Eagle Pharmaceuticals v. Slayback & Apotex: Federal Circuit Affirms Bendamustine Patent Unpatentable
Eagle Pharmaceuticals failed to defend US11103483B2 — a patent on bendamustine formulations — against a challenge brought by generic entrants Slayback Pharma and Apotex. The Federal Circuit affirmed the unpatentability finding in a Rule 36 judgment after 440 days of appellate proceedings, leaving the patent invalidated and the generic pathway clear.
Generic challengers prevail as Federal Circuit upholds bendamustine patent invalidity
Eagle Pharmaceuticals, Inc. brought this appeal to the U.S. Court of Appeals for the Federal Circuit (Case No. 23-1110) on November 2, 2022, seeking to overturn a determination that US11103483B2 — covering formulations of bendamustine, an alkylating agent used in oncology — was unpatentable. The defendants, Slayback Pharma LLC and Apotex Corp./Inc., had successfully challenged the patent at the tribunal below, clearing a path for generic versions of Eagle’s bendamustine product.
On January 16, 2024, the Federal Circuit issued a Rule 36 judgment affirming the lower decision without a written opinion. Under Fed. Cir. R. 36, the court may enter an affirmance when a written opinion would lack precedential value — meaning the panel found no reversible error but declined to elaborate further. The practical effect is that the unpatentability finding is now final and binding, removing US11103483B2 as an enforcement tool for Eagle and leaving the generic competitors free to proceed.
The 440-day duration from filing to affirmance is consistent with contested pharmaceutical patent appeals at the Federal Circuit. Because the court acted under Rule 36, no detailed reasoning is publicly available, making it impossible to identify precisely which invalidity grounds — novelty, obviousness, or written description — were determinative. The absence of a written opinion also means the decision carries no direct precedential weight, though it confirms the outcome adverse to Eagle at every level of review.
Filing to affirmance in 440 days
440 days from filing to Federal Circuit affirmance — above the median for ANDA-related pharma appeals
Federal Circuit affirms: what the unpatentability ruling means for both parties
Rule 36 affirmance: lower decision stands without written opinion
A Federal Circuit Rule 36 judgment affirms the decision below in full where the panel concludes a written opinion would add no precedential value. It is not a summary dismissal — the court reviewed the record and found no reversible error. For US11103483B2, this means the unpatentability determination is now final across all levels of review, with no further reasoning available to the public from the appellate stage.
Affirmed — no reversible error foundEagle loses its bendamustine exclusivity shield
With US11103483B2 confirmed unpatentable, Eagle Pharmaceuticals cannot use this patent to block or delay generic bendamustine products. The affirmance exhausts Eagle’s appellate options at the Federal Circuit level. Any further challenge would require a petition for certiorari to the U.S. Supreme Court — a rarely granted and high-bar avenue that typically requires a circuit split or constitutional question, neither of which is apparent here.
Patent invalidated — enforcement pathway closedSlayback and Apotex secure a clear generic pathway
For Slayback Pharma and Apotex, the Federal Circuit affirmance removes the primary patent obstacle to commercialising generic bendamustine formulations. The unpatentability finding, now affirmed at every level, substantially reduces litigation risk for these generic entrants. While other IP or regulatory hurdles may remain — including potential alternative patents in Eagle’s portfolio — US11103483B2 can no longer be asserted against them.
Generic pathway confirmed — patent barrier removedOncology drug formulation patents face heightened invalidity scrutiny
This outcome suggests that formulation patents on established oncology APIs like bendamustine face significant validity risk when challenged by well-resourced generic entrants. Brand pharmaceutical companies should audit whether formulation patents in their portfolios have robust, differentiated claims that go beyond obvious modifications to known drug compositions. The Rule 36 disposition, while non-precedential, reinforces the trend of Federal Circuit deference to IPR-level or district-level unpatentability findings in ANDA-adjacent disputes.
Formulation patents — rising invalidity riskFull party and counsel information
| Role | Name | Type | Detail |
|---|---|---|---|
| Plaintiff | Eagle Pharmaceuticals, Inc. | Company | Specialty oncology pharmaceutical company — holder of US11103483B2 covering bendamustine formulationsSearch in Eureka ↗ |
| Defendant | Slayback Pharma, LLC | Company | Generic pharmaceutical manufacturers Slayback Pharma LLC and Apotex Corp./Inc., challenging patent validitySearch in Eureka ↗ |
| Co-Defendant | Apotex Corp. | Company | Search in Eureka ↗ |
| Co-Defendant | Apotex, Inc. | Company | Search in Eureka ↗ |
| Plaintiff counsel | Daniel Brown | Attorney | Counsel for Eagle Pharmaceuticals, Inc.Search in Eureka ↗ |
| Plaintiff counsel | David Kowalski | Attorney | Counsel for Eagle Pharmaceuticals, Inc.Search in Eureka ↗ |
| Plaintiff counsel | Gabriel K. Bell | Attorney | Counsel for Eagle Pharmaceuticals, Inc.Search in Eureka ↗ |
| Plaintiff counsel | Gregory G. Garre | Attorney | Counsel for Eagle Pharmaceuticals, Inc.Search in Eureka ↗ |
| Plaintiff counsel | Michael Clemente | Attorney | Counsel for Eagle Pharmaceuticals, Inc.Search in Eureka ↗ |
| Plaintiff law firm | Latham & Watkins, LLP | Law Firm | Representing Eagle Pharmaceuticals, Inc.Search in Eureka ↗ |
| Defendant counsel | Constance Huttner | Attorney | Counsel for Slayback Pharma, LLCSearch in Eureka ↗ |
| Defendant counsel | Jason Aaron Lief | Attorney | Counsel for Slayback Pharma, LLCSearch in Eureka ↗ |
| Defendant law firm | Windels Marx Lane & Mittendorf, LLP | Law Firm | Representing Slayback Pharma, LLCSearch in Eureka ↗ |
| Presiding judge | Judge N/A | Judge | Court of Appeals for the Federal CircuitSearch in Eureka ↗ |
Official order — verbatim text
The Rule 36 judgment — ‘AFFIRMED. See Fed. Cir. R. 36’ — is the Federal Circuit’s most economical form of affirmance. It confirms the panel reviewed the full record and found no reversible error under the applicable standard of review, without elaborating on the reasoning. For patent validity challenges, the appellate court typically reviews legal conclusions de novo and factual findings for clear error. The absence of a written opinion means the decision is non-precedential but fully binding on the parties, rendering the unpatentability of US11103483B2 final.
US11103483B2 — Bendamustine Hydrochloride Intravenous Formulations
US11103483B2, filed under application number US16/509920, claims formulations of bendamustine — an alkylating chemotherapy agent used to treat chronic lymphocytic leukaemia and certain lymphomas. Formulation patents of this type typically protect specific excipient combinations, concentrations, or stability profiles that differentiate a branded product from earlier-known drug compositions. The patent’s position in Eagle’s portfolio suggests it was intended to extend commercial exclusivity beyond the compound itself.
Bendamustine formulation patents sit in a crowded and heavily litigated segment of pharmaceutical IP, where generic entrants routinely challenge secondary patents as obvious extensions of prior art compositions. The invalidation of US11103483B2 is commercially significant for Eagle because formulation patents often represent the last line of patent defence after compound patents expire or are challenged. Companies developing next-generation oncology infusion products should treat this outcome as a signal that formulation claims require unusually strong differentiation from the prior art to survive inter partes or post-grant review.
Should you run an FTO against US11103483B2 before entering the bendamustine market?
While US11103483B2 has been found unpatentable and that finding is now affirmed, any company developing or commercialising bendamustine formulations — including biosimilar or reformulated intravenous products — should confirm that no related continuation, divisional, or family patents from Eagle remain in force. A single affirmed invalidity finding does not clear the entire patent family, and Eagle may hold granted claims in related applications that cover overlapping formulation territory.
PatSnap Eureka’s FTO Search Agent enables R&D and regulatory teams to map Eagle’s full bendamustine patent family, identify claim scope across continuation applications, and flag any granted or pending claims that could cover reformulated intravenous bendamustine products. The tool cross-references prosecution history, family member status, and citation data to surface residual risk that a single case outcome may obscure — giving product teams a defensible clearance position before launch.
Run a freedom-to-operate analysis on US11103483B2 to assess your product’s exposure
Run FTO in Eureka →Similar Federal Circuit appeals in pharmaceutical formulation patent disputes
Cases where the Federal Circuit reviewed unpatentability findings in ANDA-driven pharmaceutical formulation disputes, with Rule 36 or written affirmances.
What this case signals for the oncology pharmaceutical IP landscape
The affirmance of unpatentability in a bendamustine formulation dispute underscores the fragility of secondary pharmaceutical patents against coordinated generic challenges.
Rule 36 affirmances are terminal — plan prosecution strategy accordingly
A Federal Circuit Rule 36 judgment leaves no written reasoning to work with on remand or in related proceedings. Brand pharma patent owners should treat the risk of a silent affirmance as a real scenario when deciding whether to appeal — particularly where the lower tribunal’s unpatentability analysis rests on established obviousness case law that the Federal Circuit routinely affirms.
Generic coalitions amplify invalidity pressure on formulation patents
The pairing of Slayback Pharma and Apotex as co-challengers is consistent with a coordinated ANDA litigation strategy that pools resources and legal arguments. Brand companies facing multi-defendant ANDA suits should anticipate that combined challenger briefs can raise the quality and breadth of invalidity arguments, increasing the risk that even well-drafted formulation claims are found unpatentable.
Eagle v Slayback — key questions answered
The Federal Circuit affirmed the lower tribunal’s finding that US11103483B2, Eagle Pharmaceuticals’ patent covering bendamustine formulations, was unpatentable. The court issued a Rule 36 judgment on January 16, 2024, meaning it found no reversible error but did not publish a written opinion. The patent is now finally invalidated.
A Rule 36 judgment allows the Federal Circuit to affirm a lower decision without a written opinion when the panel concludes one would add no precedential value. For US11103483B2, it means the unpatentability finding is fully affirmed and final. The decision binds the parties but is non-precedential — it cannot be cited as controlling authority in other cases.
Eagle held US11103483B2 (application US16/509920) covering formulations of bendamustine hydrochloride for intravenous use. The Basis of Termination recorded is ‘Unpatentable,’ consistent with an obviousness or anticipation finding. Because the Federal Circuit acted under Rule 36, the specific grounds are not elaborated in the appellate record.
Eagle could petition the U.S. Supreme Court for a writ of certiorari, but this is a rarely granted avenue and typically requires a circuit split or a significant legal question. No such grounds are apparent from the public record of this case. Practically, the Federal Circuit’s Rule 36 affirmance is the end of the road for US11103483B2.
The affirmance removes US11103483B2 as a barrier for Slayback Pharma and Apotex. However, other patents in Eagle’s portfolio, FDA regulatory exclusivities, or additional ANDA approval requirements may still apply. Companies seeking market entry should conduct a full freedom-to-operate review across Eagle’s complete bendamustine patent family before commercialising.
PatSnap Eureka searches patents and litigation data to answer instantly.