Merck (MRK) is facing a patent dispute that threatens its plans to sell a new version of Keytruda, its top-selling cancer drug, the Wall Street Journal reported Wednesday.

The report said the new version, which allows for injection instead of intravenous use, is at the center of the conflict with Halozyme Therapeutics (HALO), which claims Merck infringes on its patents for the enzyme Mdase.

Merck has petitioned US patent regulators to reconsider the patents, arguing they are too broad, according to the Wall Street Journal report.

Halozyme insists Merck must sign a licensing agreement to sell the new version, the sources told the Journal. While Merck maintains it developed the enzyme independently.

A Merck spokesman told the Journal the enzyme the company used was developed independently from Halozyme.

Merck has been working to modify Keytruda to make it easier to use and protect future revenue as its US patent protection expires in 2028, allowing competitors to sell copycat versions.

Merck, Halozyme did not immediately respond to MT Newswires' request for comment.

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