UPDATE 1-Gilead says U.S. upholds one Viread patent

(Adds background, analyst comment, company comment, stock price)

LOS ANGELES, May 20 (Reuters) - Gilead Sciences Inc GILD.O said on Tuesday that U.S. patent officials have upheld one of the four challenged patents covering its AIDS medicine Viread.

The U.S. Patent & Trademark Office (PTO) has not announced its ruling on the remaining three patents, the company said.

Bear Stearns analyst Mark Schoenebaum said in a research note, “This reduces a minor overhang on Gilead shares ... Although the risk had been low that the patents would be rejected in a final manner, we are nonetheless relieved by the news.”

The nonprofit Public Patent Foundation had challenged the Viread patents last year. The group said it submitted evidence that the scientific knowledge on which the patents were based had existed before they were granted to Gilead.

Viread, or tenofovir disoproxil fumarate, is also sold as part of Gilead's two-drug combination AIDS pill Truvada and as a component of the newer Atripla, which combines Truvada with Bristol-Myers Squibb Co's BMY.N Sustiva into a single pill.

“We have always believed that the U.S. PTO would recognize that Viread is a novel product, and we remain confident that the office will rule similarly on the remaining three patents under review,” Gregg Alton, Gilead’s general counsel, said in a statement.

Shares of Gilead, which rose 1.2 percent to close at $53.87 on Nasdaq, were unchanged in extended trading. (Reporting by Deena Beasley; editing by Tim Dobbyn)