METHSUXIMIDE Drug Patent Profile

Which patents cover Methsuximide, and what generic alternatives are available?

Methsuximide is a drug marketed by Novitium Pharma and is included in one NDA.

The generic ingredient in METHSUXIMIDE is methsuximide. There is one drug master file entry for this compound. Two suppliers are listed for this compound. Additional details are available on the methsuximide profile page.

DrugPatentWatch^® Litigation and Generic Entry Outlook for Methsuximide

A generic version of METHSUXIMIDE was approved as methsuximide by NOVITIUM PHARMA on May 1^st, 2023.

Summary for METHSUXIMIDE

US Patents:	0
Applicants:	1
NDAs:	1
Finished Product Suppliers / Packagers:	1
Raw Ingredient (Bulk) Api Vendors:	1
Patent Applications:	3,469
DailyMed Link:	METHSUXIMIDE at DailyMed

Pharmacology for METHSUXIMIDE

Drug Class	Anti-epileptic Agent
Physiological Effect	Decreased Central Nervous System Disorganized Electrical Activity

US Patents and Regulatory Information for METHSUXIMIDE

Applicant	Tradename	Generic Name	Dosage	NDA	Approval Date	TE	Type	RLD	RS	Patent No.	Patent Expiration	Product	Substance	Delist Req.	Exclusivity Expiration
Novitium Pharma	METHSUXIMIDE	methsuximide	CAPSULE;ORAL	217213-001	May 1, 2023	AB	RX	No	No

Methsuximide Market Dynamics, Patent Status, FDA Exclusivity, and Financial Outlook

Last updated: September 9, 2026

Methsuximide is a mature, low-volume antiseizure medicine with limited commercial upside. The product, historically marketed as Celontin, treats refractory absence seizures and competes in a narrow, specialist market. Its active ingredient has been available for decades, leaving no meaningful composition-of-matter patent barrier. Current value is concentrated in regulatory continuity, supply reliability, physician familiarity, and the ability to maintain a small generic portfolio rather than in patent exclusivity or brand expansion.

Public sources do not disclose reliable product-level revenue for methsuximide. The financial trajectory is therefore best assessed through market structure, generic competition, regulatory status, supply risk, and the absence of enforceable exclusivity.

What is methsuximide and how is it used?

Methsuximide is an oral succinimide anticonvulsant used for absence seizures that are refractory to other therapies. The historical branded product is Celontin, supplied as a 300 mg capsule. The FDA label identifies methsuximide as an adjunctive or alternative treatment for petit mal seizures that have not responded adequately to other anticonvulsants.[1]

Methsuximide is not a broad epilepsy platform product. Its use is constrained by:

• A narrow indication centered on refractory absence seizures.
• Competition from ethosuximide, valproate, and lamotrigine.
• Limited use in common focal-onset or generalized tonic-clonic epilepsy.
• A mature safety and prescribing profile with little opportunity for premium pricing.
• Low physician switching because patients stabilized on an older antiseizure drug may remain on therapy for long periods.

The product is generally relevant to neurology practices, pediatric epilepsy specialists, hospital formularies, and pharmacies serving patients with established treatment histories.

What is the FDA regulatory status of methsuximide?

Methsuximide was approved in the United States decades ago under the Celontin brand. FDA labeling identifies the dosage form as a 300 mg capsule.[1] The product does not have the regulatory profile of a recently approved specialty drug. It has no known modern orphan-drug commercial structure, no biologic component, and no active innovation-based exclusivity period comparable with a new molecular entity.

The principal regulatory issues are product availability and continuity of supply. For an old antiseizure medicine, a temporary manufacturing interruption can have a disproportionate clinical impact because replacement treatment may require titration, monitoring, or specialist supervision.

FDA regulatory characteristics

Item	Methsuximide profile
Active ingredient	Methsuximide
Historical U.S. brand	Celontin
Dosage form	Oral capsule
Historical strength	300 mg
Therapeutic class	Anticonvulsant; succinimide
Primary use	Refractory absence seizures
FDA pathway	Legacy small-molecule approval
Biologic status	Not applicable
Biosimilar pathway	Not applicable
Current commercial driver	Generic supply and regulatory maintenance

The FDA Orange Book is the principal source for determining whether approved products have listed patents or regulatory exclusivity.[2] For methsuximide, the commercial analysis is not driven by an active patent estate. The relevant question is whether an approved product remains commercially available and whether one or more manufacturers continue to support the product.

What patents protect methsuximide?

No commercially material U.S. composition-of-matter patent protection should remain for methsuximide. The active ingredient was introduced in the mid-20th century, placing any original molecule patent far beyond the ordinary 20-year patent term measured from the earliest effective filing date under modern U.S. patent law.[3]

The practical patent position is as follows:

Patent category	Current commercial significance
Composition-of-matter patent	Expired decades ago
Original therapeutic-use patent	Expired or commercially obsolete
Basic formulation patent	No identified barrier to ordinary generic capsules
Manufacturing-process patent	Potentially relevant only if a supplier relies on a protected process
Device or delivery patent	Not relevant to the standard capsule
Patent-term extension	Not commercially relevant to this legacy product
Orange Book-listed patent risk	No meaningful barrier identified from the public product profile

A process patent could theoretically affect a specific supplier, but it would not recreate market exclusivity for methsuximide as a drug. Competing manufacturers could use a non-infringing process, obtain active pharmaceutical ingredient from another source, or challenge the patent.

How many patents cover methsuximide?

Methsuximide does not have a meaningful active U.S. patent estate comparable with newer antiseizure medicines. Publicly available drug and FDA sources do not indicate a current portfolio of Orange Book-listed patents that would delay ordinary generic entry.[1,2]

The absence of an active patent estate changes the competitive analysis:

1. A manufacturer cannot rely on patent litigation to exclude generic competitors.
2. A new supplier can compete primarily through abbreviated regulatory filings, manufacturing economics, and supply execution.
3. Product economics are more sensitive to market size than to patent duration.
4. Any price increase is exposed to substitution and payer resistance.

Are formulation patents a barrier?

Standard methsuximide capsules have limited formulation differentiation. The product is an immediate-release oral capsule rather than a complex injectable, inhaled product, extended-release system, or drug-device combination. That substantially reduces the potential value of formulation patents.

A manufacturer could pursue an alternative dosage form or modified-release product, but the commercial opportunity would be uncertain. A new formulation would face:

• Small patient volume.
• Limited willingness to switch stable patients.
• Clinical and regulatory costs.
• Competition from established antiseizure drugs.
• Reimbursement pressure unless the product solved a material adherence, tolerability, or pharmacokinetic problem.

When does methsuximide lose exclusivity?

Methsuximide lost meaningful exclusivity many years ago. Its original brand protection and any early regulatory exclusivity are no longer relevant to market access. The current market is effectively genericized.

Unlike a recent small-molecule approval, methsuximide does not present a defined future “loss of exclusivity” date. The key commercial dates are historical:

Event	Timing
Initial U.S. development and approval	Mid-20th century
Brand commercialization as Celontin	Historical
End of original molecule exclusivity	Decades ago
Generic competition	Established over time
Current exclusivity expiration	No meaningful future event
Current strategic risk	Supply interruption and market withdrawal

FDA approval of a generic version does not require the generic sponsor to wait for a current patent cliff if no enforceable patent or exclusivity remains. FDA’s ANDA framework permits generic approval based on pharmaceutical equivalence and bioequivalence, subject to applicable patent certifications and regulatory requirements.[4]

Which companies are challenging methsuximide patents?

No material current Paragraph IV patent challenge is central to methsuximide’s market. The reason is structural: the product is already a legacy genericized medicine without a commercially important patent barrier.

Paragraph IV litigation generally matters when an ANDA applicant seeks approval before expiration of an Orange Book-listed patent and certifies that the patent is invalid, unenforceable, or not infringed.[4] That framework is unlikely to generate meaningful litigation for standard methsuximide capsules where no blocking patent estate is publicly evident.

The competitive threats instead come from:

• Generic manufacturers entering or exiting the market.
• API supplier concentration.
• Contract manufacturing failures.
• Pharmacy purchasing decisions.
• Wholesaler stocking policies.
• Substitution by other antiseizure medicines.

What is the Orange Book status of methsuximide?

The Orange Book remains the relevant FDA source for approved products, therapeutic equivalence, patents, and exclusivity information.[2] Methsuximide should be analyzed as a legacy small-molecule product rather than as an active brand protected by Orange Book patents.

The key Orange Book diligence points are:

• Whether the listed product has a current marketing status.
• Whether equivalent generic products are approved.
• Whether any patents are listed against the reference product.
• Whether a generic has a “B” rating or other substitution limitation.
• Whether the reference product has been discontinued for commercial or safety reasons.

An Orange Book listing does not guarantee commercial availability. A product may remain FDA-approved while experiencing limited distribution, intermittent manufacturing, or practical market withdrawal.

What generic entry risks exist for methsuximide?

Generic entry risk is already realized rather than prospective. The commercial risk is not a sudden patent-driven erosion event. It is a low-volume commodity market in which the number of active suppliers can change quickly.

Main generic-market risks

Risk	Effect on market
Multiple generic suppliers	Reduces pricing power
Small patient population	Limits manufacturing scale
Low unit volume	Increases risk of commercial discontinuation
API concentration	Raises supply vulnerability
Thin margins	Discourages redundant capacity
Low clinical substitutability for some patients	Increases shortage impact
Pharmacy inventory rationalization	Can reduce practical access
Brand withdrawal	Removes a premium-priced alternative

The product may retain some pricing stability because patients with refractory seizures are clinically sensitive to treatment changes. That does not create durable exclusivity. It creates a supply-and-continuity premium that can benefit a reliable manufacturer, but only while competitors remain constrained.

How does methsuximide compare with competing antiseizure drugs?

Methsuximide occupies a narrow position relative to better-known treatments.

Drug	Main relevance to absence seizures	Market position
Ethosuximide	Established first-line option for absence seizures	More directly associated with absence seizure treatment
Valproate	Broad-spectrum antiseizure therapy	Larger clinical and commercial market
Lamotrigine	Used across several seizure types	Broader modern prescribing base
Methsuximide	Refractory or difficult-to-control absence seizures	Niche, mature, low-volume
Levetiracetam	Broad use, but not a direct substitute in every absence-seizure case	Large overall antiseizure market

Methsuximide’s commercial niche depends on patients for whom more common therapies are ineffective, poorly tolerated, contraindicated, or clinically unsuitable. Its differentiation is historical treatment utility, not superior commercial positioning or a protected delivery technology.

What is the financial trajectory for methsuximide?

Publicly available sources do not provide audited, product-level revenue for methsuximide. The product has not been reported as a major revenue contributor by a large pharmaceutical company in the manner of a branded blockbuster. Financial analysis therefore depends on market structure rather than disclosed sales.

The likely trajectory is:

Historical phase: branded specialty product

Celontin was commercialized as a specialist anticonvulsant for a narrow patient population. Revenue was likely limited by the small indication and competition from other antiseizure therapies.

Genericization phase: price and share erosion

Once generic competition became available, brand economics weakened. The product shifted from innovation-driven revenue to supply-driven revenue. Manufacturers could earn recurring sales, but the addressable market remained small.

Current phase: maintenance economics

The current commercial model is likely based on:

• Low-volume recurring prescriptions.
• Regulatory maintenance of an established product.
• Contract manufacturing or third-party supply.
• Limited promotional spending.
• Inventory and distribution management.
• Pricing determined by generic competition and supply availability.

Forward phase: stable niche or gradual contraction

The base-case outlook is stable-to-declining demand, with periodic revenue volatility caused by supplier exits, shortages, or price changes. A sharp growth scenario would require a new clinical use, a reformulated product with clear therapeutic value, or a supply disruption that temporarily reallocates demand. None is an established growth driver.

What revenue exposure does methsuximide create for manufacturers?

Methsuximide is unlikely to represent material revenue exposure for a diversified pharmaceutical company. Its financial importance can still be disproportionate for a small generic manufacturer if the product has limited competition and a reliable supply position.

Exposure by business model

Manufacturer type	Likely exposure
Large diversified pharmaceutical company	Immaterial portfolio contribution
Generic company with a broad CNS portfolio	Small adjunct product
Specialty supplier with few products	Potentially meaningful concentration
API manufacturer	Limited value unless it supplies several customers
Distributor or wholesaler	Low-margin inventory product
Contract manufacturer	Revenue depends on a small number of purchase commitments

The principal diligence question is not peak sales. It is whether the product generates adequate gross margin after API costs, quality systems, batch-size inefficiency, pharmacovigilance, and distribution expenses.

What manufacturing and intellectual-property barriers affect methsuximide?

Methsuximide has low apparent formulation complexity but may have non-trivial operational barriers. The active ingredient must meet pharmaceutical quality requirements, and a small market can make validation, testing, and batch production expensive per unit.

Relevant barriers include:

• Qualified API sourcing.
• Stability data for the capsule product.
• Compliance with current good manufacturing practices.
• Validation of capsule filling and packaging.
• Pharmacovigilance obligations.
• Small-batch economics.
• Backup supplier availability.
• Regulatory costs relative to annual demand.

These are commercial and operational barriers, not durable patent barriers. A company with an established approved product and dependable supply chain may have an advantage over a new entrant even when the drug itself is legally unprotected.

What patent litigation and settlement agreements affect methsuximide?

No material current patent litigation or settlement agreement is central to the methsuximide market. The drug’s age and genericized status reduce the likelihood of a conventional brand-versus-generic patent dispute.

Settlement risk would become relevant only if a company introduced a new protected formulation, delivery system, or method of use. That scenario would require a distinct product strategy and would not apply to ordinary 300 mg methsuximide capsules.

Is there biosimilar risk for methsuximide?

No. Methsuximide is a chemically synthesized small molecule, not a biologic. Biosimilar approval under the Public Health Service Act does not apply.[5] Competition occurs through generic-drug and pharmaceutical-equivalence pathways, not through biosimilar development.

What geographic markets matter for methsuximide?

The United States is the most relevant market for Orange Book and ANDA analysis. International markets may have separate registrations, local brands, different availability, and different reimbursement rules. A U.S. patent conclusion does not establish freedom to operate in Europe, Canada, Japan, or other jurisdictions.

For a global manufacturer, the main geographic issues are:

• Country-specific product registrations.
• Local generic substitution rules.
• National reimbursement prices.
• Import and API controls.
• Market withdrawal requirements.
• Local shortage reporting.
• Whether the product is commercially marketed in each jurisdiction.

Because methsuximide is a niche medicine, geographic expansion would likely produce modest incremental revenue unless bundled with a broader epilepsy portfolio.

How strong is the methsuximide patent estate?

The patent estate is weak for commercial exclusivity purposes. Methsuximide has:

• No meaningful remaining molecule patent term.
• No identified modern Orange Book patent moat.
• No biologic exclusivity.
• No apparent device-based protection.
• Limited formulation differentiation.
• Potentially relevant but narrow manufacturing know-how.

Its defensibility is operational rather than legal. The strongest position belongs to a supplier that can maintain quality, avoid shortages, secure API capacity, and preserve pharmacy access.

Key Takeaways

• Methsuximide is a mature anticonvulsant used mainly for refractory absence seizures.
• Celontin is the historical U.S. brand; the market is now genericized.
• No meaningful current composition-of-matter or Orange Book patent barrier drives the market.
• Paragraph IV litigation and patent settlements are not central commercial risks.
• Biosimilar competition is irrelevant because methsuximide is a small-molecule drug.
• Public product-level revenue is not disclosed, so financial assessment relies on market structure.
• The likely revenue profile is small, recurring, and vulnerable to supplier exits.
• Manufacturing reliability, API sourcing, and regulatory continuity matter more than patent rights.
• Competitive pressure comes from ethosuximide, valproate, lamotrigine, and other antiseizure therapies.
• The principal investment thesis is supply resilience in a niche market, not patent-led growth.

FAQs

Is methsuximide still commercially available?

Availability depends on current manufacturer and distributor inventory. FDA approval status and practical pharmacy availability are separate issues for older generic medicines.

Can a company obtain exclusivity for a new methsuximide formulation?

Potentially, but any new formulation would require a distinct patentable technical feature and an FDA-supported commercial rationale. A new patent would not restore exclusivity to ordinary methsuximide capsules.

Does methsuximide qualify for an ANDA?

A generic sponsor may pursue an ANDA if it can demonstrate pharmaceutical equivalence, bioequivalence, quality compliance, and the required patent certifications under the applicable FDA framework.[4]

Why can an old generic drug remain commercially relevant?

A drug can remain relevant when a small group of patients depends on it, alternatives are clinically imperfect, and few manufacturers maintain supply. The commercial value comes from continuity and limited competition, not innovation exclusivity.

What would increase methsuximide revenue?

Revenue could rise through a supply disruption affecting competitors, a new clinically useful formulation, broader specialist adoption, or inclusion in a larger epilepsy portfolio. None is a current base-case growth driver.

References

1. U.S. Food and Drug Administration. (n.d.). Celontin (methsuximide) capsules prescribing information.
2. U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations: Orange Book.
3. United States Code. (2018). 35 U.S.C. § 154: Contents and term of patent; provisional rights.
4. U.S. Food and Drug Administration. (2018). Abbreviated new drug application submissions: Refuse-to-receive standards.
5. U.S. Food and Drug Administration. (n.d.). Biosimilar and interchangeable biosimilar products.