US Patent 11,795,178: Claim Scope, Ensitrelvir Impurity Control, and Patent Landscape

US Patent 11,795,178 is a Shionogi patent directed to compositions containing ensitrelvir together with specified process-related impurities. Its commercial value lies in controlling the impurity profile of ensitrelvir drug substance or finished product, rather than in claiming the active pharmaceutical ingredient broadly. The claims can cover ensitrelvir mixtures containing trace levels of an aniline impurity, selected pyridazinone and dicarbonimidic diamide impurities, and other manufacturing byproducts.

The patent does not, based on the supplied claims, claim a method of treating COVID-19, a pharmaceutical dosage regimen, or ensitrelvir as a chemical entity in isolation. It is a composition and analytical-control patent.

What drug does US Patent 11,795,178 protect?

The principal compound in the claims is ensitrelvir, also known as S-217622 and marketed in Japan as Xocova. Ensitrelvir is an orally administered SARS-CoV-2 main protease, or 3CL protease, inhibitor developed by Shionogi & Co., Ltd.

The claimed active is:

1-{4-[1-(2,6-difluorobenzyl)-5-dimethylaminomethyl-3-(6-methoxypyridazin-3-yl)-2,4-dioxo-1,2,3,4-tetrahydrothieno[2,3-d]pyrimidin-6-yl]phenyl}-3-methoxyurea

The claim text supplied contains OCR and spacing errors, including constructions such as "441-" and "1444142." Those expressions should be read as chemical nomenclature corresponding to substituted phenyl groups, principally "4-[...]" and related bracketed structures.

The principal impurity in claims 1 through 13 and 23 through 33 is the corresponding 4-aminophenyl compound:

6-(4-aminophenyl)-1-(2,6-difluorobenzyl)-5-dimethylaminomethyl-3-(6-methoxypyridazin-3-yl)thieno[2,3-d]pyrimidine-2,4(1H,3H)-dione

This impurity is structurally related to ensitrelvir and can arise during synthesis, particularly during formation or transformation of the methoxyurea-containing aryl substituent.

What is the independent claim scope of US 11,795,178?

Claim 1 covers an ensitrelvir-plus-impurity mixture

Claim 1 requires a mixture comprising:

ensitrelvir, or a salt or solvate of ensitrelvir; and
the specified 4-aminophenyl impurity.

The term "comprising" makes the claim open-ended. A product can fall within claim 1 even if it contains excipients, additional impurities, polymorphs, residual solvents, or other active or inactive materials, provided that the two recited compounds are present.

Claim 1 does not state a minimum or maximum concentration for the 4-aminophenyl impurity. On its face, the claim therefore reaches a mixture containing the active and the named impurity at any amount capable of satisfying the claim's compositional requirements. The practical scope will depend on issues such as detectability, claim construction, product composition, and whether the impurity is present as part of the accused mixture.

Claim 25 is a narrower composition claim

Claim 25 also requires ensitrelvir and the 4-aminophenyl impurity, but expressly recites both compounds without the broad "or a salt or solvate thereof" language used in claim 1.

Claim 25 is therefore narrower in wording than claim 1, although the commercial distinction may be limited if the marketed product contains the same active and impurity in the claimed form.

Claims 14, 17, 18, 21, and 22 add alternative impurities

The patent claims mixtures containing ensitrelvir and additional specified impurities:

Claim group	Added impurity or limitation	Commercial significance
14-16	Pyridazinone analog of ensitrelvir	Covers a degradation or synthetic intermediate related to the 6-methoxypyridazine substituent
17	A dimethoxydicarbonimidic diamide impurity	Covers a urea-related process impurity
18-20	N,N'-dimethoxydicarbonimidic diamide impurity	Adds a second urea-derived impurity family
21	6-(4-nitrophenyl) analog	Covers a nitro precursor or incompletely reduced intermediate
22	Ethyl carbamate-related thiophene intermediate	Targets a different process-stage byproduct

Claims 14, 17, 18, 21, and 22 are dependent on claim 1. Each requires ensitrelvir plus the specifically named additional compound. They do not appear to claim a general impurity class.

What impurity concentrations are covered?

Claims 2 through 13 and 23 through 33 create nested concentration ranges for the 4-aminophenyl impurity. The relevant limits are:

Claims	HPLC concentration of 4-aminophenyl impurity
2	About 4.45% or less
3	About 0.25% or less
4	About 0.23% or less
5	About 0.22% or less
6	About 0.20% or less
7	About 4.45% to about 0.20%
8	About 0.25% to about 0.20%
9	About 0.16% or less
10	About 0.07% or less
11	About 0.05%
12	About 0.16% to about 0.05%
13	About 0.07% to about 0.05%
23	About 0.25% to about 0.05%
24	About 0.25% to about 0.07%
29	About 0.05% or less
30	About 4.45% to about 0.05%
31	About 4.45% to about 0.25%
32	About 0.25% to about 0.16%
33	About 0.20% to about 0.05%

The claims are drafted as a descending impurity-control ladder. The broadest practical concentration limitation is approximately 4.45% or less. The most stringent listed limitation is approximately 0.05% or less, with claim 11 reciting approximately 0.05%.

The word "about" creates potential claim-construction questions around measurement tolerance and analytical variability. The patent also defines the HPLC method in later dependent claims, which may be important when determining whether a measured batch falls within a particular concentration range.

What HPLC method is protected?

Claims 34 through 87 claim the composition together with a specified reverse-phase HPLC method. The method limitations include:

a reverse-phase chromatography column;
a first mixed solvent containing phosphate buffer solution and acetonitrile;
a second mixed solvent containing phosphate buffer solution and acetonitrile;
separation of the 4-aminophenyl impurity from the remainder of the mixture;
ultraviolet detection at approximately 230 nm;
a C18 column;
approximately 3.5 micrometer silica particles; and
a column having an inner diameter of approximately 4.6 mm and a length of approximately 100 mm.

The stated solvent ratios are:

Parameter	Claimed condition
First mixed solvent	Phosphate buffer solution:acetonitrile of about 31:9 by volume
Second mixed solvent	Phosphate buffer solution:acetonitrile of about 12:13 by volume
Detection wavelength	About 230 nm
Column chemistry	C18
Particle size	About 3.5 micrometers
Column dimensions	About 4.6 mm x 100 mm

These claims are not broad standalone HPLC claims. They depend on composition claims, including claims 2, 8, 23, 30, 32, and 33. Infringement would require both the claimed composition and the specified analytical method limitations.

The method claims can be relevant in a dispute over product testing, release specifications, or regulatory documentation. Their value is narrower than a direct composition claim because an accused party may use a different validated analytical method, subject to the doctrine of equivalents and other legal considerations.

Does the patent cover pure ensitrelvir?

No, not based on the supplied claims.

Every independent composition concept requires ensitrelvir together with at least one specified additional compound. A batch containing only ensitrelvir, with none of the named impurities, would not satisfy the literal requirements of claim 1 or claim 25.

Claims 26 through 28 do not change that result. They require the mixture to be in solid form or to comprise crystals, including ensitrelvir crystals. They remain dependent on claim 1 and therefore retain the requirement for the named impurity.

This structure creates a central enforcement issue: the patent is strongest against products that contain both ensitrelvir and one of the specifically recited impurities, not against the manufacture, sale, or use of impurity-free ensitrelvir as such.

What formulations are protected by US 11,795,178?

The claims do not recite a conventional pharmaceutical formulation architecture. They do not specify:

tablets;
capsules;
oral suspensions;
excipient classes;
dosage strengths;
release profiles;
coatings;
particle-size distributions;
solid dispersions; or
particular salt or polymorph forms, except for the general salt, solvate, solid, and crystal language.

The coverage is therefore composition-focused rather than dosage-form-focused.

A tablet, capsule, or other finished dosage form could potentially fall within the claims if it contains the required ensitrelvir and impurity combination. The presence of excipients would not avoid claim 1 because of its open-ended "comprising" language.

How strong is the patent estate for ensitrelvir?

US 11,795,178 has moderate commercial relevance but a narrower technical perimeter than a composition-of-matter patent.

Strengths

Claim 1 does not impose a stated impurity concentration.
The claim covers salts and solvates of the active.
The open-ended "comprising" language permits additional ingredients and impurities.
The patent targets manufacturing reality, where trace impurities may be present.
Claims 2 through 33 provide multiple fallback concentration positions.
Claims 34 through 87 provide a defined analytical framework for several impurity limits.
Claims 26 through 28 extend the claim set to solid and crystalline mixtures.

Weaknesses

The claims require the presence of specific impurities.
The patent does not claim ensitrelvir alone.
The impurity structures are narrow and chemically defined.
An alternative synthetic route may avoid one or more named impurities.
A purified product below the detection or claim-relevant threshold may avoid the concentration-dependent claims.
The HPLC claims are vulnerable to nonidentical analytical methods if the composition claims are not independently infringed.
"About" and HPLC measurement conditions may create disputes over analytical precision and claim boundaries.

The patent is best characterized as a process-control and product-quality patent. It can create a barrier for a manufacturer using a process that produces the claimed impurity profile, but it is unlikely to block every route to ensitrelvir manufacture.

When does US Patent 11,795,178 lose exclusivity?

The patent number and claim text alone do not establish the legally operative expiration date. The date depends on the earliest effective nonprovisional priority date, patent term adjustment, terminal disclaimers, and any applicable patent-term extension.

The patent is identified as US 11,795,178 B2. A formal freedom-to-operate analysis should use the USPTO Patent Center record and the patent's continuity, priority, terminal-disclaimer, and PTA data rather than calculate expiration from the grant date. The 20-year term generally runs from the earliest effective US nonprovisional filing date, subject to statutory adjustments under 35 U.S.C. § 154 [1].

No conclusion about FDA regulatory exclusivity follows from the patent grant. Patent term and FDA exclusivity are separate rights.

What is the FDA status and Orange Book position?

Ensitrelvir was approved in Japan as Xocova for COVID-19 treatment, but the supplied materials do not establish an FDA-approved US New Drug Application for ensitrelvir. The FDA Orange Book lists patents and exclusivity associated with approved products and approved NDA submissions. A patent is not placed in the Orange Book merely because it claims a drug or impurity profile [2].

Based on the claim subject matter, US 11,795,178 should not be treated as an Orange Book-listed patent without confirmation in the relevant NDA patent listing. If ensitrelvir has no approved US NDA, there is no ordinary US Orange Book listing against which an ANDA applicant would file a Paragraph IV certification.

The patent is also not a biologic patent. Ensitrelvir is a small-molecule drug, so biosimilar provisions and the FDA Purple Book are not relevant. Any future US generic would proceed through the ANDA pathway, not the biosimilar pathway [3].

Which companies are challenging the patent?

No Paragraph IV challenger, ANDA litigation, or federal infringement action can be established from the claims supplied. The absence of a US-approved reference product would make conventional ANDA-based Paragraph IV litigation less likely during the period in which ensitrelvir lacks US approval.

A potential generic manufacturer could still challenge validity or enforceability through other mechanisms, including:

a declaratory judgment action;
an inter partes review petition;
a post-grant review petition, if statutory timing permitted;
a challenge during future ANDA litigation; or
a noninfringement position based on impurity avoidance.

The principal litigation theories would likely concern written description, enablement, anticipation, obviousness, indefiniteness of "about," and whether the accused product contains the claimed impurity in the claimed form and amount.

What generic launch risks exist?

The patent creates three principal launch scenarios.

Scenario 1: Generic uses a different manufacturing route

If the alternative process does not generate the claimed 4-aminophenyl impurity or the other named impurities, the primary composition claims may not be infringed. This is the most direct design-around strategy.

Scenario 2: Generic contains the impurity above a claimed threshold

A product containing the 4-aminophenyl impurity at, for example, 0.20% or less could implicate multiple nested claims, depending on the actual measured amount and the applicable HPLC method. A product at a higher concentration could implicate claim 2 and broader range claims while falling outside narrower low-level claims.

Scenario 3: Generic purifies the active but retains a trace impurity

Trace contamination may remain commercially relevant. Claims 1, 25, and 26 through 33 do not require a high impurity concentration. A manufacturer would need batch-level analytical data and a claim chart mapping the actual impurity to the precise claimed structure.

The claims therefore create greater risk for process replication than for independent development of ensitrelvir using a materially different route.

How does this patent compare with core ensitrelvir patents?

US 11,795,178 should be separated from the broader ensitrelvir patent estate.

Patent category	Typical subject matter	Relative blocking power
Composition-of-matter patent	Ensitrelvir chemical structure and broad analogs	Highest for any manufacture or use of the active
Salt, solvate, and crystal patents	Specific solid forms or salts	High for the claimed form, narrower outside it
Process patents	Synthetic intermediates, reaction sequences, purification	High where the process is copied
Formulation patents	Tablets, capsules, excipients, release profiles	Product-specific
Method-of-use patents	COVID-19 treatment, timing, patient populations	Relevant to labeled or intentional use
US 11,795,178	Ensitrelvir mixtures containing specified impurities	Targeted to impurity profile and manufacturing quality

The patent's value depends on the status of the broader Shionogi ensitrelvir patent family. A complete landscape would normally include US continuations, divisionals, national-stage applications, related Japanese and PCT filings, granted composition-of-matter patents, crystal-form patents, process patents, and any terminal disclaimers linking the family.

Key Takeaways

US 11,795,178 is an impurity-profile patent for ensitrelvir mixtures.
Claim 1 requires ensitrelvir plus a specified 4-aminophenyl impurity.
Claims 2 through 13 and 23 through 33 narrow the impurity concentration, reaching levels from about 4.45% down to about 0.05% or less.
Claims 14, 17, 18, 21, and 22 cover additional named ensitrelvir-related impurities.
Claims 26 through 28 cover solid and crystalline mixtures, but do not claim pure ensitrelvir.
Claims 34 through 87 require specific reverse-phase HPLC conditions and are dependent composition claims.
The patent is relevant to manufacturing-route selection, purification, release testing, and generic design-around analysis.
It is weaker than a composition-of-matter patent because a process that avoids the named impurities may avoid literal infringement.
Ensitrelvir is a small molecule, so biosimilar law and the Purple Book do not apply.
An Orange Book listing and Paragraph IV risk cannot be inferred from the patent grant alone.

FAQs

Can a generic avoid US 11,795,178 by reducing the impurity below 0.05%?

Possibly, but the answer depends on claim 1 and claim 25, which do not state a numerical minimum, as well as the precise impurity identity and analytical evidence. Avoiding the low-level dependent claims would not automatically eliminate all composition-claim risk.

Does a different HPLC method avoid infringement?

It may avoid the method-specific claims in claims 34 through 87, but it does not necessarily avoid the underlying composition claims. Claims 1 through 33 do not require the specific HPLC column, solvent ratios, or detection wavelength.

Does the patent cover ensitrelvir fumarate?

Claim 1 expressly includes a salt or solvate of the active, but the mixture must also contain the named impurity. The claim does not independently cover every ensitrelvir fumarate product that lacks that impurity.

Is US 11,795,178 a method-of-treatment patent?

No. The supplied claims are directed to mixtures, impurity concentrations, solid or crystalline form, and analytical characterization. They do not recite administering ensitrelvir to a patient.

Can an API manufacturer be exposed even if the finished tablet is purified?

Yes. If the API or finished dosage form contains a claimed impurity, the composition claims may apply. Purification reduces risk only if it removes the claimed compound or brings the product outside the relevant claim scope.

References

United States Patent and Trademark Office. (n.d.). 35 U.S.C. § 154: Contents and term of patent; provisional rights. https://www.uspto.gov
U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations, Orange Book. https://www.accessdata.fda.gov/scripts/cder/ob/
U.S. Food and Drug Administration. (n.d.). Purple Book: Lists of licensed biological products with reference product exclusivity and biosimilarity or interchangeability evaluations. https://purplebooksearch.fda.gov
United States Patent and Trademark Office. (2023). US Patent No. 11,795,178 B2. Google Patents/USPTO patent record.

Applicant	Tradename	Generic Name	Dosage	NDA	Approval Date	TE	Type	RLD	RS	Patent No.	Patent Expiration	Product	Substance	Delist Req.	Patented / Exclusive Use	Submissiondate
Sumitomo Pharma Am	MYFEMBREE	estradiol; norethindrone acetate; relugolix	TABLET;ORAL	214846-001	May 26, 2021		RX	Yes	Yes	⤷ Start Trial	⤷ Start Trial	Y	Y			⤷ Start Trial
Sumitomo Pharma Am	ORGOVYX	relugolix	TABLET;ORAL	214621-001	Dec 18, 2020		RX	Yes	Yes	⤷ Start Trial	⤷ Start Trial	Y	Y			⤷ Start Trial
>Applicant	>Tradename	>Generic Name	>Dosage	>NDA	>Approval Date	>TE	>Type	>RLD	>RS	>Patent No.	>Patent Expiration	>Product	>Substance	>Delist Req.	>Patented / Exclusive Use	>Submissiondate

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	092707	⤷ Start Trial
Canada	2881132	⤷ Start Trial
China	104703992	⤷ Start Trial
China	109053766	⤷ Start Trial
China	113372358	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 11,795,178

Which drugs does patent 11,795,178 protect, and when does it expire?

Summary for Patent: 11,795,178