The US Food and Drug Administration has cleared CV Therapeutics’ heart drug Ranexa (ranolazine) for the treatment of chronic angina, marking the first new class of anti-anginal therapy in the US to be approved in more than 20 years.

But the path to market has not been smooth, and the drug is not suitable for all angina patients. “Because Ranexa prolongs the QT interval, it should be reserved for patients who have not achieved an adequate response with other anti-anginal drugs,” the company said in a statement.

Chronic angina is a serious and debilitating heart condition, usually associated with coronary artery disease and marked by repeated and sometimes unpredictable attacks of chest pain. It affects approximately 6.4 million people in the US alone.