(Reuters) – Gilead Sciences Inc will invest $5.1 billion to raise its stake in Galapagos NV and partner with the Belgian-Dutch biotech firm to develop and commercialize its treatments for a ten-year period, the companies said on Sunday.

The investment in Galapagos, which comes nearly four years after the firms partnered to develop a drug targeting inflammatory diseases, includes an upfront payment of $3.95 billion and a $1.1 billion equity investment from Gilead.

“Gilead gains exclusive access to all current and future compounds in Galapagos’ rich pipeline while Galapagos is able to expand its research activities and build commercial infrastructure,” Gilead Chief Executive Officer Daniel O’Day said in a statement.

The U.S. drugmaker will pay 140.59 euros ($158.43) per new share in Galapagos to increase its stake to 22% from 12.3%. That represents a premium of nearly 10% to the close of Galapagos shares on Friday.

Galapagos said it would also seek shareholder approval to allow Gilead to further increase its ownership to up to 29.9%. After that, their agreement includes a 10-year standstill clause that would prevent Gilead from accumulating any additional stake.

In 2015, Gilead signed a deal worth more than $2 billion with Galapagos, which included a $725 million upfront payment for the development of filgotinib, as well as an equity stake in the European firm.

Filgotinib is an experimental compound being advanced for rheumatoid arthritis and inflammatory bowel disease and Gilead intends to submit its new drug application to the U.S. Food and Drug Administration this year.

Galapagos will now have greater involvement in filgotinib’s global strategy and participate more broadly in the commercialization of the product in Europe, according to their statement on Sunday.

The companies will share future development costs for filgotinib equally, replacing the 80-20 cost split provided by the original agreement, they said.

Galapagos will use the proceeds to expand and speed up its research and development programs.

Gilead will nominate two people to Galapagos’ board.

Barclays, Centerview Partners and Lazard acted as financial advisers to Gilead while Moelis & Co and Morgan Stanley advised Galapagos.

The Wall Street Journal reported the deal earlier on Sunday.

($1 = 0.8874 euros)