In a precedential decision issued on November 18, 2025, the Court of Appeals for the Federal Circuit issued its decision in Duke Univ. v. Sandoz Inc. (No. 2024-1078), where it reinforced the standard for meeting the written description requirement for genus/species claims.

Background

Duke University and Allergan Sales, LLC (“Allergan”) together own all rights in U.S. Patent No. 9,579,270 ("the '270 patent") and certain related patents. The '270 patent is entitled, “Compositions and Methods for Treating Hair Loss Using Non-Naturally Occurring Prostaglandins,” and relates generally to treating hair loss using compositions containing prostaglandin F (“PGF”) analogs.

Allergan developed a cosmetic treatment for eyelash hypotrichosis (inadequate or thin lashes), which gained FDA approval in 2008.  Allergan marketed the product as Latisse®, with bimatoprost (a PGF analog) as the active ingredient.

Since late 2016, Sandoz Inc. (“Sandoz”) has manufactured and sold a generic version of Latisse®.  In 2018, Allergan sued Sandoz, alleging its generic drug product infringes Claim 30 of the ’270 patent.

Sandoz stipulated to infringement, but challenged validity.  After a trial, the jury rejected Sandoz's defenses, concluded that the Sandoz product infringed Claim 30 of the '270 patent, and awarded $39M in damages.  Sandoz appealed to the Court of Appeals for the Federal Circuit.

Written Description

Whether a patent provides adequate written description for a claim presents a question of fact:  specifically, whether the patent specification discloses that a person of ordinary skill in the art would conclude the inventor possessed "the full scope of the invention” at the time of their patent application.  See Carnegie Mellon Univ. v. Hoffmann-La Roche Inc., 541 F.3d 1115, 1124 (Fed. Cir. 2008) (“[T]o satisfy the written description requirement for a claimed genus, a specification must describe the claimed invention in such a way that a person of skill in the art would understand that the genus that is being claimed has been invented, not just a species of the genus.”).

The written description requirement reflects the notion that an inventor may obtain a patent only if she discloses her invention to the public in sufficient enough detail that a person of ordinary skill in the art would understand that the inventor actually “possessed the invention as claimed.” Regents of the Univ. of Minn. v. Gilead Scis., Inc., 61 F.4th 1350, 1355 (Fed. Cir. 2023).  As such, “[t]he purpose of the written description requirement is to prevent an applicant from later asserting that he invented that which he did not.” Id. (citations and internal quotation marks omitted).

Sandoz argued that Claim 30 covers the use of a specific subgenus of PGF analogs to grow hair, but that the specification fails to show a skilled artisan that the inventors were in possession of that full subgenus, including the specific embodiment at issue in this litigation, bimatoprost.  Sandoz argued that the specification does not provide a single example of an actual compound claimed by Claim 30 and also fails to identify sufficient commonalities of structure to provide a skilled artisan with the necessary “blaze marks” to lead them to the claimed compounds.

"Blaze Marks"

"Blaze marks" is a term from hiking, where there are painted, carved or otherwise affixed symbols on trees or rocks that guide hikers, indicating trail continuity, directions, turns, or junctions, with different colors marking different trails.  Sometimes the application of blaze marks on a trail is called "trail blazing" or "way marking."

In patent law, "blaze marks" are specific disclosures in a patent application that guide a skilled artisan to a particular invention, ensuring the original text adequately describes what's later claimed.

At trial, both sides' experts agreed that the specification is written in such a manner that the disclosed chemical structure could encompass billions of compounds.

In order to have adequate written description, the specification of the ’270 patent needs to allow a skilled artisan to understand how to identify this subgenus of claimed compounds.

The Federal Circuit has provided guidance regarding similar patents, holding, “Written description of an invention claimed as a genus of chemical compounds, as here . . . requires description not only of the outer limits of the genus but also of either a representative number of members of the genus or structural features common to the members of the genus, in either case with enough precision that a relevant artisan can visualize or recognize the members of the genus.”  Regents of the Univ. of Minn. v. Gilead Scis., Inc., 61 F.4th 1350, 1356 (Fed. Cir. 2023).

So, in this case, the specification had to provide sufficient indication as to how a skilled artisan would narrow the disclosed universe of billions of compounds described in the specification to the subset of just 1,620 to 4,230 compounds actually claimed.

The Arguments on Appeal and Federal Circuit’s Finding of Inadequate Written Description

The Federal Circuit found that “no reasonable juror could find anything other than clear and convincing evidence that the ’270 patent fails to describe either a representative number of species of claim 30’s subgenus or structural features common to all members of that subgenus."

Allergan did not argue that the patent satisfies the first of these alternatives, since the ’270 patent “does not expressly disclose even a single embodiment of claim 30."

Instead, Allergan argued that the specification of the ’270 patent satisfies the "common-structural-features" test because it discloses three features that are common to all members of the claimed subgenus.  However, the Federal Circuit held that no reasonable juror could agree with Allergan's argument.

The first feature to which Allergan pointed was the “hairpin” structure of the prostaglandin.  The Federal Circuit reasoned that the written description was inadequate because "the ’270 patent, at best, discloses two [generic hairpin] structures and a menu of available atoms, moieties, and functional groups from which a skilled artisan could populate the [multiple variable] positions of those structures."  The Federal Circuit cited its precedent as articulated in In re Ruschig, 379 F.2d 990, 995 (CCPA 1967) (finding inadequate written description where specification leaves skilled artisan needing “to select[] from the myriads of possibilities encompassed by the broad disclosure, with no guide indicating or directing that th[e] particular selection[s] should be made rather than any of the many others which could also be made”)."

Allergan failed to identify how the common structural feature was unique to the claimed subgenus, as opposed to the entire genus described in the specification.  Also, Allergan did not show that a skilled artisan would be able to “visualize” the thousands of compounds claimed in Claim 30, from among the billions of compounds described in the specification, based on the written description of the widely-shared common hairpin structure.

The second feature to which Allergan pointed was amides being disclosed at the C1 position of the structure.  However, the Federal Circuit concluded that a reasonable juror would necessarily have found that the specification fails to provide sufficient blaze marks with respect to the C1 position.  As the Federal Circuit reasoned, "In reality, the specification’s guidance with respect to the C1 position resembles a path with 13 branches, and most of those branches lead to additional branches, yielding in the end a vast number of options for C1."

The Federal Circuit likened this situation to its previous holding from Regents, 61 F.4th at 1357: “Following [such a] maze-like path, each step providing multiple alternative paths, is not a written description of what might have been described if each of the optional steps had been set forth as the only option.”  The Federal Circuit offered an alternative analogy from a previous case, Purdue Pharma L.P. v. Faulding Inc., 230 F.3d 1320, 1326 (Fed. Cir. 2000): “One cannot disclose a forest in the original application, and then later pick a tree out of the forest and say ‘here is my invention.’”

Instead, the Federal Circuit reasoned that satisfaction of the written description requirement with a patent like the ’270 patent requires disclosure of sufficient “blaze marks directing the skilled artisan to that tree.” Id.; see also Regents, 61 F.4th at 1356. In this case, the Federal Circuit found that the only blaze marks provided by the specification for the C1 selection point away from the combinations as recited in Claim 30.

In particular, certain embodiments described in the specification of the ’270 patent called out “preferred” or “more preferred” options for C1, but none of these included the option a skilled artisan would need to choose to reach the claimed invention. So, the “preferred” and “more preferred” blaze marks actually directed a skilled artisan away from, rather than toward, the claimed subgenus, which would again lead the artisan to conclude the inventors did not actually possess what they claimed.

Only if a skilled artisan initially chose a non-preferred option would other blaze marks point the skilled artisan toward the claim invention.  But the Federal Circuit reasoned that “Allergan has not identified any persuasive reason why an artisan who ignored the first set of blaze marks, effectively directing her away from [the claimed amide], would then follow the blaze marks given for [a particular] location of this non-preferred [amide] group.”

In trying to identify part of the written description that would assist an ordinarily skilled artisan at the C1 position, Allergan pointed to portions of the specification that teach methods for compound synthesis, including four examples of synthesizing compounds containing amides.

However, the Federal Circuit reasoned that only few of numerous options would steer a skilled artisan to the placement of an amide at C1 and, while some formulas in the specification called out amides as examples for C1, even these formulas did not indicate that amides should be preferred at C1.

Finally, Allergan pointed to the feature that the Z position of the chemical structure would require a phenyl group.  However, the Federal Circuit noted that “[w]hile Allergan contends that ‘the specification discloses only eight categories of options for Z, and . . . expresses a preference for phenyl,’ in fact the specification makes clear that (as with C1) each of these eight categories requires additional embedded choices[.]”

The Federal Circuit noted that, “While the specification does identify phenyl as the most preferred aromatic group, this guidance is only pertinent once the artisan selects an aromatic group from among the eight initially described options, which nothing in the specification directs such an artisan to do.”

The Federal Circuit also noted that, although some of the examples in the patent identified a phenyl group in the Z position, “[t]he specification lists 95 example compounds and gives no reason to prefer the ten examples containing phenyl. Ten is not even the greatest number of appearances of a compound at the [Z position.]”

As the Federal Circuit concluded, "The specification fails to provide the relevant artisan with sufficient blaze marks or structural commonalities among the claimed compounds to lead her to conclude that the inventor actually possessed the claimed invention."

Lessons Learned

This precedential Federal Circuit decision emphasizes the importance of providing sufficient description to meet the written description requirement for a chemical genus claim.

Patent drafters should recognize that a broad disclosure of a genus having numerous different substituents, without providing further examples or guidance for selecting a particular subset of substituents, may be challenged under the written description requirement of 35 U.S.C. § 112 as potentially insufficient to demonstrate possession of a subgenus contained within the disclosed broader genus.  In other words, broad genus disclosures rarely support later-asserted, narrowly tailored species claims unless the specification clearly demonstrates possession of those species at filing.

Additionally, it is critical to disclose and claim your preferred compound(s).  Include "blaze marks" in the specification, so that there are clear indicators that can guide a skilled artisan from a broad genus to a narrower subgenus or specific compound.

When preparing later applications, review the specification to ensure that disclosed embodiments cover the compounds you intend to protect. Prepare structures of varying scope to create clear blaze marks that lead from the genus to the species you may later assert.

To satisfy written description, a specification claiming a genus of chemical compounds must do more than define boundaries — it must either disclose a representative number of species or identify structural features common to the genus.

A specification that lists categories at multiple variable positions—without prioritizing the combinations that actually define the claimed genus or subgenus—risks failing the written description requirement.

A specification that describes preferences that point away from the claimed combination may weigh against possession of the claimed subgenus.

The decision is the latest in a line of Federal Circuit cases that tightens the written description standard for genus and subgenus claims in the chemical and life sciences.

In Biogen International GmbH v. Mylan Pharmaceuticals Inc., 18 F.4th 1333 (Fed. Cir. 2021), the court affirmed a lack of written description finding for method of treatment claims to a particular dosage, where the specification mentioned that dose only once within a broader dosage range and did not meaningfully single it out or support it with data.

In Regents of the University of Minnesota v. Gilead Sciences, Inc., 61 F.4th 1350 (Fed. Cir. 2023), the court held that earlier applications did not provide direct support or clear blaze marks to a later-claimed subgenus of certain prodrugs, rejecting a disclosure that required following a “maze-like path” through multiple dependent claims and lists of substituents.

In Seagen Inc. v. Daichii Sankyo Company, Ltd., No. 2023-2424, 2024-1176 (Fed. Cir. Dec. 2, 2025), the Federal Circuit found that, absent any clear blaze marks, there was insufficient written description for the claimed gly-phe-only tetrapeptide, where there was a broad discussion in the specification of tetrapeptides assembled from 39 different amino acids, amounting to more than 47 million distinct structures, while only 81 gly-phe-only tetrapeptides were encompassed by the claims).

Each of these cases provides helpful guidance when considering how to meet the written description requirement in patents involving genus and subgenus claims.