United States Patent 5,679,709: Leflunomide Claims, Patent Scope, Expiration, and Generic Risk

U.S. Patent No. 5,679,709 covers leflunomide, the active pharmaceutical ingredient in Arava. Its claims include the chemical compound, physiologically tolerated salts, dosage-form compositions, and therapeutic methods for autoimmune diseases other than systemic lupus erythematosus. The patent issued on October 21, 1997, and its original 17-year patent term expired on October 21, 2014. The patent therefore no longer creates an enforceable barrier to generic leflunomide entry in the United States.[1]

The patent remains commercially important as the foundational U.S. patent for leflunomide, but its current value is historical and landscape-related rather than exclusionary.

What compound does U.S. Patent 5,679,709 protect?

Claim 1 covers N-(4-trifluoromethylphenyl)-2-cyano-3-hydroxycrotonamide, the compound internationally known as leflunomide.

Item	Description
Generic name	Leflunomide
Brand name	Arava
Chemical class	Isoxazole immunomodulator precursor and dihydroorotate dehydrogenase inhibitor
Molecular formula	C12H9F3N2O2
Molecular weight	270.21 g/mol
Patent	U.S. Patent No. 5,679,709
Patent holder at issuance	Hoechst Marion Roussel, Inc.
Issue date	October 21, 1997
Original expiration	October 21, 2014
FDA approval	Arava NDA 020905, 1998
Current status	Expired

Leflunomide is rapidly converted in vivo to teriflunomide, also known as A771726. Teriflunomide is the pharmacologically active metabolite that inhibits dihydroorotate dehydrogenase and suppresses de novo pyrimidine synthesis in activated lymphocytes.[2]

Claim 1 is a compound claim. It does not depend on a particular indication, dose, formulation, route of administration, salt, or manufacturing process. A product containing the claimed molecular structure would fall within the literal scope of claim 1, subject to ordinary claim-construction and infringement principles.

How broad is claim 1 of U.S. Patent 5,679,709?

Claim 1 is narrow by chemical genus standards but broad in commercial effect. It claims one defined molecule rather than a Markush class.

The claim does not require:

a particular crystalline form;
a specified purity level;
a particular particle size;
a specific excipient;
a particular dosage strength;
oral, injectable, or rectal administration;
treatment of a specific autoimmune disease;
conversion to teriflunomide after administration.

A pharmaceutical product containing leflunomide as the active ingredient would have implicated claim 1 during the patent term regardless of whether it was sold as a tablet, capsule, solution, suspension, or another dosage form.

The claim also covers the molecule when used as an intermediate or active pharmaceutical ingredient, although infringement analysis would depend on the accused activity and applicable statutory provisions. The claim does not expressly cover teriflunomide. Teriflunomide is a separate chemical compound and cannot be treated as identical to leflunomide merely because it is leflunomide's active metabolite.

What salts are protected by claim 2?

Claim 2 covers physiologically tolerated salts of leflunomide.

The claim extends the chemical protection beyond the neutral compound in claim 1, but only to salts that satisfy both elements of the limitation:

the salt must be formed from leflunomide; and
the salt must be physiologically tolerated.

The claim does not identify specific counterions. Its coverage could therefore include pharmaceutically acceptable acid-addition or base-derived salt forms, depending on the compound's ionization characteristics and the technical meaning of "salt" in the patent specification.

Claim 2 does not automatically cover every derivative, prodrug, metabolite, ester, tautomer, or covalent modification of leflunomide. A chemically modified compound generally requires a separate claim basis or an infringement theory under the doctrine of equivalents.

A practical point is that claims 3 through 5 refer to "the compound of claim 1," not expressly to "the compound of claim 1 or a salt thereof." On the claim language supplied, the dosage-form claims are more clearly directed to neutral leflunomide than to the salts covered by claim 2.

What formulations and dosage forms are protected?

Claims 3, 4, and 5 are composition claims defined partly by dosage amount and route of administration.

Claim	Dosage form or route	Dose limitation	Principal scope
3	Solid dosage unit	10 to 200 mg	Solid unit containing leflunomide
4	Injection	1 to 30 mg	Injectable composition containing leflunomide and a physiologically acceptable carrier
5	Rectal administration	50 to 300 mg	Rectal composition containing leflunomide

What does claim 3 cover?

Claim 3 covers a solid dosage unit containing 10 to 200 mg of leflunomide. The claim is broad enough on its face to include tablets, capsules, powders, granules, and other solid unit-dose presentations, provided the product meets the claim's composition and quantity limitations.

The claim does not require a particular excipient, dissolution profile, coating, release mechanism, or manufacturing method. A sustained-release tablet would not escape claim 3 merely because it had a modified release profile, unless another claim limitation or prosecution-history limitation applied.

What does claim 4 cover?

Claim 4 covers an injectable composition containing leflunomide, a physiologically acceptable carrier, and a dose of 1 to 30 mg.

The claim requires an injection-related dosage presentation. It is narrower than claim 1 because an accused product must satisfy the route or dosage-form limitation. It also requires a carrier. A pure active pharmaceutical ingredient sold without a pharmaceutical carrier would not meet the composition limitation as written.

The claim is commercially less significant than claim 3 because Arava was marketed primarily as an oral tablet. The patent's injectable coverage would have been relevant to a parenteral development program, but it did not define the principal commercial product.

What does claim 5 cover?

Claim 5 covers a leflunomide composition at a 50-to-300-mg dose for rectal administration.

The claim is route-specific and dose-specific. It would not ordinarily cover an oral tablet merely because the tablet contained 50 to 300 mg. It also would not necessarily cover a rectal product outside the stated dose range.

Claims 3 through 5 are formulation and presentation claims, but they are not sophisticated formulation claims. They do not recite specific excipient systems, particle engineering, coating chemistry, release kinetics, stability conditions, or device components.

What autoimmune diseases are covered by claims 6 through 8?

Claims 6 through 8 are method-of-treatment claims. All three exclude systemic lupus erythematosus.

Claim	Therapeutic subject matter	Key limitation
6	Treatment of autoimmune diseases	SLE expressly excluded
7	Reduction of B-cell-produced self-antibodies	Patient must have an autoimmune disease other than SLE
8	Restoration of inhibited T-lymphocyte proliferation to normal response levels	Patient must have an autoimmune disease other than SLE

Claim 6: treatment of autoimmune diseases

Claim 6 covers administering an effective amount of leflunomide or a physiologically tolerated salt to treat an autoimmune disease other than systemic lupus erythematosus.

The claim is indication-broad. It is not limited to rheumatoid arthritis, even though rheumatoid arthritis became the principal approved indication for Arava. Depending on the evidence and claim construction, the claim could reach use in conditions such as:

rheumatoid arthritis;
psoriatic arthritis;
multiple sclerosis;
inflammatory bowel disease;
autoimmune thyroid disease;
autoimmune uveitis;
other immune-mediated disorders.

The exclusion of systemic lupus erythematosus is material. Treatment of SLE cannot satisfy claim 6 as written, even if the treatment uses leflunomide and otherwise meets the claim.

Claim 7: reduction of B-cell-produced self-antibodies

Claim 7 adds a functional immunological result. The treatment must reduce self-antibodies produced by B cells in a patient suffering from an autoimmune disease other than SLE.

This claim is narrower than claim 6 because it requires a specific biological effect. Evidence of leflunomide administration alone would not necessarily establish infringement. The claimant would need to show that the treatment was directed to, or produced, the claimed reduction in B-cell-produced self-antibodies.

The phrase "self-antibodies" is functionally important. It distinguishes antibodies directed against self-antigens from ordinary protective antibodies or antibodies generated during vaccination or infection.

Claim 8: restoration of T-lymphocyte proliferation

Claim 8 covers administration of leflunomide or a tolerated salt to restore inhibited T-lymphocyte proliferation to normal response levels in an autoimmune-disease patient other than an SLE patient.

The claim is also functional. It requires an immunological response involving T-cell proliferation and a restoration toward normal levels. A product label that merely identifies an immunomodulatory mechanism would not necessarily establish practice of claim 8 without evidence relating to the claimed therapeutic result.

When did U.S. Patent 5,679,709 lose exclusivity?

U.S. Patent 5,679,709 expired on October 21, 2014, based on the pre-Uruguay Round Agreements Act patent-term rule applicable to the patent's filing date and issuance history.[1]

The principal timeline is:

Date	Event
Pre-1997	Patent application and foreign priority proceedings
October 21, 1997	U.S. patent issued
1998	FDA approved Arava for rheumatoid arthritis
2003	FDA approved Arava for psoriatic arthritis
October 21, 2014	Original patent term expired
After 2014	Generic leflunomide products marketed without this patent as a blocking right

A patent term adjustment or pediatric extension would have to appear in the official USPTO and FDA records to change the operative date. The core patent record identifies the patent as expired, and the patent is not a current barrier to approval or launch of a leflunomide generic.

FDA approval of an ANDA can occur before commercial launch. A generic applicant may obtain approval based on an ANDA and wait for business, manufacturing, litigation, settlement, or market conditions before selling the product.

What is the Orange Book status of leflunomide?

The FDA Orange Book lists approved drug products and relevant patent and exclusivity information for approved products.[3] Arava was approved under NDA 020905, and leflunomide tablets are available through multiple abbreviated new drug applications.

The original compound patent was the principal patent associated with the active ingredient. Because U.S. Patent 5,679,709 expired in 2014, it no longer supports a current Paragraph IV enforcement action against a generic applicant.

Orange Book analysis must distinguish among:

listed patents;
regulatory exclusivity;
patent expiration;
pediatric exclusivity;
pending or approved ANDAs;
current commercial marketing.

An expired Orange Book patent can remain important for historical analysis but cannot independently delay a new generic approval today. Orange Book listings also do not establish infringement. They identify patents that the NDA holder represented as relevant to the approved product.

Were there Paragraph IV challenges to leflunomide?

Leflunomide was subject to the standard generic-drug pathway under section 505(j) of the Federal Food, Drug, and Cosmetic Act. A generic applicant could certify that listed patents had expired, would expire before commercial marketing, were invalid, or would not be infringed.

During the patent term, a Paragraph IV certification to U.S. Patent 5,679,709 could have triggered the statutory 30-month stay if the NDA holder filed a timely infringement action. After expiration, the patent could no longer support a meaningful Paragraph IV-based launch delay.

The key commercial effect was the transition from patent-protected Arava to multiple generic leflunomide tablet suppliers. Because the patent covered the active molecule, a successful Paragraph IV strategy would have required invalidity, noninfringement, or a negotiated launch date. Once the patent expired, the central compound barrier disappeared.

Which companies compete in the leflunomide market?

Leflunomide is a small-molecule generic market, not a biosimilar market. Companies that have marketed or obtained U.S. approvals for generic leflunomide have included major generic manufacturers such as:

Teva Pharmaceuticals;
Mylan, now part of Viatris;
Sandoz;
Apotex;
Dr. Reddy's Laboratories;
Zydus;
other ANDA sponsors depending on product strength and market period.

The competitive product is generally an oral tablet in 10-mg, 20-mg, and 100-mg strengths. The 100-mg tablet is commonly used for loading-dose therapy, while the 20-mg tablet is the principal maintenance strength. A 10-mg strength is used where dose reduction is clinically appropriate.[2]

The existence of approved generic tablets does not establish that every manufacturer practices every formulation claim in the patent. It does establish that the expired compound claims no longer prevent ordinary generic leflunomide commercialization.

Is there biosimilar risk for leflunomide?

No. Leflunomide is a chemically synthesized small molecule, not a biologic. Biosimilar provisions under the Public Health Service Act do not apply.

The relevant competitive risks are:

ANDA-approved generic tablets;
additional generic suppliers;
pharmacy benefit substitution;
hospital and distributor contracting;
price erosion;
inventory and supply reliability;
potential reformulations or alternate dosage forms.

Teriflunomide, marketed as Aubagio, is a related but separate product. It is not a biosimilar to leflunomide. Teriflunomide has its own FDA approval, patents, regulatory exclusivity history, and generic competition profile.

How strong is the patent estate for leflunomide?

The patent estate was strong during the period when U.S. Patent 5,679,709 was enforceable because claim 1 covered the active molecule itself. A composition-of-matter claim generally creates a broader commercial barrier than a claim limited to a particular formulation or indication.

Its principal strengths were:

direct coverage of leflunomide;
coverage of physiologically tolerated salts;
broad autoimmune-disease method language;
no dependence on a particular tablet excipient or manufacturing process.

Its limitations were:

expiration in 2014;
express exclusion of SLE;
narrower dosage-form claims;
functional limitations in claims 7 and 8;
no express claim to teriflunomide;
no clear coverage of every possible derivative, metabolite, polymorph, or formulation.

After expiration, the patent estate has no remaining blocking strength in the United States. Later patents, if any, would have required independent validity and enforceability and could not revive the expired compound claim.

What patent litigation and settlements affected generic entry?

The principal litigation risk during the protected period would have centered on ANDA applicants certifying against the compound patent and any later-listed Arava patents. The likely litigation issues included:

validity of the compound claim;
infringement by generic leflunomide tablets;
patent-term calculation;
Orange Book listing status;
timing of generic launch;
potential settlement or license dates.

A current infringement case based solely on U.S. Patent 5,679,709 would be barred by expiration. Historical settlement agreements could have controlled launch dates before expiration, but those agreements do not extend the patent term or create a current statutory exclusion beyond the parties and applicable agreement terms.

No current biosimilar litigation is relevant because leflunomide is not a biologic. Any live dispute involving a leflunomide product would more likely concern manufacturing, labeling, trademark, supply, regulatory compliance, or a later patent rather than the expired compound patent.

What generic launch scenarios existed?

During the patent term, generic entry could have occurred through four principal scenarios:

Paragraph III certification: launch after patent expiration.
Paragraph IV certification: earlier launch after a favorable invalidity or noninfringement outcome, or after a settlement.
Authorized generic or license: launch under an agreement with the patent owner.
At-risk launch: launch before final resolution, exposing the manufacturer to damages and injunction risk.

After October 21, 2014, the fourth scenario ceased to be relevant for this patent. The market shifted to ordinary post-expiration generic competition.

What geographic coverage did the patent provide?

U.S. Patent 5,679,709 provided rights only in the United States. It did not directly block sales in Europe, Japan, Canada, or other territories.

The corresponding international landscape depended on national counterparts, national prosecution, local patent-term rules, supplementary protection certificates, pediatric extensions, and country-specific validity outcomes. A U.S. expiration date cannot be transferred automatically to foreign jurisdictions.

For global freedom-to-operate analysis, the relevant questions are:

whether a national compound patent issued;
whether the national patent expired;
whether a supplementary protection certificate extended protection;
whether a formulation or process patent remains in force;
whether the target country permits regulatory stockpiling or early working;
whether local litigation or settlement agreements restrict launch.

Do manufacturing patents still create barriers?

U.S. Patent 5,679,709 includes process-related disclosure, but the claims supplied do not include an independently recited manufacturing process. The asserted claims are directed to the compound, salts, compositions, and therapeutic methods.

A manufacturer could still face separate patent risk from later patents covering:

synthetic routes;
starting materials;
impurity control;
crystalline forms;
particle-size distributions;
tablet formulations;
stability systems;
manufacturing scale-up;
specific indications or dosing regimens.

Those rights must be analyzed separately from the expired patent. They cannot be inferred from the claims of U.S. Patent 5,679,709.

Key Takeaways

U.S. Patent 5,679,709 is the foundational U.S. patent for leflunomide.
Claim 1 covers the leflunomide molecule itself.
Claim 2 covers physiologically tolerated leflunomide salts.
Claims 3 through 5 cover specified solid, injectable, and rectal dosage presentations.
Claims 6 through 8 cover autoimmune-disease treatment, B-cell self-antibody reduction, and T-cell proliferation effects, but exclude systemic lupus erythematosus.
The patent issued on October 21, 1997, and expired on October 21, 2014.
The patent is no longer an enforceable U.S. barrier to generic leflunomide.
Leflunomide is a small molecule and has no biosimilar pathway.
Current competition is driven by ANDA-approved generic tablets, pricing, supply, and any later patents rather than by Patent 5,679,709.

FAQs

What is the active ingredient protected by U.S. Patent 5,679,709?

The patent protects leflunomide, chemically identified as N-(4-trifluoromethylphenyl)-2-cyano-3-hydroxycrotonamide.

Does U.S. Patent 5,679,709 cover teriflunomide?

No. Teriflunomide is leflunomide's active metabolite but is a separate chemical compound. The patent claims supplied do not expressly claim teriflunomide.

Can a generic company market leflunomide without licensing the patent owner?

Yes, because U.S. Patent 5,679,709 expired on October 21, 2014. A separate license could still be required for an unexpired later patent or contractual right, but not for this expired patent alone.

Does the patent cover treatment of lupus?

No. Claims 6 through 8 expressly exclude systemic lupus erythematosus.

Are injectable leflunomide products commercially blocked by this patent?

No. Claim 4 covered a defined injectable composition during the patent term, but the claim expired with the patent on October 21, 2014.

References

U.S. Patent and Trademark Office. (1997). U.S. Patent No. 5,679,709: N-(4-trifluoromethylphenyl)-2-cyano-3-hydroxycrotonic acid amide, a process for its preparation and its use. https://patents.google.com/patent/US5679709
U.S. Food and Drug Administration. (2023). Arava (leflunomide) prescribing information. Sanofi-Aventis U.S. LLC.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book. https://www.accessdata.fda.gov/scripts/cder/ob/index.cfm

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
Germany	35 34 440.7	Sep 27, 1985

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	122033	⤷ Start Trial
Austria	96669	⤷ Start Trial
Australia	588629	⤷ Start Trial
Australia	6316786	⤷ Start Trial
Canada	1275251	⤷ Start Trial
Cyprus	2033	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,679,709

Summary for Patent: 5,679,709