United States Patent 4,591,592: Scope, Claims, Expiration, and Thienopyridine Patent Landscape

U.S. Patent No. 4,591,592 protects stabilized pharmaceutical compositions containing acid-addition salts of thienopyridine derivatives, most importantly ticlopidine hydrochloride. The patent does not broadly claim ticlopidine as a chemical entity. Its protection is directed to compositions containing specified nonvolatile organic acids, especially citric acid, together with excipients and, in narrower claims, defined formulation percentages.

The patent issued on May 27, 1986, and, because it was subject to the pre-Uruguay Round patent-term rule, its ordinary term ended 17 years after issuance, on May 27, 2003. It is therefore expired and cannot currently block generic manufacture, sale, or formulation of ticlopidine products in the United States. The patent remains relevant as prior art and as a technical reference for stability strategies involving thienopyridine salts. [1]

What does U.S. Patent 4,591,592 protect?

The patent covers stable pharmaceutical compositions comprising four principal elements:

A therapeutically effective amount of a pharmaceutically acceptable acid-addition salt of a defined thieno-pyridine compound.
A non-toxic, pharmaceutically acceptable, nonvolatile acidic stabilizer.
At least one pharmaceutically acceptable excipient.
In the narrower formulation claims, specified weight percentages of active ingredient, acid, lubricant, disintegrant, binder, and diluent.

The principal commercial compound identified in the claims is ticlopidine hydrochloride:

Chemical description: 5-(2-chlorobenzyl)-4,5,6,7-tetrahydrothieno[3,2-c]pyridine hydrochloride
International nonproprietary name: ticlopidine
Therapeutic class: platelet aggregation inhibitor
Historical U.S. brand: Ticlid
Original U.S. regulatory sponsor: Roche
Later U.S. generic suppliers: multiple manufacturers

The patent’s inventive focus is formulation stability. The claims address degradation or instability of thienopyridine acid-addition salts and identify selected organic acids as stabilizing agents.

What compounds fall within claim 1?

Claim 1 uses a Markush definition covering a family of substituted tetrahydrothienopyridine derivatives. The claimed structure includes:

Claim element	Scope
R	Phenyl or benzyl
R substituents	One to three halogen, C1-C6 alkyl, C1-C6 alkoxy, hydroxy, or nitro groups
R1	Hydrogen, halogen, hydroxy, or C1-C6 alkyl
R2	Hydrogen or halogen
n	1 or 2
Salt form	Pharmaceutically acceptable acid-addition salt
Stabilizer	Ascorbic, benzoic, citric, fumaric, or tartaric acid
Dosage form	Any pharmaceutical composition meeting the claimed elements

The claim is composition-specific rather than compound-specific. A product containing one of the covered salts without the claimed stabilizing acid would not satisfy claim 1. Conversely, a formulation containing a covered salt and one of the five listed acids could fall within the claim even if it used a different dosage form or excipient system, provided the composition was stable and contained the required elements.

The term “stable” may create an evidentiary issue in litigation. The claim does not define stability through a numerical degradation limit, shelf-life period, temperature condition, or analytical method. A patent owner would need to establish that the accused formulation meets the ordinary technical meaning of a stable pharmaceutical composition, supported by the specification, formulation data, and relevant pharmaceutical standards.

What is the scope of claim 2?

Claim 2 narrows claim 1 by requiring the acidic stabilizer to be present at 0.5% to 5.0% by weight relative to the active ingredient.

This limitation is measured against the active ingredient rather than necessarily against the total tablet weight. For example, a formulation containing 100 mg of ticlopidine hydrochloride and 1 mg to 5 mg of citric acid could fall within the numerical range, subject to the claim’s other requirements.

Claim 2 excludes formulations using the listed acids below 0.5% or above 5.0% relative to active ingredient. It also excludes formulations using acids outside the five expressly recited stabilizers.

What does claim 3 cover?

As supplied, claim 3 states:

“The composition of claim 3 wherein said compound is…”

This is an apparent dependency error. It likely was intended to depend on claim 2 or claim 1. The substantive limitations identify:

Ticlopidine hydrochloride;
Citric acid as the stabilizer; and
Citric acid at 1% to 1.5% by weight.

If corrected to depend from claim 2, claim 3 would be a narrow species claim covering a ticlopidine hydrochloride composition with citric acid at 1% to 1.5% relative to active ingredient.

The dependency error matters in claim construction. Courts generally interpret claims in view of the specification and prosecution history, but they cannot automatically rewrite an issued claim. Whether claim 3 is enforceable as written would depend on the prosecution record, the nature of the error, and whether the intended dependency is reasonably apparent. Because the patent expired in 2003, the issue has no current blocking effect in the United States.

What does claim 4 protect?

Claim 4 is a composition claim based on a complete formulation profile:

Component	Claimed amount by weight
Thienopyridine acid-addition salt	40% to 90%
Selected organic acid	0.5% to 5%
Lubricant	0.2% to 5%
Disintegrant	5% to 15%
Binder	1% to 5%
Diluent	Remainder

The covered acids remain limited to ascorbic acid, benzoic acid, citric acid, fumaric acid, and tartaric acid.

Claim 4 is broader than claim 5 because it does not require ticlopidine hydrochloride, citric acid, or magnesium stearate specifically. It could encompass other thienopyridine acid-addition salts and any pharmaceutically acceptable lubricant, provided the formulation falls within all stated ranges.

The “remainder” limitation means the diluent occupies the balance after accounting for the other listed components. A formulation with additional unlisted excipients could raise an infringement issue depending on whether the claim is interpreted as permitting additional ingredients and whether the composition still has a diluent constituting the remainder.

What does claim 5 protect?

Claim 5 narrows claim 4 to a formulation containing:

Ticlopidine hydrochloride;
Citric acid;
Magnesium stearate as the lubricant;
The percentage ranges inherited from claim 4.

This is the most commercially specific claim in the supplied set. It targets a conventional tablet formulation using magnesium stearate as lubricant, a disintegrant, a binder, and a diluent.

The claim does not expressly identify a particular disintegrant, binder, or diluent. A formulation could therefore remain within claim 5 while using different excipients for those categories, assuming the ingredient functions technically as the claimed excipient and the formulation satisfies the percentage ranges.

How strong was the patent estate for ticlopidine?

The estate had two distinct layers:

Layer	Protection	Commercial relevance
Compound patents	Thienopyridine chemical entities and salts	Controlled the original active ingredient
Formulation patent	Stabilized compositions using specified organic acids	Addressed shelf stability and product formulation

U.S. Patent No. 4,591,592 was a secondary formulation patent. It did not recreate the basic chemical-entity monopoly. Its practical value would have depended on whether the marketed formulation used one of the five named acids and fell within the claimed concentration ranges.

The relevant distinction is between technical scope and market exclusivity. A formulation patent can cover a commercial product even after the compound patent expires, but only if the marketed composition practices the formulation limitations. Here, the patent’s term ended in 2003, so it no longer created an entry barrier.

The underlying ticlopidine compound patent is generally identified with U.S. Patent No. 4,051,141. That earlier patent addressed thienopyridine derivatives and provided the chemical foundation for ticlopidine development. Its term also expired before or around the beginning of the U.S. generic market for ticlopidine. [2]

When did U.S. Patent 4,591,592 expire?

The patent issued on May 27, 1986. For a U.S. patent filed before June 8, 1995, the applicable term was generally 17 years from grant, subject to statutory adjustments and terminal disclaimers.

Event	Date or status
Patent issued	May 27, 1986
Ordinary 17-year term	May 27, 2003
Current status	Expired
Current blocking value	None in the United States

The patent therefore cannot support a current infringement action against a ticlopidine generic, a reformulated ticlopidine product, or a manufacturer using the claimed excipient architecture.

What was the FDA and Orange Book status?

Ticlopidine hydrochloride was approved in the United States under the Ticlid brand for reduction of thrombotic stroke risk in certain patients. The FDA label warned of serious hematologic adverse events, including neutropenia, thrombotic thrombocytopenic purpura, and aplastic anemia. Those safety concerns reduced ticlopidine’s commercial position after the introduction of clopidogrel. [3]

Ticlopidine tablets were also approved through abbreviated new drug applications. FDA Orange Book records historically included ticlopidine hydrochloride products, but current commercial availability is limited and the drug is no longer a significant branded product. [4]

The Orange Book listing of a product does not itself extend an expired patent. Any patent listing associated with the original product would have lost practical exclusionary effect when the patent term ended.

Were there Paragraph IV challenges?

The major Paragraph IV risk for ticlopidine arose from the expiration of the underlying chemical and formulation patents. Generic applicants could challenge listed patents by asserting that they were invalid, unenforceable, or not infringed.

For U.S. Patent 4,591,592, the central generic arguments would have included:

Noninfringement because the formulation did not contain one of the five listed organic acids.
Noninfringement because the acid concentration fell outside the claimed range.
Noninfringement because the product used a different thienopyridine salt.
Invalidity based on anticipation or obviousness over earlier thienopyridine formulations.
Indefiniteness or claim-construction issues arising from “stable” and the defective dependency in claim 3.
Lack of practical remedy because the patent had already expired.

There is no current Paragraph IV launch risk associated with this patent. Any historical Paragraph IV dispute would be moot as to prospective exclusion once the patent expired.

How does ticlopidine compare with clopidogrel and prasugrel?

Ticlopidine, clopidogrel, and prasugrel are thienopyridine antiplatelet agents, but their patent estates are separate.

Drug	Primary development position	Patent landscape	Current commercial position
Ticlopidine	First-generation thienopyridine	Early compound patents plus formulation patents such as U.S. 4,591,592	Largely displaced by newer agents
Clopidogrel	Second-generation thienopyridine	Separate compound, stereochemical, salt, and formulation patents	Broad generic availability
Prasugrel	Later-generation thienopyridine	Separate compound and product-specific patents	Generic and branded competition
Ticagrelor	Non-thienopyridine P2Y12 inhibitor	Distinct chemical and formulation estate	Competes clinically but does not practice ticlopidine claims

Clopidogrel is not a chemical equivalent of ticlopidine and would not ordinarily fall within claim 3 or claim 5. A formulation containing clopidogrel bisulfate and citric acid would not infringe merely because it uses the same stabilizer. The active ingredient must satisfy the claimed thienopyridine structure.

What formulations are covered by the patent?

The strongest literal coverage is directed to oral solid dosage forms containing:

Ticlopidine hydrochloride;
Citric acid at 1% to 1.5% relative to active ingredient;
Magnesium stearate;
A disintegrant;
A binder; and
A diluent.

Typical tablet excipients could include lactose, microcrystalline cellulose, starch, povidone, sodium starch glycolate, and magnesium stearate. The patent does not require any specific one of these examples except magnesium stearate in claim 5.

Potentially outside the claims are:

Ticlopidine free base without an acid-addition salt;
Ticlopidine formulations stabilized with only a nonlisted acid;
Formulations with no stabilizing acid;
Products using citric acid outside the claimed concentration range;
Products using a different active ingredient outside the Markush structure;
Formulations that do not include the required excipient categories in claim 4 or claim 5.

What manufacturing and IP barriers remain today?

The patent creates no current U.S. manufacturing barrier. Ticlopidine hydrochloride is an established small-molecule active ingredient, and the relevant composition patent is expired.

Remaining barriers are commercial and regulatory rather than patent-based:

Limited demand because of safety concerns;
Physician preference for clopidogrel and other P2Y12 inhibitors;
Manufacturing economics for a low-volume product;
FDA requirements for active-ingredient, dissolution, impurity, and stability controls;
Potential supplier qualification and analytical-method requirements;
Product discontinuation or limited availability by manufacturers.

No biosimilar pathway applies. Ticlopidine hydrochloride is a small molecule regulated through the generic drug framework, principally an abbreviated new drug application, not a biologics license application or biosimilar application.

What is the current generic launch risk?

The current patent-related launch risk is effectively zero because U.S. Patent 4,591,592 expired in 2003.

A historical launch analysis would have focused on three scenarios:

Scenario	Patent exposure
Generic using a different stabilizer	Lower risk under this patent
Generic using citric acid outside the claimed range	Potential noninfringement position
Generic matching ticlopidine hydrochloride, citric acid, and magnesium stearate within the ranges	Highest historical exposure

The patent’s narrowest commercial claim, claim 5, would have been most relevant to a generic that copied the reference product’s excipient profile. It would not have blocked all ticlopidine products.

Key Takeaways

U.S. Patent 4,591,592 is a formulation patent covering stabilized acid-addition salts of thienopyridine derivatives.
Ticlopidine hydrochloride is the principal disclosed and claimed commercial species.
The five permitted stabilizers are ascorbic acid, benzoic acid, citric acid, fumaric acid, and tartaric acid.
Claim 3 contains an apparent dependency error and is directed substantively to ticlopidine hydrochloride with 1% to 1.5% citric acid.
Claim 5 is the narrow commercial formulation claim requiring ticlopidine hydrochloride, citric acid, magnesium stearate, and the inherited excipient ranges.
The patent expired on May 27, 2003.
It has no current Orange Book exclusionary effect, no current Paragraph IV leverage, and no current generic-launch blocking power.
The patent does not cover clopidogrel, prasugrel, or ticagrelor merely because those drugs belong to the broader antiplatelet or P2Y12-inhibitor market.
No biosimilar pathway is relevant because ticlopidine is a small-molecule drug.

FAQs

Does U.S. Patent 4,591,592 claim ticlopidine itself?

No. It claims pharmaceutical compositions containing specified acid-addition salts of a thienopyridine Markush class. Ticlopidine hydrochloride is a specifically identified species, but the patent is not a standalone compound patent.

Can a formulation use citric acid and avoid the patent claims?

Historically, possible design-around positions included using citric acid outside the claimed concentration range, omitting the required excipient combination, or using a different active salt. The patent is now expired, so the question has no current U.S. infringement consequence.

Is claim 3 invalid because it depends on itself?

The wording supplied contains an apparent self-dependency error. The likely intended dependency is claim 2 or claim 1. The legal effect would depend on the issued patent record and claim-construction principles.

Does the patent cover clopidogrel bisulfate tablets?

No, not merely because clopidogrel is an antiplatelet thienopyridine-related drug. The claimed active ingredient must satisfy the specific structural definition, and clopidogrel is a distinct chemical entity.

Is ticlopidine subject to biosimilar competition?

No. Ticlopidine hydrochloride is a small-molecule drug. Competition proceeds through generic-drug pathways, principally abbreviated new drug applications, rather than the biosimilar framework.

References

U.S. Patent No. 4,591,592. (1986). Pharmaceutical compositions containing thieno-pyridine derivatives. United States Patent and Trademark Office.
U.S. Patent No. 4,051,141. (1977). Thieno[3,2-c]pyridine derivatives. United States Patent and Trademark Office.
U.S. Food and Drug Administration. (2001). Ticlid (ticlopidine hydrochloride) prescribing information. FDA.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. FDA.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	22530	⤷ Start Trial
Australia	546492	⤷ Start Trial
Australia	7351881	⤷ Start Trial
Canada	1176170	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,591,592

Summary for Patent: 4,591,592