US Patent 5,866,590: Tiagabine Antioxidant Formulations, Claim Scope, Expiration, and Patent Landscape

US Patent 5,866,590 covers stabilized pharmaceutical compositions containing tiagabine or a pharmaceutically acceptable salt. Its principal inventive concept is the use of ascorbic acid, or a combination of alpha-tocopherol and ascorbyl palmitate, to reduce tiagabine degradation. The patent also covers preparation processes involving mixing, melt granulation, and low-moisture and low-oxygen processing conditions.

The patent was assigned to Novo Nordisk A/S and issued on February 2, 1999. Based on its US filing date, the 20-year patent term ended on December 20, 2016. The claims are therefore expired and do not currently block generic manufacture, formulation, sale, or use in the United States. The patent remains commercially relevant as a historical formulation patent and as a reference point for freedom-to-operate analysis involving tiagabine products.

What does US Patent 5,866,590 protect?

The patent protects two related formulation platforms:

Tiagabine plus ascorbic acid.
Tiagabine plus alpha-tocopherol and ascorbyl palmitate.

Both platforms require a pharmaceutically acceptable carrier. The patent also claims manufacturing processes for making those formulations.

Category	Claimed subject matter
Active ingredient	Tiagabine or a pharmaceutically acceptable salt
Primary antioxidant system	Ascorbic acid
Alternative antioxidant system	Alpha-tocopherol plus ascorbyl palmitate
Carrier	Any pharmaceutically acceptable carrier within the claim scope
Specific antioxidant ratio	5-25 parts by weight per 100 parts of tiagabine hydrochloride
Physical form	Antioxidant in powder form
Manufacturing method	Mixing active ingredient, antioxidant system, and carrier
Granulation method	Melt granulation
Processing environment	Low water vapor pressure and low oxygen pressure

The patent is a formulation and manufacturing patent. It does not claim tiagabine as a chemical compound, its mechanism of action, or the treatment of epilepsy as such.

What are the independent claims in US 5,866,590?

The patent has four principal independent claims: claims 1, 4, 7, and 10.

Claim 1: Tiagabine and ascorbic acid composition

Claim 1 requires:

tiagabine or a pharmaceutically acceptable salt;
ascorbic acid in an amount sufficient to stabilize tiagabine; and
a pharmaceutically acceptable carrier.

This is the broadest composition claim in the ascorbic-acid group. It does not require a particular tablet, capsule, granule, dosage strength, excipient, or manufacturing process. It also does not require the 5-25 parts ratio found in claim 3.

The phrase “in an amount sufficient to stabilize” is a functional limitation. A product would need to satisfy both the structural requirement, the presence of ascorbic acid, and the functional requirement that the quantity has a stabilizing effect on tiagabine or its salt.

Claim 4: Process for making the claim 1 composition

Claim 4 covers a process comprising formation of a mixture containing:

tiagabine or its salt;
ascorbic acid; and
a pharmaceutically acceptable carrier.

The claim does not require melt granulation or a controlled-atmosphere process. Those limitations appear in dependent claims 5 and 6.

Claim 7: Tiagabine and combined antioxidant composition

Claim 7 requires:

tiagabine or a pharmaceutically acceptable salt;
a mixture of alpha-tocopherol and ascorbyl palmitate in an amount sufficient to stabilize tiagabine; and
a pharmaceutically acceptable carrier.

The use of both alpha-tocopherol and ascorbyl palmitate is material. A formulation containing only alpha-tocopherol, only ascorbyl palmitate, or a different antioxidant combination would not literally satisfy claim 7.

Claim 10: Process for making the claim 7 composition

Claim 10 covers mixing:

tiagabine or its salt;
alpha-tocopherol and ascorbyl palmitate; and
a pharmaceutically acceptable carrier.

Claims 11 and 12 add melt granulation and low-moisture, low-oxygen processing, respectively.

How broad are the composition claims?

Claims 1 and 7 are relatively broad within the formulation field because they do not specify:

dosage strength;
tablet weight;
release profile;
particular filler or binder;
specific carrier;
particle size;
tablet hardness;
dissolution profile;
manufacturing equipment; or
a particular tiagabine indication.

The principal narrowing elements are the antioxidant identity and the functional stabilization requirement.

A conventional immediate-release tablet could fall within the claims if it contains tiagabine, the required antioxidant system, and a carrier. The claims are not limited to the Gabitril brand formulation or to a particular tablet presentation.

The claims also reach salts other than tiagabine hydrochloride, subject to the requirement that the claimed composition contain tiagabine or a pharmaceutically acceptable salt. Claim 3 and claim 9, however, expressly measure the antioxidant quantity against tiagabine hydrochloride.

What do the dependent claims add?

Claim	Added limitation	Practical effect
2	Ascorbic acid is in powder form	Narrows claim 1 by physical form
3	5-25 parts ascorbic acid per 100 parts tiagabine hydrochloride	Adds a quantitative ratio
5	Melt granulation	Narrows the claim 4 manufacturing process
6	Low water vapor pressure and low oxygen pressure	Adds controlled processing conditions
8	Antioxidant system is in powder form	Narrows claim 7 by physical form
9	5-25 parts antioxidant mixture per 100 parts tiagabine hydrochloride	Adds a quantitative ratio
11	Melt granulation	Narrows the claim 10 manufacturing process
12	Low water vapor pressure and low oxygen pressure	Adds controlled processing conditions

The dependent claims could have been useful during prosecution or litigation because they provide more specific fallback positions. They are less likely than claims 1 and 7 to read on a formulation that uses the same antioxidant concept but materially different quantities or manufacturing conditions.

What formulation technologies are protected?

Ascorbic-acid stabilization

The first claim group treats ascorbic acid as a stabilizer for tiagabine. The claim does not require that ascorbic acid be the only antioxidant or that it be present in a particular physical arrangement. It may be incorporated into a powder blend, granulated formulation, or other dosage form containing a carrier.

Claim 3 creates a defined concentration window: 5-25 parts by weight per 100 parts of tiagabine hydrochloride. That ratio corresponds to approximately 5%-25% by weight relative to the tiagabine hydrochloride reference amount, although the exact finished-product percentage depends on the total formulation weight.

Alpha-tocopherol and ascorbyl palmitate

The second claim group requires the combination of alpha-tocopherol and ascorbyl palmitate. The claims do not state a required ratio between those two antioxidants. Claim 9 instead specifies the combined amount relative to tiagabine hydrochloride.

This structure gives the patent broader flexibility over the internal ratio of the two antioxidants, while imposing the requirement that both be present.

Melt granulation

Melt granulation is a process in which a binder or other formulation component is softened or melted to agglomerate powder particles. Claims 5 and 11 do not identify a single apparatus or a particular temperature. The process would be assessed based on whether the manufacturing operation falls within the technical meaning of melt granulation and includes the ingredients required by the parent claims.

Low oxygen and low water vapor pressure

Claims 6 and 12 add environmental process controls. These limitations may be relevant where oxidation or moisture is a principal degradation pathway. They are narrower than the basic mixture claims because infringement requires proof of the specified processing environment in addition to the composition ingredients.

When did US Patent 5,866,590 expire?

Event	Date
Earliest listed priority date	December 20, 1995
US filing date	December 20, 1996
Patent issued	February 2, 1999
Patent number	US 5,866,590
Statutory term endpoint	December 20, 2016
Current enforceability	Expired

The patent was filed after the 1995 transition to the 20-year term measured from the earliest effective US filing date. The foreign priority date does not ordinarily reset the patent term to 20 years from the priority filing date. The patent record identifies December 20, 1996, as the relevant US filing date for term purposes.[1]

No current patent-enforcement risk arises from the expired claims. The patent can still matter in historical validity analysis, prior-art analysis, and review of legacy Gabitril formulation technology.

What was the relationship between this patent and the tiagabine compound patent?

US Patent 5,866,590 is distinct from the earlier tiagabine compound and therapeutic patents.

Patent category	Representative patent	Subject matter	Current status
Tiagabine compound and related use	US 5,236,708	Tiagabine chemical subject matter and related pharmaceutical use	Expired
Stabilized tiagabine formulation	US 5,866,590	Antioxidant-containing compositions and manufacturing processes	Expired
FDA regulatory exclusivity	Gabitril NDA 20-646	FDA approval and labeling exclusivity	Expired

The earlier compound patent created the primary chemical exclusivity position. US 5,866,590 was a later formulation-layer patent. A generic applicant could not have relied solely on the expiration of the compound patent if the proposed product infringed an unexpired formulation patent. That concern ended when US 5,866,590 expired in 2016.

What was the Orange Book status of tiagabine and Gabitril?

Gabitril, the original tiagabine product, was approved by FDA under NDA 20-646 for adjunctive treatment of partial seizures in adults and children aged 12 years and older, according to the prescribing information.[2]

US 5,866,590 was associated with the historical formulation protection for tiagabine products. Any Orange Book listing for the patent could not extend beyond the patent’s statutory expiration. After December 20, 2016, the patent no longer created a listed-patent barrier to an ANDA applicant.

Tiagabine is a small-molecule drug. The relevant abbreviated approval pathway is an ANDA under section 505(j) of the Federal Food, Drug, and Cosmetic Act, not the biosimilar pathway under section 351(k) of the Public Health Service Act. Biosimilar risk is therefore not applicable to this product.

Were there Paragraph IV challenges or patent settlements?

Paragraph IV certifications would have been relevant while the tiagabine patents remained unexpired. A generic applicant could have challenged listed patents by asserting that they were invalid, unenforceable, or would not be infringed.

The principal commercial barriers were the underlying tiagabine compound patent and formulation protection such as US 5,866,590. Once those patents expired, an ANDA applicant no longer needed to rely on a Paragraph IV challenge to enter based on patent invalidity or noninfringement.

The public patent record for US 5,866,590 does not establish a reported landmark infringement decision or a commercially material settlement that remains operative today. Any historical settlement would not revive the expired patent or prevent lawful post-expiration entry.

How strong was the patent estate for the formulation?

The estate was moderate as a formulation patent and limited as a long-term exclusivity asset.

Strengths

Claims 1 and 7 covered broad ingredient combinations.
The claims were not limited to a particular brand, dosage strength, or indication.
Functional stabilization language could reach formulations using different excipient systems.
Process claims covered both ordinary mixing and more specialized melt-granulation operations.
The claims addressed both composition and manufacturing activities.

Limitations

The patent did not cover tiagabine itself.
The antioxidant systems were specifically identified.
Claims 3 and 9 used defined quantitative ranges.
Claims 5, 6, 11, and 12 required additional manufacturing facts.
Functional stabilization language could create proof issues involving test methods and causation.
Alternative antioxidants or non-infringing stabilization strategies could avoid literal infringement.
The patent had no remaining term after 2016.

From a litigation perspective, the most vulnerable issues would have included anticipation or obviousness based on pharmaceutical antioxidant formulations, definiteness of “sufficient to stabilize,” enablement across the full carrier universe, and whether a process met the low-pressure limitations.

What generic entry risks existed?

Before expiration, generic entry risk depended on the proposed formulation.

Generic product design	Historical risk under US 5,866,590
Tiagabine plus ascorbic acid	Potential claim 1 issue
Tiagabine plus 5-25 parts ascorbic acid per 100 parts tiagabine HCl	Potential claims 1 and 3 issue
Tiagabine plus alpha-tocopherol and ascorbyl palmitate	Potential claim 7 issue
Same dual-antioxidant system in the 5-25 parts range	Potential claims 7 and 9 issue
Melt-granulated product	Potential process claims 5 or 11 issue
Product made under controlled low oxygen and moisture conditions	Potential claims 6 or 12 issue
Tiagabine with a different stabilizer	Lower literal infringement risk
Tiagabine without either claimed antioxidant system	Lower literal infringement risk

Today, the patent creates no generic launch barrier. The remaining commercial questions are FDA bioequivalence, manufacturing economics, market size, supply reliability, and any other unexpired patents or regulatory exclusivities unrelated to US 5,866,590.

How does US 5,866,590 compare with biologic patent estates?

US 5,866,590 has a simpler risk profile than a biologic patent estate.

A small-molecule formulation patent typically turns on ingredient identity, concentration, dosage form, and manufacturing steps. A biologic estate may include composition, sequence, glycosylation, cell line, purification, formulation, device, method-of-treatment, and manufacturing patents. Tiagabine has no biosimilar interchangeability issue, no biologic reference-product exclusivity, and no patent dance under the Biologics Price Competition and Innovation Act.

The relevant competitive products are generic tiagabine hydrochloride tablets rather than biosimilars. FDA’s therapeutic-equivalence framework and ANDA approval process control the principal regulatory pathway.[3]

Key Takeaways

US 5,866,590 covers tiagabine formulations stabilized with ascorbic acid or with alpha-tocopherol plus ascorbyl palmitate.
Claims 1 and 7 are the principal composition claims.
Claims 4 and 10 cover corresponding mixing processes.
Dependent claims add powder form, antioxidant ratios, melt granulation, and low-moisture, low-oxygen processing.
The patent does not claim tiagabine as a chemical compound.
The patent expired on December 20, 2016.
It does not currently block generic tiagabine entry in the United States.
Tiagabine is a small molecule, so biosimilar analysis does not apply.
Historical Paragraph IV risk focused on whether a generic product used the claimed antioxidant systems and manufacturing methods.
The patent remains relevant for historical formulation, validity, and freedom-to-operate reviews, but has no current exclusionary value.

FAQs About US Patent 5,866,590

Does US 5,866,590 cover Gabitril by brand name?

No. The claims cover defined tiagabine compositions and processes, not the Gabitril trademark or a particular branded product.

Can a generic use ascorbic acid in a tiagabine tablet today?

Yes, because the patent claims expired in 2016. A current product must still satisfy FDA quality, safety, and bioequivalence requirements.

Does the patent cover tiagabine hydrochloride API?

No. It covers pharmaceutical compositions containing tiagabine or a pharmaceutically acceptable salt with specified antioxidant systems and a carrier.

Is melt granulation required for infringement?

No. Melt granulation is required only for dependent claims 5 and 11. The broader composition claims do not require that process.

Is a product containing only alpha-tocopherol within claim 7?

Generally, no. Claim 7 requires a mixture of alpha-tocopherol and ascorbyl palmitate. A formulation containing only alpha-tocopherol would not literally satisfy that limitation.

References

United States Patent and Trademark Office. (1999). US Patent No. 5,866,590, Pharmaceutical compositions comprising tiagabine and an antioxidant.
U.S. Food and Drug Administration. (2011). Gabitril (tiagabine hydrochloride) prescribing information.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations.
United States Patent and Trademark Office. (1993). US Patent No. 5,236,708, Tiagabine and related pharmaceutical compositions.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
Denmark	0523/95	May 05, 1995

PCT Information
PCT Filed	April 29, 1996	PCT Application Number:	PCT/DK96/00192
PCT Publication Date:	November 07, 1996	PCT Publication Number:	WO96/34606

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	003418	⤷ Start Trial
Austria	211911	⤷ Start Trial
Australia	5644196	⤷ Start Trial
Australia	707508	⤷ Start Trial
Brazil	9608120	⤷ Start Trial
Canada	2220019	⤷ Start Trial
China	1092959	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,866,590

Summary for Patent: 5,866,590