Patentees and practitioners, if relying on lexicography and deviation from the ordinary meaning of a term, should remain aware that any new definition of a term ultimately controls.

One of the many tenets of United States patent law is that a patentee may choose to be his or her own lexicographer and define terms that deviate from their ordinary meaning. This gives the patent practitioner a “poetic license” to use and define terms and the flexibility to craft the underlying patent as they see fit. However, as recently enunciated in Alnylam Pharmaceuticals, Inc. v. Moderna, Inc., No. 2023-2357, 2025 WL 1572001 (Fed. Cir. June 4, 2025), defining terms as used in the specification to deviate from their ordinary meaning can have unintended consequences and unduly narrow the claim scope of the granted patent.

Background and Case History

Alnylam asserted its U.S. Patent Nos. 11,246,933 and 11,382,979, directed to cationic lipids that protect nucleic acids from nuclease degradation during the delivery of the nucleic acids into a cell (e.g., via mRNA vaccines), against Moderna for Moderna’s commercialization of its cationic lipid, SM-102. An exemplary structure of a cationic lipid is depicted below:

.

Id. at *2.

In the above structure, a set of groups R¹², M¹ and R¹³ defines a hydrophobic tail. Of note is the M¹ biodegradable group and the immediately adjacent α-carbon of the R¹³ group. The asserted claims recite, inter alia, that at least one hydrophobic tail is a “branched alkyl” in which the branching occurs at the α-position carbon with respect to group M¹ and that group R¹³ is a “branched C₁₀-C₂₀ alkyl.” At issue was whether the α-carbon of group and its degree of substitution of Moderna’s allegedly infringing product was encompassed by the term “branched alkyl” as asserted.

During claim construction, Alnylam argued for the plain and ordinary meaning of the term “branched alkyl”: a nonstraight chain alkyl group. Under a plain and ordinary meaning construction, the α-position carbon in which the branching occurs to form the branched alkyl would be a secondary, tertiary and quaternary carbon atom, thus having one, two or three other carbon atoms[1] bound to the α-carbon:

Id. Moderna, in contrast, sought a narrower construction based on the specific definition for “branched alkyl” in the specification: “alkyl … 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.” U.S. Patent No. 11,246,933 col. 412 l. 15-17; see Alnylam Pharm., 2025 WL 1572001, at *3.

The United States District Court for the District of Delaware agreed with Moderna’s narrower  proposed construction, holding that the definition of “branched alkyl” in the specification as “[u]nless otherwise specified, the terms ‘branched alkyl,’ ‘branched alkenyl,’ and ‘branched alkynyl’ refer to an alkyl, alkenyl, or alkynyl 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” is the inventor’s lexicon and, therefore, any departure from such lexicon must be clear and unmistakable. U.S. Patent No. 11,246,933 col. 412 l. 15-17; see Alnylam Pharm., 2025 WL 1572001, at *3 (emphasis added). Following claim construction, the parties stipulated to noninfringement and sought entry of a final judgment. The district court entered the final judgment of noninfringement and Alnylam appealed the district court’s construction of “branched alkyl” to the United States Court of Appeals for the Federal Circuit.

The Federal Circuit’s Decision

On appeal, Alnylam argued that the intrinsic record adequately demonstrated that Alnylam did not intend to limit the scope of the term “branched alkyl” and that secondary carbons are covered and fall under the “[u]nless otherwise specified” category of the definition.

The court disagreed, holding that the language regarding “branched alkyl” was definitional for a number of reasons, including that the term fell under the heading “Definitions,” was offset by quotation marks suggesting what followed was a definition, and that the use of “[u]nless otherwise specified” laid out a general rule to be applied.

Further noting the use of the phrase “[u]nless otherwise specified” in the definition of “branched alkyl” and the connotations of particularity and preciseness from the ordinary meaning of “specified,” the court agreed with the district court that there must be some form of clarity to support a deviation from a definition established via lexicography and that there must be a similarly high threshold to deviate from a definition as there was to establish said definition. Id. (“[O]nce the high threshold for lexicography is met in a patent, it makes sense that a high threshold would have to be met before finding a departure from that controlling definition.”) Public policy also dictates such a high bar that must be met to deviate from a definition established using lexicography as the public is entitled to notice as to what falls under and what is excepted from the definition so it can ascertained what constitutes infringement.

The court further noted that a carbon atom within a linear alkyl chain (i.e., a nonterminal carbon atom) is already a secondary carbon because it is bound to two other carbon atoms within the alkyl such that any branching requires at least a tertiary or quaternary carbon center. Alnylam thus adopted the position at oral argument that branching could only occur without the carbon center being bonded to three other carbon atoms when the carbon center is in the α-carbon position. The court proceeded to review the specifications of the asserted patents and prosecution history thereof to ascertain whether there was sufficient clarity to expand the scope of the term “branched alkyl” as used in the asserted claims to encompass a secondary carbon at the α-position relative to the biodegradable group M¹.

Alnylam argued that there was intrinsic evidence to support that a branched alkyl at the α-carbon position could involve a secondary carbon atom. First, Alnylam identified an unasserted dependent claim that specifically recited one carbon atom having bonds to three other carbon atoms to be narrower than the asserted independent claim from which it depends, such that the asserted independent claim thus encompassed a carbon atom having bonds to two other carbon atoms. The court did not agree, however, holding that the unasserted dependent claim recited one carbon atom having the bonds to three other carbon atoms—whereas the asserted independent claim did not limit the number of carbon atoms having bonds to three carbon atoms.

Alnylam further referred as intrinsic evidence to the following Formula (II) in the specification (dashed box annotation supplied):

Here, the carbon center shown within the dashed box that is bonded to hydrogen (H), R^Z and Z¹ satisfies the bonding to two other carbon atoms when R^Z and Z¹ are alkyl groups and M¹ does not further supply a carbon-carbon bond. Id. at *7. The court noted that the asserted claims did not claim Formula (II). The court further noted that the skilled person in the art would only find that Formula (II) refers to specific embodiments that include a secondary carbon center depending on the choices for R^Z and Z¹, thus failing to “specify” any exception to the definition established via lexicography.

Alnylam also referred to specifically defined embodiments in the specification as well as reference to specifically defined branched alkyl groups. Despite having a secondary carbon atom at the α-carbon position, the specifically defined embodiments fall outside the scope of the asserted claims because these embodiments include a carbon-carbon double bond that is outside the definition of of a branched alkyl, instead being alkenyl. The specifically defined branched alkyl groups, while providing examples of branching involving α-secondary carbon atoms, failed to meet the claimed C₁₀-C₂₀ carbon atom range for a branched alkyl.

In reviewing the prosecution history of the great-grandparent application, the court noted that Alnylam distinguished from prior art compounds, which did not include branching and included an example of a secondary α-carbon bonded to two other carbon atoms. However, in view of the explicit definition provided in the specification and the context of the exemplified embodiments that merely show possible embodiments of a branching species, the court declined to hold that Alnylam had established an exception to its definition of “branched alkyl” with sufficient specificity.

Based on the foregoing, the court affirmed the district court’s narrower construction of “branched alkyl” as being limited to tertiary and quaternary carbon centers as specifically defined in the specification of the asserted patents.

Conclusion and Practice Points

While permitting patentees (and the practitioners assisting with preparing the underlying patent application) to be their own lexicographer affords the flexibility and freedom to define terms used in the application as they see fit to facilitate the application drafting process, patentees and practitioners must be aware that there can be perils to such practice and, as demonstrated in Alnylam Pharmaceuticals, the use of lexicography can unduly narrow the scope of the claimed subject matter.

Patentees and practitioners, if relying on lexicography and deviation from the ordinary meaning of a term, should remain aware that any new definition of a term ultimately controls. The use of the phrase “unless otherwise specified” is not meant to be used as a backstop or a catchall phrase to suddenly and opportunistically change the scope of the underlying definition to fit the desired purpose. It should be made clear that for exceptions to the newly established definition to be “specified,” such exceptions need to be much more explicitly and precisely described such that the metes and bounds of the provided definition are clearly defined to ensure that the public has adequate notice as to what is intended to be covered by the claims of a patent.

For More Information

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Notes

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