The Center for Life Science Boston, the 704,000-square-foot lab tower in Boston’s Longwood Medical Area, is close to securing a $350 million mortgage.

John Fowler, executive managing director of Holliday Fenoglio Fowler LP, said Wednesday his firm has acquired financing for the building from two life insurance companies and one bank. The mortgage may close as soon as the end of this month.

"I think this may be one of the biggest loans done this year, in the whole country," Fowler said. "One of the biggest, if not the biggest. Certainly for a single asset. There just aren’t that many large transactions going on."

The five-year mortgage has an interest rate below 8 percent, Fowler said.

Biomed Realty Trust, a San Diego-based REIT that owns lab space on both coasts, is the owner of the building. The $350 million mortgage will replace a construction loan of more than $500 million.

The 18-story Class A building is 91 percent leased to a tenant roster that includes Beth Israel Deaconness and Children’s Hospitals, as well as the Dana-Farber Cancer Institute.

Debt markets have had little appetite for deals larger than $50 million, even for properties with healthy lease rosters owned by solid landlords. Notably, in April, Boston Properties was able to secure only a $215 million construction loan for its $500 million Russia Wharf project in Boston. Five banks partnered to supply the $215 million loan. The 854,000-square-foot, mixed-use project is 82 percent leased and anchored by Wellington Management.