Blaze Marks and Written Description: Patent Drafting Lessons from Duke v. Sandoz

On November 18, 2025, the U.S. Court of Appeals for the Federal Circuit (“Federal Circuit”) issued a precedential decision in Duke University, Allergan Sales, LLC v. Sandoz Inc., holding claim 30 of U.S. Patent No. 9,579,270 (“the ’270 patent”) invalid for lack of written description. The ’270 patent describes the use of prostaglandin F (PGF) analogs characterized by multiple variable positions on a common backbone for treating hair loss. Focusing on the claimed subgenus of PGF analogs, the Federal Circuit concluded that the specification did not provide sufficient “blaze marks” or common structural features to show that the inventors were in possession of the claimed invention.

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