US Patent 5,814,335: Claim Scope, Expiration, Orange Book Status, and Liposomal Vincristine Patent Landscape

US Patent 5,814,335 covers a pH-gradient loading process for putting alkaloid drugs, principally vincristine, into liposomes made from sphingomyelin and cholesterol. Its core limitations are the lipid composition, acidic internal buffer, higher-pH external buffer, transmembrane pH gradient, and defined vesicle characteristics. The patent is expired and does not create a current US exclusionary barrier to generic or competing liposomal vincristine development.

What does US Patent 5,814,335 protect?

The patent protects a liposome produced using an active-loading process rather than merely a liposome containing vincristine.

The central process requires:

Forming a liposome from sphingomyelin and cholesterol.
Using a first buffered aqueous solution with an acidic pH above pH 2.
Suspending the liposome in a second buffered solution with a higher pH.
Creating a transmembrane pH gradient.
Using that gradient to facilitate transfer of an alkaloid therapeutic compound into the liposome.

The patent is directed most clearly to liposomal vincristine technology later commercialized in Marqibo, a vincristine sulfate liposome injection approved by the FDA in 2012. The claims also identify swainsonine, but swainsonine has no comparable commercial product significance in the FDA-approved oncology market.

Patent identification

Item	Data
Patent	US 5,814,335
Title	Liposome compositions for delivery of alkaloid therapeutic compounds
Issue date	September 29, 1998
Technology	pH-gradient loading of alkaloid drugs into liposomes
Principal drug disclosed	Vincristine
Other claimed drug	Swainsonine
Key lipids	Sphingomyelin and cholesterol
Key internal buffer	Citrate, approximately pH 4.0
Claimed vesicle form	Unilamellar liposomes
Claimed diameter	Approximately 0.05 to 0.45 microns, with a narrower range of 0.05 to 0.2 microns
FDA product association	Marqibo, vincristine sulfate liposome injection
Current US enforceability	Expired

The patent should be analyzed as a process-defined composition patent. Claim 1 begins with "A liposome" but defines the claimed article by the process used to produce it. That language creates a product-by-process claim structure.

How broad is independent claim 1?

Claim 1 is the controlling claim. It contains both composition and manufacturing limitations.

A competing product would generally need to address each of the following elements:

Claim element	Scope
Liposome	The claimed end product must be a liposome
Alkaloid therapeutic compound	The payload must fall within the claim's alkaloid category
Sphingomyelin	Required lipid component
Cholesterol	Required lipid component
First buffered aqueous solution	Required during liposome formation
First solution pH	Acidic and greater than pH 2
Second buffered solution	Required for the suspension step
pH differential	Second solution must have a higher pH than the first
Transmembrane gradient	The process must form a pH gradient across the liposome membrane
Loading function	The gradient must facilitate transfer of the drug into the liposome

The claim does not require vincristine specifically. It reaches any qualifying alkaloid therapeutic compound, subject to the claim's other limitations. Claims 5 through 7 narrow the compound to vincristine or swainsonine.

The claim also does not expressly require a particular phospholipid other than sphingomyelin, a particular external buffer, a specific manufacturing apparatus, or a particular drug-to-lipid ratio. Those omissions leave room for design differences in buffer identity, processing equipment, concentration, and loading conditions, provided the claimed pH-gradient process is still practiced.

What do claims 2 through 12 add?

The dependent claims narrow the technology through purification, lipid ratio, payload, morphology, particle size, and internal pH.

Claim	Limitation	Practical significance
2	Separating drug-loaded liposomes from unentrapped drug	Covers a post-loading purification step
3	Cholesterol at 30% to 50% of total lipid molar proportion	Narrows membrane composition
4	Sphingomyelin/cholesterol ratio of about 55/45 mol%	More specific formulation limitation
5	Alkaloid is vincristine or swainsonine	Narrows the payload
6	Alkaloid is vincristine	Core commercial product limitation
7	Alkaloid is swainsonine	Narrow, commercially less important branch
8	Liposomes are unilamellar	Excludes multilamellar vesicles
9	Mean diameter of about 0.05 to 0.45 microns	Broad size limitation
10	Mean diameter of about 0.05 to 0.2 microns	Narrower size limitation
11	Liposome interior at pH 2 to pH 5	Narrows the internal aqueous phase
12	Interior contains citrate buffer at approximately pH 4.0	Most specific internal-buffer limitation

Claims 3, 4, 8, 9, 10, 11, and 12 are formulation and product-profile claims layered onto the process of claim 1. They are commercially significant because a liposomal vincristine product may use the same general lipid and pH-gradient architecture even if it attempts to avoid a broader process limitation.

Does the patent cover Marqibo?

Marqibo is a liposomal formulation of vincristine sulfate for the treatment of Philadelphia chromosome-negative acute lymphoblastic leukemia. The product uses a sphingomyelin-based liposome platform and is manufactured through active loading of vincristine using a pH gradient.

The FDA approved Marqibo under NDA 203189 on August 9, 2012. The product was originally associated with Talon Therapeutics and later commercialized through Spectrum Pharmaceuticals and subsequent hematology-focused owners. FDA labeling identifies Marqibo as a single-dose, preservative-free liposomal vincristine sulfate injection. [1]

The formulation described in the product labeling is closely aligned with the technical limitations of US 5,814,335, particularly:

sphingomyelin and cholesterol membrane components;
an acidic internal aqueous phase;
active loading using a transmembrane pH gradient;
a defined liposome size profile; and
vincristine as the encapsulated alkaloid payload.

The patent is therefore best understood as foundational technology for the commercial liposomal vincristine platform, not as a broad patent on all liposomal oncology products.

What is the Orange Book status of US 5,814,335?

US 5,814,335 has been listed in connection with Marqibo in the FDA Orange Book patent records. The Orange Book listing historically provided notice of patent rights asserted against an approved drug product and supported a potential Paragraph IV certification by an ANDA applicant. [2]

The listing does not extend the patent's enforceable term. Orange Book inclusion is an FDA regulatory record, not a determination that the patent remains valid or enforceable.

The relevant legal distinction is:

FDA approval links the patent to an approved drug product.
The patent's term determines whether the patent can block commercial activity.
An expired Orange Book patent cannot support an injunction against a new generic launch.
A patent listing does not establish infringement or validity.

The patent's ordinary US term expired years ago. Its status is therefore historical from a commercial-exclusivity perspective.

When did US Patent 5,814,335 lose exclusivity?

The patent issued in 1998 but patent expiration is generally calculated from the earliest effective nonprovisional filing date or applicable international filing date, not from the issue date. The patent's base statutory term ended approximately 20 years after its earliest effective filing date. USPTO records identify the patent as expired. [3]

The practical consequence is that US 5,814,335 no longer prevents:

manufacture of a liposomal vincristine product;
use of the claimed pH-gradient loading process;
sale of a product that would previously have fallen within the claims; or
submission of an ANDA relying on the patent's prior Orange Book listing.

Any patent-term extension or pediatric exclusivity would need to be reflected in the specific USPTO and FDA records. The patent is not a current blocking patent in the United States.

What Paragraph IV risks did US 5,814,335 create?

Before expiration, an ANDA applicant seeking approval for a generic or follow-on version of Marqibo could have used a Paragraph IV certification alleging that the patent was invalid, unenforceable, or not infringed. A Paragraph IV notice could have triggered patent litigation under the Hatch-Waxman framework and a 30-month FDA approval stay, subject to the statutory conditions in effect at the time.

The most credible Paragraph IV strategies would have included:

Noninfringement arguments

A challenger could argue that its process does not use:

sphingomyelin;
a first acidic buffer and second higher-pH buffer;
a transmembrane pH gradient;
vincristine or another claimed alkaloid;
a unilamellar vesicle;
the specified cholesterol range; or
the claimed citrate and particle-size limitations.

The strongest noninfringement position would ordinarily involve a process that loads vincristine without the claimed pH differential or uses a materially different lipid system.

Invalidity arguments

Potential validity challenges could have focused on:

anticipation by earlier liposome-loading references;
obviousness based on active loading using pH gradients;
lack of written description for the full alkaloid genus;
enablement across all alkaloid therapeutic compounds;
indefiniteness of terms such as "about," "facilitates," and "mean diameters"; and
product-by-process claim interpretation.

The dependent claims are narrower and may have presented stronger validity positions where the specification disclosed specific sphingomyelin/cholesterol ratios, citrate buffer conditions, and vincristine loading data.

Because the patent has expired, those issues no longer present a forward-looking US launch barrier except in unusual circumstances involving historical damages or pre-expiration conduct.

What patent families are related to US 5,814,335?

The patent belongs to a broader liposomal drug-delivery family involving sphingomyelin-containing membranes and active loading of weakly basic compounds. Related US continuation or family patents may contain claims directed to:

liposomal compositions;
methods of loading vincristine;
pharmaceutical formulations;
specific lipid ratios;
particle-size distributions;
treatment methods; and
manufacturing processes.

A known related patent associated with the Marqibo platform is US 7,001,536. Family members must be reviewed separately because a continuation may have different claim language, prosecution history, terminal disclaimers, expiration treatment, and Orange Book status. The existence of a related family member does not automatically preserve the enforceability of the claims in US 5,814,335.

Geographic coverage

US 5,814,335 provides US rights only. International counterparts may have existed in Europe, Canada, Australia, or other jurisdictions, but each jurisdiction has its own:

filing date;
prosecution history;
claim scope;
patent-term calculation;
supplementary protection certificate rules;
litigation history; and
lapse or annuity status.

The US expiration does not establish the status of corresponding foreign patents.

How strong was the patent estate for liposomal vincristine?

The estate was technically strong during its enforceable term because the claims targeted the core platform rather than an incidental manufacturing detail. The most commercially relevant combination was:

vincristine;
sphingomyelin;
cholesterol;
acidic internal citrate buffer;
higher-pH external environment;
pH-gradient loading;
unilamellar morphology; and
controlled particle size.

That combination closely tracked a commercial liposomal vincristine product.

Its limitations also reduced the estate's breadth. The patent did not cover every liposomal vincristine formulation. A competitor could potentially avoid infringement by changing the membrane lipid, loading mechanism, internal buffer, pH architecture, or vesicle structure. The product-by-process format also created fact-intensive infringement questions concerning how a competing product was made.

The estate's current strength is effectively zero for prospective US exclusivity because the patent has expired. Technical relevance remains high for freedom-to-operate analysis, prior-art review, and development of alternative liposomal loading methods.

Are biosimilar or generic competitors a risk to Marqibo?

Biosimilar law is not the applicable pathway. Marqibo is a small-molecule drug product, not a biologic licensed under the Public Health Service Act. A competing applicant would generally use the 505(j) ANDA pathway if it could establish the required pharmaceutical equivalence and bioequivalence, or the 505(b)(2) pathway if it relied on a different formulation, delivery system, or clinical bridge.

The main development barriers are technical rather than patent-based:

demonstrating equivalent liposome size and distribution;
controlling encapsulation efficiency;
reproducing vincristine loading and release behavior;
establishing stability;
proving bioequivalence for a complex liposomal product;
matching administration and dosing requirements; and
meeting FDA expectations for complex injectable products.

FDA's product-specific guidance and complex-product policies are more important to current entry risk than the expired patent. [4]

What manufacturing and intellectual-property barriers remain?

The patent does not eliminate all commercial barriers. A new entrant would still need to address:

Process reproducibility. Small changes in pH, lipid hydration, temperature, extrusion, or loading time can affect encapsulation and release.
Liposomal characterization. FDA review may examine size distribution, morphology, lamellarity, drug loading, free-drug content, and release kinetics.
Sterile injectable production. A compliant facility must support aseptic processing, validation, and control of particulates and endotoxins.
Regulatory comparability. A nominally similar liposome may not be therapeutically or pharmaceutically equivalent without extensive characterization.
Residual patent rights. Later patents, continuation patents, manufacturing patents, trademarks, and trade secrets must be reviewed independently of US 5,814,335.
Supply-chain control. Consistent pharmaceutical-grade sphingomyelin and validated sterile-fill capacity can affect launch timing.

These barriers can delay entry even when the foundational patent is expired. They do not restore exclusivity to US 5,814,335.

What litigation and settlement issues affect this patent?

No current US injunction or settlement based solely on US 5,814,335 should be treated as an active barrier after patent expiration. Historical litigation, if any, would need to be separated into:

infringement actions filed before expiration;
ANDA litigation concerning Marqibo;
settlements containing launch dates or licenses;
covenant-not-to-sue agreements;
continuation-patent disputes; and
post-expiration damages claims.

A settlement involving a related patent would not necessarily affect the freedom to practice the expired claims of US 5,814,335. Conversely, expiration of this patent would not terminate obligations under a separate license covering know-how, trademarks, manufacturing technology, or later patent rights.

How does US 5,814,335 compare with competing liposomal drug patents?

Technology	Payload	Core protection	Relation to US 5,814,335
US 5,814,335	Vincristine and other alkaloids	Sphingomyelin/cholesterol liposome with pH-gradient loading	Foundational liposomal vincristine platform
Doxil-related patents	Doxorubicin	PEGylated liposomes and remote loading	Different payload and lipid architecture
Onivyde-related patents	Irinotecan	Liposomal irinotecan formulation and use	Different drug and formulation estate
Generic liposomal products	Various	Product-specific formulation and manufacturing claims	May avoid the claimed sphingomyelin/pH-gradient combination
Marqibo-related continuations	Vincristine	Related formulation, process, or product claims	Must be assessed separately by patent number

The main competitive distinction is that US 5,814,335 claims a specific active-loading architecture. It does not establish a general monopoly over liposomal drug delivery.

What is the commercial exposure associated with this patent?

The patent's commercial exposure was concentrated in Marqibo because vincristine liposome injection was the principal approved product aligned with the claims. The product addressed relapsed or refractory Philadelphia chromosome-negative acute lymphoblastic leukemia, a specialized hematology market.

Revenue exposure was therefore narrower than for patents covering a mass-market drug. The principal commercial risk during the patent term was an ANDA or 505(b)(2) entrant capable of reproducing the complex liposome and satisfying FDA requirements. After expiration, market entry risk depends on regulatory execution, manufacturing scale, clinical adoption, contracting, and any later unexpired rights.

Key Takeaways

US 5,814,335 covers pH-gradient loading of alkaloid drugs into sphingomyelin/cholesterol liposomes.
Vincristine is the principal commercial payload covered by the claims.
The most important limitations are the acidic internal buffer, higher-pH external buffer, transmembrane pH gradient, lipid composition, and liposome structure.
Claims 3, 4, and 12 create narrower formulation positions centered on cholesterol content, a 55/45 sphingomyelin/cholesterol ratio, and citrate buffer at about pH 4.
The patent is associated with the Marqibo liposomal vincristine platform.
Its US patent term has expired, eliminating current prospective exclusivity from this patent.
Biosimilar law does not apply; competing products would generally proceed under ANDA or 505(b)(2) pathways.
Regulatory and manufacturing complexity remains the principal barrier to a competing liposomal vincristine product.
Related continuation patents, foreign counterparts, licenses, trade secrets, and later manufacturing patents must be evaluated independently.

FAQs

Can a company practice the pH-gradient loading method in the United States?

Yes. US 5,814,335 is expired, so its claims do not currently block practice of the disclosed method in the United States.

Does the patent cover all liposomal vincristine products?

No. The claims require specified process and formulation characteristics, including sphingomyelin, cholesterol, an acidic-to-higher-pH gradient, and active transfer of the alkaloid into the liposome.

Is Marqibo protected by biosimilar exclusivity?

No. Marqibo is a small-molecule liposomal drug product. A competing applicant would generally pursue an ANDA or, for a materially different product, a 505(b)(2) application.

Can an expired Orange Book patent delay FDA approval?

An expired patent cannot support a current 30-month stay or an injunction. Historical Orange Book listing does not revive expired patent rights.

Does expiration of US 5,814,335 clear all IP risk for liposomal vincristine?

No. Later patents, continuation patents, foreign rights, manufacturing patents, trademarks, licenses, and trade secrets may create separate restrictions.

References

U.S. Food and Drug Administration. (2012). Marqibo (vincristine sulfate liposome injection) prescribing information and approval materials. FDA.
U.S. Food and Drug Administration. (n.d.). Approved drug products with therapeutic equivalence evaluations: Orange Book. FDA.
United States Patent and Trademark Office. (n.d.). Patent Center record for U.S. Patent No. 5,814,335. USPTO.
U.S. Food and Drug Administration. (2022). Drug products, including biological products, that contain nanomaterials: Guidance for industry. FDA.
U.S. Patent No. 5,814,335. (1998). Liposome compositions for delivery of alkaloid therapeutic compounds. United States Patent and Trademark Office.

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	248586	⤷ Start Trial
Australia	2709495	⤷ Start Trial
Canada	2193502	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,814,335

Summary for Patent: 5,814,335