On March 6, 2025, the Federal Circuit issued its opinion in ImmunoGen, Inc. v. Stewart, affirming the district court’s ruling that denied ImmunoGen’s patent application for a dosing regimen of its antibody drug conjugate (ADC), IMGN853. The court’s decision, rooted in key principles of intellectual property law, particularly focused on the doctrines of indefiniteness and obviousness. This case highlights ongoing challenges in securing patents for pharmaceutical dosing regimens and underscores the high bar for patentability in the biotechnology sector.

Background of the Case

ImmunoGen sought a patent for a specific dosing regimen of IMGN853, a drug used to treat ovarian and peritoneal cancers. The key claim at issue involved administering IMGN853 at a dose of 6 milligrams per kilogram (mg/kg) of adjusted ideal body weight (AIBW). The U.S. Patent and Trademark Office (USPTO) rejected the application, a decision that was subsequently affirmed by the Patent Trial and Appeal Board (PTAB). ImmunoGen then pursued relief in the Eastern District of Virginia under 35 U.S.C. § 145, which allows applicants to challenge USPTO decisions in district court.

Following a bench trial, the district court ruled against ImmunoGen, determining that the claimed invention was fatally indefinite, obvious in light of prior art, and unpatentable under the doctrine of obviousness-type double patenting. ImmunoGen appealed to the Federal Circuit, which affirmed the lower court’s ruling based on obviousness.

Key Legal Issues

1. Obviousness Under 35 U.S.C. § 103

The Federal Circuit upheld the rejection of ImmunoGen’s patent claims on the grounds that they were obvious in light of prior art. The court applied the well-established Graham v. John Deere Co. framework, which requires assessing:

• The scope and content of the prior art
• Differences between the prior art and the claims at issue
• The level of ordinary skill in the art
• Any secondary considerations (such as commercial success or unexpected results)

The court found that a person of ordinary skill in the art would have been motivated to try the claimed dosing regimen based on existing knowledge of ocular toxicity risks associated with ADCs, prior dosing methodologies, and available clinical data. The decision emphasized that AIBW was a known dosing methodology in the prior art, and adjusting dosage to reduce toxicity was a routine optimization.

2. The Role of Prior Art in Determining Obviousness

The court found that:

• Existing literature disclosed IMGN853 and its dosing regimens based on total body weight (TBW).
• Previous studies had explored adjusted dosing strategies (including AIBW) to reduce toxicity in related drug classes.
• A person of ordinary skill in the art would have been able to transition from TBW to AIBW dosing as an obvious variation, especially since AIBW dosing had been previously used to reduce ocular toxicity for other drugs.

Ultimately, because the claimed dose of 6 mg/kg AIBW was within the range of known effective doses, the court determined that the invention was a predictable result of routine experimentation rather than an unexpected innovation.

3. Indefiniteness Under 35 U.S.C. § 112

While the Federal Circuit did not rule on indefiniteness, the district court’s decision noted that the application failed to define AIBW adequately, leaving ambiguity in the claim’s scope. Courts require patent claims to be sufficiently clear so that a person of ordinary skill in the art can ascertain the precise bounds of the invention.

The district court found that multiple formulas for AIBW existed, creating uncertainty about which formula applied to ImmunoGen’s claims. This contributed to the court’s ruling that the patent claims were fatally indefinite, making them unenforceable.

4. Obviousness-Type Double Patenting

The government also argued that the claims were unpatentable under the doctrine of obviousness-type double patenting, which prevents an inventor from obtaining multiple patents on obvious variations of the same invention. The district court found that ImmunoGen’s claims were largely duplicative of its previous patents, reinforcing the determination of obviousness. However, on appeal, both parties agreed that the double patenting issue rose and fell with the obviousness analysis, making further discussion unnecessary.

Implications for the Biotechnology and Pharmaceutical Industries

This decision underscores the high threshold for patentability in pharmaceutical dosing regimens. Courts have consistently ruled that dosing adjustments—particularly those based on well-known methodologies—are often seen as routine optimizations rather than true inventions. This presents significant challenges for drug developers seeking patent protection for new dosing strategies.

Key takeaways from this case include:

• Drug manufacturers must provide clear evidence of unexpected results to differentiate dosing regimen patents from routine experimentation.
• Precision in claim drafting is critical to avoid indefiniteness challenges.
• Applicants should consider alternative strategies, such as method-of-use claims tied to specific patient populations or therapeutic effects, to enhance patentability.

Conclusion

The Federal Circuit’s ruling in ImmunoGen, Inc. v. Stewart reinforces longstanding principles in patent law regarding obviousness and claim definiteness. While pharmaceutical innovators continue to seek patent protection for dosing regimens, this case demonstrates the difficulties of overcoming obviousness rejections, particularly where prior art provides clear guidance on dosage adjustments. Moving forward, companies seeking similar patents will need to present strong, non-obvious justifications to withstand legal scrutiny and secure valuable intellectual property protections.

By Charles Gideon Korrell