Developer Forest City plans to start construction in the fall on a long-awaited, 250,000-square-foot build-to-suit project for an international pharmaceutical company’s expansion in East Cambridge.

Millennium/The Takeda Oncology Co. will occupy the majority of what will be largely an office building with some research space and 15,000 square feet of retail space at 300 Massachusetts Ave. The firm will likely move in during the latter part of 2015, according to Forest City officials.

Millennium, which manages Japan’s Takeda Pharmaceuticals’ worldwide oncology business, is in growth mode, and the new building allows for the elbow room the company is craving, said Peter Calkins, executive vice president and COO for Forest City Boston. 

The pharmaceutical affiliate currently occupies Forest City’s 35 and 40 Landsdowne St. properties, and has office scattered through two or three other buildings. Millennium will continue to occupy the Landsdowne Street properties and most of the others, although it could fold some of the smaller offices into the new building, Calkins said.

As part of negotiations with the city, the developer has agreed to build 25 units of affordable housing when the 300 Mass Ave. project is complete. But if the firm cannot locate the real estate for the units, it has the option of contributing $5 million to the city’s affordable housing fund. Additionally, Forest City agreed to retain 168 affordable units scattered throughout its University Park campus along Sidney Street as affordable units for the life of the 75-year ground lease Forest City signed with MIT to develop the property.

Forest City must now gain the city’s Article 19 special permit review approval, standard for new construction of 50,000 square feet or more in Cambridge.