Photo courtesy of Robert Benson Photography

A clinical-stage biopharma company that’s preparing to seek FDA approval for a schizophrenia treatment is tripling its footprint in a downtown Boston tower.

Karuna Therapeutics signed a 10-year lease for nearly 51,000 square feet at 99 High St., where it maintains its current 25,443-square-foot headquarters.

In March, Karuna announced the closing of a 2.9 million share public stock offering raising $460 million. The firm has a schizophrenia drug in phase 3 trials expected to seek FDA approval in mid-2023.

The annual base rent for the expansion space is $3.5 million, according to an SEC filing, and includes a $9.2 million tenant improvement allowance.

The lease for Karuna’s existing 25,443-square-foot offices on the 26th floor expires in December 2025, according to company filings. The new lease includes the entire 20th and 21st floors, and a right of first offer on the 22nd floor.

The 32-story, 730,000-square-foot tower bordering the Rose Fitzgerald Kennedy Greenway is owned by Nuveen Real Estate, which acquired the property for $273 million in 2005.

The office vacancy rate in the Financial District is 19.4 percent, according to a recent report by brokerage Hunneman, with average asking rents of $59 per square foot.