By Manas Mishra and Manojna Maddipatla

(Reuters) – The U.S. patent office will review patents on Alexion Pharmaceuticals Inc’s blood disorder treatment Soliris, after Amgen Inc challenged them, court filings showed on Friday.

The move deals a blow to Alexion’s efforts to ward off competition for its top-selling drug, which accounted for nearly 82% of the drugmaker’s total revenue in the latest quarter.

Alexion’s shares fell 10.4% to $100.51, while Amgen’s were marginally higher at $208.21.

Amgen is contesting the U.S. patents to Soliris that extend the drug’s market exclusivity to 2027 from 2022.

The Patent Trial and Appeal Board, an administrative court run by the U.S. patent office, said on Friday it was instituting an inter partes review (IPR) on the patents.

The IPR is seeking to invalidate new patents covering the composition of the main ingredient of Soliris, eculizumab, its formulation, and its use to treat blood disorder proxysmal nocturnal hemoglobinuria (PNH).

Soliris is approved for four indications and brought in sales of $980.8 million in the quarter ended June 30.

Alexion said it will “vigorously defend” its patents and address the court’s specific concerns in the IPR proceedings.

The drugmaker is trying to retain its stronghold in the PNH market by converting patients to its Soliris-successor Ultomiris, which won U.S. approval last year.

Alexion has also been seeking to expand label for Soliris, which won U.S. approval to treat autoimmune disease neuromyelitis optica spectrum disorder (NMOSD) in June.

Besides PNH and NMOSD, Soliris is approved for atypical hemolytic uremic syndrome and generalized myasthenia gravis (gMG).

“Regardless of the ultimate outcome here, Soliris has orphan drug designation in the U.S. for gMG through late 2024 and for NMOSD through mid-2026,” the company said.

The U.S. Food and Drug Administration’s Orphan drug status comes with incentives, including a period of marketing exclusivity, to developers of rare disease treatments.

A decision on the validity of the new Soliris patents is expected in Q3 2020, as the U.S. patent office may require a full year for its assessment, SVB Leerink analyst Geoffrey Porges said.

Analysts were expecting Alexion to prevail over Amgen in the United States, where Soliris raked in about 51% of its total sales in the last quarter.

Amgen declined to comment on the court’s decision to review Soliris patents.

Alexion also awaits an impending European Patent Office (EPO) decision on its patent applications for Soliris, to extend the drug’s PNH market exclusivity in the European Union to 2027.

(Reporting by Manojna Maddipatla and Manas Mishra in Bengaluru; Editing by Arun Koyyur and Shinjini Ganguli)