United States Drug Patent 3,966,962: Claim Scope, Expiration, and Prostaglandin Formulation Patent Landscape

United States Patent 3,966,962 protects a liquid pharmaceutical dosage form in which a PGE-type prostaglandin is dissolved in triacetin. The strongest commercial embodiment is a soft gelatin capsule containing 0.1 to 10 mg/mL, with narrower concentration ranges of 0.5 to 5 mg/mL for specified methylated PGE2 analogs. The patent issued in 1976 under the pre-Uruguay Round patent term and expired in 1993. Its claims have no current exclusionary force in the United States.

What does United States Patent 3,966,962 protect?

The patent protects a formulation architecture rather than a new prostaglandin molecule. Its central combination has three elements:

A PGE-type prostaglandin or specified PGE2 analog.
Triacetin as the solvent or liquid vehicle.
A stable dosage form containing the resulting solution.

Claim 1 is the independent claim and provides the broadest protection:

“A stable dosage form of a prostaglandin-like compound of the PGE-type comprising a solution of said compound in triacetin.”

The claim does not require a capsule, gelatin, a particular dose, or one of the specific compounds listed in claim 5. On its face, claim 1 can cover a stable dosage form containing any compound within the relevant PGE-type class, provided the active ingredient is in solution in triacetin.

The patent is therefore directed to stability and delivery of chemically labile prostaglandins. It does not claim:

The chemical composition of PGE2 itself.
A method for synthesizing PGE2 or its analogs.
Triacetin as a general pharmaceutical excipient.
A prostaglandin treatment method.
A dosage regimen or route of administration independent of the claimed formulation.
A solid formulation that does not contain the active ingredient dissolved in triacetin.

How are the claims structured?

Claim	Scope	Principal limitation
1	Independent formulation claim	PGE-type prostaglandin in solution in triacetin
2	Claim 1 narrowed by concentration	0.1 to 10 mg/mL
3	Claim 1 narrowed by container	Capsule made from pharmaceutically acceptable water-dispersable material
4	Claim 3 narrowed by capsule type	Soft, elastic capsule containing gelatin
5	Claim 1 narrowed by active ingredient	Eight listed PGE2 compounds or esters
6	Claims 4 and 5 combined	Listed compound in a soft gelatin capsule at 0.1 to 10 mg/mL
7	Claim 4 narrowed by compound and concentration	16,16-dimethyl-PGE2 or its methyl ester at 0.5 to 5 mg/mL
8	Claim 4 narrowed by compound and concentration	15-methyl-PGE2 or its methyl ester at 0.5 to 5 mg/mL
9	Claim 4 narrowed by compound and concentration	15(R)-15-methyl-PGE2 or its methyl ester at 0.5 to 5 mg/mL

Claims 2 through 9 are dependent claims. They inherit every limitation of the claims from which they depend.

What compounds fall within claim 5?

Claim 5 identifies eight active ingredients:

PGE2
PGE2 methyl ester
16,16-dimethyl-PGE2
16,16-dimethyl-PGE2 methyl ester
15-methyl-PGE2
15-methyl-PGE2 methyl ester
15(R)-15-methyl-PGE2
15(R)-15-methyl-PGE2 methyl ester

The claim uses a closed list for these named compounds. A formulation containing a different prostaglandin analog would not literally satisfy claim 5, although it could still fall within claim 1 if the compound qualifies as a “prostaglandin-like compound of the PGE-type.”

The distinction between claims 1 and 5 is commercially important. Claim 1 potentially reaches a broader class of PGE-type compounds. Claim 5 provides greater chemical specificity but is easier to design around by selecting an unlisted analog, subject to the scope of claim 1 and possible doctrine-of-equivalents issues.

What formulation features are required?

Triacetin must function as the solution vehicle

The claims require the prostaglandin compound to be in a solution “in triacetin.” A formulation in which triacetin is present only as a minor excipient, while the active ingredient is suspended or dissolved primarily in another vehicle, presents a weaker literal infringement case.

The key factual questions would be:

Whether the active ingredient is molecularly dissolved.
Whether triacetin is the solvent for that solution.
Whether another solvent is necessary to achieve dissolution.
Whether the finished product remains a triacetin solution at the time of administration.
Whether the formulation contains a separate phase or suspension.

The claims do not expressly require triacetin to be the only solvent. A mixed-solvent formulation could still raise infringement issues if the active compound is dissolved in a vehicle containing triacetin and the claim does not require exclusivity. The precise result would depend on claim construction and the prosecution history.

Concentration limitations apply only to narrower claims

Claim 1 has no stated concentration limitation. Claim 2 requires 0.1 to 10 mg/mL. Claims 7, 8, and 9 require 0.5 to 5 mg/mL.

A formulation at 0.05 mg/mL would fall outside claim 2, but it could still fall within claim 1 if all other limitations are met. A formulation at 12 mg/mL would similarly avoid claims 2 and 6 but would not automatically avoid claim 1.

Capsule limitations are cumulative

Claim 3 requires a capsule made from a pharmaceutically acceptable water-dispersable material. Claim 4 narrows this to a soft, elastic capsule containing gelatin.

A hard gelatin capsule, a non-gelatin polymer capsule, a tablet, a vial, or a prefilled syringe would not literally satisfy claim 4. Such products could still fall within claim 1 or claim 2 because those claims do not require a capsule.

How strong is the patent estate for the claimed technology?

The patent estate appears narrow in product architecture but broad within that architecture. Its practical strength can be assessed as follows:

Issue	Assessment
Core formulation concept	Broad within PGE-type compounds dissolved in triacetin
Chemical coverage	Broad in claim 1; expressly limited to eight compounds in claim 5
Dosage-form coverage	Claim 1 is not limited to capsules
Softgel protection	Strongly defined in claims 4 and 6-9
Concentration protection	Moderate; only dependent claims contain numerical ranges
Manufacturing protection	No express process claim in the supplied claims
Method-of-use protection	None in the supplied claims
Product-by-process protection	None
Current enforceability	None because the patent expired
Design-around potential	High through non-triacetin vehicles, non-solution forms, alternative compounds, or different dosage forms

The absence of manufacturing and method-of-use claims limits the patent’s ability to control upstream production or downstream clinical use. The patent primarily targeted a finished dosage form.

When did United States Patent 3,966,962 lose exclusivity?

United States Patent 3,966,962 issued on June 29, 1976. Under the pre-1995 patent term applicable to the patent, the term was generally 17 years from issuance. The resulting nominal expiration date was June 29, 1993.[1][2]

Event	Date or status
Patent issuance	June 29, 1976
Applicable term framework	17 years from issuance
Nominal expiration	June 29, 1993
Current enforceability	Expired
Current Paragraph IV exposure	None for this patent
Current Orange Book blocking effect	None

Any terminal disclaimer, disclaimer of claims, or unusual prosecution event could affect historical term calculations. Nothing in the supplied claims indicates such an event. The patent is far beyond its statutory term in any event.

What is the Orange Book status of Patent 3,966,962?

The patent should not be treated as a current Orange Book barrier. The FDA Orange Book lists patents submitted by sponsors for approved drug products, including patents covering drug substances, drug products, and approved methods of use under 21 C.F.R. § 314.53.[3]

Patent 3,966,962 is an old formulation patent and expired in 1993. It therefore cannot support a current 30-month stay under the Hatch-Waxman framework. It also cannot support a current listed-patent certification strategy against an ANDA applicant.

A product sponsor could historically have listed the patent if it covered an approved drug product and met FDA listing requirements. Historical listing status cannot be inferred from the claim language alone. Current commercial relevance is unaffected because an expired patent does not block FDA approval or generic launch.

Did the patent create Paragraph IV or generic-launch risk?

During its term, an ANDA applicant seeking approval for a product that met the listed formulation limitations could have faced a patent certification issue. A Paragraph IV certification would have asserted that the patent was invalid, unenforceable, or would not be infringed.

The main noninfringement positions would have included:

The product does not use triacetin as the solvent.
The prostaglandin is suspended rather than dissolved.
The active compound is outside the claimed PGE-type class.
The product uses a concentration outside a dependent claim.
The product is not a capsule where a capsule is required.
The capsule is not soft, elastic, or gelatin-containing.
The formulation does not contain one of the compounds listed in claim 5.

The principal validity challenges would likely have focused on:

Anticipation by earlier prostaglandin formulations.
Obviousness of using triacetin as a solvent or stabilizer.
Written description and enablement for the breadth of “PGE-type.”
Indefiniteness of “stable,” “prostaglandin-like,” and “water-dispersable.”
Lack of an adequately supported full scope across the claimed compounds and concentrations.

Because the patent expired decades ago, those issues no longer create a launch barrier.

What patent litigation or settlement agreements affect this patent?

The supplied material establishes no litigation, settlement, license, or covenant-not-to-sue involving United States Patent 3,966,962. The claim set alone does not establish whether the patent was asserted against a particular manufacturer.

Any historical litigation assessment would require docket-level records, assignment records, prosecution history, and FDA product records. No present-day settlement or litigation consequence can arise from an expired patent except as part of historical patent or licensing diligence.

How does this patent compare with later prostaglandin drug patents?

Patent 3,966,962 is materially different from later patents that protect:

New prostaglandin analog molecules.
Specific salts, hydrates, or crystalline forms.
Vaginal, ophthalmic, injectable, or oral delivery systems.
Controlled-release inserts or implants.
Treatment methods and dosing regimens.
Manufacturing processes.
Combination therapies.
Device-drug combinations.

For dinoprostone, also known as PGE2, later commercial protection has generally centered on route-specific products and approved dosage forms rather than the expired triacetin softgel concept. Examples include vaginal inserts, vaginal gels, and other dosage forms. A product such as a vaginal insert would generally avoid claims 3 through 9 because it is not a soft gelatin capsule. It could only raise a claim 1 issue if it used a PGE-type compound dissolved in triacetin.

Misoprostol is a PGE1 analog and is not one of the compounds expressly named in claim 5. Whether it falls within claim 1 would depend on construction of “PGE-type” and the formulation’s use of triacetin. Patents directed to misoprostol chemistry, solid-state forms, or specific products would be separate rights.

Is biosimilar risk relevant?

No. The patent concerns small-molecule prostaglandin formulations, not a biologic. Biosimilar approval under the Public Health Service Act is not the relevant pathway.

Competitive entry would be assessed through the ANDA or, where appropriate, the 505(b)(2) pathway under the Federal Food, Drug, and Cosmetic Act. An applicant could develop a different vehicle, dosage form, concentration, or route of administration without confronting this expired patent.

What geographic coverage remains?

The patent’s United States rights ended with expiration. Any corresponding foreign patents would have been governed by local patent terms and may have expired earlier or later. A United States patent does not establish protection in Europe, Japan, Canada, or other jurisdictions.

For current freedom-to-operate work, geographic review should distinguish:

United States formulation rights.
European national or regional family members.
Canadian, Japanese, and other national counterparts.
Continuations or divisionals.
Later patents covering the same active ingredient or dosage form.

The supplied information does not identify a patent family or foreign counterpart list. Patent 3,966,962 itself provides no current geographic exclusivity.

What generic launch scenarios exist today?

A manufacturer can launch a product without licensing this patent because the patent is expired. Current product strategy may still consider later patents covering:

Dinoprostone or another prostaglandin analog.
A specific finished dosage form.
A controlled-release system.
A treatment method.
A manufacturing process.
An FDA-listed product patent held by another sponsor.

The expired patent does not prevent:

A non-triacetin formulation.
A triacetin formulation.
A softgel containing a listed prostaglandin.
A product using a different PGE-type analog.
An ANDA or 505(b)(2) product, subject to current regulatory and patent requirements.

Key Takeaways

United States Patent 3,966,962 claims stable PGE-type prostaglandin solutions using triacetin.
Claim 1 is the broadest claim and is not limited to capsules or a specified concentration.
Claims 4 and 6-9 focus on soft elastic gelatin capsules.
Claim 5 expressly lists eight PGE2 compounds and methyl esters.
Claims 7-9 narrow the concentration to 0.5 to 5 mg/mL for specified analogs.
The patent issued June 29, 1976 and nominally expired June 29, 1993.
It has no current United States exclusivity, Orange Book blocking effect, Paragraph IV relevance, or biosimilar relevance.
The patent contains no supplied method-of-use or manufacturing claims.
Current freedom-to-operate risk must come from later prostaglandin, dosage-form, process, or FDA-listed patents.

FAQs

Can a current manufacturer use triacetin with PGE2 without a license?

Yes. Patent 3,966,962 expired in 1993, so it cannot require a current license in the United States.

Does the patent cover every PGE2 dosage form?

No. It requires the active compound to be in a solution in triacetin. Solid, aqueous, non-triacetin, suspension, vaginal insert, and other products may fall outside the claims.

Does claim 5 cover misoprostol?

Not expressly. Claim 5 lists specific PGE2 compounds and esters. Misoprostol would require a separate analysis under the broader language of claim 1.

Could an expired patent still affect FDA approval?

The patent cannot create a current patent stay or block approval. FDA review may still consider current patents, exclusivity, safety, efficacy, labeling, and pharmaceutical equivalence requirements.

Are soft gelatin prostaglandin capsules still protected by this patent?

No. The soft gelatin capsule embodiments in claims 4 and 6-9 are no longer enforceable because the patent term expired.

References

United States Patent No. 3,966,962. (1976). United States Patent and Trademark Office.
Patent Act of 1952, 35 U.S.C. § 154, as applicable to patents issued before the Uruguay Round Agreements Act.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations.
Code of Federal Regulations. (2024). 21 C.F.R. § 314.53, submission of patent information.
Drug Price Competition and Patent Term Restoration Act of 1984, Pub. L. No. 98-417, 98 Stat. 1585.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Australia	1194876	⤷ Start Trial
Belgium	840075	⤷ Start Trial
Canada	1044140	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 3,966,962

Summary for Patent: 3,966,962