United States Patent 3,728,382: Scope, Claims, Expiration, and Carboprost Patent Landscape

U.S. Patent No. 3,728,382 covers 15-methyl prostaglandin F2α compounds, including optically active and racemic 15-methyl-PGF2α. The patent was granted on April 17, 1973, and its United States term expired no later than April 17, 1990, under the pre-Uruguay Round patent-term regime. It is therefore no longer an enforceable barrier to manufacture, marketing, or use of carboprost or its related 15-methyl-PGF2α compounds in the United States. The patent is historically important because 15-methyl-PGF2α is the prostaglandin analogue later commercialized as carboprost, including carboprost tromethamine in Hemabate. [1]

What drug and chemical subject matter does U.S. Patent 3,728,382 cover?

U.S. Patent 3,728,382 is directed to 15-methyl prostaglandins, including 15-methyl-PGF2α. The commercial pharmaceutical product most closely associated with this chemistry is carboprost, also identified as 15-methyl prostaglandin F2α.

Carboprost is a synthetic prostaglandin analogue. Carboprost tromethamine is the tromethamine salt used in the approved injectable product Hemabate. The active prostaglandin moiety, rather than the tromethamine counterion, is the central subject of the original compound claims.

The supplied claim text identifies two relevant claim categories:

An optically active 15-methyl-PGF2α compound in which R1 is hydrogen and R2 is methyl.
Racemic 15-methyl-PGF2α with the same R1 and R2 substitutions.

The chemical notation in the supplied text is incomplete because the Greek alpha and claim numbering appear to have been lost in transcription. The patent’s full specification and claim 1 are necessary to reconstruct every variable in the generic structural formula. The identified species, however, is sufficiently clear: the claims target the 15-methyl substitution pattern on the prostaglandin F2α framework.

What are the key claims of U.S. Patent 3,728,382?

The disclosed claims distinguish between stereochemically defined and racemic forms of 15-methyl-PGF2α.

Claim subject	Structural limitation	Commercial relevance
Optically active 15-methyl-PGF2α	R1 = hydrogen; R2 = methyl; optically active prostaglandin compound	Closely corresponds to the active carboprost stereochemical framework
Racemic 15-methyl-PGF2α	R1 = hydrogen; R2 = methyl; racemic mixture	Covers the corresponding non-single-enantiomer form
Generic claim 1	Parent claim defining the broader prostaglandin structural formula	Determines the dependency and interpretive scope of the species claims

The optically active claim is narrower in molecular configuration but potentially stronger against a product using the claimed stereochemical form. The racemic claim addresses a mixture containing stereoisomers and prevents avoidance merely by producing the same substitution pattern without resolving the stereochemistry.

How does the optical-activity limitation affect claim scope?

An optically active claim generally requires the claimed stereochemical form or a composition that falls within the claimed stereochemical definition. A racemic claim reaches a racemic mixture rather than only one enantiomer or diastereomer.

For a modern carboprost product, the principal infringement question would have been whether the marketed active ingredient had the claimed 15-methyl-PGF2α structure and stereochemistry. The answer would have turned on the product’s chemical identity, not on the presence of the tromethamine salt alone.

The claims do not appear, from the supplied text, to be limited to:

A particular vial or syringe;
A specific concentration;
A particular excipient system;
Intramuscular or intramyometrial administration;
Treatment of postpartum hemorrhage;
A manufacturing process; or
A commercial product name.

Those limitations would have required separate claims or a narrower construction based on the specification.

When did U.S. Patent 3,728,382 expire?

The patent was granted on April 17, 1973. At that time, United States utility patents generally ran for 17 years from grant. The resulting ordinary expiration date was April 17, 1990. [1]

Event	Date
U.S. patent grant	April 17, 1973
Applicable term regime	17 years from grant
Estimated statutory expiration	April 17, 1990
Current enforceability	Expired
Current blocking effect	None as an enforceable U.S. patent right

The 1995 change to a 20-year term from the earliest effective nonprovisional filing date did not retroactively extend an already expired patent. The Uruguay Round Agreements Act contained transition rules, but those rules did not revive the expired term of this patent.

Does the patent still create generic entry risk?

No. U.S. Patent 3,728,382 cannot support a present-day infringement action because its term expired more than three decades ago. A generic manufacturer does not face current patent damages or injunctive exposure under this patent.

The patent may still matter for:

Historical freedom-to-operate analyses;
Prior-art mapping;
Patent-family reconstruction;
Validity assessments of later continuation or improvement patents;
Determining the origin of carboprost chemistry; and
Evaluating whether later patents claim genuinely new subject matter.

It has no remaining blocking value by itself.

What is the FDA regulatory status of carboprost?

Carboprost tromethamine is associated with Hemabate, an injectable drug approved by FDA for certain obstetric indications, including control of postpartum hemorrhage caused by uterine atony that has not responded to conventional management. The product is administered parenterally and is not a biologic. [2]

The FDA regulatory framework is separate from the expired patent right.

Regulatory issue	Status
Active ingredient	Carboprost, a 15-methyl-PGF2α analogue
Commercial salt	Carboprost tromethamine
Dosage form	Sterile injectable solution
Product category	Small-molecule prescription drug
Reference product association	Hemabate
FDA pathway for a generic	Abbreviated New Drug Application, if an applicable reference product and product-specific requirements are satisfied
Biologic pathway	Not applicable
Biosimilar pathway	Not applicable

Carboprost is a small molecule. A competing product would generally proceed through the generic-drug framework rather than the biosimilar pathway under the Public Health Service Act.

What is the Orange Book status of U.S. Patent 3,728,382?

U.S. Patent 3,728,382 is not a current Orange Book patent barrier. Its 1990 expiration predates the modern commercial relevance of the Orange Book listing system for current drug-product exclusivity disputes.

An Orange Book listing, where present, would relate to an approved drug product and a patent that claims the drug substance, drug product, or an approved method of use. An expired 1973 composition patent cannot prevent FDA approval of a later generic product.

For carboprost, the relevant regulatory questions are more likely to concern:

Whether an FDA-listed reference product remains available;
Whether current patents cover the formulation or delivery system;
Whether a generic applicant must file a Paragraph IV certification against any listed patent;
Whether the reference product is discontinued or commercially available;
Whether the applicant qualifies for a suitability petition or another abbreviated pathway; and
Whether FDA has current product-specific guidance.

The original patent does not create a current Paragraph IV filing obligation.

Are there Paragraph IV challenges or generic litigation associated with this patent?

No current Paragraph IV challenge can be directed at U.S. Patent 3,728,382 because the patent expired in 1990. A Paragraph IV certification is relevant only to an unexpired patent listed in the FDA Orange Book.

The patent therefore cannot support:

A new Hatch-Waxman infringement suit;
A 30-month stay based on its listing;
A statutory first-filer exclusivity dispute;
A patent settlement restricting generic launch; or
An injunction against a current carboprost applicant.

Historical litigation involving carboprost products could have concerned other patents, regulatory exclusivity, manufacturing technology, or product-specific claims. Such disputes would not extend the term of Patent 3,728,382.

What formulations are protected by the patent?

The identified claims are composition claims to the prostaglandin compound. They do not, based on the supplied claim language, expressly claim a formulation.

A formulation claim would normally recite elements such as:

Carboprost or carboprost tromethamine;
A specified concentration;
An aqueous vehicle;
A buffer;
An antioxidant;
A stabilizer;
A pH range;
A container or closure system; or
A defined impurity or degradation profile.

The patent’s compound claims could have reached a formulation containing the claimed active compound, depending on claim construction and the presence of other claim limitations. They would not automatically establish a separate monopoly over every formulation, container, concentration, or administration device.

This distinction matters because later pharmaceutical patents often focus on formulation stability, sterilization, packaging, concentration control, or delivery technology after the active compound has entered the public domain.

Does the patent cover methods of use?

The supplied claims do not identify a therapeutic method-of-use limitation. They recite the chemical compound.

A compound claim can cover the compound regardless of whether it is used in obstetrics, gynecology, reproductive medicine, or another medically relevant setting. A method-of-use patent, by contrast, would need to claim administering the compound for a defined indication, patient population, dose, route, or treatment outcome.

Accordingly, Patent 3,728,382 should be classified as a foundational composition patent, not as a postpartum-hemorrhage method patent.

What manufacturing and intellectual-property barriers remain after expiration?

The original compound patent does not block manufacture. Commercial entry may still require control of technical and regulatory risks.

Chemical manufacturing

Potential manufacturing issues include:

Stereochemical control at the 15-methyl position;
Protection and deprotection of prostaglandin functional groups;
Control of geometric isomers;
Separation or limitation of racemic and diastereomeric impurities;
Oxidation and degradation of the prostaglandin side chains;
Reproducible salt formation with tromethamine; and
Analytical methods capable of distinguishing closely related prostaglandin impurities.

These are technical barriers, not continuing rights under Patent 3,728,382.

Product manufacturing

An injectable carboprost product may require:

Sterile manufacturing;
Validated aseptic processing;
Container-closure integrity;
Stability data;
Control of visible and subvisible particles;
Extractables and leachables evaluation;
Tight assay and impurity specifications; and
Demonstration of pharmaceutical equivalence.

A manufacturer may face later patents covering one or more of these features, but those rights must be evaluated separately from the 1973 compound patent.

How strong is the patent estate for carboprost?

The original estate was strong during its active term because it claimed the core 15-methyl-PGF2α molecule. Its commercial value was based on compound-level exclusion rather than a narrow formulation or method claim.

Its present strength is zero as an enforceable patent right.

Patent-estate factor	Assessment
Core active-compound coverage	Historically strong
Stereochemical coverage	Meaningful, with separate optical and racemic treatment
Formulation coverage	Not established from supplied claims
Method-of-use coverage	Not established from supplied claims
Manufacturing-process coverage	Not established from supplied claims
Current term	Expired
Current litigation leverage	None under this patent
Biosimilar relevance	None
Generic entry barrier	None from this patent

The commercial protection profile therefore shifted from compound exclusivity to regulatory execution, manufacturing quality, supply reliability, and any later patent families.

What licensing deals or settlement agreements are tied to this patent?

The available patent and regulatory information does not establish a current licensing obligation or settlement agreement tied to U.S. Patent 3,728,382. Because the patent expired in 1990, any historical license would no longer create a patent-term restriction on ordinary U.S. manufacture or sale.

A separate contractual restriction could theoretically survive patent expiration, but no such restriction can be inferred from the patent claims or patent record alone. Patent ownership and licensing history should not be confused with current enforceability.

How does carboprost compare with other prostaglandin drugs?

Product or active ingredient	Relationship to carboprost	Patent and regulatory profile
Carboprost	15-methyl-PGF2α analogue	Original compound patent expired
Carboprost tromethamine	Salt form used in Hemabate	Small-molecule injectable product
Dinoprostone	Prostaglandin E2	Different active compound and patent history
Misoprostol	Synthetic PGE1 analogue	Different structure, oral and vaginal uses
Gemeprost	Prostaglandin analogue	Different compound and jurisdictional history
Dinoprost	PGF2α	Parent prostaglandin with different substitution pattern

The 15-methyl substitution distinguishes carboprost from native PGF2α and gives the molecule a separate chemical identity. A competitor using dinoprostone, misoprostol, or another prostaglandin would not practice the 15-methyl-PGF2α compound claims merely because the products share a prostaglandin pharmacologic class.

What generic launch scenarios exist?

Three commercial scenarios are relevant:

A manufacturer launches a carboprost product after satisfying FDA abbreviated-application requirements and without facing an unexpired Orange Book patent.
A manufacturer develops a product using a different salt, formulation, or presentation and must establish the applicable pharmaceutical and bioequivalence pathway.
A manufacturer encounters later product-specific patents or regulatory obstacles unrelated to Patent 3,728,382.

The original compound patent does not delay any of these scenarios. A generic applicant would not need to wait for its expiration because the patent has already been expired since 1990.

Geographic coverage and international relevance

U.S. Patent 3,728,382 has territorial effect only in the United States. Its expiration does not determine the status of corresponding foreign patents.

A global carboprost freedom-to-operate review would need to examine:

Patent families claiming 15-methyl prostaglandins;
National-phase filings;
Patent-term adjustments or supplementary protection certificates;
Foreign formulation patents;
Manufacturing-process patents;
Regulatory exclusivity in each market; and
Local requirements for injectable generic approval.

The U.S. expiration date cannot be applied automatically to Europe, Canada, Japan, China, or other jurisdictions.

Key Takeaways

U.S. Patent 3,728,382 covers 15-methyl-PGF2α, including optically active and racemic forms identified by R1 = hydrogen and R2 = methyl.
The patent is a foundational composition patent associated with carboprost chemistry.
It was granted on April 17, 1973, and expired on April 17, 1990.
It does not create a current U.S. patent barrier to carboprost manufacture, sale, or use.
The supplied claims do not establish separate formulation, method-of-use, or manufacturing-process protection.
Carboprost is a small-molecule drug. Biosimilar analysis is not applicable.
Any current generic risk must arise from later patents, FDA requirements, manufacturing controls, supply constraints, or commercial factors.
The patent’s historical scope should not be confused with the current Orange Book or Hatch-Waxman position.

FAQs

Is carboprost the same drug as 15-methyl-PGF2α?

Carboprost is the nonproprietary name associated with 15-methyl prostaglandin F2α. Carboprost tromethamine is the salt form used in the Hemabate injectable product.

Can an expired compound patent still block a generic drug application?

No. An expired patent cannot support a Paragraph IV lawsuit, a 30-month stay, or an injunction. FDA approval may still require compliance with applicable product, manufacturing, and bioequivalence requirements.

Does carboprost require a biosimilar application?

No. Carboprost is a chemically synthesized small molecule. A competing product would generally use the generic-drug framework rather than the biosimilar pathway.

Do the claims cover carboprost tromethamine?

The compound claims may reach the carboprost active moiety, depending on their precise construction. They do not necessarily claim every salt, formulation, or dosage form as a separate invention. In any event, the patent expired in 1990.

Does U.S. Patent 3,728,382 cover postpartum hemorrhage treatment?

The supplied claims are compound claims, not method-of-use claims. They do not expressly limit the invention to postpartum hemorrhage, uterine atony, or any other clinical indication.

References

U.S. Patent No. 3,728,382, 15-Methyl prostaglandins (issued Apr. 17, 1973), United States Patent and Trademark Office.
U.S. Food and Drug Administration. (n.d.). Hemabate (carboprost tromethamine) injection prescribing information. FDA-approved product labeling.
U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations. FDA Orange Book.
United States Code, 35 U.S.C. §§ 154, 271, and 282.
United States Code, 21 U.S.C. § 355(j).

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Argentina	193032	⤷ Start Trial
Belgium	766682	⤷ Start Trial
Canada	959485	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 3,728,382

Summary for Patent: 3,728,382