Cheshire, Conn.-based Alexion Pharmaceuticals, striving to expand its stable of products beyond the world’s most expensive drug, will need to wait a little longer for the U.S. launch of a second treatment because the FDA wants extra time to look over its application.

Alexion’s Kanuma (sebelipase alfa) was scheduled for a final decision next week under the FDA’s priority review program, but the company said that the agency has added another 90 days to the process in order to examine requested additional information on the drug’s manufacturing. The three-month delay almost negates the benefit of the priority tag, which shortens the usual 10-month review process to six months.

Kanuma, a treatment for the ultra-rare lysosomal acid lipase deficiency (LAL-D), was the focal point of Alexion’s $8.4 billion acquisition of Synageva earlier this year. Alongside the Alexion-invented Strensiq (asfotase alfa), scheduled for FDA approval later this year, Kanuma is a key to the biotech’s planned growth. The company made its reputation with Soliris, a rare disease treatment that costs more than $500,000 annually, and Alexion now is looking to build a stable of additional therapies.

Strensiq and Kanuma won tandem European approvals this week, and Alexion plans to initiate a nation-by-nation rollout in the next few months. The company hasn’t divulged pricing information on the two new drugs, but Barclays analyst Geoff Meacham penned a note to investors indicating that Alexion likely will charge roughly $375,000 annually for each.

LAL-D is caused by a genetic mutation that results in a loss of LAL enzymes, contributing to dangerous buildups of cholesterol and triglycerides that can cause severe damage to the liver. Kanuma, which is an enzyme-replacement therapy, works by counteracting that process.