Aug 12, 2026
In vitro release rate (IVRT) assessment of liposome/lipid-based nanomedicines, and a core comparison of USP 2/4/7.
In-Vitro Liposome Release Rate (IVRT) Assessment: In-Depth Comparison and Selection Strategies of USP 2 vs. USP 4 vs. USP 7
For liposome drug products/liposomal formations, including conventional liposomes, PEGylated liposomes such as Doxil/Caelyx, and nucleic acid lipid nanoparticles (LNPs), in vitro drug release testing (IVRT) is a critical quality attribute (CQA) that is of utmost concern to regulatory authorities in various countries (FDA 2018 Liposome Drugs Guidance, EMA Liposomes Guidance).
The core physical characteristics of liposomes are their small particle size (typically 50–150 nm), the susceptibility of their lipid bilayer membrane to external shear forces and surfactant damage, and the minimal difference in particle size between free and encapsulated APIs. Therefore, when evaluating liposomes, it is crucial to strictly prevent physical shear damage to the carrier, microporous membrane blockage, and pseudo-diffusion delay (Membrane Artifact/Lag Time) caused by dialysis membranes.
I. Comparison of Liposome Formulations: USP 2 vs. USP 4 vs. USP 7 (Full-Dimensional Comparison)
Comparison Dimensions | USP 2 (Paddle Method) | USP 4 (Flow-Through Cell Method) | USP 7 (Reciprocating Holder Method) |
Main configuration modes (for liposomes) | Dialysis bag suspension method : Place the liposomes in a dialysis bag or dialysis clamp, and suspend them in a 500–900 mL dissolving cup with the clamp and stir. | Microdialysis Adapter Cell Mode : Liposomes are encapsulated in a specially designed dialysis chamber, and solvent is continuously flushed outside the chamber by laminar flow (micro-volume cell or 22.6 mm cell). | Microdialysis Cassette/Holder : The liposomes are placed in a microdialysis capsule and a reciprocating rack moves them vertically back and forth in a 5–50 mL microtubule for rinsing. |
The Influence of Flow Field and Shear Force on Liposomes | High turbulence and boundary shear : Rotating paddles easily generate vortices. If dispersed directly (without bags), the liposomes will be agitated and sheared; if bagged, asymmetrical turbulence will form around the bag. | Gentle, uniform laminar flow : The flow rate can be steplessly controlled in micro-steps (0.5–8.0 mL/min), and the fluid shear force can be precisely calculated to best match the shear force of subcutaneous or vascular microcirculation without damaging the lipid bilayer. | Gentle Dipping (Low Shear Temperature) With a reciprocating frequency of 5–30 dpm, extremely low speed and no eddy shear, it has minimal destructive effect on fragile liposome structures. |
Stirred vs. Unstirred Water Layer (UWL) | High resistance (significant pseudo-delay) : The inside of the dialysis bag is completely static and undisturbed, forming a thick unstirred water layer (UWL). The drug release rate is limited by the "diffusion rate through the membrane" rather than the "release rate of the liposomes". | Extremely low resistance : Stable high-linear-velocity laminar flow on the outside can compress the outer diffusion boundary layer thickness to the greatest extent, significantly reducing membrane lag time. | Medium resistance : The vertical up-and-down reciprocating motion can cause the liquid inside the microtube to back up, breaking some of the boundary layer, but there is still some micro-stagnation inside the dialysis capsule. |
Fouling and lipid adsorption | severe : Liposomes are deposited by gravity at the bottom of the dialysis bag, and lipids accumulate in the membrane pores, causing pore clogging. | From slight to negligible : The vertically upward laminar flow carries the liposomes to circulate and suspend within the cavity, significantly reducing the "cake formation" effect of lipids on the membrane surface. | Extremely slight : Vertical reciprocating motion has a hydraulic impact and self-cleaning effect, which can prevent liposomes from depositing and accumulating on a single surface. |
Solvent volume and detection sensitivity (for trace amounts of highly active drugs) | Extremely unfavorable (over-dilution) : Standard volume 500–1000 mL. Liposome anticancer drugs (such as doxorubicin and paclitaxel) are used in low doses, and the concentration of released free drug is often below the HPLC/UV detection limit (LOD). | Elasticity (supports closed-loop micro-cycles) : It can be used in an open loop (maintaining an infinite leakage range) or in a closed loop with low-volume circulation (10–50 mL), balancing leakage range and detection sensitivity. | Excellent (high concentration in trace amounts) : The standard test tube contains only 5–15 mL, and the free drug is enriched and can be directly injected for HPLC/UPLC analysis, saving the amount of expensive liposome samples. |
Feasibility of long-term slow release/multi-slot switching | Extremely poor : The solvent evaporates quickly, and experiments lasting several weeks require frequent manual rehydration. Changing the solvent can easily tear the dialysis bag, causing artificial leakage. | medium : Closed loops require large-volume storage tanks, and over long periods of time, trace amounts of protein or surfactants can easily accumulate in the pipeline. | Excellent (designed for long-lasting performance) : The automatic line-changing mechanism can continuously change multiple rows of buffer solutions at different time points or different pH values (such as lysosomal acidity environment simulating endocytosis, pH 7.4 → 6.5 → 5.0). |
Equipment availability and regulatory maturity | Highest : It is a standard feature in laboratories worldwide, but its methodological validity and in vitro-in vivo correlation (IVIVC) are often the worst. | high : The FDA recommends it for use in complex injectables, and it is the standard platform for European and American pharmaceutical companies to develop NDDS. | medium : It is mainly found in laboratories specializing in long-cycle release of long-acting injectables (LAIs), implants, microspheres, etc. |
II. Technical Bottlenecks and Solutions of the Three Major Systems in Liposome Experiments
1. USP 2 (Traditional Oar Method + Dialysis Bag): Worst Clinical Relevance (IVIVC)
Membrane-Controlled Release, not Liposome Release Control : In USP 2 dialysis bag experiments, the measured release curves often exhibit a first-order diffusion model, which essentially reflects the "physical diffusion resistance of free molecules across the semipermeable membrane (MWCO 10k–100k Da)". The actual release differences caused by changes in liposome prescriptions (such as cholesterol ratio, phase transition temperature $T_m$) are easily masked by membrane resistance.
Leakage damage and sedimentation : When the specific gravity of the liposomes is greater than that of the medium, they will settle at the bottom of the bag and clump together, causing a significant reduction in the effective surface area and affecting the uniformity of dissolution.
2. USP 4 (Flow Cell Method + Microdialysis Chamber): The Gold Standard for Immediate-Release/Intravenous Liposomes
Precise laminar flow simulation of microvascular flow : The external medium continuously washes away the membrane surface at a fixed linear velocity, maximizing the concentration gradient between the inside and outside of the membrane and completely solving the lag effect of dialysis.
Open Loop technology solves the problem of leakage in poorly soluble anticancer drugs : such as paclitaxel or docetaxel liposomes, which have extremely low solubility in water, can be continuously injected with fresh media containing 0.1–0.5% surfactant (such as Polysorbate 80 or Albumin) without any concern about the effects of saturation concentration.
3. USP 7 (Reciprocating rack method + microdialysis clamp): The best solution for long-acting library-type liposomes and micro-dose formulations.
pH gradient simulation of intracellular pathways : After liposomes enter the human body via veins, they often undergo endocytosis via the reticuloendothelial system (RES) or tumor cells. The USP 7 can schedule automatic switching of the reciprocating frame.
First row of tubing: physiological blood environment (pH 7.4)
Second row of tubes: Tumor microenvironment (pH 6.5)
Third row of tubes: Lysosomal acidity environment (pH 4.5–5.0)
It can automatically obtain the release kinetic curves of acid-sensitive liposomes under different cellular microenvironments.
Micro-volume protection of detection limits : In early prescription development (such as mRNA-LNP), expensive sample volumes were only a few microliters (µL). The USP 7's 5 mL test tube can provide sufficient analytical signals without the need to be diluted to 500–900 mL as with the USP 2.
III. Summary of the Selection Matrix for Liposome R&D Laboratories
Liposome types | Top recommendation | Second choice | Eliminated/Not Recommended |
Routine intravenous injection of liposomes (e.g., Doxorubicin, Amphotericin B) | USP 4 (Microdialysis flow cell, open loop/micro-closed loop) | USP 7 (Microdialysis clamp) | USP 2 (The membrane boundary effect masks the true value of release, resulting in excessive dilution) |
mRNA-LNP (lipid nanoparticles) (Early research samples were in trace amounts and extremely fragile) | USP 7 (Microtubules 1–5 mL, ultra-low shear to prevent breakage) | USP 4 (Low-flow-rate micro-flow cell) | USP 2 (High shear directly destroys the LNP structure) |
Long-lasting library-type liposomes/in-situ gel liposomes (The release period lasts from several days to several weeks) | USP 7 (Automatic pipe switching, anti-evaporation, low maintenance) | USP 4 (Requires continuous pump operation for extended periods, resulting in high solvent consumption) | USP 2 (Unable to perform long-cycle automatic multi-stage media replacement) |
IV. Authoritative Documents, Guidelines, and Data Sources
1. Regulatory Guidelines
US Food and Drug Administration (FDA) (2018) :
Liposome Drug Products: Chemistry, Manufacturing, and Controls; Human Pharmacokinetics and Bioavailability; and Labeling Documentation – Guidance for Industry. US Department of Health and Human Services, Food and Drug Administration, Center for Drug Evaluation and Research (CDER), April 2018.
European Medicines Agency (EMA) (2007) :
Reflection paper on the data requirements for intravenous liposomal products following a planned change in manufacturing process or for generic products. Committee for Medicinal Products for Human Use (CHMP), Document EMA/CHMP/QWP/28383/2007 Rev. 1.
US Food and Drug Administration (FDA) (2022) :
Drug Products, Including Biological Products, that Contain Nanomaterials – Guidance for Industry. CDER/CBER, April 2022.
2. International Pharmacopeia Standards
United States Pharmacopeia (USP) :
USP General Chapter <711> Dissolution (Specifications for Apparatus 1, 2, and 4)
USP General Chapter <724> Drug Release (Calibration and Operating Parameter Specifications for Apparatus 4 Flow Pool Method and Apparatus 7 Reciprocating Frame Method)
USP General Chapter <1724> Semi-Solid Drug Products – Performance Tests (Microdialysis Cell Membrane Resistance and Diffusion Area Calibration Guidelines)
European Pharmacopoeia (Ph. Eur.) :
Ph. Eur. Chapter 2.9.3 Dissolution Test for Solid Dosage Forms
Ph. Eur. Chapter 2.9.4 Dissolution Test for Transdermal Patches and Specialized Formulations (including technical specifications for Flow-Through and Reciprocating devices)
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