United States Patent 5,635,172: Scope, Claims, Expiration, and Ophthalmic Patent Landscape

United States Patent No. 5,635,172 covers sustained-release ophthalmic compositions and topical treatment methods that combine a specified intraocular-pressure-lowering drug with an anionic mucomimetic polymer and a cation-exchange resin. The patent is formulation-focused rather than molecule-focused. Its central limitation is the combination of drug, polymer, resin concentration, and pH.

The patent does not broadly cover all sustained-release glaucoma products. It reaches products that satisfy each required element, including the defined polymer chemistry, molecular-weight range, cation-exchange-resin range, and pH range. As a United States patent issued in 1997, its ordinary patent term has expired unless a term adjustment or other unusual term event materially extended it. No current United States exclusivity value should be assigned to the patent without confirming the USPTO patent-term record and any terminal disclaimer.

What does United States Patent 5,635,172 cover?

The patent covers two claim categories:

A sustained-release ophthalmic pharmaceutical composition.
A method of lowering or controlling intraocular pressure by topically administering that composition to the affected eye.

The claimed formulation must contain:

A therapeutically effective amount of at least one listed drug.
An anionic mucomimetic polymer with carboxylic acid functional groups.
A polymer with 2 to 7 carbon atoms per functional group.
A polymer molecular weight from 50,000 to 6 million.
A cation-exchange resin at approximately 0.05% to 10% by weight.
A pH from approximately 3.0 to 8.5.

The listed active agents are:

Pilocarpine
Epinephrine
Proepinephrine
Norepinephrine
Pronorepinephrine
Clonidine
p-Aminoclonidine
p-Acetoamidoclonidine

The patent therefore protects a delivery-system combination. It does not claim pilocarpine, clonidine, or the other listed active ingredients as chemical entities.

What are the key limitations in claim 1?

Claim 1 is an apparatus-style pharmaceutical composition claim. Each limitation must be present in an accused product for literal infringement.

Claim element	Scope
Dosage form	Sustained-release ophthalmic pharmaceutical composition
Therapeutic purpose	Control and lowering of human intraocular pressure
Active ingredient	One or more compounds from the eight-membered Markush group
Polymer	Anionic mucomimetic polymer
Functional groups	Carboxylic acid functional groups
Carbon range	2 to 7 carbon atoms per functional group
Polymer molecular weight	50,000 to 6 million
Ion-exchange component	Cation-exchange resin
Resin concentration	Approximately 0.05% to 10% by weight
pH	Approximately 3.0 to 8.5

The composition claim requires the cation-exchange resin expressly. A formulation containing the listed drug and polymer but no cation-exchange resin would fall outside the literal scope of claim 1.

The claim also requires an anionic mucomimetic polymer. A neutral viscosity enhancer, a nonionic mucoadhesive, or a cationic polymer would not satisfy this limitation unless a court treated the material as an equivalent under the doctrine of equivalents.

What are the key limitations in claim 2?

Claim 2 is a method-of-treatment claim. It requires:

A human patient with elevated or clinically relevant intraocular pressure.
Topical administration to the affected eye.
A composition containing the listed active ingredient, anionic mucomimetic polymer, and cation-exchange resin.
The specified resin concentration and pH range.

Claim 2 is narrower in use than a general formulation claim because it requires administration for controlling or lowering intraocular pressure in humans. It may be relevant to products sold with a glaucoma or ocular-hypertension indication, but its practical enforcement depends on the product labeling, instructions, promotional conduct, physician use, and evidence of induced or contributory infringement.

A product label that directs topical ocular use for glaucoma could create greater method-claim exposure than a product marketed only for a non-therapeutic laboratory or veterinary use. The claim language, however, expressly refers to humans.

How should the polymer limitation be interpreted?

The polymer limitation is one of the principal claim-construction issues.

The patent requires an "anionic mucomimetic polymer having carboxylic acid functional groups." The polymer must also have a molecular weight between 50,000 and 6 million. This language is directed to high-molecular-weight, negatively charged polymers capable of interacting with the ocular mucin layer and increasing ocular residence time.

The phrase "2 to 7 carbon atoms per functional group" appears to define the carbon content associated with the carboxylic acid functionality. Depending on the specification and prosecution history, this limitation may be applied to the repeating structural unit, the acid-bearing monomer, or the polymer composition as a whole. That issue would be material in litigation.

Potential polymer candidates could include certain polyacrylic-acid-type or acrylic-acid-containing polymers, subject to the patent specification's definitions and examples. The claim should not automatically be read to cover every commercial carbomer, polycarboxylic acid, or mucoadhesive polymer. Molecular weight, acid-group structure, crosslinking, and composition must be evaluated against the intrinsic evidence.

Does the claim require a particular polymer brand?

No. The claims are written generically and do not identify a commercial polymer supplier or brand. A commercial carbomer, crosslinked polyacrylic-acid polymer, or other polymer could be relevant if it satisfies the structural and molecular-weight limitations.

A supplier's trade name is not determinative. The relevant analysis would require:

Chemical identity
Acid-functional-group structure
Molecular-weight measurement
Crosslinking characteristics
Polymer composition
Product specification and batch data
Interpretation of the 2-to-7-carbon limitation

What is the role of the cation-exchange resin?

The cation-exchange resin is a required formulation component in both claims.

The claim gives a concentration range of approximately 0.05% to 10% by weight. The resin can affect:

Drug binding and release
Ocular residence time
Formulation stability
Drug solubility
Ionic interactions
Release kinetics
Tolerability and viscosity

The claimed range is broad. A formulation with 0.05%, 1%, 5%, or 10% resin may fall within the literal numerical range, subject to the meaning of "about" and the product's measured composition.

The claim does not identify a particular resin chemistry, particle size, counterion, crosslinking agent, or commercial product. Those details may be supplied by the specification and prosecution history. A nonionic particulate excipient would not ordinarily satisfy the cation-exchange-resin limitation.

What drugs are covered by the patent?

The patent uses a Markush group. A formulation containing one listed active ingredient can satisfy the active-agent limitation, assuming all other elements are present.

Drug or class	Commercial relevance
Pilocarpine	Established miotic and glaucoma therapy
Epinephrine	Historical ophthalmic pressure-lowering agent
Proepinephrine	Prodrug or derivative category identified in the patent
Norepinephrine	Historical adrenergic ophthalmic agent
Pronorepinephrine	Prodrug or derivative category identified in the patent
Clonidine	Alpha-adrenergic agonist with ocular-pressure relevance
p-Aminoclonidine	Clonidine-related compound
p-Acetoamidoclonidine	Clonidine-related compound

The claim does not extend to unrelated modern glaucoma agents such as latanoprost, bimatoprost, travoprost, brimonidine, dorzolamide, or timolol merely because they lower intraocular pressure. A modern product would need to contain one of the specifically listed active agents, or potentially an equivalent under a fact-specific doctrine-of-equivalents analysis.

What is the patent's likely expiration date?

United States Patent 5,635,172 issued on June 3, 1997. Under the modern United States patent-term framework, a utility patent generally expires 20 years from the earliest effective nonprovisional filing date, subject to patent-term adjustment, patent-term extension, terminal disclaimers, and transitional rules.[1][2]

Because the patent issued in 1997, its enforceable term would ordinarily have ended by 2015 at the latest in the absence of an unusual extension or later effective filing date. The issue date alone does not establish the precise expiration date.

Is United States Patent 5,635,172 currently enforceable?

The patent should be treated as expired for ordinary commercial-risk screening unless the USPTO Patent Center record shows a specific term adjustment or extension that changes the result. An expired patent cannot support a new United States infringement action for post-expiration conduct.

The patent's historical scope remains relevant for:

Prior-art analysis
Freedom-to-operate opinions concerning continuation patents
Patent-family mapping
Validity analysis of later formulation patents
Interpretation of disclosed sustained-release ophthalmic technologies
Review of historical licensing and product-development strategies

What is the Orange Book status of United States Patent 5,635,172?

No Orange Book listing can be inferred from the patent number or claim text alone.

The Orange Book lists patents submitted by approved NDA holders for approved drug products. A patent is not an Orange Book patent simply because it covers an FDA-regulated pharmaceutical composition. The relevant questions are whether:

The patent was submitted to FDA for a specific approved NDA.
The patent claimed the drug substance, drug product, or approved method of use.
FDA accepted the listing.
The listing remained active before expiration or delisting.

The patent's broad formulation language and historical compounds do not establish an Orange Book listing. The FDA Orange Book and the applicable NDA patent submissions must be checked directly for any historical listing.[3]

Did the patent create FDA regulatory exclusivity?

No. Patent rights and FDA regulatory exclusivity are separate.

A patent may block commercial activity without creating FDA exclusivity. Conversely, FDA approval may be protected by:

New chemical entity exclusivity
New clinical investigation exclusivity
Pediatric exclusivity
Orphan-drug exclusivity
Three-year exclusivity for certain applications

The claimed formulation technology does not, by itself, establish any FDA exclusivity period. The compounds listed in the claims are mostly older active ingredients and do not receive regulatory protection merely because they appear in this patent.

Are Paragraph IV challenges relevant?

A Paragraph IV challenge is relevant only if the patent is listed in the Orange Book for an approved reference-listed drug and an ANDA applicant seeks approval for a product that implicates the listing.[4]

For Patent 5,635,172, the practical relevance appears limited because:

The patent issued in 1997.
Its ordinary term has likely expired.
The claimed drug group contains older compounds.
The patent is not shown by the claim text to be tied to a particular currently marketed reference product.
A Paragraph IV certification generally addresses an unexpired listed patent.

A generic applicant could still encounter formulation patents, method-of-use patents, or continuation patents related to a commercial ophthalmic product. Those later patents would require separate analysis.

Which companies are likely to face infringement risk?

Risk would depend on the formulation, not simply on the therapeutic indication.

Potentially relevant parties include:

Manufacturers of pilocarpine ophthalmic products
Developers of sustained-release pilocarpine formulations
Companies developing clonidine-derived ocular products
Developers using polycarboxylic-acid mucoadhesive systems
Manufacturers using cation-exchange resins to control ocular drug release
Contract manufacturers supplying an infringing formulation

A conventional immediate-release pilocarpine solution would likely lack the claimed sustained-release polymer-resin system. A sustained-release ocular insert, gel, suspension, or viscous drop would require a detailed element-by-element comparison.

What formulations are most likely to fall within the claims?

High-risk formulation profile

A product presents the strongest historical claim overlap if it has all of the following:

Pilocarpine or clonidine as the active agent
Topical ocular administration
Sustained release or prolonged ocular residence
A negatively charged carboxylic-acid polymer
Polymer molecular weight within 50,000 to 6 million
A cation-exchange resin at 0.05% to 10% by weight
pH between 3.0 and 8.5

Lower-risk formulation profile

Risk is materially reduced where the product:

Uses a listed drug but has no cation-exchange resin.
Uses an immediate-release aqueous solution.
Uses a polymer outside the molecular-weight range.
Uses a non-anionic or non-carboxylic mucoadhesive.
Uses resin below or above the claimed concentration.
Has a pH outside the claimed range.
Uses an active ingredient outside the eight-compound group.

The doctrine of equivalents could still matter for insubstantial changes, but the numerical ranges and expressly recited resin limitation create potential prosecution-history and vitiation issues.

How strong is the patent estate?

The patent estate represented by these claims is technically meaningful but commercially narrow.

Factor	Assessment
Chemical composition protection	Limited
Active-ingredient coverage	Limited to eight named compounds
Formulation coverage	Broad within the specified polymer-resin architecture
pH limitation	Broad, approximately 3.0 to 8.5
Resin limitation	Material and potentially difficult to avoid if central to release design
Method-of-use coverage	Limited to topical human intraocular-pressure treatment
Modern glaucoma-product relevance	Low unless a product uses the specified older actives
Current enforceability	Likely none after ordinary patent expiration
Historical technology value	Moderate
Current blocking value	Low absent an unexpired related family member

The absence of a polymer quantity limitation in the supplied claims increases breadth in one respect. The claims require only "an amount" of the anionic mucomimetic polymer. That breadth is constrained by the polymer's chemical and molecular-weight definitions and by the requirement that the overall composition be sustained release.

The claims also do not specify a particular release period, dosage volume, viscosity, particle size, resin identity, or manufacturing process. Those omissions increase theoretical coverage but can create enablement, written-description, indefiniteness, and claim-construction issues depending on the disclosure and prosecution history.

What validity issues would affect enforcement?

Written description and enablement

The patent claims a broad class of drug-polymer-resin compositions. A validity challenge could examine whether the specification describes and enables the full combination of:

Eight active ingredients
A broad polymer class
Molecular weights from 50,000 to 6 million
Resin concentrations from 0.05% to 10%
pH from 3.0 to 8.5

The issue would be whether a skilled formulation scientist could make and use the claimed scope without undue experimentation.

Indefiniteness

Potential terms requiring interpretation include:

"Sustained release"
"Therapeutically effective amount"
"An amount" of polymer
"About" 0.05% to 10%
"About" pH 3.0 to 8.5
"2 to 7 carbon atoms per functional group"

The phrase concerning carbon atoms per functional group is particularly important. Its meaning would likely depend on the patent specification, examples, prosecution amendments, and expert testimony.

Anticipation and obviousness

Prior-art searching should focus on combinations rather than isolated elements. Relevant prior art would include:

Ocular pilocarpine and clonidine formulations
Mucoadhesive ophthalmic polymers
Cation-exchange resin drug-delivery systems
Sustained-release ocular gels and suspensions
Ocular pH-adjusted formulations
Ion-exchange resin complexes for controlled release

A reference disclosing only pilocarpine with a mucoadhesive polymer would not necessarily anticipate the claims if it lacks the cation-exchange resin. A reference disclosing a resin-based controlled-release system would need to disclose the claimed ophthalmic use, active drug, polymer, molecular-weight range, concentration, and pH, either expressly or inherently.

Obviousness risk would increase if the prior art taught combining ocular mucoadhesion with ion-exchange resin release control and identified the claimed active agents.

What patent litigation affects United States Patent 5,635,172?

The supplied information does not establish a reported infringement action, ANDA case, settlement agreement, or license involving Patent 5,635,172. A patent-number search should distinguish:

Litigation naming the patent directly
Litigation involving a continuation or divisional
FDA listing disputes
Contract disputes involving a license
Patent cases citing the patent only as prior art

Because the patent is from the 1990s, any current litigation significance is more likely to arise through a related patent family or historical prior-art use than through direct enforcement of the patent itself.

Are biosimilar risks relevant?

No material biosimilar issue is apparent.

Biosimilars address biologic products. The claims concern small-molecule ophthalmic drugs and formulation components. The relevant competitive threats would be:

Generic ophthalmic solutions
Generic suspensions
Reformulated sustained-release products
505(b)(2) applications
New drug applications for ocular delivery systems

A 505(b)(2) applicant could be more relevant than a biosimilar developer if it sought approval for a modified ocular delivery system containing an older active ingredient.[5]

What generic launch scenarios exist?

Immediate-release generic

A conventional immediate-release generic containing pilocarpine or another listed drug may avoid the claims if it lacks the required sustained-release polymer-resin system.

Sustained-release generic

A sustained-release generic could face formulation-patent risk if the reference product or listed patent required the claimed combination. The applicant would need to assess each element and determine whether a Paragraph IV, Paragraph III, or section viii certification is appropriate, depending on the patent's listing and scope.

505(b)(2) reformulation

A reformulated ocular product could use a different active ingredient, polymer, resin, pH, or release mechanism. FDA approval would not remove independent patent risk from an unexpired related patent.

Non-U.S. launch

The United States patent does not control foreign markets. Patent rights must be assessed separately in each country, including national-phase members, continuations, utility models, and supplementary protection rights where applicable.

How does this patent compare with modern glaucoma patent estates?

Modern glaucoma patent estates generally emphasize:

Prostaglandin analogs
Fixed-dose combinations
Preservative-free formulations
Sustained-release implants
Punctal plugs
Ocular inserts
Biodegradable drug-delivery systems
Device-enabled delivery
New indications and dosing schedules
Manufacturing and particle-engineering methods

Patent 5,635,172 is different. It is an older formulation patent centered on ionically interactive mucoadhesion and controlled release for older adrenergic or cholinergic agents.

Patent category	Patent 5,635,172	Modern glaucoma estate
Active agents	Pilocarpine, epinephrine, clonidine-related compounds	Prostaglandins, beta blockers, carbonic anhydrase inhibitors, rho-kinase inhibitors
Delivery	Sustained-release ophthalmic composition	Implants, inserts, gels, devices, depot systems
Core protection	Polymer plus cation-exchange resin	Molecule, device, formulation, dosing, manufacturing
Regulatory pathway	Historical small-molecule products	NDA, ANDA, 505(b)(2), device-drug combination
Current term	Likely expired	Often includes later unexpired continuations
Biosimilar exposure	None	Relevant only for biologic glaucoma products

What is the commercial significance and revenue exposure?

Direct revenue exposure from Patent 5,635,172 is likely low because the patent's ordinary term has ended or is near the end of any possible adjusted term. The relevant commercial exposure would instead arise from:

A later unexpired patent claiming the same product
A continuation with narrower formulation claims
A license agreement tied to the technology
Trade secrets covering manufacturing or resin processing
Regulatory exclusivity attached to a particular approved product
Product-specific patents not apparent from the two claims supplied

The patent itself would not ordinarily justify a current valuation premium for an ophthalmic product unless a live related patent family member remains enforceable.

Key Takeaways

Patent 5,635,172 claims a sustained-release ophthalmic formulation and treatment method.
The required combination is a listed intraocular-pressure-lowering drug, an anionic carboxylic-acid polymer, a cation-exchange resin, the specified resin concentration, and pH 3.0 to 8.5.
The patent does not claim pilocarpine, clonidine, or the other drugs as chemical compounds.
A conventional immediate-release ophthalmic solution is less likely to satisfy the claims.
The polymer identity, molecular weight, acid-group structure, and interpretation of the carbon limitation are central infringement issues.
The patent appears to have reached the end of its ordinary United States patent term. Direct current enforcement value is therefore likely low.
No Orange Book listing, Paragraph IV litigation, biosimilar issue, license, or settlement can be established from the claim text alone.
Current product diligence should focus on related continuations, FDA-listed patents, 505(b)(2) products, formulation patents, and foreign family members.
The patent remains useful as historical prior art for sustained-release ocular delivery and ion-exchange resin technology.

Frequently Asked Questions

Can a pilocarpine eye drop infringe Patent 5,635,172?

Only if the product also satisfies the sustained-release, anionic-polymer, cation-exchange-resin, concentration, molecular-weight, and pH limitations. An ordinary immediate-release pilocarpine solution would generally lack several required elements.

Does the patent cover brimonidine ophthalmic products?

No. Brimonidine is not one of the eight active agents listed in the claims. A brimonidine product would not literally satisfy the active-drug limitation.

Does use of a carbomer automatically create infringement risk?

No. Carbomer identity alone is insufficient. The product must also contain a listed active agent and cation-exchange resin and meet the relevant molecular-weight, concentration, pH, and sustained-release limitations.

Can an expired patent block FDA approval?

An expired patent cannot ordinarily block post-expiration commercial activity. FDA approval may still require a patent certification if a related unexpired patent is listed for the reference product.

Is the patent relevant to a new ocular implant?

Possibly, but only if the implant or associated formulation contains every required claim element, including one of the listed drugs, the defined anionic mucomimetic polymer, and cation-exchange resin. A device using a different active ingredient or release mechanism may fall outside the claims.

References

United States Patent and Trademark Office. (n.d.). Patent term adjustment and patent term calculation. https://www.uspto.gov/patents/laws/patent-term-adjustment
United States Patent and Trademark Office. (1997). United States Patent No. 5,635,172.
U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations. https://www.fda.gov/drugs/drug-approvals-and-databases/orange-book
U.S. Food and Drug Administration. (n.d.). Abbreviated new drug application submissions: Patent certifications and exclusivity. https://www.fda.gov/drugs/abbreviated-new-drug-application-anda
U.S. Food and Drug Administration. (n.d.). Applications covered by section 505(b)(2). https://www.fda.gov/drugs/development-approval-process-drugs/FDA-approach-505b2-applications

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Australia	1607495	⤷ Start Trial
Australia	4575389	⤷ Start Trial
Australia	621692	⤷ Start Trial
Australia	688854	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Summary for Patent: 5,635,172