US Patent 4,789,724: Claim Scope, Expiration, and Polyanhydride Patent Landscape

US Patent 4,789,724 protects a process for producing high-purity polyanhydride copolymers by separately synthesizing and purifying the anhydride monomers before polymerization. Its claims also cover the resulting copolymers when they are made from individually purified mixed anhydrides. The patent is expired under the 17-year term applicable to pre-1995 US patents, so it does not currently block generic, 505(b)(2), device, or polymer manufacturing activity. Its principal historical relevance is to biodegradable polyanhydride drug-delivery systems, including polymers related to the carmustine implant Gliadel.

What does US Patent 4,789,724 protect?

The patent has two principal protection categories:

A manufacturing process for preparing high-purity anhydride copolymers.
A product claim directed to copolymers made from individually synthesized and purified mixed anhydride monomers.

The central technical concept is process separation. Each diacid is converted separately into its corresponding polymerizable anhydride intermediate. Unreacted acetic anhydride and unreacted diacid are removed from each intermediate before the intermediates are combined and polymerized.

That sequence distinguishes the claimed process from:

Direct polymerization of untreated diacids.
One-pot reactions in which several diacids are exposed to acetic anhydride together.
Polymerization of incompletely purified monomer mixtures.
Processes that combine monomers before removal of residual diacid or acetic anhydride.
Processes using a different activating reagent, unless the resulting method still satisfies every claim limitation.

The patent’s stated objective is control of polymer composition and purity. Residual diacid, acetic anhydride, acetic acid, and incompletely converted intermediates can affect molecular weight, degradation behavior, drug release, crystallinity, and reproducibility.

What are the elements of independent claim 1?

Claim 1 is a method claim. A process must satisfy all of the following limitations to fall within its literal scope.

Claim element	Scope
At least two diacids	The process must use at least two separately identifiable diacid starting materials.
Individual synthesis and purification	Each diacid must be individually synthesized and purified before the anhydride-forming step.
Separate reaction with acetic anhydride	Each diacid is reacted separately with acetic anhydride.
Formation of individual mixed anhydrides	Each reaction produces a separately prepared polymerizable anhydride intermediate.
Separate removal of residual materials	Unreacted acetic anhydride and unreacted diacid must be removed from each individual intermediate.
Combination after purification	The purified individual mixed anhydrides are combined only after the separate purification steps.
Polymerization	The combined purified monomers are polymerized into a copolymer.

The timing limitation is commercially important. A manufacturer that prepares two monomers separately but combines them before purification may avoid literal infringement of claim 1. The same may apply to a manufacturer that purifies the starting diacids but does not separately purify the anhydride intermediates.

The claim does not appear to require a specific polymerization catalyst, temperature, solvent, molecular-weight range, molar ratio, or final dosage form. Those omissions make the claim broad within the defined manufacturing sequence, but the claim remains vulnerable to noninfringement arguments based on process order and chemical identity.

What diacids are covered by claim 2?

Claim 2 narrows claim 1 to a listed group of diacids. The supplied text contains typographical errors. The chemically recognizable compounds are:

Sebacic acid.
1,3-bis(p-carboxyphenoxy)propane.
1,6-bis(p-carboxyphenoxy)hexane.
Isophthalic acid.
1,4-phenylene dipropionic acid.
Adipic acid.
Dodecanedioic acid.

The exact construction of the two bis(carboxyphenoxy) compounds should be confirmed against the issued patent specification and drawings. The supplied wording appears to omit parenthetical and positional information.

Claim 2 is narrower than claim 1 because it limits the diacid combination to the listed compounds. A process using a different aromatic or aliphatic diacid may remain outside claim 2 while potentially raising issues under claim 1, depending on the full claim language and prosecution history.

What product is protected by claims 3 through 5?

Claims 3 through 5 concern the copolymer product rather than only its manufacture.

Claim 3: purified mixed-anhydride copolymer

Claim 3 covers a highly pure anhydride copolymer consisting essentially of individually synthesized and purified monomers selected from mixed anhydrides of aliphatic and aromatic diacids.

The phrase "consisting essentially of" is narrower than "comprising" but broader than "consisting of." It generally permits components that do not materially affect the basic and novel characteristics of the claimed copolymer. The likely basic characteristic is a high-purity polyanhydride made from individually prepared and purified anhydride monomers.

The product claim contains a process-oriented limitation: the diacids must have been individually reacted with acetic anhydride, and residual diacid and acetic anhydride must have been removed. This creates a potential product-by-process dispute.

A competitor may argue that its chemically identical polymer is not within claim 3 because it was made through a different process. The patent holder may argue that, under US product-by-process doctrine, a product claim can cover the same product regardless of manufacturing method if the claimed process limitations define the product’s identity or distinguishable characteristics. The outcome would depend on the claim construction, prosecution history, and whether the process produces measurable structural or compositional differences. See Abbott Laboratories v. Sandoz, Inc., 566 F.3d 1282 (Fed. Cir. 2009).

Claim 4: recrystallized individual mixed anhydrides

Claim 4 adds several process details:

Separate refluxing of each diacid with acetic anhydride.
Removal of excess acetic anhydride.
Recrystallization of each individual mixed anhydride.

This is a narrower product claim because the claimed monomers must be made through the specified reflux and recrystallization sequence.

Claim 5: recrystallized polymer

Claim 5 adds a further purification step in which the copolymer itself is recrystallized. It is the narrowest claim supplied. A polymer that uses purified monomers but is not itself recrystallized may fall outside claim 5 while remaining potentially relevant to claim 3 or claim 4.

How strong is the patent estate for US Patent 4,789,724?

The patent was technically important but is no longer legally enforceable because its term has expired.

Item	Assessment
Patent number	US 4,789,724
Technology	High-purity biodegradable polyanhydride copolymers
Claim types	Manufacturing method and polymer product
Apparent grant period	1988
Applicable US term	Generally 17 years from grant for a pre-1995 patent
Estimated expiration	December 2005
Current enforceability	Expired
Current Paragraph IV risk	None for this expired patent
Current biosimilar relevance	None
Historical commercial relevance	High for biodegradable implant and controlled-release polymer platforms

The patent’s historical strength came from its focus on process purity rather than a single drug or a single dosage form. A process that depended on separately purified monomers could have been difficult to reproduce consistently without adopting similar purification logic.

Its current legal strength is zero because an expired patent cannot support an infringement injunction or damages claim for post-expiration conduct. The patent may still be relevant as prior art against later patent applications, especially applications directed to purification, monomer preparation, or polyanhydride manufacturing.

When did US Patent 4,789,724 lose exclusivity?

US Patent 4,789,724 likely expired in December 2005 under the 17-year term measured from its grant date. The patent number indicates a 1988 grant, and patents filed before June 8, 1995 generally received the longer of 17 years from grant or 20 years from the earliest effective US nonprovisional filing date. For patents of this period, the 17-year grant-based term commonly controlled. [1]

No current patent term extension should be assumed for this process patent. Patent term extension under 35 U.S.C. § 156 is directed to regulatory review delays for eligible products, while patent term adjustment under 35 U.S.C. § 154 applies under different statutory rules. A manufacturing process patent of this age would not ordinarily provide present-day exclusivity.

What formulations are associated with this patent technology?

The chemistry is associated with biodegradable polyanhydride matrices used for controlled drug release. The most commercially recognized related platform is a copolymer based on:

1,3-bis(p-carboxyphenoxy)propane, often abbreviated CPP.
Sebacic acid, often abbreviated SA.

A commonly discussed composition is a CPP-SA copolymer in a high-sebacic-acid ratio, including the 20:80 composition associated with polifeprosan 20.

These materials can be processed into:

Intracranial wafers.
Biodegradable implants.
Microspheres.
Injectable or implantable matrices.
Localized drug-delivery depots.

The patent claims supplied do not themselves require carmustine, a wafer, intracranial administration, a particular CPP-SA ratio, or a particular drug payload. A product containing those features could fall within the historical technology area without falling within the expired claims.

What is the Orange Book status of US Patent 4,789,724?

US Patent 4,789,724 should not be treated as a current Orange Book barrier. The patent is expired and does not provide present-day exclusivity for an approved drug product.

For a drug-device combination such as a biodegradable wafer, current Orange Book analysis must be conducted at the product level. Relevant issues include:

Whether the product has an approved NDA.
Which patents were timely listed for the product.
Whether listed patents cover the drug substance, formulation, delivery system, method of use, or manufacturing process.
Whether any listed patent remains unexpired.
Whether an ANDA or 505(b)(2) applicant must make a Paragraph IV certification.

The expired 1980s process patent cannot independently support a current Paragraph IV challenge. Paragraph IV certifications apply to listed patents that are unexpired or otherwise relevant to the applicant’s certification obligations under the Hatch-Waxman framework. [2] The FDA Orange Book identifies approved drug products and listed patent information, but it does not revive an expired patent. [3]

Are there biosimilar risks for this patent?

No. Biosimilar law is not the relevant pathway because the claimed invention concerns synthetic polyanhydride chemistry rather than a biological product.

Competitive entry would more likely proceed through:

An abbreviated new drug application, if the product can qualify as a conventional generic drug.
A 505(b)(2) application for a modified dosage form, route, drug-delivery system, or local-release product.
A full NDA for a materially different combination product.
A medical-device or combination-product pathway, depending on the primary mode of action and FDA classification.

A polymer implant with a new drug, release profile, implantation method, or manufacturing process could face regulatory requirements beyond simple pharmaceutical equivalence.

Which companies historically commercialized related polyanhydride systems?

The commercial history of biodegradable polyanhydride drug delivery is associated with academic work from the Massachusetts Institute of Technology and commercialization efforts involving Guilford Pharmaceuticals. Guilford developed Gliadel, a biodegradable carmustine wafer for local treatment of malignant glioma. Gliadel was later associated with MGI PHARMA and Eisai through corporate transactions and product ownership changes. [4][5]

The relationship between those products and US Patent 4,789,724 must be analyzed carefully:

The patent’s chemistry is relevant to the manufacturing platform.
The patent does not, based on the supplied claims, claim carmustine specifically.
It does not claim the Gliadel wafer as a dosage form.
It does not claim the clinical use of the wafer.
Its expiration predates current commercial lifecycle management for the product.

Later patents could have covered composition ratios, drug-loaded wafers, dimensions, release characteristics, sterilization, packaging, implantation methods, or manufacturing controls. Those later patents, rather than US 4,789,724, would have been the more important barriers to a modern competing product.

What patent litigation affects this technology?

US Patent 4,789,724 cannot support a new US infringement action because it is expired. Litigation risk must therefore be evaluated against later patents covering:

Specific polyanhydride compositions.
Drug-polymer combinations.
Implant geometry.
Local administration.
Controlled-release profiles.
Sterilization and packaging.
Manufacturing and quality-control procedures.
Approved methods of treatment.

No current Paragraph IV or biosimilar litigation can be attributed to the expired patent itself. A historical infringement dispute could still be relevant for ownership, prosecution history, prior-art interpretation, or licensing analysis, but it would not create current exclusionary rights.

What licensing deals are associated with the patent platform?

The underlying polyanhydride technology originated in academic research and was commercialized through licensing and company formation around biodegradable drug-delivery systems. MIT-related technology was licensed into commercial development involving Guilford Pharmaceuticals and its successors. [4]

The precise license terms for US Patent 4,789,724 are not established by the claim text. Patent ownership, exclusive-license scope, royalty rates, field-of-use restrictions, and sublicensing rights must be taken from the relevant assignment records and license agreements. Patent expiration also does not automatically terminate every contractual obligation. A license may contain surviving know-how, confidentiality, milestone, royalty, or improvement provisions.

For current diligence, the patent has no standalone royalty value based on exclusionary patent rights. Any remaining value would derive from trade secrets, manufacturing know-how, regulatory history, validated processes, or contractual rights.

How does this patent compare with later drug-delivery patents?

Issue	US 4,789,724	Later product or formulation patents
Main protection	Purified monomer preparation and polymerization	Drug-polymer composition, dosage form, use, or release profile
Claim focus	Manufacturing sequence and purified intermediates	Commercial product characteristics
Drug-specific limitation	None apparent	Often present
Dosage-form limitation	None apparent	May include wafer, implant, microsphere, or depot
FDA exclusivity impact	Historical only	May affect ANDA or 505(b)(2) entry
Current enforceability	Expired	Depends on each patent’s expiration
Design-around potential	Change purification sequence or monomer chemistry	May require a different formulation, device, or clinical use
Biosimilar relevance	None	Usually none for synthetic polymers

The older patent is best understood as a platform manufacturing patent. Later patents are more likely to control a commercial product’s launch timing because they can attach directly to the approved formulation, delivery system, or method of treatment.

What generic launch scenarios exist?

Because US Patent 4,789,724 is expired, three principal launch scenarios remain:

Conventional generic pathway

A conventional generic may be possible if the product can demonstrate pharmaceutical equivalence, bioequivalence, equivalent dosage form, and comparable quality attributes. For a locally acting implant, conventional bioequivalence may be difficult to establish.

505(b)(2) pathway

A 505(b)(2) application may be more practical where the applicant changes:

The polymer composition.
Drug loading.
Release period.
Implant dimensions.
Route or site of administration.
Manufacturing process.
Drug substance or combination.

This pathway may require new clinical or pharmacokinetic evidence and may encounter later listed patents.

Full NDA or combination-product pathway

A materially different biodegradable implant or drug-delivery device may require a full NDA or combination-product submission. The regulatory burden would depend on whether the primary mode of action is pharmacological, device-based, or a combination of both. [6]

What manufacturing and IP barriers remain?

The expired patent does not eliminate practical barriers. A competing manufacturer may still need to solve:

Reproducible synthesis of high-purity anhydride monomers.
Control of moisture-sensitive intermediates.
Molecular-weight and polydispersity control.
Residual solvent and residual reagent limits.
Sterile processing or terminal sterilization.
Drug-loading uniformity.
In vitro and in vivo release correlation.
Stability of the polymer and drug during storage.
Implant handling and surgical placement.
Scale-up of recrystallization and purification.
Regulatory comparability for a complex local-delivery product.

These barriers can support trade-secret protection even where patent protection has expired. A manufacturer that follows the expired claimed sequence does not infringe the expired patent, but it may still need to avoid later patents and independently establish regulatory and quality performance.

Key Takeaways

US Patent 4,789,724 covers separate preparation and purification of anhydride monomers before copolymerization.
Claim 1 is a sequence-specific process claim with significant limitations on reagent handling and purification order.
Claims 3 through 5 extend protection to copolymers made from individually purified mixed anhydrides, with narrower claims for reflux, recrystallization, and polymer purification.
The listed chemistry includes sebacic acid, CPP, bis(carboxyphenoxy)hexane, isophthalic acid, adipic acid, and dodecanedioic acid.
The patent expired under the pre-1995 17-year US patent term, likely in December 2005.
It creates no current Paragraph IV, biosimilar, or injunction risk.
Current competition would be controlled by later patents, FDA requirements, manufacturing know-how, and product-specific formulation or device rights.
The technology is historically relevant to biodegradable polyanhydride systems and CPP-SA drug-delivery platforms associated with Gliadel.
The patent’s present commercial value is historical and technical, not exclusionary.

FAQs

Can a company use the purification process in US Patent 4,789,724 today?

Yes. The patent’s expiration removes its US patent-based infringement risk. The company must still evaluate later patents, contractual restrictions, and regulatory requirements.

Does using sebacic acid and CPP create infringement risk?

Not under the expired patent alone. The current risk depends on later unexpired patents covering the specific polymer ratio, drug payload, implant, release profile, manufacturing process, or treatment method.

Does claim 3 cover every copolymer made from purified mixed anhydrides?

No. Claim 3 contains limitations concerning the individual synthesis and purification of the monomers and the removal of residual diacid and acetic anhydride. The scope also depends on construction of "highly pure," "consisting essentially of," and "mixed anhydrides."

Could a polyanhydride implant qualify for an ANDA?

Possibly, but complex local-release implants may require a 505(b)(2) application or full NDA if equivalence cannot be demonstrated through conventional generic methods. The regulatory pathway depends on the product’s formulation, route, release mechanism, and FDA classification.

Does patent expiration eliminate the need for freedom-to-operate analysis?

No. It eliminates this patent as a current exclusionary right, but later composition, formulation, device, method-of-use, manufacturing, and packaging patents may still affect launch timing.

References

United States Patent and Trademark Office. (n.d.). Patent term adjustment and patent term calculation guidance. https://www.uspto.gov
United States Code. (2023). 35 U.S.C. § 355: New drugs.
U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations. https://www.fda.gov/drugs
U.S. Food and Drug Administration. (1996). Gliadel wafer prescribing information.
Eisai Co., Ltd. (2011). Annual report and product information relating to Gliadel.
U.S. Food and Drug Administration. (2022). Applications covered by section 505(b)(2). https://www.fda.gov/drugs-development-approval-process/ета

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	285432	⤷ Start Trial
Canada	1278141	⤷ Start Trial
Germany	266603	⤷ Start Trial
Germany	3752381	⤷ Start Trial
European Patent Office	0266603	⤷ Start Trial
European Patent Office	0692510	⤷ Start Trial
Japan	2626717	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 4,789,724

Summary for Patent: 4,789,724