CAFC Affirms Moderna’s Win, Holding Alnylam Narrowly Defined ‘Branched Alkyl’

In a precedential decision issued June 4, 2025, the U.S. Court of Appeals for the Federal Circuit (CAFC) affirmed a district court’s finding of noninfringement in favor of Moderna, Inc. in its ongoing patent battle with Alnylam Pharmaceuticals over mRNA-based COVID-19 vaccine technology. The court held that Alnylam had acted as its own lexicographer in defining the term “branched alkyl” in its patent specifications and that definition foreclosed Alnylam’s infringement allegations against Moderna’s SPIKEVAX® vaccine.

The Road to the Federal Circuit

At the center of the case was whether Alnylam’s patents, U.S. Patent Nos. 11,246,933 and 11,382,979, were infringed by Moderna’s use of SM-102-a cationic lipid used in Moderna’s COVID-19 vaccine formulations. Alnylam asserted that SM-102 fell within the scope of its claims, specifically limitations requiring a “branched alkyl” group in the hydrophobic tail of a cationic lipid. SM-102 enables intracellular delivery of mRNA in Moderna’s vaccine, making it central to Alnylam’s infringement theory.

Alnylam initiated two suits in the District of Delaware in 2022, one shortly after each patent issued. The suits were consolidated, and the case proceeded through claim construction. At the Markman hearing, Moderna argued that Alnylam’s definition of “branched alkyl” required a tertiary carbon at the alpha position, and the court agreed. That ruling proved dispositive once the parties stipulated to noninfringement under that construction, leading the district court to enter final judgment in Moderna’s favor.

The district court had ruled that Alnylam’s specification contained a clear definition of “branched alkyl”: a carbon atom in the group must be bound to at least three other carbon atoms (i.e., a tertiary or quaternary carbon). Under that construction, the parties agreed that SM-102 did not infringe the asserted claims. Alnylam appealed, arguing that the definition should not apply universally or that the claims at issue constituted an exception.

The Federal Circuit disagreed, holding that Alnylam had unambiguously defined the term in its specification under a heading labeled “Definitions.” The panel noted the use of quotation marks, definitional language (“refer to”), and a qualifying phrase (“unless otherwise specified”), all of which signaled a lexicographic intent. Finding no “otherwise specified” language in the asserted claims or the rest of the intrinsic record to override the stated definition, the CAFC affirmed the judgment of noninfringement.

Lexicography Controls: No Room for Implication

Writing for the unanimous panel, Judge Taranto walked through five key features of the definition that supported a finding of lexicography:

1. Heading “Definitions” in the specification;
2. Quotation marks around the term “branched alkyl”;
3. Use of the phrase “refer to”, which courts often treat as definitional;
4. Contrast with other terms in the section that used non-limiting language like “e.g.” or “include”; and
5. The clause “Unless otherwise specified,” imposes a strict standard: any departure must be clearly and unambiguously specified elsewhere in the intrinsic record.

Importantly, the panel explained that the phrase ‘unless otherwise specified’ does not invite ambiguity or speculation—it sets a high bar requiring an explicit, unambiguous exception elsewhere in the intrinsic record. The court further emphasized that once a patentee includes an express definition, it governs the claim term unless a clear exception is explicitly provided. Alnylam argued that its disclosed embodiments and prosecution history suggested broader claim scope. The panel rejected those arguments, noting that none of the embodiments cited by Alnylam were unambiguously covered by the asserted claims or clearly labeled as exceptions to the definition.

The CAFC also addressed dependent claim 14 of the ‘933 patent, which Alnylam argued implied a broader interpretation of the independent claim. But the panel found that this dependent claim, which required “only one carbon atom” bound to three others, was still narrower than the independent claim, which could cover lipids with multiple tertiary carbons.

On the prosecution history front, Alnylam pointed to a diagram and examiner argument in which it had identified a secondary carbon as part of a branched alkyl. While acknowledging the evidence as “some support,” the panel found it insufficient to override the express definition in the specification, particularly given the lack of clarity or specificity in those arguments.

This case underscores the power–and potential peril–of lexicography in patent drafting. The Federal Circuit reaffirmed that patentees may define claim terms, but once patentees provide an express definition, it governs the meaning of the term throughout the patent unless a clear and specific exception is stated. “Unless otherwise specified” is not a license for ambiguity; rather, any deviation from a stated definition must itself be clearly specified.