US Patent 4,128,658: Ranitidine Scope, Claims, Expiration, and Patent Landscape

US Patent No. 4,128,658 is the foundational United States compound patent for ranitidine and related histamine H2-receptor antagonists. It was granted to Allen & Hanburys Limited on December 5, 1978, with a priority date of March 4, 1975. The patent expired on December 5, 1995, under the pre-1995 United States patent-term rule of 17 years from grant. Its claims covered a broad Markush class of substituted furan derivatives, specific ranitidine analogs, ranitidine hydrochloride, pharmaceutical compositions, and therapeutic use against H2-receptor-mediated conditions.

The patent no longer creates an enforceable United States exclusivity barrier. Its historical importance remains substantial because claim 45 specifically covers ranitidine hydrochloride, the active pharmaceutical ingredient used in Zantac and generic ranitidine products.

What drug does US Patent 4,128,658 protect?

Claim 45 covers the hydrochloride salt of ranitidine:

N-[2-[[[5-(dimethylamino)methyl-2-furanyl]methyl]thio]ethyl]-N'-methyl-2-nitro-1,1-ethenediamine hydrochloride.

That chemical structure is ranitidine hydrochloride, commonly identified by the following characteristics:

Attribute	Description
Active ingredient	Ranitidine
Salt form	Ranitidine hydrochloride
Therapeutic class	Histamine H2-receptor antagonist
Principal indications	Peptic ulcer disease, gastroesophageal reflux disease, gastric hypersecretory conditions
Original innovator	Glaxo, through Allen & Hanburys
Original brand	Zantac
Core US patent	US 4,128,658
Grant date	December 5, 1978
Nominal expiration	December 5, 1995
Current status	Expired

The patent also covers a large number of structurally related compounds, including thioureas, guanidines, nitroethenediamines, sulfur-linked analogs, methylene-linked analogs, and homologated side-chain variants.

What is the scope of claim 1?

Claim 1 is a broad genus claim covering substituted heterocyclic compounds with H2-receptor antagonist activity. Its scope is defined through several independent structural variables.

Core structural elements

The claim requires:

A substituted heterocyclic aromatic core represented by the formula in the patent.
Two nitrogen substituents, R1 and R2, that may include hydrogen, alkyl, cycloalkyl, alkenyl, aralkyl, or oxygen-interrupted alkyl groups.
A linker containing an alkylene chain, Alk, of one to six carbon atoms.
A terminal functional group represented by Y, including:
- Thiocarbonyl, ═S
- Carbonyl, ═O
- Imino, ═NR5
- Nitroalkene-related, ═CHR6
A heteroatom or methylene connector represented by X:
- Sulfur
- Oxygen
- Methylene
A substituted amino or nitrogen-containing side chain controlled by m and n.
Physiologically acceptable salts, N-oxides, and hydrates.

The claim is drafted as a classic pharmaceutical Markush claim. It does not protect only ranitidine. It covers a chemical genus from which the later specifically claimed compounds are selected.

Breadth of the substituent definitions

Claim 1 gives unusually broad definitions to the principal substituents:

Variable	Permitted scope
R1 and R2	Hydrogen, lower alkyl, cycloalkyl, lower alkenyl, aralkyl, oxygen-interrupted alkyl, substituted aminoalkyl
R3	Hydrogen, lower alkyl, lower alkenyl, alkoxyalkyl
X	CH2, oxygen, or sulfur
Y	Sulfur, oxygen, NR5, or CHR6
Alk	Straight or branched C1-C6 alkylene
R5	Hydrogen, nitro, cyano, alkyl, substituted phenyl, alkylsulfonyl, arylsulfonyl
R6	Nitro, arylsulfonyl, or alkylsulfonyl
m	2, 3, or 4
n	0, 1, or 2, depending on X

The breadth allows multiple pharmacophore arrangements around the same general heterocyclic framework. Claims 2 through 13 progressively narrow these variables.

How do claims 2 through 13 narrow the invention?

Claims 2 through 13 are dependent species-selection claims. They reduce the scope of claim 1 by limiting substituents, linker length, heteroatoms, or chain geometry.

Claim	Principal narrowing
2	Limits R1/R2 to hydrogen, alkyl, phenylalkyl, or dialkylaminoalkyl; restricts Alk to C1-C4
3	Defines lower alkyl as C1-C8
4	Defines alkenyl as C3-C6
5	Requires n + m to equal 3 or 4
6	Requires Alk to be methylene
7	Requires R1 to be hydrogen or C1-C4 alkyl and R2 to be C1-C4 alkyl
8	Requires R1 and R2 to be methyl or ethyl
9	Requires X to be sulfur
10	Requires X to be methylene
11	Narrows R1/R2, Alk, Y, R3, and n + m
12	Further limits X to sulfur or methylene and n to 1
13	Selects the principal ranitidine-related functional groups, including ═NCN, ═NNO2, and ═CHNO2

Claim 13 is particularly relevant to ranitidine because it selects:

Small alkyl groups on the terminal nitrogen atoms;
A methylene linker;
Sulfur or methylene as X;
Nitroethenediamine, nitroguanidine, or cyanoguanidine functionality;
A short alkylene segment;
n = 1 and m = 2.

The dependent claims create fallback positions if the broad genus in claim 1 were challenged. They also establish a progression from broad chemical class to compounds closely associated with the commercial H2 antagonist program.

Which claims specifically cover ranitidine?

Claim 45 is the most direct ranitidine claim. It covers the hydrochloride salt of ranitidine as a defined chemical entity.

Claim 16 also covers the free-base chemical structure of ranitidine:

N-[2-[[[5-(dimethylamino)methyl-2-furanyl]methyl]thio]ethyl]-N'-methyl-2-nitro-1,1-ethenediamine.

The relationship is:

Claim	Subject matter
Claim 1	Broad genus of H2-receptor antagonist compounds
Claim 13	Narrower genus covering selected nitroethenediamine and guanidine compounds
Claim 16	Ranitidine free base
Claim 34	Physiologically acceptable acid addition salts
Claim 35	Hydrochloride salts
Claim 45	Ranitidine hydrochloride specifically

Claims 36 through 41 extend protection from the chemical compound to pharmaceutical compositions. Claims 42 through 44 cover therapeutic use.

What formulations are protected by US 4,128,658?

Claims 36 through 41 cover pharmaceutical compositions containing a compound within claim 1.

Claim	Formulation scope
36	Pharmaceutical composition with active compound and carrier or diluent
37	Oral, topical, parenteral, or suppository administration
38	Oral tablets
39	Slow-release tablets
40	Slow-release tablets containing 20 to 200 mg active ingredient per tablet
41	Topical spray, ointment, or cream

The formulation claims are generic composition claims. They are not limited to a particular excipient, coating system, dissolution profile, manufacturing process, or release mechanism. Their commercial relevance was therefore narrower than the compound claims. A later formulation patent with a specific controlled-release matrix, coating, particle size, or dosing profile could have provided independent protection, but claim 39 alone would not necessarily cover every later sustained-release formulation unless the product met the claim limitations.

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

Claims 42 through 44 cover treatment methods involving histamine H2 receptors.

Claim	Use
42	Treating an H2-receptor-mediated condition
43	Treating peptic ulceration
44	Treating an allergic skin condition

These claims do not identify only ranitidine. They apply to administration of any compound within claim 1. Claim 43 is the principal therapeutic-use claim for the commercial ulcer indication.

The claims predate modern United States method-of-treatment drafting practices. They do not contain the detailed dosing, patient-selection, biomarker, or combination-therapy limitations often used in later pharmaceutical patents.

When did US Patent 4,128,658 lose exclusivity?

The patent expired on December 5, 1995.

Patent-term timeline

Event	Date
Priority filing in the United Kingdom	March 4, 1975
US application filed	1976
US patent granted	December 5, 1978
Pre-1995 patent term	17 years from grant
Expiration	December 5, 1995
Post-expiration status	Public domain

Patent-term adjustment and patent-term extension regimes applicable to later patents were not available in the same form when US 4,128,658 was granted. The patent therefore did not receive the modern 20-year term measured from the earliest effective nonprovisional filing date.

What was the historical Orange Book status?

US 4,128,658 was the principal listed patent associated with ranitidine hydrochloride and Zantac during the product’s protected period. Later Orange Book listings associated with ranitidine included patents directed to related manufacturing, formulation, or product matters.

The Orange Book patent listing was commercially significant during the abbreviated new drug application period. Once the core compound patent expired, generic applicants could rely on the expired patent position and pursue ANDA approval without waiting for the compound claim to lapse further. The relevant regulatory pathway was an ANDA with a Paragraph III certification where an unexpired listed patent remained, or a Paragraph IV certification where the applicant challenged a listed patent as invalid, unenforceable, or not infringed.

Current Orange Book relevance is limited because:

US 4,128,658 is expired.
Ranitidine products were removed from the US market after FDA identified unacceptable NDMA contamination risk.
FDA requested withdrawal of all prescription and over-the-counter ranitidine products in April 2020.
The regulatory issue is now centered on product safety and market authorization rather than the expired compound patent.

(FDA, 2020a, 2020b)

Which companies challenged ranitidine patent protection?

Generic ranitidine competition developed after the core patent expired. Major generic manufacturers included:

Mylan;
Watson Pharmaceuticals, later Actavis;
Sandoz;
Teva;
Dr. Reddy's Laboratories;
Perrigo in relevant over-the-counter channels;
Other ANDA applicants supplying tablets, capsules, and injectable forms.

The principal commercial barrier was the expiration of the compound patent, rather than a durable Paragraph IV litigation campaign against an active basic patent. Generic entry followed the loss of the core patent exclusivity, subject to FDA approval, manufacturing capacity, and later product-quality concerns.

Because US 4,128,658 expired in 1995, a present-day Paragraph IV challenge directed to this patent has no practical litigation value. A Paragraph IV certification is used against an unexpired listed patent. The patent cannot currently block an ANDA.

What related patents formed the ranitidine patent landscape?

The commercial estate around ranitidine extended beyond US 4,128,658. It included related patents directed to compositions, salts, processes, and formulation technology. The most important historical categories were:

Patent category	Business purpose
Basic compound patents	Covered ranitidine and analogs
Salt patents	Covered pharmaceutically acceptable salts and specific salt forms
Process patents	Protected preparation of ranitidine and intermediates
Formulation patents	Covered tablets, injectable products, and modified-release products
Composition patents	Covered dosing forms and excipient systems
Manufacturing patents	Protected scale-up and impurity-control processes

The later estate was more relevant to generic launch timing than the broad analog claims once ranitidine became commercially established. A generic company could avoid some process and formulation claims by using a noninfringing synthesis or dosage-form design, but it could not avoid an unexpired compound claim covering the active ingredient itself.

The key distinction is between the expired foundational compound patent and later patents that may have had shorter, narrower, or product-specific enforcement windows.

How strong was the patent estate?

Chemical scope

The chemical scope was strong for its period. Claim 1 used broad functional definitions and covered multiple H2-antagonist architectures. Claims 14 through 33 provided numerous individual compounds, while claims 16 and 45 directly identified ranitidine and its hydrochloride salt.

Claim layering

The patent had a useful claim hierarchy:

Broad Markush genus;
Intermediate subgenera;
Specific chemical species;
Salt claims;
Pharmaceutical compositions;
Dosage forms;
Therapeutic methods.

That hierarchy reduced dependence on a single claim. Even if a broad genus claim faced prior-art problems, the specific ranitidine and salt claims created narrower fallback positions.

Modern enforceability

The estate has no current blocking strength in the United States because the claims expired. Its remaining value is historical, technical, and freedom-to-operate related only to legacy products or jurisdictions where corresponding patent rights had different expiration dates.

Did biosimilar risk apply to ranitidine?

No. Ranitidine is a small-molecule chemical drug, not a biologic. Biosimilar provisions under the Biologics Price Competition and Innovation Act do not apply.

The relevant competitive pathways were:

ANDA approval under section 505(j) of the Federal Food, Drug, and Cosmetic Act;
505(b)(2) applications for products relying partly on published data but differing in formulation, route, or clinical use;
Full new drug applications for materially different products.

What generic launch scenarios existed?

The commercial launch scenarios were straightforward after expiration:

Immediate post-expiration generic entry

A manufacturer could launch an approved generic ranitidine product after December 5, 1995, assuming no other unexpired patent blocked the relevant dosage form or manufacturing process.

Formulation-specific delay

A generic company using a patented sustained-release design, specific coating, or protected manufacturing process could face a separate patent risk even after expiration of the basic compound patent.

Alternative process launch

A company could reduce infringement exposure by using a distinct synthesis route or independently developed intermediate process. Process patents were more avoidable than a compound patent because the active ingredient itself was no longer protected by US 4,128,658.

Post-2019 market withdrawal

NDMA contamination concerns disrupted the entire ranitidine market. FDA action in 2020 caused manufacturers to withdraw products, making regulatory compliance and product quality more important than patent exclusivity (FDA, 2020a).

What geographic coverage did the patent provide?

US 4,128,658 provided rights only in the United States. Corresponding foreign filings created separate national rights. The commercial ranitidine estate was international, with counterpart patents pursued in major pharmaceutical markets, including Europe, Japan, Canada, and other jurisdictions.

Expiration dates could differ because of:

Different national filing and grant dates;
National patent-term rules;
Patent prosecution delays;
Supplementary protection certificates in Europe;
Patent-term extensions or restoration mechanisms;
Separate process and formulation patents.

A US expiration date cannot be applied automatically to foreign counterparts.

Key Takeaways

US 4,128,658 is the foundational US ranitidine patent.
Claim 45 specifically covers ranitidine hydrochloride.
Claim 16 covers ranitidine free base.
Claim 1 broadly covers a genus of substituted furan H2-receptor antagonists.
Claims 36 through 41 cover pharmaceutical compositions, including tablets and slow-release tablets.
Claims 42 through 44 cover H2-receptor-mediated treatment methods, including peptic ulceration.
The patent granted December 5, 1978, and expired December 5, 1995.
No current US generic company can be blocked by this patent.
Ranitidine is a small molecule, so biosimilar rules do not apply.
Later process and formulation patents historically affected generic risk, but the core compound patent is no longer enforceable.
FDA’s 2020 ranitidine withdrawal action created a regulatory barrier independent of patent protection.

FAQs

Is US 4,128,658 still enforceable?

No. The patent expired on December 5, 1995.

Does claim 45 cover Zantac?

Yes. Claim 45 specifically covers ranitidine hydrochloride, the active pharmaceutical ingredient used in Zantac.

Does the patent cover famotidine or cimetidine?

No. The claims are directed to a defined class of substituted furan derivatives. Famotidine and cimetidine are chemically distinct H2-receptor antagonists and are covered by separate patent estates.

Could a company file a Paragraph IV challenge against US 4,128,658 today?

No practical Paragraph IV challenge is required because the patent has expired. An expired patent cannot prevent FDA approval or commercial launch.

Are ranitidine formulation patents still a commercial barrier?

The historical formulation patents may have had separate expiration dates, but they do not restore exclusivity under US 4,128,658. Current ranitidine commercialization is principally constrained by FDA safety and product-approval issues.

References

Allen & Hanburys Limited. (1978). Furan derivatives and pharmaceutical compositions containing them (U.S. Patent No. 4,128,658). United States Patent and Trademark Office. https://patents.google.com/patent/US4128658A
Food and Drug Administration. (2020a, April 1). FDA requests removal of all ranitidine products (Zantac) from the market. U.S. Department of Health and Human Services. https://www.fda.gov/news-events/press-announcements/fda-requests-removal-all-ranitidine-products-zantac-market
Food and Drug Administration. (2020b). Questions and answers: NDMA impurities in ranitidine (commonly known as Zantac). U.S. Department of Health and Human Services. https://www.fda.gov/drugs/drug-safety-and-availability/questions-and-answers-ndma-impurities-ranitidine-commonly-known-zantac
Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. U.S. Department of Health and Human Services. https://www.fda.gov/drugs/drug-approvals-and-databases/orange-book-data-files
United States Patent and Trademark Office. (2024). Patent term adjustment and patent term extension. U.S. Department of Commerce. https://www.uspto.gov/patents/laws/patent-term-adjustment-patent-term-extension

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
United Kingdom	32465/76	Aug 4, 1976
United Kingdom	50685/76	Dec 6, 1976
United Kingdom	20187/77	May 13, 1977

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	375931	⤷ Start Trial
Austria	381937	⤷ Start Trial
Austria	A322880	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,128,658

Summary for Patent: 4,128,658