US Patent 5,919,455: Claim Scope, Expiration, Enforcement Risk, and PEGylation Patent Landscape

US Patent 5,919,455 covers branched, substantially non-antigenic polymers, principally branched poly(ethylene glycol) derivatives, designed for covalent attachment to proteins, peptides, enzymes, polypeptides, and drug molecules. Its core structure has two or three polymer arms connected through a defined linker to either a carboxylic acid or a nucleophile-reactive functional group.

The patent is a platform patent rather than a patent directed to one approved drug. Its principal commercial significance was the use of branched PEG architectures to increase hydrodynamic size, reduce immunogenicity, extend biological half-life, and create activated intermediates for protein or small-molecule conjugation.

The patent issued July 6, 1999. Based on its 1997 nonprovisional filing date, the ordinary 20-year term ended in 2017, subject to any applicable patent-term adjustment or terminal-disclaimer effects. The patent is no longer an enforceable US exclusivity barrier. The claims remain relevant as prior art and as evidence of the historical scope of the originating PEGylation technology (USPTO, 1999; USPTO, n.d.-a).

What does US Patent 5,919,455 cover?

The patent has four principal claim groups:

Claim group	Claims	Subject matter	Commercial relevance
Branched PEG-polymer intermediates	1-14	Branched non-antigenic polymers with a terminal carboxylic acid	Covers core chemical intermediates
Conjugates and conjugation methods	15-23	Polymer conjugates with biological or therapeutic nucleophiles	Addresses PEGylated proteins, peptides, enzymes, and drugs
Activated branched polymers	24-33	Branched polymers bearing nucleophile-reactive groups or groups convertible into such groups	Covers activated PEG reagents used in manufacturing
Manufacturing processes	34-40	Conversion of functionalized branched polymers into carboxylic acids, active carbonates, and succinimidyl carbonates	Provides process protection for reagent preparation

The independent composition claims are claims 1 and 24. Claims 15, 16, 23, 33, and 34-40 provide independent or functionally independent coverage of conjugation, treatment, and manufacturing activities.

What chemical structures are protected by the patent?

Core branched polymer structure

Claim 1 requires a polymer having the general structure:

(R)n-L-COOH

The required elements are:

R is a water-soluble, substantially non-antigenic polymer.
Each polymer contains a C1-C4 terminal group.
n is 2 or 3, creating a branched architecture.
L is a defined linker containing one or more heteroatom or carbonyl-containing structures.
The polymer terminates in a carboxylic acid.
The variable p is a positive integer, and a is approximately 1 to 5.
The variable X may be oxygen, nitrogen, sulfur, sulfoxide, or sulfone.

The claims focus on a central branching unit carrying two or three polymer arms. The most commercially important embodiment is a di-branched PEG construct in which two methoxy-terminated PEG chains are attached through a linker to a carboxylic acid or activated derivative.

Polymer identity and molecular weight

Claims 3-8 narrow the polymer component to poly(alkylene oxide), particularly PEG. The claimed molecular-weight ranges are broad:

Claim	Molecular-weight limitation
5	About 200 to 80,000 Da
6	2,000 to about 42,000 Da
7	5,000 to about 20,000 Da
8	Each arm is PEG

Claim 4 includes:

PEG homopolymers;
alkyl-capped PEG;
block or copolymeric poly(alkylene oxides).

Claim 10 specifies a methoxy terminal group. A typical covered construct would therefore contain two methoxy-terminated PEG arms linked to a central branching group and bearing a carboxylic acid or activated carbonate at the opposite end.

Linker and spacer scope

Claims 12 and 13 address spacer structures proximal to the linker. Claim 12 identifies spacer groups that may contain:

amide-linked alkylene chains;
PEG spacers;
substituted aromatic or heterocyclic structures;
chain lengths from approximately one to 18 units.

This language expands the claim beyond a single chemical scaffold. A design that changes the distance between the branching center and reactive group may remain within the claims if it retains the claimed linker architecture and polymer-arm arrangement.

How broad are claims 1 and 24?

Claim 1 is a composition claim to a branched polymer ending in a carboxylic acid. Claim 24 is broader in functional terms because it substitutes A for COOH.

Under claim 24, A may be:

A functional group capable of bonding with a biological nucleophile; or
A moiety capable of being functionalized to react with that nucleophile.

This can encompass activated PEG derivatives used to form:

amide bonds with lysine or other amino groups;
carbamate bonds with amino groups;
urea-type linkages;
thioether or disulfide linkages where the claimed functional group and linker requirements are met;
other covalent conjugation products supported by the specification and claim construction.

Claim 24 is therefore the principal platform claim for activated branched PEG reagents. It does not require the polymer to be in its final conjugated form.

The breadth is limited by the structural definition of L, the requirement for two or three polymer branches, and the requirement that the polymer be water-soluble and substantially non-antigenic. A linear PEG derivative, a four-arm PEG derivative, or a branched polymer using a chemically unrelated linker would not necessarily fall within the literal scope.

Which claims cover PEGylated drug conjugates?

Claims 16-23 address the resulting conjugates and therapeutic use.

Conjugate claims

Claim 16 covers a polymer conjugate prepared by reacting the branched polymer of claim 1 with a nucleophile. Dependent claims narrow the conjugate to:

poly(alkylene oxide), particularly PEG;
molecular weights up to about 80,000 Da;
PEG on each polymer arm;
proteins, peptides, and polypeptides as the nucleophile;
specified therapeutic classes.

Claim 21 identifies broad drug categories, including antineoplastics, anti-infectives, analgesics, anti-inflammatory agents, cardiovascular agents, and central nervous system agents.

Claim 22 specifically identifies:

paclitaxel and taxanes;
docetaxel and taxotere;
taxoid molecules;
camptothecin compounds;
anthracyclines;
methotrexates.

The inclusion of a drug name in claim 22 does not, by itself, cover every PEGylated version of that drug. The accused product must also satisfy the structural limitations inherited from claim 16 and claim 1, including the branched polymer, linker, terminal-group, and molecular-weight requirements.

Method-of-treatment claim

Claim 23 covers administering a therapeutically effective amount of the branched polymer conjugate to a mammal. The claim requires the conjugate to fall within claim 16. It is not an independent claim to treatment with any PEGylated drug.

Because the patent term has ended, these treatment claims do not create current US exclusivity. They remain relevant when analyzing historical license positions, prior art, and the scope of later patent families.

What manufacturing methods are protected?

Claims 34-40 cover preparation of reactive branched PEG intermediates.

Carboxylic-acid preparation

Claim 34 covers:

Reacting a branched polymer containing a functional group with an alkyl haloacetate in the presence of a base.
Forming an alkyl ester.
Hydrolyzing the ester with acid to produce the carboxylic-acid derivative.

Claims 35-38 narrow the haloacetate to tertiary alkyl haloacetates, including tertiary butyl haloacetate. These claims are process claims and require performance of the specified reaction sequence or an equivalent process under applicable infringement principles.

Active-carbonate preparation

Claim 39 covers reaction with p-nitrophenyl chloroformate followed by reaction with N-hydroxysuccinimide. The result is an active carbonate suitable for reaction with biological nucleophiles.

Claim 40 covers preparation of a succinimidyl carbonate active ester using N-hydroxysuccinimide and a condensing agent.

These claims are narrower than the core composition claims. A manufacturer using a different activation chemistry may avoid literal infringement of claims 39 or 40, although a separate later patent could cover the alternative route.

How does the patent apply to PEGylated proteins?

The patent is most directly relevant to branched PEGylation of proteins and peptides. The principal biological objective is to attach a larger hydrophilic polymer to a therapeutic molecule while reducing renal clearance and shielding portions of the molecule from proteolysis or immune recognition.

Potentially relevant targets include:

cytokines;
enzymes;
antibody fragments;
recombinant proteins;
peptides;
coagulation factors;
growth factors;
therapeutic hormones.

A product-by-product analysis requires more than identifying "PEGylated." The following structural questions control claim coverage:

Issue	Claim relevance
Number of PEG arms	Must generally be two or three
PEG architecture	Branched rather than merely linear
Terminal group	C1-C4 group, with methoxy expressly claimed
Linker	Must fall within the defined `L` structures
Reactive group	Must be capable of nucleophile bonding or functionalization
Conjugate linkage	Must arise from reaction with a claimed nucleophile
Polymer molecular weight	Must fall within a claimed range where the dependent claim is asserted
Manufacturing route	Must match the steps of claims 34-40 for process infringement

A linear mPEG reagent, a four-arm PEG, or a star PEG product may avoid the principal claims depending on the exact linker and branching structure.

What is the patent expiration date?

The patent issued from a US application filed in 1997. The standard US patent term is 20 years from the earliest effective nonprovisional filing date for applications filed after June 8, 1995, subject to patent-term adjustment and other statutory adjustments (35 U.S.C. § 154; USPTO, n.d.-b).

The ordinary term for US 5,919,455 therefore ended in 2017. The patent should be treated as expired for current US enforcement analysis. Any historical patent-term adjustment should be confirmed in the USPTO Patent Center record before relying on an exact day-level expiration date.

Because the patent is expired:

no current Paragraph IV challenge is required against this patent;
no current injunction should be available based solely on this patent;
the claims cannot block current generic or biosimilar entry;
later patents in the same technology area may still create commercial barriers;
know-how, regulatory exclusivity, and manufacturing patents may remain relevant.

What is the Orange Book status of US Patent 5,919,455?

US Patent 5,919,455 is not an Orange Book drug patent. It is a platform patent covering polymers, conjugates, and manufacturing methods.

The FDA Orange Book lists patents submitted by sponsors for approved small-molecule drug products, including composition, formulation, method-of-use, and certain manufacturing patents. A general PEG platform patent is not automatically listed against every drug that uses PEGylation (FDA, 2024a).

The patent is also not a Purple Book listing. The Purple Book identifies licensed biological products and biosimilar or interchangeable-product information, not a complete register of platform patents that may cover biologic conjugation technology (FDA, 2024b).

Are there Paragraph IV challenges involving this patent?

No current Paragraph IV challenge should be expected for US 5,919,455 because the patent term has ended and the patent is not an Orange Book-listed product patent.

Paragraph IV litigation would arise against a listed patent for a specific approved drug, not against an expired platform patent in isolation. For a PEGylated small-molecule product, the relevant litigation analysis would focus on:

the drug-specific composition patent;
formulation and dosage-form patents;
method-of-use patents;
linker and conjugation patents;
manufacturing patents;
regulatory exclusivity.

For a PEGylated biologic, the relevant framework would include the Biologics Price Competition and Innovation Act, Purple Book status, licensed biologic exclusivity, biosimilar patent-exchange activity, and later patent families.

Does the patent create biosimilar risk?

US 5,919,455 does not create current biosimilar risk because it is expired. It also does not independently establish biosimilar exclusivity.

The patent can have historical relevance to products such as PEGylated proteins, but current biosimilar exposure depends on later patents covering:

the specific therapeutic protein;
PEG attachment site;
degree of PEGylation;
branched versus linear PEG architecture;
linker chemistry;
formulation;
purification;
manufacturing controls;
therapeutic use.

A biosimilar applicant could potentially use a different PEG reagent or conjugation process and avoid later process claims while still producing a molecule with similar pharmacological properties. Conversely, a product can avoid the expired patent but remain exposed to later patents directed to the final conjugate or manufacturing process.

Which companies and patent families are relevant to the surrounding landscape?

The relevant competitive landscape is the PEGylation platform market, not a single generic-drug market.

Company or group	Historical relevance
Shearwater Polymers	Original assignee associated with the branched PEG platform
Nektar Therapeutics	Successor to Shearwater’s PEGylation platform and later PEG-related licensing activities
Enzon Pharmaceuticals	Commercialized PEGylated biologics and developed PEG modification technology
Amgen	Developed and commercialized PEGylated biologic products, including pegfilgrastim
Roche and related entities	Developed PEGylated biologic products, including PEG-modified interferon and other biologics
Merck and Schering-Plough	Developed and commercialized PEGylated interferon products
Specialized PEG reagent suppliers	Supply linear, branched, multi-arm, and site-specific PEG reagents

The key later patent clusters generally fall into five categories:

Branched and multi-arm PEG architecture.
Site-specific PEGylation.
Linkers that provide controlled hydrolysis or intracellular release.
PEGylated drug products and defined conjugate structures.
Manufacturing, purification, and characterization of PEG conjugates.

US 5,919,455 should be analyzed as an early platform layer. Later patents may claim narrower commercial embodiments that remain enforceable after the 2017 expiration of the originating patent.

How strong is the patent estate?

Historical strength

The patent had substantial historical breadth because it combined:

composition claims;
activated-reagent claims;
conjugate claims;
treatment claims;
process claims;
coverage of both proteins and chemotherapeutic molecules;
broad PEG molecular-weight ranges;
multiple linker and spacer options.

The strongest historical claims were likely claims 1 and 24 because they targeted the polymer reagent itself. A supplier or developer making the claimed branched PEG reagent could have faced infringement exposure before expiration even if the reagent was later sold to another party for conjugation.

Current strength

Current enforceability is zero for the expired US patent. Its practical value is now:

prior-art significance;
historical interpretation of branched PEG chemistry;
evidence of ownership and licensing history;
a baseline for mapping later continuation and improvement patents;
a technical reference for freedom-to-operate analysis.

Claim vulnerabilities

The claims contain several potential construction and validity pressure points:

"substantially non-antigenic" is functional and potentially fact-dependent;
"water soluble" may require technical evidence for borderline polymers;
"about" creates molecular-weight and numerical-range interpretation issues;
L is defined through complex Markush structures;
p is stated as a positive integer without a clear upper limit;
claims 15 and 33 use broad nucleophile classes;
claim 21 covers broad therapeutic classes rather than a single defined product;
claim 39 contains a specific two-step active-carbonate preparation that may be vulnerable to nonmatching process designs.

These issues would have mattered during the patent term. They do not restore enforceability after expiration.

What generic launch scenarios exist?

Scenario 1: Linear PEG substitution

A product using a linear PEG rather than a two- or three-arm branched PEG would generally be outside the central branched-polymer limitation. It could still face later patents covering the final drug or a different PEGylation method.

Scenario 2: Alternative branching topology

A four-arm, star-shaped, dendritic, or otherwise differently connected PEG may avoid claims requiring n=2 or 3, subject to the exact structure and any later patent rights.

Scenario 3: Different linker chemistry

A reagent using a linker outside the claimed L Markush structures may avoid literal infringement. The commercial risk then shifts to later linker patents.

Scenario 4: Different activation route

A manufacturer that does not use the claimed alkyl haloacetate, p-nitrophenyl chloroformate, or succinimide chemistry may avoid the process claims, although the resulting composition could still fall within claims 1 or 24.

Scenario 5: Same final conjugate, different process

A different manufacturing route does not necessarily avoid a composition claim. If the final branched PEG conjugate falls within an enforceable composition claim, process substitution alone would not eliminate composition-claim exposure. For US 5,919,455, that distinction is historical because the composition claims are expired.

What geographic coverage does the patent provide?

US 5,919,455 provides US coverage only. It does not establish rights in Europe, Japan, Canada, China, or other jurisdictions.

Foreign protection would depend on corresponding national applications, continuations, divisionals, and international filings. The existence of a US patent does not prove that corresponding foreign patents issued or remain in force.

For a global product launch, the relevant geographic review should separately evaluate:

US patent families and continuations;
European Patent Office grants and validations;
Japanese and Canadian counterparts;
Chinese patents;
country-specific supplementary protection or regulatory exclusivity;
local process and manufacturing patents.

What licensing deals are associated with this technology?

Shearwater’s PEGylation technology was incorporated into Nektar Therapeutics’ platform following the company’s corporate development and was used in licensing and collaboration arrangements involving PEGylated therapeutics. Nektar’s public filings describe PEGylation technology, collaborations, milestone payments, royalties, and product-related licensing arrangements, but a public filing does not establish that every agreement licensed US 5,919,455 specifically (Nektar Therapeutics, 2001).

The distinction matters:

a platform license may cover a portfolio rather than one patent;
a product license may include rights to later improvement patents;
a reagent-supply agreement may not transfer patent rights;
a sublicense may contain field-of-use or territory restrictions;
an expired patent may remain referenced in historical agreements without creating current exclusion.

No conclusion that a marketed product is licensed under US 5,919,455 should be drawn without reviewing the agreement, patent schedules, and continuation-family records.

Key Takeaways

US 5,919,455 covers branched non-antigenic polymers, especially branched PEG, and their activated derivatives.
The core architecture contains two or three polymer arms attached through a defined linker.
Claims cover carboxylic-acid intermediates, activated PEG reagents, biological conjugates, treatment methods, and manufacturing processes.
Claims 1 and 24 are the principal platform composition claims.
Claims 16-23 address PEGylated conjugates and therapeutic use.
Claims 34-40 cover selected preparation and activation methods.
The patent’s ordinary US term ended in 2017. It is not a current enforceable barrier.
The patent is not an Orange Book-listed drug patent and does not require a current Paragraph IV challenge.
It does not independently create biosimilar exclusivity or current biosimilar risk.
Current commercial risk must be assessed against later patents covering the final drug, linker, conjugation site, formulation, manufacturing process, and therapeutic use.
The patent has historical importance as an early branched-PEG platform patent but has no present US blocking strength by itself.

FAQs About US Patent 5,919,455 and Branched PEG Patent Protection

Can an expired PEG platform patent still affect a drug launch?

Yes, as prior art and as part of the historical record, but an expired patent cannot independently block a current US launch. Later patents may still cover the product or manufacturing process.

Does a PEGylated biologic infringe US 5,919,455 automatically?

No. The product would need to satisfy the relevant structural limitations, including branched architecture, two or three polymer arms, linker structure, and other claim requirements. The patent is also expired.

Are methoxy-terminated PEG arms required?

Methoxy is expressly claimed in dependent claims 10 and 30. The broader independent claims require a C1-C4 terminal group, which may include terminal groups other than methoxy if the remaining limitations are met.

Do claims 21 and 22 cover all PEGylated taxanes and methotrexates?

No. The drug-class language is subordinate to the structural limitations of the polymer and conjugate claims. A product must satisfy those limitations as well as the specified therapeutic-molecule limitation.

Is US 5,919,455 relevant to current PEG reagent suppliers?

It is relevant historically and as a reference point for patent-family analysis. Current supplier risk depends on later, unexpired patents covering specific branched PEG reagents, linkers, activation chemistries, or manufacturing processes.

References

Food and Drug Administration. (2024a). Approved drug products with therapeutic equivalence evaluations: Orange Book. U.S. Department of Health and Human Services. https://www.fda.gov/drugsatfda
Food and Drug Administration. (2024b). Purple Book: Database of licensed biological products. U.S. Department of Health and Human Services. https://purplebooksearch.fda.gov
Nektar Therapeutics. (2001). Annual report. U.S. Securities and Exchange Commission.
U.S. Patent No. 5,919,455. (1999). Branched poly(ethylene glycol) derivatives. United States Patent and Trademark Office.
United States Patent and Trademark Office. (n.d.-a). Patent Center. https://patentcenter.uspto.gov
United States Patent and Trademark Office. (n.d.-b). Patent term adjustment and patent term calculation. https://www.uspto.gov
35 U.S.C. § 154. Patent term.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	243723	⤷ Start Trial
Austria	303412	⤷ Start Trial
Australia	6463098	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Summary for Patent: 5,919,455