Key Takeaways
- Leaving pH measurement conditions undefined let the court apply a default standard temperature, which defeated literal infringement
- Narrowing the claim to “13 or higher” created prosecution history estoppel barring recovery of lower pH ranges under equivalents
- Precise drafting matters, especially for technical limits, because small ambiguities can determine the outcome
Infringement of patents often hinges on subtle details, particularly where scientific measurements intersect with claim construction. In Actelion Pharms. Ltd. v. Mylan Pharms. Inc., No. 2024-1641 (Fed. Cir. May 13, 2026), the Federal Circuit confronted one such subtlety and ultimately affirmed a district court decision that turned on how a pH limitation should be interpreted. The case illustrates how seemingly technical ambiguities, such as the conditions under which pH is measured, can determine the outcome of patent infringement disputes. Ultimately, this case reinforces:
- The importance of a robust specification when disclosing technical details, particularly if specific conditions are necessarily tied to claimed elements; and
- Prosecution History Estoppel can have very real implications and thus claim amendments must be approached cautiously, to avoid unwanted scope surrender
Actelion Pharmaceuticals owns U.S. Patent Nos. 8,318,802 and 8,598,227, both directed to pharmaceutical compositions containing epoprostenol. (Id.). Epoprostenol, an agent used to treat cardiovascular disease, is chemically unstable in water and readily degrades into inactive compounds (Id. at 2–3). The asserted claims are product-by-process claims directed to freeze-dried pharmaceutical compositions and methods of making and administering epoprostenol (Id. at 6). Importantly, each independent claim requires that the composition be “formed from a bulk solution having a pH of 13 or higher” or “greater than 13” (Id.); however, the conditions under which the pH is measured were not specified in either or Actelion’s patents. Furthermore, during prosecution, Actelion narrowed the pH limitation from “greater than 12” to “13 or higher” in response to an obviousness rejection, which created prosecution history estoppel barring it from recapturing lower pH values under the doctrine of equivalents. (Id. at 15-16).
Mylan Pharmaceuticals sought approval to market a generic version of Actelion’s drug by submitting an abbreviated new drug application (ANDA) to the FDA before Actelion’s patents expired (Id.). Actelion sued for patent infringement in the Northern District of West Virginia under 35 U.S.C. § 271(e)(2) and 21 U.S.C. § 355(j), alleging that Mylan’s proposed product infringed the asserted claims either literally or under the doctrine of equivalents (Id. at 2, 7). The district court found no literal infringement, concluding that the claim term “a pH of 13 or higher” refers to a pH measured at a standard temperature in the field, namely, 25±2°C (Id. at 8, 13). The court also held that Actelion was barred from relying on the doctrine of equivalents (Id. at 8).
On appeal, the Federal Circuit followed a structured sequence for claim interpretation, first finding that the plain claim language (“pH of 13 or higher”) is ambiguous because it does not specify measurement conditions. The court then turned to intrinsic evidence (the specification and prosecution history) which consistently assumes standard temperature pH measurements. Finally, it confirmed that understanding with extrinsic evidence (including USP standards and expert testimony) showing that skilled artisans measure pH at 25±2°C unless otherwise specified, thereby affirming no literal infringement (Id. at 10-15).
Finally, the Federal Circuit affirmed the district court’s rejection of Actelion’s doctrine of equivalents claim (Id. at 15–18). The court held that prosecution history estoppel barred Actelion from recapturing subject matter it had surrendered during prosecution by narrowing its claims to require a pH of at least 13 (Id. at 15–17). Actelion’s attempt to invoke the tangentiality exception failed because the amendment was directly related to the claimed pH limitation (Id. at 16–17). The disclosure-dedication rule independently barred Actelion’s theory, as the specification disclosed but did not claim lower pH ranges, thereby dedicating them to the public (Id. at 17–18).
The Federal Circuit’s decision underscores how critical it is for patent drafters to specify measurement conditions for quantitative limitations, especially in chemical and pharmaceutical contexts. By leaving the temperature of pH measurement undefined, Actelion allowed the court to apply a default industry standard that ultimately foreclosed infringement. The case serves as a cautionary reminder that even seemingly minor ambiguities in claim language can have dispositive consequences.