List of Excipients in Branded Drug CELONTIN

Company	Tradename	Ingredient	NDC	Excipient	Potential Generic Entry
Parke-Davis Div of Pfizer Inc	CELONTIN	methsuximide	0071-0525	D&C YELLOW NO. 10
Parke-Davis Div of Pfizer Inc	CELONTIN	methsuximide	0071-0525	FD&C YELLOW NO. 6
Parke-Davis Div of Pfizer Inc	CELONTIN	methsuximide	0071-0525	GELATIN
Parke-Davis Div of Pfizer Inc	CELONTIN	methsuximide	0071-0525	SILICON DIOXIDE

CELONTIN Excipient Strategy and Commercial Opportunities

Last updated: August 9, 2026

CELONTIN is the branded capsule formulation of methsuximide, an older succinimide anticonvulsant used for absence seizures that have not responded adequately to other therapies. Its commercial value is driven less by active-ingredient patent protection than by supply reliability, regulatory execution, formulation differentiation, and access to a small but clinically important patient population. The most credible opportunities are a generic 300 mg capsule, a lactose-free version, an oral liquid or suspension, and a patient-friendly dosage form for patients with swallowing difficulties.

What is CELONTIN and how is it formulated?

CELONTIN contains methsuximide, a synthetic anticonvulsant related pharmacologically to ethosuximide. The marketed product is a 300 mg oral capsule. The FDA labeling identifies inactive ingredients that include lactose, corn starch, magnesium stearate, gelatin, titanium dioxide, and colorants used in the capsule shell.[1]

Attribute	CELONTIN
Active ingredient	Methsuximide
Strength	300 mg
Dosage form	Immediate-release hard gelatin capsule
Route	Oral
Therapeutic category	Anticonvulsant
Primary indication	Absence seizures refractory to other therapy
FDA pathway	Original NDA product
Reference product strength	300 mg capsule
Key excipient issue	Lactose-containing solid oral dosage form
Patent profile	Legacy product with no commercially significant composition-of-matter exclusivity expected
Main commercial barrier	Small market, limited clinical demand, supply economics, and FDA approval requirements

The product is pharmacologically established but commercially narrow. Its label does not position CELONTIN as a first-line treatment for common generalized epilepsy. The restricted clinical use limits volume but may increase the importance of dependable availability because alternatives may be clinically unsuitable for some patients.[1]

What excipients are used in CELONTIN?

The core excipient system is conventional for a hard gelatin capsule. Lactose provides bulk and improves powder handling. Corn starch can support powder flow and disintegration. Magnesium stearate acts as a lubricant. The gelatin shell provides the capsule structure, while titanium dioxide and colorants support product identification.[1]

What are the main excipient risks?

The existing formulation creates several potential barriers to broad patient access:

1. Lactose may be unsuitable for patients with lactose intolerance or for institutions seeking lactose-free products.
2. Hard gelatin capsules are difficult for patients with dysphagia, pediatric patients, and some neurologically impaired patients.
3. Colorants can create avoidable sensitivity or formulary concerns.
4. The formulation may not be suitable for administration through feeding tubes without dedicated compatibility data.
5. Methsuximide dosing may require clinical flexibility that a single 300 mg capsule does not provide.

These issues do not automatically establish a regulatory deficiency. They identify areas where a differentiated product could address practical use constraints.

What excipient strategy is appropriate for a generic methsuximide capsule?

The lowest-risk strategy is a Q1/Q2-style formulation that remains close to the reference product while controlling critical quality attributes. The target should be a 300 mg immediate-release capsule with comparable dissolution, assay, content uniformity, stability, and impurity profile.

Base formulation strategy

A conventional formulation could use:

• Lactose monohydrate or anhydrous lactose as filler
• Pregelatinized starch or maize starch as disintegrant and processing aid
• Magnesium stearate or sodium stearyl fumarate as lubricant
• Hard gelatin or hypromellose capsule shell
• Minimal colorant system

A lactose-containing formulation is likely to offer the simplest manufacturing route if the reference product also uses lactose. A lactose-free formulation may have stronger commercial differentiation but can increase the need for formulation development and comparative dissolution work.

Excipient selection priorities

Formulation objective	Preferred development approach
Match reference product	Use similar filler, starch, lubricant, and capsule architecture
Improve powder flow	Evaluate silicon dioxide or a higher-functionality starch
Avoid lactose	Test microcrystalline cellulose, mannitol, dibasic calcium phosphate, or pregelatinized starch
Improve capsule robustness	Compare gelatin with hypromellose shells
Reduce patient sensitivities	Use a clear or low-colorant shell where feasible
Enable feeding-tube use	Develop a liquid or suspension product rather than relying on capsule opening
Protect stability	Control moisture, lubricant level, and capsule-shell water activity

The active ingredient's particle size, polymorphic form, bulk density, and electrostatic behavior should drive the final excipient choice. A high drug-load capsule may have less formulation flexibility than a low-dose product because excipients must compensate for flow, segregation, and fill-weight variability.

What formulation patents protect CELONTIN?

No commercially significant active patent estate is expected to protect CELONTIN today. Methsuximide and the original capsule product date from the mid-20th century, placing any original composition-of-matter and early formulation rights outside their normal patent terms.

A current diligence review should still distinguish among four possible rights categories:

Rights category	Relevance to CELONTIN
Methsuximide composition patent	Expected to be expired
Original capsule formulation patent	Expected to be expired
Method-of-use patent	Any early rights would ordinarily be expired
Modern reformulation patent	Possible only if a later party developed a new liquid, modified-release, taste-masked, or device-associated product

A patent search should cover methsuximide, Celontin, succinimide anticonvulsants, oral suspensions, taste masking, tube administration, and pediatric formulations. The absence of a known active patent does not eliminate the need to review U.S. Patent and Trademark Office records, FDA Orange Book listings, assignment histories, and litigation databases before launch.

When does CELONTIN lose exclusivity?

CELONTIN's practical exclusivity is already legacy exclusivity rather than current statutory protection. The product was approved decades ago, and any original FDA marketing exclusivity has expired. New-drug exclusivity under the Hatch-Waxman framework is not the principal barrier to entry for this product.[2]

Exclusivity type	Current commercial significance
New chemical entity exclusivity	Expired
Five-year NCE exclusivity	Not applicable to a legacy product
Three-year new clinical investigation exclusivity	No current basis identified
Orphan-drug exclusivity	No current basis identified
Pediatric exclusivity	No current basis identified
Patent term extension	No current commercial relevance expected
Active Orange Book patent	Must be confirmed against the current edition

The more relevant issue is whether an approved generic ANDA exists and whether a manufacturer can economically supply the product. A drug can have no meaningful patent barrier and still have limited generic competition because of low demand, manufacturing complexity, commercial discontinuation, or inadequate wholesaler economics.

What is the Orange Book status of CELONTIN?

The FDA Orange Book is the controlling source for current reference-listed-drug status, therapeutic-equivalence codes, patents, and exclusivity listings.[3] CELONTIN should be reviewed under methsuximide and its NDA record, including any discontinued-product notation and the current reference-listed product.

For a prospective entrant, the key Orange Book questions are:

• Is the 300 mg capsule listed as the reference standard?
• Is the NDA active, discontinued, or moved to a discontinued section?
• Are any patents or exclusivity codes listed?
• Is a separate reference standard identified for an ANDA?
• Are there approved ANDAs with an “A” therapeutic-equivalence rating?

An applicant should not assume that a historical brand product remains commercially marketed merely because its NDA appears in FDA records. FDA regulatory status, commercial availability, and wholesaler distribution are separate issues.

What generic entry risks exist for CELONTIN?

Paragraph IV challenges

A Paragraph IV certification is relevant only if an ANDA applicant identifies a listed patent as invalid, unenforceable, or not infringed. If no unexpired Orange Book patent protects the reference product, an ANDA may proceed with a Paragraph III certification or a certification that no relevant patent is listed, depending on the FDA record.[2][3]

For CELONTIN, the expected litigation risk from a Paragraph IV challenge is low unless a later patent covering a reformulated product or specific method of use has been listed. The larger risk is regulatory or commercial: failure to establish bioequivalence, inability to obtain reliable API supply, or insufficient market size to support launch.

Generic launch scenarios

Scenario	Commercial effect
No approved generic	Strongest supply opportunity, but limited market size
One approved generic	Price competition remains manageable if the product is scarce
Several approved generics	Rapid price erosion and lower incentive to maintain inventory
Brand discontinuation with an active NDA	Opportunity to become the dependable source if FDA requirements can be met
Reformulated oral liquid entrant	Potential premium pricing and reduced direct capsule substitution

An ANDA applicant would generally seek approval of the same dosage form and strength as the reference product. A different dosage form, such as an oral solution, may require a 505(b)(2) application rather than a conventional ANDA if it cannot rely fully on the reference product for bioequivalence and formulation comparability.[2]

What commercial opportunities exist for CELONTIN excipients?

1. Lactose-free 300 mg capsule

A lactose-free capsule is the most straightforward differentiation strategy. Microcrystalline cellulose, mannitol, dibasic calcium phosphate, or higher-functionality starch could replace lactose. The product could be positioned for patients with lactose intolerance and institutional formularies that seek fewer inactive ingredients.

The commercial advantage is limited if prescribers and pharmacists view the product as interchangeable with a standard generic. The opportunity increases if the reference product is unavailable or if the supplier can demonstrate consistent national distribution.

2. Hypromellose capsule

A hypromellose shell could address gelatin-avoidance requirements and support vegetarian or religious dietary preferences. It may also provide a cleaner excipient profile if colorants are reduced.

The main development issues are shell moisture, compatibility with the powder blend, brittleness, dissolution, and packaging stability. A shell change alone is unlikely to justify a premium unless the product solves a documented access problem.

3. Oral liquid or suspension

An oral liquid is the strongest product-extension opportunity. It could address pediatric use, dysphagia, feeding-tube administration, and patients unable to swallow 300 mg capsules.

A liquid product would require work on:

• Methsuximide solubility and dose uniformity
• pH control
• Chemical and microbiological stability
• Preservative selection
• Taste masking
• Container closure compatibility
• Tube adsorption and recovery
• In-use stability
• Dosing-device accuracy

A suspension may be more practical than a true solution if methsuximide has limited aqueous solubility. Suspending agents such as hydroxypropyl cellulose, xanthan gum, or sodium carboxymethylcellulose could be evaluated, subject to viscosity and syringeability requirements. Sweeteners and flavor systems would need to avoid excessive excipient burden and maintain chemical stability.

4. Sprinkle or opened-capsule product

A capsule designed to be opened and sprinkled on soft food could improve administration for selected patients. This approach requires evidence that opening the capsule does not materially affect dose uniformity, stability, palatability, or occupational exposure.

It also creates labeling and handling questions. A conventional capsule should not be marketed for opening and sprinkling without supporting product-specific data.

5. Modified strengths

A 150 mg capsule could help with titration and reduce the need for capsule manipulation. The commercial case depends on clinical dosing patterns and whether the lower strength can qualify under an ANDA pathway or requires a different regulatory route. Strength expansion may be less attractive than a liquid formulation because it adds manufacturing, packaging, and inventory complexity.

How strong is the CELONTIN patent estate?

The patent estate is weak from a conventional exclusivity perspective because the product is old and its core formulation is uncomplicated. Its defensible value would come from later, narrowly drafted patents covering a new dosage form, excipient combination, taste-masking system, administration method, or manufacturing process.

Estate component	Strategic assessment
Core molecule	Low protection potential
Standard capsule	Low protection potential
Lactose-free capsule	Moderate only if formulation performance is non-obvious
Oral suspension	Higher potential if stability, taste, and dosing problems are solved
Tube-compatible formulation	Moderate to high if supported by differentiated data
Manufacturing process	Moderate if it improves impurity control or batch consistency
Brand and supply reliability	Commercial, not patent, advantage

A reformulation patent would need more than a routine excipient substitution. Patent value would be stronger where the formulation produces an unexpected stability, dissolution, bioavailability, taste, or administration benefit.

Which companies could challenge or compete with CELONTIN?

Direct competition may come from three groups:

1. Generic methsuximide manufacturers seeking approval of the 300 mg capsule.
2. Manufacturers of alternative absence-seizure therapies, including ethosuximide and valproate products.
3. Specialty pharmaceutical companies developing liquid, pediatric, or hospital-oriented formulations.

Ethosuximide is the closest pharmacologic comparator for absence seizures, while valproate and lamotrigine may compete clinically in broader generalized seizure treatment. These products do not automatically substitute for methsuximide because treatment selection depends on seizure type, prior response, tolerability, and physician judgment.

No biosimilar risk applies. Methsuximide is a small-molecule drug, not a biologic. Competition would arise through ANDAs, 505(b)(2) applications, pharmacy substitution, and therapeutic alternatives rather than the FDA biosimilar pathway.[2]

What licensing deals and litigation affect CELONTIN?

No major current licensing transaction or active high-profile patent litigation is associated with CELONTIN in the established public record. Historical ownership and distribution arrangements may involve Parke-Davis, Pfizer, or successor entities, but those relationships should not be treated as current commercial rights without review of FDA records, corporate filings, and trademark ownership data.

The expected litigation profile is low because:

• The molecule is old.
• The original capsule formulation is conventional.
• Current statutory exclusivity is not the primary barrier.
• Any new litigation would most likely involve a later reformulation, trademark issue, supply agreement, or ANDA certification.

What revenue exposure does CELONTIN create?

Publicly attributable CELONTIN revenue is difficult to isolate because the product is a niche legacy medicine and may not have current standalone reporting in issuer filings. The relevant commercial variables are annual treated-patient volume, net price, reimbursement, wholesaler stocking, API cost, minimum efficient batch size, and the number of approved competitors.

A small market can still support a profitable product when supply is constrained and competition is limited. The opposite is also true: a product with high clinical value can be unattractive if annual demand is too low to support quality systems, pharmacovigilance, inventory, and FDA compliance.

What manufacturing and IP barriers exist?

The main barriers are operational:

• Reliable methsuximide API sourcing
• Control of capsule-fill uniformity
• Impurity and degradation monitoring
• Moisture control for gelatin or hypromellose shells
• Stability through the proposed shelf life
• Validation of dissolution against the reference product
• Low-volume batch economics
• Pharmacovigilance for a central nervous system medicine
• Distribution to pharmacies that may not routinely stock the product

For an oral liquid, manufacturing and regulatory barriers are higher because of microbiological control, preservative effectiveness, dose uniformity, taste, and in-use stability. Those barriers can also create a stronger commercial position than a simple capsule generic.

Key Takeaways

• CELONTIN is a legacy 300 mg methsuximide capsule for refractory absence seizures.
• The core molecule and conventional capsule formulation are expected to have no meaningful remaining patent exclusivity.
• The FDA Orange Book and current NDA record control the precise reference-product and patent status.
• A standard generic capsule is the lowest-risk entry strategy but may face weak market economics.
• A lactose-free or hypromellose capsule offers modest differentiation.
• An oral suspension or solution has the strongest commercial potential for dysphagia, pediatric, and feeding-tube use.
• No biosimilar pathway applies because methsuximide is a small molecule.
• Expected litigation risk is low unless a later reformulation patent is identified.
• The principal barriers are API supply, low market volume, regulatory compliance, and distribution reliability.

FAQs

Is CELONTIN still protected by patents?

The original methsuximide and conventional capsule rights are expected to be expired. Current Orange Book listings should be checked for any later-listed patent before filing.

Can a company file an ANDA for methsuximide 300 mg capsules?

Yes, if the applicant can establish the applicable reference-product relationship, pharmaceutical equivalence, bioequivalence, quality, and manufacturing compliance under the FDA pathway.

Would a methsuximide oral liquid qualify as an ANDA?

Not necessarily. A materially different dosage form may require a 505(b)(2) application, particularly where the applicant cannot rely fully on the reference product for formulation and clinical bridging.

Is lactose removal alone enough to support a CELONTIN reformulation patent?

Usually not. A patent case would be stronger if the lactose-free formulation produced an unexpected technical result, such as improved stability, dissolution, bioavailability, or administration performance.

What is the most attractive CELONTIN product opportunity?

A stable, palatable, accurately dosed oral suspension with feeding-tube and in-use stability data offers the clearest differentiation. A lactose-free 300 mg capsule is easier to develop but has a lower commercial ceiling.

References

1. U.S. Food and Drug Administration. (2009). Celontin (methsuximide) capsule prescribing information. Parke-Davis Division of Pfizer Inc.

2. U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. FDA.

3. U.S. Food and Drug Administration. (2024). Orange Book: Approved drug products with therapeutic equivalence evaluations. FDA.