Gilead Sciences has signed for 100,000 sq ft at Stockley Park in Heathrow.
Gilead will take an additional 55,000 sq ft of office space on top of the 42,000 sq ft they currently lease.
Two Roundwood Avenue is currently developed by Europa Capital and Cordatus Real Estate. They purchased the building back in 2013 for £18.25m.
The previous tenant of the property was Hasbro.
Cordatus Real Estate and Europa Capital are advised by Lambert Smith Hampton and BNP Paribas Real Estate.
Gilead Sciences was advised by Cushman & Wakefield.