Federal Circuit refines obviousness framework for drug and biologic dosing regimens

The United States Court of Appeals for the Federal Circuit recently affirmed a district court ruling that a pharmaceutical dosing claim limitation was unpatentable due to obviousness-type double patenting. The court found that the “reasonable expectation of success” for the obviousness standard can be met even when there are fact-based differences between the claimed dosing regimen and prior art.

The ruling offers useful guidance for patent challengers and patent owners navigating obviousness analyses related to dosing limitations, emphasizing the importance of distinguishing secondary patent claims from prior art.

Case background

A patent claim is considered obvious if the differences between the claim and prior art would have made the claim’s subject matter obvious to a person of ordinary skill in the art at the time of the invention (35 USC § 103). To challenge a patent on these grounds, the challenger must show that a person of ordinary skill in the relevant art would have been motivated to combine or modify the prior art to achieve the claimed invention and had a reasonable expectation of success in doing so. Procter & Gamble Co. v. Teva Pharms. USA, Inc., 566 F.3d 989, 994 (Fed. Cir. 2009).

Obviousness-type double patenting is a judicially created doctrine extending from general obviousness. It prevents a patentee from obtaining separate patents which claim inventions so alike that granting both exclusive rights would effectively extend the life of patent protection for a single invention.

The underlying technology in ImmunoGen, Inc. v. Stewart concerns the patented antibody drug conjugate IMGN853 (mirvetuximab soravtansine), used in the treatment of certain cancers. ImmunoGen designed an IMGN853 dosing protocol based on a patient’s “adjusted ideal body weight” (AIBW) to minimize side effects, especially ocular toxicity. The resulting dosing regimen is the subject of US Patent Application No.14/509,809 (’809 application).

After the Patent Trial and Appeal Board (PTAB) affirmed a patent examiner’s rejection of the ’809 application on obviousness grounds, ImmunoGen sued in the US District Court for the Eastern District of Virginia. The district court affirmed the PTAB’s decision, finding the patent claims “fatally indefinite and obvious.” ImmunoGen, Inc. v. Vidal, 653 F. Supp. 3d 258, 307 (ED Va. 2023). The district court held that, in light of the prior art (especially the patentee’s other patent claiming the IMGN853 compound), a person of ordinary skill in the art would have been motivated to arrive at the claimed dosage limitation. More specifically, the district court pointed to prior art showing that ocular toxicity was a known side effect of other antibody drug conjugates, that a skilled artisan would know that varying a drug’s dosage can mitigate its side effects, and that AIBW was a known dosing strategy.

Court finds dosing regimen unpatentable due to obviousness

ImmunoGen appealed to the Court of Appeals for the Federal Circuit. Ultimately, the court affirmed the district court in holding the ’809 application obvious under obviousness-type double patenting. Because both sides admitted that the use of IMGN853 to treat cancer was known in the prior art, the only question to resolve was whether the dosing limitation itself was obvious. The court rejected each of ImmunoGen’s arguments, offering useful analysis in the process.

First, ImmunoGen argued that since the problem of ocular toxicity was unknown at the time of invention, a person of ordinary skill in the art would not have been motivated to create a dosage regimen to avoid it. The court applied KSR Int’l v. Teleflex to conclude that a solution to an unknown problem is not necessarily non-obvious. ImmunoGen, Inc. v. Stewart, No. 2023-1762, 2025 US App. LEXIS 5215, at *7 (Fed. Cir. 2025).

The court explained that a patentee’s particular motivation or purpose does not control an obviousness determination. Instead, what matters is “the objective reach of the claim." KSR Int'l Co. v. Teleflex Inc., 550 US 398, 419, 127 S. Ct. (2007). Because the prior art indicated other anticancer drugs were known to cause ocular toxicity, the court determined there was no clear error in the district court’s determination that the ’809 application was obvious.

Next, ImmunoGen argued that even if ocular toxicity were a known problem, a person of ordinary skill in the art would not be motivated to use AIBW as a specific metric. AIBW was only one of numerous dosing methods at the time of the application’s filing and it had never been used for antibody drug conjugates. Despite these facts, the court again disagreed. It cited prior art, which showed that AIBW could reduce ocular toxicity in small molecule drugs and that the toxicity of antibody drug conjugates was known to be related to dosage.

Finally, ImmunoGen contended that even if a person of ordinary skill in the art would be motivated to try AIBW, they would not have been motivated to choose the claimed dosage of six mg/kg. To bolster its position, ImmunoGen argued that the notable unpredictability of biologics would undermine any reasonable expectation of success. Once more, the court disagreed. The patentee’s own prior art revealed a dosing metric of six mg/kg of total body weight for IMGN853, which is equivalent to six mg/kg AIBW for some patients.

In regards to the unpredictability of biologics, the court held that the relative unpredictability of a field was not linked to an analysis of reasonable expectation of success: “The obviousness inquiry is generally agnostic to the particular motivation of the inventor…That means that in this case the district court was not required to determine that a person of ordinary skill in the art would have had a reasonable expectation of eliminating ocular toxicity using a six mg/kg AIBW dose.” ImmunoGen, Inc. v. Stewart at *15. Instead, the inquiry merely requires a court to determine if a person of ordinary skill in the art would have had a reasonable expectation that a dosage of IMGN853 at six mg/kg AIBW would have been effective in treating the cancers in question.

With all ImmunoGen’s arguments rejected, the court found no clear error in the district court’s determination that the dosing limitation of the ’809 application was obvious. Because this limitation was the only feature distinguishing the application from ImmunoGen’s patent-covering IMGN853, the court affirmed that obviousness-type double patenting barred the would-be secondary patent.

ImmunoGen adds nuance to the obviousness analysis of dosing regimen patents

ImmunoGen provides two main takeaways for practitioners. First, the ruling may make it easier to challenge dosing regimen patents on obviousness grounds, especially secondary patents associated with already patented pharmaceuticals and biologics. Second, it highlights the importance of distinguishing secondary patent claims from prior art – even when notable fact-based differences exist between them, such as between biologics and small molecule pharmaceuticals.

For more information, please contact the authors.