United States Drug Patent 5,637,320: Claim Scope, Exclusivity, Litigation Risk and Patent Landscape

U.S. Patent No. 5,637,320 protects controlled-release naproxen multiparticulate pellets designed for once-daily oral administration. The broadest independent claim requires five core elements: naproxen or a salt, an organic acid, a defined drug-to-acid ratio, a multilayer polymer membrane, and specified dissolution performance. Claims 12 through 17 extend protection to formulations combining the controlled-release pellets with immediate-release naproxen and to capsules or tablets containing the formulations.

The patent is formulation-specific. It does not broadly cover naproxen, naproxen sodium, once-daily dosing, or every extended-release naproxen dosage form. Infringement generally requires practicing the structural limitations and the dissolution profile recited in the relevant claim. The most commercially significant risk areas are naproxen sodium multiparticulate products, pellet systems using organic-acid cores, and capsule or tablet products combining immediate-release and controlled-release naproxen.

What does U.S. Patent 5,637,320 protect?

The patent protects a controlled-absorption naproxen dosage form based on coated pellets. Claim 1 is the central composition and performance claim.

Required element in claim 1	Scope
Active ingredient	Naproxen or a pharmaceutically acceptable salt
Dosage architecture	Multiparticulate pellets
Pellet core	Naproxen associated with an organic acid
Drug-to-acid ratio	20:1 to 1:1
Membrane	Multilayer membrane surrounding the core
Polymer	Water-insoluble film-forming polymer, optionally with a water-soluble film-forming polymer
Intended administration	Once-daily oral administration
Dissolution apparatus	USP XXI Type 1 basket
Dissolution medium	Phosphate buffer at pH 7.2
Agitation	75 rpm
Release profile	0% to 50% at one hour, 20% to 70% at two hours, and at least 50% at four hours

The claim is cumulative. A product that contains naproxen pellets but lacks the required organic-acid association, multilayer membrane, or dissolution profile does not satisfy claim 1 solely because it is extended release.

The words “comprising” and “optionally” broaden the claim. A covered formulation may contain additional excipients, coatings, release components, or dosage-form ingredients unless another limitation excludes them.

How broad is independent claim 1?

Claim 1 is broad at the ingredient and manufacturing level but narrow at the performance level.

Ingredient scope

The claim covers naproxen and pharmaceutically acceptable salts. Claim 11 expressly identifies naproxen sodium, but claim 1 already includes salts generally. The express naproxen-sodium claim provides a narrower species claim rather than a materially different active-ingredient category.

The claim does not require a particular organic acid. Claims 3 and 7 identify permitted acid classes, including:

Adipic acid
Ascorbic acid
Citric acid
Fumaric acid
Maleic acid
Succinic acid
Tartaric acid
Malic acid in claim 7

The difference between “maleic acid” in claim 3 and “malic acid” in claim 7 is material on the face of the claims. Maleic acid and malic acid are different chemical compounds. The discrepancy may reflect prosecution history, an amendment, or a drafting error. Its legal effect requires review of the patent file history and any certificate of correction.

Structural scope

The pellet core may contain naproxen and organic acid directly, or the materials may be embedded in polymers. Claims 5 through 10 cover several architectures:

Naproxen and organic acid embedded in one or more polymers.
A core using water-insoluble polymers.
Alternating or superimposed layers of powder mixture and polymer.
A central active core formed from naproxen, organic acid, and polymer.
An inert core carrying the active ingredients and polymers.
A layered structure produced with a polymer-binding solution.

This structure gives the patent potential reach across extrusion, pelletization, powder layering, solution layering, and inert-core coating processes.

Dissolution scope

The dissolution profile is the primary limiting feature. Claim 1 specifies a range rather than a single release curve:

At one hour: 0% to 50% released.
At two hours: 20% to 70% released.
At four hours: at least 50% released.

A product can fall within the claim even if its release curve differs substantially from an example, provided the measured values satisfy the limitations.

The claim also incorporates older pharmacopoeial test conditions. Apparatus type, pH, agitation speed, sampling time, and calculation of total naproxen are therefore important infringement variables. Testing under a different apparatus or buffer condition may produce different results and cannot automatically establish noninfringement.

What additional protection do claims 2 through 11 provide?

Claims 2 through 11 narrow the pellet formulation by adding dissolution, acid, ratio, polymer, core, or active-ingredient limitations.

Claim	Added limitation	Commercial relevance
2	Type 2 paddle dissolution profile at pH 7.4 and 50 rpm	Provides an alternative performance definition
3	Specific organic-acid group	Narrows the acid component
4	Naproxen-to-acid ratio of 6:1 to 1:1	Focuses on higher acid loading
5	Active and acid embedded in polymers	Covers polymer-containing cores
6	Embedded polymers are water-insoluble	Narrows polymer selection
7	Powder mixture and alternating polymer layers	Targets layered pellet construction
8	Materials built on a central active core	Covers active-core layering
9	Homogeneous blend shaped into a central core and layered with binder solution	Covers a defined manufacturing route
10	Materials built on an inert core	Covers inert-seed pellet technology
11	Naproxen sodium	Narrows the active ingredient

Claim 2 is significant because it creates a second dissolution test using a USP Type 2 paddle apparatus, phosphate buffer at pH 7.4, and 50 rpm:

20% to 70% released at one hour.
At least 50% released at two hours.
At least 75% released at four hours.

A product might satisfy claim 1 but fail claim 2, or satisfy claim 2 but fail claim 1, depending on its release behavior under the two test methods.

What formulations are protected by claims 12 through 17?

Claims 12 through 17 address a combination product containing controlled-release pellets and rapid-release naproxen.

Claim 12 requires:

Controlled-release naproxen pellets within the scope of claim 1; and
A rapid-release form of naproxen.

Claim 15 limits the rapid-release component to up to 60% by weight. Claims 13 and 14 impose additional dissolution profiles for the combined formulation.

Claim	Formulation requirement
12	Controlled-release pellets plus rapid-release naproxen
13	Type 1 basket profile with release measured at 0.5, 1, and 4 hours
14	Type 2 paddle profile with release measured at 0.5, 1, and 4 hours
15	Blend contains up to 60% by weight rapid-release naproxen
16	Capsule or tablet containing claim 1 formulation
17	Capsule or tablet containing claim 12 formulation

The combination claims are commercially important because they cover a dosage form intended to provide an initial naproxen release followed by sustained exposure. A generic product could avoid claims 12 through 17 by omitting the immediate-release component, but it would still need to assess claims 1 through 11 if it uses the covered pellets.

When did U.S. Patent 5,637,320 lose exclusivity?

The patent’s enforceable term depends on its filing date, earliest effective nonprovisional filing date, patent-term adjustment, terminal disclaimers, and any applicable patent-term extension. Under the modern patent-term statute, utility patents generally expire 20 years from the earliest effective U.S. nonprovisional filing date, subject to statutory adjustments. The patent issued on June 10, 1997, but the issue date does not determine expiration.

The patent number and claim text alone do not establish the exact expiration date, patent-term adjustment, terminal disclaimer status, or any patent-term extension. Those dates must be taken from the USPTO patent record and the patent’s front-page term information. A patent that has expired cannot support a new infringement action, although historical litigation, settlement obligations, and regulatory records may remain relevant.

Does the patent have FDA Orange Book protection?

An Orange Book listing cannot be determined from the claims alone. Orange Book eligibility generally requires a patent to claim the approved drug substance, drug product, or an approved method of use. A controlled-release formulation patent may be listed if it covers the approved dosage form and is submitted by the NDA holder in accordance with FDA requirements.

For naproxen or naproxen sodium products, the relevant review must distinguish among:

Drug-substance patents.
Drug-product formulation patents.
Method-of-use patents.
Manufacturing-process patents.
Patents listed for a specific extended-release NDA.

The patent’s formulation claims would be more likely to support a drug-product listing than a drug-substance listing. Claims 1 through 17 do not claim a therapeutic method, disease indication, or dosing method in conventional method-of-use form.

What Paragraph IV challenges could target this patent?

A generic applicant using an abbreviated new drug application could challenge a listed patent through a Paragraph IV certification. The principal invalidity and noninfringement theories would likely focus on claim construction, dissolution testing, written description, enablement, and obviousness.

Potential noninfringement positions

A generic product could attempt to design around the claims by:

Using a monolithic matrix rather than multiparticulate pellets.
Omitting the organic acid.
Using an organic acid outside the claimed selections in a dependent claim while assessing claim 1 separately.
Using a single coating rather than a multilayer membrane.
Selecting a release profile outside the claimed ranges.
Using a different active form or formulation architecture.
Providing only controlled release without the rapid-release component.
Using an immediate-release or delayed-release formulation rather than the claimed controlled-absorption profile.

The dissolution limitations require careful testing. A product that releases more than 50% at one hour may fall outside claim 1, but the same product could still require analysis under another claim or patent family member.

Potential invalidity positions

The most relevant prior-art categories include:

Sustained-release naproxen tablets.
Naproxen sodium controlled-release dosage forms.
Multiparticulate pellet systems.
Organic-acid-modified drug cores.
Water-insoluble polymer coatings.
Layered inert-seed pellet technology.
Immediate-release and controlled-release combination products.
Prior dissolution specifications for once-daily NSAID products.

The combination of pellet structure, organic acid, polymer membrane, and dissolution profile is the likely inventive concept. Obviousness analysis would ask whether a skilled formulation scientist would have combined those elements with a reasonable expectation of achieving the claimed release pattern.

How strong is the patent estate?

The claim set has moderate structural breadth but limited enforcement breadth because of its dissolution requirements.

Strength factor	Assessment
Active ingredient coverage	Broad across naproxen and salts
Pellet requirement	Narrows the claim materially
Organic-acid requirement	Creates a substantial design-around path
Multilayer membrane	Limits claim reach to specific coating architectures
Dissolution profile	Strong factual limitation but creates testing complexity
Manufacturing coverage	Claims 5 through 10 cover several pellet-building methods
Immediate-release combination	Claims 12 through 17 target a narrower commercial configuration
Product-form coverage	Capsules and tablets are expressly covered
Biosimilar exposure	Not applicable; naproxen is a small-molecule NSAID
Generic exposure	High if the patent remains listed and claims the approved product

The patent does not create biologic exclusivity. Biosimilar litigation and the Biologics Price Competition and Innovation Act are irrelevant to naproxen. The principal competitive threat is generic entry under the ANDA pathway.

What patent landscape surrounds controlled-release naproxen?

The surrounding landscape should be divided into five technology groups:

Drug-substance patents covering naproxen or naproxen sodium.
Conventional sustained-release matrices.
Multiparticulate pellet systems.
Polymer membrane and layered-core technologies.
Product-specific patents for once-daily naproxen sodium formulations.

U.S. Patent 5,637,320 occupies the third through fifth categories. Its claims are directed to the formulation architecture and release performance rather than to the naproxen molecule itself.

A complete freedom-to-operate review should map:

Continuations and divisionals.
Foreign counterparts.
Cited and citing references.
Patent-family members.
Orange Book listings for the relevant NDA.
ANDA litigation under the Hatch-Waxman Act.
Settlement agreements affecting generic launch dates.
Patent-term adjustment and terminal-disclaimer records.
Any post-grant disclaimer, reexamination, or certificate-of-correction activity.

The supplied claim text does not identify the assignee, inventors, priority application, continuation relationships, foreign family, Orange Book listing, Paragraph IV litigation, settlement agreements, or exact expiration date. Those matters cannot be established from the claims themselves.

What generic launch risks exist?

Generic launch risk depends on whether the patent remains enforceable, whether it is listed against the relevant reference product, and whether the proposed generic uses the claimed pellet technology.

Generic design	Principal risk
Naproxen sodium multiparticulate pellets with organic-acid cores	High claim 1 and claim 11 risk
Layered inert-core pellets	Claims 5, 7, and 10 risk
Active-core layered pellets	Claims 5, 8, and 9 risk
Controlled-release pellets plus immediate-release naproxen	Claims 12 through 17 risk
Matrix tablet without pellets	Lower risk under the supplied claims
Immediate-release naproxen sodium tablet	Low risk under the supplied claims
Pellet product with a different release profile	Reduced literal-infringement risk, subject to other patents
Formulation without organic acid	Reduced risk under claim 1, subject to claim construction and other patents

The highest-risk launch scenario is a capsule or tablet containing naproxen sodium pellets with an organic-acid-containing core, a multilayer water-insoluble polymer membrane, and a release curve within the claimed ranges.

Key Takeaways

U.S. Patent 5,637,320 is a controlled-release naproxen pellet patent.
Claim 1 requires naproxen or a salt, an organic acid, a defined ratio, a multilayer polymer membrane, and a specified dissolution profile.
Claims 2 through 11 narrow the invention by adding a second dissolution method, acid selections, ratios, polymer types, pellet structures, and naproxen sodium.
Claims 12 through 17 cover controlled-release pellets combined with rapid-release naproxen in capsules or tablets.
The patent is not a broad patent on naproxen, naproxen sodium, once-daily dosing, or all extended-release naproxen products.
The organic-acid, multiparticulate-pellet, multilayer-membrane, and dissolution limitations create potential design-around routes.
Naproxen is a small molecule, so biosimilar risk does not apply. Generic ANDA and Paragraph IV risk is the relevant pathway.
The exact expiration date, Orange Book status, patent family, assignee, litigation history, and settlement record cannot be determined from the supplied claim text.

FAQs About U.S. Patent 5,637,320

Does U.S. Patent 5,637,320 cover naproxen sodium tablets?

It can cover a tablet containing qualifying naproxen sodium pellets under claims 11 and 16. It does not cover every naproxen sodium tablet.

Does a sustained-release matrix tablet infringe this patent?

Not necessarily. The supplied claims require multiparticulate pellets. A matrix tablet that lacks the claimed pellet structure may fall outside the literal scope, although other patents may apply.

Is an organic acid mandatory under claim 1?

Yes. Claim 1 requires naproxen or its salt to be associated with an organic acid.

Can a formulation avoid the patent by using a different dissolution test?

Changing the test method does not alone establish noninfringement. The relevant question is whether the product satisfies the claim limitations under the specified USP methods and whether equivalent-performance arguments apply.

Does the patent cover immediate-release naproxen?

No. Claims 12 through 17 require rapid-release naproxen only as part of a formulation that also includes the claimed controlled-release pellets.

References

U.S. Patent No. 5,637,320, claims 1-17. United States Patent and Trademark Office.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. Center for Drug Evaluation and Research.
United States Patent and Trademark Office. (2024). Manual of Patent Examining Procedure: Patent term and adjustment provisions.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
Ireland	149/90	Jan 15, 1990

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	126055	⤷ Start Trial
Australia	639519	⤷ Start Trial
Australia	6933291	⤷ Start Trial
Canada	2034096	⤷ Start Trial
Germany	69111832	⤷ Start Trial
Denmark	0438249	⤷ Start Trial
European Patent Office	0438249	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,637,320

Summary for Patent: 5,637,320