In 2023, the U.S. Supreme Court invalidated a pharma company’s broad functional patent claims and set important precedent in the process.

The unanimous decision in Amgen v. Sanofi is a critical reminder for life sciences innovators that broad patent claims must be backed by enough concrete examples and instructions to let others make and use the full range of what is claimed. Companies cannot claim an entire class of products based on only a few examples.

In the wake of that ruling, many patent attorneys like me sought to find a middle ground between broad but vulnerable functional claiming and narrow but safe structural claiming. As someone who has spent years navigating these waters with biotech and pharma clients, I’ve been considering a once-overlooked patent claiming strategy. It’s called means-plus-function claiming, and recent decisions from the USPTO and the Federal Circuit suggest it deserves a fresh look.

This article will explain what means-plus-function (MPF) claims are, why they’re gaining renewed attention in pharma and biotech, and how they can fit into a comprehensive patent strategy. The catalyst for this renewed interest is the USPTO Appeals Review Panel's Ex parte Chamberlain decision and the Federal Circuit decision on appeal, which show that these claims have strategic value, particularly for biologic innovations where describing every structural variant is difficult.

The post-amgen landscape: Vulnerability of functional claiming

If you work in pharmaceutical IP, you’ve likely felt the impact of the Amgen v. Sanofi decision, in which the Supreme Court unanimously held that Amgen’s broad functional genus claims covering PCSK9-blocking antibodies were invalid for lack of enablement. Justice Neil Gorsuch wrote that while Amgen could patent the specific antibodies it discovered, it couldn’t claim an entire antibody genus “unless the patent disclosure contained enough technical information to enable a scientist skilled in the field to make and use every antibody in the genus with reasonable experimentation.”

The practical impact has been significant: you get a patent on what you actually did and described, not everything that is capable of performing the function of what you described.

Means-plus-function claims: The basics

So what exactly 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.” You get narrower literal coverage than a purely functional claim, but broader coverage than the specifically disclosed structures through statutorily included equivalents of those structures.

Here’s an example. Suppose your company develops a formulation of a monoclonal antibody, AB-101, that binds epitope X resulting in the treatment of 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, 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.

One caveat: statutory equivalents under § 112(f) must have existed at the time of patent issuance. The separate doctrine of equivalents evaluates equivalency at the time of infringement. MPF claims can potentially take advantage of both types of equivalents and capture structures known when the patent issued as well as after-arising structures. In rapidly evolving fields like biologics, that dual coverage can be valuable.

The USPTO’s Chamberlain decision: Clarity for MPF claim elements

What really caught my attention was Ex parte Chamberlain, the first decision by the newly established USPTO Appeals Review Panel (ARP) in May of 2024, which involves a Xencor patent application.

The decision underscored the strategic advantage of MPF limitations where genus claims are not practicable or are vulnerable under Amgen. The ARP, in holding that disclosure of a single antibody that performs the claimed function was sufficient to support an MPF claim, expressly held 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 risks you should understand

Like any patent strategy, MPF claims have limitations and requirements.

Prior art can anticipate an MPF limitation if it performs the identical function and discloses an equivalent structure, even if the structures aren’t identical.

The specification must both describe the corresponding structures and link them to the claimed function.

MPF claims also 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.

Written description and enablement: The good news

Here’s the strength of MPF claims. The Chamberlain ARP determined that disclosure of a single structure capable of performing the MPF function is sufficient to support an MPF claim element. The specification need not disclose equivalents of the corresponding structure to satisfy definiteness or written description requirements.

Unlike the functional claims invalidated in Amgen, with an MPF claim, you’re not trying to claim every possible structure that performs a function. You’re claiming structures you’ve actually disclosed plus their equivalents that existed at the time of patent issuance. That’s a much more defensible position.

The Federal Circuit’s In re Xencor decision in March 2025 affirmed the ARP’s analysis of the MPF limitations, though the claim at issue ultimately failed because the “treating a patient” preamble lacked adequate support. The MPF limitations were accepted, highlighting that MPF claims aren’t subject to the same support requirements as broad functional genus claims.

Practical considerations for your portfolio

So how should you think about MPF claims in practice? First, MPF claims are complementary to structural claims, not a replacement. A well-constructed portfolio might include narrow structural claims covering your lead molecule, broader structural claims covering related variants, and MPF claims that extend coverage beyond the specific structures to their equivalents.

Second, invest in your specification. For MPF claims, you need to clearly disclose the corresponding structure and link it to the recited function. Consider including descriptions of multiple structures capable of the recited function to expand the field of potential equivalents during enforcement.

Third, think carefully about the function you recite. 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 you capture equivalents while distinguishing prior art.

Looking ahead

Means-plus-function claiming isn’t a silver bullet. But in a post-Amgen world where broad functional genus claims face intense scrutiny, it offers a valuable middle path, particularly for biologics where structural complexity and unpredictability make broad claiming difficult. For pharmaceutical executives evaluating patent strategy, now is the time to ask whether MPF claims belong in your portfolio.