The U.S. Food and Drug Administration has approved NPS Pharmaceuticals Inc's drug Natpara to treat a rare hormone disorder, validating a bet by Shire Plc, which recently agreed to acquire NPS for $5.2 billion.

Natpara is designed to treat hypoparathyroidism, a condition in which the body's parathyroid gland does not secrete enough parathyroid hormone (PTH). The hormone works with vitamin D to regulate body calcium.

Analysts expect the drug to generate peak annual sales of $542 million by 2019, according to Thomson Reuters data.

The FDA approval is for patients who cannot be well-controlled on calcium supplements and active forms of vitamin D alone. It stipulates that the drug's label include a boxed warning that bone cancer has been observed in rat studies with the drug. NPS is also required to track patient use under a risk evaluation and mitigation program.

"All of that was well known and well anticipated, so there was for us no surprises in both the label and the follow-up requirements," said Shire Chief Executive Officer Flemming Ornskov.

NPS said it plans to launch sales of the drug in the second quarter of this year, but declined to comment on what the price will be. Shire's acquisition of the company is expected to close in the first quarter.

Shire, which makes Adderall and Vyvanse for attention deficit hyperactivity disorder, is seeking to expand its position in the field of drugs for gastrointestinal disorders and rare diseases.

About 180,000 people globally suffer from hypoparathyroidism, according to New Jersey-based NPS. In about 40 percent of cases, the condition cannot be controlled with the current treatment of high doses of calcium and vitamin D.

That uncontrolled population is the audience that NPS initially plans to target, which in the United States is about 20,000 patients, the company said.

Low levels of PTH can cause tingling in the fingers and toes, muscle spasms, fatigue, muscle aches, hair loss, dry skin, headaches, mood swings and memory problems.

Natpara is a bioengineered version of the hormone itself.

The condition can be caused by congenital disorders or surgery.

Data from a late-stage clinical trial of Natpara showed that 53 percent of patients treated with the drug were able to reduce their calcium and vitamin D supplements by 50 percent or more, compared with 2 percent of placebo-treated patients.

NPS has filed for approval of Natpara in Europe.