US Patent 6,395,304: Scope, Claim Analysis, Expiration, and Risperidone Microparticle Patent Landscape

US Patent 6,395,304 covers a continuous or semi-continuous manufacturing process for polymeric microparticles. Its core technology uses static mixers to emulsify an active-agent/polymer solution, split or sequentially process the emulsion, and extract solvent to form microparticles. The patent is directed primarily to process architecture, not to risperidone as a molecule or to a finished dosage form.

The patent’s statutory term expired in 2020. It therefore does not create a current enforceable patent barrier to generic or competing manufacture. Its historical relevance is greatest for long-acting injectable risperidone products, including microsphere-based products made with poly(lactide-co-glycolide), or PLGA.

What technology does US Patent 6,395,304 protect?

The patent protects methods for producing drug-loaded polymeric microparticles through controlled mixing and solvent extraction. The process has five central elements:

A first phase containing an active agent, polymer, and solvent.
A second phase.
Combination of the two phases in a first static mixer to create an emulsion.
Processing through multiple downstream static mixers.
Contact with an extraction liquid to remove solvent and solidify the microparticles.

The independent claims are drafted around manufacturing configuration rather than a particular apparatus. The patent can therefore cover different equipment arrangements if the claimed process steps are performed.

The disclosure is particularly relevant to microsphere products in which a drug is encapsulated in biodegradable PLGA. The dependent claims identify risperidone and 9-hydroxyrisperidone, benzyl alcohol, ethyl acetate, and PLGA having a lactide-to-glycolide ratio of approximately 85:15 to 50:50.

The patent was assigned to the Alkermes organization, which developed microsphere drug-delivery technologies used in long-acting injectable products.[1]

How are the claims of US 6,395,304 structured?

Claims 1 and 14 are the two principal process claims. Claims 22 and 23 are product-by-process claims.

Claim group	Subject matter	Main limitation
Claim 1	General microparticle preparation method	First static mixer, plural downstream static mixers, extraction liquid
Claims 2-8	Mixer geometry and flow configuration	Larger first mixer, flow-rate ratios, series or parallel mixer arrangements
Claims 9-13	Composition and solvent limitations	Risperidone, benzyl alcohol, PLGA, ethyl acetate
Claim 14	Split-flow microparticle method	First mixer outflow divided into at least two downstream streams
Claims 15-21	Dependent split-flow limitations	Mixer diameter, composition, equal flow rates
Claims 22-23	Microparticles made by the claimed methods	Product-by-process scope

What does claim 1 require?

Claim 1 requires each of the following:

Preparing a first phase with an active agent, polymer, and solvent.
Preparing a second phase.
Pumping both phases through a first static mixer.
Forming an emulsion in that first mixer.
Flowing the emulsion through a plurality of static mixers.
Combining the outflow with an extraction liquid.
Extracting solvent to form microparticles.

The claim is broad as to the active agent, polymer, solvents, and second phase. The risperidone-specific limitations appear only in dependent claims 10-13. A process using a different active agent could fall within claim 1 if all other limitations are met.

The claim also requires a plurality of static mixers after the first mixer. A process using only one mixer after emulsion formation would not literally satisfy that limitation.

What does claim 14 add?

Claim 14 is narrower in process architecture but potentially important for scale-up operations. It requires:

A first static mixer that produces an emulsion outflow.
Division of that outflow into at least two flow streams.
Passage of each stream through a separate second static mixer.
Combination of the streams with extraction liquid.

Claim 14 therefore focuses on a split-flow arrangement. It differs from claim 1, which can encompass sequential downstream mixers and does not expressly require division of the first mixer’s outflow into separate branches.

How do claims 1 and 14 differ?

Issue	Claim 1	Claim 14
First mixer	Required	Required
Multiple downstream mixers	Required	Required
Explicit division of first outflow	Not required	Required
Separate downstream mixers for separate streams	Not required	Required
Parallel processing	Covered through dependent claim 8 or claim 14 architecture	Central to the claim
Equal flow streams	Not required	Added by claim 21

A facility using one large first mixer followed by two parallel smaller mixers could potentially implicate both independent claims. A facility using sequential downstream mixers without a split into separate branches would be more directly analyzed under claim 1.

What formulations are protected by the patent?

The formulation-related claims identify a specific risperidone microparticle formulation platform.

Risperidone and 9-hydroxyrisperidone

Claims 10 and 17 cover an active agent selected from:

Risperidone.
9-hydroxyrisperidone.
Pharmaceutically acceptable salts of those compounds.

These claims do not cover every risperidone formulation. They cover risperidone-containing microparticles only when made through the claimed static-mixer process.

Solvent system

Claims 11 and 18 specify benzyl alcohol as the first solvent used for the active-agent solution.

Claims 13 and 20 specify ethyl acetate as the second solvent used for the polymer solution.

The claimed preparation sequence is:

Dissolve the active agent in benzyl alcohol.
Dissolve PLGA in ethyl acetate.
Blend the two solutions.
Combine the first phase with the second phase in the static-mixer system.
Extract solvent and form microparticles.

A manufacturer using a different solvent could avoid the narrower solvent claims, but not necessarily claim 1 or claim 14, which do not expressly require benzyl alcohol or ethyl acetate.

PLGA composition

Claims 12 and 19 specify PLGA with a lactide-to-glycolide molar ratio between approximately 85:15 and 50:50.

This range includes commonly used biodegradable copolymers such as 75:25 PLGA. The dependent claims therefore target a commercially relevant class of injectable microsphere polymers.

The claims do not specify every critical formulation variable. They do not expressly require a particular:

Particle-size distribution.
Drug-loading percentage.
Release duration.
Polymer molecular weight.
Injection route.
Dosage strength.
Reconstitution medium.
Final product concentration.

Those omissions give the independent process claims broader potential scope than the dependent formulation claims.

How do the static-mixer limitations affect infringement analysis?

The static-mixer limitations are the patent’s principal point of distinction.

Mixer diameter

Claims 2 and 15 require the first static mixer to have a diameter greater than the downstream static mixers. The limitation is structural and should be assessed against actual equipment specifications, not merely nominal flow capacity.

A process using mixers of equal diameter would likely avoid these dependent claims while remaining potentially relevant to claims 1 or 14.

Flow-rate ratio

Claim 3 requires the second phase to be pumped at a greater flow rate than the first phase. Claim 4 narrows the ratio to approximately 4:1 to 5:1.

For example, a first-phase flow rate of 10 mL/min and a second-phase flow rate of 40-50 mL/min would fall within the stated ratio range, assuming the claim’s “about” language is satisfied.

Flow-rate records, pump calibration data, batch records, and process-control software would be central to any historical infringement analysis.

Series and parallel configurations

Claim 7 covers a plurality of static mixers configured in series. Claim 8 covers a configuration that provides parallel flow streams.

The patent therefore addresses two scale-up approaches:

Series processing, in which the emulsion passes through sequential mixers.
Parallel processing, in which flow is divided among multiple mixers.

Claim 14 is particularly directed to the second architecture.

Equal flow rates

Claim 21 requires the divided streams to have substantially equal flow rates. This limitation may be difficult to establish in a commercial system without evidence from flow meters, pump settings, pressure readings, or validated process parameters.

Are claims 22 and 23 product claims or process claims?

Claims 22 and 23 recite “microparticles prepared by” the claimed methods. These are product-by-process claims.

Their scope depends on the legal treatment of the process language. In U.S. patent law, a product-by-process claim generally requires the claimed product to have been made by the recited process for infringement purposes, although patentability may be assessed differently.[2]

The practical consequence is that a product with identical composition and particle characteristics may not necessarily infringe claims 22 or 23 if it was manufactured through a materially different process. The process claims, rather than the product-by-process claims, are likely to have been the stronger enforcement tools.

When did US Patent 6,395,304 lose exclusivity?

US Patent 6,395,304 expired in 2020 after the end of its statutory patent term.[1] The ordinary U.S. term for a utility patent is 20 years from the earliest effective nonprovisional filing date, subject to patent-term adjustment, patent-term extension, terminal disclaimers, and other statutory provisions.[3]

The patent is therefore no longer an enforceable exclusion right in the United States. A party cannot obtain an injunction or damages for post-expiration practice of the claimed process based solely on this patent.

The expiration does not eliminate:

Trade-secret protection for undisclosed manufacturing parameters.
Know-how protection under confidentiality agreements.
Regulatory exclusivity associated with a product.
Separate patents covering formulations, devices, dosage regimens, or other manufacturing steps.
Contractual restrictions in licenses or development agreements.

What is the Orange Book status of US 6,395,304?

US 6,395,304 is a process patent. It is not an active-ingredient composition patent, and its claims are not directed to a finished risperidone dosage form as such.

The Orange Book primarily records patents submitted by NDA holders for approved drug substances, drug products, and methods of use under the Hatch-Waxman framework.[4] A manufacturing-process patent of this type does not automatically create an Orange Book barrier to an ANDA.

The patent should therefore be analyzed separately from:

Orange Book-listed formulation patents.
Orange Book-listed method-of-use patents.
FDA regulatory exclusivity.
Patent certifications made in an ANDA.

Because US 6,395,304 expired in 2020, it cannot support a current Paragraph IV litigation strategy. Any historical Paragraph IV dispute involving a risperidone product would have required analysis of the then-active patents listed for that product, not this expired patent alone.

What Paragraph IV and generic-entry risks existed?

Before expiration, the patent could have created risk for a generic or follow-on manufacturer using the claimed static-mixer process. The risk was greatest where the proposed process used:

Risperidone or 9-hydroxyrisperidone.
PLGA within the claimed lactide-to-glycolide range.
Benzyl alcohol and ethyl acetate.
A large first static mixer.
Multiple smaller downstream mixers.
A 4:1 to 5:1 phase-flow ratio.
Parallel or sequential downstream mixing.

A generic manufacturer could pursue several design-around strategies:

Use a different emulsification device, such as a high-shear rotor-stator system.
Use a single downstream mixer rather than a plurality.
Avoid dividing the first mixer outflow into separate branches.
Use different solvents.
Alter the phase-flow ratio.
Use a different polymer or polymer composition.
Form microparticles through batch emulsification rather than continuous static mixing.
Use a non-PLGA delivery platform.

After expiration, these design-around strategies remain relevant for process economics and know-how, but not because of continuing enforceability of US 6,395,304.

Which products and companies are relevant to the patent landscape?

The patent is most closely associated with long-acting injectable risperidone and microsphere manufacturing. The competitive landscape includes products based on different delivery technologies.

Product or platform	Company	Delivery technology	Relationship to US 6,395,304
Risperdal Consta	Janssen, with Alkermes microsphere technology	Intramuscular PLGA microspheres	Technically aligned with the patent’s risperidone/PLGA subject matter
Rykindo	Luye Pharma	Long-acting risperidone microspheres	Potentially relevant to microsphere process and formulation patents, but not automatically covered by this expired patent
Perseris	Indivior	Extended-release subcutaneous delivery system	Different delivery architecture
Uzedy	Teva	Extended-release subcutaneous risperidone suspension	Different delivery architecture
Generic risperidone products	Multiple manufacturers	Usually oral immediate-release or orally disintegrating dosage forms	Generally outside the microparticle process claims

FDA approval and product labeling establish regulatory status, not infringement. A competing long-acting injectable product requires separate analysis of its manufacturing process, formulation, patent portfolio, and regulatory pathway.[5-8]

How strong was the patent estate?

The patent was technically meaningful but legally narrow in several respects.

Strengths

It covered process architecture rather than one narrow equipment model.
Claim 1 was not limited to risperidone.
The claims covered both series and parallel mixer arrangements.
The patent addressed scale-up through flow splitting and multiple mixers.
Dependent claims mapped directly onto a risperidone-PLGA solvent system.

Limitations

The claims required static mixers and specific process sequencing.
The patent did not broadly claim risperidone microparticles regardless of manufacturing method.
Product-by-process claims created additional proof requirements.
Competitors could potentially use batch processing or different emulsification technologies.
The patent expired in 2020.
The patent does not by itself protect release kinetics, dosage regimens, injection devices, or every risperidone microsphere formulation.

Its historical blocking strength was therefore highest against manufacturers replicating the disclosed continuous static-mixer process. Its current blocking strength is zero because the patent is expired.

What manufacturing and IP barriers remain after expiration?

The expiration of US 6,395,304 does not make long-acting risperidone microspheres simple to manufacture. Remaining barriers include:

Reproducible particle-size control.
Encapsulation efficiency.
Control of residual benzyl alcohol and ethyl acetate.
Sterility assurance.
Aseptic processing.
In-vitro and in-vivo release matching.
Scale-up from laboratory to commercial throughput.
Control of polymer molecular-weight loss.
Batch-to-batch release consistency.
Demonstration of bioequivalence for complex injectable products.
CMC documentation and process validation.

These barriers can support commercial differentiation even when the core process patent has expired. They do not, however, restore exclusivity under the expired patent.

What litigation and settlement issues should be checked?

US 6,395,304 should not be treated as a current litigation threat. Any historical or current dispute involving a long-acting risperidone product should be separated into four categories:

Patent infringement involving unexpired formulation or process patents.
ANDA litigation involving Orange Book-listed patents.
Regulatory disputes involving product sameness, bioequivalence, or approval timing.
Contractual or licensing disputes involving Alkermes technology.

A license to Alkermes technology may have commercial significance even after patent expiration if it includes confidential know-how, technical support, manufacturing rights, or field-of-use restrictions. Patent expiration alone does not terminate those contractual obligations.

Key Takeaways

US Patent 6,395,304 covers static-mixer manufacturing methods for polymeric microparticles.
Claims 1 and 14 are the principal process claims.
Claims 10-13 and 17-20 narrow the technology to risperidone or 9-hydroxyrisperidone, benzyl alcohol, ethyl acetate, and specified PLGA.
Claim 14 focuses on dividing the first mixer outflow into separate downstream streams.
Claims 22 and 23 are product-by-process claims.
The patent expired in 2020 and does not create a current U.S. patent barrier.
The patent historically could have affected manufacturers using continuous static-mixer production of risperidone PLGA microparticles.
It does not broadly cover all risperidone long-acting injectables or all risperidone microparticles.
Current competitive analysis must focus on unexpired formulation, device, method-of-use, manufacturing, regulatory, trade-secret, and licensing rights.

FAQs

Does US 6,395,304 cover all Risperdal Consta manufacturing methods?

No. It covers only methods satisfying the claim limitations, including the specified static-mixer sequence and solvent-extraction process. It does not cover every possible method for producing risperidone microspheres.

Can a company manufacture risperidone PLGA microparticles without infringing this patent?

Yes. The patent expired in 2020. A company must still assess other unexpired patents, trade secrets, regulatory requirements, and contractual restrictions.

Is US 6,395,304 a formulation patent?

No. It is principally a process patent. The claims identify active agents, solvents, and PLGA compositions, but the claimed invention centers on the manufacturing process.

Does using a different solvent avoid every claim?

No. Different solvents may avoid claims requiring benzyl alcohol or ethyl acetate, but the independent claims do not expressly require those solvents. The full process must be compared with each claim.

Are generic oral risperidone tablets affected by this patent?

Generally no. Oral tablets do not ordinarily use the claimed microparticle manufacturing process. Separate patents and regulatory requirements may apply to particular oral products.

References

U.S. Patent No. 6,395,304. (2002). Method for preparing microparticles. U.S. Patent and Trademark Office.
U.S. Patent and Trademark Office. (2024). Manual of Patent Examining Procedure § 2113: Product-by-process claims.
35 U.S.C. § 154. (2024). Patent term.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations.
U.S. Food and Drug Administration. (2024). Risperdal Consta prescribing information.
U.S. Food and Drug Administration. (2024). Perseris prescribing information.
U.S. Food and Drug Administration. (2024). Uzedy prescribing information.
U.S. Food and Drug Administration. (2024). Orange Book: Approved drug products with therapeutic equivalence evaluations.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	286722	⤷ Start Trial
Austria	349201	⤷ Start Trial
Australia	2750801	⤷ Start Trial
Australia	3437901	⤷ Start Trial
Australia	771497	⤷ Start Trial
Australia	773734	⤷ Start Trial
Canada	2390284	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 6,395,304

Summary for Patent: 6,395,304