U.S. Patent 4,525,358: Cetirizine Claims, Patent Scope, Expiration, and Generic Competition

U.S. Patent 4,525,358 is the core compound patent for cetirizine, the antihistamine marketed primarily as Zyrtec. It covers a broad genus of substituted benzhydryl piperazine derivatives, specific cetirizine compounds and salts, pharmaceutical compositions, and therapeutic use for allergic and antihistaminic conditions. The patent issued June 25, 1985, and its original 17-year patent term expired June 25, 2002. FDA pediatric exclusivity associated with cetirizine extended certain market exclusivity rights by six months, but did not extend the patent term itself. [1-4]

What drug does U.S. Patent 4,525,358 protect?

The principal commercial compound is cetirizine, chemically identified in the patent as:

2-[2-[4-[(4-chlorophenyl)phenylmethyl]-1-piperazinyl]ethoxy]acetic acid

Cetirizine is commonly administered as cetirizine hydrochloride or cetirizine dihydrochloride, although product labeling and chemical references often use "cetirizine hydrochloride" for the marketed active pharmaceutical ingredient. The patent claims the free acid, pharmaceutically acceptable salts, specified crystalline forms, compositions, and methods of treatment.

The compound belongs to the second-generation antihistamine class. Its principal approved use is treatment of allergic rhinitis and chronic urticaria. The original U.S. product was Zyrtec, developed and commercialized by UCB. [2, 5]

What is the prosecution and expiration history of U.S. Patent 4,525,358?

Event	Date
Priority and European development period	Early 1980s
U.S. patent application filed	1983
U.S. Patent 4,525,358 issued	June 25, 1985
Original patent term	17 years from issuance
Original expiration date	June 25, 2002
FDA approval of Zyrtec NDA	1995
Pediatric exclusivity period	Six months after relevant exclusivity expiration
Post-expiration generic competition	Began after 2002, subject to FDA approval timing

Because the patent was governed by the pre-Uruguay Round Agreements Act patent-term regime, the term was generally 17 years from grant rather than 20 years from the earliest effective filing date. The patent therefore expired in 2002, absent a patent-term extension or adjustment that materially changed the date. The available Orange Book history identifies the patent as an expired product patent associated with cetirizine. [1, 3, 4]

What compounds are covered by the broad genus claim?

Claim 1 covers a class of compounds defined by a benzhydryl-substituted piperazine connected through an ethoxyacetic-acid-type side chain. The claim permits:

Independent substitution of X and X' with hydrogen, halogen, lower alkoxy, or trifluoromethyl;
One or two carbon units in each relevant chain variable, identified as m and n;
The free acid and non-toxic pharmaceutically acceptable salts.

The broad claim is a Markush claim. It does not cover only cetirizine. It reaches a family of compounds with variation at the two aromatic rings and variation in chain length. The claim also extends to salts, which is commercially important because pharmaceutical products are commonly formulated as salts rather than as the free acid.

The precise boundaries of the Markush structure depend on the chemical drawing associated with the original patent. The text supplied for the claim contains the image placeholder "STR9," so the exact atom-by-atom interpretation of m and n cannot be reconstructed from the text alone. The named dependent claims, however, establish the commercial embodiments covered by the patent.

How do claims 2 through 11 narrow the chemical scope?

Claims 2 through 11 move from the broad genus to specific chain lengths, compounds, salts, and solid-state forms.

Claim	Subject matter	Commercial significance
2	Claim 1 compounds where n is 2	Narrows the linker configuration
3	Cetirizine free acid or salt	Direct compound protection for the marketed active
4	Potassium cetirizine salt	Specific salt form
5	Diphenhydryl-substituted analogue and dihydrochloride	Non-chloro analogue
6	4-fluorophenyl analogue or hydrate	Fluoro analogue and hydrate
7	Cetirizine free acid	Independent specific compound claim
8	Substantially pure crystalline cetirizine, melting point 110-115 °C	Solid-state and purity limitation
9	Pharmaceutically acceptable cetirizine salt	Salt protection
10	Cetirizine dihydrochloride	Specific salt claim
11	Substantially pure cetirizine dihydrochloride, melting point about 225 °C	Purity and physical-form limitation

Claims 7 and 10 are particularly important because they expressly identify the cetirizine free acid and dihydrochloride. Claims 8 and 11 add physical-property limitations. A competitor making cetirizine would not necessarily avoid claims 7 or 10 merely by selecting a different polymorph, particle size, or manufacturing process. Those claims are directed to the compound or salt itself, subject to the ordinary legal analysis of claim construction, purity, and infringement.

What formulation patents are covered by U.S. Patent 4,525,358?

Claims 12 through 22 cover compositions rather than standalone chemical products.

The composition claims include:

A pharmaceutical composition containing a claimed compound or salt;
An antihistaminic composition;
An antiallergic composition;
Cetirizine-specific compositions;
Compositions containing cetirizine dihydrochloride;
Oral dosage-unit forms containing 0.5 to 250 mg of the dihydrochloride.

These claims have narrower practical relevance after expiration of the compound claims. Before expiration, they could provide overlapping protection against formulations containing the claimed active ingredient. After expiration, they no longer independently block ordinary generic cetirizine products unless an unexpired patent or regulatory exclusivity right applies.

Claim 17 is notable because it defines a broad dosage range of 0.5 to 250 mg. It does not claim a particular excipient system, tablet coating, liquid vehicle, release profile, particle size, or manufacturing process. The patent therefore is not a modern formulation patent in the typical sense. Its formulation coverage is composition-based and active-ingredient centered.

What method-of-use claims does the patent contain?

Claims 23 through 31 cover therapeutic administration. The claimed effects include:

Antiallergic activity;
Antihistaminic activity;
Bronchodilator activity;
Antispasmodic activity;
Treatment of allergic symptoms;
Oral administration of cetirizine or its salts.

Claims 24 through 27 narrow the method to cetirizine, its salts, the dihydrochloride, and oral delivery. Claims 28 through 31 address treatment of allergic symptoms using the same compound hierarchy.

These claims were relevant to the original product position because they linked the claimed compounds to therapeutic use. Their current commercial effect is limited by patent expiration. They also would have been subject to statutory and judicial analysis concerning treatment-method infringement, approved labeling, divided infringement, and induced infringement.

When did cetirizine lose patent exclusivity?

Cetirizine lost the primary U.S. patent protection from U.S. Patent 4,525,358 on June 25, 2002.

FDA pediatric exclusivity provided a separate six-month regulatory exclusivity period associated with pediatric studies. Pediatric exclusivity does not extend the patent's expiration date. It can, however, delay FDA approval of certain abbreviated new drug applications or otherwise affect the timing of generic approval under the Hatch-Waxman framework. [3, 4]

Exclusivity type	Status
Compound patent	Expired June 25, 2002
Composition claims in the same patent	Expired June 25, 2002
Method claims in the same patent	Expired June 25, 2002
Pediatric regulatory exclusivity	Six months after applicable expiration
Data exclusivity for the original small-molecule product	Limited statutory period, separate from patent term
Current ordinary generic barrier from Patent 4,525,358	None

What was the Orange Book status of cetirizine?

FDA's Orange Book identifies approved drug products and patent or exclusivity information submitted by sponsors. The cetirizine listing historically associated the innovator product with U.S. Patent 4,525,358. The patent was listed as an expired product patent after the 2002 expiration date. [1]

The Orange Book listing did not mean that every later cetirizine product was exposed to a currently enforceable patent. It identified the patent history relevant to the approved innovator product. Generic applicants could rely on the patent's expiration, subject to other listed patents, regulatory exclusivities, labeling requirements, and ordinary FDA approval conditions.

Which companies challenged cetirizine patent protection?

Generic competition followed expiration of the core patent and the related regulatory exclusivity period. Cetirizine became widely available from multiple generic manufacturers, including major U.S. and international suppliers.

Publicly visible generic competition has included companies such as:

Perrigo;
Teva;
Mylan, later part of Viatris;
Watson, later part of Actavis and Allergan;
Ranbaxy;
Sandoz;
Dr. Reddy's Laboratories;
Major private-label and retail pharmacy suppliers.

The commercial market did not remain dependent on a single Paragraph IV dispute. Once the principal patent expired, competition shifted from patent invalidity and non-infringement litigation to FDA approval, manufacturing scale, retail distribution, branding, and price.

What Paragraph IV issues applied to U.S. Patent 4,525,358?

A Paragraph IV certification asserts that a listed patent is invalid, unenforceable, or not infringed. For an ANDA applicant seeking approval before patent expiration, a Paragraph IV filing could trigger patent litigation under the Hatch-Waxman framework and potentially create 180-day exclusivity for the first qualifying generic applicant. [3]

For cetirizine, the practical end of the core patent barrier was determined principally by the fixed expiration date and associated pediatric exclusivity. The available record does not establish a currently active Paragraph IV dispute involving U.S. Patent 4,525,358 because the patent expired more than two decades ago.

A Paragraph IV challenge today would have no practical target in this expired patent. Any current patent dispute would have to involve a different patent, such as a later formulation, polymorph, combination, manufacturing, or dosage-regimen patent.

How strong was the patent estate for cetirizine?

The estate was strong during its enforceable period because Patent 4,525,358 combined several protection layers in one document:

A broad chemical genus;
Specific cetirizine compound claims;
Salt claims;
Crystalline and purity limitations;
Pharmaceutical composition claims;
Antihistaminic and antiallergic composition claims;
Treatment-method claims;
Oral dosage-unit claims.

The strongest claims were claims 7, 10, and the corresponding dependent composition claims because they directly identified cetirizine or its dihydrochloride. The broad Markush claim had greater theoretical breadth but would have faced closer scrutiny over written description, enablement, anticipation, obviousness, and claim construction.

The estate's principal weakness was temporal. Its core protection expired in 2002, leaving later competitors free to develop and market cetirizine products absent separate unexpired rights.

What manufacturing and intellectual-property barriers remain?

Patent 4,525,358 no longer creates a current manufacturing barrier. A generic manufacturer can produce cetirizine, subject to:

FDA-approved ANDA requirements;
Current good manufacturing practice compliance;
API impurity controls;
Salt formation and crystallization control;
Stability and dissolution specifications;
Bioequivalence requirements;
Labeling restrictions;
Supplier qualification and regulatory inspection.

The patent's crystalline-form and melting-point claims would historically have created process and product-characterization risks. Those risks ended with patent expiration. Current manufacturing risk is primarily regulatory and commercial rather than patent-based.

How does cetirizine compare with competing antihistamines?

Drug	Drug class	Core commercial distinction	Current patent risk from U.S. Patent 4,525,358
Cetirizine	Second-generation H1 antihistamine	Rapid allergy symptom relief; possible sedation in some patients	None; patent expired
Levocetirizine	Enantiomer of cetirizine	Selective active enantiomer product	Not covered as a commercial product by the expired cetirizine patent unless claim scope reaches the relevant stereochemical form
Loratadine	Second-generation H1 antihistamine	Low-sedation positioning	None from Patent 4,525,358
Desloratadine	Active metabolite of loratadine	Separate chemical and patent estate	None from Patent 4,525,358
Fexofenadine	Second-generation H1 antihistamine	Low-sedation positioning	None from Patent 4,525,358

The patent claims the cetirizine structure and related analogues. It does not function as a general patent over the second-generation antihistamine class.

What is the current revenue exposure from U.S. Patent 4,525,358?

The patent no longer protects current U.S. revenue. The commercial value of the patent was concentrated in the period before generic entry. After expiration, Zyrtec faced direct generic substitution and substantial price erosion.

Revenue exposure for a current company would arise from:

Branded cetirizine sales;
Authorized generic arrangements;
Private-label supply;
OTC retail distribution;
Combination allergy products;
Brand equity and channel access.

Those revenue streams are commercial assets, not rights preserved by Patent 4,525,358.

Key Takeaways

U.S. Patent 4,525,358 is the foundational U.S. patent for cetirizine.
It covers a broad Markush genus, specific cetirizine compounds, salts, crystalline forms, pharmaceutical compositions, and therapeutic methods.
Claims 7 and 10 directly target cetirizine and cetirizine dihydrochloride.
Claims 12 through 22 cover antihistaminic and antiallergic compositions, including oral dosage forms.
Claims 23 through 31 cover antihistaminic, antiallergic, bronchodilator, and antispasmodic treatment methods.
The patent issued June 25, 1985, and expired June 25, 2002.
Pediatric exclusivity affected regulatory timing but did not extend the patent term.
The patent presents no current barrier to generic cetirizine manufacture or FDA approval.
Current risk lies in separate later patents, FDA compliance, manufacturing quality, and commercial competition.

FAQs

Does U.S. Patent 4,525,358 cover cetirizine hydrochloride?

Yes. The patent claims cetirizine and non-toxic pharmaceutically acceptable salts, with specific claims directed to cetirizine salt forms, including the dihydrochloride.

Does the patent cover levocetirizine?

The patent is directed to the cetirizine chemical structure and named analogues. Whether a particular stereoisomer is covered depends on the original structural disclosure and claim construction. Patent 4,525,358 is not the principal modern U.S. patent typically associated with commercial levocetirizine protection.

Can a company make generic cetirizine without licensing UCB?

Yes, the core patent expired in 2002. A manufacturer still requires FDA approval and must meet applicable quality, bioequivalence, labeling, and manufacturing requirements.

Did the 0.5-to-250-mg dosage claim remain enforceable after 2002?

No. Claim 17 expired with the patent. The dosage range does not create current patent protection independent of the expired patent.

Are cetirizine polymorphs still subject to patent risk?

Potentially, but not from the expired Patent 4,525,358. Any current risk would have to arise from a separate unexpired patent directed to a particular polymorph, hydrate, formulation, process, or dosage regimen.

References

U.S. Patent and Trademark Office. (1985). U.S. Patent No. 4,525,358, "Piperazine derivatives, process for their preparation and pharmaceutical compositions containing them."
U.S. Food and Drug Administration. (1995). Zyrtec cetirizine hydrochloride NDA approval materials.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations, commonly known as the Orange Book.
U.S. Food and Drug Administration. (2024). Regulatory exclusivity and patent provisions under the Hatch-Waxman Amendments.
UCB, Inc. (1995). Zyrtec cetirizine hydrochloride prescribing information.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
United Kingdom	8103768	Feb 06, 1981
United Kingdom	8110990	Apr 08, 1981

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
European Patent Office	0058146	⤷ Start Trial	2001C/045	Belgium	⤷ Start Trial
European Patent Office	0058146	⤷ Start Trial	SPC/GB01/052	United Kingdom	⤷ Start Trial
Austria	8140	⤷ Start Trial
Australia	544066	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,525,358

Summary for Patent: 4,525,358