US Drug Patent 3,923,785: Pentostatin Claim Scope, Expiration, and Patent Landscape
US Patent 3,923,785 covers pentostatin as an essentially pure compound and claims a specific pentostatin-vidarabine mixture. The patent issued on December 2, 1975, under the pre-1995 patent term of 17 years from issuance. It expired on December 2, 1992, absent a terminal disclaimer or other exceptional adjustment. The patent has no current blocking effect on pentostatin manufacture, formulation, approval, or sale in the United States.[1]
The claimed compound is pentostatin, also known as 2'-deoxycoformycin, a potent adenosine deaminase inhibitor. The second and third claims concern a combination of pentostatin and vidarabine, also known as adenine arabinoside or ara-A.
What compound does US Patent 3,923,785 protect?
Claim 1 covers the compound now known as pentostatin in an essentially pure form.
| Patent claim terminology |
Modern identification |
| (R)-3-(2-deoxy-beta-D-erythro-pentofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepin-8-ol |
Pentostatin |
| Common alternative name |
2'-Deoxycoformycin |
| Drug class |
Adenosine deaminase inhibitor |
| Primary historical use |
Hairy cell leukemia |
| Product name |
Nipent |
| Molecular formula |
C11H16N4O4 |
| Molecular weight |
268.27 g/mol |
Modern chemical references generally identify pentostatin as (8R)-3-(2-deoxy-beta-D-ribofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepin-8-ol. The older patent nomenclature uses the erythro descriptor and a historical ring-system naming convention.[1,2]
Claim 1 is a product claim. It does not depend on a particular manufacturing process, formulation, dosage, indication, or route of administration. If the claim had remained enforceable, making, using, selling, offering for sale, or importing the claimed pentostatin compound in the United States could have raised infringement issues under 35 U.S.C. § 271.
The words “essentially pure” are important. The claim does not expressly require absolute chemical purity. It covers pentostatin meeting an implied level of purity consistent with the patent specification and ordinary pharmaceutical practice. The limitation could have created factual disputes over impurities, stereoisomer content, degradation products, residual solvents, and analytical methods.
What do claims 2 and 3 protect?
Claim 2 covers a mixture consisting of pentostatin and 9-(beta-D-arabinofuranosyl)adenine, commonly called vidarabine or adenine arabinoside.
Claim 3 narrows claim 2 by specifying the ratio of pentostatin to vidarabine:
Pentostatin : vidarabine = approximately 0.005:1 to 0.2:1.
This means that, for every one part of vidarabine, the mixture contains approximately 0.005 to 0.2 parts pentostatin.
| Pentostatin:vidarabine ratio |
Pentostatin as a percentage of vidarabine |
| 0.005:1 |
0.5% |
| 0.01:1 |
1% |
| 0.05:1 |
5% |
| 0.1:1 |
10% |
| 0.2:1 |
20% |
Claim 3 is not a general claim to every pentostatin-vidarabine combination. It requires the stated ratio range. A composition with a ratio below 0.005:1 or above 0.2:1 would fall outside the literal scope of claim 3, although claim 2 could present a separate issue if the composition satisfies its broader language.
Does “consisting of” exclude other ingredients?
The term “consisting of” generally limits the claimed mixture to the listed components, subject to the established patent-law treatment of incidental ingredients and excipients. Claim 2 therefore differs from a conventional pharmaceutical composition claim using “comprising.”
A commercial vial containing pentostatin, vidarabine, buffers, stabilizers, solvents, or other excipients would require a detailed claim-construction analysis. Excipients used to prepare or administer a drug do not automatically avoid a composition claim, but their effect depends on the claim language, the patent specification, and whether the excipients are treated as excluded components or incidental materials.
How strong is the patent scope of claim 1?
Claim 1 had broad historical scope because it claimed pentostatin as a compound rather than limiting the invention to a therapeutic use.
Claim 1 strengths
- It is a direct product claim.
- It identifies a single stereochemical form.
- It is not restricted to one dosage form.
- It is not limited to a particular indication.
- It is not limited to a specified manufacturing route.
- It could have reached pentostatin produced by a different synthesis if the resulting molecule met the claim.
Claim 1 limitations
- The claim is directed to the named stereochemical compound.
- “Essentially pure” creates a purity-related limitation.
- It does not expressly claim salts, solvates, hydrates, prodrugs, conjugates, or derivatives.
- It does not expressly claim formulations containing pentostatin.
- It does not expressly claim methods of treating a disease.
- It does not establish protection for later-discovered uses after patent expiration.
The compound claim would not necessarily cover a chemically modified prodrug or a structurally distinct analog. It would cover a formulation containing pentostatin only to the extent that the formulation includes the claimed compound and the relevant infringement theory applies.
What formulations are protected by US Patent 3,923,785?
The patent claims provided do not contain a conventional formulation claim.
The patent does not expressly claim:
- An injectable solution;
- A lyophilized pentostatin vial;
- A particular buffer;
- A specific pH;
- A stabilizer system;
- A preservative;
- A dosage strength;
- A reconstitution method;
- A container-closure system;
- A sustained-release formulation; or
- A nanoparticle, liposomal, or other delivery system.
A formulation containing pentostatin could historically have implicated claim 1 because the formulation incorporated the claimed compound. That would be an indirect consequence of the compound claim, not a separate formulation right. Claims 2 and 3 could similarly have applied to a formulation containing both pentostatin and vidarabine if the mixture and ratio limitations were met.
The patent therefore should not be characterized as a dedicated formulation patent.
What method-of-use patents are covered by this patent?
US Patent 3,923,785 does not claim a method of treating hairy cell leukemia, chronic lymphocytic leukemia, lymphoma, or another disease in the claims provided.
Its operative claims are:
- A compound claim;
- A mixture claim; and
- A ratio-limited mixture claim.
A later patent could have claimed pentostatin use in a particular disease, dose, schedule, patient population, combination regimen, or treatment response. Such a patent would be legally distinct from US 3,923,785. Any later method-of-use patent would also have its own filing date, expiration date, written-description limits, and potential patent-term adjustment.
When did US Patent 3,923,785 expire?
US Patent 3,923,785 expired on December 2, 1992, based on the 17-year term applicable to a patent issued before June 8, 1995.[1,3]
| Event |
Date or status |
| Patent issued |
December 2, 1975 |
| Applicable statutory term |
17 years from issuance |
| Expiration |
December 2, 1992 |
| Current enforceability |
Expired |
| Current U.S. exclusivity |
None from this patent |
The patent predates the 20-year term measured from the earliest effective nonprovisional filing date established by the Uruguay Round Agreements Act. It therefore falls under the transitional pre-1995 regime.
Patent expiration is separate from regulatory exclusivity. Even if FDA approval occurred after the patent issued, the patent term did not automatically extend to match the regulatory approval date. The Hatch-Waxman patent-term restoration provisions were not available in their current form when this patent issued and could not be assumed without a specific patent-term-extension record.
What is the FDA and Orange Book status of pentostatin?
Pentostatin was approved by FDA under the brand name Nipent for the treatment of adult patients with hairy cell leukemia.[2] The product is a small-molecule drug, not a biologic.
| Regulatory issue |
Pentostatin status |
| FDA pathway |
New drug application |
| Product |
Nipent |
| Active ingredient |
Pentostatin |
| Historical indication |
Hairy cell leukemia |
| Regulatory category |
Small molecule |
| Biosimilar pathway |
Not applicable |
| Generic pathway |
ANDA pathway may apply if a reference product and commercial market support an application |
| Patent 3,923,785 |
Expired |
| Orange Book relevance |
Any listing would have no current exclusionary effect from this expired patent |
An expired patent can remain visible in historical regulatory records, but it cannot support a current statutory stay or block a properly approvable generic application. Orange Book status must be assessed by the current reference-listed drug record and any later-listed patents, not by the existence of the 1975 patent alone.[4]
Does pentostatin face biosimilar risk?
No. Pentostatin is a chemically synthesized small molecule. Biosimilar approval under the Public Health Service Act applies to biological products, not conventional small-molecule drugs.
The relevant competitive pathway is generic-drug approval under section 505(j) of the Federal Food, Drug, and Cosmetic Act. A potential generic applicant would evaluate:
- Reference-listed-drug status;
- Availability of an ANDA pathway;
- Pharmaceutical equivalence;
- Bioequivalence requirements;
- Sterility and injectable-product controls;
- Current Orange Book listings;
- Exclusivity periods;
- Manufacturing economics; and
- Commercial availability of the reference product.
The expired compound patent does not create a Paragraph IV barrier.
Are there Paragraph IV challenges to US Patent 3,923,785?
A Paragraph IV certification is not relevant to this patent today because US Patent 3,923,785 expired in 1992.
A Paragraph IV certification alleges that a listed patent is invalid, unenforceable, or not infringed. It generally matters when a patent is listed in the Orange Book and remains unexpired. A generic applicant would not need to challenge an expired patent as a current barrier.
Historical litigation or ANDA records might refer to the patent, but the patent’s age and expiration date make a current Paragraph IV challenge commercially immaterial.
Which companies challenged or licensed the patent?
The supplied claim text does not establish a licensing transaction, assignment history, litigation outcome, or Paragraph IV filing. The patent was associated with the early development of coformycin-related nucleoside chemistry and the development of pentostatin as an adenosine deaminase inhibitor. Later commercial rights and product marketing arrangements should not be inferred solely from the issued patent.
The patent landscape should distinguish among:
- The original compound patent;
- Later process patents;
- Later formulations and injectable-product patents;
- Method-of-treatment patents;
- Regulatory exclusivity;
- Trademark and product rights; and
- Commercial supply agreements.
No current license is necessary to practice the expired claims in the United States. A license could still matter for know-how, trade secrets, manufacturing technology, foreign rights, trademarks, or later unexpired patents.
What manufacturing and intellectual-property barriers remain?
The expired compound patent removes the principal historical composition-of-matter barrier. Commercial entry can still face technical and regulatory obstacles.
Manufacturing barriers
Pentostatin is a stereochemically defined nucleoside analog. Relevant manufacturing controls include:
- Stereochemical purity;
- Nucleoside coupling selectivity;
- Control of regioisomers;
- Residual starting materials;
- Genotoxic or reactive impurities;
- Assay and related-substances testing;
- Sterility for injectable products;
- Endotoxin control;
- Stability during storage; and
- Reproducible batch scale-up.
These are regulatory and operational barriers, not rights created by claim 1 after expiration.
Potential later patent barriers
A freedom-to-operate review should search for unexpired rights covering:
- A particular synthetic route;
- Improved purification;
- Crystalline forms;
- Stable injectable compositions;
- Lyophilized products;
- Container systems;
- Combination therapy;
- Dosing schedules;
- New indications;
- Patient-selection methods; and
- Manufacturing controls.
The existence of such patents cannot be attributed to US 3,923,785. They would need separate patent numbers and claim-by-claim analysis.
How does pentostatin compare with competing leukemia drugs?
Pentostatin competes in a narrow hematologic-oncology market rather than across the full purine analog category.
| Drug |
Drug type |
Historical relevance |
Patent issue |
| Pentostatin |
Adenosine deaminase inhibitor |
Hairy cell leukemia |
Original composition patent expired |
| Cladribine |
Purine nucleoside analog |
Hairy cell leukemia and related disorders |
Separate patent and regulatory history |
| Fludarabine |
Purine analog |
Lymphoid malignancies |
Separate composition, formulation, and use history |
| Vidarabine |
Adenine arabinoside |
Antiviral and investigational oncology uses |
Specifically named in claims 2 and 3 |
| Cytarabine |
Pyrimidine analog |
Broad hematologic use |
Chemically distinct and not covered by this patent |
Pentostatin’s historical differentiation was pharmacological inhibition of adenosine deaminase. Cladribine and fludarabine have different molecular structures, mechanisms, clinical use patterns, and patent estates. None is covered by claim 1 merely because it is used in hematologic oncology.
What generic launch risks exist for pentostatin?
The patent risk from US 3,923,785 is zero as a current enforceability matter. The commercial risk is more likely to arise from product economics and regulatory execution.
| Risk category |
Current assessment |
| Expired composition patent |
No blocking risk |
| Expired mixture claims |
No blocking risk |
| Paragraph IV exposure from this patent |
None |
| Biosimilar competition |
Not applicable |
| Small-molecule generic pathway |
Potentially applicable |
| Injectable manufacturing |
Material execution risk |
| Limited market size |
Potential commercial constraint |
| Reference-product availability |
Requires current regulatory review |
| Later patents |
Must be checked separately |
| Trade-secret manufacturing know-how |
Possible non-patent barrier |
A generic launch could proceed without a license to the expired patent, but the applicant would still need to satisfy FDA requirements and avoid any valid, unexpired later patent claims.
Key Takeaways
- US Patent 3,923,785 claims pentostatin, also known as 2'-deoxycoformycin.
- Claim 1 is an essentially pure compound claim with historically broad scope.
- Claim 2 covers a pentostatin-vidarabine mixture.
- Claim 3 limits that mixture to a pentostatin-to-vidarabine ratio of about 0.005:1 to 0.2:1.
- The patent issued on December 2, 1975, and expired on December 2, 1992.
- The patent contains no express formulation or method-of-use claim.
- It cannot support a current Paragraph IV challenge, regulatory stay, or market exclusion.
- Pentostatin is a small molecule, so biosimilar law does not apply.
- Current entry analysis must focus on later patents, FDA requirements, manufacturing capability, reference-product status, and commercial market size.
- No current U.S. license is required to practice the expired claims.
FAQs About US Patent 3,923,785 and Pentostatin
What is the generic name of the compound in US Patent 3,923,785?
The compound is pentostatin, also called 2'-deoxycoformycin.
Does US Patent 3,923,785 cover Nipent?
It covers the pentostatin active ingredient used in Nipent through claim 1. It does not expressly claim the Nipent brand, container, dosage form, or a specific injectable formulation.
Can a company manufacture pentostatin in the United States without a license to this patent?
Yes. US Patent 3,923,785 expired on December 2, 1992, so it no longer blocks manufacture or sale of pentostatin in the United States.
Is the pentostatin-vidarabine mixture still patent-protected?
Not under claims 2 and 3 of US Patent 3,923,785. Those claims expired with the patent in 1992. Separate later patents would require independent analysis.
Does the patent cover 2'-deoxycoformycin salts or prodrugs?
The claims provided cover the named pentostatin compound. They do not expressly claim salts, prodrugs, conjugates, or structurally modified derivatives.
References
- U.S. Patent No. 3,923,785. (1975). Nucleosides. United States Patent and Trademark Office.
- U.S. Food and Drug Administration. (1991). Nipent (pentostatin) prescribing information.
- United States Code, 35 U.S.C. § 154.
- U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations: Orange Book. FDA.
