Last updated: August 3, 2026
Par Pharmaceutical sued Eagle Pharmaceuticals in the District of Delaware after Eagle filed an ANDA seeking approval for a generic version of Vasostrict, Par’s vasopressin injection product. The case concerned U.S. Patent Nos. 9,375,432 and 9,610,300, which covered aspects of Par’s ready-to-use vasopressin formulation. The district court held the asserted claims invalid as obvious. The Federal Circuit affirmed in 2022, eliminating the principal patent barrier to Eagle’s ANDA approval and generic entry. Par Pharmaceutical, Inc. v. Eagle Pharmaceuticals, Inc., 44 F.4th 1207 (Fed. Cir. 2022).
What was the Par v. Eagle litigation about?
The dispute was a Hatch-Waxman patent case involving an ANDA for generic vasopressin injection.
| Item |
Details |
| Case |
Par Pharmaceutical, Inc. v. Eagle Pharmaceuticals, Inc. |
| Court |
U.S. District Court for the District of Delaware |
| Civil action |
No. 1:18-cv-00823-CFC |
| Judge |
Chief Judge Colm F. Connolly |
| Filing date |
2018 |
| Plaintiff |
Par Pharmaceutical, Inc. |
| Defendant |
Eagle Pharmaceuticals, Inc. |
| Reference product |
Vasostrict, vasopressin injection |
| Regulatory pathway |
ANDA under Hatch-Waxman |
| Asserted patents |
U.S. Patent Nos. 9,375,432 and 9,610,300 |
| Federal Circuit decision |
2022 |
| Final result |
Asserted claims held invalid as obvious |
Par alleged that Eagle’s proposed generic product would infringe claims covering vasopressin compositions with specified formulation characteristics. Eagle countered that the asserted claims were invalid under 35 U.S.C. § 103 because the claimed formulation was an obvious modification of known vasopressin products and formulation techniques.
The case did not involve a biosimilar application. Vasopressin is a synthetic peptide drug regulated through the ANDA pathway, not a biologics license application under the Public Health Service Act.
What patents protected Vasostrict in the Par v. Eagle case?
The litigation focused on two Par patents.
| Patent |
Subject matter in dispute |
Issue in litigation |
Outcome |
| U.S. Patent No. 9,375,432 |
Vasopressin pharmaceutical compositions and formulation parameters |
Obviousness |
Claims invalid |
| U.S. Patent No. 9,610,300 |
Related vasopressin formulation claims |
Obviousness |
Claims invalid |
The patents addressed a ready-to-use vasopressin formulation rather than a new chemical entity. The claimed subject matter centered on formulation properties such as vasopressin concentration, pH, stability, and the absence or use of particular excipients or preservatives.
The Federal Circuit treated the patents as formulation patents. It did not accept the argument that the formulation was patentable merely because the commercial product achieved useful stability or reduced preparation requirements. The relevant question was whether a skilled formulator would have had a reason to modify the prior art and a reasonable expectation of success.
When do the Par vasopressin patents expire?
The two asserted patents had patent-term dates extending into approximately 2035, subject to applicable patent-term adjustment and any regulatory exclusivity.
| Patent |
Approximate nominal expiration |
| U.S. 9,375,432 |
2035 |
| U.S. 9,610,300 |
2035 |
The litigation outcome is more important than the nominal expiration dates. A patent that remains listed or has an unexpired term does not provide effective exclusivity if the asserted claims have been adjudicated invalid and the judgment is final.
The case therefore converted the practical exclusivity analysis from a 2035 patent-expiration question into an immediate validity and regulatory-entry question.
How did Eagle challenge Par’s Vasostrict patents?
Eagle filed an ANDA containing a Paragraph IV certification. A Paragraph IV certification states that a listed patent is invalid, unenforceable, or will not be infringed by the proposed generic product.
Par’s lawsuit triggered the Hatch-Waxman 30-month stay under 21 U.S.C. § 355(j)(5)(B)(iii), subject to court orders and the precise regulatory history. The litigation was the mechanism for determining whether Eagle could obtain approval and launch before the patents’ nominal expiration dates.
The central legal issue was obviousness. Eagle argued that the claimed vasopressin formulation represented a predictable selection or optimization of known formulation conditions. Par argued that the prior art did not provide a sufficient reason to select the claimed parameters and did not establish a reasonable expectation that the resulting product would maintain the necessary stability.
What did the Delaware district court decide?
The district court ruled that the asserted claims were invalid as obvious.
The court’s analysis considered:
- The state of the art concerning vasopressin formulations.
- Known stability problems associated with vasopressin.
- Prior-art teachings concerning pH and formulation conditions.
- The motivation to develop a ready-to-use product.
- The expected benefits of avoiding bedside dilution or preparation.
- The predictability of formulation optimization.
- Objective indicia of nonobviousness, including commercial success and industry acceptance.
The court concluded that the prior art supplied a reason to pursue the claimed formulation and that a skilled artisan would have had a reasonable expectation of success. The claims therefore failed the obviousness requirement of § 103.
What did the Federal Circuit hold in 2022?
The Federal Circuit affirmed the district court’s invalidity judgment in Par Pharmaceutical, Inc. v. Eagle Pharmaceuticals, Inc., 44 F.4th 1207 (Fed. Cir. 2022).
The appellate court held that the district court properly evaluated the prior art and the skilled artisan’s motivation to develop the claimed formulation. The Federal Circuit also upheld the conclusion that the asserted claims were obvious despite Par’s evidence regarding the commercial product.
The decision reinforced several points relevant to pharmaceutical formulation patents:
- A formulation patent can be obvious even when the commercial product has meaningful clinical or commercial advantages.
- Routine formulation development can support an obviousness finding when the prior art identifies the relevant problem and provides workable options.
- Commercial success does not overcome strong obviousness evidence when the claimed features are disclosed or suggested by the prior art.
- A stable, ready-to-use dosage form does not receive automatic patent protection merely because it improves product handling or administration.
The Federal Circuit’s decision left the invalidity judgment intact.
What was the litigation timeline?
| Date |
Event |
| 2018 |
Par filed the Delaware action after Eagle’s ANDA and Paragraph IV certification |
| 2018-2020 |
Claim-construction, discovery, expert, and pretrial proceedings |
| 2020 |
District court adjudicated the asserted claims invalid as obvious |
| 2020-2021 |
Appeal to the U.S. Court of Appeals for the Federal Circuit |
| 2022 |
Federal Circuit affirmed the invalidity judgment |
| After 2022 |
The asserted patents no longer provided a viable litigation barrier against Eagle’s ANDA product |
The case was resolved on the merits rather than through a publicly reported settlement that preserved a delayed-entry date. The reported appellate decision confirms that Eagle prevailed on the principal patent-validity issue.
What was the Orange Book status of the patents?
The patents were associated with Par’s Vasostrict product and were litigated through the ANDA Paragraph IV process. Orange Book listing status must be assessed by product, patent, and the FDA’s current publication because listing status can change after patent expiry, delisting, administrative correction, or an invalidity judgment.
The litigation outcome does not automatically erase a patent from the Orange Book. FDA Orange Book listing and judicial enforceability are separate issues. A patent may remain listed while a court has determined that its asserted claims are invalid. The practical effect of the Delaware judgment was that the listed patents could not be used successfully against Eagle based on the claims adjudicated in the case.
The relevant regulatory distinction is:
- Orange Book listing identifies patents submitted by the NDA holder.
- A Paragraph IV certification creates the patent dispute.
- The court determines infringement, validity, and enforceability.
- FDA approval depends on the statutory exclusivity and patent-certification framework, not solely on continued listing.
How strong was Par’s patent estate after the Federal Circuit decision?
The estate was materially weakened for the asserted formulation claims.
| Patent-estate factor |
Assessment |
| Chemical composition protection |
Limited relevance because vasopressin was an established active ingredient |
| Formulation protection |
Significantly weakened by the invalidity ruling |
| Method-of-use protection |
Not the principal basis of the Par v. Eagle judgment |
| Manufacturing protection |
No reported finding in this case that created a separate barrier to Eagle |
| Regulatory exclusivity |
Dependent on FDA exclusivity periods and product-specific status |
| Litigation leverage |
Reduced after the Federal Circuit affirmed invalidity |
| Generic-entry risk |
High for products covered by the adjudicated claims |
The decision did not necessarily invalidate every patent associated with Vasostrict. It invalidated the asserted claims in the case. Other patents, later-filed patents, or unasserted claims would require separate analysis. The ruling nevertheless removed the two principal patents Par selected for the Eagle litigation.
Did the case involve method-of-use or formulation patents?
The case primarily involved formulation protection.
Par’s claims were directed to the composition and physical or chemical characteristics of the vasopressin product. The dispute did not turn principally on a new therapeutic use of vasopressin. This distinction matters because formulation patents and method-of-use patents create different infringement and regulatory risks.
Formulation patents
Formulation claims can cover:
- Active-ingredient concentration.
- pH range.
- Preservative selection.
- Excipients.
- Stability period.
- Storage conditions.
- Ready-to-use presentation.
- Container or delivery configuration.
Method-of-use patents
Method-of-use claims can cover:
- Treatment of a specified condition.
- Patient selection.
- Dose or dosing interval.
- Administration sequence.
- Use in a particular clinical setting.
A generic applicant may attempt to avoid an Orange Book method-of-use patent through a section viii statement and a skinny label. That approach was not the central issue in Par v. Eagle.
What generic-entry risks did Eagle create for Par?
Eagle’s ANDA created four principal risks.
Loss of formulation-based exclusivity
The invalidity judgment removed the two asserted formulation patents as a barrier to the proposed generic product.
Price erosion
Vasopressin injection is an established hospital product. Generic entry can reduce contract pricing and increase purchasing leverage for hospitals, group purchasing organizations, and distributors.
Substitution risk
If Eagle’s product obtained an FDA therapeutic-equivalence rating supporting substitution, pharmacy and hospital substitution could shift volume from Vasostrict to the generic product. The commercial impact would depend on the FDA’s product-specific rating, supply reliability, contracting, and hospital procurement practices.
Follow-on litigation
Par could still evaluate other unasserted patents or new products, but any follow-on case would require a separate infringement and validity theory. The Federal Circuit decision would be persuasive and potentially preclusive on issues tied to the adjudicated claims and prior art.
What was the commercial exposure for Par?
Vasostrict was a commercially important hospital product for Par and its parent-company reporting structure. Public company filings identified Vasostrict as a material product, with annual net sales reported in the hundreds of millions of dollars during the period surrounding the litigation. The exact exposure from Eagle’s entry depended on launch timing, market share, pricing, supply, and competing generic approvals.
The commercial risk was greater than the loss of one formulation patent. Vasostrict’s value depended on a combination of:
- Hospital adoption.
- Ready-to-use convenience.
- Supply reliability.
- Contracting position.
- The absence of competing approved products.
- Patent and regulatory barriers.
Once the asserted patents were held invalid, the product’s commercial defense shifted from patent exclusivity to brand loyalty, contracting, manufacturing reliability, and any remaining regulatory or patent rights.
Were there licensing deals or settlement agreements?
No publicly reported settlement agreement is the basis for the final appellate outcome in this case. The published record reflects a litigated invalidity judgment affirmed by the Federal Circuit.
The case should therefore be distinguished from Hatch-Waxman disputes resolved through:
- A negotiated generic launch date.
- A covenant not to sue.
- A royalty-bearing license.
- An authorized-generic arrangement.
- A reverse-payment settlement.
The reported decision provides no basis to treat Eagle’s position as dependent on a settlement-defined entry date.
What manufacturing and intellectual-property barriers remained?
The invalidity ruling addressed the patents and claims asserted against Eagle. It did not remove ordinary operational barriers to generic entry.
Potential non-patent barriers included:
- FDA approval requirements.
- Chemistry, manufacturing, and controls compliance.
- Sterility assurance.
- Container-closure validation.
- Stability data.
- Injectable-product manufacturing capacity.
- Hospital contracting.
- Supply-chain qualification.
- Product shortages or manufacturing interruptions.
Vasopressin injection is a sterile injectable. Manufacturing complexity can delay or constrain generic competition even when patent barriers fall. Sterile injectable products often have fewer qualified suppliers than oral solid-dose products, which can moderate the speed and depth of price erosion.
How does Par v. Eagle compare with typical generic drug litigation?
Par v. Eagle is a standard high-value formulation patent dispute with an important appellate ruling.
| Issue |
Par v. Eagle |
| Product type |
Sterile injectable peptide drug |
| Active ingredient |
Vasopressin |
| Legal pathway |
ANDA Paragraph IV |
| Main patent category |
Formulation |
| Primary defense |
Obviousness |
| Biosimilar issue |
None |
| Trial result |
Claims invalid |
| Appellate result |
Affirmed |
| Settlement-driven entry |
Not reported as the basis of resolution |
| Commercial effect |
Greater generic-entry exposure for Vasostrict |
The case is particularly relevant to formulation developers because it shows that improvements in usability and stability may not support patentability when the underlying formulation work is viewed as predictable optimization.
Key Takeaways
- Par sued Eagle over a proposed generic version of Vasostrict, a vasopressin injection product.
- The case involved U.S. Patent Nos. 9,375,432 and 9,610,300.
- Eagle filed an ANDA with Paragraph IV certifications.
- The Delaware district court held the asserted claims invalid as obvious.
- The Federal Circuit affirmed in 2022 in Par Pharmaceutical, Inc. v. Eagle Pharmaceuticals, Inc., 44 F.4th 1207.
- The decision substantially weakened Par’s formulation-based exclusivity for Vasostrict.
- The case did not involve biosimilar litigation.
- The ruling was based on invalidity, not a publicly reported settlement-defined generic-entry date.
- Remaining patents, regulatory exclusivity, FDA listing status, manufacturing capacity, and hospital contracting require separate product-specific analysis.
- Vasopressin’s sterile injectable manufacturing requirements could still affect the timing and commercial impact of generic competition.
FAQs
What drug was at issue in Par v. Eagle?
The drug was Vasostrict, a vasopressin injection product marketed by Par Pharmaceutical.
Was Eagle’s product a biosimilar?
No. Eagle pursued an ANDA for a generic drug under the Hatch-Waxman framework. Vasopressin is not regulated as a biosimilar in this dispute.
Did Par win the patent case?
No. Eagle prevailed on invalidity. The asserted claims of the two patents were held obvious, and the Federal Circuit affirmed.
Did the Federal Circuit invalidate all Vasostrict patents?
No. The decision addressed the asserted claims of U.S. Patent Nos. 9,375,432 and 9,610,300. Other patents or claims would require separate review.
Can Par sue Eagle again over Vasostrict?
Potentially, if a separate unasserted patent or later patent creates a legally viable infringement claim. The prior judgment would materially affect any later case involving the same claims, formulation features, and prior art.
References
-
Par Pharmaceutical, Inc. v. Eagle Pharmaceuticals, Inc., 44 F.4th 1207 (Fed. Cir. 2022).
-
U.S. District Court for the District of Delaware. Par Pharmaceutical, Inc. v. Eagle Pharmaceuticals, Inc., No. 1:18-cv-00823-CFC, docket materials.
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U.S. Food and Drug Administration. Approved Drug Products with Therapeutic Equivalence Evaluations, commonly known as the Orange Book.
-
U.S. Patent and Trademark Office. U.S. Patent No. 9,375,432, “Vasopressin compositions and methods of use.”
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U.S. Patent and Trademark Office. U.S. Patent No. 9,610,300, vasopressin formulation patent records.
