A Newton real estate investment trust said late Monday that it will pay $1.1 billion for two waterfront towers that house the headquarters of the biotechnology company Vertex Pharmaceuticals Inc.
The sale comes about a week after Vertex, which relocated from Cambridge, formally opened its new home office in the Seaport District and represents another marker in the city’s progress in transforming the South Boston Waterfront into a biomedical and technology hub, dubbed the Innovation District.
The $682 per square foot that Senior Housing Properties Trust will pay for the two 15-story buildings eclipses other deals in the Seaport District.
New York-based TIAA-CREF, a financial services and retirement planning firm, last year paid $400 per square foot for office space in the burgeoning neighborhood, the Boston Business Journal reported.
When the Vertex project was announced in 2011, state officials and the developer, Fallon Co. of Boston, estimated it would cost $800 million to build.
The sale is expected to close by the middle of this year, said David Hegarty, president of Senior Housing Properties Trust.
“We’ve been watching the growth of the Fan Pier and Innovation District,’’ Hegarty said. “That’s the hottest market in Boston. It’s also the hottest market in the country.’’
Joseph Fallon, chief executive of Fallon Co., could not be reached for comment Monday.
Fallon has been a major developer in the Seaport District and has other projects underway in the neighborhood, including luxury residences and another office tower.
The purchase of the Vertex buildings is Senior Housing Properties Trust’s single biggest investment and includes 1.65 million square feet of space with biomedical research facilities, offices, underground parking, and street-level retail space. The trust invests primarily in senior housing and health care facilities.
Vertex moved about 1,200 employees from scattered locations to the new headquarters. The drug maker, known for developing treatments for hepatitis C and the chronic lung disease cystic fibrosis, is paying $72.5 million a year in rent on the building.
The company occupies about 96 percent of the towers and has a 15-year lease agreement. Despite recent setbacks at Vertex — including the layoffs of 370 employees worldwide and a faster-than-expected decline in sales of its hepatitis C drug Incivek — the pharmaceutical company is a stable client, Hegarty said.
Vertex has been one of the state’s fastest-growing drug makers, adding more than 800 jobs in Massachusetts since 2004.
“We think they’re very qualified from a credit perspective,’’ Hegarty said.