Introduction
Here’s a scenario that plays out more often than it should. A patent holder wins an IPR proceeding. The challenger is estopped from raising the same prior art grounds in district court. Case closed — or so it seems. Then the challenger files an ex parte reexamination on related grounds. Or files a new IPR petition on different claims with a slightly different prior art combination. Or raises an independent invalidity defence in the district court litigation that runs in parallel. The patent that survived one challenge is now under attack again from a different angle.
Or flip the scenario: your company receives a patent assertion. You file an IPR petition. Under Director Squires’ centralised institution process, the petition is denied — Fintiv applies, the district court case is moving fast, and PTAB declines to institute. Now your invalidity case has to work in front of a jury in EDTX without the PTAB’s specialised patent judges, without the lower preponderance of evidence standard, and without the prior art strategy you were planning to develop post-institution.
Both scenarios have the same underlying requirement: a prior art case that was built comprehensively from the start, charted correctly, and documented in a way that holds up across multiple proceedings, multiple forums, and multiple attempts. This article covers how to build that case — in the specific environment of 2026, where IPR institution is uncertain, district court invalidity is the primary battleground, and the prior art investment you make first is the one that determines your leverage at every stage that follows.
The New Reality: Why the Initial Prior Art Search Is Now the Decisive Investment
The PTAB environment has changed significantly. As covered in our analysis of IPR and PGR strategies for patent invalidation, institution rates under Director Squires’ centralised process have dropped to approximately 37%. More than 63% of IPR petitions filed in 2025 were denied — and the denials are not random. They’re concentrated in cases involving parallel district court litigation (Fintiv), patents over six years old (settled expectations), and petitions that don’t clear the Director’s elevated practical threshold.
The staged approach that practitioners relied on for years is no longer reliable:
Old approach (pre-2025 PTAB): Run a targeted prior art search. File an IPR petition with your strongest few references. If instituted, develop the full invalidity case with additional art. Hold back secondary references for district court use.
New reality (2026 PTAB): With 63%+ denial rates, the IPR petition needs the strongest possible prior art from day one — because there may not be a second opportunity at PTAB. And if the petition is denied, that same prior art needs to hold up in district court under the Phillips claim construction standard, without the lower preponderance of evidence burden PTAB applies.
The consequence: the initial prior art search is now the decisive investment in any invalidity campaign. It sets the foundation for the IPR petition (if viable), feeds the district court invalidity contentions (if needed), and determines whether the ex parte reexamination (if deployed as a parallel or fallback track) has the prior art to succeed. Every subsequent proceeding draws on the quality of the prior art that was identified first.
Understanding IPR Estoppel: What You Can — and Can’t — Revisit
Estoppel is the most consequential constraint on repeat invalidity attacks — and the one most often misunderstood in terms of its practical scope.
Under 35 U.S.C. § 315(e), a petitioner who has obtained a final written decision in an IPR proceeding is estopped from asserting in district court or ITC that a patent claim is invalid on any ground that was raised — or that reasonably could have been raised — during the IPR. The operative phrase is ‘reasonably could have been raised.’ Courts interpret this broadly.
What ‘reasonably could have been raised’ covers in practice: all prior art references that a skilled searcher conducting a reasonable prior art search would have identified. This isn’t limited to the references you actually submitted in the IPR. It covers references you found but decided not to include. It covers references you would have found if you had run a comprehensive search. A decision to hold back secondary references for use in district court — because you planned to develop the invalidity case further post-institution — is a decision that may result in those references being estopped anyway.
The practical consequence for prior art strategy is significant:
- Run the full prior art search before filing the IPR petition. The only way to know whether a reference is covered by estoppel is to know whether a reasonable search would have found it. Running a comprehensive search before the petition is filed gives you the complete picture — and the informed decision about which references to include.
- Don’t hold back references you’re planning to use in district court. If the reference would have been found in a reasonable prior art search — and it almost certainly would have been — holding it back creates estoppel risk without any procedural benefit.
- Consider ex parte reexamination for the references you can’t include in IPR. Ex parte reexamination is not subject to IPR estoppel. References used in ex parte reexam don’t create IPR-style estoppel for district court use. This makes ex parte reexam a valuable parallel track for secondary prior art.
Building a Prior Art Case That Holds Up in Both PTAB and District Court
The prior art case that works across both forums — and survives repeat challenges — is the one built to the higher standard from the start. Here’s what that looks like in practice.
- Claim construction first — for both forums. PTAB applies the Phillips standard (plain and ordinary meaning as understood by a person of ordinary skill in the art) — the same standard used in district court. Build your claim construction map element by element before a single database is searched. The construction determines what prior art you need, and building it for district court standards means it holds up in both forums.
- Multi-database, multi-language search. USPTO alone is not sufficient. The most powerful prior art — particularly in software, AI, wireless communications, and semiconductor technology — is frequently in Japanese (JPO/J-PlatPat), Korean (KIPO), German (DPMA), and Chinese (CNIPA) databases. A search that covers only English-language databases is structurally incomplete for any technology space with significant international R&D activity. Japanese and Korean patent documents in particular frequently predate their US counterparts in fast-moving technology sectors.
- Non-patent literature as a primary search category, not an afterthought.
- Physical products and prior public use — district court only. IPR is limited to prior art consisting of patents or printed publications. Physical products, prior commercial sales, and public use evidence are only available in district court invalidity proceedings — not at PTAB. If your invalidity case includes a strong product-based prior art argument, that argument needs to be developed and preserved for district court use, because it’s structurally excluded from IPR.
- Element-by-element claim charts for every relevant reference. A prior art reference that’s found but not charted is nearly useless in litigation. Both PTAB panels and district court judges work from claim charts — the structured mapping of prior art reference elements to patent claim limitations. Every reference that could support an invalidity argument needs a chart, built before the petition or invalidity contentions are filed, in the format that counsel can work with directly under litigation deadlines.
“The prior art case built comprehensively the first time — multi-database, multi-language, NPL-inclusive, charted to district court standards — is the one that creates leverage at every stage of the invalidity campaign. The prior art case built in a hurry to meet an IPR deadline creates estoppel risk and leaves the district court case exposed.”
When Your Competitor Files the Same Arguments Twice: Defence Patterns
Understanding the specific patterns of repeat invalidity attack helps you design the prior art foundation to withstand each one.
Pattern 1 — IPR Filed, Denied Under Fintiv, District Court Proceeds: The most common pattern in 2025–2026. IPR petition filed within one year of complaint service. PTAB denies under Fintiv because the district court schedule is moving fast. The invalidity case shifts entirely to district court — same prior art, different forum, different procedural standards. The prior art case needs to have been built to district court standards from the start, because there’s no IPR institution to develop it further.
Pattern 2 — IPR Filed, Instituted, Partial Win, Remaining Claims in District Court: IPR is instituted but results in a final written decision that cancels some claims but upholds others. The district court litigation continues on the surviving claims. The estopped grounds can’t be reused. The prior art for the surviving claims needs to come from the comprehensive prior art foundation built before the IPR petition — the references that were identified but not included in the IPR submission.
Pattern 3 — Serial Challenges Across IPR, Ex Parte Reexam, and District Court: A patent holder wins IPR. The challenger files ex parte reexamination on related grounds (no estoppel applies). The district court litigation on surviving claims continues. Each challenge draws from the prior art foundation. A comprehensive initial search that identified all relevant references — and documented which forum each reference is appropriate for — is the only way to execute a multi-forum invalidity campaign coherently.
The Ex Parte Reexamination Strategy
Ex parte reexamination is the most underutilised tool in the current invalidity environment — and its specific characteristics make it particularly valuable in the post-Squires PTAB landscape.
- No IPR-style estoppel. A failed ex parte reexamination does not bar district court invalidity arguments on the same prior art grounds. This is fundamentally different from a failed IPR, which creates broad estoppel. Ex parte reexam can be deployed, fail, and leave the district court invalidity case intact.
- Anonymous filing. Ex parte reexamination can be filed anonymously through counsel. The patent holder doesn’t know who filed the request. For situations where maintaining strategic distance from the challenge is valuable, anonymous filing preserves that distance.
- Can run in parallel with district court litigation. Unlike IPR, which creates Fintiv complications when parallel district court litigation is active, ex parte reexamination can run alongside district court proceedings. It applies prosecution pressure on the patent holder and creates a prior art record that can influence the district court invalidity analysis.
- Useful for secondary prior art. References that weren’t included in the IPR petition — either because they were held back or discovered later — can be presented in ex parte reexamination without creating IPR estoppel. This makes ex parte reexam the appropriate vehicle for prior art that the IPR petition couldn’t accommodate.
How Our Patent Invalidation Search Service Is Built for the Current Environment
Our patent invalidation search service is structured around the IPR-and-district-court dual environment — delivering prior art that meets the standards of both PTAB proceedings and district court invalidity defences from a single comprehensive engagement.
Every engagement starts with claim construction under the Phillips standard before a single database is searched — the standard that applies in district court and, since the elimination of the broadest reasonable interpretation standard at PTAB, also in IPR proceedings. The search covers 15+ databases including USPTO, Espacenet, JPO (J-PlatPat), KIPO, WIPO PATENTSCOPE, and CNIPA, using CPC and IPC classification-based methodology rather than keyword-only search. NPL sources — academic databases, conference proceedings, technical standards — are covered as a primary search category, not an afterthought. Foreign language documents are reviewed by native-language analysts for the most commercially significant references.
The output is a court-ready invalidity report: element-by-element claim charts for every relevant reference, prosecution history analysis identifying the arguments made during examination that define the claim boundaries, and a clear mapping of which references anticipate under § 102 versus render obvious under § 103. The report is formatted to work in IPR petitions, ex parte reexam submissions, and district court invalidity contentions without reformatting. Because the invalidity campaign often runs across multiple forums — and the prior art case has to hold up in all of them.
The prior art case you build first determines your leverage at every stage — IPR petition, ex parte reexam, or district court invalidity defence. Our patent invalidation search delivers the comprehensive, court-ready prior art that holds up however many times the patent gets challenged. → Contact Us
Conclusion: The Takeaway
The patents worth invalidating are the ones that get attacked more than once. In IPR. In ex parte reexamination. In district court invalidity contentions. Across different combinations of prior art, different claim construction arguments, and different procedural standards. The invalidity campaign that succeeds across all of those challenges is the one built on a comprehensive prior art foundation from the start.
In 2026, with IPR institution rates at 37% and district court invalidity as the primary battleground, the prior art search that feeds the first filing is more important than at any point since the America Invents Act created IPR in 2012. The staged approach — targeted search, develop later if needed — creates estoppel risk and leaves the district court case exposed. The comprehensive approach — full multi-database, multi-language, NPL-inclusive search, charted to court standards before any petition is filed — creates a foundation that holds up across every forum and every repeat attack.
Build it right the first time. There may not be a second.
