Andover is about to lose one of its largest life sciences employers. TransMedics Group, the organ transplant technology company on Minuteman Road, signed a long-term lease in January for a new 498,000-square-foot headquarters in Somerville — moving roughly 300 jobs about 25 miles closer to downtown Boston by 2028.

CEO Waleed Hassanein said the new site "will position the company at the heart of the Greater Boston life sciences ecosystem, with close access to leading academic institutions, major transplant centers and a skilled workforce." Somerville sweetened the deal with an $18 million tax break over 10 years, and the state added up to $18 million more in tax credits tied to creating 600 new jobs by 2031. TransMedics plans to pour up to $300 million into renovating the site.

The Minuteman Road corridor's submarket has a 48% vacancy rate — the worst of any suburban submarket in Greater Boston — after losing more than 424,000 square feet of occupied space in the past year. Rents there run about $45 a square foot, compared with $107 in Boston and $117 in East Cambridge. That gap is exactly why companies keep gravitating toward the city.

Radiopharmaceutical company AdvanCell just leased 128,000 square feet in Andover for its U.S. headquarters, calling it "an important milestone" in its expansion. Still, that's roughly a quarter of the space TransMedics is vacating, and the broader lab market remains soft nationally even as availability has started to level off.

TransMedics hasn't said what happens to its Andover space, and construction on the Somerville campus isn't expected to start until 2027.