US Patent 5,731,000: Bupropion Hydrochloride Stabilization Claims, Expiration, and Patent Landscape

US Patent 5,731,000 covers solid pharmaceutical compositions containing bupropion hydrochloride and selected acidic or reducing stabilizers. Its central limitation is not simply the presence of bupropion and an excipient. The composition must satisfy specified accelerated-storage performance thresholds and use a stabilizer whose 6% aqueous solution has a defined acidic pH.

The patent was issued on March 24, 1998, and its enforceable term has expired. The patent therefore does not create a current US barrier to generic bupropion hydrochloride products. Its historical significance is greater than its present blocking value: it addressed degradation in solid bupropion formulations and anticipated formulation strategies based on acidic stabilizers, particularly for tablets and capsules containing 50 mg, 75 mg, 100 mg, or 150 mg of active ingredient.[1]

What does US Patent 5,731,000 protect?

The patent protects two related categories:

Solid pharmaceutical compositions containing bupropion hydrochloride and specified stabilizers.
Methods of stabilizing solid bupropion hydrochloride by mixing it with those stabilizers.

The independent claims are claims 1, 2, 12, and 13.

Claim	Claim type	Storage condition	Minimum undegraded bupropion	Stabilizer requirement
1	Composition	6 weeks, 40°C, 75% RH	At least about 80% w/w	6% aqueous solution pH about 0.9-4
2	Composition	6 weeks, 50°C, 27% RH	At least about 80% w/w	6% aqueous solution pH about 0.9-4
12	Stabilization method	6 weeks, 40°C, 75% RH	At least about 80% w/w	Mixing with qualifying stabilizer
13	Stabilization method	6 weeks, 50°C, 27% RH	At least about 80% w/w	Mixing with qualifying stabilizer

The listed stabilizer classes are:

Organic acids
Certain carboxylic acids
Acid salts of amino acids
Sodium metabisulfite or sodium metabisulphite

The dependent claims narrow the invention by adding more restrictive pH ranges, higher stability performance, stabilizer concentration limits, specific chemical compounds, and dosage strengths.

What are the key technical limitations in the claims?

Bupropion hydrochloride must be in solid form

The composition claims require a solid pharmaceutical composition. The claims therefore target tablets, capsules, powders, granules, or comparable solid dosage forms rather than an aqueous solution or other liquid dosage form.

Claim 11 expressly identifies tablets or capsules containing 50 mg, 75 mg, 100 mg, or 150 mg of bupropion hydrochloride. Claim 20 repeats those dosage strengths for the second storage-condition claim set.

The patent does not require a particular tablet manufacturing process, coating, release profile, excipient system, or dissolution specification. The core product limitations are the active ingredient, stabilizer, pH characteristic, and stability result.

The stabilizer must satisfy a pH test

The pH limitation is unusually specific. The stabilizer is evaluated in an aqueous solution at approximately 6% w/w. That solution must have a pH of approximately 0.9 to 4 under claims 1, 2, 12, and 13.

Claims 3, 14, 21, and 22 narrow the range to approximately 0.9 to 2. Claims 4 and 15 narrow it further to a pH of around 1.

This limitation creates a measurable product characteristic, but it also raises analytical questions:

Whether pH is measured at a defined temperature.
Whether the 6% concentration is w/w based on the active stabilizer or the complete solution.
Whether the stabilizer is tested alone or in the presence of bupropion hydrochloride.
How mixtures of stabilizers are evaluated.
How the terms “about” and “around” are construed.

A formulation may contain an acidic excipient yet fall outside the claim if the stabilizer’s 6% solution has a pH above 4. Conversely, a formulation may present infringement risk even if the stabilizer is present at a low concentration, provided the other claim limitations are met.

The composition must meet an accelerated stability result

The claims require at least about 80% w/w undegraded bupropion hydrochloride after storage under one of two conditions:

40°C and 75% relative humidity for six weeks; or
50°C and 27% relative humidity for six weeks.

Claims 5 and 16 raise the threshold to at least 95% w/w. These are composition-by-result limitations. They require analytical proof of the undegraded active content after the specified storage protocol.

The claim does not state that all degradation products must be identified. It focuses on the percentage of undegraded bupropion hydrochloride. The analytical method used to distinguish intact bupropion from degradation products could therefore become material in an infringement or validity dispute.

Which stabilizers are expressly covered?

Organic acids

Claims 9 and 18 identify:

Tartaric acid
Citric acid
Malic acid

These compounds are expressly recited as species within the broader organic-acid category.

Carboxylic acids

Claims 10 and 19 identify:

Ascorbic acid
Isoascorbic acid

There is a drafting tension between the claims. Claim 2 excludes ascorbic acid and isoascorbic acid from its “carboxylic acid other than” category, while claim 19, which depends on claim 2, expressly recites those same compounds. The most plausible reading is that claim 19 was intended to preserve those compounds under a separate stabilizer category, but the wording creates a potential claim-construction issue.

Acid salts of amino acids

Claims 8 and 17 identify:

Cysteine hydrochloride
Glycine hydrochloride
Cystine dihydrochloride

The claims do not require a free amino acid. They require an acid salt of an amino acid.

Sodium metabisulfite

Sodium metabisulfite is expressly included in the independent claims. The patent uses both “metabisulfite” and “metabisulphite” spelling in the supplied claim text. That spelling difference ordinarily would not determine scope.

How do the dependent claims narrow the patent’s scope?

Limitation	Claims	Effect
pH about 0.9-2	3, 14, 21, 22	Narrows the stabilizer solution’s pH
pH around 1	4, 15	Further narrows pH
At least 95% undegraded active	5, 16, 21, 22	Raises the stability threshold
Stabilizer at 2.7%-27% of bupropion weight	6, 21, 22	Adds a relative concentration range
Stabilizer at 5%-16.2% of bupropion weight	7	Adds a narrower concentration range for claim 2
Specific amino-acid salts	8, 17, 21, 22	Limits the stabilizer to three named salts
Tartaric, citric, or malic acid	9, 18, 21, 22	Limits the organic-acid category
Ascorbic or isoascorbic acid	10, 19, 21, 22	Limits the carboxylic-acid category
50, 75, 100, or 150 mg dosage	11, 20, 21, 22	Limits the active dose

Claims 21 and 22 are highly restrictive combination claims. They combine multiple limitations from earlier claims, including pH, stability, stabilizer amount, named stabilizers, and dosage strength.

Their drafting is potentially problematic. Each claim appears to recite multiple stabilizer categories in a cumulative form. If read literally, a composition might need to contain an organic acid, a carboxylic acid, an amino-acid salt, and sodium metabisulfite simultaneously. That may not reflect the intended alternatives. A court could construe the language in light of the specification, prosecution history, and claim dependency, but the text creates avoidable ambiguity.

What is the infringement test for a bupropion formulation?

A product would generally need to satisfy every limitation of at least one asserted claim. For independent composition claim 1, the relevant questions would include:

Does the product contain bupropion hydrochloride?
Is it a solid pharmaceutical composition?
Does it contain a qualifying stabilizer?
Does the stabilizer’s approximately 6% aqueous solution have a pH of about 0.9 to 4?
Does the product retain at least about 80% undegraded bupropion after six weeks at 40°C and 75% relative humidity?

Claim 2 substitutes the 50°C and 27% relative-humidity test. A product may pass one storage condition and fail the other. The claims are separately enforceable in principle because they define different accelerated-stability environments.

The method claims create a separate theory of liability. They require mixing bupropion hydrochloride with a qualifying stabilizer to achieve the specified stability outcome. A manufacturer using the claimed formulation process could implicate claims 12 or 13 even if the final product claim presented a different evidentiary record.

The phrase “effective stabilising amount” is an additional limitation. The stabilizer must be present in an amount that performs the claimed stabilizing function. A trace amount of a listed acid would not automatically satisfy the claim.

What prior-art and validity issues affect patent strength?

The patent’s technical concept is relatively narrow but its claim structure is vulnerable in several areas.

Written description and enablement

The independent claims cover broad classes of organic acids, carboxylic acids, amino-acid salts, and sodium metabisulfite. The patent’s support for the full breadth of those classes would depend on the examples and disclosure in the specification.

Potential issues include whether the specification demonstrates the full claimed pH range, all listed stabilizer categories, the full concentration ranges, and both accelerated-storage protocols.

Definiteness

Potentially indefinite terms include:

“About 80%”
“About 95%”
“About 0.9 to about 4”
“Around 1”
“Effective stabilising amount”
“Undegraded”
“Pharmaceutically acceptable stabiliser”

These terms are common in pharmaceutical patents, but their enforceability depends on whether a skilled person could determine the boundaries using the specification and ordinary analytical practice.

Anticipation and obviousness

Earlier bupropion formulations, acidic excipients, sulfite stabilizers, and pharmaceutical stability testing could be combined in an obviousness challenge. The strongest patentability argument would have been the claimed stability result under the specified conditions, particularly if the prior art taught that bupropion hydrochloride degraded despite conventional excipients.

The patent’s narrow pH and performance limitations provide some distinction from generic formulations using nonacidic excipients. The breadth of the stabilizer categories weakens that distinction if prior art disclosed acidic stabilization generally.

Claim dependency and drafting defects

Claims 19 and 22 present an internal conflict because claim 2 excludes ascorbic acid and isoascorbic acid from one category while dependent claim 19 expressly recites them.

Claims 21 and 22 also combine what appear to be alternative stabilizer classes in a way that may produce an unintended cumulative limitation. These issues would have been relevant to claim construction and could reduce practical enforcement value even before expiration.

When did US Patent 5,731,000 lose exclusivity?

US Patent 5,731,000 is expired. The patent issued in 1998 from an application claiming an earlier priority date, and its ordinary US patent term ended in approximately 2015, subject to any applicable patent-term adjustment recorded by the USPTO.[1][2]

The patent therefore has no current enforceable exclusionary term against US generic manufacturers. Any historical patent listing or Paragraph IV dispute involving this patent does not create a present-day patent barrier.

Milestone	Date or status
US patent issued	March 24, 1998
Patent term basis	20 years from the relevant nonprovisional filing date, subject to adjustment
Approximate expiration	2015
Current enforceability	Expired
Current US exclusivity from this patent	None

What is the Orange Book status of US Patent 5,731,000?

The FDA Orange Book identifies patents submitted by NDA holders for approved drug products, including patents covering drug substance, formulation, or approved methods of use.[3]

For bupropion hydrochloride products, the practical regulatory question is whether any unexpired patent remains listed against the relevant NDA and whether an ANDA applicant must make a Paragraph IV certification. US Patent 5,731,000 is expired and cannot independently support a current injunction against an ANDA applicant.

An expired formulation patent can remain relevant historically because it may explain earlier Orange Book listings, patent certifications, or settlement negotiations. It does not provide current patent protection for Wellbutrin, Zyban, or generic bupropion products.

FDA approval of an ANDA remains subject to other requirements, including bioequivalence, product quality, labeling, and any currently listed unexpired patents. The expiration of this patent does not eliminate those regulatory requirements.[3][4]

Are Paragraph IV challenges or litigation associated with this patent still relevant?

A Paragraph IV certification is relevant only while a listed patent creates a potential approval or launch obstacle. Because US Patent 5,731,000 has expired, a new Paragraph IV challenge directed solely to this patent would have no current commercial purpose.

The better-known bupropion patent disputes involved extended-release formulations, release profiles, and other patents associated with Wellbutrin XL and related products. Those disputes should not be conflated with the expired stabilization patent. Formulation patents covering release mechanisms or dosage-form architecture can have materially different scope from a patent directed to chemical stabilization under accelerated storage conditions.[5]

No current litigation consequence follows from US Patent 5,731,000 alone. Any historical litigation analysis would require separating this patent from later bupropion patents and from patent settlements involving extended-release products.

Does the patent create biosimilar risk?

No. Bupropion hydrochloride is a chemically synthesized small-molecule active pharmaceutical ingredient. Competing products proceed through the generic drug pathway, principally under section 505(j) of the Federal Food, Drug, and Cosmetic Act, rather than through the biosimilar pathway under section 351(k) of the Public Health Service Act.[4]

The relevant competitive risks are:

ANDA approval
Paragraph IV litigation involving unexpired patents
Bioequivalence and formulation differences
Manufacturing scale and supply reliability
State substitution and contracting dynamics

There is no biosimilar market applicable to this patent.

What manufacturing and formulation barriers remain after expiration?

The patent’s expiration removes the legal barrier created by its claims, but commercial barriers remain.

A manufacturer seeking to reproduce a stable bupropion hydrochloride product may still need to control:

Moisture exposure during granulation and compression
Excipient compatibility
Acid-base interactions
Packaging permeability
Tablet hardness and disintegration
Degradation-product formation
Assay and impurity methods
Batch-to-batch stability
Bioequivalence performance

The patent does not claim a particular packaging system or manufacturing process. A competitor can therefore design around the expired claims or use the disclosed stabilizers without patent infringement. Regulatory reproducibility and manufacturing economics, rather than this patent, are the principal barriers.

How does this patent compare with broader bupropion patent estates?

Issue	US 5,731,000	Later bupropion formulation patents
Primary subject	Chemical stabilization	Release profile, dosage form, coating, or delivery
Dosage form	Solid composition, tablet, capsule	Often extended-release tablets or controlled delivery systems
Key limitation	Acidic stabilizer and accelerated stability	Dissolution and release characteristics
Product relevance	Immediate-release or general solid formulations	Particularly extended-release products
Current status	Expired	Must be assessed patent by patent
Generic pathway	ANDA	ANDA, with possible patent certifications
Biosimilar relevance	None	None

US Patent 5,731,000 is best viewed as an expired formulation-stability patent, not as a current platform patent covering all bupropion products.

What licensing deals affect the patent?

The patent is associated with the Glaxo Wellcome bupropion product-development history. No current patent-specific license is necessary to commercialize a US generic product because the patent has expired.

Commercial agreements involving bupropion products, authorized generics, supply arrangements, or extended-release formulations should be analyzed separately. A license covering a later patent, trademark, manufacturing process, or regulatory asset would not revive or extend the term of US Patent 5,731,000.

What generic launch scenarios exist?

The patent presents no current launch delay by itself.

Immediate-release bupropion products

A generic manufacturer can launch after satisfying FDA approval requirements and addressing any other unexpired patents or regulatory exclusivities applicable to the target product.

Extended-release bupropion products

The main risks may arise from separate patents covering release characteristics, matrix systems, coatings, dissolution profiles, or other formulation features. The expired stabilization patent does not resolve those issues.

Alternative-stabilizer products

A manufacturer can use a stabilizer outside the listed classes, subject to product-quality and stability requirements. Since the patent is expired, design-around analysis is no longer required for current US launch decisions, but it remains relevant to historical freedom-to-operate reviews and foreign jurisdictions where corresponding rights may have differed.

What is the geographic coverage of the patent landscape?

US Patent 5,731,000 had jurisdiction limited to the United States. Any foreign counterparts would require separate assessment of:

Filing and priority dates
National-phase status
Patent-term rules
Supplementary protection certificates
Opposition or revocation outcomes
Local claim scope
Expiration dates

A US expiration does not establish expiration in Canada, Europe, Japan, or other markets. For a current multinational launch, each jurisdiction requires a separate patent-register review. No foreign rights can be inferred from the US patent number alone.

Key Takeaways

US Patent 5,731,000 covers solid bupropion hydrochloride compositions stabilized with selected acidic or reducing agents.
The independent claims require both a defined stabilizer pH and a specified accelerated-storage result.
The principal stabilizers include tartaric acid, citric acid, malic acid, ascorbic acid, isoascorbic acid, cysteine hydrochloride, glycine hydrochloride, cystine dihydrochloride, and sodium metabisulfite.
Claims 12 and 13 separately cover methods of stabilizing solid bupropion hydrochloride.
Claims 19, 21, and 22 contain drafting inconsistencies involving stabilizer categories and dependency.
The patent expired in approximately 2015 and creates no current US exclusivity.
Generic bupropion products face ANDA-related requirements, not biosimilar requirements.
Current commercial risk depends on other unexpired bupropion patents, particularly those covering extended-release formulations and release profiles.
The patent’s remaining importance is historical, technical, and relevant to prior-art or formulation-development analysis rather than current US enforcement.

FAQs

Can a generic manufacturer use citric acid in bupropion hydrochloride tablets today?

Yes. US Patent 5,731,000 has expired. A manufacturer must still meet FDA quality, stability, labeling, and bioequivalence requirements and review other potentially relevant patents.

Does the patent cover bupropion hydrobromide or another bupropion salt?

No. The claims specifically recite bupropion hydrochloride. A different salt would not literally satisfy that active-ingredient limitation, although separate regulatory and patent issues could apply.

Does the patent cover a bupropion hydrochloride product with no stabilizer?

No. The independent claims require a pharmaceutically acceptable stabilizer. A product with no stabilizer would not literally satisfy the composition claims.

Are the 40°C/75% RH and 50°C/27% RH tests interchangeable?

No. They are separate claim limitations. Claim 1 and claim 12 use the first condition, while claim 2 and claim 13 use the second. A product’s performance under one condition does not automatically establish performance under the other.

Can the expired patent block a US 505(b)(2) application?

No. The expired patent cannot independently block approval or launch. A 505(b)(2) applicant would still need to address applicable FDA requirements and any other unexpired patents or exclusivities associated with the reference product.

References

United States Patent and Trademark Office. (1998). US Patent No. 5,731,000, Stabilized pharmaceutical compositions containing bupropion hydrochloride.
United States Patent and Trademark Office. (n.d.). Patent term adjustment and patent term information. https://www.uspto.gov/patents/laws/patent-term-adjustment
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations, commonly known as the Orange Book. https://www.fda.gov/drugs/drug-approvals-and-databases/approved-drug-products-therapeutic-equivalence-evaluations-orange-book
U.S. Food and Drug Administration. (n.d.). Abbreviated New Drug Application (ANDA) process. https://www.fda.gov/drugs/types-applications/abbreviated-new-drug-application-anda
U.S. Federal Trade Commission. (2002). Generic drug entry prior to patent expiration: An FTC study. https://www.ftc.gov/reports/generic-drug-entry-prior-patent-expiration-empirical-evidence-1984-2001

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
United Kingdom	9315856	Jul 30, 1993

PCT Information
PCT Filed	July 29, 1994	PCT Application Number:	PCT/GB94/01642
PCT Publication Date:	February 09, 1995	PCT Publication Number:	WO95/03791

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	232383	⤷ Start Trial
Australia	698883	⤷ Start Trial
Australia	7235294	⤷ Start Trial
Canada	2168364	⤷ Start Trial
Germany	69432121	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Summary for Patent: 5,731,000