PURINETHOL Drug Patent Profile

Which patents cover Purinethol, 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. 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

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

Last updated: July 7, 2026

Purinethol (6-Mercaptopurine) Investment Scenario and Patent/Exclusivity Fundamentals Analysis

Purinethol is an established oncology and autoimmune drug whose US profitability is dominated by long-run generic economics, narrow defensibility after early origin patent lapses, and limited live patent leverage from formulation or process improvements. Investment upside depends less on near-term monopoly pricing and more on: (1) sourcing and manufacturing continuity for a high-liquidity generic API market, (2) any enforceable secondary IP (formulations, methods of use, process) that delays specific generic entry, and (3) whether US branded share has meaningful residuals versus price erosion.

Because Purinethol is not positioned like a contemporary biologic or novel small molecule, the core “fundamentals” lens is exclusivity reality, Orange Book/IP presence, and generic launch/paragraph IV history (which determine expected ROI windows and settlement risk).

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What patents protect Purinethol (6-mercaptopurine) in the US?

Which active ingredient and key therapeutic uses drive the IP map

Purinethol is the brand name for 6-mercaptopurine (6‑MP), a purine analog used in oncology (notably acute lymphoblastic leukemia regimens) and in inflammatory/autoimmune conditions (historically, for selected disease settings). That therapeutic breadth increases the risk that method-of-use claims exist but also increases invalidity exposure because many uses are well-established.

How to think about the likely patent estate structure

For a long-marketed drug, US patentability typically clusters into:

• Formulation patents (tablet stability, dissolution, bioavailability, excipients)
• Manufacturing/process patents (synthetic routes, purification steps, impurity control)
• Method-of-use patents (specific dosing schedules, therapeutic combinations, TPMT- or metabolite-informed regimens)
• Polymorph/crystal form patents (if a solid-state change exists)

For investment screening, the critical question is whether any of these categories are still enforceable in the US during the relevant forecast window, and whether they are listed in the Orange Book.

Orange Book linkage is the gating item

The investment implication is straightforward: if the active drug is generic-dominant and Orange Book listings are sparse or expiring, expected net present value from “patent harvesting” is low, and the play shifts to manufacturing resilience, supply contracts, and litigation avoidance.

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When does Purinethol lose exclusivity in the US?

What “exclusivity” means for a legacy small molecule

For an older small molecule like 6‑MP, the most common exclusivity events are:

• Primary composition-of-matter patent expiration (often long completed)
• Orphan/other exclusivity only if applicable (less likely for a widely used oncology drug)
• 3-year/5-year exclusivity tied to new clinical investigations (usually requires an FDA pathway event and new drug submission)

Investment timing logic

• If no unexpired Orange Book-listed patents remain for the branded reference listed drug (RLD), generic entry is structurally higher-probability and price erosion accelerates.
• If a small set of method-of-use or formulation patents remains, generics may still launch “at risk” unless carved out by label restrictions, which can preserve limited branded share.

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What is the Orange Book status of Purinethol (6-MP) for tablets?

What investors should verify on the Orange Book page

For an actionable investment view, the Orange Book record must be reviewed for:

• RLD identity and strength(s) (e.g., tablets vs other dosage forms if applicable)
• US patents listed with expiration dates
• Whether each patent is tied to drug substance vs drug product vs method of use
• Whether any patents have “Hatch-Waxman litigation” history or “carved out” labels

How this drives expected generics pricing

If Purinethol’s Orange Book is “thin” (few late-expiring patents), the market is dominated by generic competition. In that case, investment fundamentals are driven by:

• procurement costs
• manufacturing yields
• impurity specifications compliance
• supply interruptions affecting pharmacy reimbursement dynamics

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How many patents cover Purinethol (6-MP) and what are their expiration dates?

Expected pattern for a legacy drug

In legacy oncology small molecules, the patent family tends to:

• expire early for core composition
• survive longer only in narrow process/formulation improvements

Investment interpretation

• A high count of patents can still be low value if claims are method-of-use-only and easy to design around with non-infringing dosing schedules.
• A small count can still be high value if the remaining patents are drug product or drug substance with broad coverage and strong enablement.

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What generic entry risks exist for Purinethol? (Paragraph IV, at-risk launches)

Paragraph IV risk profile

For a well-established drug, Paragraph IV events often reflect:

• a generic filing challenging an Orange Book-listed patent
• subsequent settlement that delays full market entry

What matters for ROI

• Whether any Paragraph IV challenges are still active or resolved
• Whether settlements include no-authorized-generic or carve-outs
• Whether the branded reference holder receives exclusivity-like payments tied to a delay

In legacy drugs with limited remaining listed patents, entry risk is lower in litigation intensity but higher in plain vanilla generic commoditization.

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What patent litigation affects Purinethol (6-MP) in the US?

Typical litigation footprint

For established small molecules, litigation may include:

• early generation composition disputes
• later, secondary patent disputes involving formulations or methods
• settlement-driven delay agreements

Why litigation history is central to investment underwriting

Litigation and settlement history inform two inputs to underwriting:

• likelihood of future challenges by other ANDA filers
• expected delay duration in years, not months
• injunctive leverage if a late-expiring patent exists

If there is no recent litigation, it usually signals a low probability of near-term “IP-driven” price protection.

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How does Purinethol compare with other 6-MP / thiopurine brands and generics?

Competitive set logic

Purinethol’s competitive set is:

• generic 6‑mercaptopurine tablets from multiple ANDA holders
• other thiopurines used in similar therapeutic contexts (azathioprine, thioguanine) that can partially substitute in practice depending on clinician preference and patient tolerance

Investment impact

• If substitution risk exists from adjacent thiopurines, branded pricing power compresses further.
• If payer formularies treat generics as equivalent, gross margin headroom depends mainly on distribution and contract pricing.

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What formulations are protected by Purinethol patents (tablets, excipients, stability)?

Formulation patent categories that can still matter

For long-marketed tablets, the remaining defenses often involve:

• dissolution profile controls
• excipient-related stability
• manufacturing steps that reduce impurities

Investor screening rule

Even if formulation patents exist, verify:

• whether generic labels can omit the patented formulation aspects
• whether the generic product can match the same dissolution specification under a different process
• whether enforceable claims remain unexpired

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What method-of-use patents protect Purinethol dosing regimens?

Dosing and biomarker informed regimens

Method-of-use IP in thiopurine therapy often clusters around:

• dosing schedules adjusted for metabolite levels (e.g., 6‑MP metabolites)
• toxicity risk mitigation
• combination protocols in oncology regimens

Design-around reality

Method-of-use claims are frequently easier to design around via:

• different dosing algorithms
• different combinations
• label carve-outs

For investment underwriting, this usually means method-of-use patents support modest incremental price protection at best, unless they are coupled with strong label restrictions or require specific administration protocols that are difficult to avoid in practice.

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What regulatory status does Purinethol have with FDA (ANDAs, reference listed drug)?

Regulatory posture of legacy small molecules

Key points:

• Purinethol likely functions as the branded reference for generic reference products
• Generic availability reduces branded exclusivity leverage
• Substitution and interchangeability drive volume and margin allocation

Investment implication

The regulatory lens is less about new approvals and more about:

• ANDA filing cadence
• manufacturing site compliance
• FDA actions affecting supply continuity (warning letters, import alerts, or quality holds)

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What commercial performance fundamentals should investors use for Purinethol?

Core metrics for a legacy generic-adjacent branded product

• Share trend in outpatient and oncology settings
• Average net price vs wholesale acquisition cost erosion
• Gross margin stability tied to supply and API input costs
• Distribution channel mix (institutional vs retail)
• Contract pricing pressures with PBMs

Where margins typically compress

For commoditized small molecules:

• competition from multiple ANDAs reduces net price
• “brand premium” persists only if the branded product is preferred for specific stability, sourcing reliability, or institutional formulary placement

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Which companies supply Purinethol’s 6-MP tablets and how does that affect pricing?

Pricing dynamics in a multi-generic environment

When multiple ANDA holders supply the same strength and dosage form:

• lead suppliers win volume through reliable supply and tender pricing
• smaller holders often cap capacity or exit during cost spikes
• shortages can temporarily lift pricing, but the structural baseline is low

Investment implication

A value play typically hinges on:

• identifying the dominant low-cost, high-compliance manufacturers
• monitoring capacity constraints and impurity-driven QA rejects
• evaluating whether supply risk creates short-lived opportunities that do not get competed away quickly

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How does Purinethol supply chain risk influence investor returns?

High-impact operational risks

For APIs and sterile-adjacent oncologic regimens, even for tablets:

• synthesis batch variability
• impurity excursions
• raw material availability
• regulatory compliance at manufacturing sites

Financial translation

Supply disruptions can:

• cause temporary inventory shortages
• raise spot pricing for a branded or preferred generic
• trigger payer policy shifts that later reverse once supply normalizes

Investment underwriting should assume commoditized pricing with operational volatility.

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Key take: What is the realistic investment scenario for Purinethol?

Base case

• Patents are mostly expired or narrow.
• Brand value is modest and largely a function of supply reliability and market access, not exclusivity.
• Investment returns are dominated by operational execution and competitive pricing, not IP-driven monopoly time.

Bull case

• Residual unexpired Orange Book-listed patents exist with broad drug product coverage, limiting generic substitution on certain strengths or label indications.
• Supply consolidation or compliance issues reduce effective competition.
• A litigation/settlement outcome delays one or more ANDA launches long enough to preserve branded share and price.

Bear case

• Generic competition is fully effective across strengths.
• Any remaining patents are vulnerable to design-around or invalidity.
• Supply improvements elsewhere expand capacity and push prices down quickly.

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Key Takeaways

• Purinethol (6‑mercaptopurine) is structurally a mature product where investment upside typically depends on supply and manufacturing resilience more than near-term exclusivity.
• The decisive diligence item is Orange Book status: unexpired, enforceable listed patents drive any realistic IP-backed return window.
• Even if secondary IP exists (formulation or method-of-use), its practical value is usually limited by design-around and label carve-outs.
• In a multi-generic market, margin is a function of competitive net pricing and input/supply compliance, not branded monopoly dynamics.

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FAQs

1) Does Purinethol still have branded exclusivity in the US, or is it effectively generic-exposed?
Legacy small molecules like 6‑MP are typically generic-exposed; any exclusivity is usually tied to remaining listed Orange Book patents, not to broad brand exclusivity.

2) Are method-of-use patents for Purinethol likely to delay generics?
Method-of-use claims can delay only if they meaningfully restrict clinical practice via enforceable label instructions. Otherwise, they tend to be easier to design around.

3) What matters more for profitability: patent protection or manufacturing compliance?
For mature generics-adjacent drugs, manufacturing compliance and supply continuity usually dominate realized profitability because price competition is structurally high.

4) How do FDA and quality actions affect Purinethol supply economics?
Quality holds, warning letters, or impurity-driven batch failures can create short-run shortages and lift pricing until supply normalizes.

5) What is the most important diligence step before funding any Purinethol-related strategy?
Confirm current Orange Book listings and the remaining expiration timeline for the exact RLD, strength, and dosage form to quantify delay scenarios for generics.

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References (APA)

1. FDA. Orange Book: Approved Drug Products with Therapeutic Equivalence Evaluations. (Accessed 2026).
2. FDA. Hatch-Waxman Act overview and ANDA framework. (Accessed 2026).