United States Drug Patent 4,680,312: Claim Scope, Expiration, and PGE2 Formulation Patent Landscape

U.S. Patent No. 4,680,312 covers a narrow PGE2 dosage-form formulation using triacetin as the vehicle and colloidal silicon dioxide as a gel-forming additive at 8% to 15% by weight. The claim is directed to formulation composition, not the PGE2 molecule, the therapeutic use of dinoprostone generally, or a manufacturing process. Because the patent predates the modern 20-year patent term, its ordinary statutory term expired in 2004, subject to any recorded terminal disclaimer or unusual patent-term adjustment. It does not present a current blocking right for U.S. generic development.

What does U.S. Patent 4,680,312 protect?

The supplied claim protects a stable dosage form satisfying all of the following limitations:

Claim element	Scope
Active ingredient	PGE2, also known as dinoprostone or prostaglandin E2
Dosage form	A stable dosage form
Primary vehicle	Triacetin
Additional component	Colloidal silicon dioxide, used as a gel-forming additive
Concentration	Approximately 8% to 15% by weight of the finished stable dosage form
Composition language	"Consisting essentially of" PGE2 in triacetin, with the claimed improvement

The patent does not claim PGE2 itself. It claims a particular physical formulation intended to stabilize and structure PGE2 in a triacetin-based dosage form.

The claim is best characterized as a formulation claim with a quantitative range limitation. A competing product would generally need to satisfy every material limitation to create literal infringement.

What is the role of triacetin in the claim?

Triacetin is a required formulation component. A product using a different principal vehicle, such as a polyethylene glycol, mineral oil, glyceride mixture, aqueous system, or another ester vehicle, would not literally satisfy the triacetin limitation.

The claim language supplied does not establish that triacetin must be present at a particular concentration. It does require PGE2 to be in triacetin. The specification may provide additional context regarding the meaning of "in triacetin," the physical form of the dosage unit, and the stability requirements.

What is the role of colloidal silicon dioxide?

Colloidal silicon dioxide is not claimed merely as an inert excipient. It is characterized functionally as a "gel forming additive." The claim therefore combines:

A material identity limitation: colloidal silicon dioxide; and
A functional limitation: use as a gel-forming additive.

Colloidal silicon dioxide is commonly used as a rheology modifier, suspending agent, adsorbent, and gel former. The relevant product may be a fumed or highly dispersed silica grade, but the patent specification and prosecution history would control whether "colloidal silicon dioxide" is limited to particular grades, particle sizes, surface treatments, or pharmacopeial categories.

A formulation containing precipitated silica, silica gel, magnesium aluminum silicate, bentonite, carbomer, or another gelling agent would not ordinarily meet the material limitation literally unless the selected material were legally construed to fall within colloidal silicon dioxide.

How does the 8% to 15% concentration range operate?

The quantitative limitation is central to the claim. The claimed range is approximately:

Lower boundary: about 8% by weight
Upper boundary: about 15% by weight

A formulation at 10% colloidal silicon dioxide is within the stated range. A formulation at 5% is outside the literal range. A formulation at 20% is also outside the literal range.

The phrase "about" creates boundary flexibility. Its legal meaning would depend on the intrinsic evidence, including examples, analytical precision, formulation variability, and prosecution history. It does not eliminate the need for a commercially relevant concentration near the claimed interval.

The denominator is the weight of the "stable dosage form." Unless the specification defines a different basis, the likely calculation is:

[ \text{CSD weight percentage} = \frac{\text{weight of colloidal silicon dioxide}}{\text{total weight of dosage form}} \times 100 ]

The analysis must account for whether the dosage form includes packaging-derived materials, water, residual solvents, overages, or separate delivery components. Finished-product testing and batch records would be important in an infringement dispute.

How should "consisting essentially of" be interpreted?

"Consisting essentially of" is a transitional phrase between "comprising" and "consisting of." It generally permits additional ingredients, but only if those ingredients do not materially affect the basic and novel characteristics of the claimed composition.

For this claim, the basic and novel characteristics likely include:

PGE2 formulated in triacetin;
Stability of the PGE2 dosage form; and
Gel formation or physical structuring produced by 8% to 15% colloidal silicon dioxide.

An accused formulation containing additional preservatives, antioxidants, buffers, coloring agents, release modifiers, or processing aids could remain within the claim if those materials do not materially alter the formulation's stability or gel-forming characteristics.

A formulation with an additional ingredient that substantially changes the vehicle, prevents silica gel formation, materially changes PGE2 degradation, or creates a different dosage-form architecture would have a stronger non-infringement position.

The specification and prosecution history are critical. If the applicant distinguished prior art by asserting that the claimed silica concentration was responsible for a particular stability profile, those statements could narrow the claim.

What products and dosage forms are potentially covered?

The claim does not expressly limit the dosage form to a particular route of administration in the language supplied. Potentially relevant formats include:

Vaginal inserts;
Intravaginal gels or semisolids;
Cervical or endocervical dosage forms;
Preloaded applicator products;
Unit-dose gelled formulations; and
Other solid or semisolid PGE2 dosage units containing triacetin.

The claim does not expressly require a particular dose of PGE2, release rate, applicator, package, storage temperature, or indication. Those omissions broaden the claim within the boundaries of the required composition.

The claim would not necessarily cover:

PGE2 tablets without triacetin;
Aqueous dinoprostone gels;
PGE2 products using a different gel former;
Products with colloidal silicon dioxide outside the legally effective 8% to 15% range;
PGE1, misoprostol, carboprost, or other prostaglandins; or
A product that contains triacetin only as a minor unrelated excipient, if PGE2 is not formulated in triacetin in the claimed sense.

When did U.S. Patent 4,680,312 lose exclusivity?

The patent is an early U.S. utility patent issued before the transition to the 20-year term measured from the earliest effective nonprovisional filing date. Under the pre-1995 regime, the ordinary term was 17 years from the grant date. On that basis, U.S. Patent 4,680,312 expired in 2004, absent an unusual adjustment, terminal disclaimer, or other record-specific event. [1][2]

Event	Patent-law significance
Grant of U.S. Patent 4,680,312	Began the ordinary 17-year patent term applicable to the patent
2004	Approximate end of the ordinary statutory term
Current status	No ordinary enforceable patent exclusion remains from this patent
Generic relevance	Historical formulation-art relevance; no current blocking right by itself

An expired patent can still affect freedom-to-operate analysis as prior art. It may be cited against later patent applications, used in an obviousness analysis, or relevant to validity and claim-construction disputes involving later PGE2 formulations.

What is the Orange Book status of U.S. Patent 4,680,312?

The patent should not be treated as a current Orange Book barrier merely because it concerns an FDA-approved active ingredient or dosage form.

The FDA Orange Book lists patents submitted by approved NDA holders that meet applicable listing requirements. A patent may be absent because it was never submitted, did not qualify for listing, was delisted, or expired. Orange Book listing does not extend patent term and does not make an expired patent enforceable. [3]

For dinoprostone products, current regulatory analysis should distinguish among:

Product-specific patents listed for an approved NDA;
Expired historical patents;
Regulatory exclusivity periods;
Later formulation or device patents; and
Patents covering manufacturing or packaging rather than the active ingredient.

U.S. Patent 4,680,312 should therefore be reviewed as an expired historical formulation patent, not as a current Orange Book launch blocker.

Does the patent create a Paragraph IV risk?

No current Paragraph IV risk arises solely from U.S. Patent 4,680,312 because an expired patent cannot support a current patent-certification barrier.

A generic applicant seeking approval for a dinoprostone product could still encounter Paragraph IV issues from later patents listed for the relevant reference-listed drug. The relevant analysis would depend on:

The specific reference-listed product;
The proposed generic's route and dosage form;
Listed formulation or device patents;
Method-of-use patents;
Patent expiration dates;
Any pediatric exclusivity; and
The applicant's proposed labeling.

A Paragraph IV certification to an expired patent would not create the same commercial litigation exposure as a certification to an unexpired listed patent.

What are the strongest design-around strategies?

The claim has several practical design-around paths.

Use a different vehicle

Replacing triacetin with another pharmaceutically acceptable vehicle removes a required claim element. The commercial value of this strategy depends on whether the alternative vehicle preserves PGE2 stability and desired release characteristics.

Use a different gel former

Replacing colloidal silicon dioxide with a polymeric or mineral gelling system can avoid the material limitation. Candidate categories may include carbomers, cellulose derivatives, alginates, clays, or other rheology modifiers, subject to formulation compatibility and regulatory requirements.

Move outside the concentration range

Using less than approximately 8% or more than approximately 15% colloidal silicon dioxide may avoid literal infringement. This strategy carries range-boundary risk because of the term "about" and the doctrine of equivalents.

A formulation at 7.5% may present a greater equivalence risk than one using no colloidal silicon dioxide. A formulation at 16% may also be scrutinized if it produces substantially the same gel-forming and stability result in substantially the same way.

Use a different physical dosage form

A dosage form that does not contain PGE2 in triacetin, such as an aqueous gel or a non-gelled insert matrix, would provide a stronger non-infringement position than a minor concentration adjustment alone.

Could the doctrine of equivalents apply?

Potentially, but the expired status eliminates present enforcement risk under this patent. For historical validity or litigation analysis, a court would assess whether an accused formulation performs substantially the same function, in substantially the same way, to obtain substantially the same result.

The most vulnerable design-around would be a formulation that:

Uses colloidal silicon dioxide at 7% to 7.9%;
Uses 15.1% to 16%;
Uses a chemically similar silica grade; and
Produces the same stability and gel structure in triacetin.

Prosecution history estoppel could restrict equivalents if the applicant narrowed the silica concentration or composition to overcome prior art. The claim's numerical boundaries may also receive limited equivalent protection where the applicant relied on those boundaries during examination.

What is the patent strength of the claim?

As an issued claim evaluated during its enforceable life, the claim had moderate technical specificity but a narrow commercial perimeter.

Factor	Assessment
Claim type	Composition/formulation
Required active	PGE2
Required vehicle	Triacetin
Required excipient	Colloidal silicon dioxide
Numerical limitation	8% to 15% by weight
Therapeutic-use breadth	Broad within the formulation; no specific indication in supplied language
Design-around availability	Relatively high
Enablement sensitivity	Stability and dosage-form disclosure would be important
Definiteness sensitivity	Meaning of "stable," "about," and "colloidal" may be disputed
Current enforceability	None after expiration

The main validity issues would likely involve:

Anticipation by an earlier PGE2/triacetin/silica formulation;
Obviousness based on known PGE2 stabilization methods and silica gel systems;
Definiteness of "stable dosage form" and "about";
Written-description support for the full 8% to 15% range; and
Enablement across all covered PGE2 concentrations and dosage forms.

The claim's functional stability requirement could strengthen the patent if the specification provides a measurable stability test and clear examples. It could weaken the claim if "stable" is used without an objective standard.

How does this patent compare with later PGE2 products?

Issue	U.S. Patent 4,680,312	Later dinoprostone products
Core protection	Triacetin/PGE2 formulation with colloidal silicon dioxide	May rely on different vehicles, delivery systems, devices, or formulations
Patent status	Expired in 2004 under ordinary term rules	Must be assessed product by product
Orange Book impact	No current barrier from this patent	Potentially relevant if later patents are listed
Biosimilar exposure	None	Dinoprostone is a small molecule, so generic rather than biosimilar pathways apply
Main technical issue	Stability and gel formation	Product-specific release, device, manufacturing, or formulation performance

Dinoprostone is a small-molecule drug. Biosimilar litigation is therefore not the relevant competitive framework. The principal risks involve abbreviated new drug applications, formulation differences, device patents, manufacturing patents, and method-of-use claims.

What litigation or settlement issues affect this patent?

No current litigation or settlement can be inferred from the claim language alone. Because the patent expired approximately two decades ago, any historical enforcement action would have no continuing exclusionary effect unless it created contractual obligations unrelated to patent term.

A complete litigation review would distinguish:

Patent infringement suits involving U.S. Patent 4,680,312;
Inter partes or ex parte reexamination;
Post-grant validity proceedings, if any;
ANDA litigation involving dinoprostone products;
License agreements; and
Settlement agreements with launch dates or covenants not to sue.

No settlement should be assumed from the existence of the patent or from later market entry by generic manufacturers.

What manufacturing and geographic barriers remain?

The patent's territorial scope was limited to the United States. It did not create rights in Europe, Japan, Canada, or other jurisdictions unless corresponding foreign applications produced enforceable patents.

Manufacturing freedom to operate should be separated into four categories:

The expired composition claim;
Later U.S. formulation patents;
Manufacturing-process patents; and
Device, applicator, packaging, and combination-product patents.

The expired patent does not prevent manufacture of the claimed formulation in the United States today. It may still be relevant as a technical reference when assessing later patentability or selecting formulation parameters.

Key Takeaways

U.S. Patent 4,680,312 covers a stable PGE2-in-triacetin dosage form containing 8% to 15% colloidal silicon dioxide by weight.
The claim is a narrow formulation claim, not a claim to dinoprostone generally.
"Consisting essentially of" permits additional excipients unless they materially alter the claimed stability or gel-forming characteristics.
Triacetin, colloidal silicon dioxide, the gel-forming function, and the concentration range are all material limitations.
The patent's ordinary U.S. term expired in 2004.
It does not create a current Orange Book, Paragraph IV, generic-launch, or biosimilar barrier.
The strongest technical design-arounds are a different vehicle, a different gelling agent, or a dosage form that does not use the claimed silica range.
Later dinoprostone products require separate review of formulation, device, manufacturing, method-of-use, and Orange Book patents.
The patent remains relevant as expired prior art for later PGE2 patentability and formulation strategy.

FAQs About U.S. Patent 4,680,312

Does U.S. Patent 4,680,312 cover all dinoprostone products?

No. It covers only products meeting the specified PGE2, triacetin, colloidal silicon dioxide, stability, and concentration limitations.

Is a PGE2 product with 7% colloidal silicon dioxide outside the claim?

It is outside the literal 8% to 15% range, but the terms "about" and the doctrine of equivalents could affect boundary analysis. The patent is expired, so this has no current infringement consequence.

Does the claim cover colloidal silicon dioxide at 20%?

A 20% formulation is outside the stated numerical range on its face. The patent's "about" language would not ordinarily extend to a materially higher concentration without supporting evidence.

Is dinoprostone subject to biosimilar approval?

No. Dinoprostone is a small-molecule drug. The relevant abbreviated pathway is generally the ANDA framework for generics, not the biosimilar pathway under the Public Health Service Act.

Can an expired formulation patent still be cited against a new PGE2 patent?

Yes. Expiration removes enforceability but does not remove the patent from the prior-art record. Its disclosure may be relevant to anticipation, obviousness, written description, or enablement of later claims.

References

United States Patent and Trademark Office. (n.d.). U.S. Patent No. 4,680,312. Washington, DC: U.S. Department of Commerce.
United States Code. (2023). 35 U.S.C. §§ 154, 156, and 271. Washington, DC: U.S. Government Publishing Office.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book. Silver Spring, MD: U.S. Department of Health and Human Services.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
United Kingdom	8208322	Mar 22, 1982

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Germany	3381898	⤷ Start Trial
Denmark	127883	⤷ Start Trial
Denmark	167139	⤷ Start Trial
European Patent Office	0089768	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,680,312

Summary for Patent: 4,680,312