UPDATE 1-Vivus's diet pill faces generic threat from Actavis
* Vivus has 7 U.S. patents for Qsymia, earliest expires in June 2020
* Actavis says patents either invalid or will not be infringed by generic
* Qsymia has disappointed with poor physician adoption and low sales
* Vivus intends to enforce its intellectual property rights (Adds details, background)
May 9 (Reuters) - Vivus Inc said on Friday drugmaker Actavis Plc plans to market a generic version of Vivus's obesity drug Qsymia that was approved in July 2012.
Actavis had already filed a U.S. marketing application for the generic version of the diet pill, according to a notice sent by the generic drugmaker on May 7, Vivus said.
Qsymia, one of the first obesity drugs to receive U.S. approval after 13 years, has disappointed investors with poor physician adoption and low sales.
Vivus has at least seven patents in the United States on Qsymia, the earliest of which expires in June 2020.
However, Actavis said in the notice that the patents were either invalid, unenforceable, or would not be infringed by its generic drug.
Vivus said it was reviewing the notice and that it had 45 days to file a patent infringement suit against Actavis.
If it filed a suit, the U.S. Food and Drug Administration would stay approval of the generic by up to 30 months, the company said.
Qsymia, which was once touted as a potential blockbuster to combat the rising epidemic of obesity in the United States, had net sales of $23.7 million in 2013.
That was well below the $128 million analysts were expecting, according to Thomson Reuters data from February 2013.
Vivus said this week that Qsymia's net sales more than doubled to $9.1 million in the first quarter ended March 31, but fewer prescriptions were dispensed in the quarter compared with the fourth quarter.
While Qsymia is Vivus's most high-profile drug, the company also has Stendra, an erectile dysfunction treatment, on the market.
Vivus said it intended to enforce its intellectual property rights on Qsymia.
Vivus shares closed at $5.38, while those of Actavis closed at $197.05 on Thursday. (Reporting by Esha Dey and Vrinda Manocha in Bangalore; Editing by Savio D'Souza)