pH, Prosecution History Estoppel, and Patent Scope: Three Lessons from the Federal Circuit's Latest Hatch-Waxman Ruling

On May 13, 2026, the U.S. Court of Appeals for the Federal Circuit affirmed a district court judgment of no infringement in Actelion Pharmaceuticals Ltd. v. Mylan Pharmaceuticals Inc., No. 24-1641. The decision offers important guidance for patent holders and generic manufacturers on the role of industry standards in interpreting scientific terminology during claim construction, prosecution history estoppel, and the disclosure-dedication rule.

Background

Actelion owns two patents that describe certain pharmaceutical compositions involving epoprostenol, the active ingredient in Actelion’s hypertension drug Veletri®. Mylan submitted an abbreviated new drug application (ANDA) to the FDA seeking approval to market a generic epoprostenol drug before the expiration of Actelion’s patents. Actelion sued Mylan for patent infringement, alleging that Mylan’s proposed generic drug was covered literally or under the doctrine of equivalents by certain claims of the patents.

The patents’ independent claims each require that the lyophilized composition be “formed from a bulk solution having a pH of 13 or higher.” The district court initially construed “a pH of 13 or higher” subject to “ordinary rounding rules,” meaning it literally encompassed pH values as low as 12.5. The parties stipulated to entry of final judgment of infringement and a permanent injunction following that ruling, and Mylan appealed. The Federal Circuit vacated that judgment in 2023, ruling that “the proper claim construction cannot be reached without the aid of extrinsic evidence” and remanding for further consideration.

On remand, the district court, after considering both intrinsic and extrinsic evidence, construed “a pH of 13 or higher” to mean “a pH of 12.98 or higher,” which Actelion did not challenge in the subsequent appeal. In February 2024, the issue of infringement was tried to the bench, where the only dispute was whether the pH of the bulk solution used to form Mylan’s ANDA product met that threshold.

Literal Infringement: The Temperature Measurement Question

The central factual dispute at trial was whether pH should be measured at the solution’s refrigerated operating temperature or at the standard pharmaceutical industry temperature of 25±2°C. It was undisputed that Mylan’s bulk solution, when measured at the standard temperature of 25±2°C, has a pH well below 12.98. However, Actelion argued that a skilled artisan would understand that pH should be measured at the refrigerated “operating temperature” of the solution, and when measured at that cold temperature, Mylan’s bulk solution has a pH above 13.

The district court disagreed, and the Federal Circuit affirmed the finding of no literal infringement. The court determined that a skilled artisan would understand the “bulk solution having a pH of 13 or higher” term to refer to a pH measurement taken at standard temperature, i.e., 25±2°C. The court placed particular weight on industry standards: the district court properly relied on the United States Pharmacopeia (USP) — which both parties’ experts agreed is an important set of standards in the pharmaceutical industry — to find that those skilled in making pharmaceutical compositions, when referring to pH, mean a measurement at standard temperature unless they indicate otherwise. Under that construction, there was undisputedly no literal infringement. The court also noted that many of the specification’s reported experimental results would “not make sense” if measurements were not taken at that standard temperature.

Doctrine of Equivalents: Multiple Independent Bars

The Federal Circuit also affirmed the district court’s ruling that Actelion was barred from asserting, and had not proven, infringement under the doctrine of equivalents on two independent grounds.

Prosecution History Estoppel. During prosecution of one of the patents, the pH limitation was amended several times, with the final amendment narrowing the representative claim’s pH limitation from “a pH of greater than 12” to “a pH of 13 or higher” following the examiner’s rejection on obviousness grounds. The prosecution history demonstrated that the examiner viewed pH 13 as the threshold for which unexpected results had been shown, and that was the “discernible objective reason for the narrowing amendment.” Accordingly, the Federal Circuit agreed that Actelion was estopped from asserting the doctrine of equivalents.

Disclosure-Dedication Rule. The disclosure-dedication rule independently barred Actelion’s equivalents theory because the specification disclosed, but did not claim, pH ranges including values below 13, such as pH 12.5 to 13 and greater than 12, which were expressly identified as alternatives to the claimed pH of 13 or higher. The Federal Circuit rejected Actelion’s argument that the rule should not apply because the disclosed alternatives were not mutually exclusive, emphasizing that overlapping alternatives are readily susceptible of being clearly disclosed and yet partially unclaimed.

Conclusion

The Actelion v. Mylan decision reinforces that unless defined otherwise by the patentee, scientific claim terms will be construed per industry standards when they supply a clear default meaning. It also demonstrates that prosecution history estoppel and the disclosure-dedication rule can independently and collectively foreclose equivalents arguments. Companies should scrutinize these doctrines early in litigation and during patent prosecution to protect or challenge patent scope effectively.