United States Patent 5,540,938: Scope, Claims, Expiration, Litigation, and Dalfampridine Patent Landscape

U.S. Patent No. 5,540,938, the "'938 patent," covered methods of treating neurological diseases with sustained-release mono- or di-aminopyridines, particularly sustained-release 4-aminopyridine, now known as dalfampridine or fampridine. Its commercial significance arose from Ampyra, Acorda Therapeutics' extended-release dalfampridine product for improving walking in patients with multiple sclerosis.

The patent's principal protection was a method-of-use combination requiring both an aminopyridine active ingredient and controlled release that maintained therapeutically effective blood levels for 12 to 24 hours with once- or twice-daily dosing. The patent expired in 2018 after patent-term extension. It no longer creates an enforceable barrier to generic dalfampridine, although its claim structure remains important for understanding later Ampyra formulation patents and the generic litigation that followed.

What does U.S. Patent 5,540,938 cover?

The '938 patent covers the use of a sustained-release mono- or di-aminopyridine medicament to treat neurological diseases involving slowed nerve impulse transmission.

Its broadest independent claim, claim 1, has four material limitations:

Treatment of a neurological disease characterized by slowed nerve impulse transmission.
Administration of a medicament containing a mono- or di-aminopyridine active agent.
Sustained release of the active agent.
Controlled absorption producing therapeutically effective blood levels over 12 to 24 hours with once- or twice-daily administration.

The claim is therefore a functional method claim rather than a claim limited to a particular tablet composition, excipient, dissolution profile, manufacturing process, or drug concentration.

Claim hierarchy and technical scope

Claim	Subject matter	Practical scope
1	Treatment of neurological disease using sustained-release mono- or di-aminopyridine	Broad genus claim
2	Disease characterized by central nervous system demyelination	Narrows claim 1 to CNS demyelinating disease
3	Multiple sclerosis	Species claim directed to MS
4	Alzheimer's disease	Alternative disease-specific claim
5	Dose titration until a tolerable state is achieved, followed by dose increases	Adds a tolerability and escalation protocol
6	Starting dose below 15 mg/day	Specific low-dose initiation limitation
7	Starting dose below 15 mg/day, followed by increases of at least 5 to 15 mg/day	More specific titration regimen
8	Active agent is 4-aminopyridine	Species claim covering 4-aminopyridine, or dalfampridine

Claim 8 is the commercially important species claim. Dalfampridine is the pharmaceutical form of 4-aminopyridine used in Ampyra and generic dalfampridine extended-release tablets.

How broad are the claims of U.S. Patent 5,540,938?

The patent has broad conceptual coverage but limited practical scope after expiration. While enforceable, claim 1 potentially reached multiple aminopyridine compounds and several neurological indications. Its boundaries depended heavily on the meaning of "sustained release," "controlled absorption," and "therapeutically effective blood levels over a 12-24 hour period."

Active-agent coverage

The claim uses the generic terms "mono- or di-aminopyridine active agent." It is not limited to 4-aminopyridine in claim 1. Claim 8 separately narrows the invention to 4-aminopyridine.

Potentially covered compounds included:

4-aminopyridine, also called 4-AP, fampridine, or dalfampridine;
Other aminopyridine regioisomers;
Certain di-aminopyridine compounds, depending on the chemical structure and claim interpretation.

The patent does not claim every use of an aminopyridine. The agent must be administered in a sustained-release medicament capable of producing the specified pharmacokinetic result.

Disease coverage

Claim 1 describes the disease functionally, by reference to slowed nerve impulse transmission. Claims 2 and 3 move toward the clinical use that became commercially relevant:

Claim 2 covers CNS demyelinating disease.
Claim 3 covers multiple sclerosis.
Claim 4 separately covers Alzheimer's disease.

The MS claim is narrower than claim 1 but commercially stronger because it maps directly to Ampyra's FDA-approved indication. The Alzheimer's claim is broader than the FDA-approved Ampyra indication from a therapeutic-use perspective, but it did not become a major commercial source of product sales.

Release and pharmacokinetic limitations

The key technical limitation is not merely that the dosage form releases drug slowly. The medicament must permit controlled absorption and achieve therapeutically effective blood levels over a 12- to 24-hour period.

This language can create infringement and validity issues because it combines:

A release property;
An absorption profile;
A duration requirement; and
A therapeutic blood-level requirement.

The claim does not state a precise dissolution curve, maximum plasma concentration, minimum plasma concentration, or area-under-the-curve threshold. That makes the claim potentially broad, but it also creates litigation risk over whether a generic product satisfies the functional limitations.

What are the priority, filing, and expiration dates for U.S. Patent 5,540,938?

The '938 patent issued on July 30, 1996. It was prosecuted before the modern 20-year-from-earliest-effective-filing-date regime applied to many U.S. applications. Its original term was extended by patent-term extension associated with regulatory review of the dalfampridine product.

Event	Date or status
Patent number	U.S. 5,540,938
Title	Treatment of neurological diseases
Issue date	July 30, 1996
Patent holder associated with commercial rights	Elan-related patent ownership and later Acorda commercialization rights
FDA product associated with the patent	Ampyra, dalfampridine extended-release tablets
FDA approval of Ampyra	February 2010
Expiration after patent-term extension	March 31, 2018
Current enforceability	Expired

The March 31, 2018 expiration date is the operative commercial date cited in the Ampyra patent and generic-entry record. Once the patent expired, the claims ceased to block manufacture, sale, or use of products that would previously have fallen within the claims.

What is the Orange Book status of U.S. Patent 5,540,938?

The '938 patent was listed in the FDA Orange Book for Ampyra, FDA application NDA 022250. It was an important listed patent because it addressed the sustained-release aminopyridine treatment method underlying the product.

Orange Book listing did not mean that every generic applicant had to wait until 2018. An ANDA applicant could submit a Paragraph IV certification asserting that the patent was invalid, unenforceable, or would not be infringed. A patent listed in the Orange Book also could be subject to a Paragraph III certification acknowledging that the applicant would wait until patent expiration.

The '938 patent's listing had limited long-term blocking value because:

Its expiration date was in 2018.
Later Ampyra patents extended the asserted patent estate beyond the '938 patent.
Generic applicants focused on the later formulation patents in Hatch-Waxman litigation.

FDA Orange Book listings should be read separately from current patent enforceability. An expired patent may remain visible in historical Orange Book records but cannot support a new infringement injunction.

When did Ampyra lose exclusivity?

Ampyra lost effective market exclusivity in stages rather than on a single date.

Exclusivity layer	Relevant date or period	Commercial effect
FDA approval	February 2010	Ampyra entered the U.S. market
New chemical entity exclusivity	Approximately four years after approval	Restricted ANDA submission during the initial period
'938 patent expiration	March 31, 2018	Removed the principal early sustained-release method patent
Later formulation patent litigation	2017-2018	Delayed or contested generic entry
Generic dalfampridine approvals	2018 onward	Created price and volume pressure
Current market structure	Generic and branded dalfampridine	Brand faces continuing substitution risk

Ampyra's principal commercial exposure was therefore tied to the end of patent protection, not only to FDA exclusivity. Acorda's 2017 revenue from Ampyra was approximately $465 million, making the 2018 patent cliff material to the company's financial performance [5].

Which later patents covered Ampyra and dalfampridine extended-release tablets?

The '938 patent was part of a larger patent estate. Later patents generally focused more narrowly on formulation design, release control, dosing, and product-specific characteristics.

U.S. Patent 8,007,826

U.S. Patent No. 8,007,826 was one of the principal later Ampyra formulation patents. It covered aspects of sustained-release 4-aminopyridine formulations and was asserted against generic manufacturers.

The patent had a later expiration date than the '938 patent and became central to the generic litigation that followed. Generic challengers argued that its claims were invalid, including on obviousness grounds.

U.S. Patent 8,663,697

U.S. Patent No. 8,663,697 was another later Ampyra patent asserted in connection with extended-release dalfampridine. It addressed formulation and product characteristics associated with maintaining the desired release profile.

The '697 patent was litigated with the '826 patent in the Acorda generic cases. The Federal Circuit affirmed judgments of invalidity based on obviousness for the asserted patents in the relevant litigation [3].

Other formulation and later-generation patents

Acorda's broader patent portfolio included additional U.S. patents directed to:

Extended-release 4-aminopyridine dosage forms;
Tablet composition and excipient selection;
Dissolution and release characteristics;
Dosing regimens;
Manufacturing and product controls;
Alternative formulations and delivery approaches.

The commercial relevance of each patent depended on whether it was listed for Ampyra, asserted against a particular ANDA, and still enforceable at the time of the proposed generic launch.

What patent litigation affected Ampyra generic entry?

Acorda and related parties brought Hatch-Waxman litigation against generic applicants, including Roxane Laboratories and other companies seeking approval of extended-release dalfampridine products.

The key dispute was not limited to whether a generic tablet contained dalfampridine. It concerned whether the generic product would infringe later formulation patents and whether those patents were valid.

Acorda Therapeutics v. Roxane Laboratories

In Acorda Therapeutics, Inc. v. Roxane Laboratories, Inc., the Federal Circuit addressed the validity of the later Ampyra patents. The court upheld the finding that asserted claims were invalid as obvious [3].

The litigation had several commercial consequences:

It weakened the remaining U.S. Ampyra formulation patent estate.
It reduced the ability to delay generic entry through later Orange Book patents.
It allowed generic applicants to proceed after resolving regulatory and litigation requirements.
It shifted competition from patent validity to price, substitution, supply, and contracting.

The case also illustrates a recurring pharmaceutical patent issue: a formulation patent may survive an early method patent but still fail if the claimed release profile would have been predictable from known pharmacokinetic and formulation techniques.

Were there Paragraph IV challenges to the Ampyra patent estate?

Yes. Generic applicants pursuing dalfampridine extended-release products used the Hatch-Waxman Paragraph IV pathway to challenge later listed Ampyra patents.

A Paragraph IV certification asserts that an Orange Book-listed patent is invalid, unenforceable, or not infringed. The certification gives the patent owner a basis to file an infringement action, which can trigger a 30-month stay of FDA approval under 21 U.S.C. § 355(j)(5)(B)(iii).

For the '938 patent, its approaching expiration reduced its value as a Paragraph IV target. The more commercially important challenges involved later patents with expiration dates extending beyond 2018.

The practical generic strategy was to attack the formulation patents while designing an ANDA product that matched the reference listed drug's extended-release profile and labeling. The generic applicant did not need to reproduce every element of Acorda's commercial manufacturing process if the ANDA product avoided enforceable patent claims.

How does claim 5's dose-titration subject matter affect infringement analysis?

Claims 5 through 7 add a treatment protocol to the sustained-release product requirement. The protocol starts with a dose below 15 mg/day, waits until the patient reaches a tolerable state, and then increases the dose at selected intervals.

Claim 7 is the narrowest titration claim because it requires dose increases of at least 5 to 15 mg/day. These claims raise several enforcement issues:

The claimed steps are performed by a physician or patient, not necessarily by the manufacturer.
Generic labeling may create inducement allegations if it instructs physicians to follow the claimed protocol.
A product label that omits or contradicts the titration steps may reduce induced-infringement exposure.
The claim requires both the starting dose and subsequent escalation behavior.
The phrase "tolerable state" is clinically functional and may require patient-specific evidence.

The commercial Ampyra label uses a 10 mg extended-release tablet administered approximately 12 hours apart and warns against taking more than two tablets in 24 hours [4]. That labeled regimen does not map cleanly onto every element of claims 5 through 7, particularly the specified escalation amounts.

Does the '938 patent create biosimilar risk?

No. Dalfampridine is a small-molecule drug, not a biologic. The relevant follow-on pathway is an ANDA under section 505(j) of the Federal Food, Drug, and Cosmetic Act, not a biosimilar application under section 351(k) of the Public Health Service Act.

The competitive risks are therefore:

ANDA generic entry;
Paragraph IV patent litigation;
Authorized generic competition;
Formulation substitutions;
Payer-driven therapeutic or pharmaceutical substitution.

No biosimilar interchangeability analysis applies to Ampyra or dalfampridine.

What FDA regulatory status applies to dalfampridine?

FDA approved Ampyra extended-release tablets in February 2010 to improve walking in adults with multiple sclerosis. The clinical benefit was measured using walking ability, including the Timed 25-Foot Walk test [4].

The approved product is a 10 mg extended-release tablet administered twice daily, approximately 12 hours apart. The label warns that:

The maximum recommended dose is 10 mg twice daily.
Doses should not be doubled after a missed dose.
Ampyra is contraindicated in patients with a history of seizures.
Ampyra is contraindicated in moderate or severe renal impairment.
Seizure risk increases with excessive exposure.
Renal function is important because dalfampridine is substantially eliminated through the kidneys.

The FDA indication is narrower than claim 1 of the '938 patent. The patent covers a broad neurological-disease category, while the approved product is labeled for walking impairment in adults with MS.

How strong was the patent estate for Ampyra?

The '938 patent was strong as an early platform patent because it linked the active ingredient, sustained release, pharmacokinetic duration, and neurological treatment. Its strengths were:

Broad aminopyridine genus language;
Direct coverage of 4-aminopyridine;
Direct coverage of multiple sclerosis;
Functional coverage of the sustained-release profile;
Early priority relative to Ampyra's commercial launch.

Its weaknesses were equally material:

Expiration in 2018;
Functional claim language without precise dissolution or pharmacokinetic thresholds;
Potential obviousness challenges based on known 4-aminopyridine use and sustained-release formulation technology;
Dependence on later patents for post-2018 protection;
Limited relevance to the current generic market after expiration.

The later Ampyra estate was commercially important but vulnerable. The Federal Circuit's obviousness decision against key later patents materially reduced the value of the remaining patent portfolio [3].

What generic launch risks exist for dalfampridine?

The principal generic risks are now commercial rather than claim-enforcement risks.

Risk	Effect on Ampyra
Multiple ANDA-approved products	Price erosion and pharmacy substitution
Low-cost generic contracting	Reduced branded formulary access
Narrow therapeutic index concerns	Manufacturing and quality-control scrutiny
Renal dosing restrictions	Label and pharmacovigilance burden
Extended-release performance	Need to demonstrate bioequivalence and release consistency
Brand loyalty and prescriber familiarity	Potentially slows, but does not prevent, substitution
Authorized generic strategy	Can moderate third-party generic pricing pressure

A generic applicant must still demonstrate bioequivalence to the reference listed drug. For an extended-release product, this can require careful control of fed and fasting pharmacokinetics, dose proportionality, release behavior, and exposure variability.

How does the '938 patent compare with later Ampyra patents?

Issue	U.S. 5,540,938	Later Ampyra patents
Primary focus	Treatment method	Formulation and product characteristics
Active agent	Mono- or di-aminopyridines; claim 8 covers 4-AP	Generally more closely tied to 4-AP formulations
Indications	Broad neurological disease; MS and Alzheimer's claims	Often product or regimen-specific
Release requirement	12-24-hour therapeutic blood levels	More detailed formulation or release limitations
Expiration	March 31, 2018	Generally later than the '938 patent
Litigation role	Foundational patent	Main post-2018 generic-entry barrier
Current status	Expired	Key patents were challenged and some asserted claims held obvious

The '938 patent was the platform method patent. Later patents attempted to convert the commercial formulation into a longer-lived patent barrier. That strategy produced litigation but did not preserve a durable monopoly after the Federal Circuit's invalidity ruling.

Key Takeaways

U.S. Patent 5,540,938 covered sustained-release treatment of neurological disease with mono- or di-aminopyridines.
Claim 8 specifically covered sustained-release 4-aminopyridine, the active ingredient in Ampyra.
Claims 2 and 3 were particularly relevant to MS treatment.
Claims 5 through 7 covered low-dose initiation and dose-escalation protocols.
The patent expired on March 31, 2018 after patent-term extension.
It was historically listed in the Orange Book for Ampyra, NDA 022250.
Ampyra's post-2018 protection depended mainly on later formulation patents.
Generic applicants challenged those later patents through Paragraph IV certifications and Hatch-Waxman litigation.
The Federal Circuit affirmed invalidity findings against key later Ampyra patents on obviousness grounds.
Dalfampridine is a small-molecule drug, so generic ANDA competition applies; biosimilar rules do not.
Current risk from the '938 patent is zero as an enforceable patent barrier. The remaining commercial issues are generic competition, product quality, labeling, payer access, and formulation performance.

FAQs About U.S. Patent 5,540,938 and Dalfampridine

What is the difference between dalfampridine and 4-aminopyridine?

Dalfampridine is the nonproprietary name used in the United States for 4-aminopyridine, also called fampridine in other markets. They refer to the same active pharmaceutical ingredient.

Did U.S. Patent 5,540,938 cover immediate-release 4-aminopyridine?

No. The claims require a sustained-release medicament capable of controlled absorption and therapeutically effective blood levels over 12 to 24 hours. An immediate-release product would not satisfy those release limitations.

Does the patent cover all multiple sclerosis treatments?

No. The patent covers treatment of MS using the claimed sustained-release aminopyridine medicament. It does not cover unrelated MS drugs, such as interferons, glatiramer acetate, natalizumab, or oral immunomodulators.

Can a generic manufacturer avoid the patent by using a once-daily formulation?

The patent is expired, so avoidance is no longer required for the '938 patent. While it was enforceable, once-daily dosing could still fall within claim 1 because the claim expressly permits once- or twice-daily administration if the other limitations were met.

What is the principal patent risk for a new extended-release dalfampridine product?

The principal historical risk was infringement of later Ampyra formulation patents. For current development, the more significant barriers are FDA bioequivalence, extended-release manufacturing control, renal safety, seizure risk, and market access rather than the expired '938 patent.

References

United States Patent and Trademark Office. (1996). U.S. Patent No. 5,540,938: Treatment of neurological diseases.
U.S. Food and Drug Administration. (2018). Approved drug products with therapeutic equivalence evaluations: Orange Book.
Acorda Therapeutics, Inc. v. Roxane Laboratories, Inc., 903 F.3d 1310 (Fed. Cir. 2018).
U.S. Food and Drug Administration. (2023). Ampyra (dalfampridine) extended-release tablets: Prescribing information.
Acorda Therapeutics, Inc. (2018). Annual report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 for fiscal year 2017.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
European Patent Office	0484186	⤷ Start Trial	C300503	Netherlands	⤷ Start Trial
European Patent Office	0484186	⤷ Start Trial	91894	Luxembourg	⤷ Start Trial
European Patent Office	0484186	⤷ Start Trial	CA 2011 00031	Denmark	⤷ Start Trial
European Patent Office	0484186	⤷ Start Trial	SPC/GB11/055	United Kingdom	⤷ Start Trial
European Patent Office	0484186	⤷ Start Trial	11C0049	France	⤷ Start Trial
European Patent Office	0484186	⤷ Start Trial	1190033-9	Sweden	⤷ Start Trial
European Patent Office	0484186	⤷ Start Trial	CR 2011 00031	Denmark	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,540,938

Summary for Patent: 5,540,938