Connecticut-based Alexion Pharmaceuticals will move its headquarters from New Haven to Boston’s Seaport District in mid-2018, bringing 400 corporate and research jobs and filling the remaining office and lab space in a speculative tower scheduled to open early next year.

“Boston will provide access to a larger biopharmaceutical talent pool and a variety of life science partners to further support development initiatives,” CEO Ludwig Hantson said in a conference call today.

The announcement comes just 18 months after Alexion cut the ribbon on a brand-new headquarters in downtown New Haven. Alexion said it will retain approximately 450 jobs in New Haven in its R&D, clinical supply and quality teams, nurse case management and enterprise business services.

The rare disease specialist will occupy 150,000 square feet on six floors on the lower levels of 121 Seaport, according to a source. The 17-story tower is being developed by Skanska.

Needham-based tech company PTC last week leased 250,000 square feet at 121 Seaport for its own new downtown headquarters and will relocate from Kendrick Street in early 2019.

Charley Leatherbee, Skanska’s head of commercial development in Boston, told Banker & Tradesman that the CBT Architects-designed tower with its open floor plates and 10-foot ceilings can accommodate both office and lab uses.

“We design all of our buildings to be incredibly flexible from a space programming and use perspective,” he said.

Newmark Knight Frank represented Skanska as the developer and owner of the project, and Cushman & Wakefield represented Alexion.

As part of a restructuring plan, Alexion will cut 20 percent of its global workforce to save $250 million annually. It will shut its manufacturing facility in Smithfield, Rhode Island by 2018 and shift manufacturing to other sites in the U.S. and Ireland and through outside contractors, executives said.

Alexion in 2013 signed on as anchor tenant for nine floors at 100 College St., a 538,000-square-foot tower developed by Concord-based Winstanley Enterprises.

Designed by Elkus Manfredi Architects, the tower at the end of the Route 34 connector replaced 250,000 square feet in three buildings at a Cheshire office park, where Alexion employed 650 people.

The New Haven headquarters was billed as a milestone in New Haven’s Downtown Crossing urban revitalization efforts and the city’s attempts to expand its life science cluster near Yale School of Medicine.

A former Yale medical professor, Leonard Bell, founded Alexion in 1992 in New Haven before the company’s move to Cheshire in 2001.

Connecticut Gov. Dannel Malloy gave the company $51 million in economic incentives to expand in New Haven in exchange for a promise to create up to 300 permanent jobs. The incentive package under Malloy’s First Five program included up to $20 million in loans forgiveable upon meeting job targets, along with a $6 million grant for construction and equipment and up to $25 million in tax credits.

Catherine Smith, Connecticut’s commissioner of economic development, said the state would seek to recoup the loan and grant money, the Hartford Courant reported.