Means-Plus-Function Claims: A Middle-Ground Patent Strategy for Biologics After Amgen v. Sanofi

The 2023 U.S. Supreme Court decision in Amgen查看公司主页 v. Sanofi查看公司主页 sent shockwaves through the pharmaceutical patent landscape. In a unanimous ruling, the Court invalidated Amgen's broad functional genus claims covering PCSK9查看药品详情-blocking antibodies, holding that the company could not claim an entire antibody genus unless the patent disclosure contained enough technical information to enable a skilled scientist to make and use every antibody in the genus with reasonable experimentation. Justice Neil Gorsuch crystallized the practical impact: companies receive patents on what they actually did and described, not everything capable of performing the function of what they described.

In the wake of that ruling, patent attorneys have sought a middle ground between broad but vulnerable functional claiming and narrow but safe structural claiming. One once-overlooked strategy is gaining renewed attention: means-plus-function (MPF) claims.

What Are Means-Plus-Function Claims?

Under 35 U.S.C. § 112(f), a patent applicant can express a claim element by describing its function rather than its structure. The tradeoff is that the claim's scope is limited to the specific structures disclosed in the specification and their equivalents. The result is narrower literal coverage than a purely functional claim, but broader coverage than the specifically disclosed structures through statutorily included equivalents.

Consider a hypothetical monoclonal antibody, AB-101, that binds epitope X to treat a disease. A structural claim might recite AB-101's precise amino acid sequence. A broad functional genus claim might cover "a pharmaceutical formulation comprising an antibody that binds epitope X" — but after Amgen查看公司主页, that claim faces serious enablement challenges. An MPF claim takes a middle path: it claims "a formulation comprising a means for binding epitope X" but is construed to cover formulations comprising AB-101 and equivalent structures.

Two Tiers of Equivalents

MPF claims can potentially access two distinct types of equivalents. Statutory equivalents under § 112(f) are part of the literal claim scope and require that structures have been available at patent issuance; the accused device must perform the identical function, and the test is whether structural differences are "insubstantial." The doctrine of equivalents, by contrast, extends beyond literal claim scope, with equivalence assessed at the time of infringement — meaning it can capture after-arising technologies. Here, the accused device need only perform "substantially the same" function, and the test asks whether it performs substantially the same function in substantially the same way to achieve substantially the same result.

A well-drafted MPF claim can thus protect against both known equivalents through § 112(f) and future innovations through the doctrine of equivalents, providing broader coverage than either pathway alone.

The Chamberlain Decision: Strategic Clarity

The catalyst for renewed interest in MPF claims is the USPTO Appeals Review Panel's Ex parte Chamberlain decision in May 2024, the first decision by the newly established ARP, involving a Xencor查看公司主页 patent application. The decision underscored the strategic advantage of MPF limitations where genus claims are not practicable or are vulnerable under Amgen查看公司主页.

The ARP expressly held that disclosure of a single antibody that performs the claimed function was sufficient to support an MPF claim, and that disclosure of equivalent antibodies was not required — even though such antibodies would be within the literal scope of MPF claims. Based on a single structure, an MPF claim can occupy space between a narrow "picture" claim and a vulnerable genus claim, potentially helping protect against biosimilars with minor modifications.

The Federal Circuit's In re Xencor查看公司主页 decision in March 2025 affirmed the ARP's analysis of MPF limitations, though the claim at issue ultimately failed because the "treating a patient" preamble lacked adequate support. Critically, the MPF limitations themselves were accepted, highlighting that MPF claims are not subject to the same support requirements as broad functional genus claims.

Risks and Requirements

MPF claims carry specific limitations. Prior art can anticipate an MPF limitation if it performs the identical function and discloses an equivalent structure, even if the structures are not identical. The specification must both describe the corresponding structures and link them to the claimed function. Additionally, MPF claims must satisfy the statute's "combination" requirement — a claim reciting only one MPF element and no other structural elements is invalid, so MPF claims should include additional structural or functional elements beyond the MPF limitation itself.

Practical Portfolio Considerations

MPF claims are complementary to structural claims, not a replacement. A well-constructed portfolio might include narrow structural claims covering the lead molecule, broader structural claims covering related variants, and MPF claims that extend coverage beyond specific structures to their equivalents.

Specification quality is critical. For MPF claims, the corresponding structure must be clearly disclosed and linked to the recited function. Including descriptions of multiple structures capable of the recited function can expand the field of potential equivalents during enforcement. The function-way-result test used for equivalence analysis asks whether structures "perform the same function in substantially the same way to achieve substantially the same result" — a precisely defined function can help capture equivalents while distinguishing prior art.

The Broader Genus-Level Landscape

A separate Federal Circuit ruling has provided additional clarity on genus-level protection in biologics. The court drew a critical distinction: a claim to a method of using a well-known class of compounds for a specific therapeutic purpose is evaluated differently than a claim to the compounds themselves. Where the underlying genus is well known in the art and is not itself the invention, written description and enablement are assessed in the context of the claimed method. When the specification establishes that all members of the genus work for the claimed purpose, and the genus itself is not novel, the disclosure requirements are substantially reduced.

Notably, the prevailing party in that case was not a traditional brand innovator but a company known primarily for generic drug manufacturing — demonstrating that any organization investing in the underlying science and prosecuting claims with precision can establish and defend robust intellectual property.

In a post-Amgen查看公司主页 world where broad functional genus claims face intense scrutiny, means-plus-function claiming offers a valuable middle path — particularly for biologics where structural complexity and unpredictability make broad claiming difficult. For pharmaceutical executives evaluating patent strategy, the question is whether MPF claims belong in their portfolio.