US Patent 4,963,555: Acyclovir Topical Formulation Claims, Scope, Expiry and Patent Landscape

US Patent 4,963,555 covers topical acyclovir formulations, particularly oil-in-water creams containing approximately 5% acyclovir and 30% to 50% propylene glycol. The patent also claims methods of treating herpesviral infections by applying those formulations two to six times daily. The patent was assigned to The Wellcome Foundation Limited and issued in 1990. Its enforceable term has expired, leaving no current US patent exclusion based on this patent.

What does US Patent 4,963,555 protect?

The patent protects two related subject-matter groups:

Topical oil-in-water emulsions containing acyclovir and a high concentration of a water-miscible polyhydric alcohol.
Methods of treating viral infections of skin or mucosa by applying those formulations.

Acyclovir is identified in the claims by its chemical name, 9-(2-hydroxyethoxymethyl)guanine. The claims also cover pharmaceutically acceptable salts, although commercial topical acyclovir products generally use the parent compound.

The central formulation concept is an aqueous phase containing:

Acyclovir in an effective antiviral amount.
Water.
30% to 50% w/w of a water-miscible polyhydric alcohol.
A dispersed oil phase forming an oil-in-water emulsion.

The claims identify propylene glycol as the principal commercial embodiment.

What is the core technical limitation?

The technical center of the patent is the combination of acyclovir with a high level of propylene glycol or another polyhydric alcohol in an oil-in-water topical formulation.

Acyclovir has limited aqueous solubility. The formulation claims use propylene glycol as a cosolvent or solubilizing component. The claimed concentration range is material because the patent does not merely cover any acyclovir cream. It focuses on formulations containing approximately 30% to 50% w/w propylene glycol, with the acyclovir solubilized in the aqueous phase.

A product that contains acyclovir but uses a different vehicle, a different concentration of propylene glycol, or a non-emulsion dosage form may fall outside particular claims.

How do the 15 claims differ?

Claims 1 to 8: Broad formulation and treatment claims

Claim 1 is the principal broad formulation claim. It requires:

An oil-in-water topical formulation.
An effective antiviral, non-toxic amount of acyclovir or a salt.
A dispersed oil phase.
A continuous aqueous phase.
Water in the aqueous phase.
30% to 50% w/w of a water-miscible polyhydric alcohol.
Solubilized acyclovir in the aqueous phase.

Claim 2 narrows claim 1 by requiring:

1% to 10% w/w acyclovir.
30% to 50% w/w propylene glycol.
20% to 40% w/w water.
The oil phase.

Claim 3 limits the polyhydric alcohol to a glycol or macrogol. Claim 4 further identifies the group as:

Propylene glycol.
Butane-1,3-diol.
Glycerol.
Polyethylene glycol.

Claims 5 to 8 are method-of-treatment claims. They require applying the claimed formulation two to six times daily to the skin or mucosa of a mammal.

Claim	Subject matter	Principal limitations
1	Oil-in-water formulation	Acyclovir, aqueous phase, oil phase, 30%-50% polyhydric alcohol
2	Narrow formulation	1%-10% acyclovir, 30%-50% propylene glycol, 20%-40% water
3	Formulation	Glycol or macrogol
4	Formulation	Propylene glycol, butane-1,3-diol, glycerol or polyethylene glycol
5	Treatment method	Viral infection of skin or mucosa; application 2-6 times daily
6	Treatment method	Herpes simplex infection
7	Treatment method	Herpes zoster infection
8	Treatment method	Herpes varicella infection

Claims 9 to 15: Commercially focused acyclovir cream claims

Claims 9 and 10 move away from the more detailed oil-in-water limitations.

Claim 9 covers a topical cream containing:

Approximately 2% to 5% w/w micronized acyclovir.
More than approximately 30% w/w propylene glycol.

Claim 10 is narrower and specifies:

Approximately 5% w/w micronized acyclovir.
Approximately 40% w/w propylene glycol.

Claims 11 and 13 cover oil-in-water emulsions containing approximately 5% acyclovir and either approximately 40% or approximately 30% to 50% propylene glycol in the aqueous phase.

Claims 12 and 14 cover treatment methods using those formulations. Claim 15 covers an oil-in-water emulsion containing:

5% to 10% acyclovir.
Approximately 30% to 50% propylene glycol.

The commercially most relevant claim concepts are claims 9, 10, 11, 13 and 15 because they correspond closely to a 5% acyclovir cream containing a high propylene glycol concentration.

What formulations are protected by US 4,963,555?

A product is most directly aligned with the patent when it has all of the following characteristics:

Formulation attribute	Relevance to the claims
Acyclovir concentration	Claims cover 1%-10%, 2%-5%, approximately 5%, and 5%-10% ranges
Propylene glycol	Core limitation in claims 2, 9, 10, 11, 13, 14 and 15
Propylene glycol concentration	Approximately 30%-50% w/w is the principal range
Dosage form	Oil-in-water emulsion for claims 1, 2, 11, 13 and 15; cream for claims 9 and 10
Acyclovir physical form	Micronized acyclovir is expressly required by claims 9 and 10
Aqueous phase	Required by the principal emulsion claims
Solubilization	Express requirement in claims 1, 11, 12, 13 and 14
Treatment site	Skin or mucosa
Treatment frequency	Two to six applications daily in claims 5 to 8

The claims do not require a particular preservative, emulsifier, oil, surfactant, packaging system, particle-size specification, pH, or brand name unless those limitations are found elsewhere in the patent specification or prosecution history.

A cream containing 5% acyclovir and 40% propylene glycol is the clearest commercial match to claim 10. A product with 5% acyclovir and 35% propylene glycol may fall within claims 11, 13 or 15 if it also satisfies the oil-in-water and aqueous-phase requirements.

When did US Patent 4,963,555 lose exclusivity?

US Patent 4,963,555 issued on October 16, 1990. As a pre-URAA US patent, its ordinary term was 17 years from issuance. On that basis, the patent expired on October 16, 2007, subject to any applicable term adjustment or terminal disclaimer recorded in the official file. Public patent databases identify the patent as expired.

Event	Date
US patent issued	October 16, 1990
Standard pre-URAA term	17 years from issuance
Standard expiration date	October 16, 2007
Current enforceability	Expired

The patent therefore does not create a current US launch barrier for generic acyclovir cream. Its historical relevance remains significant because it defined a commercially important topical formulation architecture.

What patent family and earlier acyclovir patents are relevant?

The patent sits within the broader acyclovir patent estate developed by Burroughs Wellcome and related entities.

Foundational acyclovir composition patents

Earlier patents covered acyclovir itself and its antiviral use. The most important foundational US patent was US Patent 4,199,574, which covered 9-(2-hydroxyethoxymethyl)guanine and related antiviral applications. That patent expired before US 4,963,555.

The earlier composition patents addressed the active pharmaceutical ingredient and antiviral activity. US 4,963,555 addressed the more specific pharmaceutical problem of delivering acyclovir topically in a usable cream.

Formulation patents

The relevant formulation estate included patents directed to:

Acyclovir creams.
Acyclovir solubilization.
Micronized acyclovir.
Propylene glycol-containing vehicles.
Topical administration to skin and mucosa.
Treatment of herpes simplex, herpes zoster and varicella infections.

Patent families in this area often used overlapping but distinct claim strategies. One family could claim the formulation, another the particle size or manufacturing process, and another the therapeutic use.

How strong was the patent estate?

The historical estate was commercially meaningful but technically narrow in several respects.

Strengths

The strongest features were:

Specific coverage of 5% acyclovir cream.
Direct recitation of approximately 40% propylene glycol.
Coverage of both formulation and treatment methods.
Claims directed to micronized acyclovir.
Alignment with the product configuration associated with Zovirax cream.

Claims 9 and 10 were particularly important because they did not recite every limitation of claim 1. A competitor could therefore face claim exposure even if it disputed whether its product satisfied all oil-in-water or solubilization limitations.

Weaknesses

Potential legal vulnerabilities included:

The known antiviral activity of acyclovir before the formulation patent.
The conventional use of propylene glycol in topical pharmaceutical products.
The possibility that prior art disclosed acyclovir creams or propylene glycol-containing vehicles.
Indefinite boundaries created by terms such as “about,” “effective,” “non-toxic” and “micronized.”
Potential written-description and enablement issues across broad polyhydric alcohol and concentration ranges.
Overlap among claims that could create prosecution-history estoppel or narrow construction arguments.

The commercial value depended less on the broad concept of an acyclovir cream than on whether the patent’s specific concentration, phase, particle-size and solubilization limitations matched the marketed product.

What FDA and Orange Book status applies?

Acyclovir is an FDA-approved small-molecule antiviral. It is approved in multiple dosage forms, including oral tablets, capsules, suspension, intravenous injection and topical cream or ointment products, depending on the product and application.

The Orange Book framework applies to approved small-molecule drug products. It does not apply to biologics in the same manner as the Biologics Price Competition and Innovation Act pathway.

For US 4,963,555:

The patent is expired.
It does not provide current Orange Book exclusivity.
It does not support a current Paragraph IV litigation strategy on its own.
Any historical listing would no longer prevent approval after expiration.
Current regulatory review focuses on the proposed product’s formulation, quality, bioequivalence or other applicable approval requirements.

The Orange Book should be checked at the product-application level because patent listings attach to approved drug products and applications, not to every patent that has ever covered the active ingredient.

Are there current Paragraph IV challenges involving this patent?

No current Paragraph IV challenge can create a live enforcement issue under US 4,963,555 because the patent has expired.

Historically, a generic applicant could have used a Paragraph IV certification against an unexpired listed patent covering acyclovir cream. The applicant would have needed to address:

Anticipation.
Obviousness.
Claim construction.
Non-infringement.
The scope of any listed patent claims.
Whether the patent was properly listed for the relevant product.

After expiration, the commercial question changes from patent invalidity to regulatory timing, manufacturing readiness, product quality and market competition.

What generic launch risks exist for acyclovir cream?

The direct patent risk from US 4,963,555 is zero as a current exclusivity matter because the patent has expired. Generic launch risk may still arise from other sources:

Risk category	Current relevance
Expired formulation patent	No blocking effect from US 4,963,555
Other expired acyclovir patents	Historical only unless a separate unexpired patent exists
Active formulation patent	Must be assessed separately by product and application
FDA approval requirements	Current regulatory issue
Manufacturing reproducibility	Important for emulsion stability and acyclovir uniformity
Trade dress and branding	Separate from patent rights
Product quality	Potential approval and recall exposure
Market access	Driven by approved competitors and reimbursement
Litigation	More likely to concern other patents, trademarks or regulatory conduct

A generic product designed around a different vehicle may also avoid claim-specific limitations even if US 4,963,555 had remained in force. Examples could include a non-emulsion dosage form, a different cosolvent system, a propylene glycol concentration below the claimed range, or a different active concentration. Such design-around analysis would require testing every element of each potentially asserted claim.

What manufacturing and intellectual-property barriers remain?

The expired patent does not eliminate technical barriers. Acyclovir topical products require control of:

Particle size and micronization.
Suspension or solubilization behavior.
Emulsion droplet size.
Phase separation.
Microbial preservation.
Assay and content uniformity.
Viscosity and spreadability.
Stability under temperature cycling.
Packaging compatibility.
Skin tolerability at high propylene glycol concentrations.

Manufacturing know-how may remain commercially useful even when patent rights expire. The patent specification may disclose formulation parameters, but it does not transfer all process capability required to manufacture a stable commercial cream at scale.

Trade secrets may include mixing order, shear conditions, temperature profile, emulsification endpoint, micronization equipment settings and stability-control procedures.

How does acyclovir cream compare with competing herpes treatments?

Product or active	Dosage form	Competitive position	Patent status profile
Acyclovir cream	Topical cream	Established prescription antiviral	Core US patents expired
Acyclovir ointment	Topical ointment	Alternative topical vehicle	Core compound rights expired
Penciclovir cream	Topical cream	Competing nucleoside analog	Separate historical patent estate
Docosanol cream	Topical OTC cream	Non-nucleoside topical treatment	Separate formulation and regulatory history
Valacyclovir	Oral prodrug	Systemic treatment	Separate compound and formulation patents
Famciclovir	Oral prodrug	Systemic treatment	Separate compound and formulation patents

Acyclovir cream competes primarily on familiarity, established clinical use and broad generic availability. Oral valacyclovir and famciclovir compete in situations where systemic exposure or dosing convenience is more important than topical administration.

What is the current commercial significance?

US 4,963,555 no longer generates enforceable exclusivity revenue. Its commercial significance is historical: it helped protect the formulation architecture associated with topical Zovirax during the period when branded acyclovir products faced limited direct competition.

Current revenue exposure depends on:

Whether the product is still marketed.
The number of approved generic creams.
Pricing and reimbursement.
Channel mix between prescription and retail distribution.
Competition from oral antivirals and nonprescription products.
Manufacturing cost and supply reliability.

Patent expiry generally shifts value from exclusivity to scale, cost control, regulatory compliance and distribution.

Key Takeaways

US Patent 4,963,555 covers topical acyclovir formulations and treatment methods.
The central formulation uses 30% to 50% w/w propylene glycol or another water-miscible polyhydric alcohol.
Claims 9 and 10 closely target 5% micronized acyclovir cream containing more than 30%, or approximately 40%, propylene glycol.
Claims 11, 13 and 15 focus on oil-in-water emulsions with approximately 5% acyclovir and 30% to 50% propylene glycol.
Claims 5 to 8, 12 and 14 cover topical treatment of herpesviral infections.
The patent issued October 16, 1990 and expired under the standard pre-URAA term on October 16, 2007.
It provides no current US patent barrier to generic acyclovir cream.
Acyclovir is a small molecule, so biosimilar analysis does not apply.
Current competitive risk is driven by FDA approval, manufacturing quality, pricing and other active patents, not by US 4,963,555.
The historical estate was strongest where a product matched the 5% acyclovir and approximately 40% propylene glycol configuration.

FAQs About US Patent 4,963,555

Does US Patent 4,963,555 cover Zovirax cream?

The claims closely correspond to a 5% acyclovir topical cream containing a high concentration of propylene glycol, a formulation associated with Zovirax cream. Product coverage depends on whether every asserted claim limitation is met.

Can a generic launch be blocked by US 4,963,555?

No. The patent expired in 2007 and cannot independently block a current US generic launch.

Does the patent cover acyclovir ointment?

Not necessarily. The principal claims require an oil-in-water formulation or cream characteristics. An ointment without the claimed emulsion and phase structure may fall outside those claims.

Does the patent cover penciclovir or valacyclovir?

No. The claims specifically identify acyclovir, also called 9-(2-hydroxyethoxymethyl)guanine. Penciclovir and valacyclovir require separate patent analysis.

Is a 5% acyclovir cream with 20% propylene glycol within the patent claims?

It would not meet the express 30% to 50% propylene glycol range in the principal claims. It could still require analysis against any claim with different concentration language, but US 4,963,555 does not create a current enforcement risk because it is expired.

References

United States Patent and Trademark Office. (1990). US Patent No. 4,963,555, Acyclovir formulations. U.S. Department of Commerce.
United States Patent and Trademark Office. (1977). US Patent No. 4,199,574, 9-(2-hydroxyethoxymethyl)guanine. U.S. Department of Commerce.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book. U.S. Department of Health and Human Services.
U.S. Food and Drug Administration. (2024). Drugs@FDA: FDA-approved drugs. U.S. Department of Health and Human Services.
World Intellectual Property Organization. (2024). PATENTSCOPE patent database. WIPO.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
United Kingdom	8023645	Jul 18, 1980

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	225680	⤷ Start Trial
Austria	381025	⤷ Start Trial
Austria	A318081	⤷ Start Trial
Australia	547391	⤷ Start Trial
Australia	7307381	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,963,555

Summary for Patent: 4,963,555