US Patent 6,451,340: Adefovir Dipivoxil Crystal Forms, Manufacturing Processes, Formulations, and Patent Landscape

US Patent 6,451,340 covers crystalline adefovir dipivoxil, defined solid forms, crystalline salts, crystallization methods, manufacturing processes, and tablets containing specified proportions of Form 1. The strongest commercial claims are directed to Form 1, the form used in adefovir dipivoxil drug products, and to processes that produce Forms 2 and 4. The patent was assigned to Gilead Sciences, Inc. and issued September 17, 2002.[1]

The patent term is generally understood to have expired in March 2020, subject to the applicable patent-term calculation and any regulatory extension. It therefore does not provide a current US exclusionary barrier for a generic product. Its claims remain important for historical freedom-to-operate analysis, litigation review, and understanding the technical origin of Hepsera manufacturing and formulation technology.

What does US Patent 6,451,340 cover?

The patent has 47 claims grouped into six technical categories:

Claim group	Claims	Subject matter	Commercial relevance
Crystalline compositions	1-18	Crystalline adefovir dipivoxil, Forms 1-4, and crystalline salts	High
Pharmaceutical compositions and use	19-20	Excipients and antiviral administration	Medium
General crystallization methods	21-24	Solvent-mediated crystallization	High for process implementation
Specific polymorph preparation	25-28	Water-mediated Form 2 and fumaric-acid-mediated Form 4	Medium to high
Chemical synthesis	29-34, 47	Preparation from PMEA and related intermediates	High if the claimed route is used
Tablets and granulation products	35-46	Form 1 compression, wet granulation, and specified tablet formulas	High for the original Hepsera-type product

The patent is principally a solid-state and process patent. It is not the foundational patent covering the antiviral activity of adefovir or the general chemical identity of adefovir dipivoxil.

What patents protect crystalline adefovir dipivoxil?

Claim 1 is the broadest composition claim. It covers a composition comprising crystalline adefovir dipivoxil without limiting the composition to a specific polymorph.

Claims 2 through 14 narrow the composition to four identified crystalline forms:

Form	Claims	Structural or analytical limitation	Reported thermal transition
Form 1	2-5	C-centered monoclinic cell; principal XRPD peak at about 6.9° 2θ	About 102°C
Form 2	6-8	XRPD peaks at about 9.6°, 18.3°, 22.0°, and 32.8° 2θ	About 73°C
Form 3	9-11	XRPD peaks at about 8.1°, 19.4°, 25.4°, and 30.9° 2θ	About 85°C
Form 4	12-14	XRPD peaks at about 9.8°, 15.2°, 26.3°, and 31.7° 2θ	About 148°C

The analytical language uses “about,” which gives the claims some tolerance around the stated diffraction and thermal values. In an infringement dispute, the critical question would be whether the accused material has the claimed crystal lattice or an equivalent XRPD and DSC profile, rather than merely whether it contains adefovir dipivoxil.

How strong are the Form 1 claims?

Form 1 is the most commercially important solid form. Claim 2 broadly covers a composition containing Form 1. Claim 3 adds the unit-cell parameters and space group Cc. Claims 4 and 5 add XRPD and DSC characteristics.

The claim structure creates several enforcement paths:

A composition may infringe claim 2 if Form 1 is present, even if the formulation contains additional excipients.
Claim 3 provides a crystallographic definition that can be tested by single-crystal or powder diffraction methods.
Claims 4 and 5 provide analytical fallback limitations.
Claims 35 through 39 target products made by compression or wet granulation using Form 1.
Claims 40 through 46 target specific tablet compositions and Form 1 percentages.

The principal weakness is that later claims become increasingly dependent on analytical testing, formulation composition, manufacturing history, or product-by-process evidence. A generic manufacturer could reduce exposure by using a nonclaimed polymorph, an amorphous form, a different solid-state mixture, or a formulation that does not satisfy the specific tablet limitations, subject to the breadth of claims 1 and 2 and any doctrine-of-equivalents analysis.

What formulations are protected by US 6,451,340?

Claims 19 and 20 cover a pharmaceutical composition with a pharmaceutically acceptable excipient and administration of an antiviral amount. These claims are broad but may face validity and enforcement issues because they do not identify a particularly distinctive dosage form or excipient combination.

Claims 40 through 46 are more specific. They cover tablets containing:

Adefovir dipivoxil with at least about 70% Form 1;
20 mg pregelatinized starch;
24 mg croscarmellose sodium;
Lactose monohydrate;
24 mg talc;
4 mg magnesium stearate.

The 60 mg tablet version contains 268 mg lactose monohydrate and weighs approximately 400 mg. Claims 43 and 46 raise the Form 1 threshold to at least about 80%.

Claim	Dosage or composition limitation
40	Specified excipients and at least about 70% Form 1
41	60 mg adefovir dipivoxil and 268 mg lactose
42	60 mg tablet weighing about 400 mg
43	60 mg tablet with at least about 80% Form 1
44	120 mg adefovir dipivoxil and 208 mg lactose
45	120 mg tablet weighing about 400 mg
46	120 mg tablet with at least about 80% Form 1

These formulation claims are narrower than the core crystalline composition claims. They are most relevant to a product that replicates the historical formulation architecture, not to every adefovir dipivoxil tablet.

How do the process claims operate?

Claims 21 through 28 cover crystallization methods. Claim 24 is the most technically detailed process claim. It requires a solution containing approximately 6% to 45% adefovir dipivoxil and approximately 55% to 94% crystallization solvent. The solvent options include ether mixtures, acetone, ethyl acetate, tert-butanol, toluene, isopropyl acetate, diethyl ether, di-n-butyl ether, and related combinations.

Claim 25 covers preparation of Form 2 in the presence of water. Claim 26 specifies hydration of Form 1 or crystallization in water. Claim 28 covers preparation of Form 4 in the presence of fumaric acid.

These claims may be infringed by a manufacturing process even where the final dosage form is not directly sold with manufacturing information. Process discovery would normally require production records, batch documents, regulatory filings, discovery evidence, or reliable reverse engineering.

What manufacturing route is covered?

Claims 29 through 34 cover conversion of 9-[2-(phosphonomethoxy)ethyl]adenine, commonly referred to as PMEA or adefovir, into adefovir dipivoxil using chloromethyl pivalate.

Claim 29 requires:

PMEA;
Chloromethyl pivalate;
1-methyl-2-pyrrolidinone;
A trialkylamine; and
Recovery of adefovir dipivoxil.

Claim 30 specifies triethylamine. Claim 31 limits the amount of 1-methyl-2-pyrrolidinone to approximately 5.6 to 56.8 molar equivalents. Claim 32 limits triethylamine to approximately 2 to 5 molar equivalents. Claims 33 and 34 address PMEA containing less than about 2% salt, including sodium bromide or potassium bromide.

Claim 47 covers preparation of 9-[2-(diethylphosphonomethoxy)ethyl]adenine by contacting sodium alkoxide with 9-(2-hydroxyethyl)adenine. This is an upstream intermediate process claim and would generally matter only if a manufacturer used the claimed route.

When did US Patent 6,451,340 lose exclusivity?

The patent issued September 17, 2002 from an application associated with a March 24, 2000 filing date and an earlier March 26, 1999 priority date.[1] Under the modern US patent-term framework, the relevant expiration is generally calculated from the earliest effective nonprovisional filing date rather than the priority date. Public patent records commonly identify a March 2020 expiration date for this patent family.

Event	Date
Earliest priority date	March 26, 1999
US/PCT filing period	March 24, 2000
Patent issued	September 17, 2002
Approximate ordinary patent expiration	March 2020

Patent expiration does not eliminate historical infringement liability for conduct occurring before expiration. It does eliminate the patent holder’s ability to prevent new US commercial activity solely under this patent after expiration.

What is the Orange Book status of adefovir dipivoxil?

Adefovir dipivoxil was approved by FDA as Hepsera, a 10 mg oral tablet manufactured by Gilead Sciences, for treatment of chronic hepatitis B.[2] The original NDA was 021449.

The Orange Book framework separates patent protection from regulatory exclusivity:

Drug substance and composition patents may be listed against the approved product.
Formulation patents may be listed if they claim the approved dosage form.
Method-of-use patents may be listed if they cover an approved indication.
FDA approval does not itself extend an expired patent.
Generic applicants may address listed patents through Paragraph IV certification or wait for expiration under other certification pathways.

US 6,451,340 is best understood as a solid-state, manufacturing, and formulation patent rather than a method-of-use patent for hepatitis B. Its relevance to Orange Book litigation would have depended on whether Gilead listed the patent against Hepsera and whether an ANDA applicant certified against that listing. Current Orange Book status should be checked against FDA’s active and discontinued listings because listing status can change after patent expiration, product discontinuation, or regulatory updates.[3]

Which companies challenged adefovir dipivoxil patents?

The public record does not establish a major, commercially consequential Paragraph IV litigation campaign centered specifically on US 6,451,340. Adefovir dipivoxil is a relatively small US market compared with major antiviral products, and Hepsera’s commercial opportunity was limited by the availability of newer hepatitis B therapies.

Potential generic competition could proceed through:

A Paragraph IV certification against an unexpired listed patent;
A Paragraph III certification with launch after patent expiration;
A non-infringement or invalidity position based on a different polymorph;
A formulation design-around;
A process design-around using different solvents or reaction conditions.

The most credible design-around target would be the crystal-form claims. A manufacturer could attempt to produce a different solid form, an amorphous material, or a polymorphic mixture, provided the resulting product remains pharmaceutically acceptable and does not fall within the broader composition claims.

What patent litigation affects adefovir dipivoxil?

The principal historical US patent landscape included two layers:

Patent layer	Representative protection	Relevance
Foundational adefovir and nucleotide analog patents	Earlier Gilead patents, including US 5,663,159	Chemical entity and antiviral use
Solid-state and manufacturing patents	US 6,451,340	Crystalline forms, processes, tablets
Later formulation or process patents	Related Gilead family and continuation filings	Potentially extended formulation or process coverage

US 6,451,340 should not be analyzed in isolation. A generic product could avoid this patent but still face a separate patent covering the active compound, an approved use, a salt, a formulation, or a manufacturing route. Conversely, once the relevant foundational and later patents expired, US 6,451,340 would not independently support a continuing market block.

No current US exclusivity should be inferred solely from the existence of the patent number. FDA patent listings, patent-family records, terminal disclaimers, maintenance-fee status, litigation dockets, and regulatory exclusivity must be checked separately.

How does the adefovir dipivoxil patent estate compare with competing hepatitis B drugs?

Adefovir dipivoxil has a weaker current commercial patent position than newer hepatitis B products because its principal US patent barriers have expired or aged out. Tenofovir disoproxil fumarate, tenofovir alafenamide, entecavir, and pegylated interferon products had different patent and regulatory timelines, with later-generation products generally retaining greater commercial relevance.

Product	Active ingredient	Main competitive characteristic	Current patent risk profile
Hepsera	Adefovir dipivoxil	Older nucleotide analog	Low from US 6,451,340 after expiration
Viread	Tenofovir disoproxil fumarate	Higher historical market adoption	Dependent on separate product and formulation patents
Baraclude	Entecavir	Potent oral nucleoside analog	Separate compound and formulation estate
Vemlidy	Tenofovir alafenamide	Later-generation tenofovir prodrug	Later patent and regulatory barriers

The principal commercial risk to Hepsera was therapeutic substitution rather than patent litigation. Generic entry, if commercially viable, would compete against lower-cost generic nucleoside and nucleotide therapies and against newer branded medicines with stronger clinical positioning.

What generic launch risks exist?

A generic adefovir dipivoxil launch would have faced five principal risks before the 2020 expiration period:

Crystal-form risk. The applicant’s active ingredient could contain Form 1 or another claimed form.
Process risk. The applicant could use the claimed solvent systems, water-mediated crystallization, or fumaric-acid process.
Formulation risk. The tablet could replicate the specified excipients, strengths, weight, and Form 1 content.
Orange Book risk. A listed patent could trigger a Paragraph IV notice and a 30-month stay if litigation were timely filed.[4]
Regulatory-market risk. A technically permissible launch might still face limited demand because of competing hepatitis B products.

After expiration, the first three risks become historical rather than prospective patent barriers. Trade-secret protection for manufacturing parameters, regulatory know-how, and supplier controls could remain relevant, but those rights would not recreate the expired patent monopoly.

What is the geographic coverage of this patent?

US 6,451,340 provides territorial rights in the United States only. The same inventive disclosure could support counterpart patents in other jurisdictions, but foreign patent rights require separate analysis of:

National filing and grant status;
Local patent-term rules;
Patent-term extensions;
Supplementary protection certificates;
Maintenance fees;
Opposition or revocation proceedings;
Local Orange Book equivalents and regulatory linkage rules.

A US patent cannot block manufacture, importation, sale, or use in Europe, Canada, Japan, China, or other markets without a corresponding enforceable local right.

What is the overall patent strength?

The patent was technically broad in its initial composition claims but commercially strongest where the claim language identifies Form 1 or a specific tablet or process.

Category	Relative strength
Claim 1 crystalline adefovir dipivoxil	Broad but potentially vulnerable to claim construction and prior-art challenges
Form 1 claims	Strongest commercial relevance
Forms 2-4	Narrower and easier to design around
Crystalline salt claims	Broad chemical coverage but potentially vulnerable to enablement and written-description issues across many salts
Solvent crystallization claims	Strong if process evidence is available
Form 2 and Form 4 preparation claims	Narrow but technically identifiable
Product-by-process claims 35-39	Dependent on proof of the claimed manufacturing process
Exact tablet claims 40-46	Narrow, fact-intensive, and easier to design around
Upstream synthesis claims	Relevant only to the selected manufacturing route

Key Takeaways

US 6,451,340 covers crystalline adefovir dipivoxil, Forms 1 through 4, crystalline salts, crystallization methods, synthesis routes, and Form 1 tablets.
Form 1 is the central commercial subject because the patent links it to crystallographic data, tablet compression, wet granulation, and specified 60 mg and 120 mg formulations.
Claims 21 through 34 and 47 can create process infringement risk even when the final product is chemically identical to a product made by another route.
Claims 40 through 46 are narrow formulation claims and are less likely to cover a materially different generic tablet.
The patent’s ordinary US term ended around March 2020.
Adefovir dipivoxil’s current US patent risk is primarily historical or family-dependent, not based on continuing exclusivity under US 6,451,340.
A complete launch assessment requires review of all Orange Book-listed patents, related continuations, terminal disclaimers, and litigation records, not this patent alone.

FAQs

Is US Patent 6,451,340 a patent on adefovir dipivoxil itself?

No. It primarily covers crystalline forms, salts, crystallization, manufacturing routes, and certain tablets. Earlier patents addressed the broader chemical and antiviral technology.

Does a generic adefovir dipivoxil product infringe if it contains Form 1?

Potentially, under the historical scope of claims 2 through 5 and related formulation claims. After patent expiration, those claims no longer provide a prospective US exclusionary right.

Which adefovir dipivoxil polymorph is most commercially important?

Form 1 is the most important because the patent associates it with tablet compression, wet granulation, and the specified tablet formulations.

Can a manufacturer avoid the patent by using a different crystallization solvent?

Potentially. A process using a solvent outside the listed combinations may avoid the literal scope of claim 24, although the final crystal form could still implicate a composition claim.

Does the patent cover adefovir dipivoxil treatment for hepatitis B?

Claim 20 broadly covers administration of an antiviral composition, but the patent is not principally a hepatitis B method-of-use patent. Its central protection is directed to solid forms, processes, and formulations.

References

United States Patent and Trademark Office. (2002). US Patent No. 6,451,340: Crystalline forms of adefovir dipivoxil. U.S. Department of Commerce.
U.S. Food and Drug Administration. (2002). Hepsera (adefovir dipivoxil) prescribing information.
U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations: Orange Book.
U.S. Food and Drug Administration. (n.d.). Abbreviated new drug application regulations and patent certifications.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	016548	⤷ Start Trial
Austria	228357	⤷ Start Trial
Austria	277936	⤷ Start Trial
Austria	277937	⤷ Start Trial
Australia	747163	⤷ Start Trial
Australia	8660998	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 6,451,340

Summary for Patent: 6,451,340