US Patent 5,384,310: Clofarabine Claims, Patent Scope, Expiration, and Competitive Landscape

US Patent 5,384,310 is the foundational composition patent for clofarabine, also known as 2-chloro-9-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-9H-purin-6-amine. The patent covers a genus of halogenated purine nucleosides, pharmaceutical compositions containing those compounds, and clofarabine specifically. Its US patent term expired on February 21, 2012, leaving no current US patent exclusivity under this patent.[1]

The patent was commercially important because clofarabine, marketed in the United States as Clolar, became an FDA-approved treatment for relapsed or refractory acute lymphoblastic leukemia in pediatric patients.[2] The patent does not, however, claim every later-developed clofarabine formulation, manufacturing process, dosing regimen, or method of treating leukemia.

What compound does US Patent 5,384,310 protect?

The principal compound is clofarabine:

Attribute	Clofarabine
Chemical name	2-Chloro-9-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-9H-purin-6-amine
Drug class	Purine nucleoside antimetabolite
Structure	2-chloro, 2'-fluoro arabinonucleoside of adenine
US brand	Clolar
Original sponsor	ILEX Oncology
Later commercial owner	Genzyme, now part of Sanofi
FDA approval	December 2004
Approved indication	Relapsed or refractory acute lymphoblastic leukemia in pediatric patients
Patent at issue	US 5,384,310
Patent expiration	February 21, 2012

Clofarabine combines structural elements associated with fludarabine and cladribine. The 2-chloro substitution on the purine ring and 2'-fluoro substitution on the sugar distinguish it from those earlier nucleoside analogues.

What do claims 1 through 10 cover?

The claims divide into two principal categories:

Pharmaceutical-composition claims in claims 1 through 5.
Compound or purine-nucleoside claims in claims 6 through 10.

Claim structure

Claim	Claim type	Scope
1	Pharmaceutical composition	Genus of halogenated purine nucleosides with hydrogen or protecting groups, plus a pharmaceutically acceptable carrier
2	Dependent composition	R is a protecting group
3	Dependent composition	R is hydrogen
4	Dependent composition	Z is chlorine
5	Narrow composition	Composition containing clofarabine
6	Compound	Genus of purine nucleosides with hydrogen or protecting groups
7	Dependent compound	R is hydrogen
8	Dependent compound	R is a protecting group
9	Dependent compound	Z is chlorine
10	Narrow compound	Clofarabine itself

Claims 5 and 10 are the commercially important claims because they identify clofarabine directly.

How broad is the chemical genus in US Patent 5,384,310?

Claims 1 and 6 establish a broad genus around a substituted purine nucleoside. The key variables are R and Z.

The R limitation

R may be hydrogen or a protecting group. In the claimed structure, these R groups correspond to substituents on the sugar portion of the nucleoside, subject to the exact structural definition in the patent drawings and specification.

When R is hydrogen, the claim reaches the deprotected active nucleoside, including clofarabine.

When R is a protecting group, the claim reaches protected chemical forms used in synthesis or potentially in specialized formulations. The term “protecting group” is not unlimited. Its meaning would be determined by the patent specification, technical usage, prosecution history, and whether the accused group performs the conventional function of temporarily masking a reactive hydroxyl or related functionality.

The Z limitation

Z is limited to one of three halogens:

Fluorine
Chlorine
Bromine

The claims do not expressly cover iodine or nonhalogen substituents at the corresponding position.

Claim 4 and claim 9 narrow the genus to Z being chlorine. Claim 5 and claim 10 then identify the specific chlorine-containing compound clofarabine.

Does US Patent 5,384,310 claim clofarabine itself?

Yes. Claim 10 claims clofarabine as a purine nucleoside. Claim 5 claims a pharmaceutical composition containing clofarabine and a pharmaceutically acceptable carrier.

The distinction is material:

Claim 10 is a compound claim and does not require a dosage form or carrier.
Claim 5 requires a pharmaceutical composition containing clofarabine.
Claims 1 through 4 may cover clofarabine through their dependency structure, but claim 5 is the clearest product-composition claim directed to the marketed molecule.

A product that contains clofarabine in an injectable pharmaceutical formulation would have been within the literal scope of claim 5 during the enforceable patent term, assuming the other claim elements were met.

What formulations are protected by the patent?

Claim 1 reaches a pharmaceutical composition containing a claimed nucleoside and a pharmaceutically acceptable carrier. The claim expressly permits administration by:

Oral administration
Topical administration
Parenteral administration

The claim does not require a particular concentration, excipient, container, pH, stability profile, dosage regimen, or injection device.

As a result, the composition claim is broad at the dosage-form level. It can potentially cover a clofarabine formulation even if the formulation uses different excipients from the original commercial product, provided that the product contains the claimed compound and a pharmaceutically acceptable carrier.

The claim does not expressly recite:

A sterile aqueous solution
A specific clofarabine concentration
A particular pH range
A lyophilized formulation
A preservative
A salt-selection limitation
A polymorph
A particle-size limitation
A sustained-release system
A specific container or administration device

Those omissions limit the patent’s ability to block later products that avoid the claimed compound, fall outside the defined formulation elements, or rely on independently patented technology.

What does the patent not claim?

US 5,384,310 is not a comprehensive clofarabine product franchise. It does not expressly claim:

A method of treating acute lymphoblastic leukemia
A specific pediatric dosing schedule
Combination treatment with another antineoplastic agent
A particular infusion duration
A manufacturing process for clofarabine
A method for purifying clofarabine
A specific crystalline form
A formulation defined by pH or excipient concentration
A commercial vial configuration
A method of preventing or treating a particular toxicity

The patent also does not claim all nucleoside analogues. Its scope is limited by the disclosed purine core, sugar configuration, halogen substitution, and R-group definitions.

When did US Patent 5,384,310 lose exclusivity?

The patent issued on February 21, 1995. Under the pre-Uruguay Round patent-term regime applicable to the patent, the ordinary term was 17 years from issuance. The resulting expiration date was February 21, 2012.[1]

The FDA-approved product also received orphan-drug exclusivity. Clolar’s seven-year orphan exclusivity period ran from its December 2004 approval and expired in December 2011.[2] Orphan exclusivity was regulatory exclusivity, not patent protection. It could block FDA approval of the same drug for the same indication during the exclusivity period, subject to statutory exceptions, but it did not extend the patent term.

Exclusivity timeline

Event	Date
US Patent 5,384,310 issued	February 21, 1995
Clolar FDA approval	December 2004
Seven-year orphan exclusivity ends	December 2011
US Patent 5,384,310 expires	February 21, 2012

No current US exclusivity remains under Patent 5,384,310.

What was the Orange Book status of clofarabine?

Clolar was listed in the FDA Orange Book as a prescription drug product subject to patent and exclusivity information supplied by the NDA holder.[3] The relevant patent was US 5,384,310.

Because the patent expired in 2012, it is no longer an enforceable Orange Book barrier to an abbreviated new drug application. The practical significance of the listing is historical. At the time of a potential generic filing, an applicant could have addressed the patent through:

Paragraph III certification, acknowledging that the patent would expire before approval or launch
Paragraph IV certification, asserting that the patent was invalid, unenforceable, or not infringed, if the patent remained listed and legally relevant

A current Paragraph IV challenge to an expired patent generally has no commercial value because the patent cannot block post-expiration launch.

Were there Paragraph IV challenges to clofarabine?

The provided claim set does not establish a specific Paragraph IV filing or litigation event. Patent 5,384,310 expired in 2012, so any current generic-entry analysis must focus on later-listed patents, regulatory exclusivities, manufacturing barriers, or product-specific FDA requirements rather than this patent.

The core Paragraph IV risk during the patent’s active term would have centered on claims 5 and 10:

A generic injectable product containing clofarabine would have faced claim 5.
A manufacturer making or selling clofarabine active pharmaceutical ingredient could have faced claim 10.
Claims 1 and 6 created broader genus-based coverage that could have increased infringement risk for structurally related products.

How strong was the patent estate for clofarabine?

The patent was strong against the commercial molecule because claims 5 and 10 identified clofarabine directly. Literal infringement analysis would not require a court to decide whether the commercial product fell within a broad genus if the accused product contained the specifically claimed molecule.

Its principal limitations were temporal and technological:

Strength	Limitation
Direct claim to clofarabine	Expired in 2012
Compound claim reaches API manufacture and sale	No current patent exclusivity
Composition claim reaches carrier-containing products	No detailed formulation coverage
Genus claims cover related halogenated nucleosides	Scope depends on structural interpretation
Covers oral, topical, and parenteral compositions	No express treatment-method claims

The patent was therefore strong as a foundational molecule patent but incomplete as a modern lifecycle portfolio.

What patent litigation and settlement risks remain?

Patent 5,384,310 no longer creates a live US infringement risk because its term has expired. Any historical litigation or settlement involving the patent would have been relevant before February 2012, particularly for a generic seeking to market clofarabine before expiration.

A settlement based solely on this patent would not support a current launch restriction. A current dispute would need to involve a different patent, such as a later formulation, manufacturing, dosage, polymorph, or method-of-use patent, or a nonpatent regulatory exclusivity.

How does clofarabine compare with fludarabine and cladribine patent coverage?

Drug	Structural relationship	Commercial patent issue
Clofarabine	2-chloro, 2'-fluoro arabinonucleoside	US 5,384,310 directly claims the molecule
Fludarabine	Fluorinated purine nucleoside, commonly administered as phosphate prodrug	Separate compound and formulation estate
Cladribine	Chlorinated purine nucleoside without the same 2'-fluoro configuration	Separate compound and use claims

Patent 5,384,310 should not be treated as covering fludarabine or cladribine. Structural similarity does not establish claim overlap. Each molecule must be mapped against the claimed purine, sugar stereochemistry, substituents, and R-group limitations.

What generic entry risks exist for clofarabine?

After expiration of US 5,384,310, the principal barriers to generic clofarabine are no longer this patent. The relevant risks are more likely to involve:

FDA bioequivalence and pharmaceutical-equivalence requirements
Sterile injectable manufacturing
Stability and container-closure validation
Supply of qualified clofarabine API
Any later-listed formulation or manufacturing patents
Commercial demand in a small oncology market
Orphan-drug market economics
Hospital contracting and limited treatment volume

Biosimilar risk is not relevant to clofarabine because clofarabine is a chemically synthesized small molecule, not a biologic. The appropriate competitive pathway is an ANDA or, depending on product and regulatory facts, another small-molecule FDA pathway.

What is the geographic scope of the patent?

US Patent 5,384,310 applies only in the United States. It does not establish protection in Europe, Japan, Canada, or other jurisdictions.

International protection would depend on corresponding national applications and their individual prosecution histories, claim scope, maintenance payments, and expiration dates. A US expiration date cannot be used to determine foreign patent status.

Key Takeaways

US Patent 5,384,310 is the foundational US patent for clofarabine.
Claims 5 and 10 directly identify clofarabine.
Claims 1 through 4 cover pharmaceutical compositions containing the claimed nucleoside genus.
Claims 6 through 9 cover the underlying purine-nucleoside genus and its chlorine subset.
The patent covers hydrogen and protected forms of the sugar substituents, with Z limited to fluorine, chlorine, or bromine.
The patent expired on February 21, 2012.
Clolar’s seven-year orphan exclusivity expired in December 2011.
The patent does not claim a specific leukemia treatment method, dose, formulation pH, excipient system, polymorph, or manufacturing process.
Current clofarabine entry risk must be assessed against later patents and FDA regulatory requirements, not Patent 5,384,310.
Biosimilar analysis is inapplicable because clofarabine is a small-molecule drug.

FAQs About US Patent 5,384,310 and Clofarabine

Does US Patent 5,384,310 still block generic clofarabine?

No. The patent expired on February 21, 2012, and cannot independently block a current US generic launch.

Which claim directly covers clofarabine API?

Claim 10 directly covers clofarabine as a purine nucleoside. Claim 5 covers a pharmaceutical composition containing clofarabine.

Does the patent cover clofarabine phosphate?

Not expressly. The claims provided identify clofarabine and pharmaceutically acceptable salts, but they do not specifically recite a phosphate prodrug or phosphate ester. Coverage would depend on the exact chemical structure and claim interpretation.

Can a formulation patent extend clofarabine exclusivity after 2012?

A later formulation patent could create separate, narrower protection if it claimed a patentably distinct formulation and remained unexpired. It would not extend the term of US Patent 5,384,310 itself.

Is clofarabine subject to biosimilar competition?

No. Clofarabine is a chemically synthesized small molecule. Generic competition, rather than biosimilar competition, is the relevant US pathway.

References

United States Patent and Trademark Office. (1995). U.S. Patent No. 5,384,310, purine nucleosides and pharmaceutical compositions.
U.S. Food and Drug Administration. (2004). Clolar (clofarabine) prescribing information.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book.
U.S. Code, 35 U.S.C. §§ 154, 271, 355.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
European Patent Office	0473708	⤷ Start Trial	300247	Netherlands	⤷ Start Trial
European Patent Office	0473708	⤷ Start Trial	06C0038	France	⤷ Start Trial
European Patent Office	0473708	⤷ Start Trial	SPC/GB06/040	United Kingdom	⤷ Start Trial
Austria	147751	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,384,310

Summary for Patent: 5,384,310