US Patent 5,616,334: Claim Scope, Expiration, Orange Book Status, and Amphotericin B Lipid-Complex Patent Landscape

US Patent 5,616,334 protects high-drug-loading, non-liposomal lipid complexes of polyene antifungals, particularly amphotericin B associated with dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol. The patent’s core commercial embodiment corresponds closely to amphotericin B lipid complex, marketed as Abelcet.

The patent is expired. Its claims no longer create a US patent barrier to generic development or launch. The technology remains technically relevant because a competing product must still address complex particle structure, phospholipid composition, drug loading, injectable formulation, and FDA requirements for demonstrating pharmaceutical equivalence or therapeutic comparability.

What invention does US Patent 5,616,334 cover?

US 5,616,334 covers a bioactive-agent/lipid complex called an HDLC, or high-drug-loading complex. The claimed complex has five central characteristics:

The active agent is a polyene antifungal antibiotic.
The lipid includes a phospholipid.
The structure is non-liposomal and has no captured internal aqueous volume.
The complex is substantially free of liposomes.
The active agent concentration is at least approximately 6 mole percent, with narrower claims covering 25% to 50% and approximately 33 mole percent.

The patent is directed to a lipid-associated particulate structure rather than a conventional liposome. That distinction is commercially important. A liposome generally contains an aqueous compartment enclosed by one or more lipid bilayers. The claimed HDLC instead associates the drug directly with phospholipid material without a captured aqueous volume. (U.S. Patent No. 5,616,334, 1997).

What products most closely correspond to the patent?

The claims closely track an amphotericin B lipid-complex formulation containing:

Feature	Claimed subject matter	Commercial relevance
Active agent	Amphotericin B or nystatin	Amphotericin B lipid complex
Lipids	Phospholipids	Drug-lipid particulate complex
Specific phospholipids	DMPC and DMPG	Core Abelcet formulation
DMPC:DMPG ratio	Approximately 7:3	Matches the identified formulation architecture
Drug loading	Approximately 6% to 50% mole percent	Broad high-loading range
Preferred loading	Approximately 25% to 50%	Narrower commercial range
Exemplary loading	Approximately 33 mole percent	Key preferred embodiment
Structure	Non-liposomal, no captured volume	Distinguishes HDLC from liposomal amphotericin B
Administration	Sterile aqueous suspension for parenteral use	Injectable product format
Particle size	Approximately 0.2 to 10 microns	Formulation and manufacturing limitation

Abelcet is an amphotericin B lipid complex, while AmBisome is a liposomal amphotericin B product. The distinction between those delivery systems is central to the patent’s scope.

How are the 23 claims structured?

The claims contain two principal composition branches and several formulation and method claims.

Claims 1 through 12: Core HDLC compositions

Claim 1 is the broadest independent claim in the first branch. It requires:

a bioactive-agent/lipid complex;
a non-liposomal structure;
no captured volume;
a phospholipid;
at least approximately 6 mole percent active agent;
a polyene antifungal antibiotic;
substantial absence of liposomes; and
lower toxicity than the free active agent.

Claims 2 through 12 narrow claim 1 by specifying the lipid system, drug loading, and active agent.

Claim	Principal limitation
1	Broad HDLC composition with reduced toxicity and at least 6 mole percent polyene
2	Saturated phospholipid
3	Phosphatidylcholine plus phosphatidylglycerol
4	DMPC plus DMPG
5	DMPC:DMPG ratio of approximately 7:3
6	Polyene concentration of approximately 6% to 50%
7	Polyene concentration of approximately 25% to 50%
8	Polyene concentration of approximately 33%
9	Amphotericin B
10	Amphotericin B at approximately 6% to 50%
11	Amphotericin B at approximately 25% to 50%
12	Nystatin

Claims 13 through 18: Pharmaceutical compositions

Claim 13 requires the HDLC plus a pharmaceutically acceptable carrier or diluent. Claims 14 through 18 add:

sterile aqueous suspension;
parenteral administration;
amphotericin B;
25% to 50% amphotericin B loading;
particle size of approximately 0.2 to 10 microns; and
nystatin as an alternative antifungal.

These claims are more product-oriented than claim 1. They are relevant to injectable finished products rather than only to the underlying lipid complex.

Claims 19 through 21: Treatment methods

Claim 19 covers treating an infectious disease by administering an effective amount of the pharmaceutical composition. Claim 20 narrows the method to fungal infections. Claim 21 refers to a viral infection, although the supplied text states "vital infection." The likely intended term is "viral infection."

The treatment claims depend on claim 13. Consequently, they inherit the HDLC, non-liposomal, phospholipid, drug-loading, and pharmaceutical-composition requirements.

Claims 22 and 23: Separate high-loading composition branch

Claim 22 is an independent composition claim. It covers a polyene antifungal/phospholipid complex with:

no captured volume;
substantial absence of liposomes; and
approximately 25% to 50% active-agent loading.

Claim 22 does not expressly repeat the reduced-toxicity limitation found in claim 1. That omission gives claim 22 a potentially broader functional scope, although it remains limited by the structural and loading requirements.

Claim 23 narrows claim 22 to:

amphotericin B; and
DMPC and DMPG.

What is the practical scope of claim 1?

Claim 1 is broad in drug identity and phospholipid identity but narrow in structure. A product would need to satisfy every recited limitation for literal infringement.

The most important limitations are:

Non-liposomal structure

The product must be distinguishable from a conventional liposome. A formulation using vesicles with an internal aqueous compartment would face a substantial argument that it falls outside the claim. The term "substantially free of liposomes" also supports a product-composition limitation rather than a mere process limitation.

No captured volume

This limitation excludes structures that encapsulate or retain an aqueous internal volume. Characterization methods could include electron microscopy, cryo-electron microscopy, light scattering, density measurements, or other structural analyses. The claim does not define a single mandatory analytical test, which could create claim-construction and evidentiary disputes.

At least 6 mole percent drug loading

The lower threshold is quantitative. A product below approximately 6 mole percent would have a strong non-infringement position under claim 1, subject to measurement methodology and claim construction.

Claims 6 through 8 and 10 through 11 create narrower ranges. A product at approximately 33 mole percent amphotericin B with the specified phospholipid system would fall within the preferred claimed ranges if the structural limitations were also met.

Reduced toxicity

Claim 1 requires toxicity lower than the free form of the bioactive agent. This is a functional limitation. It may require comparative evidence using an identified toxicity endpoint, such as hemolysis, renal toxicity, cytotoxicity, animal tolerability, or clinical adverse events.

The limitation can operate in two directions:

It narrows the claim if the patent requires proof of a specific toxicity reduction.
It can create infringement uncertainty if the claim does not identify the assay, comparator, dose, exposure, or statistical threshold.

Claim 22 does not contain the express reduced-toxicity requirement, making it the more important independent claim for a high-loading, non-liposomal amphotericin complex.

Which formulations are protected by claims 4, 5, 15, 16, 17, and 23?

The narrowest commercially important formulation is a parenteral amphotericin B suspension containing an HDLC made from DMPC and DMPG at approximately a 7:3 mole ratio, with amphotericin B loading in the 25% to 50% range.

Claim 23 is especially significant because it combines the principal active ingredient and the two named phospholipids. The claim does not expressly require the 7:3 ratio, sterile aqueous suspension, or particle-size range. Those limitations appear in separate dependent claims.

A product using different phospholipids, such as distearoylphosphatidylcholine or hydrogenated soy phosphatidylcholine, could avoid the narrow DMPC/DMPG claims. It could still face the broader claims if it retained the required non-liposomal, no-captured-volume, high-loading architecture.

When did US Patent 5,616,334 lose exclusivity?

The patent expired under the 20-year patent-term regime. The relevant US patent term ran from the applicable nonprovisional filing date and ended in approximately 2014. The patent is therefore not a current US exclusivity barrier.

Event	Timing
Earliest priority period	Early 1990s
US patent filing and prosecution	1990s
Patent issued	April 1, 1997
Statutory patent term	Approximately 20 years from the applicable nonprovisional filing
Patent expiration	Approximately 2014
Current status	Expired
Current Paragraph IV relevance	None for this expired patent

Patent-term adjustment, terminal disclaimers, patent-term extension, and the exact effective filing date should be confirmed in the USPTO Patent Center record. The patent’s commercial exclusionary term, however, has ended. (United States Patent and Trademark Office, n.d.-a; U.S. Patent No. 5,616,334, 1997).

What is the FDA regulatory status of the covered product?

The principal FDA product associated with the claimed technology is amphotericin B lipid complex injection, marketed as Abelcet. The product was approved for treatment of invasive fungal infections in patients who are refractory to or intolerant of conventional amphotericin B or who have renal impairment or other reasons for avoiding conventional therapy.

FDA labeling identifies Abelcet as an amphotericin B lipid complex rather than a liposomal formulation. The formulation is administered intravenously as a suspension. (U.S. Food and Drug Administration, n.d.-a).

Is Abelcet a biosimilar?

No. Amphotericin B is a small-molecule antifungal, and Abelcet is regulated through the drug-approval framework rather than the biologic biosimilar pathway.

A competing product would generally pursue one of the following routes:

an ANDA under section 505(j), if it can establish the required pharmaceutical-equivalence and bioequivalence framework; or
a 505(b)(2) application, if the product differs materially in formulation, delivery system, clinical evidence, or reference-product reliance.

A complex lipid formulation may present greater analytical and bioequivalence challenges than conventional amphotericin B deoxycholate. Key variables include particle-size distribution, morphology, lipid composition, drug loading, free-drug fraction, release characteristics, sterility, and injectable-suspension performance.

What is the Orange Book status of US Patent 5,616,334?

The patent is expired and cannot support a current Orange Book exclusivity period. Orange Book listing history for Abelcet must be separated from current enforceability. A patent may have been listed during the commercial life of the product, but expiration eliminates its present patent-blocking effect.

Orange Book issue	Assessment
Current patent protection from US 5,616,334	None
Current 30-month stay based on this patent	None
Current Paragraph IV challenge to this patent	Not commercially relevant because the patent expired
Current pediatric exclusivity	Not established by the patent itself
Current regulatory exclusivity	Must be assessed separately from patent status
Biosimilar exclusivity	Not applicable

FDA’s Orange Book identifies approved products, therapeutic equivalence information, listed patents, and relevant exclusivity. It does not convert an expired patent into a current barrier. (U.S. Food and Drug Administration, n.d.-b).

Were there Paragraph IV challenges or litigation involving this patent?

A Paragraph IV certification would have been relevant only while the patent was listed and unexpired. After expiration, an applicant would not need to make a commercially meaningful Paragraph IV challenge to this patent.

The patent’s principal litigation significance historically would have involved:

whether Abelcet or a competing amphotericin lipid complex was a liposome;
whether the formulation had a captured aqueous volume;
whether the drug loading met the 6%, 25%, 33%, or 50% thresholds;
whether the composition was substantially free of liposomes;
whether the toxicity limitation in claim 1 was met; and
whether a generic or follow-on formulation had the same DMPC/DMPG architecture.

No current injunction, settlement, or enforceable exclusionary right arises from US 5,616,334 after expiration. Patent-specific litigation and settlement searches should distinguish this expired patent from later patents covering manufacturing processes, formulation stability, particle-size control, or other amphotericin delivery systems.

How does this patent compare with liposomal amphotericin B patents?

The patent is narrower in physical structure but broader in its distinction from liposomal systems.

Issue	US 5,616,334 HDLC	Liposomal amphotericin B estate
Structure	Non-liposomal complex	Lipid vesicle or liposome
Captured volume	None	Generally present
Drug association	Direct association with phospholipid complex	Encapsulation or incorporation in liposomes
Core product example	Abelcet	AmBisome
Main technical risk	Complex morphology and drug loading	Vesicle composition, size, encapsulation, stability
Biosimilar pathway	Not applicable	Not applicable
Current status of US 5,616,334	Expired	Separate patent families may have had different terms

A formulation that is clearly liposomal should not be treated as automatically covered by the HDLC claims. Conversely, a non-liposomal amphotericin complex can present an infringement question even if it does not copy the commercial name, dosage, or manufacturing process.

What manufacturing and intellectual-property barriers remain after expiration?

Expiration removes the core patent barrier but does not eliminate technical or regulatory barriers.

Manufacturing barriers

A competing manufacturer must control:

reproducible DMPC/DMPG or alternative phospholipid ratios;
high amphotericin B loading;
aggregation and particle-size distribution;
suspension homogeneity;
sedimentation and redispersibility;
free amphotericin B content;
degradation products;
sterility and endotoxin levels; and
container-closure compatibility.

The 0.2-to-10-micron particle-size range in claim 17 is particularly relevant to formulation development, even though that claim is expired. Particle size affects injectability, distribution, sedimentation, tissue exposure, and toxicity.

Regulatory barriers

FDA may require extensive analytical comparability for a complex lipid product. A generic applicant cannot assume that matching the amount of amphotericin B alone establishes equivalence. The applicant may need to characterize the entire drug-lipid complex and demonstrate that the product performs consistently.

Later patent risks

Potential later patent categories include:

improved amphotericin lipid compositions;
manufacturing methods;
freeze-drying or reconstitution methods;
particle-size control;
stabilizers and excipients;
infusion protocols;
combination therapies; and
new methods of treating resistant fungal infections.

These rights must be analyzed separately from US 5,616,334. The 1997 patent does not immunize a product against later-filed patents.

Which companies are competing in amphotericin B lipid delivery?

The principal competitive categories are:

Product or category	Delivery format	Patent relationship
Abelcet	Amphotericin B lipid complex	Closely aligned with the HDLC technology
AmBisome	Liposomal amphotericin B	Distinct liposomal technology
Amphocil or Abelcet-type alternatives	Non-liposomal lipid or colloidal delivery	May overlap technically but not necessarily literally
Conventional amphotericin B deoxycholate	Non-lipid conventional formulation	Generally outside the HDLC claims
Generic amphotericin B products	Usually conventional formulation	Different formulation and regulatory profile

Commercial competition depends on more than patent expiration. Amphotericin B lipid products have different renal-toxicity profiles, infusion requirements, dosing practices, hospital protocols, acquisition costs, and clinical preferences.

What generic launch scenarios exist?

Scenario 1: Conventional amphotericin B generic

A conventional amphotericin B deoxycholate product does not appear to practice the HDLC limitations. It is unlikely to implicate claims requiring a non-liposomal phospholipid complex and high drug loading.

Scenario 2: Liposomal amphotericin B generic

A liposomal product would generally be evaluated against liposomal patent and regulatory issues rather than this HDLC patent. The structural distinction remains central.

Scenario 3: ABLC follow-on product

A product using amphotericin B with DMPC and DMPG at high loading could reproduce the core commercial technology. Because US 5,616,334 is expired, the principal risks are FDA equivalence, formulation know-how, trade secrets, manufacturing reproducibility, and later patents.

Scenario 4: Design-around lipid complex

A manufacturer could pursue different phospholipids, a different drug-loading range, a distinct particle structure, or a different delivery mechanism. A design-around must still avoid the broad structural limitations of claims 1 and 22, not merely the specific DMPC/DMPG combination in claim 23.

How strong is the patent estate for US 5,616,334?

The historical claim estate was technically focused and commercially relevant. Its strongest features were:

direct coverage of high-loading amphotericin lipid complexes;
specific coverage of DMPC/DMPG systems;
composition, pharmaceutical, and treatment claims;
quantitative drug-loading ranges; and
separation from conventional liposomal delivery.

Its weaknesses included:

the potentially difficult "reduced toxicity" limitation in claim 1;
ambiguity around "no captured volume";
potential measurement disputes for mole-percent loading;
reliance on structural characterization for non-liposomal status; and
expiration of all patent rights.

Current patent strength is effectively zero as an exclusionary right because the patent has expired. Its remaining value is primarily technical and historical: it identifies the formulation architecture that a competitor or licensing party must understand when evaluating amphotericin B lipid-complex products.

Key Takeaways

US Patent 5,616,334 covers high-drug-loading, non-liposomal phospholipid complexes of polyene antifungals.
Amphotericin B with DMPC and DMPG, especially at approximately 33 mole percent loading and a 7:3 phospholipid ratio, is the central commercial embodiment.
Claims 1 and 22 are the principal independent composition claims.
Claim 22 is potentially broader than claim 1 because it does not expressly require reduced toxicity.
Claims 13 through 18 cover pharmaceutical formulations, sterile suspensions, parenteral use, and particle size.
Claims 19 through 21 cover treatment methods.
The patent expired in approximately 2014 and is not a current US exclusivity barrier.
No biosimilar pathway applies because amphotericin B is a small-molecule drug.
A follow-on product would face FDA complex-formulation and bioequivalence issues even without an active patent barrier.
Later patents covering manufacturing, stability, particle size, or improved lipid systems require separate analysis.
The main current commercial barriers are formulation know-how, manufacturing control, regulatory evidence, and market access rather than US 5,616,334.

References

U.S. Patent No. 5,616,334. (1997). High drug loading lipid complexes. United States Patent and Trademark Office.
United States Patent and Trademark Office. (n.d.-a). Patent term calculator and patent term adjustment resources. https://www.uspto.gov
U.S. Food and Drug Administration. (n.d.-a). Abelcet amphotericin B lipid complex injection prescribing information. DailyMed.
U.S. Food and Drug Administration. (n.d.-b). Approved drug products with therapeutic equivalence evaluations: Orange Book. https://www.fda.gov
U.S. Food and Drug Administration. (n.d.-c). 505(b)(2) applications. https://www.fda.gov
U.S. Food and Drug Administration. (n.d.-d). Abbreviated new drug application submissions and patent certifications. https://www.fda.gov

Country	Patent Number	Estimated Expiration	Supplementary Protection Certificate	SPC Country	SPC Expiration
Austria	113468	⤷ Start Trial
Austria	118169	⤷ Start Trial
Australia	1799088	⤷ Start Trial
Australia	622405	⤷ Start Trial
Canada	1330199	⤷ Start Trial
Canada	1337273	⤷ Start Trial
>Country	>Patent Number	>Estimated Expiration	>Supplementary Protection Certificate	>SPC Country	>SPC Expiration

Details for Patent: 5,616,334

Summary for Patent: 5,616,334