How To Kill a Weak Patent: 10 Strategies After Being Sued

If your company is sued for patent infringement, the dispute is not only about whether your product practices the claims; it is often about whether the patent should have been issued in the first place. U.S. law gives defendants multiple tools to invalidate, narrow, or render patents unenforceable. Success depends on selecting the right tool(s) based on the facts, timing, costs, and overall litigation strategy.

Below are 10 of the most common and effective ways defendants seek to kill weak patents after they have been sued for infringement.

1. Inter partes review: Active challenger participation

Inter partes review (IPR) allows defendants to ask the U.S. Patent and Trademark Office (USPTO), specifically the Patent Trial and Appeal Board (PTAB), to re-evaluate issued claims using prior patents and printed publications under 35 U.S.C. §§ 102 and 103. The IPR process is structured and technical and often leads to claims being cancelled, narrowed, or found unpatentable and therefore unenforceable. The challenger remains an active participant throughout the proceeding by briefing issues, submitting expert declarations, and responding to the patentee’s arguments. Courts may stay district court litigation while IPRs are pending. IPRs must be filed within one year of being sued, and the earlier the better to maximize the chance of staying the case and obtaining a validity ruling before trial. However, recent USPTO policy changes (including the application of “settled expectations” and expanded discretionary denials) have made institution of IPRs significantly harder and less predictable.

In practice, IPR remains a powerful tool against recently issued weak patents with strong paper prior art, but ex parte reexamination may be a more prudent or complementary option.

2. Ex parte reexamination: Minimal challenger participation

Ex parte reexamination (EPRx) allows anyone to ask the USPTO to re-examine a patent when prior patents or printed publications raise a substantial new question of patentability. After the request is filed, the requester’s participation is limited, but the USPTO can still cancel or significantly narrow claims. Unlike IPR, the proceeding is largely examiner-driven, and the challenger does not actively brief or litigate issues once the request is submitted. EPRx can be requested at almost any time, including after litigation begins, and it often runs in parallel with it. It is particularly useful when the IPR window has closed, when the risk of IPR institution denial is high, or when a lower-cost validity attack makes strategic sense.

Think of EPRx as a way to reopen prosecution of a patent when strong prior art surfaces — especially in the current IPR climate.

3. Anticipation: One piece of prior art discloses the claim

If a single prior-art reference is shown to disclose every element of a claim, the claim is said to be “anticipated” and therefore invalid for lack of novelty under § 102. In IPR and EPRx, the challenger is limited to relying on patents and printed publications to show anticipation. In district court, however, defendants can also rely on system art, including prior public uses, on-sale products, working systems, internal documentation, and source code supported by witness testimony. Anticipation arguments are typically developed early through invalidity contentions and then pressed at summary judgment or trial, often in parallel with invalidity proceedings at the USPTO (e.g., IPR, EPRx). Because anticipation asks whether the exact invention was already publicly disclosed or practiced, it can be one of the fastest paths to invalidity.

If someone else already did it, the claimed invention may be found anticipated.

4. Obviousness: A combination of prior art makes the claim obvious

Even when no single reference discloses every element, a claim can still be said to have been “obvious” to a person of ordinary skill in the art (POSITA) at the time of the alleged invention who would have combined known teachings with a reasonable expectation of success, and therefore invalid under § 103. In IPR and EPRx, the challenger is limited to relying on patents and printed publications to show obviousness. In district court, defendants can also use system art. Obviousness is framed early in invalidity contentions, refined through expert reports, and then positioned for summary judgment, trial, or USPTO review. It remains one of the most common and effective validity challenges.

If the claimed invention was just the next predictable step, it may be found obvious.

5. Ineligibility: Cannot claim abstract ideas, natural phenomena, or laws of nature

Section 101 bars patents on abstract ideas, natural phenomena, and laws of nature unless the claims add a genuine technological improvement. Courts regularly invalidate patents under § 101 that simply computerize conventional processes or automate routine decision-making without a specific technical advance. Section 101 motions are often filed very early, sometimes at the pleading stage, because they can resolve cases before costly discovery. When successful, they remove the patent from the case entirely. Challenging eligibility remains a strong early defense for business method or software-heavy patents.

If the claimed invention is merely a business method dressed up as technology, it may be found ineligible.

6. Indefiniteness: Claim is too vague to enforce

A patent must clearly define what it covers and what it does not. If a claim fails to inform a POSITA of its boundaries with reasonable certainty, it is said to be “indefinite” and is invalid under § 112(b). Vague, functional, or internally inconsistent language is especially vulnerable. Indefiniteness issues are usually presented during the claim construction process — i.e., the portion of the case when the court determines the meaning of disputed claim terms — and decided around that stage or at summary judgment. When successful, the claims cannot be enforced.

If the court determines that a POSITA could not tell what the patent claim covers, the patentee may be unable to enforce it.

7. Lack of written description: Patent claims more than was invented

The written description requirement ensures that the patent claims only what the inventor possessed. When patentees broaden claims during prosecution to capture competitor technologies that they never described in their patent specifications, defendants may challenge this lack of written description under § 112(a). Written description challenges are often built through expert testimony and argued at summary judgment and sometimes at trial. The analysis focuses on what the patent initially disclosed versus how far the claims later stretched.

If the specification does not demonstrate that the inventor actually possessed the claimed invention, the claim may be found to lack sufficient written description.

8. Lack of enablement: Patent fails to teach what it claims

Enablement requires the patent to teach skilled artisans how to make and use the full scope of the claims without undue experimentation. Patents that claim results, rather than concrete implementations, are especially vulnerable to a finding of lack of enablement under § 112(a). Enablement defenses typically mature during expert discovery and are decided at summary judgment or trial. Recent decisions reinforce that patentees cannot monopolize an entire field while providing only narrow working examples.

If the patent does not teach how to build what it claims, the claims may be found to lack enablement.

9. Break the priority chain: Open the door to newer prior art

Many patents rely on a chain of earlier filings to claim an older effective filing date. If one application in that chain does not truly support the asserted claims, the asserted patent loses the earlier priority, and newer, intervening prior art may become invalidating. Priority challenges are typically built through document discovery and expert analysis and then presented at summary judgment. When successful, they can instantly convert marginal references into decisive invalidity weapons.

Break one link in the chain and the claims become exposed to newer, intervening prior art.

10. Inequitable conduct: Render the patent unenforceable

A patent obtained through intentional deception of the USPTO can be deemed unenforceable. This can occur, for example, if the applicant withheld material prior art or submitted misleading statements with the intent to deceive. This doctrine, known as inequitable conduct, is judge-made rather than statutory. It is usually developed through fact discovery and tried to the court, often alongside or after trial. Although the standard for showing inequitable conduct is demanding, a successful finding can taint an entire patent family and cripple the plaintiff’s credibility.

Findings of inequitable conduct are rare, but when they hit, they can be devastating.

Your checklist for killing weak patents

• Strong prior patents/publications and within one year of being sued → Inter partes review
• New prior art but IPR unavailable or risky → Ex parte reexamination
• Single reference discloses every limitation of the claim → Anticipation (§ 102)
• Combination of references discloses every limitation of the claim → Obviousness (§ 103)
• Patent reads like an abstract concept, not technology → Ineligibility (§ 101)
• POSITA cannot tell what the claim covers → Indefiniteness (§ 112(b))
• Claims broader than what patent described → Lack of written description (§ 112(a))
• Patent fails to teach what it claims → Lack of enablement (§ 112(a))
• Weak link in continuation chain → Challenge priority date (§ 120)
• Evidence of deception at USPTO → Inequitable conduct

Takeaways

Killing a weak patent requires attacking it from every viable angle, in each forum that matters. When the pressure is sustained and properly sequenced, weak patents tend to collapse under their own weight.