**Abstract**

Several efforts have been focused on targeted drug delivery systems for delivering a drug to a particular region of the body for better control of systemic as well as local action. Liposomes have proven their efficiency as a choice of carrier for targeting the drugs to the site of action. The main reason for continuous research on liposomes drug delivery is they largely attributed to the fact that they can mimic biological cells. This also means that liposomes are highly biocompatible, making them an ideal candidate for a drug delivery system. The uses found for liposomes have been wide-spread and even include drug delivery systems for cosmetics. Several reports have shown the applicability of liposomal drug delivery systems for their safe and effective administration of different classes of drugs like anti tubercular, anti cancer, antifungal, antiviral, antimicrobial, antisense, lung therapeutics, skin care, vaccines and gene therapy. Liposomes are proven to be effective in active or passive targeting. Modification of the bilayer further found to increase the circulation time, improve elasticity, Trigger sensitive release such as pH, ultrasound, heat or light with appropriate lipid compositions. The present chapter focuses on the fundamental aspects of liposomes, their structural components, preparation, characterization and applications.

**Keywords:** liposomes, phoipsholipids, cholesterol, stealth liposomal technology, vaccines, doxorubicin

#### **1. Introduction**

Liposomes are microscopic vesicles containing aqueous volume enclosed by lipid bilayer membrane [1]. A.D. Bangham and R.W. Thorne first described about liposomes in 1964 when observed under electron microscope while analyzing phospholipids dispersion in aqueous environment [2]. They observed spontaneous arrangement of phospholipids into "bag-like" circular structures. Gerald Weissman, one of the colleagues of Bangham suggested the structures as liposomes [3]. This discovery helped as a multipurpose tool in several fields like biology, biochemistry and medicine. Liposomes gained popularity in vesicular research due their attributes of biocompatibility and similar structural features of biological cells (See **Figure 1**). The amount of drug loaded into the liposomes and the size of the liposomes play pivotal roles in the pharmacokinetic and pharmacodynamic parameters of the drug. The size scale of liposomes varies with typical a mean size of 100 nm. Due to their size and hydrophobic and hydrophilic character liposomes are promising systems for drug delivery.

Several reports showed the applicability of liposomes for the safe and effective administration of therapeutic molecules of different classes like antitubercular, anticancer, antifungal, antiviral, antimicrobial, antisense, lung therapeutics, skin

**Figure 1.** *Structure of liposomes.*

care, vaccines, genes etc. [4]. Liposomes have proven their commercial importance from the first product 'Doxil', a PEGylated doxorubicin liposomal formulation [5, 6] to the latest 'Marqibo', vincristine sulfate liposomal formulation [7, 8]. Liposome properties differ considerably with lipid composition, surface charge, size, and the method of preparation. The nature of phospholipid bilayer determines the 'rigidity' or 'fluidity' and the charge of the vesicles. Further modifications of bilayer help in modulation of circulation time, permeability, stimuli response drug release from the liposomes.
