By Jonathan Stempel

April 3 (Reuters) - Mylan Inc on Thursday sued Celgene Corp to stop the latter's effort to keep generic versions of two drugs that generate $4.5 billion of annual sales off the market.

The lawsuit accuses Celgene of maintaining unlawful monopolies over Revlimid, which treats disorders caused by poorly formed blood cells; and Thalomid, which treats lesions associated with a variation of Hansen's Disease, or leprosy.

Revlimid is a branded version of lenalidomide, and is a derivative of thalidomide, a drug introduced in the 1950s for which Thalomid is a branded version. Mylan said both drugs can cost more than $100,000 for a year's supply.

In its lawsuit filed in the U.S. District Court in Newark, New Jersey, Mylan accused Celgene of using federal limits on the distribution of Revlimid and Thalomid, which were designed to promote the drugs' safety, as a pretext to keep generic drug makers from obtaining their own samples.

Mylan said this prevents generic drug makers from conducting "bioequivalence" tests required by U.S. regulators before generic drugs can be launched.

"The effect of Celgene's conduct is that no generic manufacturer, including Mylan, has been able to bring generic versions of Thalomid and/or Revlimid to market," Mylan said. "Through its illegal actions, Celgene has foreclosed Mylan from even attempting to enter the market."

Greg Geissman, a Celgene spokesman, declined to comment. Celgene is based in Summit, New Jersey, and Mylan is based in Canonsburg, Pennsylvania.

The lawsuit seeks to force Celgene to sell Mylan enough Revlimid and Thalomid at market prices to allow for bioequivalence testing. It also seeks compensation for Mylan's inability to sell generic versions of both, and triple damages.

In a February 13 regulatory filing, Celgene said it sold $4.28 billion of Revlimid and $244.5 million of Thalomid in 2013, together accounting for nearly 70 percent of all revenue.

Celgene also said its exclusive U.S. rights, whether through patents or primary regulatory approvals, to Revlimid and Thalomid expire in 2027 and 2023, respectively.

The case is Mylan Pharmaceuticals Inc v. Celgene Corp, U.S. District Court, District of New Jersey, No. 14-02094.

(Reporting by Jonathan Stempel in New York; editing by Andrew Hay)