United States Drug Patent 5,744,496: Claim Scope, Cilazapril Coverage, Expiration, and Patent Landscape

U.S. Patent No. 5,744,496 covers methods of treating cardiac insufficiency with a defined class of substantially pure angiotensin-converting enzyme inhibitors, principally cilazapril and cilazaprilat. The patent issued on April 28, 1998, and its 17-year patent term expired on April 28, 2015, absent any patent-term adjustment or extension. The claims are method-of-treatment claims, not composition-of-matter claims. They do not independently block manufacture or sale of cilazapril for every indication.

The commercial significance of the patent is now historical. Cilazapril was not approved as a marketed drug in the United States, and the patent does not create a current U.S. generic-entry barrier. No biosimilar analysis applies because cilazapril is a chemically synthesized small molecule, not a biologic.

What drug does U.S. Patent 5,744,496 protect?

The patent principally covers cilazapril and cilazaprilat used to treat cardiac insufficiency.

Claim reference	Chemical subject	Commercial name or description
Claim 3	N-(1-S-carbethoxy-3-phenylpropyl)-S-alanyl-2S,3aR,7aS-octahydroindole-2-carboxylic acid	Cilazapril, the ethyl ester prodrug
Claim 6	N-(1-S-carboxy-3-phenylpropyl)-S-alanyl-2S,3aR,7aS-octahydroindole-2-carboxylic acid	Cilazaprilat, the active diacid
Claim 1	A Markush class of ACE inhibitors defined by the structural formula and R2 substituent	Broad genus claim
Claim 2	The claim 1 genus limited to R2 = hydrogen or ethyl	Includes cilazaprilat and cilazapril
Claims 4, 5, 7 and 8	Administration route or formulation conditions	Oral, parenteral, excipient-containing treatment

Cilazapril is converted in vivo to cilazaprilat, which inhibits angiotensin-converting enzyme and reduces formation of angiotensin II. The claimed indication is cardiac insufficiency, a term generally corresponding to heart failure or impaired cardiac pumping function.

The claims do not expressly require a particular dose, treatment duration, disease severity, patient age, salt form, or dosage form.

What is the patent’s legal status and expiration date?

U.S. Patent No. 5,744,496 issued on April 28, 1998. Its statutory term was governed by the pre-URAA 17-year-from-issuance rule applicable to qualifying applications filed before June 8, 1995. On that basis, the patent expired on April 28, 2015. The U.S. Patent and Trademark Office patent record is the controlling source for the term calculation and any recorded adjustment or disclaimer. (USPTO, 1998)

Event	Date or status
U.S. patent number	5,744,496
Issue date	April 28, 1998
Patent type	Utility patent
Claim category	Method of treatment
Principal active ingredients	Cilazapril and cilazaprilat
Base term	17 years from issuance
Base expiration	April 28, 2015
Current status	Expired
Patent-term extension relevance	No commercially relevant U.S. extension is associated with cilazapril

Because the patent has expired, its claims cannot support a current U.S. infringement action for conduct occurring after expiration. The expiration also removes the patent as a present barrier to a generic or other competitor practicing the claimed cardiac-insufficiency treatment method.

How broad is independent claim 1?

Claim 1 is a genus method claim. It requires all of the following:

A mammalian patient.
Recognized need for treatment of cardiac insufficiency.
Administration for the purpose of treating that condition.
An effective amount of the claimed substantially pure ACE inhibitor.
The specific chemical structure shown in the patent.
A permitted R2 substituent of hydrogen, methyl, ethyl or benzyl.
The active compound or a pharmaceutically acceptable salt.

The claim is narrower than a claim to all ACE inhibitors. It does not cover unrelated ACE inhibitors such as enalapril, lisinopril, ramipril, benazepril, quinapril, perindopril or trandolapril unless a particular compound falls within the precise chemical formula.

The term “substantially pure” adds a product-quality limitation. It points toward use of a defined stereochemical compound rather than a racemic or materially contaminated mixture. The stereochemical descriptors in claims 3 and 6 are important. A compound with the same connectivity but different stereochemistry would not necessarily satisfy those narrower claims.

The phrase “recognized need” also supplies a patient-selection limitation. The claim is directed to treatment of a patient known to have cardiac insufficiency, rather than incidental exposure to the drug or administration for an unrelated condition.

What do claims 2 through 8 add?

What does claim 2 cover?

Claim 2 restricts R2 to hydrogen or ethyl. This removes the methyl and benzyl alternatives from claim 1. It is a narrower genus claim and corresponds directly to the two specifically identified forms:

Hydrogen: cilazaprilat.
Ethyl: cilazapril.

What does claim 3 cover?

Claim 3 identifies cilazapril by chemical name. It requires use of cilazapril or a pharmaceutically acceptable salt to treat cardiac insufficiency.

This is a species method claim. It is narrower than claim 1 in chemical scope but potentially easier to prove because the accused compound can be identified directly through analytical testing.

What does claim 4 cover?

Claim 4 covers oral or parenteral administration. It does not limit the claim to tablets, capsules, injections, infusions or a specific release profile. The claim confirms that both systemic oral treatment and injection-based treatment are within the claimed method.

What does claim 5 cover?

Claim 5 covers administration with a pharmaceutically suitable excipient or acceptable organic or inorganic substance. This language captures conventional pharmaceutical formulations, including solid oral dosage forms and injectable preparations, but it does not create an independent composition claim.

What does claim 6 cover?

Claim 6 identifies cilazaprilat, the active carboxylic-acid form. It covers cilazaprilat or a pharmaceutically acceptable salt used for cardiac insufficiency.

Cilazaprilat has limited oral absorption compared with the ester prodrug, so the practical commercial focus of this claim would more likely involve parenteral use or experimental formulations. The claim itself is not limited to a particular route.

What do claims 7 and 8 cover?

Claims 7 and 8 narrow claim 1 to oral and parenteral administration, respectively. They are separate fallback claims. An oral-use product could implicate claim 7, while an injectable or other nonoral administration could implicate claim 8.

Does the patent cover formulation patents or manufacturing methods?

No. The supplied claims do not claim:

A tablet, capsule, injection, or other dosage form as a composition.
A particular excipient combination independent of the treatment method.
A controlled-release formulation.
A salt-selection process.
A manufacturing process for cilazapril or cilazaprilat.
A crystalline or polymorphic form.
A process for making the active ingredient.
A medical device or delivery system.

Claim 5 reaches administration with an excipient, but only as a dependent method claim. It does not prevent development of an alternative formulation after patent expiration, and it would not have created a standalone formulation monopoly even during the patent term.

Earlier cilazapril composition-of-matter and chemical-process patents would have been more important to pre-2015 freedom-to-operate analysis. Those patents could have affected manufacture or sale of the molecule regardless of the cardiac-insufficiency method claims. Their terms also would generally have expired by now, subject to individual patent records.

What was the patent’s relationship to FDA exclusivity and the Orange Book?

U.S. Patent No. 5,744,496 did not provide FDA regulatory exclusivity. Patent rights and FDA exclusivity are separate legal mechanisms.

Regulatory issue	Assessment
FDA-approved U.S. reference product for cilazapril	No marketed U.S. reference product is generally identified
Orange Book listing	No commercially consequential Orange Book listing for cilazapril is associated with this patent
New chemical entity exclusivity	Not applicable to a U.S.-marketed cilazapril product
Three-year clinical-investigation exclusivity	No relevant U.S. product exclusivity identified
Pediatric exclusivity	None identified
Patent expiration	April 28, 2015
Paragraph IV exposure	No meaningful current exposure because the patent is expired and cilazapril lacks a U.S. marketed reference product

Orange Book-listed patents generally relate to FDA-approved drug products and are submitted by the NDA holder under FDA procedures. A patent covering an unapproved active ingredient or an unmarketed foreign product does not automatically create an Orange Book-listed barrier. (FDA, 2024a)

Were there Paragraph IV challenges or patent litigation?

A Paragraph IV certification is relevant to an ANDA referencing an FDA-approved listed drug. Because cilazapril was not established as a marketed U.S. reference product, this patent did not generate the ordinary Orange Book-driven Paragraph IV litigation pattern associated with major U.S. cardiovascular drugs.

The patent could theoretically have been asserted in ordinary patent litigation against conduct falling within its claims while it was in force. The most likely litigation theory would have involved a competitor’s use of cilazapril or cilazaprilat to treat cardiac insufficiency. The claims would not support an action based solely on manufacturing, importing or selling cilazapril without the claimed treatment method, unless other claims or patents supplied that coverage.

No current litigation or settlement agreement can preserve enforceable rights after the patent’s expiration. Any historic settlement would be commercially relevant only as evidence of past market behavior, not as a present patent barrier.

How does this patent compare with competing ACE-inhibitor patent estates?

The patent’s estate is narrower than the estate historically associated with leading U.S. ACE inhibitors.

Drug	Principal U.S. patent strategy	Current position
Cilazapril	Chemical patents plus cardiac-insufficiency method claims, including U.S. 5,744,496	No current U.S. patent barrier identified
Enalapril	Composition, prodrug, formulation and method patents	Core patents expired
Lisinopril	Composition and therapeutic-use patents	Core patents expired
Ramipril	Composition, formulation and method patents	Core patents expired
Benazepril	Composition and treatment patents	Core patents expired
Quinapril	Composition and formulation patents	Core patents expired
Perindopril	Composition, salt, formulation and method patents	Core patents expired or no longer commercially blocking in the U.S.
Trandolapril	Composition and therapeutic-use patents	Core patents expired

The key distinction is that U.S. Patent 5,744,496 is not a foundational compound patent. It is a later-use patent directed to cardiac insufficiency. A compound patent would have created broader leverage across indications. The ’496 patent would have been avoidable by:

Using a different ACE inhibitor.
Using cilazapril for an indication outside cardiac insufficiency.
Challenging the cardiac-insufficiency limitation.
Establishing that the compound did not satisfy the claimed structure or stereochemical requirements.
Developing a noninfringing active ingredient or therapeutic class.

Those strategies were relevant before expiration. They have no current U.S. patent-avoidance value because the patent term has ended.

What generic launch scenarios existed, and what risks remain?

Before April 28, 2015, a competitor considering cilazapril for U.S. cardiac-insufficiency treatment would have faced three separate questions:

Whether a U.S. regulatory pathway existed without an approved reference product.
Whether earlier composition or process patents remained active.
Whether the competitor’s proposed label and clinical use would practice claims 1, 3, 6, 7 or 8.

After expiration, the ’496 patent no longer limits launch. The remaining barriers are commercial and regulatory rather than patent-based:

Cilazapril lacks an established U.S. commercial market.
A sponsor would need an appropriate FDA approval pathway.
Clinical development would be required for the proposed indication if no reference-product pathway were available.
Manufacturing, analytical validation and supply-chain costs could exceed the commercial opportunity.
Foreign-country patent status would need separate review.

A foreign patent family member could have had a different expiration date, claim scope or legal status. U.S. expiration does not establish freedom to operate in Europe, Japan, Canada, Australia or other jurisdictions.

How strong was the patent estate?

The ’496 patent had moderate historical strength as a targeted method patent and low present strength because it is expired.

Factor	Assessment
Chemical specificity	Moderate to high for cilazapril and cilazaprilat
Indication specificity	High; limited to cardiac insufficiency
Route coverage	Broad in claim 1, narrowed to oral or parenteral routes in dependent claims
Dose limitation	None stated
Formulation protection	Weak; formulation appears only as a dependent treatment limitation
Manufacturing protection	None in the supplied claims
Design-around potential	Material before expiration
Current enforceability	None after expiration
U.S. commercial impact	Limited by absence of a marketed cilazapril reference product

The strongest claims were claims 3 and 6 because they name cilazapril and cilazaprilat expressly. Claim 1 had broader chemical reach but presented greater construction and infringement issues because the full structural formula, stereochemistry and meaning of “substantially pure” would control its boundaries.

What are the key takeaways?

U.S. Patent 5,744,496 is a cardiac-insufficiency method patent focused on cilazapril and cilazaprilat.
Claims 1 and 2 are genus claims based on the disclosed ACE-inhibitor structure and R2 substituents.
Claims 3 and 6 specifically cover cilazapril and cilazaprilat.
Claims 4, 7 and 8 address oral and parenteral administration.
Claim 5 reaches use with pharmaceutical excipients but is not a standalone formulation claim.
The patent does not claim manufacturing processes, crystalline forms or compositions independent of treatment.
The patent issued April 28, 1998, and its base term expired April 28, 2015.
No current U.S. exclusivity, Orange Book barrier or biosimilar issue is associated with the patent.
Any present U.S. cilazapril launch risk would be regulatory and commercial, not based on enforceability of the ’496 patent.
Foreign patent-family rights require separate country-by-country review.

FAQs about U.S. Patent 5,744,496 and cilazapril

Is U.S. Patent 5,744,496 a composition-of-matter patent?

No. The supplied claims are methods of treating cardiac insufficiency. They do not independently claim cilazapril as a chemical composition.

Does the patent cover cilazapril for hypertension?

Not expressly. The claims require treatment of cardiac insufficiency. Use of cilazapril for hypertension would not satisfy that limitation solely because the same compound was administered.

Does the patent cover cilazaprilat?

Yes. Claim 6 expressly covers cilazaprilat, or a pharmaceutically acceptable salt, for treatment of cardiac insufficiency.

Could a company launch cilazapril in the United States after 2015?

The expired patent would not block launch. The company would still need FDA approval and a commercially viable regulatory pathway.

Is cilazapril a biosimilar product?

No. Cilazapril is a chemically synthesized small-molecule ACE inhibitor. A generic-drug framework, rather than the biosimilar framework, would apply.

Are the claims still relevant outside the United States?

Potentially, but only through corresponding foreign patents. U.S. Patent 5,744,496 has no continuing enforceable effect outside the United States.

References

U.S. Patent and Trademark Office. (1998). U.S. Patent No. 5,744,496: Method for treating cardiac insufficiency with angiotensin-converting enzyme inhibitors. U.S. Department of Commerce.
U.S. Food and Drug Administration. (2024a). Approved drug products with therapeutic equivalence evaluations (Orange Book). U.S. Department of Health and Human Services.
U.S. Food and Drug Administration. (2024b). Drugs@FDA: FDA-approved drugs database. U.S. Department of Health and Human Services.
U.S. Food and Drug Administration. (2024c). Purple Book: Database of licensed biological products. U.S. Department of Health and Human Services.
U.S. Code, 35 U.S.C. §§ 154, 271 and 282. (2024). Patent term, infringement and validity provisions.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	171376	⤷ Start Trial
Austria	98128	⤷ Start Trial
Australia	4104885	⤷ Start Trial
Australia	585502	⤷ Start Trial
Canada	1246457	⤷ Start Trial
Cyprus	2163	⤷ Start Trial
Germany	3413710	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,744,496

Summary for Patent: 5,744,496