Quality Study of Liposomes

Liposomes are one of the most studied nanoparticles and effective drug carriers. The preparation and production of liposomes involve several steps. The more complex their structures, the more they need to monitor process control and quality attributes. The quality research includes particle size and size distribution, morphology and layering, surface charge, encapsulation efficiency, release rate, physical stability, and so on.

1. Particle size and size distribution

The size and uniform distribution of liposome particle size directly affect the function of liposomes in body tissues. The particle size of liposomes is generally nanometer, and the particle size and distribution can be roughly measured by optical microscopy and electron microscopy. Dynamic light scattering (DLS) is a commonly used technique to measure particle size and distribution. The particle size of liposomes may change during storage, and the change can be reduced by adding positively charged groups, such as phosphatidylglycerol and phosphoplasic acid, into the membrane of liposomes. Considering the mass of liposomes, the optimal particle size range is 80-200nm.

2. Encapsulation Efficiency

The encapsulation efficiency is the most important index to evaluate the quality of liposomes and is also the key to whether liposomes can play a more efficient and less toxic role than normal preparations. The key to determining the encapsulation efficiency is to separate the free drug from the liposome. Common separation methods include column chromatography, dialysis, ultracentrifugation, ultrafiltration membrane filtration, etc. The encapsulation efficiency of drugs in liposomes is mainly determined by drug properties, including solubility, charge, and relative molecular weight of the drug. Different preparation methods of liposomes can also affect encapsulation efficiency. The encapsulation efficiency of liposomes prepared by the traditional injection method and membrane dispersion method is generally low, not higher than 70%. However, the preparation process is optimized according to the nature of the drug. For example, the encapsulation efficiency of doxorubicin liposomes prepared by the pH-gradient method and ammonium sulfate gradient method can be more than 90%, which greatly improves the utilization of the drug.

3. Release Rate

In vitro release is an important quality index of liposome preparation. The drug release rate in liposomes is related to the permeability of liposomes. The measurement of the drug release rate in vitro can preliminarily understand the permeability of liposomes. A stereoscopic in vitro correlation experiment was established to justify the use of in vitro release tests to predict in vivo performance.

4. Physical Stability

Liposomes are thermodynamically unstable, so they tend to fuse or aggregate during storage. In most cases, the stability of liposomes is evaluated by changes in particle size. The surface charge can be assessed by measuring zeta potential to understand liposome-liposome and liposome-membrane interactions. Charged liposomes can be prepared by adding anion or cationic molecules to the membrane component of the liposome, or by coating the surface of the liposome with charged polymer. Charged liposomes can stabilize the fusion or aggregation of liposomes by electrostatic repulsion.

References

1. Nakhaei P; et al. Liposomes: Structure, Biomedical Applications, and Stability Parameters With Emphasis on Cholesterol. Front Bioeng Biotechnol. 2021 Sep 9; 9: 705886.
2. Filipczak N; et al. Recent advancements in liposome technology. Adv Drug Deliv Rev. 2020; 156: 4-22.