Liposome
Liposome is an artificial membrane. The hydrophilic head of the phospholipid molecule is inserted into the water, and the hydrophobic tail of the liposome extends into the air. After stirring, a spherical liposome with a double layer of lipid molecules is formed, with a diameter ranging from 25 to 1000nm.
Liposome classification
Liposomes are divided into unilamellar liposomes and multilamellar liposomes according to the number of lipid bilayers contained.
Small unilamellar liposomes (SUV): particle size is about 0.02~0.08um; large unilamellar liposomes (LUV) are single-layer large vesicles with a particle size of 0.1~lum.
Multi-layer bilayer vesicles are called multilamellar liposomes (MIV), with a particle size between 1~5um.
Preparation of liposomes by microfluidizer
The currently reported preparation methods (such as high-pressure emulsion filtration, injection method, and rotary film hydration method) are all feasible for process amplification, but in the early stages of research, it is necessary to focus on the research of instruments and equipment required for large-scale industrial production and key process links. According to current research experience, the products after rotary film hydration are generally not very uniform and need to be homogenized (such as ultrasound, high-pressure emulsification).
Microfluidizer is a common device in the preparation of high-pressure emulsions. The maximum homogenization pressure can reach 30,000 psi, which can greatly reduce the number of treatments and improve work efficiency.
Working principle of microfluidizer
Quality control and evaluation of liposomes
Morphology, particle size and distribution
Measured by scanning electron microscopy, laser scattering or laser scanning. Different particle sizes are required according to different routes of administration. For example, the particle size of liposomes for injection should be less than 200nm, and the distribution should be uniform, normal, and the span should be small
Encapsulation rate and drug loading
Encapsulation rate: Encapsulation rate = (drug encapsulated in liposomes/total amount of drug in liposomes) × 100%
Generally, separation methods such as dextran gel, ultracentrifugation, and dialysis are used to separate free drugs and liposomes in the solution, and they are measured separately to calculate the encapsulation rate. It is usually required that the drug encapsulation rate of liposomes is more than 80%.
Drug loading: Drug loading = [amount of drug in liposomes/(total amount of drug in liposomes + carrier)] × 100%
The size of the drug loading directly affects the clinical application dose of the drug, so the larger the drug loading, the easier it is to meet clinical needs. The drug loading is related to the properties of the drug. Generally, lipophilic or hydrophilic drugs are easier to make into liposomes.
Liposome stability
1. Physical stability: mainly expressed by leakage rate.
Leakage rate = (amount of drug in the medium before placement - amount of drug in the medium after placement) / amount of drug in the preparation x 100%
Cholesterol can strengthen the lipid bilayer membrane, reduce membrane flow, and reduce leakage rate.
2. Chemical stability:
(1) Phospholipid oxidation index: oxidation index = A233nm = A215nm; generally, the phospholipid oxidation index should be less than 0.2.
(2) Determination of phospholipid content: Based on the fact that each phospholipid molecule contains 1 phosphorus element, the phospholipid in the sample is converted into inorganic phosphorus by chemical method, and then the phosphorus molar amount (or weight) is determined to deduce the phospholipid content.
Measures to prevent oxidation
General measures to prevent oxidation include filling with nitrogen, adding antioxidants-tocopherol, metal chelating agents, etc.; hydrogenated saturated phospholipids can also be directly used.
