United States Patent 5,284,662: Carbamazepine Osmotic Delivery System, Claim Scope and Patent Landscape

U.S. Patent No. 5,284,662 covers a controlled-release oral osmotic dosage form containing carbamazepine. Its independent claim requires a highly specific combination of core excipients, a semipermeable membrane, and a delivery orifice. The patent issued on February 8, 1994, and its ordinary 17-year term from grant would have expired on February 8, 2011, absent an unusual term adjustment or other extension. The patent therefore does not present a current U.S. blocking right.

The commercial significance of the patent lies in its formulation architecture rather than in carbamazepine itself. The claimed system combines carbamazepine with mixed-viscosity hydroxyalkyl celluloses, a C6 sugar alcohol, a mono- or disaccharide, and a membrane engineered for osmotic release.

What does U.S. Patent 5,284,662 protect?

The patent protects a physical dosage form with three mandatory components:

A carbamazepine-containing core.
A semipermeable wall surrounding the core.
A hole through the wall connecting the core to the external environment.

The independent claim is a product claim. It does not claim carbamazepine as a chemical compound, a particular therapeutic indication, or a dosing method. It claims the structure and composition of a controlled-release osmotic tablet.

The core must include:

Required core element	Claim requirement
Active ingredient	Carbamazepine in an amount sufficient to deliver an effective amount over the intended delivery time
Crystal habit modifier	Selected from specified cellulose derivatives or gelatin
Mixed hydroxyalkyl celluloses	About 2% to about 15% of total core weight
Viscosity relationship	Higher-viscosity cellulose to lower-viscosity cellulose ratio of about 2:1
Sugar alcohol	C6 sugar alcohol
Sugar	Mono- or disaccharide
Combined carbohydrate content	About 15% to about 60% of total core weight
Sugar alcohol:sugar ratio	From about 1:9 to about 9:1
Membrane	Semipermeable wall permeable to water or gastric fluid
Orifice	Hole through the membrane

A formulation that omits any required element may avoid literal infringement of claim 1, even if it produces a similar release profile.

How narrow is independent claim 1?

Claim 1 is compositionally and structurally narrow. A competing product would need to satisfy several quantitative limitations simultaneously.

Carbamazepine requirement

The claim requires carbamazepine as the active ingredient. Claims 2 and 3 narrow this requirement by specifying:

100 mg to 400 mg of carbamazepine, calculated as free base; and
hydroxypropyl cellulose as the crystal habit modifier at approximately 9% to 15% by weight relative to carbamazepine free base.

The independent claim does not specify a particular carbamazepine polymorph, particle-size distribution, salt form, or crystal form. It also does not require a particular release rate or pharmacokinetic profile.

Crystal habit modifier

The modifier must be selected from:

C1-C4 alkyl cellulose;
hydroxypropyl cellulose;
hydroxypropyl-C1-C4 alkyl cellulose;
sodium carboxymethyl cellulose;
sodium carboxymethyl-C1-C4 alkylcellulose; or
gelatin.

The modifier limitation is important because it is separate from the mixed-viscosity cellulose limitation. A formulation may therefore need to contain both:

A qualifying crystal habit modifier; and
Two hydroxyalkyl cellulose grades in the claimed ratio.

Hydroxypropyl cellulose can perform both roles only if the formulation also satisfies the separate mixed-cellulose requirement.

Mixed-viscosity cellulose limitation

Claim 1 requires 2% to 15% of the total core weight of a mixture of at least two different hydroxy-C1-C4 alkyl celluloses. The claimed alternatives include:

different hydroxyalkyl cellulose chemistries; or
different viscosity grades of the same hydroxyalkyl cellulose.

Claim 4 confirms that different grades of the same cellulose are covered. Claim 5 narrows the group to hydroxyethylcellulose, and claim 6 identifies grades including 250H, 250L, 250J, 250E, 250G, 250K, 250M, 250MH, 250H4, and 250HH.

The ratio of higher-viscosity to lower-viscosity hydroxyalkyl cellulose is approximately 2:1 in claim 1. Claim 23 presents a narrower commercial-style embodiment using hydroxyethylcellulose 250L and 250H, with a stated 250L:250H range of 1:4 to 4:1. That range appears broader and directionally different from the general higher-to-lower viscosity ratio in claim 1. Claim construction would depend on the specification, the identified grades, and whether the “about 2:1” limitation is treated as controlling for claim 23.

What formulations are protected by the dependent claims?

The dependent claims build a specific osmotic tablet platform.

Claim	Added limitation
2	100-400 mg carbamazepine
3	Hydroxypropyl cellulose at about 9%-15% relative to carbamazepine free base
4	Different grades of the same hydroxy-C1-C4 alkyl cellulose
5	Hydroxyethylcellulose
6	Listed hydroxyethylcellulose grades
7-8	Mannitol, sorbitol, or galactitol; specifically mannitol in claim 8
9-10	Broad sugar list; specifically dextrates NF in claim 10
11	Magnesium stearate
12	Sodium lauryl sulfate
13	Coloring agent in the core
14	Cellulose-based membrane with poly-C2-C4 alkylene glycol
15-16	Polyethylene glycol 8000, at 6%-10% of wall weight
17-19	Hydroxypropylmethylcellulose, at 6%-10% of wall weight and approximately 10-20 cps
20-22	Two cellulose acetates, including 320S NF and 398-10 NF in defined ratios
23	A detailed carbamazepine, excipient, membrane, and orifice formulation

Claims 14 through 22 are particularly relevant to formulation design because they define the membrane rather than the active-containing core.

What membrane technology does the patent claim?

The membrane must be semipermeable and permeable to water or gastric fluid. Claim 14 identifies a membrane containing:

at least one cellulose acetate;
a hydroxy-C1-C4 alkyl cellulose or C1-C4 alkyl cellulose; and
a poly-C2-C4 alkylene glycol.

The preferred membrane subcombination includes:

hydroxypropylmethylcellulose;
polyethylene glycol 8000;
cellulose acetate.

Claims 20 through 22 specify two cellulose acetate grades:

cellulose acetate 320S NF; and
cellulose acetate 398-10 NF.

The claimed ratio is approximately 5:1 to 8:1, with a narrower range of approximately 6.62:1 to 6.64:1 in claim 22.

This membrane limitation creates a substantial design-around path. A competing osmotic tablet using a different membrane polymer, a single cellulose acetate grade, a different pore-former, or a different ratio may avoid the narrower dependent claims. It would still need to be assessed against claim 1 and claim 14.

How would infringement be analyzed?

Literal infringement

Literal infringement would require the accused product to contain every limitation of an asserted claim. For claim 1, the principal questions would be:

Does the product contain carbamazepine?
Is a listed crystal habit modifier present?
Are at least two qualifying hydroxyalkyl celluloses present?
Do they satisfy the approximately 2:1 viscosity ratio?
Are the sugar alcohol and mono- or disaccharide within the combined-weight and ratio ranges?
Is there a semipermeable wall?
Is there a hole through that wall?

The analysis would require product composition records, manufacturing specifications, membrane composition data, and microscopy or destructive testing to confirm the orifice and multilayer structure.

Range limitations

The use of “about” creates ordinary claim-construction questions around the numerical ranges. Courts generally interpret “about” in view of the specification, technology, measurement precision, and prosecution history. A formulation outside the literal range may still raise a doctrine-of-equivalents issue, but prosecution-history estoppel and the substantiality of the numerical difference would matter.

Doctrine of equivalents

A product using a functionally similar viscosity modifier or membrane pore former could face an equivalents argument. The risk is lower where the substitute changes the claimed formulation mechanism or where the patent prosecution history shows that the patentee narrowed the claim to obtain allowance.

A substantially different controlled-release technology, such as a hydrophilic matrix tablet, multiparticulate bead system, nonosmotic membrane system, or extended-release capsule without a drilled orifice, would have a stronger noninfringement position against these claims.

When did U.S. Patent 5,284,662 lose exclusivity?

U.S. Patent 5,284,662 issued on February 8, 1994. For a pre-URAA patent, the general term was 17 years from issue, subject to statutory adjustments and applicable extensions. On that basis, the ordinary expiration date was February 8, 2011.[1]

The patent’s term was not governed by the post-1995 rule of 20 years from the earliest effective nonprovisional filing date. A patent-term adjustment, terminal disclaimer, or regulatory extension would need to be confirmed from the official USPTO patent record before modifying that date. The patent is now outside its ordinary enforceable term.

Exclusivity timeline

Event	Date or status
U.S. patent issued	February 8, 1994
Ordinary pre-URAA term	17 years from issue
Ordinary expiration	February 8, 2011
Current patent enforceability	Expired under ordinary term calculation
Current Paragraph IV exposure	None based solely on this expired patent
Current Orange Book blocking effect	None based solely on this expired patent

Patent expiration does not invalidate historical infringement claims arising before expiration. It eliminates prospective exclusionary rights after expiration.

What is the Orange Book status of the patent?

A patent is relevant to the FDA Orange Book only when it is submitted by an NDA holder and meets FDA listing criteria for the approved drug product. A patent covering a formulation concept is not automatically Orange Book-listed.

The patent claims an osmotic carbamazepine dosage form. The principal U.S. carbamazepine products include immediate-release tablets, extended-release tablets or capsules, and suspension products. Products such as Tegretol, Tegretol-XR, Carbatrol, and Equetro have had different sponsors, dosage forms, and formulation technologies. An Orange Book listing for one of those products would not establish that U.S. Patent 5,284,662 covers the product.

The relevant regulatory conclusions are:

The patent itself is expired.
Expired patents cannot support a current Paragraph IV stay.
A current generic applicant would focus on unexpired Orange Book-listed patents associated with the particular reference-listed drug.
FDA approval of an ANDA does not resolve private claim-construction or infringement disputes involving patents that are not Orange Book-listed.

FDA patent listings and exclusivity periods must be reviewed by reference-listed drug and dosage form, not by active ingredient alone.[2]

Which companies are challenging the patent?

The patent’s ordinary term expired in 2011. There is no current commercial Paragraph IV challenge directed at an enforceable right under U.S. Patent 5,284,662 because an expired patent cannot create a present launch bar.

Historical ANDA litigation, if any, would have to be tied to a particular carbamazepine reference product and a specific FDA Orange Book listing. The patent number alone does not establish that a generic company challenged it, that litigation occurred, or that a settlement restricted generic entry.

The practical conclusion is that the patent does not presently control U.S. generic competition. Any current challenge would concern later patents, listed patents for a specific product, or non-patent regulatory exclusivity.

How strong is the patent estate for carbamazepine osmotic delivery?

The patent is technically specific but commercially weak today because it is expired.

Factor	Assessment
Claim breadth	Narrow to moderate
Core formulation specificity	High
Membrane specificity	High in dependent claims
API protection	None independent of dosage-form structure
Polymorph protection	None apparent from the supplied claims
Method-of-use protection	None in the supplied claims
Manufacturing-process protection	None in the supplied claims
Current enforceability	None after ordinary expiration
Design-around difficulty	Moderate to low
Current generic blocking value	None

The strongest historical protection was concentrated in the combination of:

carbamazepine;
crystal habit modification;
mixed-viscosity hydroxyalkyl cellulose;
mannitol and a sugar such as dextrates;
a cellulose acetate membrane;
hydroxypropylmethylcellulose;
polyethylene glycol 8000; and
a drilled delivery orifice.

The claim set does not broadly cover all extended-release carbamazepine products.

How does this patent compare with competing carbamazepine technologies?

Osmotic tablet versus hydrophilic matrix tablet

An osmotic tablet uses water ingress through a semipermeable membrane and drug release through an orifice. A hydrophilic matrix tablet relies on polymer hydration, gel formation, erosion, diffusion, or a combination of those mechanisms.

A matrix tablet lacking the claimed membrane and orifice would generally present a stronger noninfringement position. It could still be subject to separate patents covering polymer matrices, release profiles, tablet manufacture, or therapeutic dosing.

Osmotic tablet versus extended-release capsule

An extended-release capsule containing coated beads, pellets, or multiparticulates may avoid the tablet and drilled-orifice structure. The relevant patent risks would shift to coating layers, bead compositions, capsule architecture, and release-modifying polymers.

Osmotic tablet versus immediate-release product

An immediate-release carbamazepine tablet lacks the controlled-release architecture required by claim 1. The patent does not claim carbamazepine generally.

What generic launch risks remain?

U.S. generic launch risk from this patent is effectively zero after expiration. Commercial risk may remain from:

later formulation patents;
product-specific Orange Book listings;
pediatric or clinical exclusivity;
patent claims covering a different carbamazepine dosage form;
manufacturing patents;
trade secrets involving coating orifice formation and process controls; and
regulatory requirements for demonstrating bioequivalence to an extended-release reference product.

The patent may still have historical value in freedom-to-operate analyses, prior-art studies, and invalidity searches. It should not be treated as a live exclusionary asset.

What licensing deals or patent transactions are relevant?

The supplied claims identify a carbamazepine osmotic formulation but do not establish a license, assignment history, settlement, or commercialization agreement. Patent ownership should be distinguished from product sponsorship. A patent assigned to an osmotic-delivery company would not establish that the sponsor of a carbamazepine product licensed the patent.

The most relevant diligence documents are the USPTO assignment record, FDA Orange Book patent submissions, ANDA litigation filings, and any historical settlement agreements. None of those legal relationships can be inferred from the claim language alone.

Key Takeaways

U.S. Patent 5,284,662 claims an osmotic carbamazepine dosage form, not carbamazepine as a compound.
Claim 1 requires a specific core, mixed-viscosity cellulose system, sugar alcohol/sugar combination, semipermeable membrane, and delivery orifice.
Claims 14-22 narrow the membrane to cellulose acetate, hydroxypropylmethylcellulose, polyethylene glycol 8000, and specified cellulose acetate ratios.
Claim 23 describes a detailed formulation using carbamazepine, hydroxypropylmethylcellulose, hydroxyethylcellulose 250L and 250H, mannitol, dextrates NF, sodium lauryl sulfate, and magnesium stearate.
The patent issued February 8, 1994, and its ordinary term expired February 8, 2011.
The patent has no current Paragraph IV or generic-launch blocking value.
A matrix tablet, multiparticulate capsule, or nonosmotic delivery system may avoid the claimed structure, subject to separate patent review.
Current carbamazepine patent risk must be assessed against later patents and product-specific Orange Book listings.

FAQs

Does U.S. Patent 5,284,662 cover Tegretol-XR?

Not automatically. Coverage depends on the actual Tegretol-XR formulation and whether the product satisfies every limitation of an asserted claim. The patent’s osmotic membrane and orifice requirements are not present in every extended-release carbamazepine product.

Can an expired patent still affect a generic launch?

It cannot ordinarily block launch after expiration. It may remain relevant to historical damages, prior-art analysis, or interpretation of later patents, but it does not provide a current exclusionary right.

Does the patent cover carbamazepine capsules?

Only if the capsule product contains the claimed osmotic dosage form, including the required core, semipermeable wall, and orifice. A conventional extended-release capsule with coated beads would not ordinarily satisfy those structural limitations.

Is polyethylene glycol 8000 mandatory?

It is not mandatory under claim 1. It becomes relevant under dependent claims 14-16, where the membrane includes a poly-C2-C4 alkylene glycol and claim 15 specifically identifies polyethylene glycol 8000.

Does the patent protect the manufacturing process for drilling the delivery hole?

The supplied claims do not claim a manufacturing method. They claim the finished dosage form with a hole through the semipermeable wall. Separate process claims, if present elsewhere in the patent or in related patents, would require independent review.

References

United States Patent and Trademark Office. (1994). U.S. Patent No. 5,284,662.
U.S. Food and Drug Administration. (2023). Approved drug products with therapeutic equivalence evaluations, commonly known as the Orange Book.
United States Patent and Trademark Office. (2024). Manual of Patent Examining Procedure: Patent term and expiration.
U.S. Food and Drug Administration. (2024). Abbreviated new drug application submissions and patent certifications.

Details for Patent: 5,284,662

Summary for Patent: 5,284,662