US Patent 5,935,591: Scope, Claims, Expiration and Equine Antiparasitic Patent Landscape

US Patent 5,935,591 covers veterinary methods using nitazoxanide-related 2-benzamido-5-nitro-thiazole compounds to treat equine protozoal myeloencephalitis, Sarcocystis infection, and broader protozoal or helminth infections. Its principal compound is nitazoxanide, also identified in claim 10 as 2-(2'-acetoxy)-benzamido-5-nitro-thiazole. The patent is a method-of-treatment patent, not a composition, manufacturing, or product patent. Its US term expired in December 2017, eliminating current US infringement risk from practicing the claimed methods. [1]

What does US Patent 5,935,591 protect?

The patent protects administration of specified 2-benzamido-5-nitro-thiazole compounds to horses for antiparasitic treatment. The claims require four central elements:

A horse as the treated subject.
A protozoal, helminth, or Sarcocystis-related infection or risk state.
Administration of a covered compound in an amount described as eradicating-effective.
In selected dependent claims, specified dose, treatment duration, particle size, acid excipients, granulating agents, or active ingredients.

The patent does not claim nitazoxanide as a chemical compound in the abstract. It claims the use of the compound in a horse for specified therapeutic purposes.

Claim group	Subject matter	Principal limitation
Claims 1-20	Treatment of equine protozoal myeloencephalitis	Sarcocystis spp.; compound genus; optional formulation limitations
Claims 21-28	Treatment of equine protozoal or helminth infection	Broader disease category; horse limitation remains
Claim 29	Treatment of a horse infected with Sarcocystis spp.	Specific pathogen; compound genus

The independent claims are claims 1, 21, and 29. Claims 1 and 29 are directed to Sarcocystis-related disease. Claim 21 is broader because it covers protozoal or helminth infection, but it still requires treatment of a horse.

Which compounds fall within the patent claims?

The claimed chemical genus is defined by a benzamido-5-nitro-thiazole structure with substituents R1 through R5. At least one substituent must be an acyloxy or hydroxy group. The remaining positions are generally hydrogen, with certain claims permitting an alkoxy or additional hydroxy substituent.

The expressly identified compounds are:

Compound	Patent claim	Commercial or scientific relevance
2-(2'-acetoxy)-benzamido-5-nitro-thiazole	Claim 10 and claim 26	Nitazoxanide
2-(2'-propionyloxy)-benzamido-5-nitro-thiazole	Claim 11 and claim 27	Propionyl analog
2-(2'-hydroxy)-benzamido-5-nitro-thiazole	Claim 12 and claim 28	Tizoxanide or the deacetylated metabolite
Mixture of acetoxy and hydroxy compounds	Claim 20	Nitazoxanide/tizoxanide combination

Claim 20 contains the phrase “acetolyloxy,” while the corresponding chemical designation elsewhere is acetoxy. The claim should be read in the context of the specification, prosecution history, and chemical disclosure rather than in isolation.

How broad is the compound genus?

Claims 1, 21, and 29 do not limit treatment to nitazoxanide alone. They cover one or more compounds meeting the R-group definition. The genus therefore extends to:

Nitazoxanide;
Tizoxanide;
The propionyl analog;
Compounds bearing one or two acyloxy groups;
Compounds bearing one or two hydroxy groups;
Certain compounds with alkoxy substitution.

The genus is narrower than all nitrothiazole antiparasitic compounds because the claims require the benzamido-5-nitro-thiazole core and specified oxygenated substitution. A compound outside that structural definition would not literally satisfy the compound limitation, even if it had the same therapeutic effect.

How do the independent claims differ?

Claim 1: Equine protozoal myeloencephalitis

Claim 1 is directed to treating a horse afflicted with, or at risk of being afflicted with, equine protozoal myeloencephalitis, or EPM. The treatment must use a claimed compound in an amount sufficient to eradicate Sarcocystis spp.

The claim has a disease-specific limitation and a pathogen-specific therapeutic result. A treatment directed solely to a different equine parasite would not fall within claim 1 unless the facts also establish the claimed EPM or Sarcocystis elements.

Claim 21: Protozoal or helminth infection

Claim 21 expands the disease category to a protozoal or helminth infection. It omits the express requirement that the disease be EPM or that the result be eradication of Sarcocystis spp.

Its scope remains limited by:

The horse subject;
The claimed compound genus;
An eradicating-effective amount;
Treatment of an existing protozoal or helminth infection.

Claim 21 is potentially the broadest independent claim in disease coverage, but its broad functional language creates greater validity and enforcement questions than the more pathogen-specific claims.

Claim 29: Sarcocystis infection

Claim 29 returns to a narrower pathogen-specific claim. It requires a horse infected with Sarcocystis spp. and administration of a covered compound.

This claim may be easier to map against a treatment protocol when laboratory, clinical, or label evidence identifies Sarcocystis infection. It does not expressly require EPM, so its potential coverage is not limited to neurologic disease if the infection and other claim elements are present.

What dosage and treatment regimens are protected?

Claims 7, 8, 9, 23, and 24-25 narrow the independent claims by dosage or duration.

Limitation	Claims	Scope
Average dose of 25-200 mg/kg body weight	7, 23	Dose limitation
Treatment duration of 5 days to 3 months	8, 24	Broad duration limitation
Treatment duration of 7-30 days	9, 25	Narrower duration limitation

The dose claims require an average dosage during the treatment period, not necessarily a uniform dose at every administration. The duration claims create narrower fallback positions. A protocol outside the stated dose or duration may avoid those dependent claims but could still implicate claims 1, 21, or 29 if the independent claim elements are met.

The claims use “amount sufficient to eradicate” or “eradicating effective amount.” That language makes efficacy part of the infringement analysis. Mere administration of nitazoxanide to a horse would not automatically establish infringement if the treatment was not directed to the claimed infection or was not performed in an amount capable of meeting the claimed therapeutic result.

What formulation patents and particle-size limitations are covered?

Claims 13-19 add a formulation and particle-engineering subestate. These claims do not independently cover a veterinary formulation. They depend on the underlying treatment claims.

Claim	Formulation limitation
13	Mean particle size of 10-200 micrometers
14	Mean particle size of 20-50 micrometers
15	Less than 10% of solid particles larger than 100 micrometers
16	Pharmaceutical composition containing a pharmaceutically acceptable acid
17	Specific acid list
18	Acid-to-solid-particle weight ratio of 0.01-0.5
19	Granulating agent, including polyvinylpyrrolidone, water, alcohol, sucrose, hydroxycellulose, or mixtures

These limitations appear designed to address delivery, dispersion, dissolution, or bioavailability of poorly water-soluble nitrothiazole compounds. They can materially narrow the claims against a competing formulation.

The formulation claims do not cover every oral dosage form containing nitazoxanide. A competing product would need to satisfy the applicable particle-size, acid, ratio, and granulation limitations, in addition to the treatment and horse limitations.

When did US Patent 5,935,591 expire?

US Patent 5,935,591 issued on August 10, 1999. Its effective US filing date was in December 1997, producing a standard 20-year patent term that ended in December 2017. The patent is expired and cannot presently block a US equine treatment, veterinary formulation, or commercial launch based solely on this patent. [1]

Event	Date
US filing or national-stage entry	December 1997
Patent issued	August 10, 1999
Expected statutory expiration	December 2017
Current status	Expired

The expiration analysis concerns the patent identified by US Patent No. 5,935,591. Separate patents, regulatory exclusivities, trademarks, state veterinary-practice rules, or later patents would require independent review.

What is the Orange Book status of US Patent 5,935,591?

US Patent 5,935,591 is not an Orange Book patent for nitazoxanide. The patent claims an equine veterinary method, while the Orange Book covers approved human drug products and patent/exclusivity information submitted for those products. The FDA approved nitazoxanide under the brand Alinia for treatment of diarrhea caused by Giardia lamblia or Cryptosporidium parvum, not for equine EPM or general equine parasitic disease. [2]

Animal-drug patent information is not managed through the Orange Book in the same manner as human drug listings. Veterinary products are reviewed through the FDA Center for Veterinary Medicine and recorded through animal-drug approval systems, including the Green Book. [3]

Does nitazoxanide have FDA approval for equine EPM?

No. FDA approval of nitazoxanide concerns human indications. It does not establish FDA approval for equine EPM, Sarcocystis infection, or equine helminth disease. Use in horses would therefore need to be analyzed under the applicable veterinary prescribing and extralabel-use framework, including the Animal Medicinal Drug Use Clarification Act and 21 C.F.R. Part 530. [4]

Are there Paragraph IV challenges to this patent?

No current Paragraph IV challenge has practical relevance because US Patent 5,935,591 expired in 2017. Paragraph IV certification is an ANDA mechanism directed to unexpired Orange Book-listed patents. This patent is neither a current Orange Book-listed human-product patent nor an unexpired patent.

The patent could have been challenged during its enforceable term through:

A declaratory-judgment action;
An invalidity defense in infringement litigation;
A reexamination proceeding;
A pre- or post-grant administrative challenge, depending on the applicable statutory regime.

No continuing US exclusivity remains under this patent.

What patent litigation affects US Patent 5,935,591?

The patent’s commercial importance is historical rather than current. No active infringement litigation is required to resolve the present freedom-to-operate question because the patent has expired.

The most relevant litigation issues during its term would have included:

Whether nitazoxanide administration to horses satisfied the horse-treatment limitation.
Whether the treated condition was EPM, Sarcocystis infection, or another protozoal or helminth infection.
Whether the administered product fell within the chemical genus.
Whether the dose was eradicating-effective.
Whether a formulation met the particle-size and acid-excipient limitations.
Whether the broad genus and broad disease language were enabled across the full claim scope.

The independent claims contain no manufacturing step. A company could therefore avoid direct method infringement by selling a compound without practicing or directing the claimed equine treatment. Risks could still have arisen from induced infringement if marketing materials specifically encouraged performance of the patented method while the patent was enforceable.

How strong was the patent estate?

The patent had moderate historical blocking value and weak current value because of expiration.

Strength factor	Assessment
Claim type	Method claims only
Covered species	Limited to horses
Covered disease	EPM, Sarcocystis, or broader parasite infection depending on claim
Chemical coverage	Broad genus with named nitazoxanide and related compounds
Formulation coverage	Narrow dependent claims
Manufacturing coverage	None
Current enforceability	None after expiration
Biosimilar relevance	None
Generic-drug relevance	Limited; veterinary method use rather than human Orange Book protection
Historical design-around potential	Moderate to high through compound, species, indication, or formulation changes

The strongest historical claims were likely claims 1, 21, and 29 because they did not require the particle-size or excipient features of claims 13-19. The formulation claims were narrower but could have been useful against a product intentionally matching the disclosed particle and acid system.

The principal validity pressure points were the breadth of the compound genus, the breadth of claim 21, and the functional requirement of eradication. Prior disclosure of nitazoxanide as an antiparasitic compound could not necessarily defeat the claims if the equine use and disease-specific application were novel, but it could have supported obviousness arguments when combined with prior EPM or Sarcocystis treatment literature.

How does this patent compare with competing equine EPM products?

The main FDA-approved equine EPM products are based on different active ingredients and are not covered by US Patent 5,935,591.

Product	Active ingredient	EPM relevance	FDA status
Marquis	Ponazuril	Equine EPM treatment	Approved animal drug
Protazil	Diclazuril	Equine EPM treatment	Approved animal drug
Alinia	Nitazoxanide	Human giardiasis and cryptosporidiosis	Human FDA approval; not equine EPM

FDA-approved EPM products have separate regulatory and patent histories. Ponazuril and diclazuril are triazine-based anticoccidial compounds, whereas nitazoxanide is a nitrothiazole benzamide. A competitor using ponazuril or diclazuril would not fall within the compound limitation of Patent 5,935,591. [2, 3, 5, 6]

What generic launch risks exist?

For this patent, patent-based generic launch risk is no longer material in the United States. A sponsor could commercialize an equine nitazoxanide product without infringing an unexpired claim of Patent 5,935,591.

Commercial barriers remain separate from patent barriers:

FDA animal-drug approval requirements;
Demonstration of safety and effectiveness;
Prescription and veterinary distribution controls;
Manufacturing quality systems;
Residue and food-animal considerations, where applicable;
Product liability;
Brand competition from approved EPM products;
Veterinary acceptance and clinical evidence.

A human nitazoxanide generic and an equine nitazoxanide product would also face different regulatory pathways. The existence of an approved human product does not automatically authorize an equine indication.

What biosimilar risk exists?

There is no biosimilar issue. Nitazoxanide is a small-molecule active pharmaceutical ingredient, not a biologic. A follow-on product would be evaluated as a generic or other small-molecule veterinary drug, not under the biosimilar provisions of the Public Health Service Act.

What geographic coverage does the patent provide?

The patent provided protection only in the United States. Foreign family members, if any, required separate validity and expiration analysis in each jurisdiction. The US patent did not create European, Canadian, Australian, or Latin American rights.

Because the US patent expired in 2017, the relevant geographic question is now whether any foreign counterpart remains active. A global launch assessment cannot rely on the US expiration date alone.

What licensing deals are associated with the patent?

The patent itself does not create a current licensing requirement in the United States. Any historical license, collaboration, or assignment involving the patent would not extend its statutory term or revive enforceability after expiration.

Nitazoxanide has been commercially associated with Romark Laboratories and the Alinia brand for human use. That commercial relationship does not establish that a particular equine product is licensed, approved, or covered by this expired patent. [2, 7]

Key Takeaways

US Patent 5,935,591 covers methods of treating horses with nitazoxanide and related 2-benzamido-5-nitro-thiazole compounds.
The principal named compound is nitazoxanide.
Claims 1, 21, and 29 are the core independent claims.
The claims cover EPM, Sarcocystis infection, and, in claim 21, broader protozoal or helminth infections.
Claims 13-19 add particle-size, acid-excipient, ratio, and granulation limitations.
The patent is a method patent and does not claim manufacturing or a standalone veterinary composition.
The US patent term expired in December 2017.
It is not a current Orange Book barrier and has no current Paragraph IV significance.
Nitazoxanide is FDA-approved for certain human infections, not equine EPM.
Ponazuril and diclazuril products compete in the equine EPM market under separate regulatory and patent estates.
No biosimilar pathway applies because nitazoxanide is a small molecule.

FAQs

Can a company sell nitazoxanide for horses after Patent 5,935,591 expired?

Yes, the expired patent no longer blocks US practice of its claimed equine treatment methods. FDA approval and veterinary-drug requirements remain separate issues.

Does treating a horse with tizoxanide infringe the patent?

Historically, tizoxanide was expressly identified in claim 12 and related claims. The patent is now expired, so those claims have no current US exclusionary effect.

Does the patent cover ponazuril or diclazuril?

No. The claims require a 2-benzamido-5-nitro-thiazole compound. Ponazuril and diclazuril are chemically distinct active ingredients.

Are the particle-size claims useful against a modern nitazoxanide suspension?

Only if the patent were enforceable and the product met every applicable limitation, including the claimed particle-size parameters and treatment-use elements. The patent is expired in the United States.

Is an equine nitazoxanide product automatically approved because Alinia is FDA-approved?

No. Alinia’s human approval does not authorize an equine indication or establish approval of an animal-drug product.

References

United States Patent and Trademark Office. (1999). U.S. Patent No. 5,935,591: Treatment of equine protozoal myeloencephalitis. https://patents.google.com/patent/US5935591A/en
U.S. Food and Drug Administration. (n.d.). Alinia: Prescribing information. https://www.accessdata.fda.gov/drugsatfda_docs/label/
U.S. Food and Drug Administration. (n.d.). Green Book: FDA approved animal drug products. https://animaldrugsatfda.fda.gov/adafda/views/
Electronic Code of Federal Regulations. (n.d.). 21 C.F.R. Part 530: Extralabel drug use in animals. https://www.ecfr.gov/current/title-21/chapter-I/subchapter-E/part-530
U.S. Food and Drug Administration. (n.d.). Marquis, NADA 141-199. https://animaldrugsatfda.fda.gov/adafda/views/
U.S. Food and Drug Administration. (n.d.). Protazil, NADA 141-455. https://animaldrugsatfda.fda.gov/adafda/views/
Romark Laboratories. (n.d.). Alinia and nitazoxanide product information. https://www.romark.com/

Details for Patent: 5,935,591

Summary for Patent: 5,935,591