List of Excipients in Branded Drug RIMANTADINE HYDROCHLORIDE

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Generic Drugs Containing RIMANTADINE HYDROCHLORIDE

Company	Ingredient	NDC	Excipient
Amneal Pharmaceuticals of New York LLC	rimantadine hydrochloride	0115-1911	CELLULOSE, MICROCRYSTALLINE
Amneal Pharmaceuticals of New York LLC	rimantadine hydrochloride	0115-1911	FD&C YELLOW NO. 6
Amneal Pharmaceuticals of New York LLC	rimantadine hydrochloride	0115-1911	HYPROMELLOSES
Amneal Pharmaceuticals of New York LLC	rimantadine hydrochloride	0115-1911	MAGNESIUM STEARATE

What are the Most Frequently-Used Excipients in RIMANTADINE HYDROCHLORIDE?

# Of NDCs	Excipient
5	CELLULOSE, MICROCRYSTALLINE
5	FD&C YELLOW NO. 6
1	HYPROMELLOSE
4	HYPROMELLOSES

Rimantadine Hydrochloride Excipient Strategy and Commercial Opportunities

Last updated: August 3, 2026

Rimantadine hydrochloride is an off-patent antiviral with limited U.S. commercial potential in influenza because circulating influenza A viruses have widespread adamantane resistance and CDC guidance no longer recommends rimantadine for seasonal influenza treatment or prevention. The strongest opportunities are low-cost, stable oral products for markets where rimantadine remains approved or available, institutional stockpiles, veterinary or research applications where permitted, and reformulations that improve pediatric dosing, palatability, or supply reliability. Any U.S. launch would face a weak demand outlook despite low intellectual-property barriers.

What is rimantadine hydrochloride and how is it used?

Rimantadine hydrochloride is the hydrochloride salt of rimantadine, an adamantane antiviral related to amantadine. It was marketed in the United States under the brand name Flumadine for treatment and prophylaxis of influenza A in adults and pediatric patients, subject to age and labeling restrictions.[1]

The commercial product historically used immediate-release oral dosage forms:

Attribute	Rimantadine hydrochloride
Active ingredient	Rimantadine hydrochloride
Pharmacologic class	Adamantane antiviral
Historical U.S. brand	Flumadine
Historical dosage forms	100 mg tablets; oral liquid availability in some markets
Main indication	Influenza A treatment and prophylaxis
Mechanism	Inhibition of the influenza A M2 proton channel
Key limitation	High prevalence of resistance among circulating influenza A strains
U.S. patent position	Core product and basic oral formulation rights are expired or commercially inactive
U.S. market status	No meaningful current branded market; product is not a preferred influenza therapy

Rimantadine is not effective against influenza B. It also has no established role comparable to neuraminidase inhibitors such as oseltamivir or baloxavir in current U.S. influenza treatment guidelines.[2]

When does rimantadine lose exclusivity?

Rimantadine’s core U.S. pharmaceutical exclusivity has expired. The historical Flumadine product was approved by the FDA in 1993 under NDA 018890. The product’s commercial protection is no longer the primary barrier to entry.

The relevant protection layers are:

Protection layer	Current commercial significance
Active-ingredient patent	Expired
Basic oral tablet formulation	Expired or no longer commercially material
Original NDA exclusivity	Expired
Pediatric exclusivity	Expired
Orphan exclusivity	Not applicable to the historical influenza indication
FDA approval status	Historical approval; current marketing availability is limited
New formulation patents	Potentially available only for genuinely novel delivery systems or compositions

A new developer would therefore rely mainly on regulatory compliance, manufacturing cost, product quality, supply reliability, and market access. A conventional rimantadine hydrochloride tablet would generally have limited ability to command premium pricing unless it solved a defined clinical or logistical problem.

What is the FDA and Orange Book status of rimantadine hydrochloride?

The FDA’s historical approval of Flumadine established rimantadine hydrochloride as an approved antiviral product. The original product is not a current growth asset in the U.S. market. The FDA has reported that discontinuation of Flumadine was not based on safety or effectiveness concerns, which is distinct from a formal withdrawal for safety reasons.[3]

The Orange Book should be reviewed for current listed products, discontinued products, therapeutic-equivalence codes, and any remaining reference-product information before filing an ANDA. A discontinued reference product can create regulatory complexity where no currently marketed reference standard is readily available. The absence of an active branded product does not eliminate the need to establish pharmaceutical equivalence, bioequivalence, stability, and appropriate labeling.

What regulatory pathway would a new manufacturer use?

The likely pathways are:

1. ANDA pathway: Appropriate if a suitable reference-listed drug and active FDA-approved reference product can be identified.
2. 505(b)(2) NDA: Potentially appropriate for a novel oral liquid, modified-release product, pediatric formulation, or other dosage form relying partly on published or FDA findings.
3. Full NDA: Relevant if the proposed product has a new indication, materially different clinical profile, or insufficient reliance pathway.
4. Non-U.S. national filing: Potentially more practical in countries where rimantadine remains registered or where local antiviral supply policies support the product.

A sponsor should not assume that a conventional tablet has a commercially viable ANDA route merely because the active ingredient is old. Reference-product availability, current FDA listings, dissolution comparability, and labeling requirements determine execution risk.

What excipients are suitable for rimantadine hydrochloride tablets?

Rimantadine hydrochloride is suited to conventional immediate-release solid dosage forms. The excipient system should prioritize chemical stability, low manufacturing cost, rapid dissolution, and acceptable tablet robustness.

A practical tablet platform may include:

Formulation function	Candidate excipient classes	Commercial rationale
Diluent	Microcrystalline cellulose, lactose, mannitol, dibasic calcium phosphate	Controls tablet weight and compressibility
Binder	Povidone, copovidone, pregelatinized starch	Supports granule and tablet strength
Disintegrant	Croscarmellose sodium, crospovidone, sodium starch glycolate	Promotes rapid release
Lubricant	Magnesium stearate, sodium stearyl fumarate	Supports ejection and manufacturing throughput
Glidant	Colloidal silicon dioxide	Improves powder flow
Taste or mouthfeel modifier	Mannitol, sucralose, flavor systems	Important for chewable or liquid products
Film coating	Hypromellose, polyethylene glycol, titanium dioxide or approved colorants	Improves handling, appearance, and swallowing

A direct-compression formulation could minimize processing cost if the drug-excipient blend has adequate flow and content uniformity. Wet granulation may provide better control where the active load, powder flow, or compression behavior is poor. The final choice should be driven by compatibility data and dissolution performance rather than by the lowest nominal excipient cost.

What excipient risks matter most?

The principal development risks are:

• Moisture exposure: Water activity and packaging selection should be assessed during accelerated stability studies.
• Acid-base compatibility: Rimantadine hydrochloride is a salt, so excipients with strong pH-modifying behavior should be screened.
• Dissolution variability: Hydrophobic lubricants, excessive binder, or over-compression can delay release.
• Content uniformity: Low-dose presentations may require geometric dilution or a granulation process.
• Taste: An oral liquid or chewable tablet may require taste masking because the active ingredient can produce an unpleasant medicinal taste.
• Excipient regulatory status: Colorants, sweeteners, preservatives, and flavors must comply with the target jurisdiction’s permitted-use limits.

A low-risk initial tablet screen would compare microcrystalline cellulose with lactose or mannitol as the primary diluent, crospovidone with croscarmellose sodium as the disintegrant, and magnesium stearate with sodium stearyl fumarate as the lubricant. Development should include impurity monitoring, assay, dissolution, friability, hardness, water content, and container-closure compatibility.

What formulations are protected by new rimantadine patents?

The conventional rimantadine hydrochloride tablet is unlikely to support meaningful new patent protection by itself. A new patent position would need to arise from a demonstrably novel formulation, manufacturing process, dosage regimen, or combination.

Potential formulation concepts include:

Pediatric oral solution

An oral solution or suspension could use:

• Purified water
• Glycerin or sorbitol
• Buffer system
• Sweetener such as sucralose or a jurisdictionally acceptable alternative
• Flavor
• Suspending agent for a suspension
• Preservative where required
• Chelating agent where justified by stability data

The commercial value would depend on pediatric labeling, palatability, dosing accuracy, microbial control, and shelf life. A ready-to-use liquid would be more convenient but may impose higher packaging, shipping, and stability costs than a powder for reconstitution.

Powder for reconstitution

A dry powder could improve stability and reduce shipping weight. It would require controlled reconstitution volume, sedimentation control, dose uniformity, acceptable redispersibility, and clear caregiver instructions. This format may be more defensible than a basic tablet if it solves a documented stability or pediatric-use problem.

Chewable or orally disintegrating tablet

These formats could improve administration for children and patients who cannot swallow tablets. Mannitol, crospovidone, flavors, sweeteners, and taste-masking polymers may be useful. The key technical risk is balancing rapid disintegration against bitterness and friability.

Modified-release product

A sustained-release product could reduce dosing frequency, but its commercial rationale is weak in an antiviral with limited current guideline support. A modified-release patent would also face a higher regulatory burden and may not overcome limited clinical demand.

How strong is the patent estate for rimantadine hydrochloride?

The legacy patent estate is weak from a blocking-rights perspective. The original active-ingredient and conventional product claims are old, and generic entry is not likely to be restricted by an unexpired core patent.

A new entrant could still encounter:

• Process patents covering a specific crystalline form or manufacturing route
• Formulation patents covering a liquid, taste-masked, or multiparticulate product
• Method-of-use patents for a new viral indication
• Combination-product patents involving another antiviral or immunomodulator
• Patent rights in specific non-U.S. jurisdictions

These rights would need claim-by-claim review. A patent directed only to a narrow manufacturing process may not block an alternative synthesis route. A formulation patent may be avoidable through a different excipient system, dosage form, or release profile.

Which companies are challenging rimantadine hydrochloride exclusivity?

No major current U.S. Paragraph IV campaign is associated with rimantadine hydrochloride. The molecule’s commercial problem is demand erosion and clinical obsolescence rather than an active branded patent dispute.

Paragraph IV risk is therefore likely to be limited to a future 505(b)(2) or reformulated product that obtains new patent listings. If a developer launches a novel liquid, modified-release product, or combination product, subsequent ANDA applicants could challenge listed patents under Paragraph IV. The financial value of those patents would depend on whether the new product achieved meaningful reimbursement and prescribing demand.

What patent litigation and settlement agreements affect rimantadine?

No major current U.S. patent litigation or settlement agreement materially affects the commercial launch of a conventional rimantadine hydrochloride tablet. Historical disputes, if any, would have little practical effect on a new generic product because the core exclusivity period has ended.

For diligence, the relevant litigation review should cover:

• FDA Orange Book patent listings
• Patent Trial and Appeal Board proceedings
• Federal district court cases involving the active ingredient or proposed formulation
• Paragraph IV notices
• ANDA settlements and launch dates
• Foreign litigation in countries where rimantadine remains commercially relevant

The absence of active litigation reduces legal entry risk but does not improve the underlying market outlook.

What generic launch risks exist for rimantadine hydrochloride?

Generic launch risk is low on patent grounds and high on commercial grounds.

Risk category	Assessment
Core patent blocking	Low
Formulation patent exposure	Low for a conventional tablet; higher for a novel dosage form
Reference-product access	Moderate
Bioequivalence execution	Low to moderate for an immediate-release tablet
Manufacturing cost	Low
Demand from influenza treatment	Low in the U.S.
Resistance risk	High
Reimbursement	Uncertain and likely limited
Inventory risk	High if seasonal demand is overestimated
International opportunity	Country-specific

The product could be commercially unattractive even with a low-cost supply chain. Influenza antivirals are seasonal, and wholesalers may avoid carrying an older agent with limited guideline support.

What commercial opportunities remain for rimantadine hydrochloride?

The most credible opportunities are outside a conventional U.S. retail launch.

Regional generic supply

Some markets may retain rimantadine in formularies or national registrations. A manufacturer with low-cost production and local regulatory capabilities could supply those markets through licensing, contract manufacturing, or tender channels.

Institutional or government procurement

Stockpiling may create episodic demand, but procurement agencies are more likely to prioritize antivirals with current activity against circulating strains. Any stockpile strategy would require strain-susceptibility data and a defined public-health rationale.

Pediatric and geriatric dosage forms

A palatable liquid, dispersible tablet, or unit-dose product could address administration problems. This is the clearest formulation-led opportunity, although market size remains constrained by clinical use.

Veterinary products

Veterinary commercialization would require species-specific safety, efficacy, residue, and regulatory review. Human approval does not establish veterinary authorization. The opportunity should be evaluated separately from human pharmaceutical development.

Research-use and reference-standard supply

Small-volume sales for analytical laboratories, pharmacology research, and method development may be viable. This is a specialty chemical opportunity rather than a conventional pharmaceutical opportunity.

Combination or repurposing programs

A new antiviral combination or a non-influenza indication could create intellectual-property value, but it would require new clinical evidence. A combination patent alone would not establish commercial demand.

How does rimantadine compare with competing influenza antivirals?

Product	Class	Current clinical position	Patent and commercial profile
Rimantadine	M2 ion-channel inhibitor	Limited by resistance; not preferred in U.S. guidance	Core rights expired; weak demand
Amantadine	M2 ion-channel inhibitor	Similar resistance limitations	Generic and low commercial attractiveness
Oseltamivir	Neuraminidase inhibitor	Broad use for treatment and prophylaxis	Mature generic market with stronger demand
Zanamivir	Neuraminidase inhibitor	Alternative inhaled therapy	Administration limitations
Baloxavir marboxil	Cap-dependent endonuclease inhibitor	Current branded influenza option	More active branded and patent-driven market
Peramivir	Intravenous neuraminidase inhibitor	Hospital or selected severe cases	Institutional and injectable market

Rimantadine’s principal disadvantage is not price. It is reduced clinical relevance caused by resistance and the availability of better-supported alternatives.[2]

What revenue exposure should investors expect?

A conventional rimantadine hydrochloride product should be modeled as a low-revenue, seasonal generic unless it has a secured non-U.S. tender, government contract, or differentiated formulation. Revenue sensitivity will depend on:

• Countries where the product remains registered
• Current influenza treatment guidelines
• Wholesale and tender access
• Resistance surveillance
• Seasonal influenza severity
• Competitor supply interruptions
• Pediatric formulation uptake
• API cost and minimum batch size

A sponsor should avoid assuming that a low-cost API automatically produces attractive margins. The product’s low unit value, seasonal inventory, regulatory maintenance, and limited prescriber demand can compress returns.

What manufacturing and intellectual-property barriers remain?

Manufacturing barriers are manageable but not irrelevant. The API supply chain must meet applicable GMP requirements, and the finished product must demonstrate consistent assay, content uniformity, dissolution, impurity control, and stability.

The most practical manufacturing strategy is:

1. Source a qualified API with multiple suppliers.
2. Use a simple immediate-release tablet platform.
3. Select excipients with established compendial quality and broad regulatory acceptance.
4. Use high-barrier packaging if moisture sensitivity is demonstrated.
5. Validate a low-cost process suitable for seasonal production.
6. Maintain a liquid or dispersible formulation only if a target market supports it.

Geographic IP coverage is likely to be limited for the legacy molecule. New rights would depend on jurisdiction, filing date, claim scope, patent-term adjustment, and any country-specific SPC or supplementary protection mechanism. A freedom-to-operate review should focus on new formulation and process claims rather than the historical active ingredient.

Key Takeaways

• Rimantadine hydrochloride is an old, off-patent antiviral with limited current U.S. commercial value.
• The main barrier is influenza A resistance and weak guideline support, not patent exclusivity.
• A conventional immediate-release tablet has the lowest development and manufacturing risk but also the weakest differentiation.
• Pediatric liquids, powders for reconstitution, chewables, and orally disintegrating tablets offer the most credible formulation-led opportunities.
• No major current U.S. Paragraph IV campaign or patent litigation materially blocks a conventional generic launch.
• The strongest commercial routes are regional supply, institutional contracts, specialty manufacturing, and carefully targeted pediatric products.
• A new formulation patent would need claims tied to a genuine technical or clinical advantage.
• Revenue should be modeled conservatively because demand is seasonal, guideline-dependent, and vulnerable to resistance.

FAQs

Is rimantadine hydrochloride still FDA-approved?

Rimantadine hydrochloride has a historical FDA approval through Flumadine. Current commercial availability and active reference-product status must be evaluated through current FDA databases before filing.

Can rimantadine hydrochloride be sold as an over-the-counter antiviral?

No. Rimantadine was developed and regulated as a prescription antiviral, and an over-the-counter switch would require a separate FDA review supported by appropriate safety, labeling, and consumer-use data.

Is a rimantadine hydrochloride oral suspension commercially attractive?

It may be attractive in a defined pediatric or international market, particularly where palatability and dosing flexibility are unmet needs. Its value is limited where clinicians no longer use rimantadine because of resistance.

Can an excipient combination create patent protection?

An excipient combination can support patent protection only if the claims are novel, non-obvious, adequately supported, and tied to a measurable technical result such as improved stability, taste masking, dissolution, or dose uniformity.

Does rimantadine hydrochloride have biosimilar risk?

No. Rimantadine hydrochloride is a small-molecule drug, so biosimilar regulation does not apply. Competition would arise through generic, 505(b)(2), or jurisdiction-specific small-molecule pathways.

References

1. U.S. Food and Drug Administration. (1993). Flumadine (rimantadine hydrochloride) prescribing information. FDA.

2. Centers for Disease Control and Prevention. (2024). Influenza antiviral medications: Summary for clinicians. CDC.

3. U.S. Food and Drug Administration. (2013). Determination that FLUMADINE (rimantadine hydrochloride) tablets, 100 milligrams, was not withdrawn from sale for reasons of safety or effectiveness. Federal Register.

4. National Library of Medicine. (2024). Rimantadine hydrochloride. PubChem Compound Summary.

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