United States Patent 4,771,041: Foscarnet Scope, Claim Analysis, Expiration, and Patent Landscape

US Patent 4,771,041 covers the use of phosphonoformic acid, commonly called foscarnet, and its physiologically acceptable salts for treating virus-induced disease, inhibiting retroviral replication, and preparing oral pharmaceutical compositions. The patent issued on September 13, 1988, under the pre-URAA 17-year patent term and therefore expired on September 13, 2005, absent a terminal disclaimer or applicable term adjustment. It does not create a current U.S. barrier to generic foscarnet, follow-on formulations, or unpatented methods of using the compound.[1]

What drug and technology does US Patent 4,771,041 cover?

US 4,771,041 covers phosphonoformic acid and physiologically acceptable salts as antiviral agents. The commercially relevant salt is foscarnet sodium, chemically trisodium phosphonoformate.

Foscarnet is an inorganic pyrophosphate analogue. Its antiviral activity results primarily from direct inhibition of viral DNA polymerases and reverse transcriptase. Unlike nucleoside analogues, it does not require intracellular phosphorylation for antiviral activity.

The patent has three principal technical categories:

Category	Claims	Covered subject matter
Virus-induced disease treatment	1-4	Administration of phosphonoformic acid or a salt to inhibit transformation of virus-infected cells
Retroviral replication inhibition	5-6	Administration to inhibit replication of a retrovirus
Oral pharmaceutical compositions	7-8	Oral human pharmaceutical preparations containing phosphonoformic acid or a salt

The claims are use and composition claims. They are not claims to a new chemical compound, a specific crystalline form, a manufacturing process, a dosage regimen, or a particular tablet or capsule design.

What does claim 1 protect?

Claim 1 covers a method of treating virus-induced disease in animals, including humans, by administering phosphonoformic acid or a physiologically acceptable salt in an amount effective to inhibit transformation of virus-infected cells.

Its material limitations are:

A virus-induced disease must be present.
The subject must be an animal, including a human.
The administered active ingredient must be phosphonoformic acid or a physiologically acceptable salt.
The quantity must be effective to inhibit transformation of infected cells.

The claim does not specify:

A particular virus;
A particular disease;
A dose;
A dosing frequency;
A route of administration;
A formulation;
A specific salt;
A viral polymerase target; or
A required level of clinical response.

The phrase “inhibiting transformation” is narrower than simply inhibiting viral replication. In virology, transformation generally refers to virus-associated alteration of host-cell growth or phenotype, including oncogenic transformation. A product or method that only suppresses viral replication without inhibiting transformation could fall outside claim 1, although the scope would depend on the evidence and claim construction in a particular dispute.

What do claims 2 through 4 add?

Claims 2 through 4 narrow claim 1 by specifying the salt or route.

Claim	Additional limitation	Practical scope
2	The active ingredient is the tri-sodium salt	Covers foscarnet sodium treatment
3	The tri-sodium salt is orally administered	Covers oral foscarnet sodium treatment
4	Phosphonoformic acid or a physiologically effective salt is orally administered	Covers oral administration more broadly than claim 3

Claim 3 is narrower than claim 4 because it requires both the tri-sodium salt and oral administration. Claim 4 reaches oral administration of the parent acid and other physiologically effective salts, subject to satisfaction of the claim 1 therapeutic limitation.

The oral-route claims are commercially significant because foscarnet is best known clinically as an intravenous product. An oral composition or oral method could have been important to the original patent strategy, but the expiration of the patent removes the exclusivity effect.

What does claim 5 protect?

Claim 5 covers administering phosphonoformic acid or a physiologically acceptable salt to inhibit replication of a retrovirus.

Compared with claim 1, claim 5 changes the required therapeutic result:

Claim 1 requires inhibition of transformation of virus-infected cells.
Claim 5 requires inhibition of retroviral replication.

Claim 5 does not require a virus-induced disease, although the patient must be “in need of such treatment.” It also does not specify a route, dose, salt, viral species, or formulation.

The claim is broad enough, on its face, to encompass treatment directed to retroviruses including HIV, subject to the patent’s disclosure, enablement, and written-description support. It is not limited to cytomegalovirus, herpes simplex virus, or any other specific pathogen.

Claim 6 narrows claim 5 to oral administration.

What do claims 7 and 8 protect?

Claim 7 covers an orally administrable pharmaceutical composition for humans containing phosphonoformic acid or a physiologically acceptable salt as the active ingredient, together with a pharmaceutically acceptable carrier.

The composition must be:

Effective against virus infection or disease;
Effective to inhibit viral replication or multiplication of virus-transformed cells;
Suitable for oral human administration; and
Free of pharmaceutically unacceptable impurities.

Claim 8 narrows claim 7 by requiring the active ingredient to comprise approximately 0.1% to 50% of the preparation.

The claim does not require a specific excipient, dosage form, release profile, particle size, pH, coating, or manufacturing process. The “free of pharmaceutically unacceptable impurities” limitation is a quality-related limitation, not a defined impurity profile. Its interpretation would depend on the specification, pharmaceutical standards, and the allegedly infringing product.

The composition claims are not limited to foscarnet sodium. They encompass the parent acid and other physiologically acceptable salts if the remaining limitations are met.

How strong is the claim scope?

The patent had broad nominal scope when enforceable, but the claims also contained several potential validity and infringement pressure points.

Breadth of the therapeutic claims

Claims 1 and 5 cover classes of diseases or viruses rather than a single pathogen. That breadth could support an infringement theory against a product marketed for a covered use. It could also create enablement and written-description issues if the patent specification did not demonstrate or adequately teach the full range of claimed viruses, diseases, salts, animals, and therapeutic outcomes.

The functional terms “effective to inhibit” and “sufficient for inhibiting” are common in pharmaceutical claims. They generally require proof that the administered amount produces the claimed biological effect. They do not necessarily require a fixed dose.

Transformation versus replication

Claim 1 focuses on transformation of infected cells. Claim 5 focuses on replication of a retrovirus. These are separate limitations. Evidence of antiviral activity alone would not automatically establish infringement of claim 1 unless the activity also corresponds to inhibition of transformation.

Claim 5 is potentially more commercially relevant to antiretroviral uses, but it requires a retrovirus. A method directed solely to a nonretroviral DNA virus would not meet that limitation.

Salt coverage

The expression “physiologically acceptable salt” is broad but not unlimited. The salt must be suitable for administration and must retain the claimed therapeutic function. Claim 2 specifically identifies the tri-sodium salt, while claims 1, 4, 5, 6, and 7 cover broader salt categories.

Oral composition limitations

Claims 7 and 8 require an oral human composition. An injectable foscarnet product would not literally satisfy those claims. A product intended only for veterinary use could also present a claim 7 issue because claim 7 requires suitability for oral administration to humans.

The 0.1%-50% concentration range in claim 8 is broad. A formulation outside that range would not literally meet claim 8, but it could still fall within claim 7 if the other limitations are satisfied.

When did US Patent 4,771,041 lose exclusivity?

US 4,771,041 expired in 2005.

Event	Date
Patent issued	September 13, 1988
Governing term	17 years from issuance under pre-URAA law
Patent expiration	September 13, 2005
Current enforceability	Expired
Current Paragraph IV risk	None based solely on this patent

The patent cannot support a new U.S. infringement action for conduct occurring after expiration. Patent expiration also eliminates the ability to block FDA approval or commercial launch through this patent.

Any later patent would need to be analyzed separately. A later patent on a formulation, dosage regimen, crystalline form, manufacturing process, or new therapeutic use would not revive the expired claims of US 4,771,041.

What is the FDA and Orange Book status of foscarnet?

Foscarnet sodium is marketed in the United States as Foscavir, an intravenous antiviral product. The FDA approved Foscavir for treatment of cytomegalovirus retinitis in patients with AIDS and for acyclovir-resistant mucocutaneous herpes simplex virus infections in immunocompromised patients.[2]

The FDA-approved product is an intravenous injection, while claims 3, 4, 6, 7, and 8 principally address oral administration. That distinction matters because the patent’s oral claims do not directly map onto the approved intravenous presentation.

FDA Orange Book treatment is separate from patent validity. A listed patent may affect an ANDA applicant during its term, but an expired patent does not create a continuing launch prohibition. The Orange Book’s patent listing system also does not determine whether a claim is valid or infringed.[3]

The regulatory position can be summarized as follows:

Issue	Status
Active ingredient	Foscarnet sodium
U.S. product	Foscavir
FDA pathway	New drug application
Principal approved use	CMV retinitis in patients with AIDS
Additional approved use	Acyclovir-resistant mucocutaneous HSV in immunocompromised patients
Dosage form	Intravenous injection
US 4,771,041 status	Expired
Biosimilar pathway	Not applicable; foscarnet is a small molecule
Generic pathway	ANDA, subject to the reference product and current FDA requirements

Are Paragraph IV challenges relevant?

A Paragraph IV certification would have been relevant only while an unexpired patent listed for the reference product remained enforceable or commercially relevant.

For US 4,771,041, a present Paragraph IV challenge has no practical blocking function because the patent expired in 2005. An ANDA applicant would not need to defeat this patent to launch after expiration. A historical Paragraph IV notice, if one occurred, would not restore enforceability or create current exclusivity.

The principal current generic risks are regulatory and commercial rather than patent-based:

FDA review of pharmaceutical equivalence;
Sterility and injectable manufacturing controls;
Supply of the active pharmaceutical ingredient;
Product quality and impurity control;
Limited market size; and
The technical burden of developing an intravenous product for a narrow clinical market.

What formulations are protected by the patent?

The patent’s formulation protection is limited to oral human compositions.

Covered formulation concepts

Potentially covered compositions include oral preparations containing:

Phosphonoformic acid;
Trisodium phosphonoformate;
Another physiologically acceptable salt;
A pharmaceutically acceptable carrier; and
An active concentration within approximately 0.1%-50% for claim 8.

Formulations outside the clearest scope

The following would not directly satisfy all limitations of claim 7:

Intravenous-only formulations;
Topical preparations not suitable for oral human use;
Veterinary-only oral preparations;
Compositions lacking a pharmaceutically acceptable carrier;
Preparations that do not contain phosphonoformic acid or a covered salt.

The patent does not appear, based on the supplied claims, to claim a specific formulation architecture. It therefore does not provide modern formulation-level protection comparable to claims directed to sustained release, nanoparticulate delivery, a defined polymorph, or a specific impurity threshold.

What manufacturing and intellectual-property barriers remain?

US 4,771,041 does not create a current manufacturing monopoly. Its claims do not cover a specific synthetic route, process parameter, reactor configuration, purification sequence, or starting material.

A generic manufacturer would still face technical barriers associated with foscarnet sodium:

Control of inorganic and organic impurities;
Salt formation and assay consistency;
Stability of the aqueous product;
Sterile manufacturing;
Container-closure compatibility;
Control of pH and osmolality;
Extractables and leachables;
Injectable-product microbiological controls; and
Demonstration of pharmaceutical equivalence.

These are manufacturing and regulatory barriers, not continuing rights under the expired patent.

Which companies are challenging foscarnet exclusivity?

No current patent-based exclusivity challenge is required against US 4,771,041 because the patent expired nearly two decades ago. The competitive field is more likely to involve a small number of manufacturers capable of supplying an injectable antiviral for a specialized hospital market than a large generic competition event.

Foscarnet also has no biosimilar competition because biosimilar regulation applies to biological products, not small-molecule foscarnet sodium. Any competitor would generally use the small-molecule generic pathway or pursue an alternative NDA strategy.

What patent litigation and settlements affect US 4,771,041?

The expiration of US 4,771,041 eliminates current infringement exposure under the patent. It also means that any historical litigation, license, or settlement involving the patent would have no continuing exclusionary effect unless a separate unexpired patent or contractual restriction applied.

The supplied claim set does not identify a later patent family, terminal disclaimer, reissue, certificate of correction, or continuation that extends the term of the issued claims. The patent’s expiration should therefore be treated as final for the claims identified here.[1]

How does US 4,771,041 compare with modern foscarnet patent strategies?

Patent strategy	US 4,771,041	Modern commercial strategy
Broad therapeutic method	Yes	Usually narrowed to a defined disease, patient population, or dosing regimen
Retroviral use	Yes	Likely limited by clinical data and approved indication
Oral composition	Yes	Often replaced by specific excipient, release, salt, or stability claims
Injectable formulation	Not apparent from supplied claims	May be claimed through concentration, container, stability, or administration parameters
Manufacturing process	Not apparent from supplied claims	Process patents can protect yield, purity, and scalable production
New chemical entity	No clear current barrier	NCE protection would depend on separate compound patents
Biosimilar protection	Not applicable	Not relevant to this small molecule
Current enforceability	No	Depends on later patent families

Does the patent create current generic-entry risk?

US 4,771,041 does not create a current generic-entry risk because it is expired. A generic foscarnet sodium applicant would need to address the FDA’s current requirements for the applicable reference product, but it would not need to wait for this patent to expire.

The strongest remaining commercial barriers are likely:

Low demand for a specialized antiviral;
Limited manufacturer economics;
Injectable-product compliance;
Active pharmaceutical ingredient sourcing;
Hospital procurement requirements; and
Potential unlisted or later patents unrelated to US 4,771,041.

These barriers can delay entry without creating patent exclusivity.

Key Takeaways

US 4,771,041 covers foscarnet and physiologically acceptable salts for antiviral treatment, retroviral replication inhibition, and oral pharmaceutical compositions.
Claims 1-4 focus on inhibiting transformation of virus-infected cells.
Claims 5-6 focus on inhibiting retroviral replication.
Claims 7-8 cover oral human compositions, with claim 8 requiring approximately 0.1%-50% active ingredient.
The patent issued on September 13, 1988, and expired on September 13, 2005.
Foscarnet sodium is marketed as Foscavir, an FDA-approved intravenous product for CMV retinitis and acyclovir-resistant HSV infections.
The patent does not provide current Orange Book exclusivity or a present Paragraph IV barrier.
Biosimilar competition is irrelevant because foscarnet sodium is a small molecule.
Current entry barriers are primarily regulatory, manufacturing, supply-chain, and commercial rather than patent-based.

FAQs

Is foscarnet sodium still patent protected in the United States?

Not by US 4,771,041. That patent expired on September 13, 2005. Separate later patents would require independent analysis.

Does US 4,771,041 cover intravenous Foscavir?

The supplied claims do not clearly cover the approved intravenous presentation. Claims 3, 4, 6, 7, and 8 require or focus on oral administration, while claims 1, 2, and 5 do not specify a route.

Could a company patent a new oral foscarnet formulation?

A later patent could potentially claim a novel oral formulation, dosage regimen, salt, stability profile, delivery system, or manufacturing process if it satisfies patentability requirements. Such a patent would not extend US 4,771,041.

Is foscarnet eligible for an FDA biosimilar application?

No. Foscarnet sodium is a small-molecule drug. A competing product would generally use the ANDA pathway or another small-molecule FDA pathway, not the biosimilar pathway.

Does the 0.1%-50% range in claim 8 cover all oral foscarnet products?

No. Claim 8 requires the active ingredient to comprise approximately 0.1%-50% of the preparation and also requires satisfaction of claim 7. Oral products outside that concentration range could still raise claim 7 issues, but not literal claim 8 issues.

References

United States Patent No. 4,771,041. (1988, September 13). Methods and pharmaceutical compositions using phosphonoformic acid. United States Patent and Trademark Office.
U.S. Food and Drug Administration. (2023). Foscavir (foscarnet sodium) injection prescribing information. FDA.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book. FDA.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
Sweden	7607496	Jul 1, 1976

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	369014	⤷ Start Trial
Austria	A465477	⤷ Start Trial
Australia	2650577	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,771,041

Summary for Patent: 4,771,041