US Patent 4,687,659: Claim Scope, Expiration, and MRI Contrast-Agent Patent Landscape

US Patent 4,687,659 covers amide-substituted DTPA chelates of paramagnetic metals for magnetic-resonance imaging, including gadolinium, manganese, iron and cobalt complexes. Its claims reach compositions, injectable formulations, solid-state agents, and imaging methods using diamide-DTPA complexes. The patent issued on August 18, 1987, and its 17-year statutory term expired in 2004 under the pre-Uruguay Round patent-term rule. The patent therefore has no current blocking effect, although later patents covering specific chelates, formulations, manufacturing processes or uses could have created separate historical barriers. [1]

What does US Patent 4,687,659 cover?

The patent claims a class of paramagnetic MRI contrast agents based on DTPA, or diethylenetriaminepentaacetic acid, in which one or more acetic-acid groups are converted into amides.

The core architecture is:

A-DTPA-PM(+Z)

where:

DTPA is the polyaminocarboxylate chelator;
A is an amide substituent of the general form -CONH-(CH2)n-1-CH3;
n may extend to 16 in the principal composition claims;
PM(+Z) is a paramagnetic metal ion;
the metal is coordinated by multiple donor atoms of the DTPA derivative; and
the resulting complex reduces T1 relaxation time during MR imaging.

The patent attempts to protect both the chemical entity and its use as an MRI contrast agent. The claims also pursue neutral or substantially neutral complexes, which were commercially important because reducing ionic charge can affect tolerability, osmolality and formulation characteristics.

What chemical structures are within the claims?

The principal claimed structures include:

Structure	Claim coverage	Key limitation
Monoamide-DTPA complex	Claims 1 and 7	At least one DTPA acetic acid group converted to an amide
Diamide-DTPA complex	Claims 2, 8-11 and 22	Two amide groups attached to the DTPA framework
Homodiamide	Claim 10	The two amide substituents have the same carbon-chain length
Heterodiamide	Claim 11	The two amide substituents have different chain lengths
Trivalent neutral complex	Claims 3 and 12	A +3 metal produces a zero-net-charge complex
Divalent neutral complex	Claims 4 and 13	A +2 metal is paired with a +1 inert cation
Aqueous formulation	Claims 14 and 24	Water solution or carrier vehicle
Hydrated solid	Claims 1 and 15	Solid-state agent or water of hydration
MRI imaging method	Claims 22-27	Administration followed by MR imaging

The claim language is functional as well as structural. It requires the chelator to “chemically isolate” the metal ion from the in vivo environment and requires the amide group to cooperate with that environment. Those limitations could affect claim construction because they are less chemically precise than conventional claims that identify a fully defined molecular structure.

How broad are the independent claims?

Claims 1, 7 and 22 are the principal independent claims. They divide the invention into three protection categories.

Claim 1: solid-state chemical composition

Claim 1 covers a chemically stable, physiologically tolerable contrast agent in solid form. The agent must contain:

an amide-substituted DTPA chelator;
a paramagnetic metal ion;
multiple coordination bonds between the chelator and metal; and
a function that reduces T1 relaxation time in an MR environment.

The claim is broad because it does not limit the agent to gadolinium. It covers the listed transition-metal and lanthanide ions, subject to the requirement that the resulting complex be paramagnetic and physiologically tolerable.

The claim also is broad in the number of amide substitutions. It requires at least one amide but does not restrict the substitution pattern to a particular pair of DTPA carboxyl groups.

Claim 7: pharmaceutical formulation

Claim 7 moves from a solid-state composition to a pharmacological formulation. It requires:

a paramagnetic ion;
a DTPA' chelator;
at least one covalently attached amide functional group; and
a pharmaceutically acceptable vehicle.

The claim therefore reaches injectable solutions and other dosage forms, not merely isolated chemical compounds. Claim 14 narrows the vehicle to water, while claim 15 addresses water of hydration associated with the amide-containing structure.

Claim 22: imaging method

Claim 22 covers a method comprising:

providing a diamide-DTPA-paramagnetic-metal complex;
introducing the agent into a subject;
waiting for the amide groups to interact with the in vivo environment; and
obtaining an enhanced MR image.

Claims 23-27 narrow the method to intravenous injection, a carrier vehicle, image subtraction, and imaging of the gall bladder or colon. Claim 27 specifically addresses a perspective image of the colon surface.

What metals are covered by US 4,687,659?

The patent uses broad Markush groups covering transition metals and lanthanides.

Category	Listed ions
Transition metals	Cr(III), Co(II), Mn(II), Ni(II), Fe(III), Cu(III), Fe(II), Cu(II)
Lanthanides	La(III), Gd(III), Ce(III), Tb(III), Pr(III), Dy(III), Nd(III), Ho(III), Pm(III), Er(III), Sm(III), Tm(III), Eu(III), Yb(III), Lu(III)
Separate dependent-claim listing	Gd(II)
Specific dependent claims	Fe(III), Mn(II), Co(II), Gd(III)

Gadolinium is the most commercially significant member of the claimed group. The patent’s principal commercial relevance therefore lies in its relationship to gadolinium-DTPA and gadolinium-DTPA-bisamide chemistry.

The inclusion of Gd(II) in claim 6 and claim 17 is chemically unusual. Gadolinium is principally used in the +3 oxidation state in approved MRI chelates. The Gd(II) reference may reflect a drafting or transcription issue, but it remains part of the supplied claim text and would be evaluated separately from the more commercially relevant Gd(III) claims.

What formulations are protected by the patent?

The formulation claims cover both solid and liquid presentations.

Solid-state agents

Claim 1 requires a solid-state contrast agent. Claim 15 also refers to water of hydration associated with the amide-containing structure. These provisions could reach crystalline, amorphous or hydrated forms if the product satisfied the remaining structural and functional limitations.

The claims do not expressly require:

a particular crystal form;
a particular particle size;
a specific excipient;
a defined pH;
a particular osmolality; or
a specified concentration.

That absence makes the formulation claims structurally broad but potentially vulnerable to arguments concerning enablement, written description and indefiniteness.

Injectable solutions

Claims 7 and 14 cover a pharmacological composition containing the chelate and a pharmaceutically acceptable vehicle, including water. Claim 24 similarly covers dispersing the agent in a suitable carrier vehicle before administration.

The claims do not expressly require intravenous administration in every formulation claim. Intravenous use appears in dependent claim 23 and in the imaging-method structure.

Neutral complexes

Claims 3, 4, 12 and 13 focus on zero-net-charge complexes.

For a trivalent metal such as Gd(III), the diamide-DTPA complex is claimed as neutral without requiring an inert cation. For a divalent metal, the claim adds an inert monovalent cation to achieve a zero-net-charge composition.

Neutrality is a meaningful commercial limitation because early MRI contrast-agent development focused on reducing ionic character and improving formulation properties. It is not, however, a complete definition of a commercial product. A product could avoid these claims by using a different chelator, a charged formulation, a macrocyclic structure or a different substitution pattern.

How do the claims compare with commercial gadolinium contrast agents?

The patent is most closely aligned with linear DTPA-derived gadolinium chemistry. It is less relevant to macrocyclic agents or gadolinium chelates based on different ligand frameworks.

Product or class	Relationship to claimed subject matter	Relevance
Gadopentetate dimeglumine	Gd-DTPA ionic complex	Related DTPA platform, but not necessarily an amide-DTPA product
Gadodiamide	Gd-DTPA-bisamide chemistry	Closest commercial chemical class
Gadoversetamide	DTPA-derived bisamide chemistry	Potentially related to the amide-DTPA genus
Gadoteridol	Macrocyclic non-DTPA chelate	Outside the core DTPA-amide structure
Gadobutrol	Macrocyclic non-DTPA chelate	Outside the core DTPA-amide structure
Gadoterate meglumine	Macrocyclic DOTA derivative	Outside the core DTPA-amide structure
Gadoxetate disodium	DTPA derivative with hepatobiliary targeting substituent	Requires separate analysis of its specific structure and later patents

Gadodiamide and gadoversetamide are the most relevant commercial comparators because they are DTPA-derived, nonionic or low-charge gadolinium complexes with amide-containing side chains. A product-by-product infringement assessment would depend on the exact amide substitution pattern, chain length, charge state, metal oxidation state and claim interpretation. FDA labels identify the active ingredients and structures used in approved products. [2-5]

When did US Patent 4,687,659 lose exclusivity?

The patent issued on August 18, 1987. Under the pre-June 8, 1995 rule, its term was generally 17 years from issuance. On that basis, the term ended on August 18, 2004. [1]

Event	Date
Patent issued	August 18, 1987
Ordinary statutory term	17 years from issuance
Expected expiration	August 18, 2004
Current enforceability	Expired
Current blocking effect	None from this patent alone

Patent-term adjustment did not operate under the modern 20-year-from-earliest-effective-filing regime for this 1987 patent. Patent-term extension under 35 U.S.C. § 156 is a separate issue and would require an eligible approved product and statutory approval. No current enforceable term is attributable to US 4,687,659.

The expiration does not eliminate later patents. Later patents could have protected:

a specific gadolinium-DTPA-bisamide compound;
a particular stereoisomer;
a crystal or hydrate;
a manufacturing route;
a purification method;
a dosage regimen;
a liver or gastrointestinal imaging indication;
a formulation with defined osmolality or concentration; or
a delivery system.

What is the Orange Book status of US 4,687,659?

US 4,687,659 is not a current Orange Book barrier. The patent is expired, and the Orange Book is relevant to FDA-approved drug products and listed patents rather than to a historical patent that no longer has enforceable term.

For MRI contrast agents, Orange Book analysis must be performed at the product level. The relevant question is whether a particular approved product has listed patents, pediatric exclusivity, regulatory exclusivity or a generic-drug pathway. The existence of an old composition patent does not establish current Orange Book protection.

FDA-approved gadolinium agents are regulated as drugs, generally through the NDA framework. A generic or abbreviated application for a small-molecule contrast agent would be assessed under the applicable FDA pathway, including pharmaceutical equivalence, bioequivalence and labeling requirements. [2-5]

Are there Paragraph IV challenges or generic-entry risks?

A Paragraph IV challenge to US 4,687,659 would no longer have commercial significance because the patent expired in 2004. A current generic applicant would instead analyze unexpired patents listed against the relevant reference product.

The generic-entry risk differs by product:

Risk category	Current relevance
Expired broad DTPA-amide genus	No blocking effect
Unexpired compound patent	Potentially material
Unexpired formulation patent	Potentially material
Manufacturing patent	Potentially material
Method-of-use patent	Relevant if listed and applicable to the proposed label
Regulatory exclusivity	Product-specific
Bioequivalence and injectable quality	Significant non-patent barrier

Contrast agents are small molecules, not biologics. Biosimilar litigation and the Biologics Price Competition and Innovation Act pathway generally do not apply to conventional gadolinium chelates. The commercial barriers are instead formulation equivalence, sterility, impurity control, metal-leaching control, stability, container compatibility and FDA approval requirements.

How strong is the patent estate for the claimed invention?

The patent was commercially broad at the genus level but has limited present-day value because its term expired.

Strengths

Broad coverage of monoamide and diamide DTPA complexes.
Coverage of multiple paramagnetic metals.
Separate composition, formulation and method claims.
Specific focus on neutral complexes.
Claims directed to both solid and aqueous forms.
Dependent claims covering intravenous administration and gastrointestinal imaging.

Weaknesses

Functional phrases such as “functionally cooperating with the in vivo environment” may create construction and definiteness issues.
The metal Markush group is broader than the likely demonstrated commercial examples.
The claims do not uniformly identify exact substitution positions.
The amide-chain definition is awkward and internally inconsistent in places.
The dependent claims repeat broad metal lists and include the commercially unusual Gd(II) species.
The patent expired before current MRI contrast-agent markets matured.

The patent’s historical value was strongest as an early platform claim. Its current value is primarily prior-art and freedom-to-operate context, not exclusionary rights.

What manufacturing and intellectual-property barriers remain?

Even with the patent expired, manufacturing a comparable product requires control over several technical variables:

DTPA starting-material purity.
Selective conversion of carboxylic acid groups to amides.
Control of positional isomers and over-amidation.
Gadolinium loading and removal of unchelated metal.
Residual free-ligand and free-gadolinium testing.
Sterile injectable production.
Endotoxin and particulate control.
Stability in the final container.
Control of osmolality, pH and concentration.
Demonstration of pharmaceutical equivalence and bioequivalence where applicable.

Later process patents can be commercially important even where the original composition patent is expired. A manufacturer may practice the expired chemical genus while still infringing a later, narrower process or formulation patent.

What litigation or settlement agreements affect the patent?

No current litigation or settlement right arises from US 4,687,659 because the patent expired in 2004. Historical litigation involving particular gadolinium products would need to be separated from this patent’s current status. A settlement concerning a later patent would not revive the expired rights in US 4,687,659.

There is also no biosimilar settlement framework applicable to the core small-molecule contrast agents. Any historic generic dispute would more likely have involved Hatch-Waxman patent certifications, ANDA litigation, or product-specific formulation and process claims. [6]

How does US 4,687,659 compare with later MRI contrast-agent patents?

The patent is a platform patent. Later patents generally narrowed the field by claiming a particular active ingredient, substituent, formulation or use.

Patent strategy	Example subject matter	Relative position
Broad chelate genus	Amide-DTPA-metal complexes	US 4,687,659
Specific active ingredient	Defined Gd-DTPA or DTPA-bisamide compound	Later product patents
Macrocyclic chelate	Gadolinium-DOTA or related ligand	Separate chemical class
Targeted imaging agent	Hepatobiliary or tissue-specific substituent	Later use and composition patents
Formulation	Concentration, pH, osmolality, excipient or container	Product-specific
Manufacturing	Chelation, purification or crystallization process	Process-specific
Safety or administration	Dose, infusion rate or patient population	Method-of-use

The practical freedom-to-operate question is therefore not whether a product falls within the expired genus. It is whether the product practices any later unexpired claim covering its exact molecule, formulation, method or production process.

Key Takeaways

US Patent 4,687,659 covers amide-substituted DTPA complexes with paramagnetic metals for MRI.
Its broadest commercial relevance is to gadolinium-DTPA and gadolinium-DTPA-bisamide chemistry.
Claims 1 and 7 cover chemical compositions and pharmaceutical formulations.
Claim 22 covers imaging methods using diamide-DTPA-paramagnetic-metal complexes.
The patent covers monoamides, diamides, homodiamides, heterodiamides and neutral complexes.
The patent issued on August 18, 1987, and its ordinary 17-year term expired on August 18, 2004.
The patent has no current enforceable exclusivity or standalone Paragraph IV significance.
It is not a current Orange Book barrier.
Biosimilar rules generally do not apply because the relevant contrast agents are small molecules.
Current commercial risk depends on later product, formulation, manufacturing and method-of-use patents.

FAQs

Is gadodiamide covered by the claims of US 4,687,659?

Gadodiamide is chemically related to the claimed DTPA-bisamide class and would be the closest commercial comparison. Exact coverage depends on the molecular structure, amide substituents, charge state and construction of the relevant claims.

Can a company commercialize an amide-DTPA gadolinium contrast agent today based solely on patent expiration?

The expired patent alone does not block commercialization. The company must still address later patents, FDA approval, sterile manufacturing, impurity limits, free-gadolinium control and product-specific regulatory requirements.

Does the patent cover macrocyclic gadolinium agents such as gadobutrol?

The core claims require a DTPA or DTPA'-type chelator. Macrocyclic agents based on different ligand frameworks generally fall outside the central chemical structure, subject to claim construction and the exact molecular architecture.

Does expiration of US 4,687,659 eliminate all patent risk for MRI contrast agents?

No. Later patents may cover specific active ingredients, formulations, manufacturing processes, dosage regimens, imaging indications or delivery systems.

Is US 4,687,659 relevant to current gadolinium safety litigation?

It may be relevant as historical prior art concerning chelation and MRI contrast chemistry. It does not create current enforceable exclusivity and does not, by itself, determine product-liability or regulatory issues involving gadolinium retention.

References

United States Patent and Trademark Office. (1987). United States Patent No. 4,687,659.
U.S. Food and Drug Administration. (2018). Magnevist (gadopentetate dimeglumine) injection prescribing information.
U.S. Food and Drug Administration. (2018). Omniscan (gadodiamide) injection prescribing information.
U.S. Food and Drug Administration. (2018). OptiMARK (gadoversetamide) injection prescribing information.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations.
U.S. Congress. (1984). Drug Price Competition and Patent Term Restoration Act of 1984, Pub. L. No. 98-417, 98 Stat. 1585.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Germany	3590496	⤷ Start Trial
Germany	3590578	⤷ Start Trial
European Patent Office	0200750	⤷ Start Trial
European Patent Office	0203962	⤷ Start Trial
United Kingdom	2177086	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,687,659

Summary for Patent: 4,687,659