Allergan PLC's unique plan to shield the intellectual property relating to its blockbuster eye treatment Restasis failed to win over the U.S. Patent and Trademark Office, potentially opening the door to competition from Mylan NV.

Dublin-based Allergan had signed a deal with the St. Regis Mohawk Tribe to transfer all six of the Restasis patents listed in the U.S. Food and Drug Administration's Orange Book to protect them from legal challenges with the help of sovereign immunity.

Sovereign immunity is a legal principle that grants sovereign entities, such as Native American tribes, immunity from civil suit or criminal prosecution without their consent or Congressional waiver.

The move was meant to help Allergan maintain the exclusivity of the drug in the market; However, the U.S. Patent and Trademark Office ruled the St. Regis Mohawk Tribe could not claim sovereign immunity to dismiss Mylan's patent challenge against Restasis.

In a 42-page ruling, the patent board said even if sovereign immunity is applicable, the inter partes review proceedings could still go on without the tribe since Allergan retained ownership interests in the patents.

After the patents were transferred, the St. Regis Mohawk Tribe granted Allergan exclusive licenses to the intellectual property of Restasis and filed a motion to dismiss an ongoing inter partes review against them.

Mylan said an oral hearing on the merits is tentatively scheduled by the patent board April 3, with the final written decision on the patent challenge expected to be available by June 6.

Restasis generated net revenues of $1.41 billion in the U.S. for 2017.