U.S. Patent 6,562,375: Claim Scope, Patent Expiration, Litigation Risk, and Pharmaceutical Patent Landscape

U.S. Patent No. 6,562,375 protects oral sustained-release matrix formulations using high-viscosity or high-molecular-weight polyethylene oxide and ferric oxide as a stabilizing agent. The patent covers both matrix incorporation and film-coating approaches, plus manufacturing and stability-improvement methods. The claims are composition- and process-focused rather than drug-specific.

The patent issued May 13, 2003. Its ordinary 20-year patent term has elapsed, making the patent a historical freedom-to-operate reference rather than a likely source of current U.S. blocking rights. The principal residual risk is technical: later patents may claim specific drug products, formulations, coatings, manufacturing processes, or improvements that overlap the same polyethylene oxide hydrogel platform.

What technology does U.S. Patent 6,562,375 protect?

The patent addresses physical and release-rate instability in oral sustained-release tablets based on polyethylene oxide, commonly abbreviated PEO.

The claimed platform has four central technical components:

A drug.
A hydrophilic base meeting a defined solubility threshold.
PEO used as the sustained-release matrix-forming polymer.
Yellow ferric oxide, red ferric oxide, or both, used in an amount effective to stabilize the preparation.

The invention is directed to hydrogel-forming matrices. Upon contact with gastrointestinal fluid, PEO hydrates and forms a gel layer that controls drug diffusion and matrix erosion. The patent claims ferric oxide as a stabilizer for the physical structure and release behavior of that matrix.

The claims do not identify a particular active pharmaceutical ingredient. A formulation can therefore fall within the technical class without containing a named drug, provided it satisfies the remaining limitations.

How many independent claims does the patent have?

The patent contains four principal independent claim groups:

Claim group	Claim	Subject matter	Core limitations
Composition	1	Stable oral pharmaceutical composition	Drug, hydrophilic base, PEO matrix, ferric oxide
Manufacturing	7	Method of producing the composition	Adding ferric oxide by granulation, mixing, or coating
Release stabilization	13	Method of preventing changes in drug release	Ferric oxide added to a PEO matrix
Physical stability	14	Method of increasing physical stability	Ferric oxide admixed with oral PEO composition

Claims 2 through 6 narrow claim 1. Claims 8 through 12 narrow claim 7. Claims 15 through 21 narrow claims 14 and 13 by adding manufacturing techniques, dosage ranges, or coating concentrations.

The broadest practical claim analysis should begin with claims 13 and 14, not claim 1. Those claims omit several of the numerical limitations that narrow the principal composition claim.

What are the required polyethylene oxide limitations?

Claim 1 and claim 7

Claims 1 and 7 require PEO meeting at least one of two technical thresholds:

A viscosity of at least 2,000 cP as a 2% aqueous solution at 25°C; or
A viscosity-average molecular weight of at least 2,000,000.

The claim also requires that the drug and hydrophilic base be dispersed in the PEO. The PEO must function as the base of the sustained-release preparation, not merely as a minor excipient or coating polymer.

These thresholds distinguish the claimed technology from lower-molecular-weight polyethylene oxide used as a binder, viscosity modifier, or processing aid.

Claims 13 and 14

Claims 13 and 14 are broader in drafting:

Claim 13 requires a sustained-release matrix containing a drug, hydrophilic base, and PEO.
Claim 14 requires an oral composition containing a drug, hydrophilic base, and PEO.

Neither claim expressly repeats the 2,000 cP or 2,000,000 molecular-weight threshold. A potential infringement analysis would therefore need to assess whether the limitations of claim 1 are implicitly required by the claim language, specification, or prosecution history. The absence of an express numerical PEO threshold materially broadens the literal text of claims 13 and 14.

What hydrophilic base is covered?

Claims 1 and 7 define the hydrophilic base functionally by solubility:

The amount of water needed to dissolve 1 gram of the hydrophilic base is 5 mL or less at 20±5°C.

This corresponds to a relatively water-soluble hydrophilic material. The claim does not depend on a named excipient. Potentially relevant materials could include water-soluble sugars, polyols, salts, and other soluble pharmaceutical excipients, but coverage depends on the specific material and its measured solubility under the claimed conditions.

The claim requires the hydrophilic base to be dispersed in PEO. A formulation that contains a soluble excipient outside the claimed solubility range may avoid claims 1 and 7, although claims 13 and 14 do not state the numerical solubility threshold.

What ferric oxide amounts are claimed?

The patent divides ferric oxide use into matrix incorporation and film coating.

Use location	Yellow ferric oxide	Red ferric oxide
Physical mixing in matrix, claim 1	1-20 wt%	3-20 wt%
Preferred matrix range	3-10 wt%	5-20 wt%
Narrow red oxide range	Not specified	10-15 wt%
Film coating per tablet, claim 1	0.3-2 wt%	0.3-2 wt%
Preferred coating range	0.5-1.5 wt%	0.5-1.5 wt%
Ferric oxide concentration in film material	5-50%	5-50%

The matrix percentages are high compared with conventional pigment loading. The coating route uses lower amounts measured against total tablet weight but permits a high concentration within the coating formulation.

Claims 13 and 14 use the phrase “an amount effective to stabilize.” That language creates a functional limitation without a fixed numerical range. Claims 16 through 21 then add the express ferric oxide ranges.

What formulations are protected by the patent?

A formulation is most directly aligned with claim 1 if it has all of the following:

An oral dosage form;
A sustained-release matrix;
A drug;
A hydrophilic base with the claimed water-solubility characteristic;
PEO meeting the viscosity or molecular-weight threshold;
Drug and hydrophilic base dispersed in PEO;
Yellow ferric oxide, red ferric oxide, or both;
Ferric oxide within the applicable matrix or coating range; and
A stabilizing effect on the hydrogel-forming matrix.

The patent does not require a particular tablet shape, dose strength, active ingredient, compression force, granulation solvent, release profile, or commercial brand.

A tablet with ferric oxide only as an ornamental colorant may still present risk if the claim’s stabilization and amount limitations are met. The claim does not require that ferric oxide be marketed or labeled as a stabilizer.

What manufacturing methods are protected?

Claim 7 covers adding ferric oxide to the formulation by one of three routes:

Granulation;
Mixing; or
Film coating.

Claims 8 through 12 add the same concentration limits used in claims 2 through 6.

Claims 14 and 15 are broader process-oriented claims. Claim 15 expressly identifies film coating, granulation, and mixing as covered methods. A manufacturer could therefore face method-claim exposure even where the final product composition is difficult to characterize, if the manufacturing records establish that ferric oxide was added in the claimed manner.

The method claims raise proof issues. A product claim can be assessed from the finished dosage form. A method claim requires evidence of the manufacturing process, including batch records, process instructions, coating composition, or discovery evidence.

How strong is the patent estate?

The patent’s technical scope is moderately broad, but its current commercial strength is limited by expiration.

Strengths

The claims combine several functional and structural limitations:

High-viscosity or high-molecular-weight PEO;
A hydrophilic soluble base;
A hydrogel-forming sustained-release matrix;
Specific ferric oxide species;
Defined loading ranges; and
A stabilizing effect.

The patent is also not restricted to one active ingredient. This gives the claim set platform relevance across multiple oral sustained-release drugs.

Claims 13 and 14 are potentially broader than claim 1 because they omit several numerical limitations. They could have been useful against products that used ferric oxide in a PEO matrix but fell outside the narrower percentages.

Weaknesses

The patent has several vulnerability points:

“An amount effective to stabilize” is functional and potentially subject to indefiniteness or proof disputes.
“Physical stability” and “preventing changes in drug release” require a defined comparison or test protocol.
The claim language does not always specify the relevant measurement method for viscosity, molecular weight, or release stability.
Claims 13 and 14 may face written-description or enablement scrutiny because of their broader formulation scope.
Ferric oxide was known as a pharmaceutical pigment and excipient. The inventive contribution depends on whether its use to stabilize this particular PEO matrix would have been obvious from the prior art.
The claims may be avoided by using a non-PEO matrix, a lower-molecular-weight polymer, a different stabilizer, or a formulation outside the stated ferric oxide ranges.

The strongest validity position would likely depend on comparative stability data showing that ferric oxide materially reduces release-rate drift or physical deterioration relative to otherwise identical formulations.

When did U.S. Patent 6,562,375 lose exclusivity?

U.S. Patent No. 6,562,375 issued in 2003, but patent expiration is ordinarily calculated from the effective nonprovisional or international filing date, not the issue date. The patent is now beyond the ordinary 20-year term associated with its filing period. It should therefore be treated as expired in the United States, subject to confirmation of the official term record, any patent-term adjustment, and any terminal disclaimer in the USPTO file.[1]

The patent’s expiration means that a new product generally cannot infringe an expired patent. Its claims remain relevant for:

Prior-art analysis;
Claim interpretation of later patents;
Patent-family tracking;
Invalidity searches;
Technology licensing history; and
Assessment of whether later patents are continuations or improvement patents.

What is the Orange Book status of U.S. Patent 6,562,375?

The patent is not drug-specific and does not identify an approved active ingredient, dosage strength, or brand product. It therefore does not function as a conventional Orange Book product patent.

Orange Book listing is generally tied to an approved drug application and patents that claim the drug substance, drug product, or approved method of use. A broad platform patent covering a class of oral sustained-release compositions is not, by itself, an Orange Book-listed patent for every drug that could use the technology.[2]

No Paragraph IV analysis can be performed solely from this patent. A Paragraph IV certification would relate to patents listed for a specific reference-listed drug, not to an unlinked formulation platform patent.

What generic entry risks exist?

Because the patent is expired, it does not create a current standalone generic-entry barrier. Historical risk would have been highest for a generic or branded product using:

PEO as the principal sustained-release matrix;
A soluble hydrophilic base;
Yellow or red ferric oxide;
The claimed loading ranges; and
A release profile stabilized by the ferric oxide.

Current risk may arise from later patents covering:

Drug-specific PEO formulations;
Abuse-deterrent or tamper-resistant matrices;
Multiparticulate PEO systems;
Coating systems containing iron oxides;
Manufacturing controls for release-rate consistency;
Combination products using specific hydrophilic bases; or
Commercial products with separate formulation and method-of-use patents.

A design-around based solely on removing ferric oxide would address the principal inventive element of this patent but would not address later patents covering equivalent formulation architectures.

Which companies are challenging the patent?

The supplied record does not establish a current Paragraph IV challenger, district-court case, Federal Circuit appeal, or settlement agreement involving U.S. Patent 6,562,375. Because the patent has expired, active litigation directed solely to this patent would not ordinarily be expected.

The absence of an identified challenger does not establish that no historical litigation occurred. A complete litigation conclusion requires a review of USPTO Patent Center, PACER, PTAB records, and relevant appellate dockets.[1,3]

How does this patent compare with competing sustained-release patent strategies?

Strategy	Main protected feature	Typical design-around
U.S. 6,562,375	Ferric oxide stabilization of PEO hydrogel matrix	Use another stabilizer or non-PEO polymer
Drug-specific matrix patent	Particular active ingredient and excipient ratios	Change drug salt, ratio, or matrix composition
Coating patent	Polymer, pigment, or barrier coating architecture	Alter coating polymer or layer sequence
Release-profile patent	Defined dissolution profile or kinetic range	Modify release mechanism or dosage form
Manufacturing patent	Granulation, compression, or coating process	Change process sequence or equipment
Abuse-deterrent patent	Mechanical or chemical resistance properties	Use an alternative deterrence platform

The patent is broader than a conventional drug-specific formulation patent in active-ingredient coverage, but narrower in its dependence on PEO and ferric oxide.

What geographic coverage does the patent have?

U.S. Patent 6,562,375 provides rights only in the United States. Foreign protection would require separate national or regional patents in the relevant family.

For commercial launch planning, the key geographic questions are:

Whether corresponding patents issued in Europe, Japan, Canada, China, or other target markets;
Whether those foreign patents expired on the same schedule;
Whether national prosecution narrowed the claims;
Whether continuation, divisional, or improvement applications survived; and
Whether local patent-term extensions or supplementary protection mechanisms applied.

A U.S. expiration does not establish freedom to operate in foreign markets.

What manufacturing and formulation barriers remain after expiration?

Patent expiration does not eliminate technical barriers. PEO matrices can present manufacturing challenges involving:

Poor powder flow;
High compression force;
Tablet swelling;
Sticking and picking;
Granule over-wetting;
Inconsistent hydration;
Dose dumping under altered conditions; and
Release-rate variability during storage.

Ferric oxide can affect blend uniformity, tablet color, coating opacity, density, and dissolution behavior. The commercial value of the invention may therefore persist as know-how even though the patent right has expired.

A manufacturer should separate patent clearance from process validation. A noninfringing formulation can still fail stability, dissolution, bioequivalence, or scale-up requirements.

Key Takeaways

U.S. Patent 6,562,375 covers oral PEO hydrogel-forming sustained-release compositions stabilized with yellow ferric oxide, red ferric oxide, or both.
Claim 1 requires a drug, soluble hydrophilic base, high-viscosity or high-molecular-weight PEO, and ferric oxide within specified loading ranges.
Claims 13 and 14 are broader method claims and omit several numerical limitations found in claim 1.
The patent covers ferric oxide incorporated into the matrix and ferric oxide used in a film coating.
The patent is beyond its ordinary 20-year U.S. term and should not be treated as a current blocking patent.
The patent is not tied to a particular active ingredient and is not, by itself, a conventional Orange Book product patent.
Current commercial risk is more likely to arise from later drug-specific, coating, manufacturing, or release-profile patents.
The strongest historical infringement scenario involved a PEO sustained-release tablet using red or yellow ferric oxide at the claimed concentration and demonstrating stabilization of release or physical properties.

Frequently Asked Questions

Does a PEO sustained-release tablet infringe this patent without ferric oxide?

Generally, no. Ferric oxide is a central limitation of the composition and method claims. A PEO matrix lacking yellow or red ferric oxide would not meet that limitation, although later patents could apply.

Does titanium dioxide substitute for ferric oxide under the claims?

Titanium dioxide is chemically different from yellow or red ferric oxide and would not ordinarily satisfy the literal ferric oxide limitation. Equivalents analysis would depend on the patent record, prosecution history, and the technical function of the substitute.

Are red iron oxide and yellow iron oxide both covered?

Yes. The claims expressly cover yellow ferric oxide, red ferric oxide, and combinations of the two, subject to the applicable concentration and use limitations.

Can a coating-only formulation fall within the patent?

Yes. Claims 1, 4, 5, and 6 expressly address ferric oxide in a film coating. The coating route uses 0.3-2 wt% ferric oxide per tablet, with preferred loading of 0.5-1.5 wt% and 5-50% concentration in the film material.

Does expiration of this patent clear all PEO sustained-release patent risk?

No. Expiration clears this patent only. Later patents may claim specific PEO formulations, active ingredients, coating systems, manufacturing processes, dissolution profiles, or commercial products.

References

United States Patent and Trademark Office. (2003). U.S. Patent No. 6,562,375, Stable hydrogel-forming pharmaceutical composition for oral use. https://patents.google.com/patent/US6562375
U.S. Food and Drug Administration. (2023). Approved drug products with therapeutic equivalence evaluations, Orange Book. https://www.fda.gov/drugs/drug-approvals-and-databases/approved-drug-products-therapeutic-equivalence-evaluations-orange-book
United States Patent and Trademark Office. (n.d.). Patent Center. https://patentcenter.uspto.gov/
U.S. Food and Drug Administration. (n.d.). Abbreviated new drug application process and patent certifications. https://www.fda.gov/drugs/abbreviated-new-drug-application-anda/abbreviated-new-drug-application-anda

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	324909	⤷ Start Trial
Australia	6182100	⤷ Start Trial
Australia	769807	⤷ Start Trial
Canada	2387705	⤷ Start Trial
China	1193792	⤷ Start Trial
China	1373673	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 6,562,375

Summary for Patent: 6,562,375