The US patent estate for fosphenytoin is no longer an enforceable barrier to US generic entry. US Patent No. 4,260,769 covers phosphoryloxymethyl derivatives of 5,5-diphenylhydantoin, including the compound now known as fosphenytoin and its pharmaceutically acceptable salts. The patent issued April 7, 1981, and expired April 7, 1998 under the pre-Uruguay Round patent-term rule. FDA regulatory exclusivity for the fosphenytoin product also expired years ago. Current commercial risk therefore turns on FDA approval, manufacturing capability, supply reliability, and any later formulation or process patents, not on US 4,260,769.

US Drug Patent 4,260,769: Fosphenytoin Scope, Claims, Expiration, and Patent Landscape

What drug and technology does US Patent 4,260,769 protect?

US 4,260,769 protects phosphorylated prodrugs of phenytoin, the anticonvulsant 5,5-diphenylhydantoin. The commercially important compound within the claims is 3-phosphoryloxymethyl-5,5-diphenylhydantoin, commonly known as fosphenytoin.

Fosphenytoin is administered as a water-soluble prodrug of phenytoin. The phosphate group increases aqueous solubility and permits intravenous or intramuscular administration. After administration, enzymatic cleavage converts fosphenytoin to phenytoin.

The patent is associated with the technology later commercialized as Cerebyx, an injectable fosphenytoin sodium product marketed by Parke-Davis and later Pfizer. The FDA approved Cerebyx in 1996 for use in situations requiring parenteral phenytoin therapy, including status epilepticus and seizure prophylaxis or treatment in neurosurgical settings (FDA, 1996).

What are the key claims of US 4,260,769?

The three claims create a layered chemical scope.

Claim	Scope	Commercial significance
Claim 1	5,5-Diphenylhydantoin compounds in which at least one hydantoin nitrogen bears a phosphoryloxymethyl substituent; includes specified salts, quaternary salts, and N-oxides	Broad genus claim
Claim 2	Compounds in which one nitrogen bears hydrogen and the other bears the phosphoryloxymethyl group	Covers monosubstituted compounds, including the principal fosphenytoin structure
Claim 3	3-Phosphoryloxymethyl-5,5-diphenylhydantoin	Narrow species claim directed to fosphenytoin itself

The central substituent is:

--CH(R1)--O--P(O)(OH)2

where R1 is hydrogen or a C1-C7 straight- or branched-chain alkyl group.

The proviso that the two R groups cannot both be hydrogen excludes unsubstituted phenytoin. At least one hydantoin nitrogen must carry the phosphoryloxymethyl-type substituent.

How broad is claim 1?

Claim 1 is materially broader than fosphenytoin sodium. It covers:

Monosubstituted derivatives, where one nitrogen remains hydrogen.
Potentially disubstituted derivatives, where both nitrogen atoms bear qualifying substituents.
Variants in which R1 is hydrogen or C1-C7 alkyl.
Pharmaceutically acceptable acid-addition or basic salts.
Certain C1-C4 alkylhalide quaternary salts.
N-oxide forms.

The claim is therefore a genus claim around a phosphate-linked prodrug platform. It is not limited to the disodium salt, injectable dosage form, concentration, buffer, container, route of administration, or clinical indication.

What does claim 2 add?

Claim 2 narrows the genus to compounds having one unsubstituted nitrogen and one nitrogen bearing the phosphoryloxymethyl group. This is the substitution pattern associated with 3-phosphoryloxymethyl-5,5-diphenylhydantoin.

Claim 2 has less breadth than claim 1 but remains product-focused. It does not require a particular formulation or manufacturing process.

What does claim 3 cover?

Claim 3 identifies the specific chemical entity:

3-phosphoryloxymethyl-5,5-diphenylhydantoin.

In commercial use, fosphenytoin is generally supplied as fosphenytoin sodium, often described chemically as the disodium phosphate salt of the fosphenytoin anion. Claim 1 expressly reaches pharmaceutically acceptable basic salts, which is the relevant mechanism for covering the sodium salt.

What is the patent expiration date for US 4,260,769?

US Patent 4,260,769 expired April 7, 1998.

Event	Date
Patent issued	April 7, 1981
Applicable US patent term	17 years from issue
Patent expiration	April 7, 1998
Current status	Expired and unenforceable

The patent was granted before the 1995 change that generally shifted US utility patent terms to 20 years from the earliest effective nonprovisional filing date. For patents in this period, the governing term was generally 17 years from grant, absent unusual terminal-disclaimer or extension issues.

There is no current patent-term-extension value associated with US 4,260,769 that would restore exclusivity. Patent-term extension under 35 U.S.C. §156 was not available in a way that would extend this patent into the modern generic-entry period.

When did fosphenytoin lose FDA market exclusivity?

Fosphenytoin’s FDA regulatory exclusivity ended well before the present generic market.

Cerebyx received FDA approval in 1996. If treated as a new chemical entity for Hatch-Waxman purposes, its five-year NCE exclusivity would have ended in 2001. In any event, that period has long expired. Regulatory exclusivity is separate from patent protection and does not extend the life of US 4,260,769.

Exclusivity type	Approximate end date	Current status
US 4,260,769 patent term	April 7, 1998	Expired
Potential five-year NCE exclusivity for the 1996 approval	2001	Expired
Three-year new clinical investigation exclusivity, if applicable to later supplement activity	No current effect	Expired
Orphan-drug exclusivity	Not the principal protection for fosphenytoin	No current barrier

The key commercial point is that an ANDA applicant today does not face a live US 4,260,769 patent claim.

What is the Orange Book status of fosphenytoin?

The reference product is Cerebyx, an injectable fosphenytoin sodium product. FDA Orange Book analysis must distinguish between:

Historical patent listings associated with the reference product.
Patents that remain unexpired and could require a Paragraph IV certification.
Expired patents that have no current exclusionary effect.

US 4,260,769 falls into the third category. Even if it appears in historical regulatory or patent records associated with the product, its expiration prevents it from blocking an ANDA based on a current Paragraph IV certification.

Orange Book issue	Assessment for US 4,260,769
Drug substance patent	Historical coverage of fosphenytoin chemistry
Formulation patent	Not established by the claims supplied
Method-of-use patent	Not established by the claims supplied
Current unexpired patent barrier	No
Current Paragraph IV relevance	None for this patent
Generic entry effect	No delay attributable to this patent

FDA’s Orange Book identifies patents and exclusivity associated with approved products, but it does not convert an expired patent into an enforceable right. A generic applicant may still need to address any separate, later-expiring patents listed for a reference product, but that analysis is distinct from US 4,260,769 (FDA, 2024).

Does US 4,260,769 cover fosphenytoin sodium?

Yes, the claims are drafted to cover the underlying fosphenytoin compound and qualifying salts rather than only a neutral free-acid laboratory form.

Claim 3 names the parent phosphoryloxymethyl compound. Claim 1 separately includes pharmaceutically acceptable basic salts. Sodium is the commercially relevant counterion because fosphenytoin is supplied as a highly water-soluble sodium salt for parenteral use.

The patent does not appear limited to:

A disodium stoichiometry.
A particular pH.
A specific concentration.
A particular vial or syringe.
Intravenous administration.
Intramuscular administration.
Treatment of status epilepticus.
Any particular excipient system.

That distinction matters. The patent’s protection is principally composition-of-matter protection. It does not, based on the supplied claims, provide independent protection for later-developed dosage forms or manufacturing methods.

What formulations are protected by the patent?

The claims supplied do not claim a formulation in the conventional pharmaceutical sense. They claim chemical compounds and salts.

A formulation claim would typically identify elements such as:

Active ingredient concentration.
Buffer system.
pH range.
Tonicity agent.
Preservative.
Container or closure.
Freeze-dried or liquid presentation.
Stability parameters.
Administration device.

None of those limitations appears in claims 1 through 3. A product could therefore fall within the chemical claims regardless of whether it was supplied in a vial, prefilled syringe, infusion bag, or another parenteral presentation, provided the active compound met the claimed structure. That protection ended when the patent expired.

What method-of-use protection does US 4,260,769 provide?

The supplied claims contain no method-of-use claim. They do not recite treatment of epilepsy, status epilepticus, neurosurgical seizure prevention, loading-dose therapy, or any other therapeutic indication.

The patent therefore does not independently protect:

Treating generalized tonic-clonic seizures.
Treating status epilepticus.
Preventing perioperative seizures.
Replacing injectable phenytoin with fosphenytoin.
A specific dosing regimen.
A specific infusion rate.

Any method-of-use protection would have to arise from a separate patent. Such a patent would also need to be unexpired and listed or otherwise enforceable against the relevant generic product.

Were Paragraph IV challenges relevant to this patent?

Paragraph IV challenges are no longer relevant to US 4,260,769 because the patent expired in 1998.

A Paragraph IV certification is used when an ANDA applicant asserts that a listed patent is invalid, unenforceable, or will not be infringed. For an expired patent, the applicant does not need to litigate the patent as a current barrier to approval. The patent cannot support the 30-month stay that applies to a timely infringement action involving an unexpired listed patent.

Historical ANDA filings for fosphenytoin may have addressed other listed patents or exclusivity periods. Those proceedings do not revive US 4,260,769 or extend its term.

Which companies have challenged or competed with Cerebyx?

The relevant competitive set consists of:

Pfizer, through Cerebyx.
FDA-approved generic fosphenytoin sodium injection manufacturers.
Manufacturers supplying hospital and institutional channels.
Potential suppliers of injectable phenytoin, which remains a therapeutic alternative despite different tolerability and administration characteristics.

Generic competition is commercially feasible because the core compound patent expired more than two decades ago. The principal barriers are technical and regulatory rather than basic composition-of-matter exclusivity.

A generic applicant must demonstrate pharmaceutical equivalence and bioequivalence or otherwise satisfy the applicable ANDA requirements. For an injectable product, the review also focuses on sterility assurance, particulate control, container closure, extractables and leachables, stability, manufacturing controls, and supply-chain consistency.

How strong is the patent estate for fosphenytoin?

The estate created by US 4,260,769 was strong historically but has no present exclusionary strength in the United States.

Patent dimension	Historical assessment	Current assessment
Core molecule	Strong composition-of-matter coverage	Expired
Fosphenytoin sodium	Reached through compound and salt language	Expired
Alternative salts	Broadly addressed by claim 1	Expired
Formulations	Not directly claimed in supplied claims	Must be assessed separately
Therapeutic uses	Not claimed in supplied claims	Must be assessed separately
Manufacturing process	Not claimed in supplied claims	Must be assessed separately
US generic blocking power	High before expiration	None from this patent

The patent was commercially important because it covered the prodrug itself rather than merely one use or one presentation. That structure reduced opportunities to design around the patent while it was alive. After expiration, the same breadth became a public-domain asset available to generic developers.

What design-around options exist for competitors?

Before expiration, a competitor seeking to avoid the patent would have needed to alter the protected chemistry materially. Potential design-around paths could have included:

Using a non-phosphate prodrug of phenytoin.
Modifying the hydantoin ring or phenyl substitution pattern.
Using a different linker between the hydantoin nitrogen and solubilizing group.
Developing a formulation of phenytoin that did not contain the claimed phosphoryloxymethyl compound.
Pursuing a different active pharmaceutical ingredient with comparable clinical utility.

A product containing the claimed fosphenytoin structure could not avoid the composition claims merely by changing the sodium counterion, vial, concentration, or administration route.

Today, design-around pressure is less important because the claims are expired. Developers can use the claimed fosphenytoin chemistry, subject to FDA requirements and any separate unexpired intellectual-property rights.

What manufacturing and intellectual-property barriers remain?

The expired patent does not eliminate development barriers. Fosphenytoin is a phosphate prodrug requiring controlled synthesis, purification, salt formation, and sterile drug-product manufacture.

Important technical issues include:

Control of phosphate ester impurities.
Residual solvent and elemental impurity limits.
Control of hydrolysis and degradation products.
Sterile filtration or aseptic processing.
Compatibility with the selected container closure.
Control of pH and osmolality.
Prevention of particulate contamination.
Demonstration of conversion to phenytoin within the expected pharmacokinetic profile.

These requirements can limit the number of reliable suppliers even when no blocking patent remains. They are manufacturing and regulatory barriers, not rights created by US 4,260,769.

What is the current litigation and settlement position?

US 4,260,769 cannot support current infringement litigation because it expired in 1998. Any historical litigation or settlement involving fosphenytoin would have to be analyzed against the patent term and the separate patent rights asserted at that time.

No settlement can extend the statutory life of an expired patent against unrelated third parties. A private agreement could affect contracting parties, but it would not recreate market exclusivity for the compound generally.

For current diligence, the relevant litigation questions are:

Whether any later formulation patent remains unexpired.
Whether a process patent is being asserted against a manufacturer.
Whether a supplier has entered a settlement affecting launch timing.
Whether FDA has approved a new dosage form with separate exclusivity.
Whether a product-specific injunction or supply agreement affects distribution.

Those issues are outside the claims supplied for US 4,260,769.

How does fosphenytoin compare with phenytoin patent protection?

Fosphenytoin and phenytoin occupy different patent and commercial positions.

Attribute	Phenytoin	Fosphenytoin
Active chemistry	5,5-Diphenylhydantoin	Phosphate prodrug of phenytoin
Water solubility	Limited	Substantially improved
Injectable use	Requires formulation controls, including alkalinity concerns	Designed for parenteral prodrug delivery
US 4,260,769 coverage	Parent structure excluded when both R groups are hydrogen	Directly covered by claims 1-3
Current core patent barrier	Expired	Expired
Main commercial issue	Established generic competition	Injectable manufacturing and hospital supply

US 4,260,769 did not claim ordinary unsubstituted phenytoin because its proviso excludes the case in which both R groups are hydrogen. The patent instead covered the phosphate prodrug strategy.

Key Takeaways

US 4,260,769 covers phosphoryloxymethyl derivatives of 5,5-diphenylhydantoin.
Claim 3 specifically covers 3-phosphoryloxymethyl-5,5-diphenylhydantoin, the fosphenytoin active moiety.
Claim 1 reaches pharmaceutically acceptable salts, including the sodium salt used in Cerebyx.
The patent issued April 7, 1981 and expired April 7, 1998.
The patent contains composition-of-matter claims, not formulation or method-of-use claims.
It cannot support a current Paragraph IV dispute, 30-month stay, injunction, or generic launch delay.
Fosphenytoin’s FDA exclusivity has also expired.
Current commercial exposure is concentrated in injectable manufacturing, sterile production, regulatory compliance, hospital contracting, and any separate later-expiring patents.
The patent estate was historically strong but has no current US exclusionary value.

FAQs

Does US 4,260,769 cover the disodium salt of fosphenytoin?

Yes. Claim 1 covers pharmaceutically acceptable basic salts of the claimed phosphoryloxymethyl hydantoin structure. The commercially used disodium salt falls within that salt-related scope, but the patent expired in 1998.

Can a generic manufacturer use the fosphenytoin molecule today?

Yes, US 4,260,769 no longer prevents use of the claimed compound. The manufacturer must still obtain FDA approval and comply with sterile injectable manufacturing, quality, stability, and bioequivalence requirements.

Did US 4,260,769 protect Cerebyx’s intravenous dosing regimen?

No. The supplied claims are compound claims. They do not recite intravenous administration, dose, infusion rate, indication, or clinical regimen.

Does changing the fosphenytoin counterion avoid claim 1?

During the patent term, changing the counterion would not necessarily have avoided claim 1 because the claim includes pharmaceutically acceptable basic salts. That issue has no current commercial significance because the patent is expired.

Is fosphenytoin subject to biosimilar regulation?

No. Fosphenytoin is a chemically synthesized small molecule, not a biologic. Generic fosphenytoin products are reviewed under the ANDA framework rather than the biosimilar pathway under the Public Health Service Act.

References

Food and Drug Administration. (1996). Cerebyx (fosphenytoin sodium) injection: Prescribing information. U.S. Department of Health and Human Services.
Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations, 44th edition. U.S. Department of Health and Human Services.
U.S. Patent and Trademark Office. (1981). US Patent No. 4,260,769: Phosphoryloxymethyl derivatives of 5,5-diphenylhydantoin. U.S. Department of Commerce.
U.S. Code. (2024). 35 U.S.C. §§ 154 and 156: Patent term and patent term extension. Government Publishing Office.
U.S. Code. (2024). 21 U.S.C. § 355: New drugs and abbreviated applications. Government Publishing Office.

Details for Patent: 4,260,769

Summary for Patent: 4,260,769