A speculative Seaport District office tower that attracted headquarters locations by Alexion Pharmaceuticals and PTC has been sold to Norges Bank Real Estate Management and American Realty Advisors for $455 million.

The 17-story, 400,000-square-foot 121 Seaport tower was 100 percent leased before completion, with Alexion opting to relocate from New Haven, Connecticut, and PTC moving from Needham.

The building also includes 50,000 square feet of retail condominiums in a 2-story podium, which was not part of this week’s transaction.

At nearly $1,138 per square foot, the price is just short of the record $1,216 per square foot that Commonwealth Partners paid this summer for another new Seaport tower, 200 Pier Four, which leased all of its office space to Boston Consulting Group and Cengage.

“121 Seaport’s unique and sustainable design and its lease-up to two pioneering companies, PTC and Alexion served as a model for what we can achieve by pushing the traditional boundaries of development in Boston,” said Charley Leatherbee, executive vice president for Skanska, which developed the elliptical tower designed by CBT Architects.

Newmark Knight Frank represented Skanska in the sale.

This report was updated to clarify the ownership of the retail portion of 121 Seaport.