United States Patent 5,855,906: Claim Scope, Patent Expiration, and Intravaginal Estradiol Delivery Landscape

U.S. Patent No. 5,855,906 protects a platform for delivering esterified 17β-estradiol from a cylindrical intravaginal device made with a hydrophobic elastomer, principally silicone, for at least three weeks at a substantially zero-order release rate. The claims cover the device, manufacturing process, use of the device, and specific estradiol ester embodiments, including estradiol 3-acetate and estradiol 17-acetate.

The patent is a formulation-and-device patent rather than a broad estradiol composition patent. Its strongest limitations are the combination of: an intravaginal cylindrical geometry; a hydrophobic elastomeric matrix; a readily cleavable estradiol blocking group; defined lipophilicity and hydrophilicity thresholds; and sustained substantially zero-order release for at least three weeks.

Based on the patent’s 1990s filing date and ordinary U.S. patent-term rules, the original U.S. exclusionary term has ended. The patent therefore has historical relevance to products such as estradiol vaginal rings, but it should not presently block a new U.S. product unless an unusual term adjustment or restoration affected its expiration. The exact expiration date should be confirmed against the USPTO patent-term record before relying on the conclusion in litigation or an acquisition.

What does U.S. Patent 5,855,906 claim?

The patent claims four related categories:

Claim category	Claims	Core subject matter
Intravaginal device	1-14, 20, 23-24	Cylindrical device containing a blocked 17β-estradiol precursor in a hydrophobic elastomer
Manufacturing process	15-16, 21	Mixing the precursor with polymer, cross-linker and catalyst, then curing the matrix
Product-by-process	17, 22	Device defined by preparation through the claimed process
Use and release methods	18-19	Incorporating the precursor into the device and inserting the device for at least three weeks

The independent device claim, claim 1, is the principal technical claim. It requires every element below:

A cylindrical intravaginal drug-delivery device.
Administration to a female mammal.
A 17β-estradiol precursor.
The precursor in a biocompatible hydrophobic elastomeric polymer matrix.
Substantially zero-order release for at least three weeks.
Blocking of one or both estradiol hydroxyl groups.
A minimum lipophilicity threshold.
A minimum hydrophilicity threshold.
In-vivo removal of the blocking group.
A nontoxic cleavage product.

A product that lacks any one of these limitations would not literally satisfy claim 1, although it could still raise infringement issues under a doctrine-of-equivalents analysis.

How does the patent define the estradiol precursor?

The patent does not claim unmodified estradiol generally. It claims a 17β-estradiol moiety in which one or both hydroxyl groups are blocked by a group that is removed in vivo.

The relevant hydroxyl positions are:

The 3-hydroxyl group on the aromatic A ring.
The 17β-hydroxyl group on the D ring.

Claims 7 through 11 narrow the invention to blocking at either position. Claims 10 and 11 identify specific acetate and propionate compounds.

Claim	Estradiol modification
7	Blocking group at the 3-hydroxyl
8	Blocking group at the 17-hydroxyl
9	Acetyl or propionyl group at the 3-hydroxyl
10	Estradiol 3-acetate or estradiol 3-propionate
11	Estradiol 17-acetate or estradiol 17-propionate
23	Estradiol 3-acetate
24	Estradiol 17-acetate

Claim 2 excludes the use of acetyl groups on every hydroxyl group simultaneously. That proviso matters because it prevents claim 2 from covering a fully diacetylated estradiol species where both hydroxyls are acetylated.

What chemical groups are covered?

Claims 2 through 6 define broad classes of acyl blocking groups.

Saturated acyl groups

Claim 4 lists:

Formyl
Acetyl
Propionyl
Butyryl
Isobutyryl
Oxalyl
Malonyl
Succinyl
Glutaryl

The drafting structure covers both monoacyl and, where chemically applicable, diacyl substitution. The claims are not limited to acetate, even though estradiol 3-acetate is the most commercially significant named embodiment.

Unsaturated acyl groups

Claim 6 lists:

Acryloyl
Propioloyl
Methacryloyl
Crotonoyl
Isocrotonoyl
Maleoyl
Fumaroyl
Citraconoyl
Mesaconoyl

The claim language also requires that the group be readily removed from estradiol in vivo and that the resulting cleavage product be nontoxic. A chemically possible ester therefore would not automatically fall within the claim.

What physical-property thresholds limit claim 1?

The patent uses two independent lipophilicity tests and one hydrophilicity test.

Property	Claimed threshold
Solubility in liquid silicone	At least 0.1 mg/100 ml
Alternative standard k value	At least 0.1 μg/day/mm
Solubility in distilled water	At least 1 μg/100 ml

The k value is defined as:

[ k = 2 C_S D \pi ]

where:

(C_S) is the precursor’s saturation solubility in the polymer matrix.
(D) is the diffusion coefficient in the polymer matrix.

These limitations distinguish the claimed precursors from compounds that are either too insoluble in the elastomer or too insoluble in aqueous vaginal conditions.

The claim therefore contains a functional formulation screen. A candidate ester must have enough affinity for the silicone-like matrix to load and diffuse through it, while retaining enough aqueous compatibility to support delivery and conversion to active estradiol.

What device structures are covered?

The patent covers two principal geometries.

Hollow annulus with core and sheath

Claim 14 covers a device in which:

The polymer matrix forms a hollow annulus.
A central member occupies the interior.
A sheath surrounds the polymer matrix.

This configuration is structurally important because the sheath can control exposed surface area and release rate. The central member can provide mechanical support or help define the annular geometry.

Core with surrounding sheath

Claims 20 and 21 cover a simpler structure in which:

The polymer matrix forms a core.
A sheath surrounds the core.

The claims associate the annular configuration with inducing hyper-estrogenism and fertility control, while the core-and-sheath configuration is associated with treating or preventing hypo-estrogenism, including hormone-replacement therapy.

The patent does not require a particular silicone brand, sheath thickness, cross-linking chemistry, device diameter or drug loading in the claims provided. Those parameters would generally be evaluated through the specification, prosecution history and claim construction.

What does “substantially zero order” mean in this patent?

“Substantially zero order” describes a release profile in which the device releases drug at an approximately constant rate over time. Claim 1 requires that profile to persist for at least three weeks.

The limitation is functional rather than numerical. The claim does not state a percentage deviation from constant release, a permitted daily fluctuation, or a specific release-rate range. In an infringement analysis, the specification, experimental data and prosecution history would be important in determining how much deviation is permitted.

The three-week period is a minimum duration. A device releasing for 90 days could satisfy the temporal limitation if the release remains substantially zero order for at least the first three weeks and meets the other limitations.

How broad are the independent claims?

Claim 1: device claim

Claim 1 is broad in chemical scope but narrow in delivery architecture. It reaches multiple estradiol esters and acyl groups, but only when they are used in the specified cylindrical intravaginal hydrophobic-elastomer system.

Its principal narrowing limitations are:

Intravaginal administration.
Cylindrical geometry.
Hydrophobic elastomeric matrix.
At least three weeks of substantially zero-order release.
Specific physicochemical thresholds.
In-vivo cleavage and nontoxicity.

Claim 15: process claim

Claim 15 requires:

Combining the estradiol precursor with a biocompatible hydrophobic elastomer.
Adding a cross-linking agent.
Adding a curing catalyst.
Forming a mix.
Curing the mix into a polymer matrix.

A manufacturer using a preformed matrix and subsequently loading estradiol could fall outside the literal wording of claim 15 because the claim requires combining the precursor with the polymer before curing.

Claims 18 and 19: method claims

Claim 18 covers incorporating the precursor into the device. Claim 19 requires both incorporation and insertion into the vagina for at least three weeks.

These claims may be less commercially significant than the device claims because they require proof concerning the manufacture, use or intended use of the product. They can still matter where a supplier, manufacturer or healthcare provider directly performs the claimed steps.

What formulations are protected?

The protected formulation concept is a prodrug-in-silicone system, not an ordinary estradiol formulation.

The principal formulation classes are:

Formulation class	Covered subject matter
Estradiol 3-acetate	Expressly covered by claims 10 and 23
Estradiol 3-propionate	Expressly covered by claim 10
Estradiol 17-acetate	Expressly covered by claims 11 and 24
Estradiol 17-propionate	Expressly covered by claim 11
Other saturated acyl esters	Claims 2-4
Other unsaturated acyl esters	Claims 5-6
Estradiol ester plus progestogen	Claims 12-13
Core-and-sheath device	Claims 20-21
Hollow annular device with central member and sheath	Claims 14 and 16

Claim 12 allows a progestogen in the polymer matrix. Claim 13 identifies norethisterone-17-acetate and levonorgestrel. The claim therefore extends beyond estrogen replacement to combination contraceptive or fertility-control applications.

Which products could have been technically relevant?

The patent’s most relevant commercial comparators are vaginal rings containing estradiol or sex hormones.

Femring

Femring contains estradiol acetate in a vaginal ring and is indicated for menopausal symptoms and vulvar or vaginal atrophy. Its estradiol acetate chemistry makes it the closest commercial product to claims 10 and 23. Femring’s regulatory and patent position, however, depends on the specific approved formulation, manufacturing process and Orange Book listings rather than on chemical identity alone.

Estring

Estring contains estradiol in a silicone vaginal ring and releases approximately 7.5 micrograms per day over a 90-day period, according to FDA labeling. It is technically relevant because it uses the same broad intravaginal elastomeric-ring concept, but unmodified estradiol would not satisfy the blocked-precursor limitation in claim 1.

NuvaRing and Annovera

NuvaRing and Annovera are relevant delivery-platform comparators but do not ordinarily fall within the estradiol-specific claims:

NuvaRing uses etonogestrel and ethinyl estradiol.
Annovera uses segesterone acetate and ethinyl estradiol.

Their patents may overlap in ring geometry, polymer design or manufacturing concepts, but the specific estradiol-precursor limitations of U.S. Patent 5,855,906 materially distinguish this patent.

Does the patent cover estradiol itself?

No. The claims require a blocked 17β-estradiol precursor.

A device containing unmodified estradiol could potentially raise issues under a doctrine-of-equivalents theory only if the blocked precursor limitation were treated as nonessential and the accused system performed substantially the same function in substantially the same way. That argument would face significant obstacles because the patent emphasizes the prodrug’s physicochemical properties and in-vivo deblocking.

What is the patent’s likely expiration status?

U.S. Patent 5,855,906 issued on January 5, 1999. It originated from a 1990s application and is subject to the pre-TRIPS and post-1995 U.S. patent-term framework applicable to its filing history. For an application filed after June 8, 1995, the ordinary term is generally 20 years from the earliest effective nonprovisional U.S. filing date, subject to patent-term adjustment, patent-term extension and terminal disclaimers. [1]

The patent is now more than 25 years past issuance. On that basis, the patent’s standard U.S. term has ended. No current blocking right should be assumed without verifying:

The earliest effective nonprovisional filing date.
Any continuation or divisional relationship.
Patent-term adjustment.
Patent-term extension under 35 U.S.C. § 156.
Terminal disclaimers.
Maintenance-fee status.
Any unusual correction or reexamination event.

The patent is not a current Orange Book-listed drug patent merely because its claims concern a drug-delivery device. Orange Book listing is tied to approved drug products and qualifying patents submitted by the NDA holder under FDA regulations. [2]

What is the Orange Book status?

The claims are directed to a device, manufacturing process and method of use. They do not claim:

A new active ingredient.
A drug substance composition.
A conventional drug product composition independent of the device.
A dosage form in the regulatory sense without the claimed device architecture.

For that reason, the patent would not automatically qualify for Orange Book listing. A patent may be listed against an approved drug product only if it meets the statutory and regulatory listing categories, including drug substance, drug product or method-of-use requirements applicable to the NDA. [2]

A vaginal ring NDA may have separate listed patents covering:

The active ingredient.
The ring formulation.
Drug release.
Manufacturing.
A method of treating an FDA-approved indication.

Those listings must be checked product by product in the current FDA Orange Book. The existence of U.S. Patent 5,855,906 alone does not establish an Orange Book listing or a Paragraph IV pathway.

What Paragraph IV risks would apply?

A generic or follow-on applicant could file a Paragraph IV certification only against an unexpired Orange Book-listed patent. If U.S. Patent 5,855,906 is expired and not listed against the relevant product, it would not support a present Paragraph IV challenge.

For a live related patent, the most important design-around positions would include:

Design-around strategy	Potential effect
Use unmodified estradiol	Avoids the blocked-precursor limitation
Use a non-acyl prodrug	May avoid claims 2-6, but claim 1 remains relevant if the group satisfies its broad language
Use a non-elastomeric matrix	Avoids the hydrophobic elastomer limitation
Use a non-cylindrical device	May avoid the cylindrical-device limitation
Use a reservoir rather than a matrix	May avoid matrix-based claims, depending on construction
Release for less than three weeks	Avoids the minimum-duration limitation
Use a non-zero-order profile	May avoid the release-profile limitation
Deliver through a nonvaginal route	Avoids the intravaginal limitation

These strategies would require a full claim chart. A design-around is not established merely because one claim element differs; dependent claims and equivalents must also be assessed.

What is the biosimilar risk?

Biosimilar law is not the principal framework for this patent. Estradiol is a small-molecule active ingredient, and products such as vaginal rings generally proceed through the abbreviated new drug application pathway when the statutory requirements are met, rather than through the Biologics Price Competition and Innovation Act pathway.

The more relevant risks are:

Generic drug-device combination development.
Product-specific bioequivalence.
Local and systemic pharmacokinetics.
Device performance and release-rate equivalence.
Manufacturing process equivalence.
FDA requirements for combination products.

The prodrug and delivery system can make a product technically different from a conventional estradiol vaginal insert, even where the therapeutic target is the same.

What patent litigation and settlements affect the patent?

The claim text alone does not establish current litigation, settlement or licensing activity. The patent’s age and likely expiration make historical prosecution and litigation more relevant than current enforcement.

A diligence review should distinguish among:

Litigation involving U.S. Patent 5,855,906 itself.
Litigation involving Femring or another estradiol vaginal ring.
Litigation involving later continuation patents.
ANDA litigation involving Orange Book-listed patents.
Contractual settlements that did not result in reported court opinions.

A patent search should cover PACER, CourtListener, USPTO Patent Examination Data System records, FDA Orange Book historical listings and assignment records. A litigation-free result in a commercial database should not be treated as proof that no confidential settlement or unreported dispute existed.

How strong is the patent estate?

The patent was technically meaningful when live because it combined chemistry, device structure and release performance. Its present commercial strength is limited by expiration.

Strength factor	Assessment
Chemical breadth	Moderate; covers multiple estradiol acyl derivatives
Device breadth	Moderate to narrow; requires a cylindrical intravaginal elastomeric matrix
Functional scope	Significant; requires physicochemical thresholds and zero-order release
Dependent-claim specificity	High for estradiol acetate and propionate
Manufacturing coverage	Moderate; focused on in-situ curing of the drug-loaded matrix
Combination-product coverage	Present through progestogen claims
Current enforceability	Likely none after ordinary patent-term expiration
Design-around difficulty during term	Moderate
Relevance to modern products	Historical and freedom-to-operate context

The patent’s key value was the interaction between prodrug selection and silicone-matrix release. A competitor could not necessarily avoid the patent by changing only the device dimensions or drug loading if it retained the same claimed chemistry and release architecture.

How does this patent compare with competing vaginal-ring estates?

Technology	Estradiol precursor	Delivery system	Relationship to U.S. 5,855,906
U.S. 5,855,906	Blocked 17β-estradiol precursor	Hydrophobic elastomeric cylindrical device	Direct subject matter
Estring	Estradiol	Silicone vaginal ring	Same broad platform, but lacks the claimed blocked precursor
Femring	Estradiol acetate	Vaginal ring	Closest commercial chemistry and device comparator
NuvaRing	Ethinyl estradiol plus etonogestrel	Hormonal vaginal ring	Different active ingredients and contraceptive estate
Annovera	Ethinyl estradiol plus segesterone acetate	Reusable vaginal system	Different actives and later platform patents

The highest historical overlap is with estradiol acetate vaginal-ring products. The lowest direct overlap is with rings using non-estradiol hormones or non-elastomeric delivery systems.

What manufacturing and intellectual-property barriers remain?

Even after expiration, commercial entry can face non-patent barriers:

FDA approval of the combination product.
Establishment of release specifications.
Demonstration of dose uniformity.
Control of silicone curing and cross-linking.
Extractables and leachables testing.
Device biocompatibility.
Stability and packaging.
Local tolerability.
Bioequivalence or comparative clinical evidence.
Trade secrets concerning mixing, curing and sheath formation.

Later patents may protect improvements that were not present in U.S. Patent 5,855,906, including specific sheath compositions, ring dimensions, drug loading, curing conditions, release-control structures, packaging and clinical regimens.

Key Takeaways

U.S. Patent 5,855,906 claims a cylindrical intravaginal device using a blocked 17β-estradiol precursor in a hydrophobic elastomeric matrix.
The core claim requires substantially zero-order release for at least three weeks.
Estradiol 3-acetate and estradiol 17-acetate are expressly claimed.
Claims also cover estradiol propionate, other saturated and unsaturated acyl groups, and combinations with norethisterone-17-acetate or levonorgestrel.
The patent covers device, manufacturing, product-by-process and use claims.
Unmodified estradiol is outside the literal scope of the central blocked-precursor requirement.
The patent is not automatically an Orange Book patent because it primarily claims a delivery device and manufacturing process.
Its ordinary U.S. patent term has likely ended based on its 1990s filing history and 1999 issue date.
Femring is the closest commercial comparator because it uses estradiol acetate in a vaginal ring.
Estring is a relevant platform comparator but uses unmodified estradiol.
Current freedom-to-operate analysis must include later continuation patents, product-specific Orange Book listings and post-1999 improvement patents.

FAQs About U.S. Patent 5,855,906

Does U.S. Patent 5,855,906 cover Femring?

Femring is the closest commercial product because it contains estradiol acetate in a vaginal ring. Historical coverage would depend on the exact estradiol acetate position, ring construction, polymer, release profile and prosecution history. The patent’s present expiration status makes current infringement unlikely under this patent alone.

Does the patent cover an estradiol vaginal tablet?

Generally no. The claims require a cylindrical intravaginal drug-delivery device containing the precursor in a hydrophobic elastomeric polymer matrix. A tablet, insert or gel would not ordinarily satisfy those structural limitations.

Can a product avoid the patent by using estradiol instead of estradiol acetate?

A product containing unmodified estradiol would avoid the literal requirement for a blocked hydroxyl group. Other claim limitations and any later patents would still require separate review.

Are estradiol acetate vaginal rings eligible for generic approval?

Potentially. The regulatory pathway depends on the reference product, product formulation, device attributes, active-ingredient form, release profile and FDA determination. Expiration of this patent does not eliminate approval, bioequivalence or combination-product requirements.

Does the patent cover contraceptive vaginal rings?

Only within its estradiol-based scope. Claims 12-14 cover devices containing an estradiol precursor with specified progestogens and identify fertility control as an intended application. Rings using unrelated hormones would generally fall outside the central estradiol-precursor claims.

References

United States Patent and Trademark Office. (2024). Patent term adjustment and patent term calculation. https://www.uspto.gov/patents/laws/patent-term-adjustment
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book. https://www.fda.gov/drugs/drug-approvals-and-databases/approved-drug-products-therapeutic-equivalence-evaluations-orange-book
United States Patent and Trademark Office. (1999). U.S. Patent No. 5,855,906, Intravaginal drug delivery device.
U.S. Food and Drug Administration. (2024). Femring prescribing information.
U.S. Food and Drug Administration. (2024). Estring prescribing information.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations and patent certification requirements.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
Ireland	S940976	Dec 19, 1994
Ireland	S950247	Apr 05, 1995

PCT Information
PCT Filed	December 19, 1995	PCT Application Number:	PCT/IE95/00063
PCT Publication Date:	June 27, 1996	PCT Publication Number:	WO96/19196

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	002010	⤷ Start Trial
Austria	178204	⤷ Start Trial
Australia	4399196	⤷ Start Trial
Australia	692659	⤷ Start Trial
Canada	2207622	⤷ Start Trial
China	1138526	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Summary for Patent: 5,855,906