United States Drug Patent 4,351,841: Scope, Claims, Expiration, and Leflunomide Patent Landscape

US Patent 4,351,841 covers leflunomide, chemically identified as 5-methylisoxazole-4-carboxylic acid 4-trifluoromethylanilide. Its two claims protect the active compound in therapeutic use and pharmaceutical compositions containing it. The patent issued on September 28, 1982, and its pre-URAA 17-year patent term expired in 1999, subject to any applicable statutory adjustment or extension. It no longer creates a live US patent barrier to generic leflunomide products.

What drug does US Patent 4,351,841 protect?

The patented compound is leflunomide, marketed in the United States as Arava. Its chemical name is N-(4-trifluoromethylphenyl)-5-methylisoxazole-4-carboxamide.

Item	Details
US patent	US 4,351,841
Patented compound	Leflunomide
Chemical description	5-methylisoxazole-4-carboxylic acid 4-trifluoromethylanilide
Patent issue date	September 28, 1982
Original assignee	Hoechst AG
Brand product	Arava
FDA approval	September 10, 1998
Initial indication	Active rheumatoid arthritis
Dosage forms	Oral tablets, principally 10 mg, 20 mg and 100 mg loading-dose tablets
Current patent status	Expired
Regulatory pathway for generics	Abbreviated New Drug Application, or ANDA

Leflunomide is an oral disease-modifying antirheumatic drug. Its active metabolite, teriflunomide, inhibits dihydroorotate dehydrogenase and reduces pyrimidine synthesis in activated lymphocytes. The metabolite and its mechanism are not separately claimed in the two claims quoted from US 4,351,841.

What does claim 1 of US 4,351,841 cover?

Claim 1 is a therapeutic-use claim. It covers:

Administering an effective amount of leflunomide to a patient suffering from inflammation, rheumatism or multiple sclerosis.

The claim has four principal limitations:

A patient must be suffering from one of the specified conditions.
The treatment must involve administration to that patient.
The administered substance must be the exact leflunomide compound identified in the claim.
The amount must be therapeutically effective.

The claim does not specify:

A particular dose;
A dosing schedule;
An administration route;
A tablet, capsule or solution;
A particular salt or polymorph;
A blood concentration;
A treatment duration;
A particular disease subtype;
A combination therapy;
A manufacturing process.

The disease language is broad. “Inflammation” is a general pathological category, while “rheumatism” is an older and less precise medical term. “Multiple sclerosis” is expressly named. The claim therefore reaches beyond the later FDA-approved rheumatoid arthritis indication as a matter of claim language, although infringement still requires proof that the claimed method was practiced.

How would claim 1 be infringed?

Direct infringement would generally require administration of leflunomide to a qualifying patient for a covered condition. A manufacturer does not necessarily directly infringe a method-of-treatment claim merely by making or selling tablets. Potential indirect infringement theories could depend on labeling, instructions, promotional activity and the knowledge or intent requirements applicable under US patent law.

The claim is compound-specific. A product containing teriflunomide, another active metabolite, or a different immunomodulator would not literally satisfy the active-ingredient limitation. A nonliteral infringement theory would have faced separate legal and technical requirements, but expiration eliminates present enforcement value.

What does claim 2 of US 4,351,841 cover?

Claim 2 is a pharmaceutical-composition claim. It covers a composition containing:

An effective amount of leflunomide; and
A pharmaceutically acceptable excipient or diluent.

The claim identifies four therapeutic classifications:

Antirheumatic;
Antiphlogistic, meaning anti-inflammatory;
Antipyretic;
Analgesic.

Unlike claim 1, claim 2 does not require administration to a patient. It is directed to the composition itself. A tablet, capsule, powder, suspension or other pharmaceutical preparation could fall within the claim if it contains the exact active compound and a pharmaceutically acceptable excipient or diluent.

What formulations are protected by claim 2?

The claim is broad as to formulation architecture. It does not require a particular:

Tablet hardness;
Coating;
Release profile;
Particle size;
Excipient;
Binder;
Disintegrant;
Lubricant;
Packaging system;
Manufacturing method.

A conventional leflunomide tablet would have been within the literal scope during the patent term. The claim does not, however, independently protect later-developed formulation concepts such as controlled release, specific stability systems or specialized solid forms unless those concepts also fall within the claim's broad composition language and satisfy its limitations.

The word “containing” generally indicates an open-ended composition limitation. Other ingredients may be present. The composition can therefore include additional excipients or active ingredients, subject to the remaining claim requirements.

How strong were the claims of US 4,351,841?

The patent had commercially significant breadth because both claims centered on the active pharmaceutical ingredient rather than on a narrow formulation or manufacturing process.

Claim characteristic	Assessment
Active ingredient	Narrow chemical identity, but commercially important
Therapeutic-use scope	Broad disease categories
Dosage limitation	None
Route limitation	None
Formulation limitation	Minimal
Manufacturing limitation	None
Composition scope	Broad, subject to an excipient or diluent
Design-around potential during term	Limited for products using leflunomide itself
Current enforceability	None after expiration

The main strength was molecule-level coverage. A generic company could not avoid the claims merely by changing the excipient, tablet shape, dose, coating or packaging if the product still used leflunomide and met the applicable claim elements.

The principal limitations were also clear. The patent did not cover every treatment of rheumatoid arthritis, every anti-inflammatory compound, or every formulation containing a related isoxazole. It covered the identified compound and the specified therapeutic or composition context.

When did US Patent 4,351,841 lose exclusivity?

US 4,351,841 issued before the 1995 change from a grant-based patent term to a filing-based term. The ordinary term for a pre-URAA patent was 17 years from grant. Based on the September 28, 1982 issue date, the ordinary term ended on September 28, 1999. USPTO patent records identify the patent as expired.

The relevant exclusivity timeline is:

Event	Date
US patent issued	September 28, 1982
Ordinary 17-year patent term ends	September 28, 1999
FDA approves Arava	September 10, 1998
Five-year new chemical entity exclusivity	Through approximately September 10, 2003
Current patent barrier	None

FDA regulatory exclusivity and patent exclusivity are separate. The five-year NCE period attached to Arava's FDA approval did not extend the patent term. It restricted certain ANDA submissions even after the patent had expired, although the practical impact was limited because the regulatory exclusivity period ran only until 2003. FDA's Orange Book records distinguish patent listings from regulatory exclusivity periods. [FDA, 2024a]

What is the Orange Book status of leflunomide?

Leflunomide is an FDA-approved small-molecule drug regulated through the NDA and ANDA systems. It is not a biologic and therefore is not subject to the biosimilar pathway under section 351(k) of the Public Health Service Act.

The Orange Book significance is as follows:

Arava was approved under NDA 020905.
Generic leflunomide products are approved through ANDAs.
Any historical listing of US 4,351,841 no longer creates a live patent expiry date.
Current generic approval decisions depend on FDA bioequivalence and labeling requirements, not on an enforceable US 4,351,841 patent.
The patent does not provide a current basis for a 30-month stay.

The Orange Book should be used for the current listed-patent and exclusivity record because historical listings and regulatory exclusivities can differ from the patent document's face-term calculation. [FDA, 2024a]

Were there Paragraph IV challenges to leflunomide?

Leflunomide was subject to generic competition after the expiration of the core compound patent and the end of Arava's regulatory exclusivity. ANDA applicants could use Paragraph III certifications after patent expiration or Paragraph IV certifications where they alleged that listed patents were invalid, unenforceable or not infringed.

The commercial importance of a Paragraph IV challenge to US 4,351,841 is now historical. A Paragraph IV certification against an expired patent cannot preserve an ongoing exclusionary right for the patent holder. It may have affected the timing of an ANDA review or litigation at the time of filing, but it does not restore enforceability.

There is no current biosimilar challenge risk because leflunomide is a chemically synthesized small molecule. The relevant competitive pathway is generic substitution through ANDAs.

Which companies challenged or competed with Arava?

The US market has included generic leflunomide products from multiple ANDA sponsors. Generic competition has involved companies such as Teva and Mylan among the established generic manufacturers, with market participation changing over time. The relevant legal distinction is between:

Historical ANDA certifications and patent litigation; and
Current commercial competition after the expiration of the core patent.

The expired status of US 4,351,841 means that no company currently needs to design around its claims for US commercial entry.

What patent litigation affected leflunomide?

Any historical litigation involving US 4,351,841 would have focused on the validity, enforceability or infringement of the compound, treatment-method or composition claims. Typical issues would include:

Whether the ANDA product contained leflunomide;
Whether the proposed labeling induced performance of claim 1;
Whether a composition contained the claimed active ingredient and excipient;
Whether prior art anticipated or rendered the claims obvious;
Whether the patent had expired or was otherwise unenforceable.

The patent's current expiration makes historical litigation commercially inactive. A prior lawsuit, settlement or Paragraph IV notice does not extend the patent term unless a separate, enforceable patent remained in force. No such extension can be attributed to the two claims quoted from US 4,351,841.

Did Arava have formulation or method-of-use patents beyond US 4,351,841?

The quoted patent is not a specialized formulation patent. It does not claim:

A specific tablet formulation;
A controlled-release system;
A particular crystalline form;
A stability-enhancing excipient system;
A manufacturing route;
A dosing regimen;
A combination with methotrexate;
A biomarker-defined patient population.

Later patents, applications or regulatory exclusivities could have addressed separate subject matter. They must be analyzed independently by patent number, claim language, expiration date and Orange Book status. They cannot be inferred from US 4,351,841.

For freedom-to-operate purposes, the critical distinction is between the expired core compound patent and any later patent that might claim a separate formulation, process or method. An expired foundational patent does not invalidate a later patent, but a later patent also cannot revive the expired claims of US 4,351,841.

What manufacturing and intellectual-property barriers remain?

The expired patent does not prevent manufacture of leflunomide in the United States. Remaining barriers are primarily regulatory and operational:

Barrier	Current significance
Core compound patent	No active US barrier
FDA ANDA approval	Required for a generic finished product
Bioequivalence	Required for approval
API quality and impurity control	Commercial and regulatory requirement
Process know-how	May affect yield, cost and scale-up
Foreign patents	Must be assessed country by country
Later formulation patents	Must be checked separately
Trade secrets	May restrict access to proprietary manufacturing information
Supply-chain qualification	May affect launch timing
Brand loyalty and contracting	May reduce generic uptake

Geographic coverage is territorial. US 4,351,841 affected US rights only. European, Japanese, Canadian and other national family members had their own filing dates, terms and legal outcomes. A US expiration date does not establish freedom to operate in another jurisdiction.

How does leflunomide compare with biologic antirheumatic drugs?

Leflunomide has a fundamentally different patent and regulatory profile from biologic rheumatoid arthritis therapies.

Attribute	Leflunomide	Biologic antirheumatic
Product type	Small molecule	Biologic
Approval pathway	NDA/ANDA	BLA/351(k) biosimilar pathway
Core competition	Generic substitution	Biosimilar or interchangeable biosimilar competition
Patent focus	Compound, formulation, method, process	Sequence, formulation, manufacturing, dosing, device
US 4,351,841 relevance	Expired core compound patent	Not applicable
Manufacturing barrier	Chemical synthesis and impurity control	Cell line, fermentation, purification and characterization
Substitution	Generally pharmacy-level under state law	Depends on interchangeability and state law

Leflunomide's expired compound patent places it in a lower patent-risk category than branded biologics with active formulation, device and manufacturing estates. Its commercial risk is generic price erosion rather than biosimilar interchangeability litigation.

What generic launch scenarios exist for leflunomide?

The realistic US launch scenarios are:

Immediate generic availability following FDA approval and the absence of blocking patents.
Multi-sponsor generic competition, producing rapid price pressure.
Continued brand sales in limited channels based on physician preference, contracting or supply considerations.
Market concentration if only a small number of API or finished-dose suppliers remain active.

A new entrant does not need to invent around US 4,351,841. Its principal tasks are to demonstrate pharmaceutical equivalence, bioequivalence, adequate manufacturing controls and compliant labeling.

Key Takeaways

US 4,351,841 covers leflunomide, the active ingredient in Arava.
Claim 1 is a broad method-of-treatment claim covering administration for inflammation, rheumatism or multiple sclerosis.
Claim 2 covers pharmaceutical compositions containing leflunomide with a pharmaceutically acceptable excipient or diluent.
The patent does not claim a specific dose, route, tablet design, formulation technology or manufacturing process.
The ordinary US patent term ended in 1999.
Arava's five-year NCE exclusivity ended approximately in 2003 and did not extend the patent.
Leflunomide is a small molecule and has generic, not biosimilar, competition.
Current commercial barriers arise from FDA approval, bioequivalence, API supply and any later patents, not from US 4,351,841.
Foreign patent rights and later formulation or process patents require separate jurisdiction-specific analysis.

FAQs

Does US 4,351,841 cover teriflunomide?

No. The claims identify leflunomide, not teriflunomide. Teriflunomide is leflunomide's active metabolite and requires separate patent and regulatory analysis.

Can a company avoid claim 2 by changing the excipient?

Changing the excipient would not avoid the claim if the resulting product still contained leflunomide and a pharmaceutically acceptable excipient or diluent. The patent is expired in the United States in any event.

Does the patent cover veterinary use of leflunomide?

The quoted claims refer to treatment of a patient and specified human therapeutic categories but do not expressly state “human.” Veterinary applicability would depend on claim construction, product use and the governing jurisdiction. The expired US term makes the issue commercially immaterial.

Is leflunomide subject to a biosimilar approval pathway?

No. Leflunomide is a chemically synthesized small molecule. Generic products are approved through ANDAs under section 505(j) of the Federal Food, Drug, and Cosmetic Act.

Can an expired compound patent block a new leflunomide formulation?

No, the expired patent cannot block activity based on its own claims. A separate later patent could create an independent barrier if its claims cover the new formulation and remain enforceable.

References

U.S. Patent and Trademark Office. (1982). 5-Methyl-isoxazole-4-carboxylic acid 4-trifluoromethylanilide and pharmaceutical compositions containing it (U.S. Patent No. 4,351,841).
U.S. Food and Drug Administration. (1998). Arava (leflunomide) approval history and prescribing information. FDA.
U.S. Food and Drug Administration. (2024a). Approved drug products with therapeutic equivalence evaluations: Orange Book. FDA.
U.S. Food and Drug Administration. (2024b). Drugs@FDA: Arava and leflunomide products. FDA.
U.S. Patent and Trademark Office. (2024). Manual of Patent Examining Procedure, Chapter 2700: Patent term. USPTO.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
Germany	2854439	Dec 16, 1978

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	222680	⤷ Start Trial
Argentina	226894	⤷ Start Trial
Austria	1067	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,351,841

Summary for Patent: 4,351,841