Sept 6 (Reuters) - Bristol Myers Squibb has been accused in a new lawsuit of using fraudulent patents and other illegal tactics to maintain its monopoly on blockbuster blood cancer drug Pomalyst for years after it should have faced generic competition.

In a complaint filed on Tuesday in Manhattan federal court, Blue Cross Blue Shield of Louisiana said the company violated U.S. antitrust law and had caused purchasers of the drug to overpay "by many hundreds of millions, if not billions, of dollars."

The health insurer brought the claim on behalf of a proposed nationwide class of entities that paid for Pomalyst since October 2020, when it claimed generic versions of the multiple myeloma treatment would have launched if not for the illegal scheme.

It is seeking three times the amount of the alleged overcharge, which is permitted under the federal Sherman Act antitrust law.

A spokesperson for Bristol Myers did not immediately respond to a request for comment.

Pomalyst is a top seller for Bristol Myers, bringing in nearly $3.5 billion out of $46.16 billion of its total revenue last year. The company in July forecast sales of the drug to fall this year, however, as more patients receive it for free through a patient assistance program.

The drug was developed by Celgene, a company acquired by Bristol Myers in 2019. Tuesday's lawsuit claims that Celgene fraudulently obtained multiple patents on it based on information that was already in the public domain, a fact it concealed from the U.S. Patent and Trademark Office while it was applying for the patents.

The company then used those patents to file "sham lawsuits" against generic drugmakers to prevent them from launching their own versions of Pomalyst, the lawsuit said.

Bristol Myers eventually reached settlements with several generic companies including Teva Pharmaceutical Industries , Aurobindo Pharma, Breckeridge Pharmaceutical and Natco Pharma under which they agreed delay the launch of generics until 2026.

Blue Cross Blue Shield of Louisiana said that while the exact terms of the agreements were "cloaked under an effort at absolute secrecy," the economic incentives for the companies suggest they must have been "large, unjustified" payments in exchange for not launching generics, which can be illegal.

The generic companies, which are also named as defendants in the lawsuit, did not immediately respond to requests for comment. (Reporting By Brendan Pierson in New York, Editing by Alexia Garamfalvi and Bill Berkrot)