United States Patent 6,703,408: N-Formyl Paroxetine Claims, Scope, Expiration, and Patent Landscape

US 6,703,408 protects isolated N-formyl paroxetine, specified stereochemical and purity characteristics, and pharmaceutical compositions containing N-formyl paroxetine alone or with paroxetine or a paroxetine salt. The patent is directed primarily to a defined paroxetine-related chemical entity and compositions containing it, rather than to paroxetine treatment broadly.

The patent term has expired. The claims therefore have historical relevance for freedom-to-operate analysis, invalidity review, patent-family tracing, and assessment of generic paroxetine development, but they do not provide an enforceable exclusionary right in the United States today.[1-3]

What does US Patent 6,703,408 protect?

The patent covers N-formyl paroxetine in isolated form and in compositions containing the compound. Its claim architecture has four principal layers:

Claim group	Subject matter	Commercial significance
Claims 1-4	Substantially pure, isolated N-formyl paroxetine	Protects the chemical compound and selected physical, purity, and stereochemical characteristics
Claims 5-9	Mixtures of N-formyl paroxetine with paroxetine or specified paroxetine salts	Reaches impurity-containing or deliberately blended compositions
Claims 10-11	Compositions with excipients, including calcium phosphate	Covers pharmaceutical dosage-form compositions
Claims 12-15	Pharmaceutical and liquid compositions, including binders, fillers, carriers, and preservatives	Extends coverage to drug-product and formulation contexts

The central independent product claim is claim 1. Claims 5, 12, and 13 are composition claims. The patent does not claim paroxetine generally. It claims a formylated paroxetine derivative and compositions in which that derivative is present.

What is the scope of claim 1 for N-formyl paroxetine?

Claim 1 requires a compound that is:

N-formyl paroxetine;
substantially pure;
isolated; and
within the structural formula identified as formula (1).

The formula is essential to claim interpretation. The supplied claim text does not reproduce the chemical drawing, so the precise position of the formyl group and the complete stereochemical depiction must be taken from the issued patent.[1] A product would need to fall within that structural formula, not merely contain paroxetine and a generic formyl impurity.

“Substantially pure” and “isolated” are material limitations. The claim is not written as a method claim covering every manufacturing process that generates N-formyl paroxetine. It is a product claim directed to the compound as an identifiable and separable substance.

Claim 3 supplies a more specific purity limitation: impurities must not exceed 10%. This generally means the N-formyl paroxetine preparation contains at least approximately 90% of the claimed compound, subject to the patent’s analytical method and definitions.

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

Claim 2: oil form

Claim 2 limits the compound to an oil. This is a physical-state limitation. A crystalline or otherwise solid preparation would not satisfy the claim unless the product also met the ordinary legal meaning of “in the form of an oil” at the relevant time.

The claim may have practical relevance to isolated process intermediates, liquid reaction products, or noncrystalline preparations. It is narrower than claim 1 and does not independently cover every physical form of N-formyl paroxetine.

Claim 3: maximum impurity level

Claim 3 requires no more than 10% impurities. The claim does not, on its face, identify a particular impurity profile. A preparation containing 9% total impurities could fall within the literal limitation if the remaining limitations are met.

The main enforcement issue would be analytical proof. A party would need to establish:

the identity of the isolated compound;
the percentage of N-formyl paroxetine;
the identity and quantity of impurities;
the analytical method used; and
whether the specification defines purity on a weight, chromatographic-area, molar, or other basis.

Claim 4: trans-3S,4R enantiomer

Claim 4 narrows the compound to the trans-3S,4R enantiomer. This is important because paroxetine has stereochemical complexity, and an N-formyl derivative with different relative or absolute stereochemistry would fall outside this dependent claim.

Claim 4 does not necessarily limit claim 1 to that enantiomer. Unless the formula in claim 1 independently contains the same stereochemical restriction, claim 1 may have broader stereochemical scope.

What mixtures are covered by claims 5 through 9?

Claim 5 covers a composition containing:

N-formyl paroxetine; and
0.1% to 99.97% of a paroxetine compound,

with the percentage calculated from the combined weight of N-formyl paroxetine and the paroxetine compound.

Claim 9 defines “paroxetine compound” as one of:

paroxetine;
paroxetine hydrochloride;
paroxetine maleate;
paroxetine acetate; or
paroxetine mesylate.

The percentage ranges create overlapping dependent claims:

Claim	Required paroxetine compound content
Claim 5	0.1% to 99.97%
Claim 6	1% to 99.89%
Claim 7	1% to 99.8%
Claim 8	10% to 99.7%

The percentages are based only on the combined weight of N-formyl paroxetine and the paroxetine compound. Excipients are excluded from that calculation unless the specification requires a different interpretation.

These claims could reach:

paroxetine drug substance containing N-formyl paroxetine;
paroxetine hydrochloride containing the impurity;
laboratory reference mixtures;
process intermediates;
stability samples; and
finished dosage forms, when the relevant composition limitations are met.

The claims do not necessarily cover every paroxetine product with trace N-formyl paroxetine. Claim 5 requires at least 0.1% of a defined paroxetine compound. A composition containing only trace N-formyl paroxetine as an incidental process impurity would still require analysis under the claim language, including whether the composition contains the claimed N-formyl compound in the required form and whether the asserted product falls within the relevant percentage and purity limitations.

What formulation products are protected?

Claims 10 through 15 add conventional pharmaceutical ingredients and dosage-form characteristics.

Claim 10 requires at least one pharmaceutically acceptable excipient. Claim 11 identifies calcium phosphate as an example. Claim 13 covers compositions containing N-formyl paroxetine, a paroxetine compound within the stated range, and one or more binders, fillers, carriers, or preservatives.

Claim 14 narrows claim 13 to compositions containing 0% to 5% paroxetine compound. This range is potentially significant because it covers compositions consisting primarily of N-formyl paroxetine, with little or no additional paroxetine compound.

Claim 15 covers a liquid composition. The claim could reach a solution, suspension, emulsion, or other liquid dosage form, depending on the specification’s definitions and the construction of “liquid form.” It does not automatically cover every liquid containing paroxetine. The composition must include the N-formyl paroxetine and the other limitations inherited from claim 13.

What does claim 12 cover for therapeutic use?

Claim 12 covers a pharmaceutical composition for treating an SSRI-treatable disease or condition. It requires:

an effective amount of a paroxetine agent;
at least one pharmaceutically acceptable excipient; and
a paroxetine agent consisting of N-formyl paroxetine and 0% to 99.97% of a paroxetine compound.

The claim is composition-based, although it includes a therapeutic-purpose limitation. It does not list specific diseases such as depression, panic disorder, obsessive-compulsive disorder, social anxiety disorder, or premenstrual dysphoric disorder. The phrase “SSRI-treatable disease or condition” is broad but may create construction and enablement issues depending on the specification.

The claim also uses “effective amount,” which can create an infringement dispute if the asserted product is not labeled or administered for an SSRI-treatable condition. For an approved paroxetine product, FDA labeling and actual use would be relevant evidence, but the expired status of the patent removes current US enforcement risk.

When did US 6,703,408 lose exclusivity?

US 6,703,408 was issued on March 9, 2004.[1] Its effective patent term ended no later than the applicable 20-year term measured from the earliest effective nonprovisional filing date, subject to patent-term adjustment and any applicable patent-term extension.[2]

Public patent records place the enforceable term in the early 2020s, and the patent should be treated as expired in the United States. The patent is not a current barrier to generic manufacture, formulation, importation, or sale of products that would otherwise fall within the claims.

The patent also does not create current regulatory exclusivity. FDA drug exclusivity is separate from patent rights and is governed by the Federal Food, Drug, and Cosmetic Act.[3]

What is the Orange Book status of US 6,703,408?

US 6,703,408 is not understood to be a current Orange Book-listed patent protecting an approved paroxetine product.

The Orange Book generally lists patents submitted by an NDA holder for an approved drug, including patents claiming the drug substance, drug product, or approved method of use. An impurity or chemical-derivative patent is not automatically Orange Book eligible merely because it relates to the active ingredient.[4]

The patent’s subject matter is also poorly aligned with the principal Orange Book categories:

it does not claim paroxetine hydrochloride as the active drug generally;
it does not claim an approved paroxetine indication in conventional method-of-use language;
it does not claim a marketed tablet or capsule by product-specific composition;
it does not claim a specific approved dosage regimen; and
its central compound is N-formyl paroxetine, not paroxetine itself.

An Orange Book listing would therefore require separate confirmation in the relevant historical and current FDA listings. The patent’s expiration independently removes any present US exclusivity.

Were Paragraph IV challenges likely for this patent?

A Paragraph IV certification would be relevant only if the patent were listed in the Orange Book for an ANDA-referenced paroxetine product. Because this patent is directed to N-formyl paroxetine rather than the approved paroxetine drug product, it was not the ordinary type of patent expected to drive ANDA litigation.

Generic paroxetine companies typically faced the foundational paroxetine compound, salt, polymorph, formulation, and method-of-use patents identified for the relevant NDA, rather than an impurity patent such as US 6,703,408.

A generic manufacturer could still have faced a patent infringement theory if its product literally contained the claimed N-formyl paroxetine in the claimed concentration, purity, physical form, and composition. That would have been a conventional patent dispute, not necessarily a Paragraph IV case.

What patent litigation and settlements affect this patent?

No material current US litigation or settlement is associated with US 6,703,408 in the principal paroxetine generic-entry disputes. The important commercial litigation involved paroxetine product and method-of-use rights held by the originator and challenges by generic manufacturers to those rights.

The patent’s claim set creates several litigation vulnerabilities:

Issue	Potential defense or attack
Formula (1)	Structural noninfringement if the accused impurity is a different formylated species
“Substantially pure”	Lack of the required purity or isolation
“Oil”	Physical-state noninfringement under claim 2
3S,4R stereochemistry	Different stereoisomer or unresolved stereochemical identity
Percentage ranges	Composition outside the specified concentration range
“Paroxetine compound”	Product uses a different salt or derivative
“Effective amount”	No therapeutic use or no relevant pharmaceutical composition
Purity testing	Dispute over assay method and impurity calculation

Invalidity arguments could have focused on anticipation, obviousness, written description, enablement, indefiniteness, and the patentability of a compound characterized primarily as an impurity or process-related derivative. The strength of any such challenge would depend heavily on the patent specification, priority chain, analytical data, and pre-filing disclosures.

How strong is the patent estate for N-formyl paroxetine?

The estate is narrow in subject matter but broad in claim format. Its strongest historical protection was claim 1, assuming the compound and formula were novel and adequately supported. Claims 5, 12, and 13 attempted to extend that protection into drug-substance mixtures and formulations.

The estate had several limitations:

The patent did not control paroxetine itself.
It did not control the approved paroxetine salts generally.
It did not claim every manufacturing route for paroxetine.
It required the presence of N-formyl paroxetine in the claimed form and, for some claims, within specified concentration ranges.
Several terms are potentially fact-intensive, including “substantially pure,” “isolated,” “effective amount,” and “liquid form.”
The patent has expired.

Its present commercial strength is therefore zero as an enforceable US exclusion right, although its disclosure may remain relevant to impurity control, analytical methods, and process-development diligence.

How does this patent compare with the broader paroxetine landscape?

Area	US 6,703,408	Broader paroxetine estate
Active ingredient	N-formyl paroxetine derivative	Paroxetine base and salts
Primary protection	Compound and compositions	Drug substance, polymorphs, formulations, uses, and manufacturing
FDA relevance	Limited and indirect	Directly relevant to NDA and ANDA products
Generic risk	Historical impurity-related risk	Historical Paragraph IV and product-launch risk
Biosimilar risk	None	None; paroxetine is a small molecule
Current enforceability	Expired	Depends on individual patent and jurisdiction
Manufacturing barrier	Analytical and impurity-control issues	Process, solid-state, formulation, and regulatory issues
Commercial impact	Limited to products containing the claimed derivative	Potentially material during originator exclusivity

There is no biosimilar pathway issue because paroxetine is a synthetic small-molecule drug. Generic competition proceeds through the ANDA pathway, not through a biosimilar application.

What generic launch risks remain?

US 6,703,408 does not create a current generic launch barrier. Residual risks are technical and regulatory rather than exclusivity-based:

formation of N-formyl paroxetine during synthesis or storage;
failure to identify or control the impurity;
differences between the generic impurity profile and the reference listed drug;
manufacturing specifications that create an unintended claimed composition;
analytical-method disputes;
foreign patent rights in jurisdictions where related family members had different terms; and
product-liability or FDA quality issues unrelated to patent enforceability.

Geographic analysis must be performed country by country. US expiration does not establish expiration in Europe, Canada, Japan, or other jurisdictions. Patent-family members may have different priority claims, prosecution histories, term adjustments, and lapse dates.

Key Takeaways

US 6,703,408 claims N-formyl paroxetine, not paroxetine generally.
Claim 1 is the principal isolated-compound claim.
Claims 2 through 4 narrow the compound by physical form, purity, and stereochemistry.
Claims 5 through 9 cover mixtures with paroxetine and specified paroxetine salts.
Claims 10 through 15 extend coverage to excipient-containing, therapeutic, and liquid compositions.
The patent is expired and does not provide current US exclusivity.
It is not the type of patent that ordinarily drives present-day Orange Book or Paragraph IV litigation for paroxetine.
There is no biosimilar exposure because paroxetine is a small-molecule drug.
The remaining business relevance is historical, technical, and jurisdiction-specific.

FAQs About US Patent 6,703,408

Does US 6,703,408 cover paroxetine hydrochloride?

No. It covers compositions containing N-formyl paroxetine and may include paroxetine hydrochloride as the defined “paroxetine compound.” It does not claim paroxetine hydrochloride by itself.

Is N-formyl paroxetine a separate active pharmaceutical ingredient?

The patent claims N-formyl paroxetine as a chemical compound and includes therapeutic composition language. The claims do not establish that it became an FDA-approved active pharmaceutical ingredient.

Can a generic manufacturer sell paroxetine after expiration of US 6,703,408?

Yes, expiration of this patent removes its US patent barrier. The manufacturer must still satisfy FDA requirements and avoid any other unexpired patent rights.

Does the patent cover trace N-formyl paroxetine impurities?

Potentially, but only if the product meets all applicable limitations, including the structural identity, isolation or purity requirements, concentration ranges, and composition limitations. Incidental trace contamination is not automatically infringement.

Are there biosimilar applications for N-formyl paroxetine?

No. N-formyl paroxetine and paroxetine are small-molecule compounds. Relevant follow-on products use the generic-drug framework rather than the biosimilar pathway.

References

United States Patent No. 6,703,408, “N-Formyl Paroxetine,” issued March 9, 2004. U.S. Patent and Trademark Office. https://patents.google.com/patent/US6703408B2/en
35 U.S.C. §§ 154, 156. Patent term and patent-term extension provisions. https://uscode.house.gov/
Federal Food, Drug, and Cosmetic Act, 21 U.S.C. § 355. New drug applications, generic applications, and exclusivity. https://uscode.house.gov/
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book. https://www.fda.gov/drugs/drug-approvals-and-databases/orange-book-data-files-referenced-drug-listing-information

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	036860	⤷ Start Trial
Austria	285408	⤷ Start Trial
Australia	2002330771	⤷ Start Trial
Canada	2464327	⤷ Start Trial
Germany	20220955	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 6,703,408

Summary for Patent: 6,703,408