Introduction
Antisense oligonucleotides (ASOs) are among the most sequence-dependent inventions in biotechnology, with their identity, function and therapeutic effect directly tied to their nucleotide sequence. As a result, accurate sequence listing preparation is critical for patent protection. The presence of backbone, sugar and base modifications—key to ASO performance—creates additional compliance challenges under WIPO Standard ST.26. This article outlines the essential requirements for ASO sequence listings, including classification rules, modification annotation practices, ST.26 compliance tools and common errors that can lead to examination objections or legal risks.
Why ASO Sequence Listings Are Uniquely Complex
Understanding what makes ASO listings different from standard nucleic acid listings is the foundation for understanding the compliance requirements.
The Modification Problem
A standard nucleotide sequence listing records the identity and order of nucleobases – adenine, thymine, guanine, cytosine, uracil – in a linear chain. For an unmodified DNA or RNA sequence, this is straightforward. The IUPAC nucleotide codes are well-defined, WIPO Sequence handles them natively and the annotation burden is limited to biological features like coding sequences and functional regions.
Therapeutic ASOs are almost never unmodified. The modifications that define their therapeutic profile – and that are almost always the subject of patent claims – include:
Backbone modifications:
- Phosphorothioate (PS) linkages – sulfur replaces one non-bridging oxygen in the phosphodiester backbone; the most common ASO backbone modification, conferring nuclease resistance
- Phosphorodithioate linkages – both non-bridging oxygens replaced by sulfur
- Phosphonoacetate and thiophosphonoacetate linkages
- Boranophosphate linkages
- Methylphosphonate linkages – neutral backbone, eliminating the negative charge
Sugar modifications:
- 2′-O-methyl (2′-OMe) – the most common sugar modification; methyl group at the 2′ position of ribose
- 2′-O-methoxyethyl (2′-MOE) – longer alkoxy group at 2′ position; used in gapmer designs
- 2′-fluoro (2′-F) – fluorine at the 2′ position; enhances binding affinity
- Locked nucleic acid (LNA) / bridged nucleic acid (BNA) – an additional methylene bridge connects the 2′ oxygen to the 4′ carbon, locking the sugar in the 3′-endo conformation
- 2′,4′-constrained 2′-O-ethyl (cEt) – constrained ethyl modification; used in next-generation gapmers
- Morpholino – the entire ribose sugar is replaced with a morpholine ring; used in PMO (phosphorodiamidate morpholino oligomers) designs
Base modifications:
- 5-methylcytosine – methyl group on the 5-position of cytosine
- 2,6-diaminopurine – modified adenine analog
- 5-propynylcytosine and 5-propynyluracil – modified bases with enhanced binding affinity
- G-clamp (phenoxazine cytosine analog) – high-affinity cytosine replacement
End modifications and conjugates:
- 5′ and 3′ end-capping modifications to confer nuclease resistance
- GalNAc conjugates for hepatic targeting
- Cholesterol, lipid and polymer conjugates
- Phosphate group modifications at 5′ and 3′ termini
For a patent claiming a phosphorothioate ASO with a gapmer architecture – where a central DNA gap region is flanked by 2′-MOE-modified wings – simply recording the nucleobase sequence without disclosing the modification pattern is legally and scientifically inadequate. The modification pattern is an integral part of the invention’s identity.
The Sequence Listing’s Role in ASO Patent Claims
ASO patents routinely claim their inventions at least partly by reference to specific sequences – “an antisense oligonucleotide comprising the sequence of SEQ ID NO: 1,” “an oligonucleotide of 14 to 20 nucleotides in length complementary to a region of SEQ ID NO: 5,” “the compound of claim 1 wherein the oligonucleotide comprises at least one phosphorothioate internucleotide linkage.”
These claims create a direct legal dependency between the patent’s claims and the sequence listing’s accuracy. If SEQ ID NO: 1 in the sequence listing does not correctly represent the compound claimed in claim 1 – because modifications are missing, residues are misidentified, or the sequence length is wrong – the claim lacks proper support. A claim to a compound defined by reference to a sequence listing entry that inaccurately represents the compound is a claim to the wrong compound.
This is not merely a formality compliance issue. It is a written description and enablement issue under 35 U.S.C. § 112 that can invalidate the claims.
WIPO ST.26 Framework for Modified Nucleotides
WIPO Standard ST.26 provides specific mechanisms for disclosing modified nucleotide residues, but these mechanisms require understanding and correct application. The relevant tools are:
The Modified_Base Feature Key
For sequences containing modified nucleobases – 5-methylcytosine, 2,6-diaminopurine and other base-modified residues – ST.26 uses the modified_base feature key to annotate the modified position.
Implementation:
- The modified_base feature is placed at the position(s) in the sequence where the modification occurs
- The /mod_base qualifier specifies the type of modification using the controlled vocabulary from WIPO ST.26 Table 3
- For modifications not listed in Table 3’s controlled vocabulary, the value “OTHER” is used for the /mod_base qualifier, with a mandatory /note qualifier providing a chemical description of the modification
Example: For a sequence where position 5 contains 5-methylcytosine:
Feature key: modified_base
Location: 5
Qualifier: /mod_base = “5mc”
For a modification not in the controlled vocabulary (e.g., a proprietary modified base):
Feature key: modified_base
Location: 8
Qualifier: /mod_base = “OTHER”
Qualifier: /note = “5-(1-propynyl)-2′-deoxyuridine”
The Modified_DNA and Modified_RNA Sequence Type
For sequences that consist of non-standard nucleotide units – morpholino oligomers, PNA (peptide nucleic acid) sequences and other non-natural backbone architectures – ST.26’s categorization requires careful consideration.
Under ST.26, sequences are classified as:
- DNA – deoxyribonucleotide sequences
- RNA – ribonucleotide sequences
- AA – amino acid sequences
- DNA/RNA – chimeric sequences containing both deoxy- and ribonucleotides
Sequences with backbone modifications (phosphorothioate, phosphonate, morpholino) but standard nucleobases use the DNA or RNA type designation as appropriate for the base identity – the backbone modification is annotated separately through feature keys and qualifiers rather than through the sequence type designation.
This creates a compliance challenge: a phosphorothioate ASO with a DNA backbone and standard nucleobases (ACGT) is classified as type DNA in the sequence listing and the phosphorothioate modifications must be fully described in the specification text and, where required by the claims, in the sequence listing’s feature annotations.
Handling Chimeric ASOs
Many therapeutic ASOs – particularly gapmers – are chimeric constructs containing both DNA and 2′-modified RNA residues. A typical 20-mer gapmer might have:
- Positions 1–5: 2′-MOE ribonucleotides (RNA-type residues with 2′-O-methoxyethyl modification)
- Positions 6–15: 2′-deoxyribonucleotides (DNA-type gap region)
- Positions 16–20: 2′-MOE ribonucleotides
For chimeric sequences, ST.26 uses the DNA/RNA sequence type designation. Each position’s ribose or deoxyribose nature must be explicitly recorded in the sequence data, using the appropriate nucleotide codes (uppercase for DNA residues, lowercase for RNA residues in the WIPO Sequence input, or explicit designation of each position’s sugar type).
For a chimeric 10-mer (5′-ACGT[r]acgt-3′, where lowercase indicates ribonucleotides):
- Positions 1–6: DNA residues → uppercase IUPAC codes
- Positions 7–10: RNA residues → recorded as RNA-type at those positions
The 2′-MOE modification on the RNA-type residues is then annotated using the modified_base feature with the appropriate qualifier for the 2′-position modification – or through description in the specification with explicit cross-reference to the sequence entry.
The Critical Role of the Specification Description
ST.26’s annotation requirements for modified residues are extensive but not unlimited in what they can encode directly in the XML structure. For complex modification patterns – particularly variable backbone modifications across the length of an ASO – the sequence listing’s feature annotations work in conjunction with the specification’s written description to fully define the compound.
A compliance-optimal approach for complex ASO listings:
- Use ST.26 feature annotations for all modifications that have defined controlled vocabulary entries or can be clearly described in /note qualifiers
- In the specification’s detailed description, provide a complete modification table for each claimed ASO – specifying position by position the nucleobase identity, the sugar modification and the backbone linkage type
- In the claims, reference both the SEQ ID NO and the modification description: “an antisense oligonucleotide consisting of the sequence of SEQ ID NO: 1 wherein all internucleotide linkages are phosphorothioate linkages and positions 1–5 and 16–20 comprise 2′-O-methoxyethyl modified residues”
This layered approach – sequence listing for sequence identity, specification for modification pattern, claims that integrate both – provides the most robust legal and regulatory foundation.
The Ten-Residue Threshold and ASO Claim Scope
WIPO ST.26 requires inclusion in the sequence listing of all nucleotide sequences of ten or more residues. For ASO patents, this threshold has specific implications:
Short ASO Sequences Below Threshold
Some ASOs in clinical development – particularly highly modified LNA-containing oligomers – may be as short as 8–12 residues. For ASOs below the 10-residue threshold, formal sequence listing inclusion is not required. However:
- The sequence must still be fully disclosed in the specification text
- If claims reference shorter sequences, the claims must not use SEQ ID NO notation (which implies a sequence listing entry)
- Drawings may include structural representations of short sequences with full chemical modification notation
For ASOs at or near the 10-residue threshold, erring on the side of inclusion – filing the sequence in the sequence listing even if borderline – provides more robust support than omission.
Target RNA/mRNA Sequences
Most ASO patents claim not only the therapeutic oligonucleotide itself but also the target RNA sequence to which the ASO binds. The target mRNA sequence is typically much longer than 10 residues (mRNA sequences are commonly hundreds to thousands of nucleotides) and must be included in the sequence listing.
For target sequence claims, the sequence listing serves the additional function of precisely defining the claimed target region – enabling construction of the key patent claim “an antisense oligonucleotide complementary to nucleotides [X] through [Y] of SEQ ID NO: [Z],” where SEQ ID NO: Z is the target mRNA sequence entry.
Antisense Strand vs. Sense Strand
For double-stranded ASO applications (siRNA, for example), both the antisense and sense strands must be included as separate sequence entries with their own SEQ ID NOs. In the sequence listing and specification, the antisense strand is typically SEQ ID NO: N and the complementary sense strand is SEQ ID NO: N+1 or designated otherwise with explicit notation of their complementary relationship.
Multi-Target and Pan-Sequence ASO Patents
Advanced ASO patents – those claiming oligonucleotides targeting multiple sites on a target RNA, or claiming pan-family targeting of related transcripts – present unique sequence listing scope challenges.
Representative Sequence Claims
Some ASO patents claim compounds that are not limited to a single precise sequence but are defined by a representative sequence with permitted degeneracy or variation. For example: “an antisense oligonucleotide 15–25 nucleotides in length wherein the oligonucleotide is at least 80% complementary to SEQ ID NO: 5.”
For these claims, the sequence listing must include the specific sequences that are the preferred embodiments of the claimed range – not every possible sequence within the range (which could be astronomically large), but sufficient representative sequences to demonstrate possession of the claimed genus.
The specification must describe how to generate and evaluate additional sequences within the claimed range – providing enablement support for the full scope of the claimed class.
Sequence Sets for Target Regions
When an ASO patent claims oligonucleotides targeting a defined region of a target transcript – for example, “an ASO targeting the splice donor site of exon 51 of the human dystrophin pre-mRNA” – the target sequence (the dystrophin pre-mRNA region) is entered as a full-length or region-specific sequence entry and each claimed targeting ASO is entered as a separate entry.
For patents targeting a gene family (all PCSK9 family members, all ApoB-related transcripts), the target sequences for each family member should be individually entered if they are individually claimed or if differences between family members affect the claimed oligonucleotide scope.
Prosecution Considerations: Examining ASO Sequence Listings
What USPTO Examiners Look For
Examiners in the biotechnology art units (Art Units 1630, 1631, 1632, 1633 and related units) reviewing ASO patent applications focus on sequence listing compliance in several specific areas:
Completeness: Are all claimed sequences entered? Are all sequences referenced in the specification (target sequences, control sequences, comparative sequences) entered?
Modification disclosure: Is the modification pattern of each claimed ASO compound fully described, either within the sequence listing annotations or in the specification with clear cross-reference to the sequence listing entries?
SEQ ID NO consistency: Does every SEQ ID NO reference in the claims and specification correspond exactly to an entry in the sequence listing, with the correct sequence identity?
Claim-sequence alignment: Does the claimed compound correspond to what the sequence listing entry actually represents? A claim to a specific phosphorothioate ASO citing SEQ ID NO: 1, where SEQ ID NO: 1 in the listing is an unmodified DNA sequence without PS annotation, creates a gap between what is claimed and what is listed.
Common Examination Objections in ASO Cases
§ 112(a) written description: Claimed ASO compounds whose modification patterns are not adequately described – either in the sequence listing annotations or the specification – receive written description objections. The examiner’s position is that the specification does not demonstrate inventor possession of the specific modification pattern that the claims require.
§ 112(a) enablement: Broad genus claims covering all ASOs complementary to a target sequence, or all ASOs with a particular modification pattern, may receive enablement objections if the disclosure does not adequately enable the full claimed scope.
Formal drawing/specification inconsistency: When the specification’s chemical structure drawings of claimed ASO compounds are inconsistent with the sequence listing entries (different sequences, different modification patterns), examiners cite this as a specification inconsistency.
ST.26 format objections: Incorrect sequence type designations (classifying a chimeric ASO as DNA when it should be DNA/RNA), missing mandatory qualifiers for modified_base annotations and invalid XML structure generate formal objections from the USPTO’s formalities review.
Pre-Filing Compliance Checklist for ASO Sequence Listings
Before filing any patent application with ASO sequence listings:
Sequence entry completeness: ☐ Every claimed ASO compound has a sequence listing entry ☐ Every target sequence referenced in claims or specification has an entry ☐ Every control sequence, comparison sequence, or reference sequence discussed in the specification has an entry ☐ Both antisense and sense strands entered separately for dsRNA/siRNA applications ☐ All sequences ≥10 residues in length are included
Sequence type designation: ☐ DNA-type ASOs (standard deoxyribose backbone, standard bases) classified as DNA ☐ RNA-type ASOs classified as RNA ☐ Chimeric ASOs (gapmer, hemimer designs) classified as DNA/RNA ☐ PMO and other non-natural backbone sequences – classification approach documented and consistent with examiner practice
Modification annotation: ☐ All modified base positions annotated with modified_base feature key ☐ Controlled vocabulary (/mod_base) used where available; “OTHER” used for non-standard modifications with descriptive /note ☐ LNA, 2′-OMe, 2′-MOE, 2′-F and other sugar modifications annotated at each modified position ☐ PS and other backbone modification patterns described in /note qualifier or in specification with explicit cross-reference ☐ Chimeric positions (DNA vs. RNA residue identity) correctly designated throughout the sequence
Specification cross-reference: ☐ Modification table in specification for each claimed compound ☐ Each SEQ ID NO referenced in claims corresponds to a complete, accurate sequence listing entry ☐ Claim language integrates sequence identity (SEQ ID NO) with modification pattern description ☐ Chemical structure drawings of claimed ASOs are consistent with sequence listing entries
Validation: ☐ WIPO Sequence built-in validator run – no unresolved errors ☐ Standalone WIPO Sequence validator run on exported XML ☐ USPTO Patent Center pre-submission validator run (for USPTO filings) ☐ All validation errors resolved before filing
Post-Filing and Continuation Strategy for ASO Portfolios
Sequence Listing Amendment Considerations
When prosecution requires claim amendments that narrow or change the compound definitions, verify that the sequence listing entries continue to accurately support the amended claims. If claim amendments introduce new SEQ ID NO references, those entries must already exist in the sequence listing – they cannot be added post-filing as new matter.
When sequence listing amendments are needed (correction of an error, not addition of new matter), the amendment must be made in compliance with current USPTO amendment procedures for sequence listings, including submission of a replacement ST.26 XML file with a specification amendment identifying the corrections.
Continuation Strategy for ASO Portfolio Development
For companies developing ASO drugs, continuation filing strategies should account for sequence listing requirements:
Comprehensive original filing: The parent application’s sequence listing should include not only the primary development compounds but also analogs, variants and related sequences that are reasonably anticipated to be claimed in continuation applications. Adding new sequences in a continuation-in-part creates new matter for those additions – the continuation application cannot claim the benefit of the parent’s priority date for the new sequences.
Consistent numeration across the family: SEQ ID NOs in continuation applications should maintain consistency with the parent where the same sequences are carried over. Renumbering sequences in continuations without updating all specification and claim references creates the reference numeral consistency errors described in the quality control literature.
Conclusion
ASO patent sequence listings require exceptional precision due to the close relationship between sequence structure, chemical modifications and therapeutic function. Compliance with WIPO Standard ST.26 is essential to ensure accurate disclosure, strong claim support and successful patent prosecution. By correctly classifying sequences, properly annotating modifications and thoroughly validating sequence listings before filing, applicants can strengthen patent protection and reduce the risk of examination issues, validity challenges and future litigation vulnerabilities.