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Three pharmaceutical companies on Tuesday added another patent to an infringement suit against Sandoz Inc., as they try to prevent a contingent of drugmakers from bringing generic versions of the gastrointestinal drug Linzess to market.

In an eight-page complaint, Allergan USA, Forest Laboratories and Ironwood Pharmaceuticals Inc. said Princeton, New Jersey-based Sandoz had infringed as many as 28 claims on the so-called '371 patent for the popular drug, which is used to treat irritable bowel syndrome with constipation and chronic idiopathic constipation.

The companies filed the suit in U.S. District Court for the District of Delaware, after learning that Sandoz had “recently amended” its abbreviated new drug application to include allegations that the patent was invalid and unenforceable, they said in the filing.

According to court filings, Allergan, a Delaware corporation based in New Jersey, is the exclusive distributor of Linzess in the United States. Ironwood owns the patent and Forest is the exclusive licensee for marketing and selling products containing linaclotide, the drug's active ingredient.

The suit seeks monetary relief and asks that production and sale of Sandoz's planned generic be delayed until at least 2033, when the patent is set to expire.

“Sandoz has participated in the preparation and/or filing of [the] ANDA seeking FDA approval to market and sell generic versions of plaintiffs' branded prescription drug product Linzess—and has plans to manufacture, distribute, market and/or sell the Sandoz generic products throughout the United States, including in this judicial district—before the '371 patent expires,” the companies said in the complaint.

The most recent lawsuit comes amid a broader campaign by Allergan, Forest and Ironwood to protect Linzess from generic competitors. In 2016, the companies sued Sandoz and fellow generic drugmakers Teva Pharmaceuticals USA Inc., Mylan Pharmaceuticals Inc. and Aurobindo Pharma of infringing up to 70 claims on nine patents for the drug.

The defendants said in the ANDAs that the patents were invalid and unenforceable and would not be impacted by the manufacture, use or sale of the generic products, according to court documents. That case remains pending before U.S. District Judge Richard G. Andrews of the District of Delaware.

A representative for Sandoz did not immediately return a call Tuesday afternoon seeking comment on the latest complaint, which was filed by Morris, Nichols, Arsht & Tunnell partners Jack B. Blumenfeld and Maryellen Noreika. It also lists attorneys from Quinn Emanuel Urquhart & Sullivan in New York and Honigman Miller Schwartz and Cohn in Chicago as representing the plaintiffs.

An online docket-tracking service did not list counsel for Sandoz; however, the company is represented in the other litigation by a team of attorneys from Phillips, Goldman, McLaughlin & Hall and Rakoczy Molino Mazzochi Siwik.

The case is captioned Forest Laboratories v. Sandoz.