Pfizer v. Somerset Therapeutics: Xeljanz Patent Suit Dismissed Without Prejudice
Pfizer and affiliated entities filed suit in Delaware against Somerset Therapeutics over alleged infringement of reissue patent USRE041783E covering Xeljanz (tofacitinib) 5 mg and 10 mg tablets. The case closed after 138 days when Pfizer voluntarily dismissed all claims without prejudice under Rule 41(a)(1)(A)(i), before Somerset had filed any answer or dispositive motion.
Pfizer’s Pre-Answer Withdrawal Leaves Tofacitinib Claims Open
On May 16, 2025, Pfizer Inc. and four affiliated entities — C.P. Pharmaceuticals International C.V., PF PRISM C.V., PBG Puerto Rico LLC, and PF PRISM IMB B.V. — filed an infringement action in the Delaware District Court against Somerset Therapeutics LLC. The suit asserted reissue patent USRE041783E, which covers Pfizer’s 5 mg and 10 mg Xeljanz (tofacitinib) tablets, a JAK inhibitor approved for rheumatoid arthritis and related indications. The case was assigned to Judge Colm F. Connolly.
On October 1, 2025 — 138 days after filing — Pfizer filed a Notice of Dismissal pursuant to Rule 41(a)(1)(A)(i) of the Federal Rules of Civil Procedure, dismissing all claims without prejudice. Critically, the notice confirms Somerset had not yet served an answer or motion for summary judgment, meaning Pfizer retained the unilateral right to withdraw without court approval. The without-prejudice designation preserves Pfizer’s ability to refile the same claims in future proceedings.
The speed of resolution — under five months, before any substantive defence was entered — is consistent with several scenarios common in Hatch-Waxman ANDA litigation: settlement of underlying commercialisation terms, a licensing agreement reached out of court, or a strategic reassessment of the patent’s enforceability. The public record does not disclose which factor drove the dismissal, and no consent order or licence terms have been filed. The open question is whether Somerset has agreed to any constraints on its tofacitinib product launch, or whether Pfizer reserves the right to refile.
Filing to Dismissed without Prejudice in 138 days
138 days — resolved before defendant filed any responsive pleading
Rule 41 dismissal without prejudice: what it means for both parties
Rule 41(a)(1)(A)(i): unilateral right to dismiss before answer
Under Federal Rule of Civil Procedure 41(a)(1)(A)(i), a plaintiff may dismiss an action without a court order by filing a notice of dismissal before the defendant serves an answer or a motion for summary judgment. Pfizer’s notice confirms Somerset had not yet done either, making this a self-executing dismissal requiring no judicial approval and leaving no merits ruling on record.
No court order requiredWithout prejudice: the distinction matters significantly
A dismissal ‘without prejudice’ means the action is terminated but Pfizer’s underlying infringement claims are not extinguished — they can be refiled in a future proceeding. This stands in contrast to a dismissal ‘with prejudice,’ which would bar relitigation. The public record is silent on whether any side agreement accompanies this dismissal; commercially, the without-prejudice designation sustains ongoing uncertainty for Somerset regarding its tofacitinib launch timeline.
Claims may be refiledSomerset faces continued patent risk despite dismissal
Somerset Therapeutics emerges from this proceeding without a court ruling in its favour and without any finding of non-infringement or invalidity. The dismissal provides no legal shield against a future Pfizer action asserting USRE041783E or related patents. Somerset’s ability to launch or continue commercialising its tofacitinib product without litigation risk depends on terms not visible in the public record — suggesting continued commercial uncertainty.
No non-infringement findingTofacitinib genericisation timeline remains uncertain
Xeljanz is a commercially significant JAK inhibitor. A without-prejudice dismissal in a Hatch-Waxman-adjacent context typically signals either ongoing negotiations or a strategic pause rather than definitive patent clearance. Other generic or speciality pharmaceutical firms monitoring this ANDA pathway should note that USRE041783E remains fully enforceable and Pfizer retains the right to refile. The outcome is consistent with a negotiated standstill rather than market resolution.
Patent remains enforceableFull party and counsel information
| Role | Name | Type | Detail |
|---|---|---|---|
| Plaintiff | Pfizer, Inc. | Company | Global pharmaceutical company — holder of USRE041783E covering tofacitinib tabletsSearch in Eureka ↗ |
| Defendant | Somerset Therapeutics LLC | Company | Somerset Therapeutics LLC — generic/specialty pharmaceutical manufacturerSearch in Eureka ↗ |
| Plaintiff counsel | Megan Elizabeth Dellinger | Attorney | Counsel for Pfizer, Inc.Search in Eureka ↗ |
| Plaintiff law firm | Morris, Nichols, Arsht & Tunnell LLP | Law Firm | Representing Pfizer, Inc.Search in Eureka ↗ |
| Presiding judge | Judge Colm F. Connolly | Judge | Delaware District CourtSearch in Eureka ↗ |
Official order — verbatim text
The dismissal notice cites Rule 41(a)(1)(A)(i) and expressly confirms Somerset had not yet served an answer or motion for summary judgment at filing — the precise precondition for a plaintiff’s unilateral, self-executing withdrawal. The without-prejudice designation is legally significant: it forecloses any res judicata or claim preclusion argument by Somerset. No merits findings were made, no invalidity ruling issued, and the patent’s enforceability is entirely unaffected. The phrasing is procedurally clean and consistent with a strategic pause or undisclosed commercial resolution.
USRE041783E — Tofacitinib (Xeljanz) JAK Inhibitor Formulation
USRE041783E is a United States reissue patent, with a corrected application number of US12/577790. Reissue patents are granted by the USPTO when the original patent is deemed wholly or partially inoperative or invalid due to a defective specification, drawing, or claims — allowing the patentee to correct errors and, in some cases, broaden claim scope. The underlying technology covers tofacitinib, a small-molecule JAK (Janus kinase) inhibitor marketed by Pfizer as Xeljanz in 5 mg and 10 mg tablet formulations, approved for rheumatoid arthritis, psoriatic arthritis, and ulcerative colitis.
Xeljanz was among the first oral JAK inhibitors approved by the FDA, giving it significant commercial and strategic weight in the immunology and rheumatology space. The reissue status of USRE041783E means claim scope may have been modified from the original grant, potentially capturing a wider range of formulations or methods than the original patent. For generic and specialty pharmaceutical companies pursuing tofacitinib ANDA pathways, this patent represents a key obstacle: its reissue history and Pfizer’s willingness to enforce it — even if briefly — signal active lifecycle management of this asset.
Should you run an FTO against USRE041783E before entering the tofacitinib market?
Any company developing, formulating, or seeking regulatory approval for a tofacitinib product — including 5 mg and 10 mg tablet equivalents — should treat USRE041783E as a live enforcement risk. This case confirms Pfizer actively monitors potential market entrants and is prepared to file suit in Delaware. The without-prejudice dismissal provides no safe harbour: the patent remains in force, and Pfizer’s affiliated entities retain full standing to refile. R&D and regulatory teams preparing ANDA or 505(b)(2) submissions should ensure claim mapping against USRE041783E is part of pre-filing due diligence.
PatSnap Eureka’s FTO Search Agent enables rapid claim-by-claim mapping of USRE041783E against your product specifications, surfacing potential infringement vectors and identifying prior art that could support invalidity arguments. Eureka’s reissue patent tracking also flags any continuation or related family members that may present parallel risk. For in-house IP teams and outside counsel managing tofacitinib portfolio exposure, Eureka provides the landscape visibility needed to assess launch risk before committing to regulatory timelines.
Run a freedom-to-operate analysis on USRE041783E to assess your product’s exposure
Run FTO in Eureka →Similar JAK inhibitor and small-molecule pharmaceutical patent cases in Delaware
Explore related Hatch-Waxman and small-molecule patent infringement actions involving JAK inhibitors and reissue patents filed in the Delaware District Court.
What this case signals for the JAK inhibitor and small-molecule IP landscape
Pfizer’s pre-answer withdrawal keeps USRE041783E active and leaves the tofacitinib competitive entry question unresolved.
Pre-answer dismissals in Hatch-Waxman cases often signal off-record negotiations
When a branded pharmaceutical plaintiff withdraws before the defendant even files an answer, it typically suggests either an agreed licensing framework or a decision to reassert later. The absence of a consent order or licence on the docket means any deal terms — if they exist — are confidential. Competitors and ANDA filers in the tofacitinib space should treat this as an unresolved patent landscape, not cleared.
USRE041783E remains fully enforceable: FTO assessment is still required
A without-prejudice dismissal creates no estoppel and establishes no prior art record against the patent. Any company developing or launching a tofacitinib product should independently assess freedom-to-operate against USRE041783E. The reissue designation suggests the patent has already been broadened or corrected post-grant, which may expand its claim scope relative to the original grant.
Pfizer v Somerset — key questions answered
The without-prejudice dismissal under Rule 41(a)(1)(A)(i) means Pfizer voluntarily ended the action before Somerset filed any answer, with no merits ruling made. USRE041783E remains fully enforceable and Pfizer retains the right to refile identical claims against Somerset or any other tofacitinib entrant. The dismissal creates no estoppel and does not affect the patent’s validity or scope.
USRE041783E is a United States reissue patent covering tofacitinib (Xeljanz) formulations. The reissue designation means the USPTO reviewed and corrected the original patent, potentially broadening its claims. For generic challengers, reissue patents can present a higher infringement risk than original grants because claim language may have been deliberately expanded during the reissue process. This makes independent FTO analysis against the reissued claims essential for any tofacitinib ANDA filer.
The public record does not disclose the reason. However, pre-answer dismissals in pharmaceutical patent litigation are often consistent with a confidential licensing or settlement arrangement, a strategic decision to refile under different circumstances, or a reassessment of claim mapping. The speed — before Somerset filed any responsive pleading — suggests Pfizer’s decision was deliberate and unilateral rather than driven by court-ordered deadlines.
Not directly. This dismissal resolves only the claims against Somerset Therapeutics LLC and has no binding effect on other potential defendants. USRE041783E remains enforceable against any party that Pfizer believes infringes its claims. Other companies pursuing tofacitinib products should conduct independent claim mapping and not infer patent clearance from this procedural outcome.
The plaintiffs are Pfizer Inc. and four affiliated entities: C.P. Pharmaceuticals International C.V., PF PRISM C.V., PBG Puerto Rico LLC, and PF PRISM IMB B.V. Multi-entity plaintiff structures in pharmaceutical patent litigation typically reflect the distribution of patent ownership, manufacturing rights, and commercialisation interests across a corporate group. All five entities joined the Rule 41 dismissal notice collectively.
PatSnap Eureka searches patents and litigation data to answer instantly.