Where the claim language and specification do not expressly identify measurement conditions, courts may look to the intrinsic record as a whole and to industry standards to determine how a person skilled in the art would understand the limitation.

Numerical patent claim limitations can be helpful to both define the metes and bounds of the claimed subject matter to deter potential infringers as well as distinguish from the prior art in advocating for patentability of the underlying subject matter during prosecution. Often, relevant chemical properties such as pH are numerically recited in patent claims. However, one aspect that may be overlooked during claim drafting is consideration of any relevant conditions at which the recited numerical claim limitations are applicable. For example, properties of compositions of matter such as pH can vary depending on the environmental conditions at which the value is measured, including temperature and pressure. Actelion Pharmaceuticals Ltd. v. Mylan Pharmaceuticals Inc., No. 2024-1641, 2026 WL 1314700 (Fed. Cir. May 13, 2026) provides an example of the pitfalls of reciting a claimed range of chemical properties without any further insight as to the conditions in which the properties are measured and the resulting treatment by the court in ascribing a meaning to such claims.

Background and Case History

Actelion owns U.S. Patent Nos. 8,318,802 and 8,598,227 (the ’802 and ’227 patents) directed to pharmaceutical compositions involving epoprostenol, the active ingredient in Actelion’s hypertension drug Veletri. Mylan submitted an abbreviated new drug application (ANDA) seeking FDA approval to market a generic epoprostenol drug before the expiration of Actelion’s patents. Actelion sued Mylan in the United States District Court for the Northern District of West Virginia alleging that Mylan’s proposed generic drug infringes the ’802 and ’227 patents.

The acidity or basicity of solutions is typically measured using the logarithmic pH scale in which lower values indicate higher acidity due to the presence of increased concentrations of hydrogen ions and higher pH values are indicative of basic solutions, which have lower concentrations of hydrogen ions. For example, the pH of water is considered neutral, having a pH of 7 under standard ambient conditions, which include an ambient temperature of 25° Celsius. Changing the temperature can alter the pH of the solution. Generally, increasing the solution temperature encourages dissociation of hydrogen ions thereby lowering the pH of the solution to be more acidic based on the increased concentration of hydrogen ions, and decreasing the solution temperature will have the opposite effect and increase the pH.

Epoprostenol is unstable in water and is susceptible to acid-catalyzed degradation. The specifications of the ’802 and ’227 patents disclosed methods for providing epoprostenol compositions that would remain stable when reconstituted with commercially available intravenous fluids and kept at room temperature versus the techniques required for preservation of another epoprostenol composition, Flolan, which is stored as a lyophilized (freeze-dried) powder, requires reconstitution with a special, more basic pH diluent than commercial intravenous fluids and refrigeration. The specifications disclose creating a highly basic pH bulk solution containing epoprostenol, arginine and sodium hydroxide or other bases, and then freeze-drying the resulting solution to form a lyophilized powder.

The asserted independent claims recite a bulk solution pH of “13 or higher” or “greater than 13.” For example, asserted claim 1 of the ’802 patent recites a lyophilized pharmaceutical composition comprising a unit dose of epoprostenol or a salt thereof, arginine and sodium hydroxide, wherein the composition is formed from a bulk solution having a “pH of 13 or higher” and is capable of reconstitution for intravenous administration with an intravenous fluid.

In contrast, Mylan’s ANDA identified a manufacturing process in which its bulk solution was refrigerated, and Mylan maintained that its generic product was manufactured from a bulk solution with a pH outside the asserted claims. Actelion asserted that when under refrigeration as part of the manufacturing process, the pH of Mylan’s bulk solution increases to a level that is encompassed by the asserted claims.

During claim construction, the district court initially construed “pH of 13 or higher” under ordinary rounding rules to encompass pH values as low as 12.5, but the United States Court of Appeals for the Federal Circuit vacated the resulting infringement and remanded for further consideration with the aid of extrinsic evidence. See Actelion Pharmas. Ltd. v. Mylan Pharmas. Inc., 85 F.4th 1167 (Fed. Cir. 2023). On remand, after considering intrinsic and extrinsic evidence, the district court construed the claim limitation “a pH of 13 or higher” to mean “a pH of 12.98 or higher.”

At trial, the district court determined that the only disputed infringement issue was whether the bulk solution used to form Mylan’s product had a “pH of 13 or higher.” Actelion advanced two theories of infringement: first, that Mylan literally infringed because the pH should be measured at the refrigerated operating temperature of Mylan’s bulk solution, where Actelion asserted the pH exceeded 13; and second, that Mylan infringed under the doctrine of equivalents because its manufacturing process allegedly performed the same function, in the same way, to achieve the same result as the claimed invention.

The district court rejected both theories and found no literal infringement or infringement by an equivalent. Actelion appealed the district court’s decision to the Federal Circuit.

The Federal Circuit’s Decision

On appeal, Actelion argued that the district court erred by construing the disputed claim term “pH of 13 or higher” to mean the pH measured at standard temperature of 25°±2° Celsius and sought a construction in which the pH is measured at the solution’s actual temperature and that it was not barred from asserting the doctrine of equivalents.[1]

The court noted that the claim language alone does not resolve the issue of how the limitation “pH of 13 or higher” should be interpreted and provides no insight as to the conditions at which the pH is measured to determine if an allegedly infringing solution meets the claim. The court rejected Actelion’s reliance on the word “having” in the context of the solution being “formed from a bulk solution having a pH of 13 or higher,” explaining that if pH would be understood in the relevant context to be measured at standard temperature, then the bulk solution having that pH means having the property of a pH of 13 or higher when measured under that standard.

Because the claims did not provide insight as to construction of the disputed claim limitation, the court next turned to intrinsic evidence, finding that the specifications of the ’802 and ’227 patents favored Mylan’s and the district court’s construction of measuring the pH at standard temperature. Although the specifications did not expressly state the conditions for the reported pH measurements, it identified an “alkaline environment” as one with “pH>7,” a statement the court understood to be accurate only at standard temperature. The court also observed that the specification presented comparative experimental results using various pH values without any indication that the values were measured at different temperatures or required temperature-specific interpretation. Because expert evidence showed that the tables reflected pH measured at standard temperature, it could be inferred that all pH were similarly measured (i.e., at standard temperature).

The court further turned to the extrinsic evidence relied on by the district court to support its finding that the limitation “pH of 13 or higher” should be attributed its plain and ordinary meaning as the pH measured at standard temperature. For example, the district court, in reaching its holding of noninfringement, relied on the United States Pharmacopeia (USP), which was agreed upon by both parties and their experts as an important set of standards in the pharmaceutical industry. The USP prescribes a default understanding that, “unless otherwise specified,” pH measurements are made at 25°±2° Celsius. The Federal Circuit held that the district court properly relied on this type of extrinsic evidence because evidence that artisans use a claim term in an established and consistent way, including evidence of well-known industry standards, can be persuasive of the term’s meaning. Similarly, the Federal Circuit agreed with the consultation of chemistry textbooks and the proposition that disclosed pH values generally refer to those pH measurements made at standard temperature and atmospheric pressure.

Based on the intrinsic and extrinsic evidence that the recitation of a “pH of 13 or higher” in the asserted claims, without more, should be construed as the pH as measured at standard temperature (25°±2° Celsius), the Federal Circuit affirmed the district court’s holding of noninfringement.

Conclusions and Practice Points

The court held that a claim recitation drawn to a “pH of 13 or higher” without providing any additional insight or context as to the conditions or stage in the process at which the pH is measured should be construed using a plain and ordinary meaning as the pH measured at standard temperature. Thus, failing to provide any special context in the claim, such as the conditions for which a recited numerical claim limitation is applicable such as a chemical property, is likely to draw an interpretation and construction as would be understood to be the property as measured under the relevant industry standards for a particular scientific field.

Actelion thus provides another reminder that numerical limitations in patent claims may depend as much on measurement conditions as on the number itself. Where the claim language and specification do not expressly identify measurement conditions, courts may look to the intrinsic record as a whole and to industry standards to determine how a person skilled in the art would understand the limitation. Practitioners should therefore consider expressly identifying relevant test conditions, such as temperature, pressure, sample preparation techniques, instrumentation and timing—and if those conditions could materially affect whether an accused product or process falls within the claims.

While not mentioned in the Federal Circuit’s opinion, Actelion raises further questions as to whether the omission of the requisite conditions at which the claimed properties are observed renders the claim indefinite and “fail[s] to inform, with reasonable certainty, those skilled in the art about the scope of the invention” as established under Nautilus, Inc. v. Biosig Instruments, Inc., 572 U.S. 898, 901 (2014). Applicants may recite in the claims that a claimed property is determined using a particular standard method, for example, the measurement of boiling ranges of petroleum products using the test method ASTM D86. Thus, a challenger, in reviewing the claims of the ’802 and ’227 patents with respect to the disputed claim term of “pH of 13 or higher,” could assert that the claims are indefinite because it cannot ascertain the means in which the pH according to the claims is measured such that it cannot determine whether its product or process would potentially be encompassed by the claim and potentially infringe. To avoid such ambiguity, practitioners should err on the side of disclosure in the specification as to the parameters by which various properties are measured so that it is clear and adequately supported as to how the claimed properties are determined. If the disclosed properties and measurements are determined as would be understood to be standard conditions in the relevant field, an overarching broad statement as to such would be sufficient and any special deviations from this standard should be noted.

For More Information

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Notes

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