United States Patent 3,839,573: Scope, Claims, Expiration, and Antifungal Patent Landscape

U.S. Patent No. 3,839,573 covers an antifungal pharmaceutical composition containing a specific tritylimidazole compound: 1-[(3-methylphenyl)diphenylmethyl]-1H-imidazole, or a pharmaceutically acceptable nontoxic salt, combined with an inert pharmaceutical carrier. The patent does not claim the compound in isolation, a treatment method, a manufacturing process, a formulation technology, or a broad class of imidazole antifungals.

The patent is expired. Assuming the patent issued on October 1, 1974, its ordinary U.S. 17-year term ended on October 1, 1991, subject to any prosecution-history adjustment, terminal disclaimer, or other term-specific event. No current U.S. composition-of-matter, Orange Book, Paragraph IV, or biosimilar exclusivity appears to arise from this patent. [1][2]

What compound does U.S. Patent 3,839,573 cover?

The claim identifies the active compound through the structure:

1-((X"-phenyl)-C(-C6H5)2-)imidazole, wherein X" is m-methyl.

In conventional chemical nomenclature, the claimed compound is:

1-[(3-methylphenyl)diphenylmethyl]-1H-imidazole

The structure contains:

Structural element	Claim requirement
Imidazole ring	Required
Attachment to imidazole	The imidazole nitrogen is attached to the substituted diphenylmethyl group
Central carbon	Bonded to the imidazole, one 3-methylphenyl group and two phenyl groups
Substitution	The methyl group must be in the meta, or 3-position, of the phenyl ring
Salt form	A pharmaceutically acceptable nontoxic salt is permitted
Pharmaceutical use	Antifungal administration to humans or animals
Formulation	Must include an inert, pharmaceutically acceptable diluent or carrier

The claim is narrower than a generic “tritylimidazole” claim. It does not cover every substituted phenyl-diphenylmethyl imidazole. The positional limitation to the meta-methyl is material.

What does claim 1 of U.S. Patent 3,839,573 require?

Claim 1 is a composition claim with four principal limitations.

1. An antifungal composition

The accused product must be a composition intended for antifungal use. A pure chemical sample or research intermediate would not, by itself, satisfy the full literal wording of the claim.

2. The specified active compound

The composition must contain the compound having the claimed 3-methylphenyl-diphenylmethyl-imidazole structure, or a pharmaceutically acceptable nontoxic salt.

The claim does not expressly cover:

Ortho-methyl or para-methyl positional isomers;
Unsubstituted tritylimidazole unless it independently falls within the stated structure;
Chloro-substituted analogs;
Methoxy-substituted analogs;
Polyhalogenated imidazole antifungals;
Structurally different imidazole or triazole antifungals.

3. A therapeutically effective amount

The compound must be present in an amount sufficient to produce antifungal activity against fungi pathogenic to humans or animals.

This limitation is functional rather than numerical. Claim 1 does not specify:

A minimum concentration;
A maximum concentration;
A dosage strength;
A dosing interval;
A treatment duration;
A particular fungal species;
A particular route of administration.

The absence of numerical dosage limits gives the claim breadth across concentrations that meet the therapeutic-effectiveness requirement, but it also creates a potential evidentiary issue if a formulation contains only a trace or subtherapeutic amount.

4. A pharmaceutically acceptable inert carrier or diluent

The active compound must be combined with an inert pharmaceutical carrier or diluent. The claim therefore reaches finished or intermediate pharmaceutical compositions rather than the active molecule alone.

Potential carriers could include conventional solid or liquid excipients, depending on the specification and the product’s actual composition. The claim does not expressly limit the dosage form to creams, tablets, capsules, powders, solutions, suspensions, or topical preparations.

What does the patent not claim?

The claim language does not independently claim the following subject matter:

Subject matter	Covered by claim 1?
The active compound as a standalone chemical	No
A method of treating a fungal infection	No express method claim
A specific dosage regimen	No
A particular topical, oral, or veterinary route	No
A manufacturing process	No
A crystalline form or polymorph	No express limitation
A specific particle size	No
A controlled-release system	No
A specific cream, gel, tablet, or capsule formula	No express limitation
A broad Markush genus of substituted tritylimidazoles	No
Other antifungal active ingredients	No, unless the claim is interpreted to permit combination products containing the claimed compound

The practical scope is therefore centered on pharmaceutical compositions containing the specified compound. The claim may read on a combination product that contains additional active ingredients, provided the claimed compound and carrier limitations are also satisfied. The claim does not expressly exclude combination therapy.

When did U.S. Patent 3,839,573 expire?

U.S. patent term

U.S. patents issued from applications filed before June 8, 1995, generally received a term of 17 years from grant under the pre-Uruguay Round patent-term regime. [2]

Event	Date or status
Patent	U.S. Patent No. 3,839,573
Subject	Antifungal compositions
Issue date	October 1, 1974
Ordinary patent term	17 years from issue
Projected ordinary expiration	October 1, 1991
Current status	Expired
Current enforceability	None, absent an unusual restoration or surviving related right

Patent-term adjustment under the modern statutory framework generally does not change the conclusion for a patent issued in 1974. Patent-term extension under 35 U.S.C. § 156 was enacted after this patent issued and would not ordinarily create a current right for a patent of this age. [2][3]

The patent’s expiration eliminates infringement risk based on claim 1 itself. It does not eliminate risk from later patents covering a particular product, formulation, manufacturing process, salt, polymorph, delivery system, or new therapeutic use.

What is the Orange Book status of U.S. Patent 3,839,573?

U.S. Patent 3,839,573 should not be treated as a current Orange Book barrier.

The FDA Orange Book lists patents submitted by sponsors for approved drug products and identifies patent certifications relevant to abbreviated new drug applications. [4] A patent issued in 1974 for a compound that is not identified as a current reference-listed drug product would not ordinarily create a live Orange Book listing or a current Paragraph IV litigation pathway.

Orange Book issue	Assessment
Current listed patent	No apparent current listing
Unexpired patent protection	No
Active ingredient exclusivity	No
Pediatric exclusivity	No apparent relevance
New chemical entity exclusivity	Expired or unavailable
Paragraph IV exposure	None based on this expired patent
ANDA blocking effect	None based on this patent

The absence of a current Orange Book listing does not establish that no later patent exists for a related product. It means that this 1974 patent is not a current statutory barrier to an ANDA.

Does the patent create Paragraph IV risk?

No current Paragraph IV risk arises from U.S. Patent 3,839,573 because the patent expired decades ago.

A Paragraph IV certification applies when an ANDA applicant asserts that a listed patent is invalid, unenforceable, or will not be infringed. The certification mechanism is relevant only to an unexpired patent listed for an FDA-approved reference product. [5]

A generic applicant seeking approval for a product containing the claimed compound would instead face ordinary regulatory, quality, safety, and bioequivalence requirements. The expired patent could remain relevant as prior art in validity analysis of later patents, but it cannot independently support an infringement suit today.

What patent landscape surrounds this compound and related antifungals?

The patent sits within the earlier generation of synthetic imidazole antifungal chemistry. Its structure is related at a high level to tritylimidazole and substituted imidazole antifungals, but structural similarity does not establish claim coverage.

Related antifungal categories

Category	Representative examples	Relationship to U.S. 3,839,573
Tritylimidazoles	Clotrimazole and related compounds	Similar central trityl-imidazole architecture, but substitution patterns differ
Aryl ether imidazoles	Miconazole, econazole, isoconazole	Structurally distinct; generally contain ether-linked and halogenated side chains
Triazoles	Fluconazole, itraconazole, voriconazole	Different ring systems and broader later-generation chemistry
Benzimidazoles	Bifonazole and related compounds	Different heterocyclic core or substitution pattern
Allylamines	Terbinafine, naftifine	Different mechanism-related chemical class
Polyenes	Nystatin, amphotericin B	Different chemical class and biological profile

Clotrimazole is the closest commercial comparison at the scaffold level because it is also a substituted tritylimidazole. Clotrimazole, however, contains a 2-chlorophenyl substituent rather than the 3-methylphenyl substituent recited in claim 1. A clotrimazole product would not literally satisfy the specific active-ingredient limitation of this claim.

How strong is the patent estate for the claimed compound?

The current patent estate is commercially weak because the foundational composition claim is expired. Historical scope was meaningful, but the present exclusionary value is zero.

Historical strengths

The claim focused on a defined antifungal pharmaceutical composition.
The active structure was identified with chemical precision.
The therapeutic amount limitation could reach multiple dosage strengths.
The carrier limitation could encompass conventional pharmaceutical dosage forms.
The claim was not restricted to one route of administration or one fungal pathogen.

Historical weaknesses

The claim did not cover the compound in isolation.
The claim did not cover manufacturing methods.
It did not specify a dosage form or formulation technology.
It did not expressly claim treatment methods.
The active ingredient was limited to one positional isomer.
The claim could face written-description, enablement, or therapeutic-effectiveness disputes depending on the specification and evidence.

Current strength

Factor	Current assessment
Term remaining	None
Enforceability	None
Orange Book leverage	None apparent
Licensing leverage	Historical only
Generic blocking power	None
Prior-art value	Potentially relevant against later patents
Freedom-to-operate relevance	Limited to historical analysis and claim construction

What formulation patents could still matter?

Although claim 1 is expired, later patents could have protected a product-specific formulation. Such patents might concern:

A cream or ointment vehicle;
Enhanced skin penetration;
A particular solvent system;
A stable suspension or emulsion;
Particle-size control;
A salt or solid-state form;
Controlled release;
A transdermal or mucosal delivery system;
A combination with another antifungal or anti-inflammatory agent;
A preservative or stabilizer system;
A specific manufacturing process.

None of those protections can be inferred from claim 1 alone. Claim 1 is broad as to the carrier in functional terms but does not itself create a separate formulation patent estate.

For a current product-specific freedom-to-operate review, later U.S. patents and published applications assigned to the relevant manufacturer would need to be evaluated separately. The expired 1974 claim cannot substitute for that analysis.

Are biosimilar risks relevant?

No. Biosimilar law applies to biological products, not conventional small-molecule imidazole antifungals. A product containing the claimed compound would proceed, if eligible, through an abbreviated small-molecule pathway such as an ANDA or another applicable FDA pathway, not through the biosimilar framework under the Biologics Price Competition and Innovation Act.

The principal regulatory issues would be active-ingredient identity, pharmaceutical equivalence, bioequivalence where applicable, manufacturing quality, labeling, and product-specific FDA requirements. [4][6]

Which companies could challenge or commercialize the product?

No current company can be identified as a Paragraph IV challenger to this expired patent because the patent no longer blocks approval.

The historical commercial landscape likely involved companies developing substituted imidazole antifungals, including manufacturers associated with clotrimazole, miconazole, econazole, and related products. Those products should be assessed by exact chemical structure, approved indication, dosage form, and surviving product-specific patents. A company’s ownership of a related antifungal patent does not establish ownership or infringement of claim 1.

No licensing or settlement conclusion can be drawn from the claim text alone. Patent assignments, recorded licenses, litigation dockets, and prosecution files would be required to establish transaction history.

What generic launch scenarios exist?

Because U.S. Patent 3,839,573 is expired, a generic launch would not need to wait for its expiration.

Launch scenario	Patent impact from U.S. 3,839,573
Oral small-molecule product	No blocking effect from this patent
Topical product	No blocking effect from this patent
Veterinary product	No blocking effect from this patent
Salt-based product	No blocking effect from this patent
Combination product	No blocking effect from this patent
Reformulated product	No blocking effect from this patent, but later formulation patents may matter
New indication	No blocking effect from this patent, but later method patents may matter

The commercial viability of a launch would depend on market demand, FDA requirements, raw-material availability, manufacturing economics, clinical or bioequivalence requirements, and any later patents unrelated to the 1974 claim.

What geographic coverage does the patent provide?

The patent provides U.S. rights only. It does not establish protection in Europe, Canada, Japan, or other jurisdictions.

International coverage would require separate national or regional applications, such as European, Canadian, Japanese, or other counterpart patents. The U.S. patent number alone cannot establish foreign filing, grant, expiration, abandonment, or enforceability status.

For present commercial planning, the U.S. patent has no remaining geographic exclusionary value. Foreign rights, if any existed, would also be expected to have expired given the patent’s 1974 U.S. issue date, but each jurisdiction’s records and term rules control.

What manufacturing and intellectual-property barriers remain?

The expired patent does not create a manufacturing barrier. A manufacturer could, in principle, produce the claimed compound without infringing this patent.

Operational barriers may still include:

Synthetic route development;
Control of regioisomeric purity;
Removal of residual solvents and catalysts;
Salt selection;
Stability and degradation testing;
Analytical reference standards;
Scale-up reproducibility;
Pharmaceutical-grade excipient compatibility;
Toxicology and regulatory documentation;
Supplier qualification.

Those are technical or regulatory barriers, not surviving patent rights under U.S. Patent 3,839,573.

Key Takeaways

U.S. Patent 3,839,573 claims a pharmaceutical composition containing 1-[(3-methylphenyl)diphenylmethyl]-1H-imidazole or a pharmaceutically acceptable nontoxic salt.
The claim requires an antifungal composition, a therapeutically effective amount, and an inert pharmaceutical carrier or diluent.
It does not claim the active compound alone, a treatment method, a manufacturing process, or a particular dosage form.
The patent’s ordinary U.S. term expired in approximately 1991.
It creates no current Orange Book, Paragraph IV, generic-launch, or biosimilar barrier.
Related antifungals such as clotrimazole and miconazole should be analyzed by exact structure and separate patent families.
Later formulation, salt, polymorph, manufacturing, or method-of-use patents could create separate risks, but they are not established by this patent’s claim.

FAQs About U.S. Patent 3,839,573

Does U.S. Patent 3,839,573 cover clotrimazole?

No. Clotrimazole has a different aryl substituent. The claim specifies a 3-methylphenyl group, while clotrimazole is a chlorinated tritylimidazole.

Can a company manufacture the claimed compound in the United States?

The expired patent does not prevent manufacture. Separate later patents, regulatory obligations, or non-patent restrictions could still affect commercialization.

Does the patent cover both human and veterinary antifungal products?

Yes, the claim expressly describes administration to humans and animals. That scope has no current exclusionary effect because the patent is expired.

Is a pharmaceutically acceptable salt separately protected by the patent?

The claim includes a pharmaceutically acceptable nontoxic salt of the specified compound. The salt language is part of the expired composition claim; it is not a separate unexpired patent right.

Could the patent still be cited against a later patent application?

Yes. An expired patent can remain prior art and may be relevant to novelty, obviousness, written description, or enablement issues in later patent prosecution. Its expiration does not remove its technical disclosure from the prior-art record.

References

United States Patent and Trademark Office. (1974). U.S. Patent No. 3,839,573, Antifungal compositions.
35 U.S.C. § 154. Patent term and conditions.
35 U.S.C. § 156. Extension of patent term.
U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations: Orange Book.
21 U.S.C. § 355(j). Abbreviated applications and patent certifications.
U.S. Food and Drug Administration. (n.d.). Abbreviated new drug application process.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	278001	⤷ Start Trial
Austria	285600	⤷ Start Trial
Belgium	720801	⤷ Start Trial
Belgium	721378	⤷ Start Trial
Brazil	6802293	⤷ Start Trial
Canada	938611	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 3,839,573

Summary for Patent: 3,839,573