United States Patent 4,021,481: Claim Scope, Expiration, and Non-Ionic X-Ray Contrast Patent Landscape

United States Patent 4,021,481 covers a broad class of non-ionic, water-soluble triiodinated X-ray contrast compounds, including compounds with acetamide, substituted carbamoyl, glucamine, hydroxyethyl, dihydroxypropyl, and related hydroxyalkyl substituents. The patent was granted on May 3, 1977, and its original 17-year term expired in 1994. It has no current blocking effect on generic manufacture, formulation, clinical use, or commercialization in the United States.

The principal commercial value of the patent was historical. It protected a chemical platform used in the development of low-osmolar non-ionic contrast media. Current freedom-to-operate risk depends on later patents covering specific active ingredients, formulations, manufacturing processes, injectors, packaging, and clinical uses, not on US 4,021,481 itself.

What does United States Patent 4,021,481 protect?

US 4,021,481 protects non-ionic X-ray contrast compounds built around a highly iodinated aromatic nucleus. The claims require:

A triiodinated benzene or benzoyl-type core;
Nitrogen-containing substituents attached to the aromatic ring;
Amide, substituted amide, aminomethylamide, or related groups;
At least one N-hydroxyalkyl substituent;
At least two hydroxyl groups in the molecule;
Alkyl, hydroxyalkyl, alkanoyloxyalkyl, and alkanoyl groups containing up to six carbon atoms.

The patent is directed to molecular compounds rather than a finished pharmaceutical composition, imaging method, dosage regimen, device, or manufacturing process.

Core structural concept

The claimed molecules combine three characteristics:

High iodine content for X-ray attenuation.
Non-ionic amide or amino-alcohol substituents intended to improve aqueous solubility and reduce ionic character.
Hydroxyl-bearing side chains intended to increase hydrophilicity.

The claim language describes the R1 and R2 substituents as hydrogen, alkyl, hydroxyalkyl, or alkanoyloxyalkyl. R3 and R4 may contain acylated amino groups, aminomethyl derivatives, or related substituted structures. The independent claim also imposes the aggregate requirement of at least one N-hydroxyalkyl group and at least two hydroxyl groups.

This combination makes claim 1 a Markush genus claim. It does not protect every iodinated contrast agent. A compound must satisfy the specified substitution pattern, functional-group definitions, carbon limits, and hydroxyl-content limitation.

How broad is claim 1 of US 4,021,481?

Claim 1 is the principal broad claim. Its scope extends across numerous structurally related non-ionic triiodinated compounds, but it is narrower than a claim to all non-ionic iodinated contrast agents.

Claim 1 limitations

Limitation	Scope
Compound type	Non-ionic X-ray contrast compound
Core	Formula reproduced in the patent specification
R1 and R2	Hydrogen, alkyl, hydroxyalkyl, or alkanoyloxyalkyl
R3 and R4	Substituted amide, aminomethylamide, or related group
Carbon limit	Alkyl, hydroxyalkyl, and alkanoyl groups have up to six carbon atoms
Required functionality	At least one N-hydroxyalkyl group
Hydroxyl requirement	At least two hydroxyl groups in the molecule

The missing chemical drawings represented by the supplied ##STR8## and ##STR9## placeholders are material to a final infringement opinion. The textual limitations establish the main boundaries, but the precise ring substitution pattern and connectivity must be read from the issued patent figures and claims.

Scope of the functional-group definitions

The claim uses broad chemical definitions:

“Alkyl” generally captures saturated hydrocarbon substituents within the specified carbon limit.
“Hydroxyalkyl” captures alkyl groups bearing one or more hydroxyl groups.
“Alkanoyloxyalkyl” captures esterified hydroxyalkyl-type substituents.
“Alkanoyl” captures acyl groups derived from aliphatic carboxylic acids.
“N-hydroxyalkyl” requires the hydroxyalkyl group to be attached to nitrogen.

The hydroxyl requirement is cumulative. A molecule with two hydroxyl groups elsewhere in the structure may satisfy the literal “at least two hydroxyl groups” limitation, but it must also contain at least one N-hydroxyalkyl group.

What do claims 2 through 4 add?

Claims 2 through 4 narrow claim 1 by selecting specific substituent classes.

Claim	Added limitation	Legal effect
2	At least one of R1 or R2 is alkyl or hydroxyalkyl	Removes compounds in which both positions are hydrogen or other permitted substituents
3	At least one R3 or R4 group is an `--NR5Ac` group with specified R5 options	Selects direct acylamido or substituted acylamido structures
4	At least one hydroxyalkyl group is beta-hydroxyethyl, dihydroxypropyl, or tris(hydroxymethyl)methyl	Narrows the hydroxyalkyl definition to named species

These dependent claims would have been useful during prosecution or enforcement because they provide narrower fallback positions if the full Markush genus were challenged for anticipation, obviousness, or insufficient disclosure.

What compounds are specifically covered by claims 5 through 9?

Claims 5 through 9 identify specific compounds or compound groups. They are narrower than claim 1 and may remain enforceable only if the accused compound falls within the exact named structure.

Claim 5

Claim 5 lists a large group of individual compounds containing combinations of:

Acetamido groups;
N-methylacetamido groups;
N-hydroxyethylacetamido groups;
N-dihydroxypropyl carbamoyl groups;
Glucamine and N-methylglucamine residues;
Ethanolamine, diethanolamine, and substituted amino-alcohol residues;
Triiodobenzoyl or triiodoacetanilide cores.

The supplied text contains typographical and OCR defects, including inconsistent hyphenation, missing parentheses, and apparent errors in “2,4,6-triiodo” and related ring descriptions. Those errors matter because chemical identity is determined by structure, not by the name alone.

Claims 6 through 8

Claims 6, 7, and 8 select individual compounds from the claim 5 list:

Claim 6 covers N-(3-acetamido-5-N-methylcarbamoyl-2,4,6-triiodobenzoyl)-N-(2-hydroxyethyl)-glucamine.
Claim 7 covers N-(3-acetamido-5-N-methylcarbamoyl-2,4,6-triiodobenzoyl)-N-methylglucamine.
Claim 8 covers 3,5-bis-[N-(2,3-dihydroxypropyl)-N-methylcarbamoyl]-2,4,6-triiodoacetanilide.

These claims are composition-of-matter claims to specified chemical entities. They do not require a particular salt, concentration, container, indication, or route of administration unless those limitations appear elsewhere in the issued claim set.

Claim 9

Claim 9 separately covers two bis-substituted adipamide compounds:

N,N'-bis-[3-di-(beta-hydroxyethyl)-aminocarbonyl-2,4,6-triiodophenyl]-adipamide.
N,N'-bis-[3-(N-methyl)-glucaminocarbonyl-2,4,6-triiodophenyl]-adipamide.

Claim 9 is structurally distinct from the monomeric compounds in claims 1 through 8. It covers dimeric molecules linked through an adipamide bridge. The claim therefore expands the patent's platform beyond single aromatic triiodinated contrast molecules.

When did US Patent 4,021,481 expire?

US 4,021,481 was granted on May 3, 1977. Because it is a pre-Uruguay Round patent, its basic term was 17 years from grant, subject to any applicable terminal disclaimer or patent-term adjustment rules. On that basis, the patent expired on May 3, 1994.[1]

Event	Date
US patent grant	May 3, 1977
Applicable historical term	17 years from grant
Approximate expiration	May 3, 1994
Current enforceability	Expired
Current Paragraph IV relevance	None for this patent

Patent expiration eliminates the right to exclude others from making, using, selling, offering to sell, or importing the claimed compounds in the United States. It does not eliminate possible rights under later patents covering particular products or processes.

What is the Orange Book status of US 4,021,481?

US 4,021,481 is not a current Orange Book patent barrier. Its 1994 expiration predates modern Orange Book listing activity for most currently marketed non-ionic contrast products, and an expired patent cannot support a current listed-patent enforcement strategy.

The Orange Book is relevant only to approved drug products and patents submitted by the applicable new drug application holder. A historical compound patent may have supported an original product but does not remain a live listed patent after expiration.[2]

Current regulatory implications

The patent's expiration does not create FDA approval. A company seeking to market a contrast agent must still establish:

Identity and quality of the active ingredient;
Manufacturing controls;
Sterility and endotoxin controls;
Chemical and microbiological stability;
Toxicology and clinical safety;
Imaging efficacy;
Labeling and risk-management compliance.

For an already approved active ingredient, an abbreviated pathway may be available where the statutory requirements are satisfied. The expired patent removes one exclusivity layer but does not remove regulatory obligations.

Does US 4,021,481 create Paragraph IV risk?

No. A Paragraph IV certification concerns a patent listed in the Orange Book for an approved reference drug. Because US 4,021,481 expired decades ago, it cannot support a present Paragraph IV dispute.

A generic or follow-on applicant could still face:

Later active-ingredient patents;
Formulation patents;
Process patents;
Crystalline-form or salt patents;
Container-closure patents;
Method-of-use patents;
Patent claims covering a different iodine-containing compound.

Those rights must be assessed product by product. They are legally separate from US 4,021,481.

What later patents and products are relevant to the landscape?

The patent sits in the first generation of non-ionic, water-soluble triiodinated contrast chemistry. The later competitive landscape developed around distinct active ingredients and product families rather than continued enforcement of the 1977 patent.

Principal commercial classes

Product or class	Typical commercial development context	Relationship to US 4,021,481
Metrizamide-type compounds	Early non-ionic contrast development	Technically adjacent; exact coverage requires structure comparison
Iohexol	Low-osmolar non-ionic monomer	Later product-specific patent estate
Iopamidol	Low-osmolar non-ionic monomer	Later composition, process, and formulation rights
Iopromide	Low-osmolar non-ionic monomer	Later product-specific rights
Ioversol	Low-osmolar non-ionic monomer	Later composition and manufacturing rights
Ioxilan	Low-osmolar non-ionic monomer	Separate later patent and regulatory history
Iodixanol	Iso-osmolar non-ionic dimer	Different dimeric platform and later patent estate

The central legal issue is whether a later commercial active ingredient is structurally identical to a claim 5, 6, 7, 8, or 9 compound. Similar therapeutic use or similar iodine content is insufficient for literal infringement.

How strong was the patent estate for US 4,021,481?

Historically, the patent had meaningful breadth because claim 1 covered a family of hydroxyalkylated triiodinated amide compounds. Its strength came from the combination of:

A defined chemical platform;
Broad substituent alternatives;
Dependent claims directed to commercially plausible hydroxyalkyl groups;
Numerous named compounds;
Separate coverage of monomeric and adipamide-linked structures.

Its limitations were equally important:

The claims were tied to a particular molecular framework.
The hydroxyl and N-hydroxyalkyl requirements excluded many iodinated contrast agents.
The exact formula drawings controlled the ring topology.
The patent had no process claims in the claims supplied.
The patent had no formulation or method-of-use claims in the claims supplied.
The term expired in 1994.

On a current basis, patent strength is zero because the patent is expired. On a historical basis, the estate was a platform composition patent with potentially substantial blocking value during the development period for early non-ionic contrast agents.

What formulations are protected by US 4,021,481?

The supplied claims do not recite a formulation. They claim compounds.

The patent therefore does not, based on the supplied claims, independently protect:

A sterile injectable solution;
A specified iodine concentration;
A pH range;
A buffer system;
A stabilizer;
A container or vial;
A prefilled syringe;
An injector protocol;
A particular dose;
A specific imaging indication.

A formulation containing a claimed compound could have been commercially relevant, but formulation infringement would depend on separate claims in the patent or later patents. Expired compound protection also means that a formulation using the same active ingredient cannot be blocked by US 4,021,481 alone.

What manufacturing and intellectual-property barriers remain?

The expired compound patent does not remove practical barriers to market entry. Current barriers can include:

Manufacturing barriers

Non-ionic iodinated contrast agents require controlled processes for:

Regioselective iodination;
Amide formation;
Protection and deprotection of polyol groups;
Control of residual iodine-containing impurities;
Removal of heavy-metal and organic impurities;
Sterile filtration or aseptic processing;
Stability under high-concentration aqueous conditions.

Process patents may protect specific reaction sequences even where the active ingredient is public-domain. Trade secrets may also cover impurity control, crystallization, purification, and scale-up.

Regulatory barriers

The FDA may require detailed evidence on:

Acute kidney injury risk;
Hypersensitivity reactions;
Neurotoxicity;
Reproductive and developmental safety;
Compatibility with imaging equipment and injection systems;
Extractables and leachables;
Container closure integrity;
Pharmacovigilance.

These regulatory requirements can delay entry after patent expiry, especially for injectable products.

Which companies have historically competed in this field?

The non-ionic contrast market has included major pharmaceutical and imaging companies such as:

GE HealthCare;
Bracco;
Bayer;
Guerbet;
Sanofi and predecessor companies;
Nycomed and predecessor companies;
Mallinckrodt and related contrast businesses.

The commercial landscape is product-specific. A company may hold rights to one active ingredient while licensing or sourcing another. Historical ownership changes do not revive expired patents.

Licensing and settlement exposure

No current licensing or settlement agreement can restore exclusivity under US 4,021,481. Any relevant license would concern:

Later patents;
Know-how;
Manufacturing technology;
Regulatory files;
Brand or supply arrangements;
Device compatibility;
Geographic rights outside the United States.

A historical license involving the original patent would generally have no ability to prevent independent manufacture after expiration unless it also included surviving know-how, confidentiality, supply, or contractual restrictions.

What is the geographic coverage of US 4,021,481?

The patent provides US rights only. Foreign counterparts, if filed, would have had separate national terms and expiration dates. A US expiration does not determine the status of corresponding patents in Europe, Japan, Canada, China, or other jurisdictions.

For a global launch, the relevant review should distinguish:

Jurisdiction	Effect of US expiration
United States	US 4,021,481 no longer enforceable
European countries	Must assess national or regional counterparts separately
Japan	Separate patent term and maintenance history
Canada	Separate Canadian patent record
China	Separate Chinese patent record
Rest of world	Country-specific family and supplementary protection analysis

Patent-family analysis should also distinguish direct equivalents from later improvement patents. A later patent may cover a marketed product even where the original platform patent has expired.

What generic launch scenarios exist?

Immediate compound entry

If the target product is covered only by US 4,021,481, the compound patent does not prevent launch. The primary constraints become FDA approval, manufacturing readiness, quality validation, and commercial supply.

Later-patent delay

A later formulation, process, or use patent may delay launch even though the original compound patent expired. This is the principal present-day risk for products descended from the chemistry covered by the patent.

Design-around

A competitor may avoid the named compounds by changing:

The N-substituent;
The hydroxyalkyl chain;
The acyl group;
The linking group;
The dimer bridge;
The iodine-substitution pattern.

A design-around must be assessed against both literal claim language and equivalents doctrine, although the age and expiration of this patent make that issue commercially irrelevant in the United States.

Biosimilar risk

Biosimilar risk is not applicable to US 4,021,481. The patent covers small-molecule chemical compounds, not biologics. Competing products would be evaluated under generic-drug or other small-molecule regulatory pathways, not the biosimilar pathway under the Public Health Service Act.

What patent litigation affects US 4,021,481?

The supplied record does not establish an active US litigation dispute involving this patent. Given the 1994 expiration date, no current infringement case based solely on US 4,021,481 would be commercially viable.

Historical litigation involving later non-ionic contrast products should not be attributed to this patent without a case-specific docket and claim chart. Product disputes in this sector have generally centered on later patents, manufacturing methods, formulations, or product-specific regulatory rights.

Key Takeaways

US 4,021,481 is a composition-of-matter patent for non-ionic triiodinated X-ray contrast compounds.
Claim 1 covers a Markush genus requiring specified amide-type substituents, at least one N-hydroxyalkyl group, and at least two hydroxyl groups.
Claims 5 through 9 narrow the scope to named compounds, including glucamine, hydroxyethyl, dihydroxypropyl, and adipamide-linked structures.
The patent was granted May 3, 1977, and its basic US term expired May 3, 1994.
It creates no current US Orange Book, Paragraph IV, generic-entry, or biosimilar barrier.
The patent does not, based on the supplied claims, independently protect formulations, manufacturing processes, devices, or methods of use.
Current freedom-to-operate analysis must focus on later patents for specific contrast agents, formulations, manufacturing processes, delivery systems, and clinical uses.
Foreign counterparts must be reviewed separately because US expiration does not determine international patent status.

FAQs

Is US 4,021,481 still enforceable against an iodinated contrast-agent manufacturer?

No. Its US patent term expired in 1994.

Does US 4,021,481 cover iohexol, iopamidol, or iodixanol?

The claim text alone does not establish coverage of any particular commercial INN. Each active ingredient must be structurally mapped against the issued formula and the limitations of claims 1 through 9.

Can a company manufacture a compound listed in claim 7 today?

In the United States, the expired patent cannot prevent manufacture. Separate later patents, FDA requirements, process rights, or contractual restrictions may still affect commercialization.

Does the patent cover non-ionic contrast-agent injections?

The supplied claims cover compounds, not injections or finished formulations. A formulation claim would require separate claim language.

Can a foreign patent corresponding to US 4,021,481 still be active?

Possibly, depending on filing date, grant date, maintenance, patent-term rules, and jurisdiction. The US expiration date does not determine foreign enforceability.

References

United States Patent No. 4,021,481. (1977). Non-ionic X-ray contrast compounds. United States Patent and Trademark Office.
United States Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. FDA.
United States Patent and Trademark Office. (2024). Patent term calculator. USPTO.
United States Food and Drug Administration. (2024). Orange Book: Approved drug products with therapeutic equivalence evaluations. FDA.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
	32699/69	Jun 27, 1969
	6130/70	Feb 9, 1970

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	318134	⤷ Start Trial
Belgium	752574	⤷ Start Trial
Brazil	7020080	⤷ Start Trial
Canada	935153	⤷ Start Trial
Switzerland	544551	⤷ Start Trial
Germany	2031724	⤷ Start Trial
Denmark	130709	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Summary for Patent: 4,021,481