Brimonidine Alone: How Patent Applicants Can Shape the Scope of Their Claims 

The Federal Circuit recently reversed a Patent Trial and Appeal Board (PTAB) decision finding unpatentable the claims of a patent directed to an eye therapy using the drug brimonidine. Eye Therapies, LLC v. Slayback Pharma, LLC, No. 23-2173 (Fed. Cir. June 30, 2025). The case turned on the deceptively simple transition phrase “consisting essentially of.” How that phrase was used and understood during patent prosecution ultimately controlled the outcome of the case.

Background 

Eye Therapies owns U.S. Patent No. 8,293,742, which describes a method of reducing eye redness using low doses of brimonidine, a drug that narrows blood vessels in the eye. During prosecution, the applicant amended the claim language from “comprising” (which is open-ended) to “consisting essentially of” (which limits what additional elements can be present). The applicant did this to avoid a prior patent that described administering brimonidine with another active ingredient, brinzolamide. The applicant then explained to the patent examiner that its method stood apart because the method did not include “any other active ingredients” other than brimonidine. The examiner agreed and granted the patent. 

Slayback challenged the patent in an inter partes review. In its final decision, PTAB held that the phrase “consisting essentially of” did not foreclose the inclusion of additional active ingredients beyond brimonidine that do not materially affect the basic and novel characteristics of the invention. Using that interpretation, the Board found that prior art references involving brimonidine combined with other agents rendered the patent obvious.  

Decision 

The Federal Circuit reversed. It held that the applicant had narrowed the meaning of “consisting essentially of” through prosecution to exclude all other active ingredients, not merely those that altered the invention’s basic and novel characteristics. The court emphasized that the applicant defined the method during prosecution as “i.e. methods which do not include administering other active agents.”  The use of “i.e.” signaled that the statement was not just a passing comment, but a definition. The court gave the applicant’s argument during examination substantial weight in interpreting the claim language even though there was no prosecution history estoppel. The court also explained that the presence of examples with multiple active ingredients did not negate the narrower construction, given the clear prosecution history statements. “The applicant secured allowance of the amended claims by arguing that the claimed methods were novel because they ‘do not require the use of any other active ingredients.’”  

Because the PTAB’s obviousness analysis relied on a broader construction, the court vacated that decision and remanded the case. On remand, the PTAB must evaluate the prior art with the understanding that the claims are limited to brimonidine alone as the active ingredient. 

Takeaways 

Eye Therapies highlights the importance of prosecution history to claim construction, including claim terms that have well-established meanings in patent law. That “consisting essentially of” has a known meaning in patent claims — one that typically allows for additional components — mattered little to the court. Indeed, in view of clear prosecution statements, the court gave very little, if any, weight to the accepted understanding of the term. The applicant’s statements during prosecution, where it distinguished the prior art and explained its understanding as limited to products with a single active ingredient, controlled.  

This decision does not, however, change the generally accepted meaning of “consisting essentially of.” The court reiterated the typical understanding of that transition phrase: Such claims are typically limited to specified material and those that do not materially affect the basic and novel characteristics of the invention. But the opinion suggests that the presumption of that meaning is a relatively weak one that can be readily overcome by inconsistent statements made during prosecution.