United States Patent 5,360,817: Zanamivir Scope, Claims, Expiration, and Patent Landscape

US Patent No. 5,360,817 covers 4-amino and 4-guanidino sialic-acid analogs, including zanamivir, the active ingredient in Relenza. The patent issued on November 1, 1994, and its ordinary US patent term expired on November 1, 2011. The patent is no longer an enforceable barrier to generic zanamivir development. Its commercial importance arose from claims covering zanamivir itself, pharmaceutically acceptable salts, and pharmaceutical formulations, including intranasal formulations.

What drug does US Patent 5,360,817 protect?

The principal commercial compound within the patent is zanamivir, also known as 4-guanidino-Neu5Ac2en.

Item	Description
US patent	5,360,817
Issue date	November 1, 1994
Protected commercial compound	Zanamivir
Chemical name	5-acetamido-4-guanidino-2,3,4,5-tetradeoxy-D-glycero-D-galacto-non-2-enopyranosonic acid
Commercial product	Relenza
Innovator	Biota-related research entities; commercialized through GlaxoSmithKline
Drug class	Neuraminidase inhibitor
Principal indication	Treatment and prevention of influenza A and B
US patent term	Generally 17 years from issue
Expiration date	November 1, 2011
Current status	Expired

Claim 5 expressly covers the zanamivir free acid and its pharmaceutically acceptable salts and derivatives. Claim 6 specifically covers the ammonium salt of zanamivir.

The patent also covers the corresponding 4-amino compound in claims 3 and 4. That compound is structurally related to zanamivir but is not the active ingredient in Relenza.

What are the key claims in US Patent 5,360,817?

The claims fall into four groups: Markush compounds, specific 4-amino compounds, specific 4-guanidino compounds, and pharmaceutical formulations.

Claim 1: Broad compound genus

Claim 1 covers compounds of formula (Ib) with:

A defined sialic-acid-like carbohydrate core;
An R3b substituent that may be an amino-containing group, cyano, or azido group;
An R4b substituent defined as NNHCOR9b;
Various alkyl, aryl, aralkyl, amidine, heterocyclic, and amino substituents;
Pharmaceutically acceptable salts and derivatives.

The claim is a genus claim. It is materially broader than zanamivir because it covers multiple substituent permutations at the R3b and R4b positions.

The breadth is limited by the required chemical scaffold and the specifically defined R3b and R4b substituents. It does not cover every neuraminidase inhibitor or every sialic-acid analog.

Claim 2: Amino and guanidino subgenus

Claim 2 narrows R3b to:

NH2; or
NHC(=NH)NH2.

The second group is the guanidino substituent present in zanamivir. This claim therefore captures the principal amino and guanidino members of the disclosed compound family.

Claims 3 and 4: 4-amino compound

Claim 3 covers:

5-acetamido-4-amino-2,3,4,5-tetradeoxy-D-glycero-D-galacto-non-2-enopyranosonic acid and its pharmaceutically acceptable salts and derivatives.

Claim 4 specifically covers the sodium salt of that compound.

These claims do not directly claim zanamivir because zanamivir has a guanidino group rather than a primary amino group at the 4-position.

Claims 5 and 6: Zanamivir and ammonium zanamivir

Claim 5 covers:

5-acetamido-4-guanidino-2,3,4,5-tetradeoxy-D-glycero-D-galacto-non-2-enopyranosonic acid and its pharmaceutically acceptable salts and derivatives.

This is the core compound claim for zanamivir.

Claim 6 specifically covers ammonium zanamivir, also called ammonium 5-acetamido-4-guanidino-2,3,4,5-tetradeoxy-D-glycero-D-galacto-non-2-enopyranosonate.

Claims 7 through 10: Formulations

Claim 7 covers pharmaceutical formulations containing a compound of claim 1 with a pharmaceutically acceptable carrier.

Claim 8 covers formulations suitable for intranasal administration.

Claim 9 narrows the formulation to the 4-amino compound.

Claim 10 narrows the formulation to the 4-guanidino compound, including zanamivir.

The formulation claims are composition claims. They are not method-of-treatment claims and do not expressly require a particular dose, influenza strain, patient population, or administration schedule.

Does US 5,360,817 cover Relenza?

Yes. Claim 5 covers the active pharmaceutical ingredient in Relenza, and claim 10 covers pharmaceutical formulations containing the 4-guanidino compound.

Relenza contains zanamivir as an inhaled dry powder. The FDA-approved product uses a Diskhaler device and blister-packaged powder. The product is administered by oral inhalation rather than intranasal administration.

The patent's intranasal claim is broader than Relenza's approved route in one respect, but the commercial Relenza product is principally relevant to claim 10 and the broader formulation claim 7. The patent does not depend on the Diskhaler device for compound protection. A formulation containing zanamivir could fall within a compound claim even if it used a different delivery device.

What is the patent expiration date for US 5,360,817?

US 5,360,817 expired on November 1, 2011, based on the 17-year term applicable to patents filed under the pre-1995 US patent-term regime.

Milestone	Date
Patent issued	November 1, 1994
Ordinary expiration	November 1, 2011
Current enforceability	Expired
Potential patent-term extension relevance	None identified for the basic patent term

The patent could not receive a modern 20-year-from-filing term because the application falls within the earlier statutory regime. FDA regulatory exclusivity for Relenza did not extend the patent term.

Patent expiration eliminates the right to exclude others from making, using, selling, offering for sale, or importing the claimed compounds and formulations in the United States. It does not eliminate separate rights under later patents covering manufacturing processes, devices, excipients, formulations, or different uses.

What is the Orange Book status of zanamivir and Relenza?

Relenza was approved by the FDA in 1999 under NDA 021036. The product was approved for treatment of uncomplicated influenza A and B in patients aged seven years and older and for prophylaxis in specified age groups and settings.

US 5,360,817 was historically associated with the Relenza patent estate and appeared in FDA patent-listing records for the product. Because the patent expired in 2011, it is not a current blocking Orange Book patent.

The relevant regulatory position is:

Regulatory issue	Status
FDA product	Relenza
Active ingredient	Zanamivir
NDA	021036
Approval pathway	NDA
Small-molecule product	Yes
Biosimilar pathway	Not applicable
Key original compound patent	US 5,360,817
Current enforceable term of US 5,360,817	None

An ANDA applicant would ordinarily address listed patents through a Paragraph IV certification if a listed patent remained unexpired. A Paragraph IV certification against US 5,360,817 would no longer create a current patent-infringement barrier because the patent has expired.

Were there Paragraph IV challenges to US 5,360,817?

The patent's expiration date removes the practical value of a current Paragraph IV challenge against this patent. Paragraph IV litigation is relevant only while a listed patent remains unexpired or while enforceable patent rights affect generic approval timing.

For an expired patent:

An ANDA applicant does not need to defeat the patent in litigation to launch after approval;
The 30-month stay mechanism under Hatch-Waxman is not a meaningful current barrier based solely on the expired patent;
A declaratory judgment action would generally have limited commercial value unless another legal issue remained;
The patent cannot support an injunction against a post-expiration commercial launch.

Historical Paragraph IV activity, if any, would not revive the patent or extend its term. The operative assessment is the current expiration status, not the existence of earlier certifications or litigation filings.

What later patents may affect generic zanamivir?

The main residual risks do not arise from US 5,360,817. They arise from later patents that may have covered formulation technology, inhalation devices, manufacturing processes, or specific therapeutic uses.

Formulation and delivery patents

Later patent families relating to neuraminidase inhibitors may claim:

Dry-powder inhalation compositions;
Lactose-based carrier systems;
Particle-size distributions;
Powder blending and dispersibility;
Blister packaging;
Inhalation devices;
Stabilized zanamivir compositions;
Intranasal or pulmonary delivery systems.

A generic company can avoid an expired compound patent while still facing a later formulation or device patent. The commercial relevance depends on whether the generic uses the same excipient system, particle engineering approach, packaging configuration, or delivery device.

Manufacturing patents

Zanamivir is a polar, highly functionalized carbohydrate analog. Manufacturing may involve:

Stereocontrolled construction of the nonulosonic acid core;
Installation of the 4-guanidino substituent;
Protection and deprotection steps;
Salt formation;
Control of anomeric and geometric purity;
Removal of process-related impurities.

A later process patent cannot prevent use of an independently developed non-infringing process after its expiration. Before expiration, however, process claims can affect active pharmaceutical ingredient sourcing even if the compound patent has expired.

Method-of-use patents

Potential later-use claims could address:

Influenza treatment;
Influenza prophylaxis;
Pandemic influenza;
High-risk patient groups;
Treatment timing;
Combination therapy;
Specific dosing regimens.

A method-of-use patent does not necessarily block an ANDA for all uses. Under section viii of the Hatch-Waxman framework, an applicant may seek approval with patented indications carved out, provided the proposed labeling does not encourage the patented use.

Is there biosimilar risk for Relenza?

No. Zanamivir is a chemically synthesized small molecule, not a biologic. The relevant competitive pathway is an ANDA for a generic drug, not a biosimilar application under the Biologics Price Competition and Innovation Act.

The principal regulatory questions are:

Whether the applicant can demonstrate pharmaceutical equivalence;
Whether the inhalation powder has equivalent performance;
Whether the device or delivery system is acceptable;
Whether any remaining listed patents or exclusivities affect approval;
Whether the proposed labeling includes or omits protected uses.

The expired compound patent substantially lowers the legal entry barrier. Device and formulation equivalence may remain the more important regulatory and commercial issues.

How strong is the patent estate for zanamivir?

The estate anchored by US 5,360,817 was strong during the protected period because it combined:

A broad compound genus;
A specific claim to the 4-guanidino active ingredient;
Salt and derivative coverage;
Pharmaceutical formulation claims;
Intranasal formulation coverage.

Its current strength is materially different:

Estate component	Current position
Zanamivir compound claim	Expired
Ammonium zanamivir claim	Expired
4-amino analog claims	Expired
General formulation claims in 817	Expired
Intranasal formulation claims in 817	Expired
Later device or formulation patents	Must be assessed individually
Manufacturing know-how	May remain commercially relevant but is not equivalent to patent exclusivity
Trade secrets	May remain relevant to supply and quality control

The patent is therefore historically foundational but no longer a standalone barrier to US generic entry.

How does zanamivir compare with competing influenza antivirals?

Product	Active ingredient	Primary US route	Patent position relevant to generic entry
Relenza	Zanamivir	Oral inhalation	Original compound patent expired
Tamiflu	Oseltamivir	Oral	Compound and formulation estate largely expired or expired in major markets
Rapivab	Peramivir	Intravenous	Separate compound, formulation, and use patents
Xofluza	Baloxavir marboxil	Oral	Later-generation patent estate with longer expected life
Amantadine/rimantadine	Adamantane antivirals	Oral	Older generic products; resistance limits use

Zanamivir's differentiating commercial issue is its inhaled delivery system. That delivery route may create greater manufacturing and device complexity than an oral capsule, even though the core compound patent has expired.

What generic launch scenarios exist?

Scenario 1: Inhaled zanamivir powder

A generic applicant develops an inhalation powder with a compatible device and obtains ANDA approval. The primary legal risk shifts from the expired compound claim to later formulation, device, and process patents.

Scenario 2: Alternative delivery system

An applicant uses a nebulized, intranasal, or other delivery system. This may avoid product-specific device claims but could implicate separate formulation or method-of-use patents.

Scenario 3: Different salt or solid form

A different pharmaceutically acceptable salt or solid-state form may avoid a narrow claim directed to ammonium zanamivir or a specific formulation. It would not avoid claim 5 if the resulting product remains within the broad compound and salt language, but claim 5 is expired.

Scenario 4: API supply by an independent manufacturer

An API supplier can produce zanamivir using a non-infringing process. The key commercial barriers would be process robustness, stereochemical purity, regulatory qualification, and scale economics rather than US 5,360,817.

What is the litigation and licensing significance?

Biota developed the underlying neuraminidase inhibitor technology and licensed commercial rights to GlaxoSmithKline. The commercial licensing relationship was central to Relenza's launch and commercialization, but the license does not extend the US patent term.

The key distinction is between:

Patent rights, which ended for US 5,360,817 in 2011;
Contractual license rights, which may have governed commercialization during the patent term;
Know-how, manufacturing, regulatory, and supply arrangements, which may continue independently of patent expiration.

No enforceable injunction or royalty obligation follows solely from the expired patent. Any current commercial restriction would need to arise from a separate live patent, contract, regulatory obligation, or trade-secret right.

Key Takeaways

US 5,360,817 covers zanamivir and related 4-amino and 4-guanidino sialic-acid analogs.
Claim 5 is the principal compound claim for zanamivir.
Claim 6 specifically covers ammonium zanamivir.
Claims 7 through 10 cover pharmaceutical formulations, including formulations containing zanamivir.
The patent issued on November 1, 1994, and expired on November 1, 2011.
The patent is no longer a current US barrier to generic zanamivir.
Relenza is an FDA-approved small-molecule drug, not a biologic, so biosimilar rules do not apply.
Current generic-entry risk depends on later formulation, device, manufacturing, and method-of-use patents rather than US 5,360,817.
A Paragraph IV challenge against this expired patent has no current blocking effect.
The commercial difficulty of zanamivir entry is more likely to involve inhalation-device development, powder performance, API manufacture, and limited market demand than the expired compound patent.

FAQs About US Patent 5,360,817 and Zanamivir

Does US 5,360,817 cover oseltamivir?

No. US 5,360,817 covers zanamivir-related sialic-acid analogs. Oseltamivir is a different neuraminidase inhibitor with a separate chemical structure and patent history.

Does US 5,360,817 cover the Relenza Diskhaler?

Not specifically as a device claim. The patent covers compounds and formulations. Separate device or delivery patents would be required to claim the Diskhaler itself.

Can a company launch generic zanamivir after the patent expired?

Yes, subject to FDA approval and any separate unexpired patents covering the proposed formulation, device, manufacturing process, or labeling.

Is ammonium zanamivir still protected by claim 6?

No. Claim 6 expired with the patent on November 1, 2011.

Does patent expiration eliminate Relenza's FDA approval?

No. Patent expiration affects exclusivity, not the validity of the NDA or the FDA approval. Relenza may remain approved even after generic competition becomes legally possible.

References

Food and Drug Administration. (1999). Relenza (zanamivir) prescribing information. U.S. Department of Health and Human Services.
Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. U.S. Department of Health and Human Services.
United States Patent and Trademark Office. (1994). U.S. Patent No. 5,360,817: 4-amino- and 4-guanidino-2,3,4,5-tetradeoxy compounds. U.S. Department of Commerce.
United States Code. (2024). 35 U.S.C. §§ 154, 156. U.S. Government Publishing Office.
World Health Organization. (2023). WHO guidelines on regulatory considerations for pharmaceutical products. World Health Organization.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
Australia	PJ9800	Apr 24, 1990
Australia	PK2896	Oct 19, 1990
Australia	PK4537	Feb 11, 1991

PCT Information
PCT Filed	April 24, 1991	PCT Application Number:	PCT/AU91/00161
PCT Publication Date:	October 31, 1991	PCT Publication Number:	WO91/16320

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
European Patent Office	0526543	⤷ Start Trial	C990030	Netherlands	⤷ Start Trial
European Patent Office	0526543	⤷ Start Trial	SPC/GB99/030	United Kingdom	⤷ Start Trial
European Patent Office	0526543	⤷ Start Trial	990030	Netherlands	⤷ Start Trial
European Patent Office	0526543	⤷ Start Trial	99C0042	Belgium	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,360,817

Summary for Patent: 5,360,817