Anticipation by inherency is a well-recognized, and generally well-understood, patent law doctrine. A prior art reference may anticipate an undisclosed feature of the claimed invention if that missing feature is: (1) necessarily present in or (2) the natural result of the elements explicitly disclosed by the prior art. Courts have typically adhered to this standard. Courts, and parties appearing before those courts, have been less successful, however, in adhering to the U.S. Court of Appeals for the Federal Circuit’s standard for the use of inherency in obviousness analyses, especially in the last 10 years.

These difficulties stem from important differences between anticipation and obviousness. Anticipation focuses on what is disclosed in the prior art and not what one of ordinary skill in the art would have understood or appreciated based on that disclosure. Thus, a prior art disclosure may inherently disclose a claim element even where a person of ordinary skill in the art would not have recognized that inherent element at the time of invention. For example, a claim to a metabolite may be inherently anticipated by the prior art administration of a compound that invariably forms that metabolite, even if formation of the metabolite is not recognized until years later. Schering Corp. v. Geneva Pharms., 339 F.3d 1373 (Fed. Cir. 2003), review en banc denied, 348 F.3d 992 (Fed. Cir. 2003).

In contrast, obviousness focuses on the knowledge of a person of ordinary skill in the art at the time of the invention. This tension between inherency (which allows for later recognition) versus obviousness (in which hindsight is forbidden) has led to confusion in applying inherency in an obviousness analysis. Recognizing this tension, the Federal Circuit has repeatedly cautioned that the use of inherency must be “carefully circumscribed” in the context of obviousness.

Notwithstanding this guidance, lingering confusion over the precise scope of the inherency doctrine in an obviousness analysis has led to improper applications of the doctrine. In some instances, inherency has been invoked as a means of justifying hindsight-driven obviousness arguments. In others, litigants have misapplied the doctrine in an effort to discount unexpected results of a combination invention by alleging that those results are merely inherent. Such arguments have generally been unsuccessful, but even the limited exceptions have sown confusion and resulted in a dramatic increase in inherent obviousness challenges arising in both district courts and before the Patent Trial and Appeal Board (PTAB). Sorting through this confusion and understanding the scope of this challenging legal doctrine requires a careful analysis of the case law and underlying fact patterns.

This article addresses recent Federal Circuit cases discussing the scope of inherency in obviousness and then analyzes its sometimes fickle application in the district courts and before the board.

Recent Evolution of the Doctrine

While inherency in obviousness is not new, 11 of the 20 cases to reach the Federal Circuit on this doctrine did so in the past eight years. This modern trend can be traced to the 2011 Federal Circuit decision In re Kao, 639 F.3d 1057 (Fed. Cir. 2011).

In Kao, the court affirmed an obviousness holding for a claim directed to a controlled-release drug formulation where the maximum concentration of drug released from the claimed formulation was higher in patients under fed versus fasted conditions. Id. at 1070. Although the claimed controlled release drug and its associated food effect were not disclosed in the prior art, the court held that the claimed formulation was obvious and the specified food effect was an inherent property that did not render the claimed formulation nonobvious. In so holding, the court relied on an admission in the patent specification that the food effect is an inherent property of the active ingredient of the drug product, present in various types of drug formulations using this ingredient. Id. Thus, while In re Kao does not provide a detailed discussion of the inherency doctrine, it nevertheless illustrates the circumstances where the Federal Circuit may find claimed features inherent in an obviousness analysis. Where, as in In re Kao, those features are necessarily present in otherwise obvious subject matter, and where there is no showing that those features are unexpected, the court has found associated patent claims inherently obvious.

The Federal Circuit revisited inherent obviousness in Santarus, Inc. v. Par Pharms., Inc., 694 F.3d 1344 (Fed. Cir. 2012), affirming the district court’s holding that claims directed to certain pharmaceutical compositions would have been obvious. The patent claims at issue recited specific blood serum concentrations following administration, and the prior art did not disclose these concentrations. The court nevertheless found that the prior art rendered the formulation obvious, because “an obvious formulation cannot become nonobvious simply by administering it to a patient and claiming the resulting serum concentrations.” Id. at 1354. The court further stated that ”[t]o hold otherwise would allow any formulation—no matter how obvious—to become patentable merely by testing and claiming an inherent property.” Importantly, the court noted that the specified blood serum concentrations were not only inherent (and thus necessarily associated with the claimed formulation) but also not unexpected in light of the dosages. Id.

The Federal Circuit offered a more expansive discussion of inherency in obviousness in Par Pharma., Inc. v. TWI Pharms., Inc., 773 F.3d 1186 (Fed. Cir. 2014). In Par, the district court held that claims directed to a method of treatment using nanoparticles of a megestrol formulation were invalid as obvious. Id. at 1192. The claims at issue recited that there was no substantial difference in maximum concentration achieved when the formulation was administered in a fed versus a fasted state. Id. at 1194. The Federal Circuit vacated the district court’s determination that the claimed food effect was inherent and that the claims were therefore obvious. As the court explained, inherency must be limited when applied to obviousness, and the limitation must be the “natural result” of the combination of prior art elements. Id. at 1195. Because the district court had not made any findings that the claimed food effect necessarily occurred, the court remanded for additional fact-finding. Upon remand, the district court made these additional findings and again concluded that the claims were obvious. The Federal Circuit affirmed. Pharma., Inc. v. TWI Pharms., Inc., 120 F. Supp. 3d 468 (D. Md., July 28, 2015), aff’d, 624 Fed. Appx. 756 (Fed. Cir. 2015).

Kao and Santarus therefore illustrate when inherency can play a role in assessing obviousness, while Par reinforces the narrow scope of the doctrine and the type of judicial findings necessary to support a holding of inherent obviousness. These guiding principles are generally applied correctly in the lower courts, but some confusion remains.

Application in the PTAB and District Courts

District court and PTAB decisions relying on inherency in an obviousness analysis typically adhere to Federal Circuit guidance and carefully and narrowly apply this doctrine. In rare cases, however, inherency has been improperly used to discount unexpected results or the knowledge of the person of ordinary skill in the art, which may incentivize continued challenges. Through all of this, certain trends have developed over the past few years providing guidance to litigants and courts on this narrow doctrine.

Patent claims directed to drug concentrations in the blood stream or release profiles after administration may be vulnerable to an inherent obviousness challenge. For example, in GeneriCo, LLC v. Dr. Falk Pharma GmbH, the board found that a release profile of 85 to 90 percent was obvious based on previous press releases discussing the administration of the same formulation. IPR2016-00297, Paper 55, *12-14 (P.T.A.B. May 19, 2017). The patent disclosure attributed the release profile to the dual-release formulation and did not identify any additional factors that would affect the release profile. The PTAB further determined that it was known in the art how to measure the release profile of a pharmaceutical formulation. The board thus held the 85 to 90 percent release profile was an inherent property of the formulation disclosed in the prior art and that the patent claim was obvious. Id. Similarly, in Purdue Pharm. Prods. LP v. Actavis Elizabeth LLC, the U.S. District Court for the District of New Jersey found a claimed plasma concentration to be inherent based on an expert’s testimony that “a POSA could easily predict offset blood concentrations.” 2015 BL 273849, *52-53 (D.N.J. Mar. 27, 2015). [Authors’ note: In each of these cases, a person of ordinary skill in the art likely would have had a reasonable expectation of success in achieving the claimed release profiles or drug concentrations. Further, there was no convincing argument of unexpected results. Thus, the claims potentially could have been held obvious based on a more conventional obviousness argument.]

Not all patent claims reciting drug concentrations, however, are at risk of being invalidated as obvious. Fresenius-Kabi USA, LLC v. AstraZeneca AB, IPR2017-01910, Paper 10, *15-16 (P.T.A.B. Dec. 11, 2017). In Fresenius-Kabi, the patent owner provided evidence that administering the drug through intramuscular, instead of subcutaneous, injections would produce unpredictable plasma level concentrations—concentrations the petitioner argued were inherent. Id. Here, the PTAB did not reach the issue of whether the claimed plasma concentrations were necessarily associated with the claimed formulation, instead holding that unexpected results would deter a person of ordinary skill from attempting intramuscular injection. Id.

In other situations, the board has applied the doctrine narrowly. In Colas Solutions Inc. v. Blacklidge Emulsions, Inc., the board rejected an inherency argument directed to a claimed range of softening points for asphalt. IPR2016-01031, Paper 38, *8-12 (P.T.A.B. Nov. 2, 2017). While the claimed softening points were not explicitly taught in the prior art, the petitioner argued it was known how to calculate the softening points of prior art asphalt compositions. The petitioner was able to show that most of the claimed softening points were present in the prior art. The PTAB rejected the inherency argument. Although only a few values fell outside the prior art range, that nevertheless established that the claimed softening points were not all necessarily present in the prior art. Id; see also, Actavis LLC v. Abraxis Bioscience, Inc., IPR2017-01100, Paper 7, *11-13 (P.T.A.B., Oct. 11, 2017) (stability of a single formulation is not sufficient to show all claimed formulations necessarily have the claimed stability, thus stability not proven to be inherent).

Despite the guidance in these recent cases, in some situations the PTAB or district court has incorrectly used an inherency analysis to discount unexpected results. The Federal Circuit has used these situations to clarify the doctrine. In Millennium Pharms., Inc. v. Sandoz Inc., the district court held that claims to a new compound, the D-mannitol ester of bortezomib, were obvious despite evidence of unexpected results and unpredictability. 862 F.3d 1356, 1364 (Fed. Cir. 2017). The inventors had lyophilized bortezomib, a known compound, in the presence of mannitol. Lyophilization involves freeze drying a substance and typically does not change the chemical structure of an active pharmaceutical ingredient. Here, the inventors prepared a lyophilized formulation for injection because liquid formulations of bortezomib were unstable. However, instead of merely freeze drying bortezomib, they created a new compound. Id. at 1361-62. There was nothing in the prior art to indicate that the specific new compound would be created by this process, or that the new compound would have superior properties that would solve the stability problems associated with bortezomib. Id. at 1364-65. The district court nevertheless determined that the ester was a natural result of freeze drying the bortezomib with mannitol, meaning the ester was the inherent, or “inevitable,” result of the combination and that Millennium had conceded this fact. The Federal Circuit disagreed with this analysis and reversed the district court’s obviousness determination. Id. at 1367, 1370.

In Millennium, the defendant was not relying on inherency to supply a missing claim limitation in an obviousness argument. Rather, the defendant argued that the principle of inherency should be used to discount the evidence of unexpected results. The court rejected this approach, explaining that the “invention is not a matter of what the inventor intended when the experiment was performed; obviousness is measured objectively in light of the prior art, as viewed by a person of ordinary skill in the field of the invention.” Id. at 1367.

The Federal Circuit more clearly explained this same point in Honeywell Int’l Inc. v. Mexichem Amanco Holding S.A., 865 F.3d 1348 (Fed. Cir. 2017). In this case, the PTAB found that the stability and miscibility of the claimed mixture of HFO-1234yf and PAG were unexpected properties. But the board nevertheless determined they were inherent and that the claims to this mixture were obvious. Id. at 1352-53. The board stated that the claims at issue were unpatentable because, due to the “overall unpredictability as to stability in the art,” a person of ordinary skill would have simply engaged in routine testing of all possibilities and ultimately would have arrived at the claimed combination. Id. at 1353. The Federal Circuit reversed, holding that the board should not have dismissed properties as being merely inherent without further looking into their unpredictability and unexpectedness. Importantly, the court explained that “all properties of a composition are inherent,” and thus when considering the obviousness of a composition what matters is whether its properties are known and expected. Id. at 1355. The court further criticized the PTAB’s reliance on the unpredictability of the art in support of its obviousness determination. The court explained that this was “reverse reasoning” because ”[u]npredictability of results equate more with nonobviousness rather than obviousness.” Id. at 1356.


The Federal Circuit has repeatedly stated that inherent obviousness is a narrow doctrine. Parties attempting to advance this argument must therefore satisfy the strict standards in the case law. An inherent limitation must necessarily be present in the combination of references. It is not enough to merely show that it is likely to be present.

Moreover, inherent properties of a new combination are only unpatentable if those properties were not unexpected at the time of the invention. While certain types of patent claims are susceptible to inherent obviousness arguments, such as those reciting blood plasmaconcentrations associated with certain pharmaceutical formulations, further expansion of the doctrine appears unlikely.

The Federal Circuit has been effectively policing the recent rise in the number of inherent obviousness arguments, requiring parties to fully meet their evidentiary burden in order to successfully make out their claims. As a consequence, for now, it appears that the dramatic increase in the number of cases asserting inherent obviousness is more of a nuisance for litigants and courts than a legitimate cause for concern.