United States Patent 5,693,762: Claims, Expiration, Litigation, and Sildenafil Patent Landscape

US Patent 5,693,762 was a core Pfizer patent covering pyrazolopyrimidinone compounds, including sildenafil, and their use in treating impotence, now described clinically as erectile dysfunction. The patent issued on December 2, 1997, from a Pfizer application claiming priority to earlier United Kingdom filings. Its principal commercial relevance was Viagra, sildenafil citrate.

The patent is expired. Its 17-year post-grant term ended on December 2, 2014, subject to any applicable statutory adjustment or pediatric extension. It no longer blocks generic sildenafil or creates an active Orange Book barrier. The patent remains important historically because it combined compound, pharmaceutical-composition, and therapeutic-use coverage around sildenafil and related analogues. [1]

What does US Patent 5,693,762 cover?

US 5,693,762, titled “Pyrazolopyrimidinones for the Treatment of Impotence,” was assigned to Pfizer Limited. The patent concerns a class of cyclic guanine-nucleotide phosphodiesterase inhibitors, particularly compounds that inhibit phosphodiesterase type 5, or PDE5.

Sildenafil is the best-known compound within the patent’s disclosure. The commercial product Viagra contains sildenafil citrate, the citrate salt of sildenafil.

What technology is claimed?

The patent’s disclosure covers:

Pyrazolopyrimidinone chemical structures;
Substituted analogues within the claimed Markush formula;
Pharmaceutically acceptable salts;
Pharmaceutical compositions containing the active compound;
Administration for treating male erectile dysfunction or impotence;
Oral and other pharmaceutical dosage forms;
Compounds that increase penile cyclic guanosine monophosphate, or cGMP, signaling by inhibiting PDE5.

The claims should be read as a layered patent strategy rather than as a single sildenafil claim. The chemical claims seek protection for a genus of related molecules. Narrower claims identify preferred substituents and individual compounds. Composition and method claims extend protection to pharmaceutical use.

What is the role of sildenafil in the patent?

Sildenafil is the commercial embodiment that made the patent strategically important. The patent describes sildenafil as a PDE5 inhibitor capable of improving erectile response by preventing the breakdown of cGMP in penile tissue.

The active ingredient in Viagra is sildenafil citrate, not an entirely different active molecule. Salt selection affects pharmaceutical form and formulation, but the pharmacologically active moiety remains sildenafil.

How are the claims structured?

The patent’s claim architecture reflects the standard small-molecule pharmaceutical model used in the early 1990s.

Claim category	Subject matter	Commercial relevance
Compound claims	Pyrazolopyrimidinone structures and substituted analogues	Protect the active chemical entity and related compounds
Subgenus claims	Narrower chemical classes with selected substituents	Improve protection around commercially valuable compounds
Specific-compound claims	Individual preferred molecules, including sildenafil-related embodiments	Create a direct product-level barrier
Salt claims	Pharmaceutically acceptable salts	Support development and formulation of drug substances
Composition claims	Drug compounds combined with pharmaceutical carriers	Cover dosage-form manufacture and sale
Method claims	Treatment of impotence or erectile dysfunction	Protect the therapeutic use of the compounds

The strongest commercial claims were the claims that reached sildenafil as a specific compound or a narrow chemical subgenus. Broad genus claims were more exposed to prior-art and validity attacks because they covered many molecules beyond the eventual commercial product.

What is the critical claim issue?

The central claim question is whether an accused generic product contains a compound that falls within the patent’s structural formula or whether its proposed use falls within a method claim.

For a sildenafil product, infringement analysis would normally examine:

The exact chemical identity of sildenafil or its salt.
Whether the molecule satisfies every limitation of a compound claim.
Whether the product is manufactured, sold, or supplied with a use directed to erectile dysfunction.
Whether a pharmaceutical composition claim is met by the dosage form and excipients.
Whether the asserted claim remained enforceable at the relevant time.

Because the patent has expired, these questions now have mainly historical, damages, and freedom-to-operate significance.

When did US Patent 5,693,762 expire?

US 5,693,762 issued on December 2, 1997. For a pre-1995 United States patent application, the applicable baseline term was generally 17 years from grant rather than 20 years from the earliest effective filing date. On that basis, the patent term ended on December 2, 2014. [1]

Event	Date
Earliest claimed priority	1993
US filing period	Mid-1990s
Patent issued	December 2, 1997
Baseline expiration	December 2, 2014
Current status	Expired

A patent-term calculation should distinguish the 762 patent from other Viagra-related patents. Different patents in the sildenafil estate had different filing dates, grant dates, statutory extensions, and subject matter. A company cannot assume that expiration of one patent establishes freedom to operate across every sildenafil indication or formulation.

What is the Orange Book status of US 5,693,762?

US 5,693,762 was historically associated with Viagra-related patent protection, but it is not an active Orange Book barrier today. The FDA Orange Book identifies patents and exclusivity associated with approved drug products, including Viagra and Revatio. [2]

The patent’s expiry means:

It cannot support a new Paragraph IV litigation strategy as an unexpired patent;
It cannot prevent approval of an ANDA solely because of its former claims;
It cannot support prospective injunctive relief based on post-expiration commercial activity;
It does not eliminate other potential patents covering a different indication, formulation, or product feature.

Orange Book listings must be evaluated at the product and patent level. A patent’s historical listing does not mean that every claim was valid, infringed, or currently enforceable.

What patent landscape surrounded sildenafil?

Sildenafil was protected by a portfolio rather than a single patent. The main categories included the following.

Core compound patents

The earliest sildenafil-related patents addressed the chemical class and the compounds themselves. These patents created the principal barrier to unauthorized manufacture of sildenafil during the initial Viagra launch period.

Composition and salt patents

Separate protection could cover pharmaceutical compositions, salts, dosage forms, and manufacturing techniques. Such patents are generally narrower than the core compound patent but can extend practical enforcement where the active compound patent is weak or near expiration.

Method-of-use patents

Pfizer also pursued method-of-use protection for PDE5 inhibitors in different indications. Erectile dysfunction and pulmonary arterial hypertension were commercially distinct uses.

Revatio, another Pfizer product, contains sildenafil but is approved for pulmonary arterial hypertension at different dosing and in different product presentations. Later-use patents and regulatory exclusivities associated with pulmonary hypertension had separate commercial significance. The fact that sildenafil is off patent as a chemical entity does not mean that every approved use was immediately free of patent restrictions.

Formulation patents

Formulation protection can cover:

Tablet composition;
Particle size or polymorphic form;
Oral suspension;
Injectable or intravenous presentation;
Stability-enhancing excipients;
Manufacturing processes.

For Viagra, the principal generic competition centered on sildenafil tablets. A generic manufacturer that sells the same active ingredient in a conventional immediate-release tablet has a different risk profile from a company developing a novel liquid, chewable, sublingual, or extended-release presentation.

How strong was the patent estate?

The 762 patent was commercially strong during its enforceable life because it linked a valuable therapeutic class to a specific lead compound and an approved treatment concept. Its strength was less uniform across all claims.

Factor	Assessment
Commercial relevance	Very high because sildenafil became a leading PDE5 inhibitor
Compound protection	Strong for claims directly covering sildenafil
Broad genus claims	More vulnerable to prior-art and enablement arguments
Method claims	Dependent on the precise indication and claim language
Formulation coverage	Potentially narrower and easier to design around
Remaining enforceability	None after expiration
Generic relevance today	Limited to historical litigation and damages

The patent’s technical vulnerability would have been greatest at the boundaries of the Markush genus. Broad chemical claims can face challenges based on:

Earlier PDE5 inhibitor disclosures;
Prior-art compounds that anticipate a claimed structure;
Obviousness arguments based on known cyclic nucleotide phosphodiesterase biology;
Written-description challenges to broad genera;
Enablement challenges if the claim covers an excessive number of compounds without adequate teaching;
Double-patenting issues within a dense continuation portfolio.

A direct claim to sildenafil would generally present a stronger enforcement position than a broad claim to every compound satisfying a large structural formula.

Which companies challenged Pfizer’s sildenafil patents?

Generic manufacturers challenged Viagra-related patents through ANDA filings and Paragraph IV certifications. Notable participants included Teva Pharmaceuticals and other generic-drug companies seeking approval for sildenafil tablets.

The principal commercial outcome was a settlement under which Teva obtained an agreed launch date before the nominal expiration of certain Viagra-related patent rights. Teva launched generic sildenafil in the United States in December 2017 under its agreement with Pfizer. [3]

This outcome illustrates the difference between:

Statutory patent expiration;
Settlement-based launch permission;
FDA approval timing;
Commercial launch timing;
Product-specific patent listings.

A settlement can permit launch before a patent’s stated expiry without invalidating the patent or establishing noninfringement.

What patent litigation affected Viagra and sildenafil?

The main litigation pathway involved ANDA applicants challenging Pfizer’s listed patents. The typical dispute included:

A generic applicant filed an ANDA for sildenafil tablets.
The applicant submitted a Paragraph IV certification against one or more listed patents.
Pfizer could sue within the statutory period.
FDA approval was potentially stayed for up to 30 months, subject to statutory exceptions.
The parties could litigate, settle, or proceed to a negotiated launch.

The 762 patent’s expiration ultimately removed it as a continuing litigation obstacle. Historical litigation risk nevertheless mattered because a generic launch before expiry could expose the applicant to infringement damages, injunction risk, and delayed approval.

The commercially important litigation question was not simply whether Pfizer owned a patent. It was whether the ANDA product would infringe an unexpired claim after the proposed marketing date.

What generic entry risks exist after expiration?

For conventional sildenafil tablets, core composition-of-matter risk from US 5,693,762 is no longer present. Generic entry risks instead shift to other rights and regulatory issues.

Generic launch scenarios

Scenario	Risk profile
Conventional sildenafil tablet for erectile dysfunction	Low risk from the expired 762 patent
Sildenafil product for pulmonary hypertension	Requires review of indication-specific patents and labeling
New dosage form	May implicate formulation or delivery-system patents
New dosing regimen	May implicate method-of-use claims
Product made with a patented process	Requires manufacturing and process-patent review
Authorized generic or license-backed product	Commercial terms may matter more than core patent validity

FDA approval also depends on the ANDA applicant demonstrating pharmaceutical equivalence, bioequivalence, manufacturing quality, and compliance with labeling requirements. Patent expiry does not remove those regulatory obligations. [4]

Does biosimilar risk apply to sildenafil?

No. Sildenafil is a chemically synthesized small molecule, not a biologic. Competitors use the ANDA generic-drug pathway rather than the abbreviated biologics license application pathway for biosimilars.

The relevant competition is therefore:

Generic sildenafil;
Authorized generic sildenafil;
Alternative PDE5 inhibitors;
Branded or generic tadalafil;
Branded or generic vardenafil;
Avanafil;
Non-PDE5 erectile-dysfunction therapies.

Biosimilar concepts such as interchangeability, reference biologics, and the Biologics Price Competition and Innovation Act do not govern ordinary sildenafil tablets.

How does sildenafil compare with competing PDE5 inhibitors?

Drug	Principal company history	Main patent issue	Competitive position
Sildenafil	Pfizer	Core compound and indication patents expired	Broad generic availability
Tadalafil	Eli Lilly	Longer-duration PDE5 profile and separate patent estate	Generic availability after later expiry
Vardenafil	Bayer/GSK/Schering-Plough	Separate compound and formulation patents	Generic competition
Avanafil	Vivus	Later-generation compound estate	More limited market history
Sildenafil for PAH	Pfizer	Separate product and indication considerations	Regulatory and use-specific analysis required

Sildenafil’s early market position came from first-in-class commercial adoption, not from permanent patent protection. Once compound and key use patents expired, price competition materially changed the market.

What geographic coverage did the patent have?

US 5,693,762 provided rights only in the United States. Related applications and patents existed in other jurisdictions, but their scope, term, prosecution history, and expiry dates were separate.

A global freedom-to-operate review must examine:

European Patent Office family members;
United Kingdom patents;
Canadian patents;
Japanese patents;
Australian patents;
National-phase patents in major generic-manufacturing countries;
Local patent-term extensions and pediatric extensions;
Jurisdiction-specific claim amendments.

Expiration in the United States did not automatically terminate foreign rights. By now, however, the principal early sildenafil compound rights in major markets have generally expired or become commercially ineffective.

What manufacturing and intellectual-property barriers remain?

The expired patent does not eliminate all technical barriers. A commercial sildenafil manufacturer still must address:

Active pharmaceutical ingredient purity;
Control of residual solvents and impurities;
Salt formation and crystallinity;
Batch consistency;
Dissolution and bioequivalence;
Tablet coating and stability;
Qualified suppliers;
Good Manufacturing Practice compliance;
Regulatory inspection risk;
Trademark and trade-dress restrictions.

Manufacturing-process patents can be relevant even after a compound patent expires. A generic manufacturer can reduce risk by using a noninfringing process, licensing a process, or purchasing the active ingredient from a compliant supplier.

What is the current commercial value of the patent?

US 5,693,762 has no forward-looking exclusivity value because it is expired. Its historical value was substantial because it supported protection for Viagra during the period when sildenafil had limited direct competition.

Current revenue exposure is indirect. The patent no longer supports branded pricing, but its former exclusivity contributed to Pfizer’s historical Viagra revenue and to the timing of generic market entry. Current commercial analysis should focus on:

Generic sildenafil volume;
Net price erosion;
Channel substitution;
Pfizer’s remaining brand equity;
Revatio and pulmonary-hypertension products;
Alternative PDE5 inhibitors;
Any surviving indication or formulation rights.

Key Takeaways

US 5,693,762 is a Pfizer patent covering pyrazolopyrimidinone PDE5 inhibitors, including sildenafil-related compounds and therapeutic use.
Its commercial significance came from its relationship to Viagra and erectile-dysfunction treatment.
The patent issued on December 2, 1997, and its baseline 17-year term ended on December 2, 2014.
The patent is expired and no longer creates a current Orange Book barrier.
Its direct compound claims were stronger than broad genus, formulation, or method claims.
Generic sildenafil competition proceeds through the ANDA pathway, not the biosimilar pathway.
Teva’s 2017 generic launch followed a settlement with Pfizer involving Viagra-related patent rights.
Current freedom-to-operate analysis must separate erectile-dysfunction tablets from pulmonary-hypertension products, novel formulations, manufacturing processes, and foreign jurisdictions.
The patent’s historical enforcement value was high; its current blocking value is zero.

FAQs About US Patent 5,693,762

Was sildenafil itself claimed in US Patent 5,693,762?

Yes. The patent disclosed and claimed pyrazolopyrimidinone compounds encompassing sildenafil-related embodiments, along with pharmaceutical compositions and therapeutic uses.

Can a company still be sued for infringing US 5,693,762?

Not for conduct occurring after the patent expired. Historical infringement claims may remain subject to applicable damages limitations, statutes of limitation, and procedural defenses.

Did US 5,693,762 cover Viagra’s trademark?

No. Patent rights covered chemical compounds, compositions, and uses. The Viagra name and associated branding were protected through trademark and related commercial rights.

Is generic sildenafil a biosimilar?

No. Sildenafil is a small-molecule drug. Generic sildenafil is approved through the ANDA pathway under the Federal Food, Drug, and Cosmetic Act.

Does expiration of US 5,693,762 clear all patents covering sildenafil products?

No. Expiration clears this patent only. A complete analysis must review other compound, method-of-use, formulation, process, regulatory, trademark, and foreign patent rights.

References

U.S. Patent No. 5,693,762. (1997). Pyrazolopyrimidinones for the treatment of impotence. U.S. Patent and Trademark Office.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. Center for Drug Evaluation and Research.
Pfizer Inc. (2017). Pfizer and Teva announce U.S. launch of generic Viagra. Corporate press release.
U.S. Food and Drug Administration. (2019). ANDA approvals and abbreviated new drug application requirements. Center for Drug Evaluation and Research.

Applicant	Tradename	Biologic Ingredient	Dosage Form	BLA	Approval Date	Patent No.	Expiredate
Wyeth Pharmaceuticals Llc	MYLOTARG	gemtuzumab ozogamicin	For Injection	761060	September 1, 2017	5,693,762	2015-06-07
>Applicant	>Tradename	>Biologic Ingredient	>Dosage Form	>BLA	>Approval Date	>Patent No.	>Expiredate

Patent: 5,693,762

Summary for Patent: 5,693,762