Akcea Therapeutics Inc. and Ionis Pharmaceuticals Inc. said the U.S. Food and Drug Administration approved Tegsedi to treat a rare genetic disorder.

Tegsedi got the agency's approval to treat polyneuropathy, or nerve damage outside the brain and central nervous system, caused by hereditary transthyretin-mediated, or hATTR, amyloidosis in adults.

The disorder results from the disintegration of a protein called transthyretin, leading to deposition of nerve-destroying amyloid protein clumps throughout the body.

Also known as inotersen, Tegsedi works by reducing the production of transthyretin protein through a once-weekly injection that patients can administer themselves through the skin.

The FDA's approval is based on a phase 3 study, called Neuro-TTR, which showed that the drug substantially reduced the production of transthyretin protein among the patients, the company said in an Oct. 5 news release.

In addition, the therapy carries a boxed warning — the FDA's most serious warning — regarding the risks of thrombocytopenia and glomerulonephritis as major side effects. Thrombocytopenia is a condition involving low blood platelet count in the blood and glomerulonephritis is inflammation of the tiny filters in the kidneys.

Tegsedi will be sold under a Risk Evaluation and Mitigation Strategy program under which the FDA requires the company to use additional measures to inform patients and healthcare providers about the drug's risk and benefits.

Carlsbad, Calif.-based Ionis sold the rights for inotersen, along with another drug for a similar condition, to its affiliate Akcea in a $1.7 billion deal earlier this year.

Following the U.S. approval, Cambridge, Mass.-based Akcea will pay $50 million as a milestone payment to Ionis either in cash or company stock and also provide Ionis 60% of the future profits or losses related to the drug.

Tegsedi has already received marketing approval in Europe and Canada. The U.S. approval comes after rival Alnylam Pharmaceuticals Inc.'s Onpattro received the FDA nod in August to become the first approved therapy for the same disease.

Akcea is partnering with Express Scripts Holding Co. unit Accredo to distribute Tegsedi and will consider including value- and outcomes-based access arrangements in addition to other contracting strategies and risk-based approaches that address access and affordability concerns of patients, said Kyle Jenne, head of U.S. commercial at Akcea, in a separate statement.