast-track clinical trials for polycystic kidney disease. Treatment with t tolvaptan (OPC-41061) causes rapid and sustained body weight reductions concurrent with increases in urine output, improves and/or normalizes serum sodium in hyponatremic patients, reduces signs and symptoms of congestion and increases thirst. However, tolvaptan (OPC-41061) has not been shown to decrease heart failure re-hospitalization or mortality. As an adjunct to standard therapy, tolvaptan (OPC-41061) is unique in that it is virtually the only novel agent tested in patients hospitalized for acute heart failure syndrome (AHFS) to reach its primary end point for short-term efficacy without causing deleterious side effects.