(Reuters) - A group of U.S. senators on Tuesday asked drugmaker Allergan Plc to produce documents relating to its agreement to transfer patents to a Native American tribe to shield them from review by an administrative court.

FILE PHOTO: A sign marks Allergan's offices in Medford, Massachusetts, U.S., July 31, 2017. REUTERS/Brian Snyder/File Photo

Democratic Senators Sherrod Brown, Maggie Hassan, Amy Klobuchar, Al Franken and Patty Murray made the request for documents in a letter to Allergan Chief Executive Brent Saunders.

The letter was critical of Allergan’s deal with New York’s Saint Regis Mohawk Tribe, saying “it is difficult to conceive of Allergan’s transaction as anything other than a sham to subvert the existing intellectual property system.”

Allergan contends the tribe’s status as a sovereign entity places the patents outside the jurisdiction of the U.S. Patent Trial and Appeal Board, or PTAB.

Allergan on Sept. 8 transferred patents on its dry eye medication Restasis to the tribe, which agreed to license them exclusively back to the company in exchange for ongoing payments. A federal court invalidated Allergan’s Restasis patents on Oct. 16, rendering the tribal deal mostly meaningless. Allergan has appealed that ruling.

In Tuesday’s letter, the senators said they were concerned that tribes could reach similar deals with other manufacturers of brand-name prescription drugs.

They asked Allergan to disclose the terms of its agreement with the tribe and explain whether it has plans for similar patent transactions.

The lawmakers also demanded to see “any legal briefs or memos prepared by Allergan regarding sovereign immunity,” particularly memos on the question of whether Allergan’s agreement could immunize patents from review by federal courts.

The lawmakers gave Allergan a December 1 deadline to produce the requested documents.

A bipartisan group of representatives from the House Oversight and Government Committee made a similar request for documents from Allergan in October.

Allergan spokesman Mark Marmur said the company welcomes the opportunity to continue to brief lawmakers about the deal.

Marmur also defended Allergan’s bid to avoid PTAB review, saying “experts across the legal, biopharmaceutical and business communities” have raised concerns about the fairness of the administrative process.

Allergan has said it does not object to its patents being reviewed in federal court, which it called a fair and time-tested process.