This is a very weird case interpreting “consisting of”. The Federal Circuit recently affirmed a ruling of non-infringement in Azurity Pharmaceuticals v. Alkem Laboratories. Azurity had sued Alkem for allegedly infringing patent claims related to a drinkable antibiotic.
The patent at issue—Azurity’s U.S. Patent No. 10,959,948—was a continuation of an earlier application that had been rejected due to prior art referencing the ingredient propylene glycol. To overcome that rejection, Azurity amended its claims to disclaim propylene glycol, arguing that its absence distinguished the invention from the prior art. Instead, Azurity used the more generic term “flavoring agent.”
How the Prosecution History Set the Trap
During prosecution, Azurity clearly and unmistakably argued that its claimed formulation excluded propylene glycol in order to overcome prior art. That argument worked—at least to secure allowance.
But prosecution disclaimer cuts both ways. Statements made to obtain a patent can later limit its scope. By distinguishing prior art based on the absence of propylene glycol, Azurity boxed itself in.
Why “Consisting Of” Left No Wiggle Room
In patent law, wording matters.
- “Comprising” is open-ended. It allows additional, unrecited elements.
- “Consisting of” is closed. It excludes anything not specifically listed.
Here, the claims used “consisting of,” which strictly limits the formulation to only the listed ingredients. When paired with the prosecution history disclaiming propylene glycol, the claim language reinforced a narrow construction.
The Stipulation That Didn’t Save the Claims
At trial, both parties stipulated that “flavoring agents” in the asserted claims could include substances with or without propylene glycol. Azurity argued that this meant a product containing a flavoring agent with propylene glycol would still infringe.
The district court disagreed. A litigation stipulation cannot erase a clear and unmistakable prosecution disclaimer. Even if “flavoring agent” could theoretically encompass propylene glycol, the intrinsic record controlled.
Federal Circuit Affirms Non-Infringement
The Federal Circuit agreed: Alkem’s product, which contained propylene glycol, fell outside the scope of claims that—through both language and prosecution history—excluded it.
The takeaway is simple but powerful. When you amend claims to overcome prior art—especially when using closed language like “consisting of”—you may be defining not just what your invention is, but what it can never cover.
For some other articles about patent claims: Patent Like You Mean It, But Also Show the Structure
Federal Circuit Clarifies Limits on Prosecution Disclaimer Across Patent Families