SACRAMENTO — Sidley Austin has withdrawn as defense counsel for Merck & Co. Inc. in preemptive patent litigation brought by rival Gilead Sciences Inc., according to documents filed in the Northern District this week.

The pharmaceutical giant will be represented instead by attorneys from Durie Tangri’s San Francisco office and a team from Fried, Frank, Harris, Shriver & Jacobson. The filings gave no reason for the switch. A Merck representative did not return a message seeking comment Wednesday.