PURINETHOL Drug Patent Profile

When do Purinethol patents expire, and when can generic versions of Purinethol launch?

Purinethol is a drug marketed by Stason Pharms and is included in one NDA.

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

DrugPatentWatch^® Litigation and Generic Entry Outlook for Purinethol

A generic version of PURINETHOL was approved as mercaptopurine by DR REDDYS LABS SA on February 11^th, 2004.

Summary for PURINETHOL

US Patents:	0
Applicants:	1
NDAs:	1
Finished Product Suppliers / Packagers:	1
Raw Ingredient (Bulk) Api Vendors:	171
Clinical Trials:	57
Patent Applications:	1,834
What excipients (inactive ingredients) are in PURINETHOL?	PURINETHOL excipients list
DailyMed Link:	PURINETHOL at DailyMed

Recent Clinical Trials for PURINETHOL

Identify potential brand extensions & 505(b)(2) entrants

Sponsor	Phase
City of Hope Medical Center	Phase 1
Pfizer	Phase 2
EsPhALL network I-BFM Study Group	Phase 3

See all PURINETHOL clinical trials

Pharmacology for PURINETHOL

Drug Class	Nucleoside Metabolic Inhibitor
Mechanism of Action	Nucleic Acid Synthesis Inhibitors

US Patents and Regulatory Information for PURINETHOL

Applicant	Tradename	Generic Name	Dosage	NDA	Approval Date	TE	Type	RLD	RS	Patent No.	Patent Expiration	Product	Substance	Delist Req.	Exclusivity Expiration
Stason Pharms	PURINETHOL	mercaptopurine	TABLET;ORAL	009053-002	Approved Prior to Jan 1, 1982	AB	RX	Yes	No

EU/EMA Drug Approvals for PURINETHOL

Company	Drugname	Inn	Product Number / Indication	Status	Generic	Biosimilar	Orphan	Marketing Authorisation	Marketing Refusal
Nova Laboratories Ireland Limited	Xaluprine (previously Mercaptopurine Nova Laboratories)	mercaptopurine	EMEA/H/C/002022Xaluprine is indicated for the treatment of acute lymphoblastic leukaemia (ALL) in adults, adolescents and children.	Authorised	no	no	no	2012-03-09

PURINETHOL Market Dynamics, Financial Trajectory, Patent Status, and Generic Competition

Last updated: September 5, 2026

Purinethol is the branded form of mercaptopurine, an oral thiopurine used primarily in acute lymphoblastic leukemia and, in some markets, inflammatory bowel disease. Its commercial profile is mature: core composition-of-matter patents expired decades ago, FDA-approved generic mercaptopurine is available, and the branded product has limited pricing power. The principal value drivers are supply continuity, hospital and specialty-pharmacy distribution, formulation reliability, and regulatory compliance rather than patent exclusivity.

The product’s standalone revenue is not publicly reported by its current commercial owner. Financial performance must therefore be assessed through market structure, generic substitution, public-company disclosures, product availability, and therapeutic demand.

What is Purinethol and how is mercaptopurine used?

Purinethol contains mercaptopurine, also known as 6-mercaptopurine or 6-MP. It is a purine antimetabolite that interferes with nucleic-acid synthesis and lymphoid-cell proliferation.

Attribute	Purinethol
Active ingredient	Mercaptopurine
Common strength	50 mg tablet
Primary oncology use	Maintenance therapy for acute lymphoblastic leukemia
Other clinical use	Crohn’s disease and ulcerative colitis in selected markets
Administration	Oral tablet
Regulatory category	Prescription drug
Drug class	Thiopurine antimetabolite
Key safety issue	Myelosuppression and hepatotoxicity
Major pharmacogenomic issue	TPMT and NUDT15 variation can increase toxicity risk

The FDA labeling identifies Purinethol for maintenance therapy of acute lymphoblastic leukemia. The label also requires attention to severe bone-marrow suppression, hepatotoxicity, and interactions involving xanthine oxidase inhibitors such as allopurinol.[1]

Mercaptopurine is usually used under specialist supervision. Genetic testing for TPMT and NUDT15 can inform dosing, but testing does not eliminate the need for blood-count and liver-function monitoring.

What is the FDA regulatory status of Purinethol?

Purinethol was approved in the United States under NDA 009053. FDA approval predates the modern Hatch-Waxman patent framework by several decades. The product is an established small-molecule drug, not a biologic, and is not subject to biosimilar competition.

The FDA-approved product is a 50 mg oral tablet. Generic mercaptopurine tablets are approved through abbreviated new drug applications, allowing ANDA applicants to rely on the reference product’s safety and efficacy findings while demonstrating pharmaceutical equivalence and bioequivalence.[2]

The regulatory economics are unfavorable for a branded product:

• The active ingredient is well characterized.
• The dosage form is a conventional immediate-release tablet.
• No complex device or delivery system creates a major regulatory barrier.
• Generic applicants can use the FDA’s established reference-drug pathway.
• Clinical switching is generally easier than for narrow-therapeutic-index biologics, although prescribers still monitor blood counts closely.

What is the Orange Book status of Purinethol?

Purinethol’s core commercial protection is not based on an active patent estate. Mercaptopurine was discovered and commercialized many decades ago, and the original chemical patents expired long before the current generic market developed.

The Orange Book identifies patents and regulatory exclusivities submitted for approved products. For a mature product such as Purinethol, the important commercial fact is the absence of meaningful remaining composition-of-matter exclusivity. Any listed protection, if present for a particular presentation or regulatory period, would not restore broad market exclusivity for mercaptopurine tablets.

Does Purinethol have active Orange Book patents?

No commercially significant, unexpired U.S. patent is generally associated with the basic mercaptopurine molecule or the conventional 50 mg tablet. The principal market barrier is therefore commercial supply and contracting, not patent enforcement.

The relevant patent history includes early thiopurine and mercaptopurine patents, but those rights expired decades ago. A generic applicant would not face the type of patent estate associated with newer oncology medicines.

What patents protect Purinethol?

The original mercaptopurine patent position is historical rather than commercially active.

Protection category	Current commercial relevance
Mercaptopurine composition of matter	Expired
Conventional 50 mg tablet formulation	No meaningful modern exclusivity identified
Method of treating leukemia	Any early patent protection expired
Method-of-use claims for inflammatory bowel disease	No broad, durable branded exclusivity comparable to a new molecular entity
Manufacturing know-how	Potentially relevant as trade secret or process know-how
Packaging and labeling	Limited value and generally nonexclusive
Pediatric or orphan exclusivity	No current exclusivity blocking generic entry

Are formulation patents a barrier to mercaptopurine competition?

Formulation patents are not a significant barrier for conventional mercaptopurine tablets. The product does not depend on a patented extended-release system, injector, inhaler, liposomal carrier, or other complex delivery platform.

Manufacturers may still protect manufacturing processes, impurity controls, tablet composition, stability methods, or supply-chain know-how as confidential information. Those protections can affect cost and reliability, but they do not prevent another company from marketing a bioequivalent tablet.

When did Purinethol lose exclusivity?

Purinethol lost meaningful market exclusivity decades ago. The relevant sequence is:

Period	Market event
1950s	Mercaptopurine developed and introduced as an antimetabolite
1957	Early U.S. patent filings covered mercaptopurine-related inventions
1960s	U.S. regulatory approval and commercial use expanded
1970s and later	Original patent protection expired
1980s onward	Generic and non-branded mercaptopurine supply developed
Current market	Mature generic market with limited branded differentiation

There is no modern loss-of-exclusivity event comparable to a major oncology product approaching a first generic launch. The competitive transition occurred long ago.

How many patents cover Purinethol?

The practical answer is zero meaningful blocking patents for the basic U.S. product. Historical patents existed, but expired rights do not constrain present generic entry.

A patent search may identify:

• Expired mercaptopurine composition patents.
• Expired treatment patents.
• Patents covering unrelated thiopurine analogs.
• Process patents with limited relevance to the marketed tablet.
• International patents that have no current force in the United States.

Patent counts should therefore be separated into historical documents and enforceable rights. A high document count would not indicate a strong current Purinethol estate.

Which companies compete with Purinethol?

Competition comes primarily from generic mercaptopurine manufacturers and from alternative thiopurines used in selected non-oncology indications.

Direct product competitors

Direct competitors include FDA-approved generic mercaptopurine tablets marketed by multiple manufacturers or labelers over time. Availability can vary by wholesaler, national contract, and shortage status.

Typical competitive factors are:

• Wholesale acquisition cost.
• Reimbursement and formulary placement.
• National supply reliability.
• Tablet availability in the 50 mg strength.
• Packaging and bottle size.
• Pharmacy substitution rules.
• Ability to maintain production during API or manufacturing disruptions.

Therapeutic alternatives

In inflammatory bowel disease, azathioprine is the closest pharmacologic alternative. In acute lymphoblastic leukemia, mercaptopurine is part of established multi-agent treatment protocols, and substitution decisions depend on the treatment regimen rather than simple retail price comparison.

Product	Active ingredient	Primary competitive relationship
Purinethol	Mercaptopurine	Branded reference product
Generic mercaptopurine	Mercaptopurine	Direct pharmaceutical equivalent
Imuran and generic azathioprine	Azathioprine	Alternative thiopurine in selected indications
Modern targeted leukemia therapies	Various	Disease-specific alternatives, not direct tablet substitutes

Modern leukemia medicines can carry much higher prices, but they do not automatically displace mercaptopurine because treatment protocols, disease subtype, age, risk group, and maintenance strategy determine use.

What is the financial trajectory of Purinethol?

Purinethol’s financial trajectory is best described as mature, low-growth, and supply-sensitive.

The brand does not have the characteristics of a high-growth pharmaceutical asset:

1. Its active ingredient is off patent.
2. Generic substitution limits branded pricing.
3. The tablet dosage form is easy to reproduce.
4. Demand is concentrated in established treatment protocols.
5. No major indication expansion is publicly associated with the product.
6. Standalone product revenue is not disclosed in the public financial reporting of the commercial owner.

Revenue is likely driven more by unit retention and market availability than by price expansion. A branded product can retain a niche where physicians or pharmacies prefer a specific source, but that position is vulnerable to generic contracting and product discontinuation.

Revenue exposure by market factor

Factor	Effect on Purinethol economics
Generic substitution	Reduces price and branded unit share
Oncology protocol demand	Supports stable baseline volume
Pediatric leukemia treatment	Creates recurring clinical demand
Supply interruptions	Can shift share rapidly between labelers
Reimbursement controls	Compresses net price
API cost changes	Can materially affect low-price generic economics
Hospital purchasing	Favors reliable suppliers and contract pricing
Brand recognition	Provides limited protection after patent expiry

For a company holding the product, Purinethol is more likely to be a portfolio or continuity asset than a major growth product. Its value may include customer relationships, manufacturing utilization, and access to oncology channels even when reported sales are modest.

What generic entry risks exist for Purinethol?

Generic entry risk is already realized rather than prospective. The product faces:

• Direct substitution by generic mercaptopurine.
• Low switching costs for pharmacies and payers.
• Price compression from multiple ANDA holders.
• Loss of shelf space if the branded product has a higher acquisition cost.
• Contract displacement in hospitals and integrated delivery networks.
• Exposure to shortages if one or more manufacturers exit.

The principal remaining commercial risk is not a future Paragraph IV launch. It is generic-market concentration and supply disruption.

Are there Paragraph IV challenges to Purinethol?

A Paragraph IV challenge is unlikely to be strategically important for Purinethol because the original patents expired long ago. Paragraph IV litigation is typically used when an ANDA applicant seeks approval before expiration of a listed patent. A mature product without meaningful unexpired Orange Book patents does not create the same incentive for a major patent challenge.

Any current ANDA filing would more likely proceed through a certification pathway that does not require challenging an active patent estate. FDA approval timing would depend on application review, manufacturing readiness, bioequivalence, and regulatory exclusivity, not on a contested patent term.

What patent litigation affects Purinethol?

No major, current U.S. patent litigation is a central commercial feature of Purinethol. The product’s competitive disputes are more likely to involve:

• Supply contracts.
• Product quality.
• Manufacturing compliance.
• Shortage allocation.
• Labeling or pharmacovigilance.
• Distribution rights.
• Reimbursement and substitution.

This sharply contrasts with newer oncology products, where patent litigation can delay generic or biosimilar entry for years.

Does Purinethol face biosimilar competition?

No. Purinethol is a small-molecule chemical drug, not a biologic. Its competitors are generic drugs approved through the ANDA pathway, not biosimilars approved under the Biologics Price Competition and Innovation Act.

The distinction matters commercially:

Issue	Purinethol
Product type	Small molecule
Main competitor pathway	ANDA generic
Biosimilar litigation	Not applicable
Interchangeability framework	State pharmacy-substitution and product-specific rules
Clinical switching barrier	Generally lower than for biologics

How strong is the Purinethol patent estate?

The patent estate is weak from a present-day exclusivity perspective. Its strength can be rated as follows:

Estate component	Assessment
Composition-of-matter protection	None currently relevant
Formulation protection	Weak or nonblocking
Method-of-use protection	Historical and nonblocking
Manufacturing barriers	Moderate operational relevance, low exclusionary power
Regulatory exclusivity	None of commercial significance
Litigation leverage	Low
Generic-entry resistance	Low

The remaining defensibility is operational. A supplier with validated manufacturing, stable API access, dependable quality systems, and established distribution may outperform competitors despite having no meaningful patent moat.

How does Purinethol compare with azathioprine?

Purinethol and azathioprine are both thiopurines, but their commercial positions differ by indication and formulation.

Factor	Purinethol	Azathioprine
Active ingredient	Mercaptopurine	Azathioprine
Main oncology role	Established leukemia maintenance therapy	Limited direct substitution
Inflammatory bowel disease role	Used in selected patients	Common alternative
Patent status	Long expired	Long expired
Competition	Generic mercaptopurine	Generic azathioprine
Commercial differentiation	Low	Low
Monitoring	Blood counts, liver function, toxicity risk	Blood counts, liver function, toxicity risk

Clinical use is not determined only by price. Physicians consider prior response, tolerability, metabolite profiles, dosing, treatment protocol, and patient-specific pharmacogenomics.

What generic launch scenarios are plausible?

Because generic competition already exists, future launch scenarios concern market entry or exit by individual manufacturers.

Base case

Multiple suppliers remain active. Prices stay compressed, and market share moves according to supply reliability, payer contracts, and wholesaler inventory.

Consolidation case

One or more manufacturers withdraw because of low margins, API costs, or compliance expenses. Remaining suppliers gain volume but face shortage risk and greater regulatory scrutiny.

Shortage case

A manufacturing interruption or API constraint reduces available supply. The branded product may regain temporary share if it remains commercially available, but the benefit would depend on inventory and contracting.

Repricing case

Payers or wholesalers favor the lowest-cost source, reducing brand demand further. A manufacturer may preserve the product only if it has strategic portfolio value or an efficient production platform.

What geographic markets matter for Purinethol?

The United States is a mature generic market. European and other international markets may retain branded mercaptopurine products under different commercial names, reimbursement systems, and national authorization structures.

Geographic performance depends on:

• National tendering.
• Hospital procurement.
• Local reference pricing.
• Authorized generic rules.
• Import requirements.
• National shortages.
• Pediatric oncology treatment protocols.
• Local registration of the brand and generic presentations.

Patent expiry is not the main geographic variable. Registration, supply authorization, reimbursement, and procurement determine market access.

What manufacturing and intellectual-property barriers remain?

Manufacturing barriers are practical rather than patent-based. Mercaptopurine production requires controlled handling of a cytotoxic or hazardous active pharmaceutical ingredient, validated impurity controls, stability data, and compliance with current good manufacturing practices.

Potential barriers include:

• Limited qualified API suppliers.
• Containment requirements.
• Batch-release testing.
• Stability and degradation control.
• Pharmacopoeial compliance.
• Serialization and packaging obligations.
• Regulatory inspection findings.
• Low commercial returns on a mature product.

These barriers can reduce the number of active suppliers even when the legal market is open to generic competition.

Key Takeaways

• Purinethol is the branded 50 mg oral tablet form of mercaptopurine.
• Its principal use is maintenance treatment for acute lymphoblastic leukemia.
• The active ingredient and conventional tablet are long off patent.
• No meaningful current U.S. patent estate is expected to block generic mercaptopurine.
• Paragraph IV litigation is not a central risk because the relevant patents expired decades ago.
• Biosimilar competition does not apply because mercaptopurine is a small molecule.
• Generic substitution, procurement, and supply reliability determine commercial performance.
• Standalone Purinethol revenue is not publicly disclosed by the commercial owner.
• The product is a mature portfolio asset with limited pricing power and low growth potential.
• Manufacturing quality, API access, and shortage resilience provide more practical protection than intellectual property.

FAQs

Is Purinethol still commercially available?

Availability varies by country, supplier, and distribution channel. Generic mercaptopurine is the principal source of supply in markets with established ANDA or equivalent generic competition.

Is mercaptopurine a generic drug?

Yes. Mercaptopurine is available as a generic small-molecule drug. Purinethol is the branded reference product.

Can a pharmacy automatically substitute generic mercaptopurine for Purinethol?

Substitution depends on U.S. state law, the product’s regulatory equivalence, payer rules, and prescriber instructions. Pharmacies generally can substitute an approved therapeutically equivalent product where permitted.

Why can an off-patent leukemia drug still experience shortages?

Low margins, limited API suppliers, manufacturing interruptions, quality investigations, and concentrated procurement can make a mature generic drug vulnerable to shortages even without patent restrictions.

Is Purinethol a high-value pharmaceutical asset?

Purinethol is unlikely to be a high-growth asset on standalone sales. Its value is more consistent with a mature specialty product that can provide recurring demand, oncology-channel access, and portfolio continuity when manufacturing and distribution are reliable.

References

1. U.S. Food and Drug Administration. (2024). Purinethol (mercaptopurine) tablets, 50 mg: Prescribing information. FDA.
2. U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations. FDA.
3. U.S. Food and Drug Administration. (2024). Drugs@FDA: Purinethol, NDA 009053. FDA.
4. U.S. Food and Drug Administration. (2024). Orange Book: Approved drug products with therapeutic equivalence evaluations. FDA.
5. National Cancer Institute. (2024). Mercaptopurine. National Cancer Institute.
6. U.S. Food and Drug Administration. (2020). Table of pharmacogenetic associations. FDA.
7. DailyMed. (2024). Mercaptopurine tablet labeling. National Library of Medicine.