WASHINGTON, Oct 2 (Reuters) - A U.S. Patent Office tribunal on Monday rejected challenges to two key patents owned by Novo Nordisk covering the active ingredient in its weight-loss and diabetes drugs Wegovy and Ozempic brought by a generic drugmaker that is hoping to sell generic versions of the blockbuster medications.

The office's Patent Trial and Appeal Board denied the requests by Mylan Pharmaceuticals, which is owned by Viatris , to review the validity of the Wegovy and Ozempic patents. Mylan had argued that the patents were obvious based on the anti-diabetes medication liraglutide and thus should be invalidated.

Mylan has also challenged a third patent related to a method of treatment using the drugs. The board's decision on whether to review that patent is due by Friday.

A spokesperson for Novo Nordisk said the company will "vigorously defend" its intellectual property. Representatives for Viatris did not immediately respond to a request for comment.

Novo's Wegovy is the first to market in a new class of highly effective weight-loss drugs. Its booming sales have led some analysts to predict the obesity market could be worth more than $100 billion by the end of this decade.

Record profits from Wegovy and type 2 diabetes drug Ozempic - which contains the same active ingredient, semaglutide - helped Denmark-based Novo become Europe's most valuable company in September.

Novo has filed several U.S. patent lawsuits against companies including Pennsylvania-based Viatris that are seeking to market generic versions of the drugs. Viatris has separately asked a West Virginia federal court to invalidate the patents as part of the litigation.

(Reporting by Blake Brittain in Washington; additional reporting by Patrick Wingrove in New York; editing by Will Dunham and David Bario)