United States Drug Patent 5,543,152: Claim Scope, Expiration, Litigation Risk, and Liposomal Patent Landscape

U.S. Patent No. 5,543,152 covers liposomes that use an acidic internal aqueous phase and a higher-pH external phase to load lipophilic therapeutic compounds, particularly vincristine, into sphingomyelin-cholesterol vesicles. The patent issued on August 6, 1996, and its enforceable term has expired. It does not currently block generic, branded, or follow-on liposomal products. Its technical disclosure remains relevant to liposomal vincristine products such as Marqibo, but later patents, regulatory exclusivity, manufacturing know-how, and product-specific patents determine current commercial risk.[1]

What does U.S. Patent 5,543,152 protect?

The patent protects two related categories:

A composition comprising a sphingomyelin-cholesterol liposome with an acidic interior and a lipophilic therapeutic compound.
A process-defined liposome made by forming the vesicle in an acidic buffer and then suspending it in a higher-pH external buffer to create a transmembrane pH gradient.

The central technical concept is remote or gradient-mediated loading. The pH differential drives a lipophilic drug from the external medium into the liposome. The patent is not limited to one drug, but the dependent claims narrow the commercial focus toward vincristine, vinblastine, and etoposide.[1]

Patent identification and legal status

Field	Information
Patent	U.S. Patent No. 5,543,152
Grant date	August 6, 1996
Technology	Acidic-interior liposomes with transmembrane pH-gradient loading
Principal membrane lipids	Sphingomyelin and cholesterol
Named drug examples	Vincristine, vinblastine, etoposide, and prodrugs
Key dosage form	Unilamellar liposomes
Claimed particle size	Approximately 0.05 to 0.45 microns; narrower claim of 0.05 to 0.2 microns
Claimed internal buffer	Citrate buffer at approximately pH 4.0
Drug-to-lipid ratio in vincristine claim	Approximately 0.01:1 to 0.2:1
Patent term	Expired
Current blocking effect	None from this patent alone

The claim set has no claim directed to a specific disease, patient population, dosing schedule, or manufacturing facility. Its commercial importance comes from the combination of lipid composition, pH gradient, particle characteristics, and therapeutic loading method.

How broad is claim 1 of U.S. Patent 5,543,152?

Claim 1 is the broadest composition claim. It requires all of the following:

A liposome intended to deliver a therapeutic compound to a mammalian host.
One or more membranes containing sphingomyelin and cholesterol.
An internal aqueous environment with a pH lower than the external environment.
A lipophilic therapeutic compound contained in the liposome.

The claim does not require:

Vincristine specifically.
A particular sphingomyelin source.
A specific cholesterol percentage.
A specific particle size.
A unilamellar structure.
Citrate as the internal buffer.
A particular administration route.
A specified cancer indication.

That structure makes claim 1 technically broad, but its practical value is limited by the expired patent term and by the need to prove every claim element in an accused product.

Claim 1 element-by-element analysis

Claim element	Scope and practical significance
Liposomal composition	Covers a product, not merely a manufacturing process
Delivery to a mammalian host	Links the composition to therapeutic use
Membrane includes sphingomyelin and cholesterol	Excludes liposomes using only phosphatidylcholine, PEG-lipid, or other membrane systems unless sphingomyelin and cholesterol are also present
Acidic interior	Requires the internal pH to be lower than the external pH
Lipophilic therapeutic compound	Requires drug partitioning or loading consistent with a lipophilic compound
Compound contained in the liposome	Free drug outside the vesicle is not enough

A liposome using sphingomyelin and cholesterol but lacking an acidic internal compartment would not meet claim 1. A liposome with an acidic interior but no sphingomyelin would also fall outside the literal scope. The patent therefore claims a defined formulation architecture rather than every pH-gradient liposome.

What formulations are protected by claims 2 through 13?

Claims 2 through 13 narrow claim 1 through formulation and product specifications. The strongest narrowing occurs in claims 3, 4, 7, 8, 10, 11, 12, and 13.

Lipid ratio claims

Claim 2 covers sphingomyelin-to-cholesterol ranges stated as 75/25 through 30/50 mol percent. Claim 3 narrows the range to 70/30 through 40/45 mol percent. Claim 4 identifies approximately 55/45 mol percent as a preferred composition.

The ratio language is unusual because the endpoints do not always read as conventional two-component totals of 100 mol percent. The most commercially relevant formulation point is the approximately 55/45 sphingomyelin/cholesterol ratio.

A product with materially different lipid ratios could avoid claims 2 through 4 while still potentially implicating claim 1 if it retains the required sphingomyelin, cholesterol, acidic interior, and lipophilic drug elements.

Drug-specific claims

Claim 5 identifies alkaloids. Claim 6 lists vincristine, vinblastine, etoposide, and prodrugs. Etoposide is pharmacologically an epipodophyllotoxin derivative rather than a classical alkaloid, but its inclusion in the claim creates an express species limitation.

Claim 7 narrows the drug to vincristine. Claim 8 adds a vincristine-to-lipid ratio of approximately 0.01:1 to 0.2:1.

These claims were commercially significant because liposomal vincristine became the principal clinical application of the disclosed loading technology. They are no longer enforceable because the patent has expired.

Additional lipid, size, and buffer claims

Claim 9 permits additional lipids, including phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylglycerol, phosphatidic acid, cardiolipin, phosphatidylinositol, ceramide, cerebroside, and ganglioside.

Claims 10 through 12 address vesicle structure and size:

Claim 10: unilamellar liposomes.
Claim 11: mean diameter of approximately 0.05 to 0.45 microns.
Claim 12: mean diameter of approximately 0.05 to 0.2 microns.

Claim 13 specifies citrate buffer at approximately pH 4.0. This claim provides a comparatively concrete formulation limitation and would have required analytical characterization of the internal buffer system.

What does claim 14 protect?

Claim 14 is a product-by-process claim directed to a liposome made by a defined loading process. The process requires:

Forming a liposome from sphingomyelin and cholesterol.
Using a first buffered aqueous solution with an acidic pH greater than pH 2.
Suspending the liposome in a second buffered solution with a higher pH.
Creating a transmembrane pH gradient.
Facilitating transfer of the lipophilic therapeutic compound into the liposome.
Producing a liposome with an internal pH from 2 to 6.

The claim is narrower than claim 1 in process terms but potentially important analytically. Product-by-process claims are generally assessed according to the product characteristics, although the process limitations can affect claim construction and infringement analysis depending on the jurisdiction and factual record.

Claim 15 adds separation of loaded liposomes from unentrapped drug. This is a conventional purification step, but its inclusion narrows the process further.

When did U.S. Patent 5,543,152 lose exclusivity?

The patent lost exclusivity after expiration of its statutory term. Because it is a pre-Uruguay Round Agreements Act patent, the applicable term was generally 17 years from grant, subject to any terminal disclaimer or patent-term adjustment. On that basis, the term ended on or about August 6, 2013.[1,2]

Milestone	Date or status
Patent issued	August 6, 1996
17-year grant term	Approximately August 6, 2013
Current enforceability	Expired
Current Paragraph IV relevance	None for this patent
Current Orange Book blocking effect	None from this patent
Public-domain status of disclosed formulation concept	Yes, subject to later patents

Expiration of U.S. Patent 5,543,152 does not invalidate later patents covering a specific product, manufacturing process, formulation excipient, dosage regimen, or device. A follow-on product can therefore be free of this patent while still facing later patent claims.

What is the Orange Book status of U.S. Patent 5,543,152?

U.S. Patent 5,543,152 is not a current Orange Book barrier. The Orange Book lists patents submitted by approved applicants for approved drug products, but an expired foundational technology patent does not create a continuing statutory stay or launch prohibition under Hatch-Waxman.[3]

For liposomal vincristine, the relevant Orange Book analysis must be performed against the approved product and its currently listed patents. The existence of a historical formulation patent does not establish that every later liposomal vincristine product has the same patent profile.

Orange Book implications

The patent cannot support a new 30-month stay.
A Paragraph IV certification against this patent would have no commercial effect today.
An abbreviated new drug application applicant would evaluate currently listed product patents, not an expired foundational patent.
If no appropriate generic reference product exists, a product may require a different FDA pathway rather than a conventional ANDA.

Which companies and products are most relevant?

The main commercial relevance is to liposomal vincristine. Marqibo is vincristine sulfate liposome injection, approved by the FDA in 2012 for adults with Philadelphia chromosome-negative acute lymphoblastic leukemia in second or greater relapse or progressive disease after two or more anti-leukemia therapies.[4]

Commercial product comparison

Product	Active ingredient	Liposomal technology	FDA status	Relationship to U.S. 5,543,152
Marqibo	Vincristine sulfate	Sphingomyelin-cholesterol liposome with gradient loading	FDA-approved	Technically aligned with the claimed platform, but later patents and product-specific rights are more relevant
Doxil	Doxorubicin hydrochloride	PEGylated liposome; ammonium sulfate remote loading	FDA-approved	Uses a different principal lipid and loading architecture
Onivyde	Irinotecan liposome injection	PEGylated liposome	FDA-approved	Different drug, lipid system, and commercial patent estate
Generic conventional vincristine	Vincristine sulfate	Non-liposomal injectable solution	FDA-approved generics exist	Does not practice the claimed liposomal formulation

Marqibo's formulation and manufacturing controls can create regulatory and technical barriers even when the foundational patent is expired. The key barriers include vesicle size distribution, encapsulation efficiency, drug-to-lipid ratio, release profile, sterility, aggregate control, and batch reproducibility.[4]

How does this patent compare with later liposomal drug patents?

U.S. Patent 5,543,152 occupies an earlier generation of liposome technology. Its principal distinctions are:

Sphingomyelin and cholesterol as core membrane components.
An acidic internal buffer.
A higher-pH external medium.
Loading of lipophilic or weakly basic therapeutic compounds.
Emphasis on vincristine and related agents.

Later liposomal patents commonly focus on:

PEGylated lipids and prolonged circulation.
Ammonium sulfate or other ion gradients.
Drug-specific loading and release behavior.
Surface ligands or targeting molecules.
Lyophilized formulations.
Stabilizers and cryoprotectants.
Manufacturing scale-up and sterile filtration.
Dosing schedules and therapeutic combinations.
Product-specific quality attributes.

Comparative patent landscape

Technology category	Typical claim focus	Relationship to 5,543,152
Sphingomyelin-cholesterol liposomes	Membrane composition and stability	Directly overlaps the patent's technical core, but the patent is expired
Acidic pH-gradient loading	Internal/external pH differential	Directly related to claims 1 and 14
Ammonium sulfate loading	Ionic gradient and drug precipitation	Adjacent technology, often outside the express claim language
PEGylated liposomes	Long circulation and reduced clearance	Usually distinct from the claimed sphingomyelin-centered platform
Liposomal vincristine	Drug-specific formulation and release	Later patents may remain relevant to commercial products
Manufacturing processes	Mixing, extrusion, sterilization, loading, and purification	Can create current freedom-to-operate risk independent of the expired patent
Method-of-use claims	Cancer indications, dosing, and combinations	Not claimed by U.S. 5,543,152

Are there Paragraph IV challenges involving this patent?

No current Paragraph IV risk attaches to U.S. Patent 5,543,152 because the patent has expired. Historical Paragraph IV activity, if any, would no longer support a launch stay or enforceable exclusionary right.

The relevant current question for liposomal vincristine is whether an applicant must address later-listed patents associated with the reference product. Those patents may involve:

The composition of the liposomal vincristine product.
A method of treating acute lymphoblastic leukemia.
A dose or administration schedule.
Manufacturing or packaging.
Stability or reconstitution.
Product-specific quality attributes.

A generic or follow-on developer should not treat the expiration of U.S. 5,543,152 as proof that all Marqibo-related patent risk has ended.

What litigation and settlement risk affects the patent?

No active litigation risk should be attributed to U.S. Patent 5,543,152 itself because the patent is expired. The patent cannot support an injunction against a current entrant.

Potential disputes involving a liposomal vincristine product would instead center on later patents and regulatory issues:

Patent-listing disputes in the Orange Book.
Declaratory-judgment actions concerning later formulation patents.
Hatch-Waxman litigation following a Paragraph IV certification.
Obviousness challenges to later liposomal compositions.
Infringement disputes over manufacturing parameters.
Regulatory disputes over whether a proposed product is pharmaceutically equivalent to the reference product.

No settlement agreement involving this expired patent has present exclusionary force.

How strong is the patent estate for the underlying technology?

The historical patent was strong as a platform patent because claim 1 combined a recognizable lipid system with a functional pH-gradient requirement and a therapeutic payload. The dependent claims gave the owner positions around vincristine, lipid ratios, vesicle size, citrate buffer, and loading ratio.

Its present legal strength is zero because the patent term has ended. Its technical strength remains moderate for prior-art and freedom-to-operate analysis:

The pH-gradient loading concept is well established.
The sphingomyelin-cholesterol combination is specifically disclosed.
Vincristine loading is expressly claimed.
Later applicants must account for the patent as prior art.
The patent can limit the scope of later patent claims through anticipation or obviousness arguments.

The patent is therefore historically important but commercially nonblocking.

What generic launch risks exist?

A conventional vincristine injection is materially different from a liposomal vincristine product. A liposomal entrant may face the following barriers:

FDA pathway uncertainty. A liposomal product may not qualify as a simple generic substitution if its formulation, release, distribution, or pharmacokinetics differ materially.
Product-specific patents. Later patents may remain active even though U.S. 5,543,152 has expired.
Manufacturing complexity. Encapsulation, size control, sterility, and release testing require specialized capability.
Clinical and pharmacology requirements. FDA may require comparative pharmacokinetic, bioequivalence, or clinical evidence depending on the proposed pathway.
Limited market size. A narrow oncology indication can reduce the commercial incentive for entry.
Biosimilar irrelevance. Marqibo is a small-molecule liposomal drug, not a biologic. Biosimilar procedures under the Public Health Service Act do not apply.[5]

Generic entry scenarios

Scenario	Patent risk	Regulatory risk	Commercial assessment
Conventional vincristine injection	Low from 5,543,152	Established ANDA pathway	Existing generic competition
Liposomal vincristine equivalent	Low from 5,543,152; potentially material from later patents	High relative to conventional vincristine	Requires specialized development
New liposomal vincristine formulation	Low from 5,543,152; new formulation patents possible	Potentially substantial	Could obtain new intellectual property and exclusivity
Non-vincristine liposomal alkaloid	No blocking risk from expired patent	Product-specific	Depends on drug and indication

What manufacturing and geographic IP barriers remain?

U.S. Patent 5,543,152 no longer creates geographic protection in the United States. The same is likely true in major jurisdictions if corresponding foreign patents expired on comparable timelines, but foreign family members must be reviewed individually.

Current geographic risk may arise from:

Later U.S. continuation patents.
European, Canadian, Japanese, or other national family members with different term calculations.
Supplementary protection certificates in Europe.
Patent-term adjustments or extensions.
National patents directed to Marqibo-specific manufacturing.
Trade secrets covering scale-up, sterile processing, and release testing.

The expired U.S. patent does not place the complete manufacturing process in the public domain if later process patents or confidential know-how remain active.

Key Takeaways

U.S. Patent 5,543,152 claims sphingomyelin-cholesterol liposomes with an acidic interior and a higher-pH exterior.
Its principal commercial application is liposomal loading of vincristine and related therapeutic compounds.
Claims 2 through 13 narrow the invention by lipid ratio, drug identity, drug-to-lipid ratio, vesicle size, lamellarity, and citrate buffer.
Claims 14 and 15 cover a pH-gradient loading and separation process.
The patent issued August 6, 1996, and expired approximately August 6, 2013.
It has no current Orange Book blocking effect and cannot support a current Paragraph IV stay.
Marqibo-related patent risk must be analyzed against later product-specific patents, not this expired patent.
Biosimilar law does not apply because liposomal vincristine is not a biologic.
Manufacturing complexity, regulatory comparability, and later patents remain the principal barriers to a liposomal vincristine entrant.

FAQs About U.S. Patent 5,543,152

Does U.S. Patent 5,543,152 cover Doxil?

No. Doxil uses a PEGylated liposomal doxorubicin platform and a different principal formulation architecture. The expired patent's claims focus on sphingomyelin-cholesterol liposomes with an acidic interior and lipophilic drug loading.

Can a company practice the vincristine formulation disclosed in U.S. 5,543,152?

Yes, this patent no longer prevents practice in the United States. A company must still assess later patents, FDA requirements, trade secrets, and foreign rights.

Is etoposide properly classified as an alkaloid in claim 6?

Etoposide is generally classified as a semisynthetic epipodophyllotoxin derivative rather than a classical alkaloid. Its express inclusion in claim 6 remains relevant to claim interpretation because the patent lists it as a claimed therapeutic compound.

Does the patent cover every liposome with an acidic internal pH?

No. The claims require sphingomyelin and cholesterol in the membrane and a lipophilic therapeutic compound. Liposomes using different membrane systems or different drug-loading mechanisms may fall outside the literal claim language.

Does patent expiration eliminate all intellectual-property risk for Marqibo?

No. Expiration eliminates the blocking effect of U.S. Patent 5,543,152. It does not eliminate later formulation, method-of-use, manufacturing, packaging, trademark, regulatory-exclusivity, or foreign patent risks.

References

United States Patent and Trademark Office. (1996). Liposomal composition for delivery of therapeutic compounds, U.S. Patent No. 5,543,152.
United States Patent and Trademark Office. (2024). Patent term adjustment and patent term expiration guidance. U.S. Department of Commerce.
U.S. Food and Drug Administration. (2024). Approved drug products with therapeutic equivalence evaluations: Orange Book. Center for Drug Evaluation and Research.
U.S. Food and Drug Administration. (2023). Marqibo (vincristine sulfate liposome injection) prescribing information. FDA Center for Drug Evaluation and Research.
U.S. Food and Drug Administration. (2024). Biosimilar and interchangeable biosimilar products. FDA Center for Drug Evaluation and Research.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	248586	⤷ Start Trial
Australia	2709495	⤷ Start Trial
Canada	2193502	⤷ Start Trial
Germany	69531701	⤷ Start Trial
European Patent Office	0804159	⤷ Start Trial
Spain	2206510	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,543,152

Summary for Patent: 5,543,152