Federal Circuit Ruling on ImmunoGen's Dosing Patent Highlights Challenges in Pharmaceutical Patent Strategy

In a precedential decision with significant implications for pharmaceutical patent strategy, the U.S. Federal Circuit has affirmed a district court's judgment that ImmunoGen's patent application for a specific dosing regimen of its antibody-drug conjugate (ADC) mirvetuximab soravtansine (IMGN853) was obvious, despite the inventors' claims of addressing a previously unrecognized toxicity issue.

The ruling, issued on March 6, 2025, upholds the Eastern District of Virginia's decision that ImmunoGen's U.S. Patent Application No. 14/509,809 contained claims that were unpatentable due to obviousness, highlighting the challenges pharmaceutical companies face when seeking protection for dosing regimens and treatment methods.

Patent Claims and Background

ImmunoGen's '809 application, filed in 2014 with priority dating back to October 2013, claimed methods for treating ovarian and peritoneal cancers by administering IMGN853 "at a dose of 6 milligrams (mg) per kilogram (kg) of adjusted ideal body weight (AIBW) of the patient." The company's FDA-approved ADC consists of an antibody (huMov19) linked to a toxic maytansinoid payload (DM4).

According to ImmunoGen, the inventors discovered that IMGN853 caused ocular toxicity at certain doses and developed "a therapeutically effective dosing regimen that results in minimal adverse effects." The company argued that since the ocular toxicity problem was unknown at the time of invention, the solution could not have been obvious.

Key Legal Reasoning

The Federal Circuit disagreed with ImmunoGen's position, establishing several important principles:

1. Unknown Problems, Obvious Solutions: The court clarified that "it does not follow that a claimed solution to an unknown problem is necessarily non-obvious." This distinction is particularly significant in U.S. patent law, which differs from European and other jurisdictions that apply a "problem-solution" approach to inventiveness.
2. Inventor Motivation Not Controlling: The court emphasized that "the obviousness inquiry is generally agnostic to the particular motivation of the inventors," citing KSR International Co. v. Teleflex Inc. What matters is whether a person of ordinary skill in the art (POSA) would have been motivated by the prior art to arrive at the claimed dose, regardless of the reason.
3. Prior Art Considerations: The court found that ocular toxicity was already a known problem with immunoconjugates containing DM4 (like IMGN853), and a POSA would have been motivated to monitor for those side effects.
4. Self-Created Obstacles: Notably, ImmunoGen was hindered by its own prior patent publication, which disclosed dosing IMGN853 at around 6 mg/kg of total body weight (TBW) and stated that "dosing regimen and dosages will depend on the particular cancer being treated... and can be determined by the physician."

Technical Analysis of Dosing Methodology

The court's technical analysis of the dosing methodology proved particularly damaging to ImmunoGen's case. The Federal Circuit agreed with the district court that for patients at their ideal body weight, a 6 mg/kg AIBW dose would be identical to a 6 mg/kg TBW dose—meaning ImmunoGen was effectively trying to patent a dosing regimen already disclosed in prior art.

"A doctor dosing a patient at his or her IBW with IMGN853 at a dose of 6 mg/kg TBW would necessarily be dosing that patient at 6 mg/kg AIBW, as claimed. This would be true regardless of whether a doctor knew of AIBW dosing," the court stated.

Industry Implications

Dr. Sarah Mendelsohn, a pharmaceutical patent attorney not involved in the case, notes the significant implications for drug developers: "This ruling further narrows the path for pharmaceutical companies seeking to extend patent protection through dosing regimens. Companies will need to be much more strategic about what they disclose in their primary compound patents."

The decision adds to what the court described as "the growing arsenal of jurisprudence on which generic drug companies may rely on attacking later-generation Orange Book-listed patents covering drug compositions and uses thereof."

Broader Patent Strategy Considerations

The case highlights several critical considerations for pharmaceutical companies:

1. Balancing Patent Scope: Companies must carefully balance the scope of primary patents covering new chemical entities against the foreseeability of filing later-generation applications on improvements like treatment regimens and dosage formulations.
2. Boilerplate Language Risks: The ruling serves as a warning about using "boilerplate" language in patent applications, particularly when describing the level of skill in the art regarding optimization of parameters.
3. International Strategy Differences: The contrast between U.S. and European approaches to inventiveness is stark—ImmunoGen successfully obtained European patent protection for similar claims using the "problem-solution" approach.

Market Response

Following the ruling, industry analysts have noted potential implications for ImmunoGen's market position. While IMGN853 (marketed as ELAHERE) received accelerated FDA approval in November 2022 for folate receptor alpha-positive, platinum-resistant epithelial ovarian, fallopian tube, or primary peritoneal cancer, the patent ruling could affect the company's long-term exclusivity strategy.

"This decision doesn't impact ImmunoGen's primary composition patents on mirvetuximab soravtansine, but it does potentially weaken their patent estate by eliminating one layer of protection they were seeking," explained Dr. James Wilson, pharmaceutical market analyst at Capital Research Group.

Looking Forward

The Federal Circuit's decision serves as an important reminder of the evolving landscape for pharmaceutical patents, particularly for method-of-treatment and dosing regimen claims. Companies developing new therapeutics will need to carefully consider their disclosure strategies in primary patents and subsequent improvement applications.

As pharmaceutical companies face increasing scrutiny over "patent thickets" and drug pricing, this ruling adds another dimension to the complex balance between innovation protection and market competition in the pharmaceutical sector.