Liposomes have been widely used in drug delivery, drug targeting, controlled release and solubility enhancement. However, the main limitation of the widespread use of this universal drug delivery system is its instability. Therefore, the successful introduction of liposome formulations depends on a rigorous stability study. Liposome stability test using effective and reliable research techniques is important.

The stability of liposomes is the major consideration in the production and administration of liposomes: from process to storage and delivery. Stable drug formulations maintain their physical integrity and do not adversely affect the chemical integrity of active ingredients during their life cycle. The stability study must include a description of the product characteristics and another part concerning the stability of the product during storage.

The stability of liposomes can be subdivided into interrelated physical, chemical, and biological stability. In general, the shelf-life stability of liposomes depends on physical and chemical stability (uniformity in size distribution and encapsulation efficiency and minimal degradation of all compounds).

-Physical stability

Liposomes stability is affected by many factors, such as size and integrity, the amount of charge, and unsaturated fatty acid groups. Defects in the lattice structure can lead to liposome fusion, aggregation, and leakage of contained drugs during storage. Therefore, the stability test should include the change in particle size, zeta potential, and integrity.

The particle size of all liposome preparations is not uniform, and the average particle size distribution of liposomes varies with storage time. Liposomes tend to fuse and grow into larger vesicles, which is a thermodynamically more favorable state. The fusion and rupture of liposomes during storage is also a key problem leading to drug leakage from vesicles. Therefore, particle morphology and size distribution are important parameters to evaluate physical stability.

-Chemical stability

Chemical stability is affected by the liposome preparation process and the storage conditions such as pH, temperature, oxygen, and light. Phospholipid oxidation and hydrolysis, phospholipid denaturation, leaks, and toxicity fundamentally cause liposome instability.

The main component of liposome formulations is lipids, which are derived from natural and/or synthetic phospholipid sources containing unsaturated fatty acids that are known to undergo oxidation reactions. Therefore, it is necessary to develop a stability program to evaluate the chemical integrity of the drug over a period of time. There are many ways to monitor lipid degradation. For example, thin-layer chromatography (TLC) can be used to measure the purity and concentration of lipids.

-Biological stability

Proteins, degrading enzymes, immune system, and even blood flow in the body can interrupt liposome stability. In vivo and in vitro evaluation of liposome biological factors is important for the preparation and optimization of liposomes, and even more during clinical applications.

Most therapeutic liposomes are non-intestinal products and must be sterilized to remove microbial contamination from the products. Therefore, it is very important to control the microbial stability of liposome formulations.