Last updated: September 3, 2026
Acetohydroxamic acid is a niche urinary stone drug marketed in the United States as Lithostat by Mission Pharmacal. Its commercial opportunity is constrained by a narrow indication, significant adverse-effect liability, limited prescriber use, and competition from antibiotics and surgical management. The product has no meaningful patent-based protection remaining, and its financial performance is not publicly disclosed. The investment case is therefore driven by durable niche demand and supply continuity rather than market expansion or exclusivity.
What is the FDA status of acetohydroxamic acid?
Acetohydroxamic acid is an FDA-approved oral urease inhibitor used as adjunctive therapy for patients with chronic infection-related struvite or carbonate apatite stones caused by urease-producing bacteria. It is marketed in 250 mg tablets under the brand name Lithostat [1].
| Regulatory attribute |
Acetohydroxamic acid |
| Active ingredient |
Acetohydroxamic acid |
| Brand |
Lithostat |
| U.S. sponsor |
Mission Pharmacal Company |
| Dosage form |
Oral tablet |
| Strength |
250 mg |
| Original FDA approval |
NDA 018651, approved in the 1980s |
| Therapeutic area |
Infection-related urinary stones |
| FDA pathway |
Full NDA |
| Biosimilar pathway |
Not applicable |
| Prescription status |
Prescription drug |
| Primary target |
Urease produced by certain bacteria |
The drug is not indicated for ordinary calcium oxalate stones or for routine prevention of all kidney stones. Its use is limited to patients whose stones are associated with chronic infection and who are not adequately managed with surgery, antimicrobial treatment, or both [1].
The labeling contains significant safety restrictions. Acetohydroxamic acid can cause hemolytic anemia, thrombosis, tremor, anxiety, depression, and other neurologic or psychiatric adverse reactions. It is contraindicated in pregnancy because of potential fetal harm and is unsuitable for patients with severe renal impairment [1].
What market does acetohydroxamic acid address?
Acetohydroxamic acid addresses a small subset of the urinary stone market. The relevant disease segment is infection-related urolithiasis, particularly struvite stones associated with organisms such as Proteus species that produce urease.
The overall kidney-stone market is large, but most patients do not qualify for acetohydroxamic acid. The product’s addressable population is narrowed by four factors:
- Infection stones are a minority of urinary calculi.
- Surgical removal and urinary drainage remain central to treatment.
- Antibiotic therapy is used to control the underlying infection.
- Toxicity limits chronic use and patient acceptance.
The drug is generally considered when complete stone removal is not possible, when infection persists, or when a patient has recurrent infection stones despite standard management. This positioning makes acetohydroxamic acid a specialist product used mainly by urologists and infectious-disease clinicians.
How large is the acetohydroxamic acid market?
Public sources do not disclose audited sales, prescription volume, market share, or patient counts for Lithostat. Mission Pharmacal is a privately held company and does not report product-level revenue in public securities filings.
The commercial market is therefore best characterized as:
- Low-volume relative to mainstream urology drugs.
- Specialist-driven rather than primary-care driven.
- Dependent on a limited number of diagnosed infection-stone patients.
- Vulnerable to supply interruptions because of narrow market scale.
- Capable of producing recurring revenue because treatment may continue for months when surgery is incomplete or contraindicated.
No reliable public source supports a precise annual U.S. revenue estimate. Any numerical estimate of Lithostat sales would be speculative.
How does acetohydroxamic acid generate revenue?
Revenue is generated primarily through prescription sales of 250 mg tablets in the United States. The financial model differs from that of a conventional branded specialty drug.
Pricing and reimbursement
The product’s commercial economics are likely influenced by:
- Low prescription volume.
- Specialty urology distribution.
- Limited therapeutic alternatives for selected patients.
- Potentially high per-patient annual cost relative to older generic medicines.
- Payer scrutiny because the drug has substantial toxicity and a narrow label.
- Reimbursement variation across commercial insurance, Medicare, Medicaid, and cash-pay channels.
Publicly available FDA and product-label sources do not provide net price, gross-to-net deductions, average selling price, or payer mix. Mission Pharmacal does not publish a public revenue breakdown for Lithostat.
The product can maintain commercial relevance without achieving blockbuster scale because its value is tied to a high-need niche. The principal financial risk is not rapid generic erosion. It is the small number of eligible patients and the possibility that clinicians choose surgical or antimicrobial approaches instead.
What patents protect acetohydroxamic acid?
The original composition-of-matter and product patents for acetohydroxamic acid are expired. The drug was approved decades ago, placing any original U.S. patent protection well beyond its term.
| Patent issue |
Current commercial implication |
| Original active-ingredient protection |
Expired |
| Original product protection |
Expired |
| New chemical entity exclusivity |
Expired |
| Pediatric exclusivity |
No current commercial relevance identified |
| Orphan exclusivity |
No current active exclusivity identified |
| Formulation exclusivity |
No publicly established active barrier identified |
| Method-of-use exclusivity |
No publicly established active barrier identified |
| Trade-name protection |
Lithostat trademark protection may remain separate from drug patent protection |
The absence of active patent protection means Mission Pharmacal’s position depends on manufacturing, distribution, brand recognition, regulatory maintenance, and the limited economic attractiveness of the market. Those factors can delay or discourage generic competition, but they do not create legal exclusivity.
What is the Orange Book status of acetohydroxamic acid?
The FDA Orange Book is the relevant source for approved drug products, therapeutic-equivalence evaluations, and listed patents or exclusivity. Acetohydroxamic acid is associated with an older NDA rather than a modern patent-protected launch product.
A patent-free Orange Book position would have several consequences:
- A competing applicant would not need to overcome an unexpired listed patent.
- A Paragraph IV certification would be unnecessary unless a later patent were listed.
- Generic entry could proceed through an abbreviated new drug application if the applicant satisfies FDA requirements.
- The primary commercial barrier would be market economics and product availability, not patent litigation.
The existence of an approved branded product does not prove that a therapeutically equivalent generic is commercially available. FDA approval, Orange Book listing, manufacturing capacity, and actual launch are separate events.
Are there Paragraph IV challenges to acetohydroxamic acid?
No major public Paragraph IV litigation involving acetohydroxamic acid or Lithostat is identified in the principal federal patent-litigation record commonly used for pharmaceutical tracking.
That result is consistent with the product’s age and the apparent absence of active listed patents. If a generic applicant were to file an ANDA today, the legal pathway would more likely involve certification that no relevant patent is listed or that listed patents have expired, rather than a contested Paragraph IV challenge.
The absence of reported litigation does not establish that no ANDA has ever been filed. FDA does not disclose every development-stage generic filing, and an applicant may choose not to launch after approval.
What formulations are protected by acetohydroxamic acid patents?
The commercial product is a conventional immediate-release 250 mg tablet. No active, commercially significant formulation patent estate is publicly established for Lithostat.
Potential technical barriers include:
- Control of tablet compression and dissolution.
- Stability of the active ingredient.
- Control of impurities and degradation products.
- Consistent tablet content uniformity.
- Manufacturing compliance for a low-volume product.
- Supplier continuity for active pharmaceutical ingredient.
These are manufacturing and regulatory execution issues rather than patent barriers. A generic sponsor would still need to demonstrate pharmaceutical equivalence and bioequivalence, or satisfy an FDA-approved alternative pathway where applicable.
How strong is the acetohydroxamic acid patent estate?
The patent estate is weak from a current exclusivity perspective. Its principal strengths are historical and operational rather than legal.
| Estate dimension |
Assessment |
| Composition-of-matter protection |
Expired |
| Remaining patent term |
None of material commercial significance identified |
| Formulation protection |
No active barrier publicly established |
| Use patents |
No active barrier publicly established |
| Litigation leverage |
Low |
| Manufacturing know-how |
Potentially relevant |
| Brand and prescriber familiarity |
Moderate niche value |
| Regulatory maintenance |
Important |
| Generic deterrence |
Driven mainly by market size and safety profile |
For an acquirer, the asset should be valued as a mature niche product with supply-chain and regulatory value, not as a patent-protected specialty pharmaceutical.
What competitive products challenge acetohydroxamic acid?
Acetohydroxamic acid competes with treatment strategies rather than with a single directly interchangeable drug.
Surgical management
Percutaneous nephrolithotomy, ureteroscopy, stent placement, and nephrostomy are central interventions for infection stones. Complete stone clearance reduces the bacterial reservoir and can remove the need for long-term urease inhibition.
Antibiotics
Antibiotics are used to treat the underlying urinary infection. They may be administered before and after procedures and during recurrent infection. Antibiotics do not always eliminate established stone material, so they are not a direct replacement in every case.
Observation and supportive care
Selected patients may receive surveillance, hydration guidance, infection monitoring, and individualized management when procedural risk is high or treatment goals are palliative.
Other stone-prevention drugs
Common urinary stone drugs, including potassium citrate and thiazide diuretics, address different stone chemistries. They are not direct substitutes for acetohydroxamic acid in urease-driven infection stones.
The product’s clinical differentiation is therefore concentrated in patients for whom infection control remains difficult and definitive stone removal is incomplete or impractical.
When does acetohydroxamic acid lose exclusivity?
Acetohydroxamic acid has already lost effective patent and regulatory exclusivity. Its commercial exclusivity is economic and operational rather than statutory.
The relevant timeline is:
| Period |
Event |
| Pre-1980s |
Development of acetohydroxamic acid as a urease inhibitor |
| 1980s |
FDA approval of Lithostat under NDA 018651 |
| Following decades |
Brand commercialization in a narrow infection-stone indication |
| Current period |
No meaningful original patent exclusivity remains; continued sales depend on niche demand and product supply |
There is no standard near-term patent cliff comparable to a modern branded drug. Generic entry could occur without waiting for a future expiration date, subject to FDA approval and commercial willingness.
What generic entry risks exist?
Generic entry risk is legally high but commercially moderate.
A potential generic sponsor would face:
- Small patient volume.
- Limited prescriber base.
- Complex safety monitoring.
- Potential product-liability exposure.
- Difficulty forecasting reimbursement.
- Low manufacturing scale.
- Limited return on bioequivalence and regulatory investment.
- Competition from surgery and antibiotics.
These factors can preserve a branded product’s share after patent expiry. They do not prevent a generic from entering. A successful generic launch could reduce unit price, increase payer substitution, and compress Mission Pharmacal’s revenue.
The most plausible launch scenarios are:
| Scenario |
Market effect |
| No generic launch |
Brand retains niche pricing and specialist distribution |
| One generic launch |
Moderate price pressure and payer substitution |
| Multiple generics |
Significant price erosion and possible supply fragmentation |
| Supply disruption at brand |
Temporary demand transfer to any available generic |
| Clinical substitution |
Long-term volume decline as surgery and infection management improve |
What is the FDA regulatory risk?
Regulatory risk is centered on safety, labeling, manufacturing, and availability.
The label’s warnings may restrict use in patients with anemia, renal dysfunction, pregnancy risk, or neurologic vulnerability [1]. A safety signal could reduce prescribing or trigger label changes. Manufacturing issues could have an outsized commercial effect because the market is small and alternative suppliers may be limited.
FDA enforcement or manufacturing remediation could affect:
- Product availability.
- Wholesale distribution.
- Hospital and specialty-pharmacy stocking.
- Prescriber confidence.
- Payer coverage.
- Generic development priorities.
The product is not a biologic, so biosimilar competition does not apply. Any competitive FDA pathway would involve an ANDA or a separate NDA, not a biosimilar application.
Which companies are challenging acetohydroxamic acid?
No major company is publicly identified as pursuing a prominent commercial challenge to Lithostat through patent litigation. The relevant competitive set includes:
- Generic-drug manufacturers that could pursue an ANDA.
- Urology companies offering stone-removal technologies.
- Antibiotic manufacturers.
- Specialty distributors and pharmacies that can influence access.
- Developers of diagnostics or interventions that improve infection-stone clearance.
The absence of a visible challenger is consistent with the product’s limited market size. It should not be interpreted as evidence of strong patent protection.
What is the financial trajectory for acetohydroxamic acid?
The likely trajectory is mature, stable-to-declining niche revenue rather than growth from broad market expansion.
Near-term outlook
Revenue can remain resilient if:
- The brand remains continuously available.
- Urologists continue treating patients who cannot undergo definitive stone clearance.
- Reimbursement supports chronic therapy.
- No generic achieves broad payer substitution.
- The manufacturer maintains regulatory compliance.
Medium-term outlook
Revenue faces gradual pressure from:
- Generic entry.
- Improved surgical management.
- Better infection control.
- Stricter safety-based prescribing.
- Population-level movement toward shorter treatment duration.
- Increased payer review of older specialty products.
Long-term outlook
The product’s long-term value is likely to remain concentrated in a small refractory-patient segment. The asset could remain profitable as a low-overhead niche product, but it is unlikely to become a major growth driver without a new indication, improved formulation, stronger evidence base, or a materially broader clinical positioning.
No public company filing provides enough information to calculate Lithostat revenue, EBITDA, net price, or valuation. Financial diligence should therefore focus on prescription volume, realized price, gross margin, inventory turns, payer concentration, regulatory history, and customer retention rather than headline market-size estimates.
How does acetohydroxamic acid compare with modern specialty drugs?
| Attribute |
Acetohydroxamic acid |
Modern specialty drug |
| Indication size |
Very narrow |
Often moderate to large |
| Patent position |
Expired |
Usually active at launch |
| Clinical use |
Refractory infection stones |
Targeted high-value disease |
| Pricing power |
Niche and payer-dependent |
Often supported by exclusivity |
| Sales model |
Specialist prescription |
Specialty or broad commercial |
| Generic risk |
Possible at any time |
Usually delayed by patents |
| Regulatory differentiation |
Limited |
Often supported by clinical development |
| Financial transparency |
Low |
Higher for public-company products |
| Main value driver |
Supply continuity and niche demand |
Exclusivity, clinical growth, and access |
Key Takeaways
- Acetohydroxamic acid is a mature, narrow-use urease inhibitor marketed as Lithostat.
- Its FDA-approved role is adjunctive treatment for infection-related struvite and carbonate apatite urinary stones.
- The product has no meaningful original patent exclusivity remaining.
- No major Paragraph IV litigation or public patent challenge is identified.
- Biosimilar risk does not apply because acetohydroxamic acid is a small-molecule drug.
- Generic-entry risk is legally significant but commercially moderated by low volume, toxicity, and specialist use.
- Mission Pharmacal does not publicly disclose Lithostat revenue or product-level financial results.
- The financial trajectory is likely stable-to-declining niche revenue, with supply continuity and prescriber retention as the main commercial drivers.
- Surgery, antibiotics, and improved infection-stone management are the principal competitive pressures.
- The asset’s value is operational and recurring, not patent-driven.
FAQs About Acetohydroxamic Acid Market and Exclusivity
Is acetohydroxamic acid still commercially available?
Lithostat has been marketed by Mission Pharmacal as a 250 mg prescription tablet. Availability can vary by distributor, pharmacy, and inventory status.
Is Lithostat a generic drug?
Lithostat is the branded product. The active ingredient itself is an old small molecule with no meaningful original patent exclusivity, but commercial generic availability is separate from patent expiration.
Can acetohydroxamic acid be used for calcium oxalate kidney stones?
The FDA-labeled use is for infection-related urinary stones associated with urease-producing bacteria. It is not a general treatment for calcium oxalate stones.
Does acetohydroxamic acid have orphan-drug protection?
No current orphan exclusivity protection of material commercial significance is identified for the marketed product.
What would most increase the value of acetohydroxamic acid?
The strongest value drivers would be reliable supply, evidence supporting use in difficult-to-treat infection stones, favorable reimbursement, limited generic competition, and expanded clinical adoption among urologists.
References
-
U.S. National Library of Medicine. (n.d.). Lithostat- acetohydroxamic acid tablet: Prescribing information. DailyMed. https://dailymed.nlm.nih.gov/
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U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations. FDA Orange Book. https://www.accessdata.fda.gov/scripts/cder/ob/
-
U.S. Food and Drug Administration. (n.d.). Drugs@FDA: FDA-approved drugs. https://www.accessdata.fda.gov/scripts/cder/daf/
-
American Urological Association. (n.d.). Medical management of kidney stones: Guideline. https://www.auanet.org/guidelines/medical-management-of-kidney-stones
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Mission Pharmacal Company. (n.d.). Lithostat acetohydroxamic acid. Mission Pharmacal. https://www.missionpharmacal.com/
