US Patent 6,051,698: Scope, Claims, Expiration, and VEGF Aptamer Patent Landscape

US Patent 6,051,698 protects an early VEGF-binding RNA aptamer platform that became the basis for pegaptanib sodium, marketed as Macugen. Its claims cover four related subject areas: 2'-fluoro-modified RNA ligands to VEGF, defined VEGF aptamer sequences and variants, PEGylated aptamer complexes, and methods for selecting or improving the pharmacokinetic properties of those ligands. The patent was granted on April 18, 2000, and its ordinary 20-year term from the nonprovisional filing date ended on October 21, 2018, absent a separate term adjustment or extension.[1]

The patent is therefore expired and does not present a current US blocking right. Its historical importance remains substantial because it established the core aptamer and PEG-conjugation architecture used for pegaptanib.

What technology does US Patent 6,051,698 protect?

The patent covers a VEGF-binding nucleic acid ligand, generally an RNA aptamer, containing 2'-fluoro-modified nucleotides. The 2'-fluoro substitutions improve nuclease resistance and help make an RNA aptamer suitable for pharmaceutical use.

The principal technical elements are:

Technical element	Covered by claims
Non-naturally occurring RNA ligand binding VEGF	1
2'-fluoro-modified nucleotides	1
VEGF aptamer sequences identified in Tables 1-4	3, 7
Homologous sequence variants with substantially similar VEGF binding	4
Structurally similar ligands with substantially similar binding	5
SELEX-type selection method	6, 7
Covalent attachment of a high-molecular-weight, non-immunogenic compound	2, 8
Polyalkylene glycol conjugates	10, 16
Polyethylene glycol conjugates	11, 17
PEG molecular weight of approximately 10-80 kDa	12, 18
PEG molecular weight of approximately 20-45 kDa	13, 19
Specific chemical conjugate structures	14, 15, 20, 21

The patent is not limited to a particular ophthalmic disease. Its claims are directed primarily to the ligand and conjugate architecture, although claim 2 expressly addresses improving pharmacokinetic properties in an ocular application.

What are the main independent claim concepts?

Claim 1: 2'-fluoro VEGF RNA ligand

Claim 1 is the central composition claim. It requires:

A purified and isolated RNA ligand;
Binding to vascular endothelial growth factor;
A non-natural origin; and
Inclusion of 2'-fluoro-modified nucleotides.

The claim does not expressly require PEG, ocular administration, a particular sequence, a particular VEGF isoform, or a particular therapeutic indication. On its face, it reaches a genus of 2'-fluoro RNA VEGF ligands.

A potential infringement theory would require proof that the accused product contains an RNA ligand that binds VEGF and includes the specified 2'-fluoro modification. A conventional unmodified RNA aptamer would not satisfy the express 2'-fluoro limitation. A DNA ligand, peptide, antibody, or protein would also fall outside the literal language of claim 1.

Claim 2: ocular pharmacokinetic improvement

Claim 2 covers a method for improving the pharmacokinetic properties of a VEGF nucleic acid ligand in an ocular application by covalently linking the ligand to a non-immunogenic, high-molecular-weight compound.

The claim has several narrowing requirements:

The ligand must be a VEGF nucleic acid ligand.
The use must be an ocular application.
The high-molecular-weight compound must be covalently linked.
The purpose must be improvement of pharmacokinetic properties.

The claim is broader than a PEG-only claim because the compound is not initially limited to polyethylene glycol. Claims 16-19 narrow the compound to a polyalkylene glycol, polyethylene glycol, and specified molecular-weight ranges.

The claim could cover improvements in residence time, nuclease protection, clearance, or systemic exposure. It does not expressly require intravitreal injection, but an accused ocular use would likely need to be established from the product label, clinical use, or other evidence.

How do claims 3 through 7 define the aptamer sequences?

Claims 3 through 5 create sequence and functional layers around the sequences disclosed in Tables 1-4, identified as SEQ ID NOS: 10-86.

Claim 3: listed sequences

Claim 3 is limited to ligands selected from the disclosed sequences. This is the most conventional sequence-specific claim in the set.

Claim 4: substantially homologous variants

Claim 4 extends to ligands substantially homologous to the listed sequences that retain substantially the same ability to bind VEGF. The claim therefore attempts to capture sequence variants, including substitutions or other changes that preserve binding activity.

The principal litigation issue would be the meaning of:

"Substantially homologous"; and
"Substantially the same ability to bind VEGF."

Those terms may require construction using the specification, experimental data, and the disclosed aptamer structures. A small sequence difference would not automatically establish infringement or noninfringement.

Claim 5: structural and functional equivalents

Claim 5 uses a structural-function formulation. It covers ligands with substantially the same structure and substantially the same VEGF-binding ability as a listed ligand.

This claim is potentially broader than a strict exact-sequence claim but remains constrained by the requirement for substantial structural and functional similarity. The claim does not appear to cover every VEGF-binding nucleic acid. It is directed to variants that remain close to the disclosed aptamer structures.

Claims 6 and 7: SELEX-derived ligands

Claims 6 and 7 describe a selection process involving:

Contacting a candidate RNA mixture with VEGF;
Partitioning higher-affinity RNA from the remainder; and
Amplifying the selected RNA to produce an enriched mixture.

This is the basic SELEX process used to identify aptamers. Claims 6 and 7 are unusual because they combine a ligand definition with the method used to identify it. Their practical enforcement value depends on whether the accused ligand was made or identified through the claimed selection steps, rather than merely whether it has similar binding characteristics.

What formulations and conjugates are protected?

Claims 8 through 21 cover complexes consisting of the VEGF RNA ligand and a high-molecular-weight compound. The preferred embodiment is PEGylated RNA.

PEG molecular-weight ranges

Claim	PEG limitation
12 and 18	Approximately 10-80 kDa
13 and 19	Approximately 20-45 kDa

The use of "about" creates ordinary claim-construction questions concerning permissible variation around the stated ranges. A PEG conjugate outside the recited range may avoid literal infringement of the narrower claims, although it could remain relevant to broader claims or equivalents analysis.

Pegaptanib uses a high-molecular-weight PEG group to increase molecular size and reduce renal clearance. FDA labeling describes Macugen as a modified RNA aptamer conjugated to a 40-kDa PEG moiety.[2]

Linker limitations

Claim 9 adds a linker between the RNA ligand and the high-molecular-weight compound. This limitation distinguishes direct conjugation from a structure in which the PEG is connected through a chemical spacer.

The precise scope depends on the patent's structural disclosure and the meaning of "linker." A linker may include a chemical spacer, a phosphodiester-related connection, or another covalent connecting group, depending on the disclosed embodiments.

Claims 14, 15, 20, and 21

These claims refer to specific structures depicted in the patent drawings. The chemical structures are not reproduced in the supplied claim text. Their scope cannot be evaluated solely from the text because the identity, attachment position, linker chemistry, and PEG architecture are structural limitations.

These claims should be analyzed separately from the generic PEG claims. A product could fall within the general conjugate genus while avoiding a particular depicted structure, or it could satisfy a depicted structure even if the commercial formulation differs in secondary details.

What is the patent's relationship to pegaptanib and Macugen?

Pegaptanib sodium is an anti-VEGF aptamer approved by the FDA for neovascular age-related macular degeneration. The FDA approved Macugen on December 17, 2004, under NDA 021756.[2,3]

Macugen's active ingredient is a chemically modified oligonucleotide that binds the 165-amino-acid isoform of VEGF. The product is administered by intravitreal injection. Its pharmacological design corresponds closely to the patent's claimed combination of:

A VEGF-binding RNA ligand;
2'-fluoro-modified nucleotides;
PEG conjugation; and
Ocular use.

The commercial development involved Eyetech Pharmaceuticals and Pfizer. The underlying aptamer technology was associated with NeXstar Pharmaceuticals and later Gilead Sciences through corporate transactions and licensing arrangements. Pfizer and Eyetech entered into a development and commercialization agreement for Macugen in 2000.[4]

When did US Patent 6,051,698 lose exclusivity?

The patent's relevant timeline is:

Event	Date
Earliest provisional priority	October 22, 1997
US nonprovisional filing	October 21, 1998
Patent grant	April 18, 2000
Macugen FDA approval	December 17, 2004
Ordinary patent-term expiration	October 21, 2018

For a US utility patent filed after June 8, 1995, the ordinary term is generally 20 years from the earliest effective nonprovisional filing date, subject to patent-term adjustment, patent-term extension, terminal disclaimers, and other statutory rules.[5]

The patent's ordinary term therefore expired in 2018. FDA regulatory exclusivity for Macugen was separate from patent protection and did not preserve the patent after expiration. The product's commercial decline also reduced the economic significance of the patent estate after newer anti-VEGF therapies became dominant.

What was the Orange Book status of Macugen?

Macugen was approved under NDA 021756 and was historically associated with patent protection for the pegaptanib composition and related technology. Orange Book listing and patent expiration information are maintained by FDA and may change based on regulatory submissions, delistings, corrections, or product status.[3]

The key distinction is:

Orange Book listing identifies patents submitted by an NDA holder.
Listing does not determine ultimate validity or infringement.
Expiration ends enforceable patent rights even if historical listing data remains visible.
FDA regulatory exclusivity and patent exclusivity are separate rights.

Because US Patent 6,051,698 expired in 2018, its historical Orange Book presence does not create a current barrier to an otherwise approvable follow-on product.

Were there Paragraph IV challenges or settlements?

No major, commercially consequential Paragraph IV litigation involving US Patent 6,051,698 is generally associated with the US market in the manner seen for major small-molecule blockbusters. Macugen did not develop a large generic market after patent expiry, and the commercial opportunity was limited by declining use of pegaptanib relative to ranibizumab, aflibercept, and off-label bevacizumab.

A Paragraph IV applicant would have been able to challenge an Orange Book-listed patent by certifying that the patent was invalid, unenforceable, or not infringed. After expiration, a Paragraph IV strategy would have had little practical value unless directed to an unexpired continuation or related patent.

The absence of a prominent challenge does not establish that the claims were unassailable. It more likely reflects the limited commercial value of entering the pegaptanib market after the product lost clinical share.

How strong was the patent estate?

The original patent was strong as a pioneering platform patent but narrower in practical scope than the broad commercial description of "VEGF aptamer technology" might suggest.

Strengths

Claim 1 directly addressed the core 2'-fluoro RNA ligand concept.
Claims 3-5 connected the invention to disclosed sequences and functional variants.
Claims 8-19 covered PEGylated embodiments relevant to pharmaceutical development.
The patent supported a product architecture that reached FDA approval.
The claims addressed both composition and pharmacokinetic modification.

Vulnerabilities

The claims depend on technical terms such as "substantially homologous," "substantially the same structure," and "substantially the same ability."
Claims 2, 6, and 7 contain method-specific limitations that may be difficult to prove against a finished product.
The exact sequences and structures are central to infringement analysis.
The patent could not broadly block antibodies, recombinant VEGF traps, or unrelated VEGF inhibitors.
Expiration eliminated its current blocking effect.

The patent would have had its greatest commercial value before Macugen approval and during the early period when PEGylated VEGF aptamers had limited direct competition.

How does this patent compare with competing anti-VEGF estates?

Product	Modality	Principal developer	US approval	Relationship to US 6,051,698
Macugen, pegaptanib	PEGylated RNA aptamer	Eyetech/Pfizer	2004	Directly aligned with claimed VEGF aptamer technology
Lucentis, ranibizumab	Anti-VEGF antibody fragment	Genentech/Roche	2006	Separate biologic antibody estate
Avastin, bevacizumab	Full-length anti-VEGF antibody	Genentech/Roche	2004 for oncology	Separate antibody estate; ophthalmic use was off-label
Eylea, aflibercept	Recombinant VEGF-binding fusion protein	Regeneron/Bayer	2011	Separate protein and formulation estate
Vabysmo, faricimab	Bispecific antibody	Genentech/Roche	2022	Separate antibody estate

The main competitive threat to Macugen was not a patent workaround around the same aptamer. It was substitution by antibody fragments and VEGF-trap proteins with stronger clinical and commercial positioning.

What generic and biosimilar entry risks exist?

Generic entry

Pegaptanib is an oligonucleotide drug rather than a conventional small molecule. A follow-on applicant would face analytical, manufacturing, sequence, impurity, PEG-conjugation, potency, sterility, and delivery challenges.

A 505(j) abbreviated new drug application may be difficult for a complex oligonucleotide product where active-ingredient sameness and therapeutic equivalence are difficult to establish. A 505(b)(2) application could provide a more practical regulatory route depending on the proposed product and differences from Macugen.[6]

Biosimilar entry

Pegaptanib is not a conventional protein biologic comparable to ranibizumab or aflibercept. A biosimilar pathway would not be the natural route for an RNA aptamer unless FDA classified the product and pathway in a manner permitting use of the Public Health Service Act framework.

The commercial risk is therefore better characterized as complex generic or follow-on oligonucleotide risk, not conventional biosimilar risk.

What manufacturing and IP barriers remain after patent expiration?

Patent expiration removes the principal composition barrier, but technical barriers remain:

Reproducible chemical synthesis of the modified RNA;
Control of 2'-fluoro substitution patterns;
Sequence fidelity and truncation control;
Site-specific PEG attachment;
Control of PEG molecular-weight distribution;
Removal of residual reagents and manufacturing impurities;
Demonstration of VEGF-binding potency;
Sterile intravitreal formulation;
Stability during storage and administration;
Comparability of ocular exposure and pharmacokinetics.

These barriers may protect an incumbent commercially without creating patent exclusivity. Separate patents covering manufacturing processes, formulations, devices, dosing schedules, or later-generation anti-VEGF products would need independent analysis.

What is the current commercial exposure?

Macugen's market position declined after the introduction of Lucentis and Eylea. The product's original opportunity was concentrated in neovascular age-related macular degeneration. Its clinical use became limited relative to broader VEGF-blocking agents and newer dosing regimens.

The expired patent creates no current US exclusivity-based revenue moat for pegaptanib. Any residual value would arise from manufacturing know-how, regulatory history, supply capability, residual demand, or licensing rights rather than US Patent 6,051,698 itself.

Key Takeaways

US Patent 6,051,698 is the foundational US patent for 2'-fluoro RNA VEGF aptamers and PEGylated VEGF nucleic acid ligands.
Claim 1 covers a non-natural, purified RNA VEGF ligand containing 2'-fluoro-modified nucleotides.
Claims 3-5 cover listed VEGF aptamer sequences and certain homologous or structurally similar variants.
Claims 8-21 cover PEG and related high-molecular-weight conjugates, including approximately 10-80 kDa and 20-45 kDa PEG ranges.
Claims 2, 6, and 7 add ocular pharmacokinetic or SELEX-process limitations.
The patent was filed October 21, 1998, granted April 18, 2000, and ordinarily expired October 21, 2018.
The patent was closely related to pegaptanib sodium, Macugen, approved by FDA in 2004.
The patent did not cover competing anti-VEGF antibodies or VEGF-trap proteins.
Current follow-on risk is primarily technical and regulatory, not based on this expired patent.
Claims 14, 15, 20, and 21 require the actual patent drawings for a complete structural analysis.

FAQs

Can a new VEGF aptamer infringe claim 1 after the patent expired?

No current US infringement liability arises from an expired claim. Before expiration, a new aptamer could have raised infringement issues if it satisfied the 2'-fluoro RNA and VEGF-binding limitations.

Does PEGylation alone make an RNA product fall within US Patent 6,051,698?

No. The relevant claims generally require both a VEGF RNA ligand and the specified high-molecular-weight conjugate. PEGylation of an unrelated RNA would not satisfy those limitations.

Does the patent cover all VEGF-binding oligonucleotides?

No. Its claim scope is directed to specified RNA ligands, 2'-fluoro-modified embodiments, disclosed sequences and variants, and related conjugates. Antibodies, proteins, DNA ligands, and unrelated oligonucleotide structures are outside the core composition claim.

Is pegaptanib eligible for a conventional biosimilar application?

Pegaptanib is an RNA aptamer, not a conventional therapeutic protein. A follow-on applicant would more likely assess an abbreviated or 505(b)(2)-type strategy than rely on the standard antibody or protein biosimilar model.

Do expired claims still affect licensing negotiations?

They can affect historical chain-of-title analysis, know-how valuation, freedom-to-operate diligence for past conduct, and interpretation of related continuation or foreign rights. They do not independently provide current US exclusionary rights after expiration.

References

United States Patent and Trademark Office. (2000). U.S. Patent No. 6,051,698, Nucleic acid ligands to vascular endothelial growth factor. https://patents.google.com/patent/US6051698A/en
U.S. Food and Drug Administration. (2004). Macugen prescribing information. https://www.accessdata.fda.gov/drugsatfda_docs/label/2004/021756lbl.pdf
U.S. Food and Drug Administration. (2025). Approved drug products with therapeutic equivalence evaluations: Orange Book. https://www.fda.gov/drugs/drug-approvals-and-databases/approved-drug-products-therapeutic-equivalence-evaluations-orange-book
Pfizer Inc. (2000). Pfizer and Eyetech enter agreement to develop and commercialize Macugen. Pfizer investor and corporate announcements.
United States Patent and Trademark Office. (2024). Patent term adjustment and patent term extension. https://www.uspto.gov/patents/laws/patent-term-calculator
U.S. Food and Drug Administration. (2023). Applications covered by section 505(b)(2). https://www.fda.gov/drugs/abbreviated-new-drug-application-anda/applications-covered-section-505b2

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
European Patent Office	0957929	⤷ Start Trial	91252	Luxembourg	⤷ Start Trial
European Patent Office	0957929	⤷ Start Trial	300234	Netherlands	⤷ Start Trial
European Patent Office	0957929	⤷ Start Trial	PA2006004	Lithuania	⤷ Start Trial
European Patent Office	0957929	⤷ Start Trial	CA 2006 00021	Denmark	⤷ Start Trial
European Patent Office	0957929	⤷ Start Trial	SPC024/2006	Ireland	⤷ Start Trial
European Patent Office	0957929	⤷ Start Trial	PA2006004,C0957929	Lithuania	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 6,051,698

Summary for Patent: 6,051,698