US Patent 8,524,880: Scope, Claims, Expiration, and Exon 45-Skipping Patent Landscape

US Patent 8,524,880 protects antisense oligonucleotides directed to the exon 45 acceptor splice site of the human dystrophin gene. Its broadest claim groups cover sequence-defined antisense oligomers, exon 45-targeting oligomers identified by the H45A(-06+20) annealing site, and oligomers containing at least 20 consecutive nucleotides of SEQ ID NO: 207. The patent also covers pharmaceutical compositions and methods for inducing dystrophin exon 45 skipping and treating Duchenne muscular dystrophy.

The patent is directly relevant to casimersen, marketed by Sarepta Therapeutics as Amondys 45. Casimersen is an exon 45-skipping phosphorodiamidate morpholino oligomer, or PMO, approved by the FDA in 2021 under the accelerated approval pathway.[1]

What does US Patent 8,524,880 protect?

The patent has 69 claims, but its substantive scope is concentrated in three independent oligonucleotide claims and three composition claims.

Claim group	Independent claim	Core subject matter	Functional limitation
Sequence-defined exon 45 oligomers	1	20- to 50-nucleotide oligomer containing at least 17 consecutive nucleotides of SEQ ID NO: 207	Must hybridize to the exon 45 acceptor splice site and induce exon 45 skipping
H45A(-06+20) oligomers	23	20- to 50-nucleotide oligomer containing at least 17 nucleotides complementary to the designated target region	Must hybridize to the acceptor splice site and induce exon 45 skipping
Broad sequence-fragment oligomers	47	20- to 50-nucleotide oligomer containing at least 20 consecutive nucleotides of SEQ ID NO: 207	No express exon-skipping limitation in the independent claim
Pharmaceutical compositions	65-67	Composition containing an oligomer of claim 1, 23, or 47 and a pharmaceutically acceptable carrier	Depends on the incorporated oligonucleotide claim
Exon-skipping method	68	Administering a composition of claim 65	Induces dystrophin exon 45 skipping
DMD treatment method	69	Administering a composition of claim 65	Treats Duchenne muscular dystrophy

The patent is therefore not limited to one precise chemical embodiment. It claims a sequence-defined targeting concept across multiple antisense chemistries, provided the relevant structural and sequence limitations are met.

How broad are claims 1, 23, and 47?

Claim 1: sequence plus biological activity

Claim 1 requires all of the following:

An isolated antisense oligonucleotide.
A length of 20 to 50 nucleotides.
At least 17 consecutive nucleotides of SEQ ID NO: 207.
Specific hybridization to the exon 45 acceptor splice site.
Induction of exon 45 skipping.
Optional substitution of thymine for uracil.

The phrase "comprising at least 17 consecutive nucleotides" creates potentially broad sequence coverage. A claimed oligomer can contain additional nucleotides before, after, or around the claimed 17-nucleotide segment. The 20- to 50-nucleotide length limitation constrains the outer boundary.

The claim also requires a functional result. An oligomer containing the required sequence may still fall outside the claim if it does not specifically hybridize to the acceptor splice site or does not induce exon 45 skipping. In an infringement dispute, this limitation could require experimental or other technical evidence.

Claim 23: target-site-defined coverage

Claim 23 uses the annealing-site designation H45A(-06+20), rather than relying solely on SEQ ID NO: 207. It covers a 20- to 50-nucleotide antisense oligomer containing at least 17 consecutive nucleotides complementary to the designated exon 45 target region.

Dependent claims 28-32 tighten the complementarity requirement. They cover:

At least 17 consecutive nucleotides with 100% complementarity.
At least 20 consecutive complementary nucleotides.
20- to 31-nucleotide oligomers with at least 17 or 20 consecutive complementary nucleotides.

This claim group is important because it may capture sequence variants that are described by their target relationship rather than by complete identity to SEQ ID NO: 207.

Claim 47: the broadest apparent sequence claim

Claim 47 covers an isolated 20- to 50-nucleotide antisense oligonucleotide comprising at least 20 consecutive nucleotides of SEQ ID NO: 207. Unlike claims 1 and 23, claim 47 does not expressly recite exon 45 hybridization or exon skipping.

That drafting difference has commercial significance. Claim 47 may be asserted against an oligomer based on sequence content alone, without the need to prove the biological activity limitations appearing in claims 1 and 23. The claim remains subject to ordinary claim-construction principles, including the meaning of "antisense oligonucleotide," "isolated," "comprising," and "SEQ ID NO: 207."

What chemical modifications are covered?

Claims 9-22, 33-46, and 51-64 extend the sequence claims across several backbone and conjugate formats.

Chemical feature	Claims	Relevance
Non-RNase H-activating oligomer	9, 33, 51	Covers steric-blocking antisense mechanisms
Non-natural backbone	10, 34, 52	Broad structural category
Modified sugar moieties	11, 35, 53	Includes morpholino chemistry
Modified internucleotide linkages	13, 37, 55	Includes modified phosphate linkages
Morpholino plus modified phosphate	15-17, 39-41, 57-59	Directly relevant to PMO-type products
2'-O-methyl RNA	18, 42, 60	Covers 2'-O-methyl-oligoribonucleotides
Peptide nucleic acid	19, 43, 61	Covers PNA formats
Uptake or distribution conjugates	20, 44, 62	Functional conjugate category
Polyamine conjugates	21, 45, 63	Specific conjugate subcategory
PEG conjugates	22, 46, 64	Specific conjugate subcategory

The claims are drafted to reach both the sequence and the chemical architecture. A competitor cannot necessarily avoid the patent by changing from a PMO to a 2'-O-methyl RNA, PNA, or another non-natural backbone if the resulting molecule still satisfies the applicable sequence and length limitations.

The modified phosphate list in claims 17, 41, and 59 includes methyl phosphonates, methyl phosphorothioates, phosphoromorpholidates, phosphoropiperazidates, and phosphoroamidates. These claims do not cover every possible antisense backbone expressly, but the higher-level claims covering a "non-natural backbone" or modified linkage may remain relevant depending on construction and written-description support.

Does the patent cover casimersen and Amondys 45?

Casimersen is an exon 45-directed PMO developed by Sarepta and approved by the FDA for Duchenne muscular dystrophy patients with a confirmed mutation amenable to exon 45 skipping.[1] Its mechanism is to bind dystrophin pre-mRNA and cause exclusion of exon 45 during RNA processing.

The commercial product falls within the technical category addressed by the patent:

It is an antisense oligonucleotide.
It is directed to the exon 45 splice region.
It is designed to induce exon 45 skipping.
It uses a morpholino-based, RNase H-independent mechanism.
It is administered as a pharmaceutical composition.

Whether a marketed product literally satisfies a particular patent claim depends on the exact sequence, chemical structure, and claim construction. The patent’s claim architecture, however, is specifically aligned with exon 45 PMO products and is highly relevant to Amondys 45 freedom-to-operate analysis.

What pharmaceutical and method-of-use protection exists?

Claims 65-67 cover pharmaceutical compositions containing oligomers falling within the three principal sequence claim groups. These claims require a pharmaceutically acceptable carrier but do not require a particular dose, concentration, route, or dosage schedule.

Claims 68 and 69 provide separate method protection:

Claim 68 covers inducing dystrophin exon 45 skipping by administering the claimed composition.
Claim 69 covers treating Duchenne muscular dystrophy by administering an effective amount of the claimed composition.

The method claims may be relevant even where a product claim is challenged. Their practical value depends on whether the accused activity involves administration in the claimed therapeutic context and whether the required composition limitations are met.

When does US Patent 8,524,880 lose exclusivity?

The patent has a March 14, 2029 expiration date based on the earliest claimed priority date and the standard 20-year patent term framework.[2] The patent issued on September 3, 2013, but the relevant expiration calculation is generally tied to the effective nonprovisional filing or priority chain, not the issue date.

Event	Date
Earliest priority date	March 14, 2008
Patent issued	September 3, 2013
Expected standard expiration	March 14, 2029
FDA approval of Amondys 45	February 25, 2021
Seven-year orphan-drug exclusivity period	Through approximately February 25, 2028
Patent expiration relative to orphan exclusivity	Patent extends approximately one year beyond orphan exclusivity

The precise enforceable term can be affected by patent-term adjustment, patent-term extension, terminal disclaimers, maintenance-fee status, or later legal events. The principal commercial planning date associated with US 8,524,880 is March 14, 2029.

What is the FDA and Orange Book status?

FDA approval does not itself establish patent rights. Patent listing and regulatory exclusivity are separate mechanisms.

Amondys 45 received accelerated approval based on an increase in dystrophin production in skeletal muscle, with continued approval dependent on verification of clinical benefit.[1] The product also received orphan-drug designation, creating a separate period of market exclusivity for the same drug and indication under the Orphan Drug Act.

For commercial entry analysis, the relevant regulatory questions are:

Whether US 8,524,880 is listed in the Orange Book for the approved casimersen product.
Whether other product, formulation, manufacturing, or method patents are listed.
The date of the first approved generic or 505(b)(2) application.
Whether a listed patent is subject to a Paragraph IV certification.
Whether the NDA holder files an infringement action within the statutory period after receiving notice.

The Orange Book must be read as a current product-specific record. A patent’s technical relevance does not automatically establish that it is listed for a particular NDA. FDA’s Approved Drug Products with Therapeutic Equivalence Evaluations is the controlling public source for listed patents and regulatory exclusivity.[3]

Are Paragraph IV challenges or generic entry risks established?

A Paragraph IV challenge would require an ANDA applicant to certify that a listed patent is invalid, unenforceable, or will not be infringed. If the NDA holder sues within the statutory period, FDA approval may be subject to a 30-month stay, subject to statutory exceptions and court developments.[4]

US 8,524,880 presents several potential challenge vectors:

Written description and enablement for broad sequence fragments.
The breadth of the "at least 17 consecutive nucleotides" limitation.
Whether the specification supports every claimed backbone and conjugate.
Indefiniteness or construction disputes involving H45A(-06+20).
Anticipation or obviousness based on earlier dystrophin exon-skipping disclosures.
Lack of infringement where a competing oligomer does not meet the sequence, length, or functional limitations.

A generic PMO manufacturer would face more than a sequence comparison. It would also need to address the specific morpholino chemistry, exon 45 targeting, formulation, manufacturing controls, and FDA requirements for demonstrating pharmaceutical equivalence and bioequivalence or an appropriate alternative.

Casimersen is a chemically synthesized oligonucleotide, not a protein biologic. A conventional biosimilar pathway under section 351(k) is therefore not the principal competitive route. A future entrant would more likely consider an ANDA, a 505(b)(2) application, or another pathway depending on product differences and FDA classification.

What other patents may affect casimersen competition?

US 8,524,880 should be treated as one layer of the estate rather than the entire protection package.

Formulation patents

A formulation patent may cover concentration, excipients, pH, stability, container closure, or injectable presentation. Such patents can remain commercially relevant after a sequence patent expires if they are properly listed and valid.

Manufacturing patents

PMO manufacturing can involve protected monomer synthesis, oligomer assembly, purification, impurity control, scale-up, and analytical release methods. These rights may create manufacturing barriers even where a competitor designs around the composition claims.

Method-of-use patents

Additional patents may cover treatment populations, dosing regimens, administration schedules, or specific dystrophin-restoration uses. Claim 69 is already a broad treatment method, but later patents may contain narrower, commercially useful improvements.

Platform and licensing rights

The exon-skipping program originated from AVI BioPharma, which later became Sarepta Therapeutics. Commercial rights may also depend on platform licenses, university rights, research agreements, or third-party chemistry rights. Patent ownership, exclusive licensing, and freedom to operate are separate questions and should not be inferred solely from the named assignee on an issued patent.[5]

How strong is the patent estate?

US 8,524,880 has meaningful blocking potential because it combines:

A disease-specific target.
A defined exon 45 splice-site region.
Sequence-fragment coverage.
Multiple antisense chemistries.
Pharmaceutical composition claims.
Exon-skipping and DMD treatment methods.

Its principal vulnerability is breadth. Claims covering sequence fragments of at least 17 or 20 nucleotides, combined with broad backbone categories, may invite written-description, enablement, anticipation, and obviousness challenges. The biological limitations in claims 1 and 23 may narrow enforcement but can also strengthen the connection between the claimed sequence and the demonstrated technical result.

The strongest commercial claims are likely claims 47-50 for sequence-defined oligomers and claims 52-64 for PMO and related chemical formats, subject to the exact sequence of the accused product. Claims 65-69 provide additional enforcement routes against formulated products and clinical use.

What generic launch scenarios exist?

Scenario	Likely timing pressure	Commercial implication
No successful challenge	After patent expiry in 2029	Entry constrained by patent and any later-listed rights
Paragraph IV litigation	Potentially before 2029	Entry depends on litigation outcome, settlement, and FDA exclusivity
Orphan exclusivity only	After approximately February 2028	Orphan exclusivity may end before the principal patent
Design-around oligomer	Before 2029 if technically viable	Requires avoiding sequence, length, chemistry, and functional claim limitations
505(b)(2) product	Case-specific	May face patent certifications and clinical bridging requirements
Post-expiry PMO competitor	After 2029	Manufacturing capability and FDA comparability become central barriers

Key Takeaways

US 8,524,880 is a core exon 45-skipping antisense patent relevant to casimersen and Amondys 45.
Independent claims 1, 23, and 47 create overlapping sequence and target-site coverage.
Claim 47 is particularly important because its independent language does not expressly require exon 45 skipping.
Dependent claims extend coverage to PMOs, modified phosphates, 2'-O-methyl RNA, PNAs, polyamine conjugates, and PEG conjugates.
Claims 65-69 cover pharmaceutical compositions, exon-skipping treatment, and DMD treatment.
The principal stated patent expiration date is March 14, 2029.
Casimersen received FDA accelerated approval on February 25, 2021 and benefits from orphan-drug exclusivity expected to run approximately through February 25, 2028.
Biosimilar competition is not the primary risk because casimersen is a chemically synthesized antisense oligonucleotide rather than a protein biologic.
Generic or 505(b)(2) entry would require analysis of Orange Book listings, Paragraph IV certifications, formulation rights, manufacturing patents, and any later Sarepta patents.
The estate is commercially significant but vulnerable to validity challenges directed at broad sequence fragments and broad chemistry coverage.

FAQs

Is US 8,524,880 limited to casimersen?

No. The claims cover a broader genus of exon 45-directed antisense oligonucleotides, including multiple backbone chemistries and sequence variants. Casimersen is a principal commercial product within the technical scope of the patent.

Does changing a PMO to a 2'-O-methyl RNA avoid the patent?

Not necessarily. Claims 18, 42, and 60 expressly recite 2'-O-methyl-oligoribonucleotides. A chemistry change must be evaluated with the sequence, length, target, and applicable dependent claims.

Can a competitor avoid infringement by using a longer antisense oligomer?

Only if the longer molecule falls outside the applicable length range or otherwise avoids the sequence and structural limitations. The principal claims reach oligomers up to 50 nucleotides.

Does patent expiration end all exclusivity for Amondys 45?

No. Patent expiration does not eliminate separate regulatory exclusivity, later-issued patents, formulation rights, manufacturing rights, or market barriers created by FDA requirements.

Is a casimersen biosimilar expected?

A traditional biosimilar pathway is unlikely to be the central route because casimersen is a synthetic antisense oligonucleotide. Competition would more likely involve an ANDA, 505(b)(2) application, or a product-specific FDA pathway.

References

U.S. Food and Drug Administration. (2021). FDA grants accelerated approval to first targeted treatment for rare Duchenne muscular dystrophy mutation.
United States Patent and Trademark Office. (2013). U.S. Patent No. 8,524,880, Antisense oligomers for inducing exon skipping in dystrophin mRNA.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations.
Federal Food, Drug, and Cosmetic Act, 21 U.S.C. § 355(j).
Sarepta Therapeutics, Inc. (2024). Amondys 45 (casimersen) prescribing information.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
Australia	2004903474	Jun 28, 2004

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	E498685	⤷ Start Trial
Cyprus	1111447	⤷ Start Trial
Cyprus	1117475	⤷ Start Trial
Germany	602005026386	⤷ Start Trial
Denmark	1766010	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 8,524,880

Summary for Patent: 8,524,880