Emerging research and clinical development trends of liposome and lipid nanoparticle drug delivery systems

Abstract

Liposomes are spherical-enclosed membrane vesicles mainly constructed with lipids. Lipid nanoparticles are loaded with therapeutics and may not contain an enclosed bilayer. The majority of those clinically approved have diameters of 50-300 nm. The growing interest in nanomedicine has fueled lipid-drug and lipid-protein studies, which provide a foundation for developing lipid particles that improve drug potency and reduce off-target effects. Integrating advances in lipid membrane research has enabled therapeutic development. At present, about 600 clinical trials involve lipid particle drug delivery systems. Greater understanding of pharmacokinetics, biodistribution, and disposition of lipid-drug particles facilitated particle surface hydration technology (with polyethylene glycol) to reduce rapid clearance and provide sufficient blood circulation time for drug to reach target tissues and cells. Surface hydration enabled the liposome-encapsulated cancer drug doxorubicin (Doxil) to gain clinical approval in 1995. Fifteen lipidic therapeutics are now clinically approved. Although much research involves attaching lipid particles to ligands selective for occult cells and tissues, preparation procedures are often complex and pose scale-up challenges. With emerging knowledge in drug target and lipid-drug distribution in the body, a systems approach that integrates knowledge to design and scale lipid-drug particles may further advance translation of these systems to improve therapeutic safety and efficacy.

Keywords: disposition; drug delivery systems; lipids; liposomes; micelle; nanoparticles; nanotechnology; pegylation; phospholipids.

Figure 1

A schematic presentation of commonly used phospholipids. Most of the commonly used lipids are presented with hydrophobic R₁ and R₂ fatty acyl tail groups and a hydrophilic head group carrying a net charge at neutral pH 7. The head group determines the charge of a phospholipid, whereas the lipid tail group contributes no charge. The lipids with head groups (oval shape shaded area) for sphingomyelin (SPH), phosphatidylcholine, and phosphatidylethanolamine exhibit neutral net charge. Phosphatidylserine and phosphatidylglycerol carry a negative net charge at neutral pH 7. The tail groups (R₁ and R₂) for each phospholipid can have various lengths (typically C14–C18) and degrees of saturation. SPH contains a sphingoid base backbone (unshaded) and the other four phospholipids contain a glycerol backbone (unshaded). In addition, R₁ of SPH is a C15-saturated carbon chain and R₂ is a fatty acid residue connected to the sphingoid base backbone through an amide functional group. The fatty acid residues for the other four phospholipids are attached to the glycerol backbone via an ester functional group. The detailed effects on the physical properties of phospholipids because of charge and variation in R₁ and R₂ are described in Table 3.

Figure 2

A schematic presentation of lipids and derivatives that form micelles and inverted micelles. (a) Lipidic micelles (hexagonal H_I) are formed because of a large hydrophilic head group, such as a lyso-phosphatidylcholine with a choline head group and a saturated fatty acid. They form stable molecular aggregates that resemble sheets of tubes with an internal lipidic core. (b) Inverted micelles (hexagonal H_II) are formed because of phospholipid with a neutral and small head group, such as phosphatidylethanolamine, with unsaturated fatty acyl tails that tend to form inverted cone structures in solution (e.g., 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine or DOPE). They form stable molecular aggregates that resemble sheets of tubes with an internal aqueous space.

Figure 3

Number of publications on liposome research in vitro and in specific animal species. Data recorded in the PubMed database were identified with the search terms “liposome AND (‘in vitro’ or specific animal species).” For “human,” the term “clinical trial” was used for the search query. Data were compiled and plotted as a bar graph for the number of publications since 1965 and the last 5 years (2008–2013). A summary of numerical data is presented in Table 5.

Figure 4

Some common chemistry for conjugating targeting ligands to lipid anchors. (a) For short peptides containing about 3–20 amino acids, a 16-carbon chain palmitoyl or other fatty acid chains may be attached to the protein through a lysine, cysteine (Cys), or glycine residue on either the N-terminal or C-terminal end of the protein sequence. (b) A sulfhydryl-containing terminal Cys on a targeting peptide ligand can be coupled to succinimidyl 3-(2-pyridyldithio)propionate-activated phosphatidylcholine (PC) lipid anchors, forming disulfide bond linkages. (c) A sulfhydryl-containing terminal Cys on a targeting peptide ligand can be coupled to succinimidyl-4-(p-maleimidophenyl) butyrate-activated PC lipid anchors, forming stable thioether linkages. (d) An activated amino group on a targeting peptide ligand (e.g., transferrin) can be coupled to an activated carboxyl group on a PC lipid anchor [e.g., N-glutaryl phosphatidylethanolamine (NGPE)-PC] through a carbodiimide reaction with ethyl(dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS), forming peptide bond linkages. Addition of NHS to EDC reactions increases efficiency. The black dot indicates atoms that form the bond of interest.