The US subsidiary of Japan’s Astellas Pharma has received approval from the Food and Drug Administration for the use of Vaprisol (conivaptan hydrochloride) to treat hospitalised patients with euvolemic hyponatremia. - also known as water poisoning.

Hyponatremia is a life-threatening condition that occurs when the body's blood sodium level falls significantly below normal, and affects up to 4% of hospitalised patients in the USA each year.

In the euvolemic form, hyponatremia occurs when the body retains water but not sodium sodium, and tends to be caused by cancer, hypothyroidism and as a side effect of some drugs such as antidepressants. Vaprisol is the first drug to be approved for this indication in the USA.

Vaprisol is an arginine vasopressin antagonist, and induces water loss without depleting sodium and other electrolytes, unlike diuretic drugs. Astellas has coined the term ‘aquaretic’ for the drug to reflect this profile. At present, hyponatremia is treated using a combination of diuretics and intravenous saline solution that can be tricky to administer.

It was not all good news for Astellas, however. The FDA issued an ‘approvable’ letter for Vaprisol in the treatment of hypervolemic hyponatremia, a related condition in which the body retains water and sodium, but more of the former. The company said it would work with the agency to try to secure approval for this form of the condition.