Making sense of the doctrine of equivalents in pharmaceutical patent litigation

This is an Insight article, written by a selected partner as part of IAM's co-published content. Read more on Insight

Companies should be mindful during prosecution of patent applications to avoid inadvertently surrendering claim scope and barring themselves from later relying on the doctrine of equivalents. Aaron Barkoff and Ben Mahon of McAndrews Held & Malloy explain why in this co-published article

Going beyond the literal scope of claims

It is axiomatic that the claims of a patent define the scope of the right to exclude. In theory, infringement can be avoided by steering clear of the literal scope of the claims – in other words, by designing around the patent. But for more than 150 years (dating back to at least the 1853 Supreme Court case Winans v Denmead), courts have found infringement even in cases where the accused party has not literally infringed the claims as written. The doctrine of equivalents arose because courts recognised the inherent limitations of language and the problem that strict ‘literalism’ could allow copyists to unfairly exploit loopholes in claims. At the same time, the Supreme Court has acknowledged the tension between the doctrine of equivalents and the public notice function of claims, stating that “the doctrine of equivalents, when applied broadly, conflicts with the definitional and public-notice functions of the statutory claiming requirement”.

The line between an insignificant change to a product and a successful design-around is blurred and leads to uncertainty for patentees and competitors. Further complicating the matter are several judicially created limitations on the doctrine of equivalents, most notably prosecution history estoppel and a complex series of exceptions to estoppel. Understanding the scope of the doctrine, its limitations and its nuances is essential to protecting patent rights and avoiding infringement. While true in all fields, this is especially the case for pharmaceuticals, where generic pharmaceutical companies must establish bioequivalence while avoiding patent infringement.

Pharmaceutical companies therefore need sharp patent counsel both to protect their own inventions and to avoid infringement. Sophisticated companies rely on counsel to consider the doctrine of equivalents and its limitations at all phases of product development. Companies should be mindful during prosecution of patent applications to avoid inadvertently surrendering claim scope and barring themselves from later relying on the doctrine of equivalents. Conversely, companies looking to design around competitors’ patents must study the specification and prosecution history carefully to determine where they can safely practice without risking infringement. And, of course, companies require litigation counsel who can navigate this complex area of patent law.

Doctrine of equivalents

To prove infringement, a patentee must show that the accused product meets every limitation of a claim, either literally or under the doctrine of equivalents. Therefore, infringement is evaluated on a limitation-by-limitation basis, not by comparing the accused product ‘as a whole’ to the claims. Further, the doctrine of equivalents does not extend so far as to cover a situation in which a claim limitation is missing altogether. This would improperly vitiate the claim limitation. For example, if a limitation requires the claimed product to be metallic, the doctrine of equivalents would not cover a non-metallic product.

The Supreme Court has approved two different tests to determine whether a feature in an accused product is equivalent to a claim limitation. The first is the ‘insubstantial differences’ test, which, as the name implies, asks whether a person of ordinary skill in the art would consider the difference between the claim limitation and the alleged equivalent feature of the accused product to be insubstantial. The second is the ‘function-way-result’ test, which looks at whether the accused equivalent performs substantially the same function, in substantially the same way, to achieve substantially the same result as the claim limitation. Both the Supreme Court and Federal Circuit have noted that the function-way-result test may be difficult to apply in chemical and pharmaceutical contexts, although they have stopped short of barring its use. Regardless of the test used, infringement under the doctrine of equivalents represents a factual enquiry for the jury (or judge in a bench trial) to assess.

Perhaps the surest way to prove equivalency is to show that the accused equivalent and the claimed feature were known in the art to be used interchangeably. For example, the Federal Circuit stated in Hilton Davis v Warner-Jenkinson that “the known interchangeability of the accused and claimed elements is potent evidence that one of ordinary skill in the relevant art would have considered the change insubstantial” and that “without such evidence, the patentee will need other objective technological evidence demonstrating that the substitute nevertheless represents a change that the ordinary artisan would have considered insubstantial at the time of infringement”.

A statement of bioequivalency on the whole (eg, between a claimed drug product and an accused drug product) is not an admission of infringement under the doctrine of equivalents. However, statements to the US Food and Drug Administration (FDA) that a particular feature of a drug product is equivalent to (or accomplishes the same function as) a feature appearing in a claim limitation may be used against the accused infringer. For example, in Intendis v Glenmark Pharms, the Federal Circuit affirmed a judgment of infringement under the doctrine of equivalents, finding “fatal to Glenmark’s argument is its own ANDA [abbreviated new drug application] submission to the FDA repeatedly referring to the claimed excipients” as equivalent to the claimed penetration enhancer. This case illustrates the difficult position that generic manufacturers may find themselves in, caught between representing to the FDA that a substituted excipient does not affect bioequivalence and arguing to a court that the substitution is significant.

But there are several important legal limitations on the doctrine of equivalents that prevent a patentee from even resorting to the doctrine in the first place, and they take on increased importance in pharmaceutical patent litigation.

Disclosure-dedication rule

In 1881, the Supreme Court established that “the claim of a specific device or combination, and an omission to claim other devices or combinations apparent on the face of the patent, are, in law, a dedication to the public of that which is not claimed”. This is now known as the ‘disclosure-dedication’ rule, which holds that where a patent specification describes alternatives but does not literally claim them, the patentee may not later resort to the doctrine of equivalents to capture the disclosed-but-unclaimed alternative. In a recent example, a Delaware court in Eagle Pharms v Hospira held that where the patent specification disclosed “a pharmaceutically acceptable fluid which contains one or more of propylene glycol, ethanol, polyethylene glycol, benzyl alcohol and glycofurol”, but claimed only propylene glycol, the patentee could not resort to the doctrine of equivalents to cover the alternative solvents. Thus, the court dismissed the case before even reaching the factual question of whether the accused solvent was equivalent to propylene glycol.


The doctrine of ensnarement prevents patentees from using the doctrine of equivalents to cover what is disclosed in, or obvious over, the prior art. To assess ensnarement, a court typically uses a hypothetical claim analysis: the claim is re-written to literally cover the accused equivalent and the court assesses whether that claim would be patentable over the prior art. For example, in Gevo v Butamax Advanced Biofuels, a Delaware court found that where a patentee narrowed a claim to cover a single bacterial enzyme to avoid an obviousness rejection, the patentee could not resort to the doctrine of equivalents because the hypothetical claim covering the alleged equivalent would have been obvious over the prior art of record.

Prosecution history estoppel

By far the most significant limitation on the doctrine of equivalents is prosecution history estoppel. Prosecution history estoppel prevents a patentee from narrowing patent claims during prosecution, then using the doctrine of equivalents in patent litigation to recapture what had been previously surrendered to secure allowance of the claims. Understanding prosecution history estoppel is imperative for a patent applicant to appreciate the risks and benefits of different patent prosecution strategies. An applicant too eager to obtain a patent by narrowing claims may find the patent easily designed around and virtually worthless. At the same time, competitors seeking to design around a patent may be able to safely evade the doctrine of equivalents by carefully analysing the prosecution history.

Prosecution history estoppel can arise when claims are narrowed during prosecution, either through claim amendments or arguments to the patent office. Although there are many interesting nuances to argument-based estoppel, this chapter focuses on the more common amendment-based estoppel, which involves a three-part analysis.

First, the accused infringer has the burden of showing that the amendment narrowed the claim limitation at issue. An amendment that merely clarifies the claim language or is ‘merely cosmetic’ is not narrowing and does not invoke prosecution history estoppel.

Second, the court must assess whether the amendment was made for the purposes of patentability. There is a rebuttable presumption (the Warner-Jenkinson presumption) that all claim amendments are made for the purposes of patentability, and that the patentee bears the burden to show otherwise. While narrowing amendments made to avoid prior art are more obviously made for the purposes of patentability, narrowing amendments made for other reasons (eg, to avoid a rejection for lack of written description or enablement under Section 112) may also give rise to estoppel.

Third, the court must assess whether the equivalent in question was within the scope of the matter surrendered. There is a second rebuttable presumption (the Festo presumption) that all claim scope between the original claim and the amended claim is surrendered and cannot be recaptured through the doctrine of equivalents. But the patentee may rebut that presumption in one of three ways:

  • The patentee may show that the equivalent in question was unforeseeable – for example, if the alleged equivalent is a new technology that arose after the claim amendment. The foreseeability of the alleged equivalent is intensely factual.
  • The patentee may show that the equivalent in question is only ‘tangentially related’ to the reasons for the amendment. For example, in Regents of the University of California v Dakocytomation California, the Federal Circuit held that where the patentee added the limitation ‘blocking nucleic acid’ (which the parties agreed literally covered DNA and RNA), prosecution history estoppel did not preclude application of the doctrine of equivalents to the accused peptide nucleic acid, because “the focus of the patentees’ arguments centered on the method of blocking – not on the particular type of nucleic acid that could be used for blocking”.
  • The patentee can provide “some other reason” that reasonably precluded them from drafting a claim to cover the alleged equivalent.

The exceptions to prosecution history estoppel are narrowly applied, and cases finding an alleged equivalent unforeseeable or another reason for rebutting the presumption have been rare. Patentees have had slightly more success with the tangentiality exception. Several recent cases illustrate what is well-established about the tangentiality exception (eg, “an amendment made to avoid prior art that contains the equivalent in question is not tangential”) and raise difficult, fact-intensive questions about its scope.

Recent cases

In January 2020, in Amgen v Amneal, the Federal Circuit affirmed a judgment of non-infringement for generic drug company Piramal Healthcare, finding that Amgen failed to show that the tangentiality exception applied. The claims in Amgen’s patent as filed originally encompassed a pharmaceutical formulation of the active ingredient cinacalcet (brand name Sensipar) comprising “at least one binder”. Facing an obviousness rejection during prosecution, Amgen narrowed that claim limitation to a group of four specific binders, after which the claim was allowed. The Federal Circuit rejected Amgen’s reliance on statements during prosecution that the amendment “was not for purposes of patentability” as self-serving and unreliable, noting that the claim amendment served no purpose if it did not lead to patentability, and held that Amgen could not rebut the Warner-Jenkinson presumption. It then found that the alleged equivalent binder in Piramal’s generic product – pre-gelatinised starch – was not tangential to the claim amendment because the prior art over which the claims were rejected disclosed the use of pre-gelatinised starch as a binder. The Federal Circuit recently denied Amgen’s request for rehearing and rehearing en banc.

In August 2019, in two related cases, Eli Lilly v Hospira and Eli Lilly v Dr Reddy’s Labs, the Federal Circuit affirmed judgments of infringement under the doctrine of equivalents, holding that the tangentiality exception to prosecution history estoppel applied. The patent claim at issue covered a method of using pemetrexed disodium (brand name Alimta) in chemotherapy. The claims were originally written broadly, covering administering ‘an antifolate’, and were rejected as anticipated over a reference disclosing a similar use of methotrexate, which is a type of antifolate. In response, Lilly narrowed the claims to the use of pemetrexed disodium, another antifolate. Both Dr Reddy’s and Hospira sought to market a generic pemetrexed tromethamine, stating to the FDA that pemetrexed dissociates in vivo and thus the type of salt used is largely irrelevant clinically.

Lilly admitted that the claim amendment (narrowing ‘an antifolate’ to ‘pemedtrexed disodium’) was made for purposes of patentability, but argued that the tangentiality exception applied because the amendment was made to narrow the claims to pemetrexed antifolates, not to any particular salt of pemetrexed. The Federal Circuit agreed, finding that “the particular type of salt to which pemetrexed is complexed relates only tenuously to the reason for the narrowing amendment”. But the decision seems to conflict with a body of Federal Circuit law holding that “there is no principle of patent law that the scope of a surrender of subject matter during prosecution is limited to what is absolutely necessary to avoid a prior art reference that was the basis for an examiner’s rejection” and injects greater uncertainty into the circumstances in which prosecution history estoppel will apply.

Both Hospira and Dr Reddy’s have petitioned the Supreme Court for certiorari, framing the issue as “whether a patentee may recapture subject matter via the doctrine of equivalents under the ‘tangential relation’ exception by arguing that it surrendered more than it needed to during prosecution to avoid a prior art rejection, even if a claim could reasonably have been drafted that would literally have encompassed the alleged equivalent”.

In a third recent case, Ajinomoto v ITC, the accused infringer, CJ CheilJedang Corp, filed a cert petition arguing that the rationale for a claim amendment must be clearly stated in the prosecution history, so as to avoid a “zone of uncertainty”. While the premise of this argument is understandable (ie, the patentee should provide the public with notice of the rationale), in practice such a requirement could be easily subverted because patentees can insert self-serving rationales into prosecution histories (as in Amgen).

The cert petitions raise interesting questions highlighting the tension between protecting patentees from blatant copying and the public notice function of claims. Prosecution history estoppel (and the other limiting doctrines) help ease this tension by limiting the doctrine of equivalents in cases where the patentee has publicly surrendered claim scope. The tangentiality exception similarly exists in a state of tension, aiming to protect patentees from inadvertently surrendering claim scope that is unrelated to the purpose of a claim amendment, without undermining the public’s ability to ascertain the scope of patents.

Businesses – especially in the pharmaceutical industry – need experienced patent counsel who understand the nuances of these doctrines at all stages (prosecution, product development and litigation) and who keep up with its rapid evolution.

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