(Bloomberg) -- Gilead Sciences Inc. agreed to purchase CymaBay Therapeutics Inc., a developer of an experimental liver disease drug, for $4.3 billion in equity value. 

Gilead offered to purchase all outstanding shares of CymaBay for $32.50 a share, according to a statement Monday, a 27% premium to Friday’s close for the Newark, California-based biotech.  

Gilead shares rose as much as 2% as of 10:10 a.m. in New York. CymaBay’s rose as much as 25%, the most intraday since June 30.

CymaBay’s seladelpar is intended to treat a rare bile duct disorder that mostly affects women. The drug received priority review from US regulators after a successful final-stage trial, with an approval decision expected by Aug. 14, the companies said in the statement.

Gilead is seeking new areas for growth to supplement its core antiviral-drug franchise. Among other products, Gilead helped develop and sells key medications to treat hepatitis C, a virus that attacks the liver.

The deal is Gilead’s largest since it agreed to buy Immunomedics for about $21 billion in 2020, according to data compiled by Bloomberg. 

CymaBay’s drug has potential to be the best in class for the rare disorder, primary biliary cholangitis, Bloomberg Intelligence analyst Michael Shah said in an email. In addition to controlling biochemical signs of the disease, it significantly reduces itchy skin, a common symptom. Ocaliva, a competing drug from Alfasigma SpA, increases itching, he noted.

Still, with potential peak sales of $650 million for the CymaBay drug, the deal value of $4.3 billion is “on the expensive side,” RBC Capital Markets analyst Brian Abrahams wrote in a note to clients.

--With assistance from Catherine Larkin.

(Updates with details on drug market starting in seventh paragraph.)

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