United States Patent 6,036,976: Sustained-Release Microsphere Claims, Scope and Patent Landscape

United States Patent 6,036,976 is a Takeda patent directed to manufacturing sustained-release biodegradable microspheres, with the strongest commercial focus on leuprorelin or leuprorelin acetate encapsulated in lactic acid/glycolic acid polymer. Its claims combine composition, formulation, and highly specific in-water drying process parameters. The patent term has expired, so the claims no longer create a current U.S. exclusion right. The technical disclosure remains relevant to freedom-to-operate analyses involving depot peptide formulations and manufacturing know-how.

What does U.S. Patent 6,036,976 cover?

The patent covers sustained-release microspheres containing a physiologically active substance and a biodegradable polymer. The claimed microspheres must be produced through an in-water drying process using a water-in-oil-in-water or oil-in-water emulsion.

The central claim limitations are:

Limitation	Claim 1 requirement
Dosage form	Sustained-release microsphere
Active ingredient	Physiologically active substance
Polymer	Biodegradable polymer
Emulsion	W/O/W or O/W
Active phase	Active substance in an inner aqueous phase
Polymer phase	Polymer in an external oil phase
Drying process	In-water drying
Microsphere concentration	About 0.1 to about 500 kg/m³ of external aqueous phase
Liquid-surface geometry	About 0.2 to about 4.5 m² per m³^(1/3)
Emulsion replacement	About 0.01 to about 10 times/minute
Gas transfer rate	About 10 to about 300 m/second
Gas replacement	At least about 0.5 times/minute

Claim 14 is the commercially narrowest and most important claim. It limits the active to leuprorelin or leuprorelin acetate and requires a lactic acid/glycolic acid polymer having a lactic acid:glycolic acid ratio of approximately 90:10 to 50:50.

When does U.S. Patent 6,036,976 lose exclusivity?

The patent is expired. U.S. Patent 6,036,976 was granted on March 14, 2000, and is subject to the post-Uruguay Round 20-year patent term measured from the applicable U.S. nonprovisional filing date, subject to patent-term adjustment, terminal disclaimers, and other statutory adjustments.[1]

Public patent records identify the patent as a Takeda Chemical Industries patent with priority in the mid-1990s. On the available filing chronology, the statutory term ended in approximately 2016 or 2017. An expired patent cannot support a new U.S. infringement action for conduct occurring after expiration, although it may remain relevant to historical validity, past damages, prosecution history, technical disclosure, and ownership-chain analysis.

Event	Date or period
Earliest priority	1995, based on the patent family chronology
U.S. grant	March 14, 2000
Patent number	U.S. 6,036,976
Expected statutory expiration	Approximately 2016-2017
Current enforceability	Expired
Current Orange Book blocking effect	None as an enforceable patent

The exact expiration date should be taken from the USPTO patent-term record because the grant document alone does not establish the final date where patent-term adjustment or disclaimer data apply.

How many patents cover the claimed microsphere technology?

U.S. Patent 6,036,976 is one patent in a broader technology group covering peptide depot formulations, biodegradable microspheres, polymer composition, particle manufacture, and therapeutic use. The patent itself has 15 claims, but the enforceable claim architecture is concentrated in three groups:

Claims 1-3 and 8-9: process-defined microspheres using manufacturing and gas-transfer parameters.
Claims 4-7 and 10-13: narrowing limitations directed to peptides, LH-RH compounds, polymer chemistry, and leuprorelin.
Claims 14-15: leuprorelin-specific microspheres and finished sustained-release preparations.

The claims are not directed to every leuprorelin microsphere. They require the specified emulsion arrangement and process conditions. A formulation made with a different encapsulation process may fall outside the literal scope even if it contains leuprorelin and PLGA.

What is the scope of claim 1?

Claim 1 is a product-by-process claim. It recites a microsphere by describing the process used to produce it. The claim is therefore broader than a pure manufacturing claim in one respect, because it is written as a microsphere, but narrower because the microsphere must result from the specified process.

The critical limitations are cumulative. A potentially infringing product would need to involve:

A sustained-release microsphere;
A physiologically active substance;
A biodegradable polymer;
A W/O/W or O/W emulsion;
The active substance positioned in an inner aqueous phase;
The polymer positioned in an external oil phase;
In-water drying;
The claimed concentration, surface-area, emulsion-replacement, gas-transfer, and gas-replacement ranges.

The claim does not require leuprorelin, PLGA, or a particular dose. Those limitations appear in dependent claims. Claim 1 therefore reaches a wider class of peptide, small-molecule, hormone, antibiotic, antitumor, and other biodegradable microsphere products if the process limitations are met.

How do the numerical limitations affect infringement?

The numerical limitations are central rather than incidental. They appear to define the manufacturing environment that allegedly improves drying efficiency, batch reproducibility, and microsphere quality.

The surface-area limitation is expressed as:

[ \frac{\sqrt{A}}{V^{1/3}} \approx 0.2 \text{ to } 4.5 ]

where A is the liquid surface area in square meters and V is the external aqueous-phase volume in cubic meters.

This is a dimensionless vessel-geometry ratio. It links exposed liquid surface to the scale of the aqueous phase. A manufacturer could potentially avoid the limitation by using a vessel geometry outside the claimed range, but the term "about" introduces a fact-dependent boundary.

The gas-transfer limitations are also unusually specific. Claim 1 requires a gas transfer rate near the liquid surface of approximately 10 to 300 m/second and gas replacement of at least 0.5 times per minute. Claims 8 and 9 narrow the transfer rate to 10-200 and 50-150 m/second, respectively.

A serious freedom-to-operate analysis would require batch records showing:

Vessel dimensions;
External aqueous-phase volume;
Microsphere loading;
Emulsion feed and replacement rate;
Gas identity and flow rate;
Local gas velocity near the liquid surface;
Gas-exchange frequency;
Polymer molecular weight and end-group chemistry;
Phase composition and drying endpoint.

What formulations are protected by claims 4 through 15?

Leuprorelin and leuprorelin acetate

Claims 13 and 14 specifically cover leuprorelin or leuprorelin acetate. Claim 14 requires all of the following:

Leuprorelin or leuprorelin acetate;
A sustained-release microsphere;
A biodegradable lactic acid/glycolic acid polymer;
A lactic acid:glycolic acid ratio of approximately 90:10 to 50:50;
The specified W/O/W or O/W in-water drying process;
The specified concentration, geometry, replacement, and gas-transfer conditions.

This is materially narrower than a claim covering leuprorelin depot products generally.

Peptides and LH-RH compounds

Claim 4 covers physiologically active peptides. Claim 5 narrows the field to LH-RH agonists or antagonists. Claim 12 recites two chemically defined peptide families, while claim 13 identifies leuprorelin and leuprorelin acetate by name.

The patent's practical center of gravity is the long-acting LH-RH depot product rather than the full list of therapeutic classes in claim 11.

Biodegradable polymers

Claims 6 and 7 identify:

Biodegradable polymers with a free terminal carboxyl group; and
Lactic acid/glycolic acid polymers.

Claim 10 narrows the polymer to a lactic acid:glycolic acid composition ratio of approximately 90:10 to 50:50. The claim does not expressly require a specific molecular weight, inherent viscosity, particle size, solvent, surfactant, or release duration.

Those omitted parameters may still matter for claim construction if they appear in the specification as necessary characteristics, but they are not express limitations of claim 14.

Is claim 15 a formulation patent?

Claim 15 covers a sustained-release preparation containing a microsphere under claims 1-14 together with one or more conventional pharmaceutical excipients. The listed excipient categories include dispersants, fillers, disintegrants, binders, lubricants, thickeners, and bases.

Claim 15 does not independently protect an ordinary injectable excipient system. It depends on the underlying microsphere satisfying claims 1-14. Adding an excipient to a noninfringing microsphere would not, by itself, create infringement.

For an injectable depot suspension, the most relevant components would likely be a dispersant, vehicle, suspending agent, buffer, or isotonicity-adjusting material. The claim language is broad, but its dependency materially limits its practical reach.

Is U.S. Patent 6,036,976 listed in the Orange Book?

The patent is unlikely to have functioned as a current Orange Book barrier because its principal claims are directed to a manufacturing process and a product-by-process microsphere. FDA regulations exclude process patents from the patent information that is required to be submitted for Orange Book listing under 21 C.F.R. § 314.53.[2]

Orange Book listing is generally relevant to patents that claim:

The approved drug substance;
The approved drug product;
A method of using the approved drug.

A process claim for manufacturing a leuprorelin microsphere normally does not qualify for listing merely because the process is used to make an approved product.

The absence of an Orange Book listing would not have eliminated infringement risk while the patent was in force. It would, however, have reduced the patent's role in an ANDA-based Paragraph IV certification and automatic 30-month stay.

What Paragraph IV challenges affected this patent?

No current Paragraph IV risk arises because the patent has expired. A Paragraph IV certification is relevant only when a listed patent remains in force and an ANDA applicant asserts that the patent is invalid, unenforceable, or not infringed.[3]

Because U.S. Patent 6,036,976 is principally process-focused and appears not to be a qualifying Orange Book product or use patent, an ANDA applicant would not ordinarily face a Paragraph IV certification directed to this patent unless FDA records had treated a claim as listable. The commercial regulatory dispute would more likely involve:

Other listed patents for leuprorelin depot products;
Drug-product equivalence;
Injectable suspension characteristics;
Sterility and container-closure systems;
Clinical or bioequivalence requirements;
Any later patent or regulatory exclusivity.

What patent litigation affects U.S. Patent 6,036,976?

The patent does not create a current U.S. litigation obstacle because its term has ended. Historical litigation risk would have depended on whether a competitor's process met the numerical limitations and whether its product had the same characteristics required under the product-by-process claim.

Potential disputes would have centered on:

Whether "about" permits variance from the stated numerical ranges;
Whether gas transfer rate means bulk gas velocity, local velocity, or another measured parameter;
Whether "external oil phase" is satisfied in a particular emulsion architecture;
Whether a product made by a different process is materially indistinguishable;
Whether the claims are enabled across the full therapeutic list;
Whether the numerical process limitations are definite;
Whether the patent term was correctly calculated.

No live settlement or litigation restriction should be inferred from the existence of the patent alone. Expiration removes prospective enforcement, but historical settlement terms, licenses, and confidential manufacturing agreements are not necessarily public.

How strong is the patent estate for leuprorelin depot microspheres?

The patent was technically narrow but commercially relevant during its term.

Factor	Assessment
Claim breadth	Moderate for generic biodegradable microspheres; narrow for leuprorelin
Process dependence	High
Numerical limitations	High; create design-around opportunities
Product-specific coverage	Strongest in claim 14
Orange Book leverage	Limited or absent if treated as a process patent
Current enforceability	None after expiration
Manufacturing relevance	High as technical know-how
Generic-entry relevance today	Low for this patent alone
Historical licensing value	Potentially significant for Takeda-related depot technology

The estate's main strength was the combination of a named active ingredient, PLGA-type polymer, and controlled drying conditions. Its main weakness was dependence on process measurements that may be difficult to reproduce consistently and easy to vary through equipment design.

How does this patent compare with leuprorelin product patents?

Older leuprorelin patents generally focused on the peptide itself, pharmaceutical compositions, or therapeutic use. U.S. Patent 6,036,976 is different because it concentrates on the manufacturing conditions for a sustained-release microsphere.

Patent category	Typical subject matter	Relevance to U.S. 6,036,976
Compound patent	Leuprorelin molecule or peptide analog	Separate technology layer
Product patent	Leuprorelin depot composition	May overlap commercially but not necessarily technically
Formulation patent	Polymer, particle, vehicle, release profile	Partial overlap
Manufacturing patent	Emulsion, drying, gas flow, scale-up	Directly aligned
Method-of-use patent	Treatment indication or dosing schedule	Outside the core claims
Regulatory exclusivity	FDA exclusivity period	Separate from patent rights

A competitor that avoids the claimed drying conditions may still need to address other formulation, device, use, or process patents. Conversely, expiration of this patent does not establish freedom from every patent affecting a leuprorelin depot product.

What generic launch risks remain for leuprorelin depot products?

The principal risks after expiration of U.S. Patent 6,036,976 are regulatory and technical rather than direct infringement risk under this patent.

FDA regulatory status

Leuprorelin acetate depot products are approved as injectable sustained-release products. FDA review may focus on the active ingredient, microsphere attributes, release profile, dose uniformity, sterility, residual solvents, particle-size distribution, reconstitution, and injection performance.[4]

A generic or follow-on applicant may need to show that its product has the same active ingredient, dosage form, strength, route, and conditions of use as the reference product, while satisfying applicable bioequivalence or product-specific requirements.

Biosimilar risk

Leuprorelin is a synthetic peptide, not a conventional monoclonal antibody or recombinant therapeutic protein. A biosimilar pathway under section 351(k) is therefore not the ordinary route for a leuprorelin acetate depot. The more relevant pathways are an ANDA or, depending on product differences, a 505(b)(2) application.

Manufacturing and know-how barriers

Patent expiration does not eliminate manufacturing difficulty. Key barriers include:

Stable formation of the inner aqueous peptide phase;
Prevention of peptide migration into the external phase;
Control of porosity and particle size;
Reproducible PLGA degradation;
Control of initial burst release;
Aseptic processing;
Scale-up of gas-liquid mass transfer;
Reconstitution and syringeability;
Long-term stability.

These barriers can preserve commercial differentiation even after patent expiry. They are not, however, enforceable exclusivity rights absent a surviving patent, trade secret, or regulatory protection.

What geographic coverage does the patent provide?

U.S. Patent 6,036,976 provides rights only in the United States. Foreign family members would require separate analysis by jurisdiction. Patent expiration in the United States does not establish expiration in Japan, Europe, Canada, or other countries.

For a global launch, the relevant review should separate:

U.S. patent term and terminal disclaimer data;
European Patent Convention validations and national post-grant status;
Japanese family-member expiration;
Patent-term extensions;
Supplementary protection certificates;
Local Orange Book or equivalent linkage systems;
National litigation and settlement records.

Patent families often contain materially different claims after prosecution. A foreign claim directed to the process may survive, expire later, or be narrowed differently from the U.S. claims.

Key Takeaways

U.S. Patent 6,036,976 covers sustained-release biodegradable microspheres made through a tightly defined in-water drying process.
Claim 14 is the most commercially specific claim because it covers leuprorelin or leuprorelin acetate in a 90:10 to 50:50 lactic acid/glycolic acid polymer.
The patent is a product-by-process patent with numerous quantitative manufacturing limitations.
Claims 1 and 14 require more than a leuprorelin depot formulation; they require the specified emulsion and drying conditions.
The U.S. patent term expired approximately in 2016-2017, eliminating current infringement exposure under this patent.
The patent is primarily a manufacturing and formulation asset, not a broad leuprorelin compound patent.
Its likely Orange Book value was limited because process patents generally are not listable under FDA patent-submission rules.
Current generic-entry risk must be assessed against later patents, FDA requirements, product-specific equivalence issues, and manufacturing know-how.

Frequently Asked Questions

Does U.S. Patent 6,036,976 cover all leuprorelin depot injections?

No. The strongest leuprorelin claim, claim 14, requires the specified PLGA composition and the claimed emulsion-drying conditions.

Can a competitor use PLGA and leuprorelin after this patent expired?

Yes, this patent no longer prevents U.S. commercial activity. Other unexpired patents, regulatory requirements, or contractual restrictions may still apply.

Is the gas-transfer-rate limitation likely to be important in a patent dispute?

Yes. It is an express limitation in claims 1, 8, 9, and 14. Measurement methodology and the meaning of "near the liquid surface" would likely be contested.

Does claim 15 protect a particular injectable vehicle?

Not independently. Claim 15 depends on a microsphere that already satisfies claims 1-14 and then adds a pharmaceutically acceptable excipient or related formulation component.

Is leuprorelin a biosimilar product?

No. Leuprorelin acetate depot products are generally addressed through small-molecule or peptide drug approval pathways rather than the standard 351(k) biosimilar pathway for biologic products.

References

United States Patent and Trademark Office. (2000). U.S. Patent No. 6,036,976, Sustained-release microsphere.
U.S. Food and Drug Administration. (2024). 21 C.F.R. § 314.53: Submission of patent information.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations.
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	219662	⤷ Start Trial
Austria	305772	⤷ Start Trial
Canada	2192782	⤷ Start Trial
Germany	69622006	⤷ Start Trial
Germany	69635249	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 6,036,976

Summary for Patent: 6,036,976