United States Drug Patent 4,983,598: Claim Scope, Expiration, and Patent Landscape for Diltiazem-Enalapril

U.S. Patent No. 4,983,598 covers a pharmaceutical combination of diltiazem and enalapril, and a treatment method using that combination for hypertension, cardiac insufficiency, or coronary insufficiency. The claims require specified dose ranges and a greater-than-additive antihypertensive effect. The patent issued on January 8, 1991, under the pre-1995 U.S. patent-term regime and expired on January 8, 2008. It no longer creates a U.S. patent barrier to generic diltiazem, enalapril, or a diltiazem-enalapril product.

What does U.S. Patent 4,983,598 protect?

The patent protects two related inventions:

A pharmaceutical composition containing diltiazem and enalapril.
A method of treating specified cardiovascular conditions by administering that combination.

The central limitation is pharmacodynamic synergy. The claimed combination must produce an antihypertensive effect greater than the sum of the effects produced by diltiazem and enalapril when administered separately.

Claim	Claim type	Required actives	Dose range	Covered conditions	Functional requirement
1	Pharmaceutical composition	Diltiazem and enalapril	Diltiazem: 10-360 mg; enalapril: 1-100 mg	Hypertension, cardiac insufficiency, coronary insufficiency	Combined antihypertensive effect must exceed the sum of individual effects
2	Treatment method	Diltiazem and enalapril	Diltiazem: 10-360 mg; enalapril: 1-100 mg	Hypertension, cardiac insufficiency, coronary insufficiency	Same greater-than-additive effect requirement

The patent is therefore narrower than a general claim to the coadministration of a calcium-channel blocker and an ACE inhibitor. It does not, on its face, claim every diltiazem-ACE inhibitor combination.

How should the diltiazem and enalapril limitations be construed?

The claim language identifies enalapril as the ACE inhibitor. The phrase “an angiotensin-converting enzyme inhibitor, enalapril” is best read as limiting the claimed ACE inhibitor to enalapril rather than extending the claim to all ACE inhibitors.

That distinction matters. A combination containing diltiazem and lisinopril, captopril, ramipril, benazepril, or another ACE inhibitor would not literally satisfy the enalapril limitation. A patent infringement analysis could still consider equivalents, but the expired status of the patent removes any present U.S. enforcement risk under this patent.

The claims also do not expressly require:

A particular diltiazem salt or polymorph.
A particular enalapril salt, although commercial enalapril products commonly use enalapril maleate.
Immediate-release or extended-release delivery.
A fixed-dose tablet.
A capsule, solution, injection, or other dosage form.
A particular administration route.
A fixed dosing frequency.
A specific diltiazem-to-enalapril ratio.
A specific patient subgroup.
A particular blood-pressure reduction threshold.

Those omissions broaden the structural scope of the claims, but the functional synergy limitation narrows infringement and validity questions.

What is the significance of the greater-than-additive effect limitation?

The synergy requirement is the most important technical limitation in both claims. The claimed effect must be greater than the sum of the effects observed from separate administration of the two drugs.

A product containing diltiazem and enalapril would not automatically fall within the claim merely because it contains the two active ingredients in the stated amounts. The claim requires proof that the combination produces the specified pharmacological relationship.

Claim-construction issues

The limitation raises several legal and technical questions:

What endpoint measures the antihypertensive effect?
Must the effect be demonstrated in humans, animals, or both?
What baseline and statistical method determine the “sum” of individual effects?
Must the synergy occur at every dose within the claimed ranges?
Is the claim satisfied if synergy occurs only in a subset of patients?
Does the claim require simultaneous administration or permit sequential dosing?
Must the combination be formulated in one dosage unit?

The patent claims do not state a numerical synergy threshold. That creates potential disputes over claim construction, proof of infringement, enablement, and written description. The specification and prosecution history would control how broadly the limitation could historically have been applied.

For present commercial purposes, the issue is largely academic because the patent expired in 2008.

When did U.S. Patent 4,983,598 expire?

U.S. Patent 4,983,598 issued on January 8, 1991. Because it was governed by the pre-June 8, 1995 patent-term framework, its ordinary term was 17 years from grant. The resulting expiration date was January 8, 2008, absent a valid patent-term extension or other unusual adjustment. The patent document does not provide a basis for treating it as an unexpired U.S. right today. (U.S. Patent No. 4,983,598, 1991).

Event	Date
Patent issued	January 8, 1991
Ordinary term	17 years from issue
Expiration	January 8, 2008
Current enforceability	Expired
Current Paragraph IV relevance	None as to this patent

A patent-term extension under 35 U.S.C. § 156 would require regulatory approval of a qualifying product and satisfaction of statutory requirements. The claimed fixed combination does not appear to have generated a recognized U.S. regulatory exclusivity period capable of extending this patent.

What is the current Orange Book status of U.S. Patent 4,983,598?

U.S. Patent 4,983,598 should not be treated as a current Orange Book barrier. The Orange Book lists patents associated with approved drug products, including patents covering drug substances, formulations, and approved methods of use. Expired patents do not provide current protection against ANDA approval or commercial launch.

Diltiazem and enalapril have historically been approved as separate small-molecule products. A diltiazem-enalapril fixed-dose combination is not widely established as an FDA-approved reference product in the same way as the individual ingredients. The absence of an approved fixed-dose reference product would create a regulatory issue for a sponsor seeking approval of a new combination product, but it would not preserve U.S. patent protection under Patent 4,983,598.

The relevant FDA pathways are therefore different:

Product concept	Likely regulatory issue
Generic diltiazem	ANDA pathway based on an approved reference product and dosage form
Generic enalapril	ANDA pathway based on an approved reference product and dosage form
Co-packaged diltiazem plus enalapril	Separate product approvals or an appropriate combination strategy
New fixed-dose diltiazem-enalapril tablet	Potential 505(b)(2) or other applicable pathway, depending on formulation and reference products
New clinical use based on synergy	Clinical and labeling support would be required; expired Patent 4,983,598 does not supply exclusivity

The Orange Book status of any later diltiazem or enalapril formulation must be assessed at the product level. The expiration of Patent 4,983,598 does not eliminate later formulation, polymorph, manufacturing, or method-of-use patents.

How does this patent compare with the core diltiazem and enalapril patent estates?

Patent 4,983,598 is a combination-use patent rather than a basic compound patent.

Diltiazem

Diltiazem is an established benzothiazepine calcium-channel blocker. Its original compound and early therapeutic patents expired decades ago. Later protection focused on:

Extended-release formulations.
Controlled-release beads or pellets.
Once-daily dosage forms.
Specific dissolution profiles.
Pharmaceutical salts and solid-state forms.
Manufacturing processes.

A generic diltiazem product may therefore have faced formulation or dosage-form patents even after the underlying active ingredient became unpatented. Those later patents are product-specific and cannot be inferred from Patent 4,983,598.

Enalapril

Enalapril is an established ACE inhibitor developed by Merck. Its foundational U.S. patent estate also expired long before the present date. Later patent activity generally concerned:

Enalapril maleate formulations.
Stability and manufacturing processes.
Combination products.
Dosage forms and excipient systems.

A product containing enalapril does not infringe Patent 4,983,598 unless it also contains diltiazem and satisfies the claim’s dose and functional requirements.

Comparative patent position

Patent category	Relevance to current U.S. entry
Original diltiazem compound patents	Expired
Original enalapril compound patents	Expired
U.S. Patent 4,983,598 combination claims	Expired January 8, 2008
Later diltiazem extended-release patents	May require product-specific review
Later enalapril formulation patents	May require product-specific review
New combination formulation patents	Potentially relevant only if separately issued and unexpired
Manufacturing patents	May remain relevant independently of drug-substance expiry

What formulations are protected by U.S. Patent 4,983,598?

The claims are not limited to a specific formulation architecture. Claim 1 is directed to a composition containing the two active ingredients in the stated ranges. It could potentially read on:

A single fixed-dose tablet.
A capsule containing both actives.
A liquid formulation.
A powder or granule composition.
A composition in which the two drugs are physically separated within one dosage form.

The claim does not clearly require that the drugs be released at the same rate or at the same time. A formulation could potentially satisfy the structural portion of the claim even if it uses different release mechanisms for the two actives, provided the synergy limitation is met.

Claim 2 is broader in one practical respect because it covers administering the claimed combination to a patient. It does not necessarily require a single dosage unit. Separate administration of diltiazem and enalapril could have been relevant if the resulting treatment met the claim language, although the phrase “pharmaceutical composition containing a combination” would create a dispute where the drugs are supplied in separate products.

Did U.S. Patent 4,983,598 create Paragraph IV litigation risk?

During its term, an ANDA applicant seeking approval for a covered diltiazem-enalapril combination could have faced a Paragraph IV issue if the patent had been listed for the relevant reference product. The applicant would have needed to certify that the patent was invalid, unenforceable, or would not be infringed, depending on the Orange Book listing and product structure. The patent’s age and expiration eliminate that issue now.

There is no current Paragraph IV risk under Patent 4,983,598 because:

The patent expired in 2008.
An expired patent cannot support a current statutory stay of ANDA approval.
A generic applicant does not need to make a Paragraph IV challenge to an expired patent.
The patent cannot block commercial launch of a product that otherwise satisfies FDA requirements.

The historical validity profile would have depended on prior art concerning calcium-channel blockers, ACE inhibitors, combination antihypertensive therapy, and evidence of unexpected synergy. Without the prosecution file and experimental data, the relative strength of the original claims against an invalidity challenge cannot be quantified.

What generic launch scenarios exist for a diltiazem-enalapril product?

The main launch scenarios are regulatory rather than patent-driven.

Separate generic products

A sponsor can market generic diltiazem and generic enalapril separately after obtaining the required approvals. Physician-directed coadministration would not be blocked by Patent 4,983,598.

Co-packaged products

A sponsor could consider a co-packaged presentation containing separately approved diltiazem and enalapril products. The regulatory and labeling requirements would differ from those for a single fixed-dose combination.

Fixed-dose combination

A new fixed-dose tablet or capsule could require a regulatory pathway addressing:

Dose selection.
Bioavailability and bioequivalence.
Drug-drug interaction.
Stability of the combined formulation.
Manufacturing controls.
Justification for the combination.
Labeling of the cardiovascular indications.

Patent 4,983,598 would not prevent such a launch. Unexpired later patents covering the dosage form, excipients, release profile, or manufacturing process would be the relevant IP risks.

New synergistic indication

A sponsor could seek protection for a newly defined patient population, dosing schedule, biomarker, or clinical outcome. Any such protection would need to arise from a later patent and would be evaluated separately from the expired combination patent.

Which companies are challenging U.S. Patent 4,983,598?

No current challenger is required because the patent is expired. The patent cannot support a live ANDA litigation strategy or a present market-exclusion claim.

The commercial competition instead comes from manufacturers of generic diltiazem and enalapril products, including companies that hold FDA approvals for the individual active ingredients or their dosage forms. A fixed-dose combination entrant would compete against separate generic products and established branded or generic extended-release diltiazem products, rather than against an enforceable right under Patent 4,983,598.

Did the patent generate licensing deals or settlement agreements?

The patent record alone does not establish a licensing transaction, royalty agreement, or Hatch-Waxman settlement. Because the patent expired in 2008, any historical license would have no current exclusionary effect unless tied to separate, unexpired rights.

No commercial diligence conclusion should attribute current market protection to a license involving Patent 4,983,598. Current rights would have to arise from later patents, regulatory exclusivity, trademarks, know-how, manufacturing controls, or commercial contracting.

How strong is the patent estate for diltiazem-enalapril today?

The estate associated specifically with Patent 4,983,598 has no remaining U.S. patent strength because the patent is expired.

Dimension	Assessment
Composition claim	Historically meaningful but narrow to diltiazem plus enalapril and specified ranges
Method claim	Historically relevant to the listed cardiovascular conditions
Synergy limitation	Technically important but potentially difficult to prove
Basic active-ingredient coverage	None remaining
Formulation coverage	Not specific enough to replace later dosage-form patents
Current blocking power	None
Biosimilar relevance	None; both products are small molecules
Generic litigation risk under this patent	None
Geographic scope	United States only

What geographic coverage did the patent provide?

U.S. Patent 4,983,598 provided rights only in the United States. A U.S. patent does not create protection in Europe, Japan, Canada, China, or other jurisdictions. Foreign protection would require separate national or regional patents, each with its own filing date, scope, term, and expiration.

The existence of a U.S. patent family member would not extend the U.S. patent term or revive the expired U.S. claims. Any foreign patent analysis must be conducted independently.

Key Takeaways

U.S. Patent 4,983,598 claims compositions and treatment methods combining diltiazem with enalapril.
The claimed doses are 10-360 mg of diltiazem and 1-100 mg of enalapril.
Both claims require a greater-than-additive antihypertensive effect.
Enalapril is the relevant ACE inhibitor identified by the claim language; the claims do not plainly cover every ACE inhibitor.
The patent issued on January 8, 1991, and expired on January 8, 2008.
It has no current Orange Book blocking effect and cannot support a current Paragraph IV stay.
The patent is not a biosimilar matter because diltiazem and enalapril are small molecules.
Current launch risk would arise from later formulation, manufacturing, dosage-form, or method-of-use patents, not from Patent 4,983,598.
The patent provided U.S. rights only and did not itself establish foreign protection.
No current licensing or settlement right can be inferred from the expired patent.

FAQs About U.S. Patent 4,983,598

Can a company market diltiazem and enalapril together in the United States?

Yes. Patent 4,983,598 expired on January 8, 2008. A company must still satisfy FDA approval, labeling, safety, efficacy, manufacturing, and any other unexpired patent requirements.

Does the patent cover diltiazem combined with lisinopril?

Not literally based on the supplied claim language. The claims identify enalapril as the ACE inhibitor. A separate infringement analysis would be required for any doctrine-of-equivalents theory, but the patent’s expiration eliminates current U.S. enforcement risk.

Does taking separate diltiazem and enalapril tablets infringe claim 2?

Historically, that issue would have depended on whether the separate products were treated as administration of the claimed pharmaceutical composition and whether the greater-than-additive effect limitation was satisfied. Today, the expired patent cannot be infringed in an actionable U.S. sense.

Is a diltiazem-enalapril combination eligible for an ANDA?

The answer depends on whether an appropriate FDA reference product and dosage form exist. A fixed-dose combination may require a different regulatory route, potentially including a 505(b)(2) application. The expired patent does not determine eligibility.

Are biosimilar applications relevant to diltiazem or enalapril?

No. Diltiazem and enalapril are chemically synthesized small-molecule drugs. Applications would generally proceed through generic-drug pathways, not the biosimilar pathway under the Public Health Service Act.

References

United States Patent and Trademark Office. (1991). U.S. Patent No. 4,983,598: Pharmaceutical composition and method comprising diltiazem and enalapril. Washington, DC: U.S. Department of Commerce.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. Silver Spring, MD: U.S. Department of Health and Human Services.
U.S. Food and Drug Administration. (2024). Orange Book: Approved drug products with therapeutic equivalence evaluations. Silver Spring, MD: U.S. Department of Health and Human Services.
United States Code. (2023). 35 U.S.C. §§ 154, 156, and 271. Washington, DC: U.S. Government Publishing Office.
United States Code. (2023). 21 U.S.C. § 355. Washington, DC: U.S. Government Publishing Office.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
France	86 16138	Nov 20, 1986

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	54253	⤷ Start Trial
Australia	596712	⤷ Start Trial
Australia	8140787	⤷ Start Trial
Canada	1302268	⤷ Start Trial
Germany	3763582	⤷ Start Trial
Denmark	607387	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,983,598

Summary for Patent: 4,983,598