Background
Amgen Inc. owns a patent covering methods of purifying proteins using hydrophobic interaction chromatography (HIC) — a technique used in pharmaceutical manufacturing to separate proteins from impurities. The patent claims specific salt combinations that facilitate the purification process. Coherus BioSciences was developing a biosimilar product and used a purification process involving a salt combination not expressly listed among the combinations claimed in Amgen’s patent.
Amgen sued Coherus for infringement, but because Coherus’s salt combination did not literally match any claimed combination, Amgen relied on the doctrine of equivalents — a legal theory that allows a patent to cover products or processes that are not literally within the claim language but are insubstantially different from what is claimed. Coherus moved to dismiss, arguing that Amgen was barred by prosecution history estoppel from asserting the doctrine of equivalents.
During prosecution, Amgen had argued to the patent examiner that its claimed salt combinations distinguished the invention from a prior art reference. Coherus contended that this argument surrendered all other salt combinations, including Coherus’s, from the scope of Amgen’s patent.
The Court’s Holding
The Federal Circuit affirmed the district court’s dismissal. The court explained that prosecution history estoppel arises in two ways: through narrowing amendments to claims (amendment-based estoppel), or through arguments made to the examiner that distinguish the invention from the prior art (argument-based estoppel). When a patent applicant clearly and unmistakably surrenders claim scope through an argument, the patentee cannot later recapture that scope through the doctrine of equivalents.
A key question was whether estoppel attached to Amgen’s prosecution argument even though that argument was not the sole reason — or even the required reason — for the examiner’s decision to allow the claims. The Federal Circuit held that it did. The court stated that “separate arguments can create separate estoppels” and that each distinct argument made during prosecution can independently surrender claim scope. The fact that Amgen raised other arguments, or that the examiner may have been persuaded by a combination of grounds, did not diminish the estoppel created by the salt combination argument.
Key Takeaways
- Each distinct argument made to a patent examiner during prosecution can independently create a prosecution history estoppel, even if that argument was not strictly necessary to secure allowance of the claims.
- Patent applicants should be careful about making broad distinctions from prior art during prosecution, because those arguments may limit the scope of the patent’s protection under the doctrine of equivalents after issuance.
- The doctrine of equivalents is a powerful infringement theory, but prosecution history estoppel is an equally powerful bar — careful prosecution strategy should account for the trade-offs between securing the claim and preserving room for equivalents arguments in litigation.
- Generic and biosimilar manufacturers can leverage prosecution arguments as a defense even when the patentee used multiple, alternative arguments during examination.
Why It Matters
Amgen v. Coherus is an important reminder that patent prosecution is not just about getting claims allowed — it shapes the boundaries of the patent for its entire life. Every argument made to distinguish prior art potentially surrenders some scope under the doctrine of equivalents. Patentees who pile on multiple arguments to overcome rejections may inadvertently surrender more claim scope than necessary.
In the biopharmaceutical industry, where manufacturing methods are often patented to protect biologics from biosimilar competition, this decision is particularly significant. Biosimilar manufacturers can scrutinize prosecution histories for argument-based estoppels even when the biosimilar’s process differs from the claims in a minor way. The decision underscores the importance of thoughtful prosecution strategy and the value of working with litigation counsel when drafting prosecution arguments — because a court’s interpretation of those arguments, years later, may determine whether the doctrine of equivalents is even available as an infringement theory.