United States Drug Patent 5,108,995: Claim Scope, Expiration, HRT Patent Landscape, and Generic Risk

U.S. Patent No. 5,108,995 covers a hormone-replacement method using repeating 20-to-35-day regimens that alternate estrogen-dominant and progestin-dominant phases. The claims reach oral, transdermal, and buccal administration and cover specific combinations involving estradiol, piperazine estrone sulfate, norethindrone, desogestrel, norgestimate, and progesterone. The patent was granted on April 28, 1992. Under the pre-URAA patent-term regime, its ordinary 17-year term expired on April 28, 2009, absent a term adjustment, terminal disclaimer, or other exceptional event.[1][2]

The patent therefore does not present a current U.S. blocking right. Its principal value is historical and analytical: it illustrates protection for phased hormone-delivery schedules rather than protection limited to a single active ingredient or dosage form.

What does U.S. Patent 5,108,995 protect?

The patent protects a method of treating a female requiring hormone replacement therapy by administering repeated cycles of daily unit doses. The central inventive concept is the alternation of short estrogen-dominant and progestin-dominant phases.

Core claim limitations

Limitation	Scope under claim 1
Patient	A female in need of hormone replacement therapy
Treatment structure	Repeating cycles
Cycle length	20 to 35 consecutive daily unit doses
Phase structure	Alternating estrogen-dominant and progestin-dominant phases
Phase duration	One to four consecutive daily doses per phase
Estrogen amount	Activity equivalent to approximately 0.3 to 2.5 mg piperazine estrone sulfate
Progestin amount	Zero to activity equivalent to approximately 5 mg norethindrone
Administration	Not limited in claim 1; dependent claims add oral, transdermal, and buccal forms
Dose relationship	Progestin activity increases in progestin-dominant phases and decreases in estrogen-dominant phases

The claim is a method claim. It does not claim estradiol, norethindrone, a patch, a tablet, or a particular product standing alone. Infringement would require practicing the claimed treatment regimen, including the specified cycling pattern and dose relationships.

How does claim 1 define the protected HRT regimen?

Claim 1 contains five material technical requirements.

First, the regimen must use repeated cycles of 20 to 35 daily doses. A 28-day regimen would fall within this numerical range, while a regimen of 36 or more daily doses would not satisfy that limitation literally.

Second, each cycle must contain alternating phases. The claim does not permit an arbitrary sequence of changing doses. It requires alternating estrogen-dominant and progestin-dominant phases.

Third, each phase must last one to four consecutive daily doses. A regimen with five consecutive days in one phase would fall outside the express phase-duration limitation.

Fourth, the estrogen and progestin quantities are defined by biological activity equivalents rather than solely by mass. This creates a potentially broad functional limitation, but it also raises proof issues because the parties would need to establish the relevant equivalence between the accused substance and the stated reference compounds.

Fifth, progestin activity must be greater in the progestin-dominant phases than in the estrogen-dominant phases. The claim is therefore directed to a changing hormonal ratio, not merely to the presence of both hormones.

Drafting issue in claim 1

The supplied text of claim 1 contains an apparent duplication in the description of the progestin-dominant phase. It states that those doses contain:

"an amount of a substance exhibiting estrogen activity and an amount of a substance exhibiting estrogen activity and an amount of a substance exhibiting progestin activity."

The second reference to estrogen activity appears facially inconsistent with the surrounding claims and with claim 16. The likely intended formulation is an estrogen amount plus a progestin amount. The issued patent and its prosecution history would control the legal interpretation. Claim 16 provides clearer language by specifying progestin activity of zero to 0.5 mg norethindrone equivalent in estrogen-dominant phases and 0.35 to 5 mg norethindrone equivalent in progestin-dominant phases.

What do claims 2 through 12 add?

Claims 2 through 12 narrow the regimen by specifying dose uniformity, active ingredients, delivery routes, and phase lengths.

Claim	Added limitation
2	Uniform estrogen amount in every daily dose
3	Progestin present in every daily dose, with alternating higher and lower amounts
4	Estrogen selected from 17β-estradiol or piperazine estrone sulfate; progestin selected from norethindrone, desogestrel, norgestimate, or progesterone
5	Oral administration
6	Transdermal administration
7	Buccal administration
8	Two estrogen-dominant days alternating with two progestin-dominant days
9	Three estrogen-dominant days alternating with three progestin-dominant days
10	Four estrogen-dominant days alternating with three progestin-dominant days
11	Three estrogen-dominant days alternating with four progestin-dominant days
12	Estrogen-dominant phases are free of progestin

Claims 8 through 11 are particularly important because they convert the general one-to-four-day phase limitation into defined dosing patterns. They would be narrower than claim 1 but easier to analyze against a commercial regimen.

Claim 12 excludes progestin from the estrogen-dominant phases. That limitation distinguishes a regimen in which progestin is completely absent during the lower-progestin phase from one in which the progestin amount merely decreases.

What formulations are protected by claims 13 through 15?

Claims 13 through 15 are the most product-like claims in the patent. They specify active ingredients, amounts, route of administration, and phase duration.

Claim	Estrogen-dominant phase	Progestin-dominant phase
13	Three oral doses, each with 0.75 mg piperazine estrone sulfate; no progestin	Three oral doses, each with 0.75 mg piperazine estrone sulfate and 0.35 mg norethindrone
14	Three transdermal doses, each releasing 0.1 mg 17β-estradiol; no progestin	Three transdermal doses, each releasing 0.1 mg 17β-estradiol and 0.35 mg norethindrone
15	Three transdermal doses, each releasing 0.1 mg 17β-estradiol and 0.15 mg norethindrone	Three transdermal doses, each releasing 0.1 mg 17β-estradiol and 0.35 mg norethindrone

Claim 13 is directed to a six-day alternating sequence if the phases are administered consecutively. Claims 14 and 15 address transdermal systems with constant estradiol delivery and changing norethindrone exposure.

The claims do not expressly require a particular patch construction, adhesive, reservoir, matrix, backing layer, release-control membrane, tablet excipient, or packaging configuration. Those features would need protection from separate formulation or device claims, if any existed in related patents.

How strong is the patent estate for U.S. Patent 5,108,995?

The patent is technically broad at the independent-claim level but legally weak as a current commercial asset because the U.S. patent term has expired.

Strengths of the claim set

The claim set has several features that historically could have created meaningful design-around risk:

It covers multiple delivery routes.
It covers several estrogen and progestin classes.
It uses functional activity equivalents rather than one fixed chemical identity.
It covers a range of cycle lengths and phase lengths.
It includes narrower claims directed to specific oral and transdermal dose schedules.

The use of activity equivalents could have expanded coverage beyond the named compounds, subject to enablement, definiteness, written-description, and claim-construction limits.

Weaknesses of the claim set

The principal weaknesses are structural:

The claims require a particular phase sequence.
The phase length is limited to one through four days.
The cycle length is limited to 20 through 35 days.
The regimen must produce a demonstrable estrogen-dominant or progestin-dominant phase.
Claims 13 through 15 require exact active-ingredient amounts and delivery formats.
Functional equivalence may create evidentiary disputes.
The apparent duplication in claim 1 creates an interpretation issue.

A product using continuous combined therapy, a conventional 28-day sequential regimen, a 12-to-14-day progestin phase, or a nonalternating titration pattern would not necessarily satisfy the claimed regimen.

When did U.S. Patent 5,108,995 lose exclusivity?

The patent was issued April 28, 1992. For a U.S. patent subject to the pre-June 8, 1995 term rule, the ordinary term was 17 years from grant. The corresponding ordinary expiration date was April 28, 2009.[1][2]

Event	Date
Patent grant	April 28, 1992
Ordinary statutory term	17 years from grant
Ordinary expiration	April 28, 2009
Current status for ordinary enforcement	Expired

The patent’s expiration eliminates current infringement liability based solely on the expired U.S. patent. It also eliminates the need for a present Paragraph IV challenge directed to this patent.

What is the Orange Book status of U.S. Patent 5,108,995?

U.S. Patent 5,108,995 should not be treated as a current Orange Book barrier merely because it claims a pharmaceutical hormone regimen.

The FDA Orange Book lists patents submitted for approved drug products under the Hatch-Waxman framework. Listed patents may include composition, formulation, method-of-use, and certain drug-substance or manufacturing patents, subject to FDA listing rules.[3][4] A method-of-use patent must be tied to an approved use of the relevant drug product and must meet the applicable submission requirements.

The patent claims a treatment method rather than a single approved product. The supplied information does not establish that the patent was listed for any particular NDA, whether any listing was delisted, or whether any patent-use code was assigned. Because the patent expired in 2009, it cannot currently block an ANDA approval even if it had historically been listed.

Are Paragraph IV challenges relevant to this patent?

A Paragraph IV certification is relevant when an ANDA applicant asserts that a listed patent is invalid, unenforceable, or will not be infringed. The certification can trigger patent litigation and, in some circumstances, a 30-month stay of approval under the Hatch-Waxman framework.[4]

That mechanism is no longer commercially relevant to Patent 5,108,995 because the patent has expired. A generic applicant would not need to invalidate the patent to obtain approval after expiration. Any historical Paragraph IV certification would be part of the patent’s enforcement history, not a current entry barrier.

The more relevant historical question would have been whether an ANDA product practiced the claimed alternating regimen. An ordinary generic hormone product with a different schedule would have had a stronger noninfringement position than a product replicating the claimed phase structure and dose amounts.

What patent litigation or settlement agreements affect the patent?

The supplied claim text does not identify litigation, a Paragraph IV notice, a settlement agreement, a license, or a consent judgment involving Patent 5,108,995. The patent record alone does not establish a commercial settlement or a continuing license restriction.

Because the patent expired in 2009, any historical dispute would now have limited commercial significance unless it involved damages, an agreement with surviving obligations, or related patents with later expiration dates. Those issues cannot be attributed to Patent 5,108,995 without a docket, settlement, or license document.

Does the patent create biosimilar risk?

No. The patent concerns small-molecule hormone replacement therapy and does not claim a biologic product, biosimilar reference product, cell line, protein sequence, antibody, or biologic manufacturing process.

The relevant competitors would have been generic drug manufacturers and hormone-therapy product sponsors proceeding under the ANDA or 505(b)(2) pathways, not biosimilar applicants under the Public Health Service Act.[5]

What manufacturing and formulation barriers does the patent create?

The expired patent does not currently create a manufacturing barrier. During its term, the principal barrier would have been regimen replication rather than chemical synthesis or device manufacture.

Potentially relevant technical issues included:

Maintaining daily estrogen delivery within the claimed activity range.
Adjusting norethindrone or another progestin between phases.
Producing oral units with distinct hormone content.
Producing transdermal systems with defined release rates.
Packaging multiple phase-specific doses in the correct sequence.
Demonstrating that the dosage schedule produced estrogen-dominant and progestin-dominant phases.

The patent does not, on the supplied claims, require a proprietary excipient, patch membrane, manufacturing step, crystal form, polymorph, impurity profile, or process parameter. Those omissions reduce formulation and manufacturing barriers compared with a patent directed to a specific drug substance or delivery platform.

How does this patent compare with typical HRT patent strategies?

Patent strategy	Typical protected subject matter	Patent 5,108,995
Active ingredient	A new estrogen, progestin, or analog	No
Composition	A defined hormone combination	Only through method claims
Formulation	Tablet, patch, gel, or controlled-release composition	Only indirectly
Delivery device	Patch construction or release mechanism	No express device claim
Method of use	Dose schedule or therapeutic indication	Yes
Manufacturing	Synthesis or formulation process	No express process claim
Regulatory exclusivity	New chemical entity or clinical exclusivity	Not established
Biosimilar protection	Biologic product or process	No

The patent’s commercial strategy was regimen protection. It sought to control how known hormones were administered over time rather than to control a new molecular entity.

What generic launch scenarios existed?

During the patent term, an entrant could have pursued several design-around strategies:

Use a cycle outside the 20-to-35-day range.
Use phases longer than four days.
Use a continuous combined regimen.
Use a conventional sequential regimen with a longer uninterrupted progestin phase.
Change the estrogen or progestin dose relationship.
Use a delivery route not covered by a relevant dependent claim while avoiding claim 1.
Use a progestin not listed in claim 4, subject to the functional scope of claim 1.
Avoid the claimed alternation between estrogen-dominant and progestin-dominant phases.

Those strategies would require claim-by-claim analysis. They do not matter for present launch timing because the patent has expired.

What revenue exposure remains?

Patent 5,108,995 creates no current U.S. revenue exposure from ordinary patent infringement. Any historical revenue exposure ended with expiration of the patent term, subject to unresolved litigation or accrued claims.

The patent also does not establish current market exclusivity for a particular branded HRT product. Revenue exposure would instead depend on other patents, regulatory exclusivity, trademarks, approved labeling, formulation rights, or commercial contracts associated with a specific product.

Key Takeaways

U.S. Patent 5,108,995 covers alternating estrogen-dominant and progestin-dominant HRT phases.
Claim 1 requires 20 to 35 daily doses per cycle and one-to-four-day alternating phases.
Claims 5 through 7 extend the regimen to oral, transdermal, and buccal administration.
Claims 13 through 15 provide the narrowest examples, including specific estradiol, piperazine estrone sulfate, and norethindrone amounts.
The patent was granted April 28, 1992, and its ordinary 17-year term expired April 28, 2009.
It is not a current U.S. blocking patent.
It does not create biosimilar risk or a current Paragraph IV barrier.
The claims protect treatment scheduling, not a new hormone molecule, patch architecture, or manufacturing process.
The apparent duplicated estrogen phrase in claim 1 creates a claim-construction issue, while claim 16 provides clearer progestin ranges.
Current commercial risk would need to come from later patents, regulatory exclusivity, or contractual rights, not from Patent 5,108,995 itself.

FAQs

Can an expired HRT method patent still block a generic launch?

No. An expired U.S. patent cannot support a new infringement injunction or prevent approval solely because the generic practices the claimed method.

Does Patent 5,108,995 cover every estradiol and norethindrone product?

No. The claims require a defined cyclic regimen, alternating phase structure, dose relationship, and, for narrower claims, specified routes and amounts.

Would a 28-day continuous-combined HRT product infringe claim 1?

Not necessarily. Continuous-combined therapy lacks the required alternating estrogen-dominant and progestin-dominant phases unless its dose pattern otherwise satisfies every limitation.

Is norethindrone acetate necessarily covered by a claim naming norethindrone?

The supplied claims name norethindrone, not norethindrone acetate. Coverage would depend on claim construction, activity equivalence, prosecution history, and whether the accused regimen satisfies the functional progestin limitations.

Does the patent protect the physical packaging of phased HRT doses?

No express packaging claim appears in the supplied claims. The claims protect administration of a specified regimen, not a package, blister card, patch construction, or tablet design standing alone.

References

United States Patent No. 5,108,995, claims 1-16 (issued Apr. 28, 1992).
35 U.S.C. § 154 (pre-URAA patent term provisions governing patents issued before the transition to the 20-year term).
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations (Orange Book).
21 C.F.R. § 314.53. Submission of patent information.
Public Health Service Act, 42 U.S.C. § 262. Regulation of biological products.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
Canada	547743	Sep 24, 1987
Canada	547943	Sep 24, 1987

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	102484	⤷ Start Trial
Austria	209919	⤷ Start Trial
Australia	2276088	⤷ Start Trial
Australia	3044892	⤷ Start Trial
Australia	630334	⤷ Start Trial
Canada	1332227	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,108,995

Summary for Patent: 5,108,995