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Federal Circuit Affirms District Court’s Claim Construction Of "Branched Alkyl" In mRNA Vaccine Patent Dispute
06/17/2025On June 4, 2025, the United States Court of Appeals for the Federal Circuit (Judges Taranto, Chen, and Hughes) affirmed the District of Delaware’s claim construction in a patent infringement dispute involving lipid components used in Moderna’s (Moderna, Inc., ModernaTX, Inc., and Moderna US, Inc.) mRNA-based COVID-19 vaccine. The case centered on the interpretation of the term “branched alkyl” as recited in U.S. Patent Nos. 11,246,933 and 11,382,979, both owned by Alnylam Pharmaceuticals, Inc.
The asserted patents relate to biodegradable lipids for delivering nucleic acids, such as the lipids used in lipid nanoparticles used to deliver mRNA vaccines like those made by Moderna. The patents describe lipids comprising three parts: a head group, a linker (referred to in the patent as a “central moiety”), and hydrophobic tails, with the claims at issue requiring at least one hydrophobic tail to include a “branched alkyl” at the “alpha position” relative to a biodegradable group, i.e., the carbon immediately adjacent to the biodegradable group.
Alnylam alleged that Moderna’s COVID-19 vaccine, which contains the cationic lipid SM-102, infringed the asserted claims. The dispute turned on whether the “branched alkyl” limitation encompassed structures where the alpha-position carbon can be a secondary carbon (bound to two other carbons) or was limited to tertiary or quaternary carbons (bound to at least three other carbons).
The district court found that the patent specification provided an explicit definition of “branched alkyl” in its “Definitions” section: “Unless otherwise specified, the term[] ‘branched alkyl’ … refer[s] to an alkyl … group in which one carbon atom in the group (1) is bound to at least three other carbon atoms and (2) is not a ring atom of a cyclic group.” The court concluded that this language constituted lexicography—meaning the patentee provided its own definition for a term in the patent. In the context of patent law, when a patentee clearly sets forth a special definition for a term in the specification, that definition governs the interpretation of the term in the claims, even if it differs from the ordinary or customary meaning. Courts will enforce such definitions, and a high threshold must be met to depart from explicit lexicography. In order to deviate from the lexicographical definition provided there must be a clear and unmistakable exception to the definition specified in the patent. Following these precepts, the district court found that no exception was “otherwise specified” for the asserted claims and concluded that the patentee’s definition of “branched alkyl” should be adopted. Under this construction, the parties stipulated that Moderna’s product did not infringe, and final judgment was entered for Moderna.
On appeal, Alnylam argued that the district court erred in finding that the patentee had defined the term as determined by the District Court, contending that the intrinsic record did not show an intent to limit the “branched alkyl” terms to carbon atoms bound to at least three other carbons. Alnylam asserted that the specification’s language was not sufficiently clear to constitute lexicography and that the definition provided in the patent specification should not override the ordinary meaning of “branched alkyl.” Alnylam further argued that the district court’s construction excluded disclosed embodiments from the scope of the claims, pointing to examples in the specification and the definitions section that, in Alnylam’s view, encompassed secondary carbons at the alpha position. Alnylam also cited portions of the prosecution history that, it claimed, supported a broader interpretation of “branched alkyl.” Alternatively, Alnylam maintained that even if the specification’s definition was deemed lexicographic, the “unless otherwise specified” clause permitted the asserted claims to cover secondary carbons at the alpha position, based on the context of the claims and the disclosed embodiments.
The Federal Circuit rejected Alnylam’s arguments and affirmed the district court’s claim construction. The court explained that a patentee acts as its own lexicographer when the intrinsic evidence—such as the patent specification—clearly sets forth or clearly redefines a claim term to put one reasonably skilled in the art on notice that the patentee intended to adopt a special definition. Here, the Federal Circuit found that the specification’s definition of “branched alkyl” was clear. The court pointed to several factors supporting lexicography: the definition appeared in the “Definitions” section, the term was set off in quotation marks, and the phrase “refer to” was used, all of which signaled an intent to define the term for the purposes of the patent. The court emphasized that, once lexicography is established, a high threshold must be met to depart from the definition—there must be a clear exception that is specifically stated in the claims, specification, or prosecution history.
The Federal Circuit rejected Alnylam’s contention that the definition was not sufficiently clear or that the “unless otherwise specified” clause allowed for a broader interpretation. The court held that the asserted claims, specification, and prosecution history did not “otherwise specify” an exception to the definition, and that the disclosed embodiments cited by Alnylam either fell outside the scope of the asserted claims or did not address the degree of branching at the alpha position. The court also found that the structure of the dependent claims did not require a broader construction, and that the prosecution history, even if it suggested some ambiguity, was not enough to override the express definition in the specification. Ultimately, the Federal Circuit concluded that patentees are bound by their own definitions unless a clear exception is provided, and that courts will enforce such definitions strictly in claim construction.
Beyond its doctrinal analysis, the Federal Circuit grounded its decision in core patent-law policies of public notice and claim clarity. The court stressed that expressly defined claim terms provide the public with predictable boundaries, foster reliance by competitors, and reflect the patentee’s unique control over its own drafting. Once a patentee has chosen to act as a lexicographer, treating that definition as authoritative serves both fairness and administrative efficiency: competitors may rely on the stated meaning, examiners can evaluate patentability with precision, and courts are spared the uncertainty that often attends “ordinary meaning” disputes. Requiring any departure from an express definition to be stated with equal specificity thus preserves the balance between the patentee’s drafting autonomy and the public’s right to clear notice of what the patent does—and does not—cover.
This decision underscores the importance of clear definitions in patent specifications and the high threshold required to depart from explicit lexicography. The Federal Circuit’s ruling affirms that, absent a clear and unmistakable exception, patentees are bound by their own definitions, and courts will enforce such definitions strictly in claim construction. The outcome also highlights the significance of claim drafting and the potential impact of definitional language on infringement analyses, especially in the context of complex chemical and pharmaceutical patents.
