Last updated: November 9, 2026
Zyflo, the brand name for zileuton, is a legacy asthma drug with limited commercial relevance in the current U.S. market. FDA approved immediate-release Zyflo in 1997 and extended-release Zyflo CR in 2007. The drug competed against inhaled corticosteroids, leukotriene receptor antagonists such as Singulair, and combination inhalers. Its market position weakened because of four-times-daily dosing for the original product, liver-monitoring requirements, generic competition, and the expansion of inhaled therapies.
Zyflo’s financial trajectory was modest compared with leading asthma products. Public filings indicate a niche commercial product rather than a major revenue contributor. The U.S. brand has since been discontinued, eliminating meaningful current branded revenue and leaving limited residual value in the active ingredient, historical regulatory approvals, and any remaining generic or institutional demand.
What is Zyflo and how does zileuton work?
Zyflo contains zileuton, a 5-lipoxygenase inhibitor. It reduces formation of leukotrienes, inflammatory mediators involved in bronchoconstriction and airway inflammation.
| Attribute |
Zyflo |
| Active ingredient |
Zileuton |
| Drug class |
5-lipoxygenase inhibitor |
| Initial FDA approval |
1997 |
| Original dosage form |
Immediate-release tablets |
| Original dosing burden |
Four times daily |
| Extended-release product |
Zyflo CR |
| Extended-release approval |
2007 |
| Main indication |
Chronic prophylactic treatment of asthma |
| Rescue therapy |
No |
| Key safety issue |
Hepatotoxicity and liver-enzyme monitoring |
| Primary market |
United States |
| Current branded position |
Discontinued U.S. product |
Zyflo was designed for chronic asthma control. It was not indicated for acute bronchospasm and did not replace short-acting rescue inhalers.
When did Zyflo lose commercial exclusivity?
Zyflo lost practical commercial exclusivity after generic competition and the decline of the original branded franchise. The product’s commercial erosion reflected both patent expiry and market substitution.
The immediate-release product had been available for years before the extended-release formulation reached the market. Zyflo CR temporarily addressed the original product’s dosing burden by reducing administration frequency, but it entered an asthma market already dominated by inhaled corticosteroids and leukotriene receptor antagonists.
The key exclusivity milestones were:
| Event |
Approximate timing |
Commercial effect |
| FDA approval of immediate-release Zyflo |
1997 |
Established the zileuton franchise |
| FDA approval of Zyflo CR |
2007 |
Extended product life through a modified-release formulation |
| Generic and therapeutic competition |
Late 2000s onward |
Reduced pricing power |
| Cornerstone acquisition by Chiesi |
2014 |
Transferred commercial ownership |
| U.S. branded discontinuation period |
Mid-2010s |
Ended active brand investment |
Public FDA records and company disclosures should be used to confirm any specific patent expiry date for a particular dosage form. The commercial conclusion is clearer than the patent chronology: Zyflo no longer has a meaningful branded-exclusivity position in the United States.
What was Zyflo’s financial trajectory?
Zyflo generated niche revenue but did not become a high-value asthma franchise. Public-company filings from Cornerstone Therapeutics reported product revenue at the portfolio level, with Zyflo CR representing one product among several respiratory assets. The disclosures do not provide a complete, audited, long-term standalone revenue series for Zyflo.
The financial trajectory can be summarized as follows:
- Initial launch period: Zyflo entered a mature asthma market with limited differentiation beyond leukotriene-pathway inhibition.
- Formulation-extension period: Zyflo CR improved dosing convenience and supported continued commercial activity.
- Portfolio commercialization period: Cornerstone promoted Zyflo CR alongside other respiratory products.
- Acquisition period: Chiesi acquired Cornerstone in 2014, consolidating the product into a broader respiratory portfolio.
- Decline and discontinuation: Brand economics deteriorated as generic alternatives and competing asthma therapies reduced demand.
Revenue scale and commercial significance
Zyflo was not comparable to blockbuster asthma brands such as Singulair, Advair, Symbicort, or Dulera. Its value was constrained by:
- a narrow prescriber base;
- extensive competition from inhaled therapies;
- monitoring requirements;
- low patient convenience for immediate-release dosing;
- limited use in treatment guidelines;
- generic pricing pressure;
- the absence of a large international franchise.
Cornerstone’s acquisition by Chiesi provided strategic portfolio value, but Zyflo was not the principal economic driver of the transaction. Chiesi’s respiratory business had greater value in products such as Curosurf, Alvesco, and other established inhaled or hospital therapies.
How does Zyflo compare with competing asthma drugs?
Zyflo competed against drugs with stronger guideline adoption, broader physician familiarity, or more convenient administration.
| Product or class |
Mechanism |
Dosing or delivery advantage |
Commercial position relative to Zyflo |
| Zyflo |
5-lipoxygenase inhibition |
Oral; immediate-release dosing was burdensome |
Niche |
| Singulair |
Leukotriene receptor antagonism |
Once-daily oral dosing |
Much stronger historical commercial position |
| Inhaled corticosteroids |
Anti-inflammatory |
Direct pulmonary delivery |
Core controller therapy |
| ICS/LABA combinations |
Anti-inflammatory plus bronchodilation |
Fixed-dose inhalation |
Stronger efficacy and guideline position |
| Rescue bronchodilators |
Bronchodilation |
Rapid symptom relief |
Different role; not direct substitutes |
| Biologics |
Targeted inflammatory pathways |
Injectable or infused |
Used in severe asthma; high cost and specialized care |
The main commercial weakness was that zileuton required liver-function monitoring and did not offer a clear efficacy advantage over more established asthma controllers. The extended-release formulation improved convenience but did not eliminate the need for safety monitoring.
What FDA regulatory issues affected Zyflo demand?
The FDA-approved labeling identified hepatotoxicity as a central risk. Liver enzymes were required before treatment and during therapy. The monitoring burden affected prescribing decisions, especially when physicians could choose alternatives without the same degree of hepatic surveillance.
Zyflo’s regulatory profile included:
- chronic asthma prevention rather than acute rescue;
- contraindication or restriction in patients with active liver disease or persistent liver-enzyme elevation;
- liver-function testing before and during treatment;
- drug-interaction considerations;
- limited role in modern stepwise asthma treatment.
The safety-monitoring requirement reduced the product’s attractiveness in primary care and contributed to low persistence relative to simpler once-daily oral or inhaled regimens.
What was the Orange Book status of Zyflo?
Zyflo and Zyflo CR were FDA-approved prescription products listed in the FDA’s drug-product and Orange Book systems during their commercial life. Orange Book relevance depended on the specific NDA, dosage form, strength, and listed patents.
The practical Orange Book assessment is:
- the product was an approved small-molecule drug, not a biologic;
- it was exposed to ANDA-based generic competition;
- there was no biosimilar pathway;
- formulation patents, if listed, applied to the relevant extended-release product rather than to all zileuton products;
- discontinuation of the brand reduced the economic value of remaining brand-linked listings.
Current Orange Book entries and FDA discontinued-product records should be reviewed by dosage form before relying on any patent expiry date or listing status. A historical Orange Book listing does not establish that the brand remains commercially available.
What patents protected Zyflo and Zyflo CR?
Zyflo’s patent protection was likely divided between active-ingredient, formulation, and manufacturing or use claims. The most commercially relevant protection for the later product was the extended-release formulation.
Active-ingredient protection
Zileuton’s original composition-of-matter protection was old by the time Zyflo CR launched. Any original compound patent could not provide a durable barrier by the 2007 launch period.
Formulation protection
Zyflo CR relied on modified-release technology to reduce dosing frequency. Formulation claims could cover:
- extended-release tablets;
- release-control matrices or coatings;
- dosage schedules;
- pharmacokinetic performance;
- treatment regimens using the extended-release product.
Formulation claims generally provide narrower protection than composition-of-matter claims. Generic manufacturers can challenge them through Paragraph IV certifications or design around them while retaining the same active ingredient.
Method-of-use protection
Zileuton method-of-use protection was commercially less important than the product’s formulation and regulatory approval. Asthma-treatment claims are difficult to monetize when physicians can prescribe multiple controller therapies and when the active ingredient is already known.
Manufacturing and IP barriers
Manufacturing barriers were limited relative to biologic products. Zileuton is a small molecule that does not require cell-line development, complex aseptic processing, or a specialized delivery device. Potential barriers were more likely to involve:
- validated extended-release manufacturing;
- dissolution-profile control;
- stability data;
- impurity specifications;
- bioequivalence testing;
- regulatory documentation.
These barriers could delay generic entry but did not support a durable premium once broad patent protection expired.
Were there Paragraph IV challenges to Zyflo?
Zileuton was exposed to the standard ANDA framework for small-molecule generic competition. Any generic applicant seeking approval before expiry of an Orange Book-listed patent could file a Paragraph IV certification and trigger potential patent litigation.
The commercial record does not support treating Zyflo as a major, high-value Paragraph IV battleground. The product’s limited market size reduced the expected return from prolonged litigation. Generic entrants could also pursue approval after patent expiry or challenge only selected formulation claims.
The likely launch scenarios were:
| Scenario |
Market effect |
| Paragraph IV challenge succeeds |
Early generic entry and rapid brand erosion |
| Settlement permits later entry |
Temporary remaining brand sales followed by price decline |
| Patent expires without major litigation |
Routine generic entry |
| Product discontinued before challenge |
Limited value of further patent enforcement |
There is no current biosimilar risk because zileuton is a chemically synthesized small molecule. The relevant competitive risk is generic, not biosimilar.
Which companies owned or commercialized Zyflo?
The product passed through several commercial phases.
| Company |
Role |
| Abbott Laboratories |
Associated with the original Zyflo commercial franchise |
| Cornerstone Therapeutics |
Commercialized Zyflo CR and other respiratory products |
| Chiesi Farmaceutici / Chiesi USA |
Acquired Cornerstone and inherited the product portfolio |
| Generic manufacturers |
Potential suppliers of zileuton following loss of exclusivity |
The Cornerstone acquisition gave Chiesi control of a respiratory-focused portfolio. Zyflo was part of the acquired assets but did not define Chiesi’s overall U.S. commercial strategy.
What patent litigation and settlements affected Zyflo?
Zyflo did not develop the profile of a major patent-litigation product such as a top-selling inhaled corticosteroid/long-acting beta agonist combination. Publicly visible commercial risk centered on generic substitution and product economics rather than a prolonged, high-value litigation campaign.
A complete litigation review would require case-level searches across PACER, district court dockets, the Federal Circuit, and FDA Paragraph IV records. The available commercial history does not indicate that litigation materially preserved Zyflo revenue after generic competition and therapeutic substitution intensified.
When was Zyflo discontinued?
The U.S. brand was discontinued during the mid-2010s commercial decline of the product. The discontinuation of a brand does not necessarily mean that every generic or international zileuton product ceased distribution at the same time.
For market analysis, three statuses must be separated:
- FDA approval status;
- manufacturer marketing status;
- commercial availability at the pharmacy or wholesaler level.
A product can remain technically approved while no longer being marketed. That distinction is important for Zyflo because its commercial value is materially lower than its historical regulatory status might suggest.
What is the current market opportunity for zileuton?
The current opportunity is limited and concentrated in selected patients who cannot tolerate or do not respond adequately to other controller therapies. Zileuton may retain clinical relevance as a leukotriene-pathway option, but it lacks the commercial characteristics required for a large branded relaunch.
A potential relaunch would face:
- generic price competition;
- low awareness among newer prescribers;
- liver-monitoring requirements;
- strong inhaled-controller competition;
- reimbursement pressure;
- limited guideline prominence;
- lack of a differentiated delivery system;
- no clear path to premium pricing.
The strongest residual opportunity would involve a niche formulation, a new delivery method, or a clearly differentiated clinical use. Existing oral zileuton alone has limited commercial upside.
How strong is the Zyflo patent estate?
Zyflo’s historical patent estate was moderate to weak from a long-term commercial perspective. It had value during the transition from immediate-release to extended-release dosing, but it lacked the durability associated with a new chemical entity or a biologic platform.
| Patent-estate factor |
Assessment |
| Composition-of-matter protection |
Exhausted or commercially irrelevant |
| Extended-release formulation |
Historically important but narrower |
| Method-of-use protection |
Limited differentiation |
| Manufacturing complexity |
Moderate to low |
| Generic design-around risk |
Meaningful |
| Litigation leverage |
Limited by modest market size |
| Current branded value |
Minimal |
What generic launch risks exist for Zyflo?
Generic launch risk is high because the product is an older small molecule with a limited clinical differentiation profile. Generic entry would likely produce rapid price compression, particularly if multiple manufacturers entered the market.
The main launch variables are:
- whether a generic targets immediate-release or extended-release tablets;
- whether FDA considers the product commercially available for reference-product purposes;
- whether any Orange Book-listed formulation patent remains enforceable;
- whether the applicant can demonstrate bioequivalence;
- whether manufacturing economics justify entry.
For investors or licensors, the key issue is not whether zileuton can be manufactured. It is whether the addressable market is large enough to support regulatory, commercial, and supply-chain investment.
Key Takeaways
- Zyflo is the brand name for zileuton, an oral 5-lipoxygenase inhibitor approved for chronic asthma.
- Immediate-release Zyflo was approved in 1997; Zyflo CR followed in 2007.
- The product’s financial contribution was niche and did not approach the scale of leading asthma brands.
- Commercial weaknesses included frequent dosing, liver monitoring, limited guideline adoption, and strong competition from inhaled therapies.
- Cornerstone Therapeutics commercialized Zyflo CR before Chiesi acquired Cornerstone in 2014.
- The U.S. branded product was discontinued during the mid-2010s.
- Zileuton has generic risk but no biosimilar risk.
- The most important historical IP was associated with extended-release formulation protection.
- Current branded revenue and patent value are minimal.
- A commercial relaunch would require meaningful clinical or delivery differentiation.
FAQs About Zyflo and Zileuton
Is Zyflo still available in the United States?
The branded U.S. product was discontinued. Availability of any generic zileuton product depends on current FDA approval and commercial distribution records.
Is Zyflo the same as Singulair?
No. Zyflo contains zileuton, which inhibits 5-lipoxygenase. Singulair contains montelukast, a leukotriene receptor antagonist. Both affect the leukotriene pathway but have different mechanisms.
Does Zyflo have a biosimilar competitor?
No. Zileuton is a small-molecule drug. Competition occurs through generic-drug pathways rather than biosimilar approval.
Why did Zyflo fail to become a major asthma product?
Its commercial performance was limited by dosing frequency, liver-function monitoring, competition from inhaled corticosteroids and combination inhalers, and the stronger commercial position of once-daily leukotriene receptor antagonists.
Could a company relaunch zileuton?
A relaunch is technically possible but commercially difficult. A viable strategy would likely require a differentiated formulation, improved safety monitoring, a new delivery system, or a narrowly defined population with an unmet treatment need.
References
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U.S. Food and Drug Administration. (1997). Zyflo (zileuton) prescribing information. FDA.
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U.S. Food and Drug Administration. (2007). Zyflo CR (zileuton extended-release tablets) prescribing information. FDA.
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U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations: Orange Book. FDA.
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U.S. Food and Drug Administration. (n.d.). Drug safety-related labeling changes and discontinued drug products. FDA.
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Cornerstone Therapeutics Inc. (2014). Annual report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934. U.S. Securities and Exchange Commission.
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Chiesi Farmaceutici S.p.A. (2014). Chiesi Farmaceutici completes acquisition of Cornerstone Therapeutics. Chiesi.
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Global Initiative for Asthma. (2024). Global strategy for asthma management and prevention. Global Initiative for Asthma.
