SACRAMENTO — Sidley Austin has withdrawn as defense counsel for Merck & Co. Inc. in preemptive patent litigation brought by rival Gilead Sciences Inc., according to documents filed in the Northern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

Gilead filed suit in U.S. District Court for the Northern District of California in August, asking a judge to declare that its Hepatitis C–treating drug, Sofosbuvir NDA, does not infringe patents co-owned by Merck and Isis Pharmaceuticals. The two companies had not sued, but Gilead executives clearly believed litigation was imminent.

Gilead, represented by Fish & Richardson, said in its complaint that a Merck official had called a Gilead counterpart in August and suggested it license two Merck patents at a cost of 10 percent of all future net sales of Sofosbuvir. Gilead sued instead.

Attorneys for the Foster City, Calif.–based Gilead argue that Merck, which makes the Hepatitis C treatment Victrelis, modified its patent claims to include compounds found in Sofosbuvir. Gilead argues that Merck’s patents are invalid.

Merck and Isis Pharmaceuticals filed counterclaims on Monday, asking U.S. District Judge Jeffrey White to find that Sofosbuvir does indeed infringe two of the companies’ patents.

Contact the reporter at cmiller@alm.com.