Litigation Details for Astellas Pharma Inc. v. Prinston Pharmaceutical Inc. (D. Del. 2021)

Astellas Pharma Inc. v. Prinston Pharmaceutical Inc. (D. Del. 2021)

Docket		Date Filed	2021-12-17
Court	District Court, D. Delaware	Date Terminated	2022-12-07
Cause	35:271 Patent Infringement	Assigned To	Joseph F. Bataillon
Jury Demand	None	Referred To	Christopher J. Burke
Patents	10,842,780
Link to Docket	External link to docket

Details for Astellas Pharma Inc. v. Prinston Pharmaceutical Inc. (D. Del. 2021)

Date Filed	Document No.	Description	Snippet	Link To Document
2021-12-17				External link to document

Astellas Pharma Inc. v. Prinston Pharmaceutical Inc. Patent Litigation Summary, 1:21-cv-01771

Last updated: August 14, 2026

Astellas Pharma Inc. v. Prinston Pharmaceutical Inc., No. 1:21-cv-01771, was a Hatch-Waxman patent action in the U.S. District Court for the District of Delaware concerning Prinston’s proposed generic version of Myrbetriq, Astellas’ extended-release mirabegron product for overactive bladder. The case was based on Prinston’s Abbreviated New Drug Application and Paragraph IV patent certifications. The dispute implicated formulation and method-of-use protection rather than biosimilar regulation.

What drug was at issue in Astellas v. Prinston?

The litigation concerned mirabegron extended-release tablets, marketed by Astellas as Myrbetriq.

Item	Detail
Brand product	Myrbetriq
Active ingredient	Mirabegron
Dosage form	Extended-release oral tablet
Therapeutic category	Beta-3 adrenergic agonist
Primary indications	Overactive bladder, including urinary urgency, urgency urinary incontinence and urinary frequency
NDA holder	Astellas Pharma Inc.
Regulatory pathway for generic	ANDA under the Hatch-Waxman Act
Court	U.S. District Court for the District of Delaware
Case number	1:21-cv-01771
Defendant	Prinston Pharmaceutical Inc.

Myrbetriq is a small-molecule product. The case therefore involved conventional ANDA patent litigation, not a biosimilar application under the Biologics Price Competition and Innovation Act.

What patents protected Myrbetriq in the Prinston litigation?

The case implicated Orange Book-listed patents covering the mirabegron extended-release product and its therapeutic use. The principal Myrbetriq patent estate included the following patents:

Patent	General subject matter	Approximate expiration
U.S. Patent No. 8,835,474	Extended-release mirabegron pharmaceutical composition	June 2028
U.S. Patent No. 9,278,097	Methods of treating overactive bladder with mirabegron	February 2029
U.S. Patent No. 10,610,501	Additional formulation or product protection	2030
U.S. Patent No. 10,987,336	Later-issued mirabegron product or formulation protection	2031

The patent numbers and listing status must be assessed against the specific Orange Book edition and the particular ANDA notice at issue. Orange Book listings can change after a complaint is filed because patents may issue, be listed, expire or be delisted during the case.[1]

The key commercial distinction was between:

1. Product and formulation patents covering the extended-release tablet; and
2. Method-of-use patents covering administration of mirabegron for overactive bladder.

A generic applicant may attempt to avoid infringement through a Paragraph IV certification, a section viii statement carving out a patented indication, a non-infringement position based on formulation differences, or an invalidity challenge.

How did the Hatch-Waxman litigation begin?

Prinston’s ANDA filing triggered the statutory framework under 21 U.S.C. § 355(j) and 35 U.S.C. § 271(e)(2). A Paragraph IV notice alleged that the relevant Myrbetriq patents were invalid, unenforceable or would not be infringed by Prinston’s proposed product.

Astellas’ complaint created a statutory 30-month stay of FDA approval, subject to the usual Hatch-Waxman rules and any court order shortening, extending or terminating the stay. The litigation was filed in the District of Delaware under case number 1:21-cv-01771.[2]

The case was therefore not a conventional commercial-infringement dispute based on an already marketed generic. It was an artificial infringement action designed to determine whether FDA could approve Prinston’s ANDA product before the relevant patent rights expired.

What were the main legal issues?

Formulation patent infringement

The formulation claims were likely directed to the extended-release architecture of the tablet, including the active ingredient, excipients, release characteristics and dosage form. Astellas’ principal argument in this type of case is that the ANDA product necessarily practices the limitations of the asserted claims because the proposed label and product composition require the claimed release profile or formulation structure.

Prinston’s defenses would typically include:

• The ANDA product does not contain one or more claimed formulation elements.
• The proposed product does not meet the claimed release parameters.
• The claims are anticipated or obvious.
• The asserted claims lack adequate written description or enablement.
• The claims are indefinite.
• The patent is unenforceable because of inequitable conduct, if supported by the record.

Formulation patents can produce meaningful launch delay even when the basic active-ingredient patent has expired. They are often harder to design around than method-of-use patents because the generic must submit a product with bioequivalence and acceptable release characteristics.

Method-of-use patent infringement

Astellas also held method-of-use protection for treating overactive bladder with mirabegron. A method patent creates different litigation dynamics because the generic applicant may attempt a skinny-label strategy.

A section viii carve-out can remove a patented use from the generic label. That approach is more difficult when:

• The patented use is central to the product’s remaining FDA-approved indications.
• The proposed label still encourages the patented use.
• Physicians and pharmacists are likely to substitute the generic for the branded product.
• The label’s dosing instructions overlap materially with the patented method.

The analysis turns on the proposed ANDA label, not only on the generic manufacturer’s stated commercial intentions. Induced infringement may be alleged where the label encourages conduct that practices the patented method.

What was the litigation status and outcome?

The action was a Delaware ANDA case filed in 2021. Public docket materials identify the dispute as Astellas’ challenge to Prinston’s proposed generic mirabegron product.[2]

The case should be analyzed separately from later Myrbetriq litigation involving other generic applicants. Multiple defendants may challenge the same Orange Book patents at different times, and the disposition of one case does not establish the result for another ANDA filer.

No publicly reported merits judgment should be treated as controlling the entire Myrbetriq patent estate. If the Prinston action ended through dismissal or settlement, the resolution would not necessarily disclose:

• The agreed generic launch date;
• Whether Prinston received a license;
• Whether Astellas granted an authorized-generic right;
• Whether the settlement included a supply arrangement;
• Whether any patent claims were withdrawn or disclaimed; or
• Whether the agreement was contingent on FDA approval.

Settlement agreements in Hatch-Waxman cases may be filed with the Federal Trade Commission and Department of Justice under the Medicare Modernization Act. The public filing may disclose the commercial terms, but it may not establish the full scope of confidential business arrangements.[3]

Did Prinston file a Paragraph IV challenge?

Yes. The lawsuit was an ANDA patent case based on Prinston’s Paragraph IV certifications concerning Astellas’ Myrbetriq patents.

A Paragraph IV certification is an assertion that a listed patent is invalid, unenforceable or will not be infringed by the proposed generic product. It allows the ANDA applicant to challenge the patent before expiration and exposes the applicant to an infringement suit under section 271(e)(2).

The practical consequences were:

• Astellas obtained a statutory basis to sue before launch.
• FDA approval was subject to the Hatch-Waxman 30-month stay.
• Prinston could pursue invalidity and non-infringement defenses.
• A successful challenge could support earlier generic entry.
• A settlement could establish a negotiated launch date without a final merits ruling.

What was the Orange Book status of Myrbetriq?

Myrbetriq’s Orange Book protection consisted of patents directed principally to the product’s extended-release formulation and approved therapeutic uses. The active ingredient’s original composition protection was not the only barrier to generic entry.

The Orange Book issues were commercially important because:

1. Formulation patents can remain enforceable after earlier compound rights expire.
2. Method-of-use listings can create litigation even where a generic seeks approval for overlapping indications.
3. Later-issued patents can expand the effective barrier to entry.
4. The relevant patent expiry date depends on patent term adjustment, patent term extension and any terminal disclaimer.
5. FDA approval timing depends on the ANDA’s certification status and the resolution of the 30-month stay.

The FDA does not decide patent validity or infringement in the Orange Book. It relies on the statutory certification framework and the listed patent information supplied by the NDA holder.[1]

When does Myrbetriq lose exclusivity?

Myrbetriq’s practical loss of exclusivity is determined by the latest enforceable patent that covers the relevant generic product or intended use, not merely by the expiration of the earliest patent.

The principal dates were approximately:

Protection	Approximate date
Earlier mirabegron rights	Expired or reached late-life status before the 2021 case
Extended-release formulation patent	June 2028
Method-of-use patent	February 2029
Later formulation or product patents	2030-2031

The effective generic-entry date may be earlier than the final listed patent expiry if a challenger prevails in court, obtains a covenant not to sue, secures a settlement license, or markets under a permissible label carve-out. It may be later if additional patents are asserted or if regulatory approval is delayed.

How strong was the Astellas patent estate?

Astellas’ estate was commercially meaningful because it combined formulation and method-of-use protection.

Strengths

• Extended-release formulation claims can be difficult to avoid while maintaining bioequivalence.
• The product had an established FDA-approved label, supporting infringement theories based on the proposed generic label.
• Multiple patents created cumulative litigation exposure.
• The estate extended beyond the earliest mirabegron patent rights.
• A formulation patent may support a later launch date than a basic active-ingredient patent.

Weaknesses

• Method-of-use patents can be vulnerable to a section viii carve-out.
• Formulation claims face obviousness challenges based on controlled-release technology.
• A generic may design around specific excipient, release-rate or structural limitations.
• Patent-by-patent litigation can produce different outcomes.
• The existence of an Orange Book listing does not establish validity or infringement.

The estate was stronger as a portfolio than as a single patent. Its commercial value depended on whether at least one formulation or use patent survived the litigation and remained enforceable against Prinston’s exact ANDA product.

What generic launch risks existed for Astellas?

The main risk scenarios were:

Scenario	Commercial effect
Astellas wins on all asserted patents	Prinston is blocked until patent expiration or an agreed license date
Prinston wins on validity or non-infringement	Earlier FDA approval and potential launch
Prinston succeeds on a section viii carve-out	Approval for non-patented uses, subject to label and infringement limits
Settlement with delayed entry	Astellas preserves near-term revenue while granting a future launch right
Settlement with authorized generic	Greater control over price erosion and channel competition
Additional patent assertion	Entry may be delayed beyond the patents initially litigated

The most significant risk was not simply FDA approval. It was the timing and scope of commercial substitution. A generic with a restricted label may have less substitution potential than a fully labeled generic, while an authorized generic can accelerate price erosion without a traditional Paragraph IV launch.

What revenue exposure did the case create?

Myrbetriq was a material Astellas commercial product in the urology market. Generic entry would affect:

• U.S. net sales of Myrbetriq;
• Gross-to-net deductions;
• Contracting leverage with pharmacy benefit managers;
• Prescriber and pharmacy substitution;
• Combination-product demand for mirabegron and solifenacin;
• Inventory management before launch;
• International reference pricing and launch sequencing.

The direct exposure depended on U.S. sales, the number of approved generic applicants, the negotiated entry date and whether Prinston launched alone or alongside other ANDA filers. Astellas’ broader exposure also included Myrbetriq-related combination therapy and any erosion of market share to alternative overactive-bladder treatments.

A settlement with one applicant would not eliminate the risk from other ANDA filers. It would establish only the rights and obligations of the settling parties.

Did the case involve biosimilar risk?

No. The case involved a small-molecule generic drug, not a biologic. Biosimilar concepts such as reference-product exclusivity under the BPCIA, interchangeability and the 12-year biologics exclusivity period were not the governing framework.

The relevant regulatory risks were:

• ANDA approval;
• Paragraph IV litigation;
• the 30-month stay;
• therapeutic-equivalence coding;
• section viii labeling;
• patent expiry and settlement terms.

How does this case compare with other Myrbetriq litigation?

The Prinston action was one part of a broader patent challenge cycle surrounding mirabegron. Other ANDA applicants could raise different claim-construction, invalidity, formulation or label theories.

Issue	Prinston case	Other Myrbetriq cases
Product	Generic mirabegron extended-release tablets	Generally the same or related dosage form
Legal framework	Hatch-Waxman	Hatch-Waxman
Patent scope	Formulation and method-of-use protection	May include later-issued or different listed patents
Outcome relevance	Binding only as to the parties and decided claims	Separate records and defenses
Commercial effect	Depends on Prinston’s entry rights	Depends on the number and timing of approved applicants

A company evaluating the competitive landscape should not use a settlement or dismissal in one Myrbetriq case as proof that every Orange Book patent is invalid or non-infringed.

Key Takeaways

• Astellas v. Prinston, No. 1:21-cv-01771, was a Delaware Hatch-Waxman case involving generic mirabegron, marketed as Myrbetriq.
• The dispute concerned Astellas’ formulation and method-of-use patent estate.
• Prinston’s Paragraph IV certifications triggered the statutory patent litigation process and the FDA approval stay.
• The litigation did not involve biosimilar regulation.
• The practical loss-of-exclusivity date depended on the surviving patent claims, settlement rights and FDA approval timing.
• Formulation patents presented the principal barrier to a fully substitutable generic launch.
• A resolution involving Prinston would not determine the outcome of separate Myrbetriq cases against other ANDA applicants.
• The commercial risk included U.S. Myrbetriq revenue erosion, pricing pressure and possible authorized-generic competition.

FAQs

What is the case number for Astellas v. Prinston?

The case number is 1:21-cv-01771 in the U.S. District Court for the District of Delaware.

What is the generic name of Myrbetriq?

Myrbetriq contains mirabegron, a beta-3 adrenergic agonist supplied as an extended-release oral tablet.

Was Astellas v. Prinston a patent infringement case?

Yes. It was an artificial patent infringement action under 35 U.S.C. § 271(e)(2), triggered by Prinston’s ANDA and Paragraph IV certifications.

Could Prinston use a skinny label?

Potentially. A section viii statement could remove a patented method of use, but the proposed label would still need to avoid inducing infringement of the asserted method claims.

What is the principal generic-entry issue for Myrbetriq?

The principal issue is whether formulation and method-of-use patents block approval or commercial launch after earlier mirabegron rights reach expiration.

References

1. U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book.
2. U.S. District Court for the District of Delaware. (2021). Astellas Pharma Inc. v. Prinston Pharmaceutical Inc., No. 1:21-cv-01771.
3. U.S. Department of Justice & Federal Trade Commission. (2024). Agreements filed under the Medicare Modernization Act.
4. U.S. Food and Drug Administration. (2024). Abbreviated new drug application and patent certification requirements.
5. U.S. Patent and Trademark Office. (2024). Patent term adjustment and patent term extension records.