Liposomes as Drug Delivery Vehicles

Due to the amphiphilic nature of lipids in liposomes, they can serve as candidate vehicles for drug delivery, and liposomes have received more attention than other systems because of their remarkable ability to deliver drugs to their target sites.

Advantages of Liposomes as Drug Delivery Vehicles

• As drug delivery vehicles, liposomes have low toxicity and immunogenicity, and the high biocompatibility and biodegradability of their components—phospholipids can increase the in vivodrug concentrations and protect drugs from degradation.
• They can be easily modified with various ligands and functional molecules for targeted delivery of drugs.

Challenges of Liposomes as Drug Delivery Vehicles

• The cost of excipients and equipment required for liposome preparation is relatively high, resulting in the cost of liposomal formulations being higher than that of many other dosage forms.
• Cationic liposomesmay be highly toxic when high doses are required for treatment.
• One drawback of liposomal formulations loaded with anticancer drugs is the lack of targeting when they are administered intravenously, which can lead to adverse effects such as hand-foot syndrome.
• Issues such as differences between batches, the need for sterilization, the need to improve the encapsulation efficiency, particle size control, shelf life and organic solvent residues are all concerned.

Drug encapsulation efficiency, sterility, preparation method, availability of large-scale production, liposome stability, cost and effectiveness all depend on the choice of drug loading methods in liposomes. There are two different methods for incorporating drugs into liposomes, namely the passive drug loading method and the active drug loading method. In the passive drug loading method, the drug is encapsulated in liposomes during the preparation process. In the active drug loading method, the drug is loaded into intact liposomes.

Passive Drug Loading Method

Whether passive drug-loading techniques are employed depends on the ability of the liposomes to encapsulate a specific volume of aqueous phase containing dissolved drugs or solutes during formation. For hydrophilic drugs, the encapsulation efficiency of passive drug-loaded liposomes is related to the volume of the aqueous phase in which the liposomes are located, and the properties of liposomes depend on the concentration of phospholipids in the dispersion system, the number of lipid layers and the morphology of liposomes.

Lipophilic drugs interact with the phospholipid bilayers; therefore, the encapsulation efficiency depends on the type of phospholipid and its concentration. Morphological parameters did not affect the encapsulation efficiency. Using passive drug loading methods, water-soluble drugs are encapsulated within the aqueous phase of the liposome, while lipid-soluble drugs are encapsulated in the lipid phase of the liposome. The lipid-soluble portion of the drug will be embedded between the phospholipids of the liposome, while their water-soluble portion is in the aqueous phase of the liposome and thus encapsulated.

Active Drug Loading Method

For the encapsulation of active pharmaceutical ingredients (APIs), the empty liposomes that have been prepared are mixed with a high concentration solution of APIs, and the molecules can be permeated into the liposome through the lipid bilayer due to the concentration gradient. Because the phospholipid bilayer is highly permeable to drug diffusion, high encapsulation efficiency can be achieved within an appropriate time.

During active loading, the water-soluble drug interacts with the polar head group of phospholipids and cannot enter the interior of the liposome. The amount of hydrophobic drug that can enter the liposome depends on the degree of space limitation of the lipid bilayer. In the case of amphiphilic drugs, they are difficult to retain within liposomes because they can rapidly penetrate the lipid bilayer.

This method has many advantages, such as no active ingredients being added in the preparation of liposomes, so the safety precautions that must be taken when dealing with toxic drugs can be minimized. The disadvantage of this method is that it is limited to a small subset of drugs that behave as amphoteric weakly basic or acidic substances, and can only penetrate the bilayer without charge, but not in the charged condition.

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