Details for Patent: 4,951,675

Summary for Patent: 4,951,675

Title:	Biodegradable superparamagnetic metal oxides as contrast agents for MR imaging
Abstract:	This invention relates to materials exhibiting certain magnetic and biological properties which make them uniquely suitable for use as magnetic resonance imaging (MRI) agents to enhance MR images of animal organs and tissues. More particularly, the invention relates to the in vivo use of biologically degradable and metabolizable superparamagnetic metal oxides as MR contrast agents. Depending on their preparation, these metal oxides are in the form of superparamagnetic particle dispersoids or superparamagnetic fluids where the suspending medium is a physiologically-acceptable carrier, and may be uncoated or surrounded by a polymeric coating to which biological molecules can be attached. These materials are administered to animals, including humans, by a variety of routes and the metal oxides therein collect in specific target organs to be imaged; in the case of coated particles, the biological molecules can be chosen to target specific organs or tissues. The biodistribution of the metal oxides in target organs or tissues results in a more detailed image of such organs or tissues because the metal oxides, due to their superparamagnetic properties, exert profound effects on the hydrogen nuclei responsible for the MR image. In addition, the dispersoids and fluids are quite stable and, in the case of the fluids, can even be subjected to autoclaving without impairing their utility. Furthermore, the materials are biodegradable and, in the case of iron oxide compounds, can eventually be incorporated into the subject's hemoglobin, making them useful in treating anemia. Thus, the materials are well-suited for in vivo use.
Inventor(s):	Ernest V. Groman, Lee Josephson, Jerome M. Lewis
Assignee:	Amag Pharmaceuticals Inc
Application Number:	US07/244,432
Patent Claim Types: see list of patent claims	Use; Composition;

United States Drug Patent 4,951,675: Scope, Claims, Expiration, and MRI Contrast-Agent Patent Landscape

US Patent 4,951,675 covers a method of obtaining an in vivo magnetic resonance image using a biodegradable superparamagnetic metal oxide contrast agent. Its central technical limitation is rapid biological clearance, demonstrated by the return of proton relaxation rates in the imaged organ or tissue to preadministration levels within about two weeks.

The patent was issued in 1990 and its US term expired in 2007. It therefore does not create a current US exclusionary right. Its historical scope was directed to MRI imaging methods rather than a standalone composition, manufacturing process, or drug product claim. Later patents could still have covered particular iron-oxide formulations, coatings, particle sizes, manufacturing processes, or clinical uses.

What does US Patent 4,951,675 cover?

US 4,951,675 covers the use of a biodegradable superparamagnetic metal oxide as an MRI contrast agent in an animal or human subject.[1]

The patent requires all of the following elements:

1. Administration of an effective amount of the contrast agent.
2. Use of a physiologically acceptable carrier.
3. Inclusion of a biodegradable superparamagnetic metal oxide.
4. Biodegradation in the subject within about two weeks or less.
5. Evidence of biodegradation through return of proton relaxation rates in the relevant organ or tissue to preadministration levels.
6. Acquisition of an in vivo MR image.

The claim is a method-of-use claim. It does not claim every superparamagnetic iron oxide composition, every injectable MRI contrast agent, or every method of manufacturing iron oxide particles.

Patent identification and term

Field	Information
US patent	4,951,675
Title	Biodegradable superparamagnetic metal oxides as contrast agents for magnetic resonance imaging
Technology	In vivo MRI contrast agents
Issue date	August 28, 1990
Primary claim type	Method of imaging
Core agent	Biodegradable superparamagnetic metal oxide
Express metal limitation	Iron in dependent claim 4
US statutory term	17 years from grant for this pre-URAA patent
Expected US expiration	August 28, 2007
Current status	Expired

The patent’s expiration means that the claims cannot presently be asserted to block US manufacture, sale, or use. The patent remains relevant as prior art and as a historical source for the development of biodegradable superparamagnetic iron oxide MRI agents.

What is the scope of independent claim 1?

Claim 1 is the controlling claim. It has a broad subject-matter category but several restrictive functional requirements.

Method of imaging

The claim covers a method for obtaining an in vivo MR image. The claim therefore requires an imaging step after administration. Administration of the agent alone is insufficient.

The method applies to:

• Human subjects.
• Nonhuman animals.
• Organs.
• Tissues.
• In vivo MRI procedures.

The claim does not expressly limit the imaging sequence, magnetic field strength, MRI instrument, anatomical site, or disease indication.

Superparamagnetic metal oxide

The contrast agent must contain a superparamagnetic metal oxide. The claim is not limited to iron in its independent form. Claim 4 narrows the invention to iron, indicating that the inventors contemplated other superparamagnetic metals or metal oxides at the independent-claim level.

The term “superparamagnetic” generally refers to nanoscale magnetic particles that exhibit strong magnetic response in an applied field but little or no remanent magnetization after the field is removed. In practice, MRI agents within this category were principally iron oxide particles.

The claim does not specify:

• Particle diameter.
• Crystalline phase.
• Magnetite versus maghemite.
• Iron oxidation state.
• Surface charge.
• Zeta potential.
• Iron concentration.
• Relaxivity value.
• Route of administration.
• Injection volume.
• Infusion time.
• Particular polymer coating.

Those omissions create breadth, but the biodegradation limitation materially narrows the claim.

Biodegradation within about two weeks

The key limitation is that the superparamagnetic metal oxide must biodegrade in the subject within about two weeks or less after administration.

This is not merely a requirement that the agent be capable of degradation in a laboratory test. The wording ties the limitation to biodegradation “in such subject.” The relevant assessment is biological behavior after administration.

The phrase “about two weeks or less” introduces an approximation around the two-week period. It does not create an unlimited clearance period. An agent that remains MRI-active for months would face a substantial claim-scope problem even if it eventually enters normal iron metabolism.

Return of proton relaxation rates

Claim 1 defines the evidence of biodegradation by reference to the return of proton relaxation rates in the organ or tissue to preadministration levels.

This is important because the claim does not simply require pharmacokinetic disappearance from blood. The relevant endpoint is restoration of the tissue’s MRI relaxation behavior. A particle could be cleared from plasma but remain in the liver, spleen, lymph nodes, or other tissue and continue to alter T1 or T2 relaxation.

The claim therefore links:

• Biological degradation;
• Loss of the relevant MRI contrast effect; and
• Return of organ or tissue proton relaxation rates to baseline.

A product with rapid plasma clearance but persistent tissue retention could fall outside the intended technical concept of claim 1.

How do claims 2, 3, and 4 narrow the invention?

Claims 2 through 4 add formulation and material limitations.

Claim	Added limitation	Practical effect
1	Biodegradable superparamagnetic metal oxide used for in vivo MRI	Broadest claim
2	Metal oxide associated with a polymeric substance	Covers polymer-associated particles
3	Polymer is a polysaccharide	Narrows claim 2 to polysaccharide systems
4	Metal is iron	Narrows claim 1, 2, or 3 to iron-containing systems

Claim 2: polymer-associated metal oxide

Claim 2 requires the metal oxide to be associated with a polymeric substance. “Associated” is broader than a narrowly defined covalent bond or a specific coating architecture. It can encompass particles coated, embedded, complexed, or otherwise physically or chemically linked to a polymer, depending on the specification and prosecution history.

The claim does not specify:

• Polymer molecular weight.
• Degree of substitution.
• Crosslinking.
• Covalent versus noncovalent association.
• Particle-to-polymer ratio.
• Coating thickness.
• Polymer biodegradation rate.

Claim 3: polysaccharide

Claim 3 narrows the polymer to a polysaccharide. This limitation points toward dextran, modified dextran, starch-derived materials, and other carbohydrate polymers commonly used to stabilize iron oxide particles.

The claim does not name dextran expressly. A formulation using a different polysaccharide could potentially fall within the claim if the other limitations were met. A synthetic non-polysaccharide polymer would not satisfy claim 3, although it could remain relevant to claim 2.

Claim 4: iron

Claim 4 narrows the metal to iron and depends on claim 1, claim 2, or claim 3. As drafted, it reaches:

• Iron oxide without a polymer;
• Polymer-associated iron oxide; and
• Polysaccharide-associated iron oxide.

Claim 4 is the most commercially relevant claim because clinical superparamagnetic MRI products historically relied primarily on iron oxide particles.

What formulations are protected by US 4,951,675?

The patent potentially covered biodegradable superparamagnetic iron oxide formulations used for MRI if they met the two-week biological degradation and relaxation-rate criteria.

The most relevant formulation categories were:

Formulation category	Relevance to claim set
Uncoated or minimally stabilized iron oxide	Potentially claim 1 or claim 4
Polymer-associated iron oxide	Potentially claim 2
Dextran-coated iron oxide	Potentially claim 3 or claim 4
Other polysaccharide-coated iron oxide	Potentially claim 3 or claim 4
Long-circulating iron oxide nanoparticles	Potentially outside the rapid-degradation limitation
Gadolinium chelates	Outside the metal-oxide limitation
Manganese-based agents	Potentially relevant to claim 1, depending on oxide and behavior
Non-superparamagnetic particles	Outside the claim
Ex vivo MRI contrast use	Outside the in vivo subject limitation

A formulation would not infringe merely because it contains iron oxide. The agent would also need to be superparamagnetic, used in the claimed imaging method, and capable of the specified biological behavior.

What MRI drugs and products were commercially related?

The patent sits within the early superparamagnetic iron oxide, or SPIO, technology field.

Feridex and ferumoxides

Feridex, known as ferumoxides, was an iron oxide MRI contrast agent used primarily for liver imaging. The product was approved by the FDA in 1996 and was associated with Advanced Magnetics and Berlex, later part of Bayer.[2]

Ferumoxides used a carbohydrate-based coating system and were designed for uptake by the reticuloendothelial system, particularly Kupffer cells in the liver and macrophages in the spleen. That profile made the product technically relevant to the patent’s emphasis on biodegradable iron oxide and organ relaxation behavior.

Feridex was discontinued in the United States. Its commercial discontinuation was separate from the expiration of US 4,951,675. The patent had already expired by the time later commercial decisions were made.

Resovist and ferucarbotran

Resovist, or ferucarbotran, was another iron oxide MRI agent developed for liver imaging and marketed outside the United States. Its commercial history illustrates that the relevant patent landscape was geographically fragmented.

A product marketed in Europe or Asia could have faced different patent rights from a US product. Expiration of US 4,951,675 did not eliminate foreign rights covering specific particle designs, coatings, manufacturing processes, or regulatory uses.

Combidex and ferumoxtran

Ferumoxtran, also known as Combidex or Sinerem in development programs, was an ultrasmall superparamagnetic iron oxide agent investigated for lymph-node and other imaging applications. It was not approved as a broadly marketed US MRI drug.

Its clinical and commercial status demonstrates the distinction between:

• The broad technical concept of biodegradable SPIO imaging;
• A particular investigational product;
• FDA approval of a specific drug product; and
• Enforceable patent claims covering a formulation or indication.

Ferumoxytol

Ferumoxytol, marketed as Feraheme, is an iron oxide nanoparticle approved by the FDA in 2009 for treatment of iron-deficiency anemia in adults with chronic kidney disease.[3] It is not approved by the FDA as an MRI contrast agent.

Ferumoxytol has generated substantial research into off-label MRI, vascular imaging, macrophage imaging, and lymphatic imaging. Its approved indication is different from the claimed method in US 4,951,675. MRI use would require analysis of later method, formulation, regulatory, and labeling rights rather than reliance on the expired patent.

What was the FDA and Orange Book status?

US 4,951,675 is not a current source of FDA regulatory exclusivity.

The patent itself did not establish FDA approval. FDA approval attaches to a drug product, sponsor, dosage form, route, manufacturing controls, labeling, and approved indication. Feridex received FDA approval in 1996, while Feraheme received FDA approval in 2009 for an anemia indication.[2,3]

Orange Book implications

The Orange Book lists patents and exclusivity associated with approved drug products where applicable.[4] A patent’s presence in the Orange Book would not extend its statutory term. Conversely, the absence of a current Orange Book listing would not by itself prove that no other patent exists.

For practical purposes:

• US 4,951,675 expired in 2007.
• It cannot support a current Hatch-Waxman patent listing or Paragraph IV enforcement strategy.
• Feridex is no longer a current US commercial product.
• Ferumoxytol is an approved iron-replacement drug, not an FDA-approved MRI contrast agent.
• A future MRI product based on an iron oxide platform would require separate analysis of current product-specific patents and regulatory exclusivities.

When did US 4,951,675 lose exclusivity?

US 4,951,675 lost enforceable US patent exclusivity on August 28, 2007, based on its August 28, 1990 issue date and the 17-year term applicable to the patent.

The timeline is:

Date	Event
August 28, 1990	US patent issued
1996	Feridex approved by FDA
August 28, 2007	Patent term expired
2009	Feraheme approved for iron-deficiency anemia
Later years	Feridex discontinued in the US; iron oxide MRI research continued

No current US generic-entry restriction can be based on this patent. Any present-day commercial risk would have to arise from later patents, regulatory requirements, trade secrets, manufacturing know-how, or product-specific intellectual property.

Were Paragraph IV challenges or patent settlements relevant?

Paragraph IV litigation is generally directed at patents listed for an approved drug product in the FDA Orange Book. US 4,951,675 is now expired and cannot support a current Paragraph IV notice or an injunction against an ANDA applicant.

Historical litigation involving Feridex, ferumoxides, or other SPIO products would need to be distinguished from this patent. A case involving a later formulation patent, manufacturing patent, or product-specific patent would not establish that US 4,951,675 itself remained enforceable.

No current settlement agreement can preserve the expired term of this patent. A private license or settlement may govern commercial conduct between parties, but it cannot revive the patent’s exclusionary rights against the public.

How strong was the patent estate?

Strengths

US 4,951,675 had several features that made it technically important when issued:

• It addressed an emerging MRI contrast-agent class.
• Claim 1 was not limited to one named polymer or one specific iron oxide phase.
• The claims covered human and animal imaging.
• The patent linked biodegradation to an objective MRI relaxation endpoint.
• Claim 4 targeted iron oxide systems that became commercially important.

Limitations

The patent also had material limits:

• It claimed an imaging method, not the composition itself.
• It required in vivo administration and actual MR imaging.
• It required biodegradation within about two weeks or less.
• It required return of tissue proton relaxation rates to baseline as evidence.
• It did not expressly claim a specific commercial product, brand, dose, particle size, or manufacturing process.
• Its term expired in 2007.

The functional biodegradation requirement could have created claim-construction and infringement-proof issues. A patent owner would need to show that the accused agent met the specified biological endpoint in the claimed subject or under a sufficiently representative testing framework.

What generic-entry risks exist today?

The direct risk from US 4,951,675 is zero because the patent is expired. A competitor may not need a license to practice the expired claims in the United States.

Current entry risks would instead include:

1. Later patents covering particle synthesis or surface modification.
2. Patents covering a specific polysaccharide coating.
3. Patents covering controlled particle size or magnetic properties.
4. Patents covering sterile injectable formulations.
5. Method-of-use patents for lymph-node, liver, vascular, inflammatory, or oncology imaging.
6. FDA requirements for clinical data, contrast-agent safety, dosing, and labeling.
7. Manufacturing barriers involving reproducible particle size, coating density, aggregation control, and endotoxin limits.
8. Trade secrets relating to scale-up and quality control.
9. Foreign patents with unexpired terms.

A biosimilar pathway does not apply to conventional iron oxide contrast agents. A competing product would generally be assessed under the applicable small-molecule or drug-product regulatory pathway rather than under the US biosimilar framework.

How does this patent compare with later iron oxide patent strategies?

US 4,951,675 is an early platform patent. Later patent strategies in the field generally became narrower and more product-specific.

Patent strategy	Typical protected subject matter	Commercial relevance
Early platform method	Biodegradable SPIO used for MRI	Broad technical coverage, now expired
Composition patent	Defined particle, coating, size, or magnetic profile	Can create product-specific barriers
Formulation patent	Injectable carrier, concentration, stability, or excipients	Relevant to regulatory product development
Manufacturing patent	Precipitation, oxidation, coating, purification, or sterilization	Can restrict scale-up routes
Method-of-use patent	Specific organ, disease, imaging protocol, or patient group	Relevant to labeled or reimbursed uses
Drug-device patent	Administration system or imaging workflow	Relevant to integrated products
Regulatory exclusivity	New drug, orphan, pediatric, or other FDA exclusivity	Independent of patent term

The expired patent therefore has historical platform value but no remaining standalone blocking power.

Key Takeaways

• US 4,951,675 covers in vivo MRI using a biodegradable superparamagnetic metal oxide.
• Claim 1 requires biological degradation within about two weeks or less and return of tissue proton relaxation rates to baseline.
• Claims 2 and 3 narrow the invention to polymer-associated and polysaccharide-associated particles.
• Claim 4 narrows the metal to iron and is the most relevant claim for historical iron oxide MRI products.
• The patent is a method patent, not a broad composition or manufacturing patent.
• Its US term expired on August 28, 2007.
• It cannot support a current Paragraph IV challenge, injunction, or Orange Book exclusivity position.
• Feridex and other SPIO products were commercially related to the claimed technology, but product-specific and later patents must be analyzed separately.
• Ferumoxytol is FDA-approved for iron-deficiency anemia, not as an MRI contrast agent.
• Current market-entry barriers would arise from later patents, regulatory requirements, manufacturing know-how, and foreign rights.

FAQs About US Patent 4,951,675

Does US 4,951,675 cover all iron oxide MRI contrast agents?

No. It covers a method using a biodegradable superparamagnetic metal oxide that meets the specified in vivo degradation and proton-relaxation criteria. The patent does not claim every iron oxide composition.

Is US 4,951,675 still enforceable?

No. The US patent term expired in 2007.

Did the patent cover Feridex?

Feridex was technically within the field addressed by the patent, particularly because it was a biodegradable iron oxide MRI agent with a carbohydrate-based coating. Product-level infringement would require analysis of the issued claims and the product’s biological behavior. The patent is now expired.

Does ferumoxytol have FDA approval for MRI?

No. Feraheme, the ferumoxytol product, is FDA-approved for treatment of iron-deficiency anemia in adults with chronic kidney disease. MRI use is not its approved indication.

Can a company commercialize a new biodegradable SPIO agent without licensing US 4,951,675?

Yes, the expired patent no longer requires a US license. A company would still need to clear later patents and satisfy FDA requirements applicable to its proposed product and indication.

References

1. United States Patent and Trademark Office. (1990). Biodegradable superparamagnetic metal oxides as contrast agents for magnetic resonance imaging (U.S. Patent No. 4,951,675).

2. U.S. Food and Drug Administration. (1996). Feridex I.V. prescribing information. FDA.

3. U.S. Food and Drug Administration. (2009). Feraheme (ferumoxytol injection) prescribing information. FDA.

4. U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. FDA.

Drugs Protected by US Patent 4,951,675

Applicant	Tradename	Generic Name	Dosage	NDA	Approval Date	TE	Type	RLD	RS	Patent No.	Patent Expiration	Product	Substance	Delist Req.	Patented / Exclusive Use	Submissiondate

International Family Members for US Patent 4,951,675

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	135920
Austria	139431
Austria	142891
Austria	143604
Austria	143814
Austria	151991
Australia	2545292