Lixivaptan is a highly potent, non-peptide, selective vasopressin 2 receptor antagonist. The oral capsule works by reducing the action of a hormone (vasopressin) that blocks fluid excretion. Lixivaptan acts specifically on the vasopressin 2 receptor in the kidney, causing water to be excreted while sparing sodium or other electrolytes. The drug, currently being studied in Phase III clinical trials, shows promise in treating heart failure patients with hyponatremia (low blood levels of sodium and other electrolytes).