One of the Seaport District’s remnants of affordable office space is expected to be sold to a national real estate investor for over $90 million late this year.

The 152,102-square-foot 51 Sleeper St. overlooking Fort Point Channel is currently 71 percent leased with existing tenants paying an average rent just under $40 per square foot, according to marketing materials by HFF, which is representing ownership. That gives the owner the opportunity to upgrade rents as leases expire, with market rates in the neighborhood now topping $53 per foot.

TIAA-CREF paid $60.2 million for the multitenant building in 2013, which at the time was fully leased, representing a record price per square foot for the emerging Fort Point neighborhood.

A commercial real estate industry source said the latest round of bidding for 51 Sleeper St. amounted to a “feeding frenzy” culminating with a national investor placing the property under agreement.

Recent leases at 51 Sleeper St. include venture-backed artificial intelligence software firm Neurala, which leased 8,487 square feet in 2017, and craft brewer Hopsters, which leased 6,000 square feet of ground-floor space.

Seaport rents have continued to climb as the neighborhood attracted corporate headquarters tenants, attracting record prices as high as $1,208 per square foot for Commonwealth Partners’ purchase of the speculative 200 Pier 4 Boulevard following leases to Cengage and Boston Consulting Group.