US Patent 6,399,632: Scope, Claim Construction, Expiration, and Fexofenadine Patent Landscape

US Patent 6,399,632 protects methods of administering fexofenadine, the active acid metabolite of terfenadine, to patients who are poor terfenadine metabolizers or who receive drugs that inhibit terfenadine metabolism. The claims target prevention of terfenadine-associated QT prolongation and ventricular tachycardia. They do not broadly claim fexofenadine as a compound, a general antihistamine, or every use of Allegra.

The patent is a method-of-treatment patent. Its commercial importance was linked to the withdrawal and replacement of terfenadine products, including Seldane, with fexofenadine products, including Allegra. The patent term has expired, and generic fexofenadine products are commercially available.

What drug and clinical problem does US Patent 6,399,632 cover?

The claimed compound is fexofenadine, chemically identified in the claims as:

4-[1-hydroxy-4-[4-(hydroxydiphenylmethyl)-1-piperidinyl]butyl]-α,α-dimethylbenzeneacetic acid

Fexofenadine is the carboxylic-acid metabolite of terfenadine. Terfenadine is metabolized primarily through CYP3A4 to fexofenadine. Patients with impaired metabolism, or patients receiving CYP3A4 inhibitors, can accumulate terfenadine. That exposure was associated with QT prolongation, torsades de pointes, and other ventricular arrhythmias.

US 6,399,632 addresses the therapeutic substitution of fexofenadine for terfenadine in that risk population. The patent therefore links three factual elements:

A histamine-mediated condition requiring antihistaminic treatment.
Abnormal or inhibited terfenadine metabolism.
Administration of fexofenadine to provide antihistaminic benefit without the cardiac liability associated with terfenadine.

The patent does not require that the patient previously received terfenadine in every independent claim. Claims 1 and 5 require that the patient be one in whom terfenadine is not metabolized at the normal rate. Claim 9 adds QT prolongation and/or ventricular tachycardia risk. Claim 10 addresses a patient who has received a product that inhibits terfenadine metabolism.

What are the independent claims in US Patent 6,399,632?

The patent contains four principal independent method claims.

Claim	Core subject matter	Additional limitation
1	Treating a histamine-mediated condition with fexofenadine in a patient with impaired terfenadine metabolism	Avoidance of terfenadine-associated cardiac arrhythmias
5	Treating a histamine-mediated condition with fexofenadine in a patient with impaired terfenadine metabolism	No express “avoiding arrhythmia” limitation
9	Providing an antihistaminic effect with fexofenadine	Patient is subject to QT prolongation and/or ventricular tachycardia with terfenadine
10	Providing an antihistaminic effect with fexofenadine	Human received a product that inhibits terfenadine metabolism

Claims 2 through 4 depend from claim 1. Claims 6 through 8 depend from claim 5.

Claims 1 and 5 are the broadest practical claims because they do not impose the dose limitations in claims 2, 3, 6, and 7. Claim 5 is potentially broader than claim 1 because it does not expressly recite avoiding cardiac arrhythmias. In litigation, however, the “impaired terfenadine metabolism” limitation remains central.

How should the fexofenadine compound limitation be construed?

The claims require administration of fexofenadine or a pharmaceutically acceptable salt. The claim language covers:

Fexofenadine free acid.
Fexofenadine hydrochloride.
Other pharmaceutically acceptable salts, subject to ordinary claim-construction principles.
Pharmaceutical compositions containing the compound, where the carrier limitation is met.

The compound identity is not limited to a particular brand, manufacturer, tablet, capsule, or dosage form. Claims 4 and 8 expressly recite administration with a pharmaceutically acceptable carrier, but those dependent claims do not expand the scope of claims 1 and 5.

The patent does not claim:

Terfenadine itself.
A CYP3A4 inhibitor.
A specific inhibitor such as ketoconazole, erythromycin, clarithromycin, or grapefruit-derived products.
A particular tablet coating or dissolution profile.
A sustained-release formulation.
A manufacturing process for fexofenadine.
A general method of treating allergic rhinitis in every patient.

What patient population is required by the claims?

The patient limitation is the principal narrowing feature.

Patients with abnormal terfenadine metabolism

Claims 1, 5, and 9 require a patient in whom terfenadine is not metabolized at the normal rate. This language can encompass a pharmacokinetic phenotype, drug interaction, disease state, or other circumstance that reduces conversion of terfenadine to fexofenadine.

A patient does not necessarily need to have a laboratory-confirmed genetic polymorphism. The claim language focuses on the metabolic result rather than a specific genotype.

Patients exposed to metabolism inhibitors

Claim 10 is directed to a human who has received a product that inhibits terfenadine metabolism. The claim is therefore interaction-focused rather than purely phenotype-focused.

Potential examples include products that inhibit CYP3A4 sufficiently to increase terfenadine exposure. The claim does not identify a closed list of inhibitors. Whether a particular product falls within the claim would depend on evidence that it inhibits terfenadine metabolism in the relevant clinical context.

Patients at cardiac risk

Claim 9 is narrower than claim 5 because it requires that the patient be subject to QT prolongation and/or ventricular tachycardia when using terfenadine. Evidence concerning ECG changes, arrhythmia risk, clinical history, or drug-interaction exposure could be relevant to this limitation.

What dosage ranges are protected?

Claims 2, 3, 6, and 7 define dosage ranges:

Claims	Daily dose
2 and 6	About 20 mg to about 800 mg per day
3 and 7	About 40 mg to about 360 mg per day

The ranges are broad relative to conventional fexofenadine dosing. Commercial adult products commonly use 60 mg twice daily, 120 mg once daily, or 180 mg once daily, depending on the indication and product labeling.

The word “about” introduces ordinary numerical flexibility, but it does not eliminate the need to establish that the administered dose falls within the claimed range. A product used at a dosage outside the range could still implicate claims 1 or 5 if all other limitations are satisfied.

What is the scope of the “avoid arrhythmias” limitation?

Claim 1 recites administration while “avoiding the concomitant liability of cardiac arrhythmias associated with the administration of terfenadine.” This language describes the therapeutic purpose and outcome associated with replacing terfenadine with fexofenadine.

The limitation has two possible litigation effects:

It may narrow claim 1 compared with claim 5 if the court treats avoidance of arrhythmia as a separate required result.
It may be construed in light of the specification as explaining the clinical advantage of fexofenadine rather than requiring proof that an arrhythmia would otherwise have occurred in the particular patient.

Claim 5 omits that express avoidance language. For infringement analysis, claim 5 is therefore the more important independent claim when the accused use involves impaired terfenadine metabolism but does not document a specific arrhythmia-avoidance result.

How does claim 10 differ from claims 1, 5, and 9?

Claim 10 is directed to a patient who “has also received a product which inhibits terfenadine metabolism.” It is distinct in three respects:

It uses a prior exposure to an inhibitor as the patient-selection criterion.
It does not expressly require the phrase “terfenadine is not metabolized at the normal rate.”
It recites a human receiving an antihistaminically effective amount of fexofenadine.

The issued claim contains apparent typographical and chemical-nomenclature irregularities, including the expression “4-[-1-hydroxy” and mismatched parentheses in the compound name. The legal effect of those errors depends on the intrinsic record, prosecution history, correction mechanisms, and claim-construction principles. The errors do not automatically eliminate the claim, but they create a greater interpretive risk than the cleaner claims 1 and 5.

What formulations are protected by US Patent 6,399,632?

The patent protects administration of fexofenadine in a pharmaceutical dosage form when the relevant method and patient limitations are met. Claims 4 and 8 expressly cover administration with a pharmaceutically acceptable carrier.

The claims are not directed to a particular formulation technology. They do not require:

A specific excipient.
A specific tablet hardness.
A particular dissolution profile.
An orally disintegrating tablet.
A liquid suspension.
A sustained-release matrix.
A specific particle size.
A coating or capsule shell.

A generic fexofenadine tablet could technically fall within a method claim only if the use is directed to the claimed patient population. The composition itself is not the claimed subject matter.

What patents protect fexofenadine and Allegra?

The relevant fexofenadine estate historically included compound, method-of-use, formulation, and regulatory patents. The principal categories are as follows.

Patent category	Representative subject matter	Commercial relevance
Compound patents	Fexofenadine and related antihistaminic compounds	Foundational protection for the active ingredient
Method patents	Treating allergic disorders and replacing terfenadine in at-risk patients	Relevant to prescribing and label-directed use
Formulation patents	Oral dosage forms and product presentations	Relevant to branded product versions
Process patents	Preparation, purification, crystallization, or salt formation	Potential manufacturing barriers
Regulatory listings	Orange Book patents associated with Allegra products	Relevant to ANDA certifications and patent litigation

US Patent 4,929,605 is widely associated with fexofenadine and related antihistaminic compounds. Its term expired before the term of US 6,399,632. The older compound protection therefore did not permanently block generic fexofenadine entry.

US 6,399,632 is narrower than a compound patent because it covers a defined treatment method. Its value depended on whether a generic product was marketed for the claimed patient population and whether the claims remained enforceable during the relevant period.

When did US Patent 6,399,632 lose exclusivity?

US Patent 6,399,632 has expired. Its effective term ended in the late 2010s, based on the ordinary 20-year patent-term framework applicable to the relevant application family and any recorded patent-term adjustment.

The patent therefore does not presently provide an enforceable exclusion against ordinary generic fexofenadine marketing. The precise term calculation should be taken from the USPTO Patent Center record, including the earliest effective nonprovisional filing date and any patent-term adjustment or disclaimer. The patent’s expiration is separate from FDA regulatory exclusivity.

Fexofenadine generic entry occurred after the expiration or resolution of the principal barriers protecting Allegra. The active ingredient is now available from multiple generic manufacturers.

What was the FDA and Orange Book status of Allegra?

Fexofenadine received FDA approval as a prescription antihistamine and later became available over the counter. Allegra products were approved for histamine-mediated conditions including allergic rhinitis and chronic idiopathic urticaria, subject to the specific product labeling.

The relevant FDA pathways were:

NDA approval for branded fexofenadine products.
ANDA approval for generic fexofenadine products.
OTC switching and nonprescription labeling for later Allegra products.

Orange Book relevance was greatest while listed patents or exclusivity periods remained active. A patent listed for an Allegra product could trigger a Paragraph IV certification by an ANDA applicant. After patent expiration, the listing no longer creates a live patent-based barrier to approval or launch.

Because US 6,399,632 is a method patent, its practical Orange Book effect depended on whether it was listed against the relevant approved drug product and whether the proposed generic labeling carved out the patented use. FDA’s patent-listing decisions and current Orange Book entries should be distinguished from the historical scope of the issued patent. [2]

Which companies challenged Allegra and fexofenadine patents?

Generic competition came from the major ANDA manufacturers, including companies such as Mylan, Teva, Barr, Ivax, Ranbaxy, and related generic entities involved in fexofenadine development or litigation activity.

The principal challenge mechanisms were:

Paragraph IV certifications asserting that listed patents were invalid, unenforceable, or not infringed.
Paragraph III certifications awaiting patent expiration.
Labeling carve-outs excluding patented indications.
Post-expiration ANDA launches.

The relevant litigation record must be separated by patent number and product. A challenge to a formulation patent does not necessarily invalidate or defeat a method patent. Conversely, expiration of the compound patent did not automatically eliminate method-of-use or formulation claims.

What Paragraph IV risks did generic fexofenadine applicants face?

A Paragraph IV certification against a listed fexofenadine patent could expose the ANDA applicant to patent litigation under the Hatch-Waxman Act. The risk depended on:

Whether US 6,399,632 was listed for the relevant Allegra product.
Whether the ANDA label instructed use in patients covered by the claims.
Whether the generic applicant proposed a section viii statement or therapeutic carve-out.
Whether the patent was valid and enforceable.
Whether the claims were infringed by the proposed product labeling and marketing.

For a method claim such as claim 5, inducement theories would generally focus on the generic label, promotional materials, and foreseeable use. A generic manufacturer does not directly administer the drug to patients, so the commercial litigation theory would commonly involve induced infringement rather than direct method performance by the manufacturer.

After expiration, Paragraph IV risk under this patent ceased to be commercially material.

How strong was the patent estate for fexofenadine?

The estate was strongest during the period when compound protection, regulatory exclusivity, and method or formulation patents overlapped. Its strength declined as follows:

Period	Estate position
Before compound-patent expiration	Strongest exclusivity position
After compound-patent expiration	Generic entry risk increased; secondary patents became more important
During method/formulation patent term	Product-specific and indication-specific litigation remained possible
After US 6,399,632 expiration	This patent no longer created a launch barrier

US 6,399,632 had meaningful historical value because it addressed a clinically important safety distinction between terfenadine and fexofenadine. Its limitations, however, were substantial:

It was limited to a defined patient population.
It did not claim the fexofenadine molecule itself.
It did not claim every antihistaminic use.
Several claims depended on clinical or pharmacokinetic facts that may be difficult to prove.
The patent did not create a durable manufacturing barrier.

The patent was therefore more useful as a targeted method-of-use asset than as a broad platform patent.

What manufacturing and intellectual-property barriers affected generic entry?

Generic manufacturers faced several potential barriers:

Active pharmaceutical ingredient

The compound patent and related process rights were the primary historical barriers. Once those rights expired, fexofenadine API manufacturing became substantially more accessible.

Salt and solid-state properties

Fexofenadine hydrochloride, polymorphs, crystallization conditions, particle properties, and purification methods could support narrower process or formulation patents. Such rights would need separate claim-by-claim analysis and would not be established merely by US 6,399,632.

Formulation and labeling

Product-specific formulation patents could affect tablets, suspensions, extended-release products, or pediatric presentations. A generic applicant could reduce risk through a different formulation or a permitted labeling carve-out.

Method-of-use claims

US 6,399,632 created a risk only for uses directed to patients with impaired terfenadine metabolism or exposure to metabolism inhibitors. A standard generic label for ordinary allergic rhinitis treatment would not necessarily practice those limitations.

Is there biosimilar risk for fexofenadine?

No. Fexofenadine is a chemically synthesized small molecule, not a biologic. Generic competition proceeds through the FDA ANDA pathway, not the biosimilar pathway under the Public Health Service Act.

The relevant competitive issues are generic substitution, ANDA approval, Paragraph IV certifications, therapeutic carve-outs, API sourcing, and formulation differentiation.

How does fexofenadine compare with competing antihistamines?

Drug	Active ingredient	Product class	Generic pathway	Patent risk today
Allegra	Fexofenadine	Second-generation antihistamine	ANDA	Historical patents expired
Claritin	Loratadine	Second-generation antihistamine	ANDA	Historical patents expired
Zyrtec	Cetirizine	Second-generation antihistamine	ANDA	Historical patents expired
Clarinex	Desloratadine	Active metabolite of loratadine	ANDA	Historical patents expired
Xyzal	Levocetirizine	Enantiomeric antihistamine	ANDA	Historical patents expired

Fexofenadine competes primarily on non-sedating use, duration of action, dosing convenience, and consumer familiarity. Its originator patent strategy was distinct because the molecule was positioned as the safer antihistaminic replacement for terfenadine.

What litigation and settlement issues remain relevant?

Historical litigation involving Allegra or fexofenadine should be assessed by:

Patent number.
NDA and product involved.
ANDA filer.
Filing date and Paragraph IV notice date.
Asserted claims.
Settlement or license terms.
Authorized-generic provisions.
Agreed launch date.
Whether the case ended in dismissal, judgment, or settlement.

A settlement involving a different Allegra formulation or method patent does not establish the status of US 6,399,632. Likewise, an authorized-generic arrangement can alter commercial launch timing without changing patent validity.

For current diligence, the decisive fact is that US 6,399,632 is expired. Historical settlements may explain why generic entry occurred when it did, but they do not create current exclusivity.

What generic launch scenarios existed under US 6,399,632?

During the patent term, four principal launch strategies were available.

Wait-and-launch: File an ANDA with a Paragraph III certification and launch after patent expiration.
Paragraph IV challenge: Assert invalidity, unenforceability, or noninfringement and seek the 180-day first-filer advantage where applicable.
Section viii carve-out: Exclude the claimed at-risk population or patented use from the labeling, if FDA requirements permitted the carve-out.
License or settlement: Obtain an agreed launch date, license, or commercial arrangement with the patent owner.

The most important risk variable was labeling. A generic label that instructs clinicians to use fexofenadine in patients exposed to terfenadine-metabolism inhibitors would create more direct method-of-use exposure than a label limited to ordinary allergic rhinitis.

Key Takeaways

US 6,399,632 is a method-of-treatment patent covering fexofenadine use in patients with impaired terfenadine metabolism or exposure to terfenadine-metabolism inhibitors.
Claims 1, 5, 9, and 10 are the principal independent claims.
Claims 2, 3, 6, and 7 add daily dose ranges of approximately 20 to 800 mg and 40 to 360 mg.
Claims 4 and 8 add a pharmaceutically acceptable carrier.
The patent does not claim fexofenadine as a compound or every use of Allegra.
Claim 5 is potentially the broadest independent claim because it omits the express arrhythmia-avoidance language found in claim 1.
Claim 9 is narrower because it requires QT prolongation and/or ventricular tachycardia risk with terfenadine.
Claim 10 focuses on prior receipt of a product that inhibits terfenadine metabolism.
The patent has expired and does not create a current barrier to generic fexofenadine entry.
Fexofenadine is a small molecule and faces generic, not biosimilar, competition.
Current commercial diligence should focus on any separate API, polymorph, formulation, process, labeling, or regulatory rights rather than US 6,399,632 itself.

FAQs About US Patent 6,399,632 and Fexofenadine

Does US Patent 6,399,632 cover all Allegra products?

No. It covers specified methods of administering fexofenadine to patients with impaired terfenadine metabolism or related cardiac-risk circumstances. It does not claim every Allegra product or every fexofenadine use.

Can a generic fexofenadine manufacturer infringe claim 5?

A manufacturer would face method-of-use risk primarily if its labeling or conduct induced administration to the claimed patient population. Ordinary treatment of patients without the specified terfenadine-metabolism condition would not necessarily meet claim 5.

Is fexofenadine hydrochloride covered by the patent?

Yes, the claims expressly cover fexofenadine and pharmaceutically acceptable salts. The patent does not, however, claim the salt as a standalone composition independent of the claimed treatment method.

Did the patent protect the conversion of terfenadine into fexofenadine?

No. The claims cover administration of fexofenadine. They do not claim a process for metabolizing terfenadine or manufacturing fexofenadine.

Does FDA approval establish infringement of US 6,399,632?

No. FDA approval and patent infringement are separate inquiries. Approval depends on the FDA regulatory pathway; infringement depends on whether the product, labeling, and conduct satisfy every limitation of an enforceable claim.

References

United States Patent and Trademark Office. (2002). US Patent No. 6,399,632, methods of using fexofenadine. https://patents.google.com/patent/US6399632
U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations, Orange Book. https://www.accessdata.fda.gov/scripts/cder/ob/
U.S. Food and Drug Administration. (1996). FDA approves Allegra (fexofenadine hydrochloride) for seasonal allergic rhinitis. FDA drug approval materials.
U.S. Food and Drug Administration. (1997). FDA announces withdrawal of Seldane and Seldane-D following availability of alternatives. FDA safety communications.
United States Patent and Trademark Office. (1990). US Patent No. 4,929,605, antihistaminic compounds. https://patents.google.com/patent/US4929605

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	194913	⤷ Start Trial
Australia	3973493	⤷ Start Trial
Australia	679910	⤷ Start Trial
Canada	2134211	⤷ Start Trial
Chile	2004000304	⤷ Start Trial
Germany	69320952	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 6,399,632

Summary for Patent: 6,399,632