United States Drug Patent 6,268,489: Claims, Scope, Expiration, and Azithromycin Patent Landscape

U.S. Patent No. 6,268,489 protected crystalline azithromycin dihydrate as a solid-state product and protected a specific crystallization process using tetrahydrofuran, a C5-C7 aliphatic hydrocarbon, and at least two molar equivalents of water. The patent’s ordinary 20-year term ended on December 4, 2018. It no longer creates a U.S. patent barrier to generic azithromycin manufacture or sale. Its historical importance was concentrated in solid-form control and manufacturing rather than in the basic azithromycin molecule.

What does U.S. Patent 6,268,489 claim?

The patent has three claims:

Claim	Subject matter	Scope
1	Crystalline azithromycin dihydrate	Product claim covering the claimed crystalline hydrate form
2	Preparation of crystalline azithromycin dihydrate	Process using amorphous azithromycin or azithromycin monohydrate, THF, a C5-C7 aliphatic hydrocarbon, and at least two molar equivalents of water
3	Process of claim 2	Narrower process in which the hydrocarbon is hexane

The patent is directed to azithromycin, the azalide antibiotic marketed by Pfizer as Zithromax. It does not claim the underlying macrolide structure generally. The original azithromycin compound patent and the later crystalline-form patent are separate layers of protection.

What is the scope of claim 1 for crystalline azithromycin dihydrate?

Claim 1 is a product-by-form claim. It covers azithromycin in a crystalline dihydrate state, meaning the crystal contains azithromycin associated with approximately two moles of water per mole of azithromycin under the relevant solid-state definition.

The claim does not require:

A particular manufacturing process;
THF;
Hexane;
A particular particle size;
A specific pharmaceutical dosage form;
A particular excipient;
A specific polymorph designation beyond the claimed crystalline dihydrate;
A particular therapeutic use.

A product could therefore have infringed claim 1 regardless of how the crystalline dihydrate was produced. A process that used ethanol, acetone, water, or another solvent would not avoid claim 1 if it produced the claimed crystalline azithromycin dihydrate during the patent term.

Product-form claim construction

The principal technical question would have been whether the accused material was the claimed crystalline dihydrate rather than:

Amorphous azithromycin;
Azithromycin monohydrate;
Anhydrous azithromycin;
Another hydrate or solvate;
A mixture of solid forms.

Proof would typically rely on powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, Karl Fischer water analysis, infrared spectroscopy, and solid-state characterization. Water content alone would not necessarily establish infringement because a material can contain water without having the claimed crystal lattice.

The most important distinction is between composition and physical form. Claim 1 does not cover every azithromycin product. It covers the crystalline dihydrate form.

What process does claim 2 protect?

Claim 2 requires each of the following elements:

Crystallization of amorphous azithromycin or azithromycin monohydrate;
A mixture containing tetrahydrofuran;
A C5-C7 aliphatic hydrocarbon;
At least two molar equivalents of water;
Formation of crystalline azithromycin dihydrate.

The process claim is narrower than claim 1. It does not cover every process that makes the dihydrate. A process using a solvent system outside the claimed combination could avoid claim 2 even if it produced the same product.

Required solvent system

The hydrocarbon must fall within the C5-C7 aliphatic range. Examples potentially within the literal carbon-number range include:

Pentane;
Hexane;
Heptane.

The claim is narrower than a generic “hydrocarbon” limitation. Aromatic hydrocarbons such as toluene and substituted aromatic solvents would not ordinarily satisfy a C5-C7 aliphatic hydrocarbon limitation.

The claim also requires THF. A process using only a hydrocarbon and water, or using another cyclic ether instead of THF, would present a noninfringement position on the literal claim language.

Water limitation

The process must be conducted in the presence of at least two molar equivalents of water. This is a quantitative limitation. The analysis would require determining the water-to-azithromycin molar ratio at the relevant stage of crystallization.

The limitation is not necessarily satisfied merely because the final solid contains two water molecules. The claim focuses on the crystallization conditions, not only the final hydrate stoichiometry.

How narrow is claim 3 using hexane?

Claim 3 depends on claim 2 and replaces the broader C5-C7 hydrocarbon limitation with hexane. It therefore requires:

Amorphous azithromycin or azithromycin monohydrate;
THF;
Hexane;
At least two molar equivalents of water;
Crystallization of crystalline azithromycin dihydrate.

Claim 3 is the most process-specific claim in the patent. A manufacturer using THF and heptane, for example, could avoid claim 3 but might still face claim 2 if all other limitations were met.

A manufacturer using THF, hexane, and water but starting from a different azithromycin form would have a potential noninfringement argument if the starting-material limitation were material under the court’s construction. Process documentation, batch records, solvent composition, water charging records, and starting-material characterization would be central evidence.

When did U.S. Patent 6,268,489 lose exclusivity?

The ordinary patent term ended on December 4, 2018, based on the U.S. filing and priority information associated with the patent. The patent issued on July 31, 2001, as U.S. Patent No. 6,268,489. The grant date did not extend the patent term beyond the statutory 20-year period except for any applicable patent-term adjustment or extension.

Event	Date
Earliest relevant U.S. filing in the patent family	December 4, 1998
Patent issued	July 31, 2001
Ordinary 20-year expiration	December 4, 2018
Current enforceability	Expired
Current blocking effect on generic entry	None

The patent therefore cannot support a new U.S. infringement action for conduct occurring after expiration. Historical damages would also be subject to the applicable limitation periods and other litigation requirements.

What was the Orange Book status of azithromycin?

Azithromycin was approved in the United States under Pfizer’s Zithromax applications, including NDA 050710 and related products. The FDA Orange Book historically identified patents associated with branded azithromycin products, including patents covering the active ingredient and solid-form technology where applicable.[2]

The Orange Book does not create patent rights. It records patents submitted by an NDA holder and affects the certification framework for abbreviated new drug applications. Once listed patents expired, they ceased to block approval on an exclusivity basis.

Azithromycin is a small-molecule drug. It has no biosimilar pathway. Generic applicants use the ANDA pathway, generally relying on the reference listed drug’s safety and efficacy findings and demonstrating pharmaceutical equivalence and bioequivalence.

What generic entry risks existed during the patent term?

During the active term, an ANDA applicant producing crystalline azithromycin dihydrate could face two principal risks:

Product risk

If the generic product itself contained crystalline azithromycin dihydrate, claim 1 could support a product infringement theory. A process change would not avoid the product claim if the final commercial drug remained within the claim.

Process risk

A generic manufacturer using THF, a qualifying C5-C7 aliphatic hydrocarbon, and at least two molar equivalents of water could face claims 2 and 3. The process risk would depend on the manufacturing process used by the applicant, not merely on the identity of the finished dosage form.

An ANDA applicant could pursue a Paragraph IV certification against an unexpired listed patent. Paragraph IV litigation under the Hatch-Waxman Act can trigger a 30-month stay of FDA approval if the NDA holder or patent owner files suit within the statutory period.[3] That mechanism is no longer relevant to Patent 6,268,489 because the patent expired in 2018.

Were Paragraph IV challenges and azithromycin litigation material?

Azithromycin had substantial generic competition after expiration of the core compound and related exclusivity rights. Historical litigation involving azithromycin focused primarily on the basic compound patent, listed solid-form patents, and regulatory timing.

For Patent 6,268,489, the relevant litigation question was whether an ANDA product:

Contained crystalline azithromycin dihydrate;
Used the patented THF/hydrocarbon/water crystallization process;
Was manufactured by a supplier using the claimed process;
Was covered by an Orange Book-listed patent at the time of the Paragraph IV certification.

A final assessment of a specific historical Paragraph IV case requires matching the ANDA number, certification date, patent listing, complaint, and settlement terms. No current litigation effect remains after expiration of the patent.

What licensing deals affected the patent?

Pfizer commercialized Zithromax, while azithromycin technology was developed and licensed through relationships involving the original inventors and European pharmaceutical companies. The existence of a commercial license or assignment does not extend patent duration.

Patent ownership, exclusive licensing, and product commercialization must be separated:

Ownership determines who could enforce the patent;
Licensing determines who could practice the technology;
Orange Book listing affects ANDA notice and certification;
Patent expiration ends the exclusionary right regardless of commercial licensing.

Public patent records identify the patent’s assignee and ownership history, but private manufacturing licenses and settlement agreements may not be fully disclosed in patent databases. The patent itself does not establish the commercial terms of any license.

How strong was the patent estate for azithromycin?

The azithromycin estate was layered rather than dependent on a single patent.

Protection layer	Function	Current status
Basic azithromycin compound patent	Covered the active pharmaceutical ingredient	Expired
Crystalline dihydrate patents	Covered solid-state form and crystallization technology	Expired
Formulation patents	Could cover tablets, suspensions, coatings, or excipient systems	Product- and patent-specific; historical rights largely expired
Method-of-use patents	Could cover specific dosing or indications	Product- and patent-specific
Regulatory exclusivity	Delayed approval independently of patent rights	Expired for the original product

Patent 6,268,489 was strongest against a generic that both marketed the claimed crystalline dihydrate and used a process within claim 2 or claim 3. Claim 1 created the broader commercial risk because it focused on the final solid form. Claims 2 and 3 were narrower and more dependent on process evidence.

The estate did not provide durable protection against all azithromycin alternatives. A manufacturer could potentially avoid the process claims through a different crystallization route, although that would not avoid claim 1 if the final product remained the claimed dihydrate.

What formulations were protected by azithromycin patents?

Patent 6,268,489 does not claim a tablet, capsule, suspension, intravenous formulation, dosage regimen, or excipient combination. It claims the active ingredient’s crystalline form and the process used to produce it.

Separate patents could address:

Immediate-release tablets;
Oral suspensions;
Pediatric powder-for-reconstitution products;
Film coatings;
Taste masking;
Extended-release delivery;
Specific dosage regimens;
Combination products.

A finished tablet could implicate claim 1 because the tablet contains crystalline azithromycin dihydrate. The presence of excipients would not remove the product from the claim if the claimed form remained present. The patent would not, however, independently protect the tablet architecture or formulation ingredients.

How does Patent 6,268,489 compare with the core azithromycin patent?

Issue	Core azithromycin patent	Patent 6,268,489
Primary subject	Chemical compound	Crystalline hydrate form
Typical claim type	Composition of matter	Product-by-form and process
Design-around potential	Low during term	Higher for process claims
Relevance to finished generic	Broad active-ingredient coverage	Depends on solid form
Relevance after expiration	None	None
Biosimilar relevance	None	None

The core compound patent would have been broader because it covered azithromycin itself. Patent 6,268,489 added a solid-state barrier that could extend commercial protection beyond the basic compound patent, subject to its own filing date and term.

What manufacturing and intellectual-property barriers remain?

No enforceable barrier from Patent 6,268,489 remains in the United States. Commercial manufacturers still must manage technical and regulatory issues:

Control of hydrate state during crystallization and drying;
Consistent water content;
Polymorph and solid-form reproducibility;
Impurity control;
Residual THF and hydrocarbon limits;
Bioequivalence;
Stability of oral suspension and solid dosage forms;
Compliance with current good manufacturing practices;
API supplier qualification.

These are manufacturing and regulatory requirements, not continuing patent exclusivity. Patent rights in other countries must be assessed separately because foreign family members may have had different filing dates, term adjustments, maintenance histories, and legal outcomes.

What is the geographic coverage of Patent 6,268,489?

The patent’s enforceable scope was limited to the United States. It did not automatically protect azithromycin dihydrate in Europe, Canada, Japan, India, or other jurisdictions.

International coverage would depend on corresponding national patents or regional applications. Each jurisdiction could differ on:

Claim wording;
Validity;
Patent-term calculation;
Supplementary protection certificates;
Maintenance fees;
Disclosure requirements;
Claim construction;
Litigation results.

The U.S. expiration date therefore does not establish the expiration date of any foreign counterpart.

Key Takeaways

Patent 6,268,489 covered crystalline azithromycin dihydrate and a specific THF/hydrocarbon/water crystallization process.
Claim 1 was the broadest claim because it focused on the final crystalline product.
Claim 2 required THF, a C5-C7 aliphatic hydrocarbon, at least two molar equivalents of water, and specified azithromycin starting materials.
Claim 3 narrowed claim 2 to hexane.
The ordinary U.S. patent term ended on December 4, 2018.
The patent no longer blocks generic azithromycin entry in the United States.
Azithromycin is a small molecule and has no biosimilar pathway.
The patent did not independently protect tablets, suspensions, dosage regimens, or excipient combinations.
Historical infringement exposure depended on the solid form in the finished product and, for claims 2 and 3, the actual API manufacturing process.
Foreign patent rights require separate family-by-family analysis.

FAQs About U.S. Patent 6,268,489

Does Patent 6,268,489 cover all azithromycin?

No. It covers crystalline azithromycin dihydrate, not every chemical or physical form of azithromycin.

Can a generic manufacturer use hexane after the patent expired?

Yes. The expired patent no longer prevents use of hexane or the claimed THF/hexane/water process in the United States, subject to other applicable rights and regulatory requirements.

Does a finished azithromycin tablet infringe claim 1?

It could have done so during the patent term if the tablet contained the claimed crystalline azithromycin dihydrate. Excipients would not necessarily avoid the product claim.

Is crystalline azithromycin dihydrate a polymorph or a hydrate?

It is principally a crystalline hydrate form. The legally relevant issue is whether the material matches the claimed crystal form and dihydrate characteristics, not whether it is informally labeled a polymorph.

Are azithromycin patents still relevant to generic approval?

Expired patents no longer block approval, but FDA requirements for pharmaceutical equivalence, bioequivalence, quality, stability, and current good manufacturing practices remain applicable.

References

U.S. Patent No. 6,268,489. (2001). Crystalline azithromycin dihydrate. United States Patent and Trademark Office.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book. FDA.
U.S. Food and Drug Administration. (2023). Hatch-Waxman amendments and abbreviated new drug applications. FDA.
United States Code, 35 U.S.C. §§ 154, 271, and 282. (2024). Patent term, infringement, and validity provisions.
United States Code, 21 U.S.C. § 355. (2024). New drug applications, abbreviated applications, and patent certifications.

Foriegn Application Priority Data
Foreign Country	Foreign Patent Number	Foreign Patent Date
	PCT/US87/01612	Jul 09, 1987

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
African Regional IP Organization (ARIPO)	44	⤷ Start Trial
African Regional IP Organization (ARIPO)	8800093	⤷ Start Trial
Argentina	243532	⤷ Start Trial
Austria	71370	⤷ Start Trial
Austria	72446	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 6,268,489

Summary for Patent: 6,268,489