United States Patent 4,551,148: Claim Scope, Expiration, and Vaginal Drug-Delivery Patent Landscape

U.S. Patent No. 4,551,148 covers bioadherent vaginal delivery systems based on liquid or semisolid adjacent unit cells. The claimed systems use common lipoidal external phases, nonlipoidal internal phases, and emulsifiers to release an active agent for at least three hours. The patent issued on November 5, 1985, and its ordinary U.S. patent term expired on November 5, 2002. The claims are therefore no longer enforceable against current products, although they remain relevant as prior art and as evidence of early vaginal bioadhesive formulation technology.[1]

The claims are directed to a formulation platform, not to a single drug. The broadest claim covers vaginal systems containing an active agent, while dependent claims narrow the composition by phase structure, active ingredient, water or glycerin content, and nonlipoidal-phase volume.

What does U.S. Patent 4,551,148 protect?

The patent protects a vaginal delivery system with five core limitations:

The system is bioadherent to vaginal surfaces.
It releases an active agent in a controlled manner.
Release continues for at least three hours.
The system consists essentially of liquid or semisolid adjacent unit cells.
The unit cells have common lipoidal external phases, nonlipoidal internal phases, and emulsifiers.

The claim is not limited to a particular dosage form, applicator, commercial product, release-rate profile, or manufacturing process. It reaches a class of multiphase vaginal formulations if the accused product satisfies each required limitation.

Claim 1: the principal platform claim

Claim 1 is the controlling claim:

"Vaginal delivery systems which are bioadherent to the vaginal surfaces and which release an active agent in a controlled manner for at least three hours ... consisting essentially of liquid or semi-solid adjacent unit cells having common lipoidal external phases, nonlipoidal internal phases and emulsifiers."

The claim has both structural and functional elements.

Claim element	Scope implication
Vaginal delivery system	The claimed product must be intended for vaginal administration or operation at vaginal surfaces
Bioadherent	The formulation must adhere to vaginal tissue or mucosal surfaces; ordinary retention caused only by shape or gravity may not satisfy this limitation
Controlled release	Release must be managed by the formulation rather than occurring immediately after administration
At least three hours	A product that releases the active agent for less than three hours falls outside the literal claim
Liquid or semisolid	Solid tablets, rings, films, and implants are not literally covered unless they contain or operate as the claimed liquid or semisolid system
Adjacent unit cells	The formulation must have a cellular or compartmental organization, rather than merely being a conventional homogeneous cream
Common lipoidal external phases	The unit cells share lipoidal outer phases
Nonlipoidal internal phases	Internal regions are nonlipoidal, including water- or glycerin-based regions
Emulsifiers	Emulsifying agents are required
"Consisting essentially of"	Unlisted ingredients may be present if they do not materially alter the basic and novel characteristics of the claimed system

The phrase "consisting essentially of" is narrower than "comprising" but broader than "consisting of." It allows excipients, preservatives, buffers, stabilizers, fragrances, penetration modifiers, and other formulation components unless their presence materially changes the claimed lipoidal/nonlipoidal unit-cell architecture or controlled-release function.

How do claims 2 through 9 narrow the patent?

Claims 2 through 9 depend from claim 1 and inherit every limitation of that claim. They do not create independent protection for the listed actives or excipients.

Claim	Added limitation	Commercial significance
2	The phases form a liquid or semisolid	Confirms the physical-state limitation but adds limited narrowing
3	The phases form an emulsion, emulsion/dispersion, double emulsion, suspension within an emulsion, or mixture	Targets multiphase emulsion technologies
4	At least part of the active agent is contained in the nonlipoidal phases	Narrows drug localization within the formulation
5	Active agent includes nonoxynol-9	Covers a spermicidal embodiment
6	Active agent includes nystatin	Covers an antifungal embodiment
7	Active agent includes a pH buffer	Covers buffering systems, whether or not the buffer is pharmacologically active
8	Nonlipoidal phases comprise at least 70% by volume	Adds a quantitative composition limitation
9	Nonlipoidal phases comprise water, glycerin, or both	Covers common hydrophilic internal phases

Claim 3 is the most technically important dependent claim because it expressly identifies several architectures. It likely provides the clearest route to infringement for a product using a double emulsion or a suspension within an emulsion.

Claim 8 may be difficult to apply without reliable compositional evidence. The 70% threshold requires a volumetric calculation, and the claim does not specify whether volume is measured before administration, after equilibration, or using a particular phase-separation method.

Claim 9 is comparatively broad at the ingredient level because water and glycerin are common vaginal formulation excipients. The remaining limitations of claim 1 still control.

What formulations are protected by U.S. Patent 4,551,148?

The claimed technology is a bioadhesive multiphase vaginal formulation. Its technical architecture can be summarized as follows:

Formulation component	Claimed role
Lipoidal external phase	Forms the common outer phase around internal units
Nonlipoidal internal phase	Houses at least some of the active agent in claim 4
Emulsifier	Stabilizes the multiphase structure
Water or glycerin	Supplies the hydrophilic internal phase under claim 9
Active agent	May include nonoxynol-9, nystatin, or another active ingredient
Bioadhesive property	Promotes retention at vaginal surfaces
Controlled release	Extends delivery for at least three hours

The patent does not require a particular polymeric mucoadhesive. It also does not expressly require carbomer, polycarbophil, cellulose derivatives, alginate, chitosan, or a specific emulsifier. A later formulation could therefore encounter literal-claim risk even if it uses a different adhesive excipient, provided the system has the claimed unit-cell and phase architecture.

When did U.S. Patent 4,551,148 lose exclusivity?

The patent issued on November 5, 1985. For a U.S. application filed under the pre-Uruguay Round patent-term regime, the ordinary term was 17 years from issue. On that basis, the patent expired on November 5, 2002.[1][2]

Event	Date
Patent issued	November 5, 1985
Ordinary 17-year term measured from issue	November 5, 2002
Current enforceability	Expired
Current freedom-to-operate value	Prior-art and historical relevance only

Patent term adjustment did not apply under the modern statutory framework to extend this patent in the same manner as later-filed patents. Any terminal disclaimer or special term event would have appeared in the USPTO record, but the ordinary expiration date is sufficient to establish that the patent is no longer an enforcement barrier.

What is the Orange Book status of U.S. Patent 4,551,148?

U.S. Patent 4,551,148 is not an Orange Book patent in the conventional regulatory sense. The patent claims a vaginal delivery system and formulation architecture rather than an approved drug substance or an NDA-specific drug product. The FDA Orange Book lists patents submitted by NDA holders for approved products; it does not create a general registry for all pharmaceutical formulation patents.[3]

The claims to nonoxynol-9 and nystatin do not, by themselves, establish Orange Book listing status. A listed patent would need to be associated with a particular approved drug product and submitted in accordance with FDA patent-listing requirements. The patent’s 2002 expiration also eliminates any current Orange Book exclusivity value.

Does the patent create Paragraph IV risk?

No current Paragraph IV risk arises from the expired patent. Paragraph IV certifications address patents listed in the Orange Book for an approved reference drug. A generic applicant could not be required to certify Paragraph IV against an expired patent as an active patent barrier.[4]

Historically, a product sponsor might have considered the patent in product-development strategy if the patent had been listed or asserted before expiration. Today, the patent can be cited as prior art in validity or patentability analysis, but it cannot support a new infringement action.

Historical Paragraph IV relevance

The patent’s claims could have been relevant to an ANDA involving:

A vaginal nonoxynol-9 product;
A vaginal nystatin product;
A bioadhesive cream or emulsion;
A controlled-release vaginal formulation;
A product using a water- or glycerin-rich internal phase.

A Paragraph IV challenge would have focused on claim construction, anticipation, obviousness, and whether the proposed product had the required adjacent unit-cell architecture. It also could have argued that the three-hour release requirement was not met.

How strong was the patent estate?

The patent appears to have been a focused platform patent rather than a large, layered estate. Its commercial strength would have depended on whether related continuation, divisional, foreign, or improvement patents extended protection around specific polymers, emulsifiers, active agents, manufacturing methods, or products.

Strengths

Claim 1 covers a broad formulation concept.
The claim is not limited to one active ingredient.
The claim reaches multiple liquid and semisolid emulsion arrangements.
Bioadhesion and controlled release are functional characteristics that may be difficult to avoid while preserving vaginal residence time.
Claim 3 expressly covers several complex multiphase structures.

Weaknesses

The claim combines numerous limitations, creating multiple noninfringement positions.
"Adjacent unit cells" may be difficult to define and prove.
"Common lipoidal external phases" imposes a specific microstructural requirement.
"At least three hours" requires release testing and may vary with assay conditions.
The patent does not clearly claim a specific adhesive polymer, emulsifier, manufacturing process, or commercial dosage form.
The nonoxynol-9 and nystatin claims remain dependent on every limitation of claim 1.
The patent expired in 2002.

The principal validity risks would have been lack of written-description support for the full breadth of claim 1, indefiniteness of terms such as "adjacent unit cells" and "bioadherent," and obviousness based on earlier emulsion, mucoadhesive, and vaginal controlled-release systems.[5]

What patent litigation affects U.S. Patent 4,551,148?

No current litigation can be based on the expired patent. The patent has no present enforcement life, and any historical litigation would require review of PACER, USPTO file history, and state or federal docket records. The supplied claim set does not identify a litigation matter, asserted defendant, settlement, or license.

The absence of a current enforceability right is commercially decisive. A company developing a vaginal formulation does not need a license merely because its product might fall within claims 1 through 9. A license could still be relevant for archived know-how, confidential formulations, trademarks, or separate unexpired patent families, but those rights are not established by U.S. Patent 4,551,148 alone.

What licensing deals and settlement agreements are associated with the patent?

The patent claims supplied do not establish an assignment, license, settlement, or commercialization agreement. A patent document may identify an assignee, but an assignee is not necessarily a licensee or commercial partner. No licensing or settlement rights should be inferred from the claims.

For transaction diligence, the relevant distinction is:

Issue	Status based on the patent itself
Patent ownership	Requires assignment-record review
Patent expiration	Expired November 5, 2002
Current patent license required	No, not for the expired claims
Know-how license	Not determined by the patent
Settlement agreement	Not established
Royalty obligation	Not established
Product-specific sublicense	Not established

How does the patent compare with later vaginal delivery patents?

Later vaginal delivery patents generally moved toward narrower and more commercially defined technologies:

Polymeric mucoadhesive tablets and gels;
Vaginal rings for sustained hormone delivery;
Films and dissolving inserts;
In situ gelling systems;
Thermoresponsive formulations;
Nanoparticle and liposomal delivery;
Specific antimicrobial, antifungal, contraceptive, or hormone products;
Manufacturing methods and dose-specific release profiles.

U.S. Patent 4,551,148 is broad at the platform level but old and structurally specific. A later patent may avoid its claims by using a solid ring, a homogeneous polymer gel without the claimed unit cells, a nonlipoidal continuous phase, or a delivery period below three hours. Conversely, a later product using a water-rich internal phase inside lipoidal emulsion cells could have fallen within the literal scope during the patent term.

What generic entry risks exist today?

There is no patent-based generic entry risk from U.S. Patent 4,551,148 today. The product risks that remain are regulatory and commercial:

Whether the active ingredient requires an NDA, ANDA, OTC monograph pathway, or another FDA route;
Whether the formulation is pharmaceutically equivalent to a reference product;
Whether a separate later patent covers the formulation or use;
Whether the product has an unexpired method-of-use, manufacturing, device, or packaging patent;
Whether the delivery system is regulated as a drug, combination product, or drug-delivery device.

The patent does not create biosimilar risk because nonoxynol-9 and nystatin are small-molecule or conventional drug actives, not biologics. The relevant competitive risk is generic or follow-on formulation competition, not biosimilar substitution.

What geographic coverage did the patent provide?

U.S. Patent 4,551,148 provided rights only in the United States. Foreign protection would have required separate national or regional applications. A U.S. patent does not establish coverage in Canada, Europe, Japan, Australia, or other markets.

For a multinational product, geographic freedom to operate must be assessed separately in each jurisdiction. Because the U.S. patent expired in 2002, it does not currently restrict U.S. manufacture, sale, importation, or use. Foreign family members, if any, would have expired under their own national terms.

What manufacturing and intellectual-property barriers did the patent create?

During its term, the patent could have created manufacturing risk for products made by processes that generated:

Adjacent liquid or semisolid unit cells;
A shared lipoidal external phase;
Nonlipoidal internal phases;
Emulsifier-stabilized multiphase structures;
At least three hours of controlled vaginal release.

The claims are product claims, not express process claims. A manufacturer could therefore have faced infringement exposure based on the resulting formulation even if its manufacturing process differed substantially from the disclosed process.

The most practical design-around strategies would have included:

Using a homogeneous gel rather than an emulsion-cell system.
Using a solid vaginal ring, tablet, film, or insert.
Making the continuous phase nonlipoidal.
Eliminating the claimed lipoidal external phase.
Designing release below the three-hour threshold, where clinically acceptable.
Using a structure that lacks adjacent unit cells.
Developing a product in which the active agent is not contained in the nonlipoidal phase, avoiding claim 4.

These strategies would have addressed literal infringement only. During the patent term, the doctrine of equivalents could still have presented risk if the alternative performed substantially the same function in substantially the same way to obtain substantially the same result.[6]

Key Takeaways

U.S. Patent 4,551,148 covers bioadherent vaginal liquid or semisolid delivery systems with multiphase unit-cell structures.
Claim 1 is the principal platform claim.
Claim 3 is the most specific structural claim because it identifies emulsions, double emulsions, suspensions within emulsions, and mixtures.
Claims 5 and 6 cover nonoxynol-9 and nystatin only as embodiments of the broader claim 1 system.
Claim 8 requires at least 70% nonlipoidal phase by volume.
Claim 9 identifies water and glycerin as nonlipoidal-phase components.
The patent issued November 5, 1985, and expired November 5, 2002.
It does not create current Orange Book, Paragraph IV, generic-entry, biosimilar, or U.S. infringement risk.
Any current risk must come from later patents, regulatory requirements, trade secrets, or separate foreign rights.
The patent remains relevant as prior art for vaginal bioadhesive, emulsion-based, and controlled-release formulation development.

FAQs About U.S. Patent 4,551,148

Can a company commercialize a vaginal nystatin product without licensing U.S. Patent 4,551,148?

Yes. The patent expired in 2002. A separate unexpired patent or regulatory exclusivity right could still affect commercialization.

Does claim 5 independently cover nonoxynol-9?

No. Claim 5 depends on claim 1 and requires every limitation of claim 1 in addition to the nonoxynol-9 limitation.

Does a vaginal gel infringe the patent automatically?

No. A gel would need to satisfy the patent’s bioadhesion, controlled-release, three-hour, unit-cell, lipoidal-external-phase, nonlipoidal-internal-phase, and emulsifier limitations.

Are vaginal rings covered by this patent?

Not ordinarily. A conventional solid vaginal ring does not appear to satisfy the claimed liquid or semisolid adjacent-unit-cell structure, although historical infringement analysis would depend on the ring’s internal formulation and operation.

Can the expired patent block FDA approval of a new vaginal drug?

No. Patent expiration does not itself block FDA approval. FDA review and patent rights are separate systems, although active patents and regulatory exclusivities may affect a product’s market entry.

References

United States Patent and Trademark Office. (1985). U.S. Patent No. 4,551,148, Vaginal delivery systems.
35 U.S.C. § 154. Patent term provisions applicable to pre-June 8, 1995 applications.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations.
21 U.S.C. § 355(j)(2)(A)(vii). ANDA patent certifications.
35 U.S.C. §§ 102, 103, 112. Patentability, anticipation, obviousness, and disclosure requirements.
Warner-Jenkinson Co. v. Hilton Davis Chemical Co., 520 U.S. 17 (1997). Doctrine of equivalents.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	68686	⤷ Start Trial
Canada	1338977	⤷ Start Trial
Germany	3584523	⤷ Start Trial
European Patent Office	0244405	⤷ Start Trial
Japan	2519029	⤷ Start Trial
Japan	S63501563	⤷ Start Trial
World Intellectual Property Organization (WIPO)	8702576	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,551,148

Summary for Patent: 4,551,148