US Drug Patent 4,374,829: Claim Scope, Enalapril Coverage, Expiration and Patent Landscape

US Patent 4,374,829 is the foundational compound patent for enalapril and related N-substituted proline angiotensin-converting enzyme inhibitors. Its claims cover broad chemical genera, specific compounds including enalapril and enalaprilat, pharmaceutical compositions, and hypertension-treatment methods. The patent is expired and presents no current US enforcement barrier to generic enalapril or enalaprilat products.

What drug does US Patent 4,374,829 protect?

US 4,374,829 principally protects enalapril, enalaprilat, and structurally related ACE inhibitors.

Product or compound	Relationship to US 4,374,829
Enalapril	Specifically recited in claim 27
Enalapril maleate	Specifically recited in claim 28
Enalaprilat	Specifically recited in claim 26
Enalaprilat-related lysyl analogues	Covered by claims 29 and 31
Enalapril diester hydrochloride	Recited in claim 30
Related proline, thiaproline and methoxyproline compounds	Covered by claims 1-25 and 32-57
Hypertension compositions	Claims 32-57
Hypertension treatment methods	Claims 58-82

The specific enalapril structure is:

N-[1-(ethoxycarbonyl)-3-phenylpropyl]-L-alanyl-L-proline

The patent’s claims also cover the corresponding active diacid, enalaprilat:

N-(1(S)-carboxy-3-phenylpropyl)-L-alanyl-L-proline

The S configuration at the carbon bearing the substituted carboxyl group is expressly recited in several claims. That stereochemical limitation is commercially important because enalapril’s ACE-inhibitory activity depends on the defined stereochemical configuration.

What is the priority, filing and expiration history?

The patent issued on February 22, 1983. Public pharmaceutical patent records and historical Orange Book listings associate the patent with enalapril and report an expiration date in 2000, generally identified as April 24, 2000. The patent therefore has no remaining enforceable US term.

Event	Date or status
US patent	4,374,829
Issue date	February 22, 1983
Technology	N-substituted proline ACE inhibitors
Commercial product	Enalapril maleate, marketed as Vasotec
Historical US patent expiration	April 24, 2000
Current status	Expired
Current generic pathway	ANDA for small-molecule drug products
Biosimilar pathway	Not applicable

The reported April 24, 2000 expiration reflects the pre-1995 US patent-term rules and the relevant effective filing date. The patent was issued before the modern 20-year-from-effective-filing-date regime applied to later-filed patents. FDA Orange Book records historically listed US 4,374,829 for enalapril maleate products, with the patent no longer presenting an exclusivity obstacle after 2000.[1]

How broad are the independent compound claims?

The broadest chemical protection appears in claims 1, 3, 7 and 9. These claims use Markush definitions to cover numerous substituent combinations rather than one commercial molecule.

Claim 1: broadest compound genus

Claim 1 covers compounds having the patent’s N-substituted dipeptide ACE-inhibitor scaffold, with:

Two terminal substituent positions, R and R6, that may be hydroxy, alkoxy, alkenoxy, amino, acylamino, aryloxy or related groups.
A central R1 substituent ranging from simple C1-C20 alkyl to substituted aralkyl, heteroaralkyl and aralkenyl groups.
Amino-acid-derived substituents at other stereocenters.
Proline-like ring systems and other nitrogen-containing cyclic structures.
Pharmaceutically acceptable salts.

The R1 definition is particularly expansive. It includes phenylpropyl-type groups, substituted phenylalkyl groups, aminoalkyl groups, heteroarylalkyl groups and sulfur-containing substituents. This is the provision that captures the structural diversity of the claimed ACE-inhibitor family.

Claim 3: restricted structural genus

Claim 3 narrows the invention by requiring:

R2 and R7 to be hydrogen.
A narrower set of R3 substituents.
A cyclic connection between R4 and R5.
Permitted ring systems including proline-type, thiaproline-type and unsaturated or substituted analogues.

This claim is closer to the commercially relevant ACE-inhibitor structures than claim 1.

Claim 7: commercially focused genus

Claim 7 limits the compound family further by specifying:

R1 as C1-C8 alkyl, substituted C1-C5 alkyl, aralkyl or heteroaralkyl.
R3 as lower alkyl or amino lower alkyl.
A cyclic R4-R5 structure.
Y as CH2, sulfur or CH-OCH3 in the ring.

This framework captures the core structure of enalapril and related candidates while eliminating many of the remote substituent possibilities in claim 1.

Claim 9: narrower proline-focused genus

Claim 9 is the most commercially relevant genus before the individual compound claims. It requires:

R2 and R7 to be hydrogen.
R3 to be methyl or aminoloweralkyl.
R4 and R5 to form proline, 4-thiaproline or 4-methoxyproline.
R and R6 to be hydroxy, lower alkoxy or aralkyloxy.
R1 to include alkyl, substituted alkyl, aralkyl or heteroaralkyl groups.

Enalapril falls within this narrower proline-based genus.

Which claims specifically cover enalapril and enalaprilat?

Claims 26-31 identify individual compounds. These claims have greater product-specific value than the broad Markush claims because they expressly name defined chemical entities.

Claim	Compound or subject matter
26	N-(1(S)-carboxy-3-phenylpropyl)-L-alanyl-L-proline, enalaprilat
27	N-(1(S)-ethoxycarbonyl-3-phenylpropyl)-L-alanyl-L-proline, enalapril
28	Enalapril maleate
29	S-carboxyphenylpropyl lysyl proline analogue
30	Ethyl N-(1(S)-ethoxycarbonyl-3-phenylpropyl)-L-alanyl-L-prolinate hydrochloride
31	The corresponding S-ethoxycarbonyl lysyl-proline compound

Claim 28 is the most direct claim to the commercial salt form. Enalapril maleate consists of enalapril combined with maleic acid. The salt claim does not merely cover the free base; it identifies a particular pharmaceutical salt of the stereochemically defined enalapril molecule.

What formulation patents are protected by US 4,374,829?

US 4,374,829 does not claim a narrowly defined tablet, capsule, coating, release profile or excipient system. Its formulation claims are composition claims requiring:

A pharmaceutically acceptable carrier.
A pharmaceutically effective amount of a covered ACE inhibitor.
Use of the composition for treating hypertension.

Claims 32-57 are composition claims. Claims 35-57 narrow those compositions to named compounds, including enalapril, enalapril maleate and enalaprilat-related compounds.

The formulation scope is therefore functional and molecule-centered. It could have covered conventional oral pharmaceutical preparations containing a claimed compound, but it would not necessarily cover a later-developed formulation that used a different active ingredient or a materially different chemical entity outside the claims.

There is no indication from the claims supplied that US 4,374,829 protects:

A particular tablet compression process.
A specific excipient ratio.
A controlled-release delivery system.
A particular particle-size distribution.
An injectable presentation.
A manufacturing impurity specification.
A specific packaging configuration.

Those matters would require separate patent claims.

What method-of-use patents are included?

Claims 58-82 cover methods of treating hypertension by administering a claimed compound in a pharmaceutically effective amount.

The method claims are broad in therapeutic purpose. They do not require:

A particular patient subgroup.
A specified dose.
A dosing interval.
A combination with a diuretic or calcium-channel blocker.
A specific blood-pressure threshold.
A defined duration of treatment.
A biomarker-selected population.

The core method limitation is treatment of a patient with hypertension using a compound within the chemical scope of the earlier claims.

Claims 60-82 identify specific compounds, including enalapril, enalaprilat-related compounds and enalapril maleate. Because the patent expired, these method claims no longer create a current US infringement risk.

How strong was the patent estate for enalapril?

The patent was commercially strong during its term because it combined several layers of protection in one document:

Protection layer	Claims	Strategic value
Broad chemical genus	1, 3, 7 and 9	Captured many related ACE inhibitors
Stereochemical definition	2, 4, 6, 8 and 10	Protected active configurations
Individual compounds	11-31	Targeted commercially relevant molecules
Pharmaceutical compositions	32-57	Covered drug products containing claimed compounds
Hypertension treatment methods	58-82	Covered therapeutic use

The strongest product-specific coverage was concentrated in claims 26-28. Claims 27 and 28 directly identify the active enalapril molecule and its maleate salt. The broad genus claims supplied fallback protection against close structural variants, while the composition and method claims extended the claim set beyond the isolated compound.

The estate had less value after expiration because generic manufacturers could rely on the expired compound disclosure and commercial formulation knowledge without needing a license from the patent owner.

What was the FDA and Orange Book status?

Enalapril maleate was approved by FDA as an antihypertensive product and marketed by Merck as Vasotec. The product is a small-molecule prescription drug regulated through an NDA, with generic versions approved through the ANDA pathway.

The FDA Orange Book historically listed US 4,374,829 against enalapril maleate products. Orange Book patent listings are relevant to the Hatch-Waxman framework because an ANDA applicant must address listed patents through certification or a statement that the patent does not apply. After expiration, the patent ceased to block approval or commercial launch.[1]

Regulatory issue	Assessment
FDA pathway	NDA for the innovator; ANDA for generics
Reference product	Vasotec, enalapril maleate
Active ingredient	Enalapril maleate
Patent listing	Historical Orange Book listing
Current patent block	None from US 4,374,829
Biologic status	Not a biologic
Biosimilar exposure	None
Generic approval risk	Primarily regulatory, manufacturing and commercial rather than patent-based

The patent does not create biosimilar risk because enalapril is a chemically synthesized small molecule. The relevant competitive pathway is generic substitution, not biosimilar interchangeability.

Which companies challenged or competed against the patent?

Enalapril generic competition developed through the ANDA framework as the patent approached expiration. The principal commercial issue was whether applicants could obtain approval and launch before or after the reported April 2000 expiration date.

A complete company-by-company Paragraph IV chronology cannot be derived from the claim text alone. The patent’s current business significance is clear, however: any Paragraph IV dispute tied solely to US 4,374,829 is historical because the patent expired more than two decades ago.

The relevant competitive set has included:

Merck, as innovator and Vasotec sponsor.
Generic manufacturers marketing enalapril maleate tablets.
Manufacturers of enalaprilat injection.
Contract manufacturers supplying active pharmaceutical ingredient and finished dosage forms.
Regional suppliers competing on price and supply continuity.

Because enalapril is an established generic product, current competition is generally driven by manufacturing economics, FDA compliance, product availability and contracting rather than by the expired compound patent.

What manufacturing and intellectual-property barriers remain?

US 4,374,829 no longer protects the active molecule, but a commercial entrant may still face non-patent barriers:

API manufacturing

Enalapril synthesis requires control of:

Stereochemical purity.
Peptide coupling reactions.
Esterification and hydrolysis conditions.
Residual solvents and reagents.
Related substances and degradation products.
Salt formation and crystallization.
Stability under storage conditions.

The patent disclosure may remain technically useful as prior art and a process reference, but it does not prevent practice of the disclosed chemistry after expiration.

Regulatory requirements

An ANDA applicant must establish pharmaceutical equivalence and bioequivalence for the relevant dosage form. Manufacturing sites, analytical methods, stability data, process validation and quality systems remain material approval requirements.

Formulation and supply

Commercial risks can arise from:

API source qualification.
Enalapril maleate particle properties.
Tablet dissolution performance.
Nitrosamine or other impurity controls where applicable.
Packaging and moisture protection.
FDA inspection findings.
Shortages among concentrated suppliers.

These risks are separate from the expired patent estate.

How does enalapril compare with other ACE-inhibitor patent estates?

Drug	Product type	Primary legal pathway	Current patent position
Enalapril	Small-molecule ACE inhibitor	ANDA	Original compound patent expired
Captopril	Small-molecule ACE inhibitor	ANDA	Original compound patents expired
Lisinopril	Small-molecule ACE inhibitor	ANDA	Original compound patents expired
Ramipril	Small-molecule ACE inhibitor	ANDA	Original compound patents expired
Perindopril	Small-molecule ACE inhibitor	ANDA	Original compound patents expired
Benazepril	Small-molecule ACE inhibitor	ANDA	Original compound patents expired

Enalapril’s original estate was unusually broad because US 4,374,829 claimed the chemical genus, defined stereochemistry, individual molecules, salts, compositions and therapeutic methods. That breadth supported exclusivity during the 1980s and 1990s. It does not distinguish enalapril from other mature ACE inhibitors in the current US market because the core patent rights have expired.

What is the current generic launch risk?

The generic launch risk attributable to US 4,374,829 is effectively zero because the patent expired in 2000.

Risk category	Current assessment
Infringement of US 4,374,829	None based on current enforceability
Paragraph IV litigation under this patent	Historical only
Enalapril compound blocking patent	None from this patent
Enalapril maleate salt blocking patent	None from this patent
Biosimilar substitution risk	Not applicable
ANDA approval risk	Product-specific and regulatory
Manufacturing risk	Moderate, depending on supplier and process
Commercial pricing risk	High in a mature generic market
Geographic patent risk	Must be assessed separately outside the US

A freedom-to-operate analysis for a modern product should still review later US patents, foreign patents, process patents, formulation patents and regulatory exclusivities. Those rights are separate from the expired compound patent and cannot be inferred from US 4,374,829 alone.

Key Takeaways

US 4,374,829 is the foundational US patent covering enalapril and related ACE inhibitors.
Claim 27 specifically covers enalapril in its active S configuration.
Claim 28 specifically covers enalapril maleate.
Claim 26 covers enalaprilat, the active diacid.
Claims 1, 3, 7 and 9 create broad chemical genera around the proline-based ACE-inhibitor scaffold.
Claims 32-57 cover pharmaceutical compositions.
Claims 58-82 cover hypertension-treatment methods.
The patent issued on February 22, 1983 and historically expired on April 24, 2000.
FDA Orange Book relevance is historical; the patent no longer blocks generic approval or launch.
Enalapril is a small-molecule drug and has no biosimilar pathway.
Current commercial barriers are manufacturing, regulatory, quality and pricing issues rather than rights under US 4,374,829.

FAQs

Does US 4,374,829 cover enalapril maleate?

Yes. Claim 28 expressly identifies the maleate salt of N-(1(S)-ethoxycarbonyl-3-phenylpropyl)-L-alanyl-L-proline, the chemical structure of enalapril maleate.

Does US 4,374,829 cover enalaprilat?

Yes. Claim 26 specifically recites N-(1(S)-carboxy-3-phenylpropyl)-L-alanyl-L-proline, corresponding to enalaprilat.

Can a company manufacture generic enalapril in the United States?

US 4,374,829 does not prevent manufacture or sale because it expired in 2000. A manufacturer must still satisfy FDA ANDA, cGMP, quality, labeling and supply requirements.

Was enalapril protected by a biosimilar exclusivity period?

No. Enalapril is a chemically synthesized small molecule. Generic competition proceeds through the ANDA pathway, not the biosimilar pathway under the Public Health Service Act.

Do the method claims still block hypertension treatment with enalapril?

No. The method claims in US 4,374,829 expired with the patent and are no longer enforceable in the United States.

References

U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations (Orange Book). FDA.
U.S. Patent and Trademark Office. (1983). U.S. Patent No. 4,374,829: N-substituted proline derivatives. U.S. Department of Commerce.
U.S. Food and Drug Administration. (n.d.). Vasotec label: Enalapril maleate. FDA.
U.S. Patent and Trademark Office. (2024). Patent Center and patent term information resources. U.S. Department of Commerce.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	231971	⤷ Start Trial
Austria	6503	⤷ Start Trial
Australia	530380	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,374,829

Summary for Patent: 4,374,829