Liposomal oral drug delivery: The use of bipolar amphiphiles to stabilize liposomes

Liposomes, composed of classical phospholipids such as DPPC and DOPE, are widely used as drug delivery systems. Besides the intravenous, intravitreal, or transdermal administration also the oral application of liposomal formulated drugs is suitable. There are numerous advantages of these liposomal formulations in contrast to the administration of usually formulated drugs, such as enhanced bioavailability, improvement or alteration of uptake and release of drugs, and reduction of side effects. However, there are still some drawbacks, namely the chemical instability of the components of liposomes (phospholipids) in the gastrointestinal fluid (low pH value), the mechanical instability of the liposome itself, i.e., their tendency to become leaky over longer periods of time, particularly when confronted with body fluids, as well as the fast opsonization and uptake by the reticuloendothelial system. Several attempts were made to overcome these disadvantages, for example the circulation half-life of liposomes intravenously administered could be enhanced using PEGylated phospholipids ("stealth liposomes”). However, the chemical and mechanical stability of liposomes in body fluids is an obligatory prerequisite-and in most cases also the limiting factor-for the long-term application of these vesicles. This stability can be enhanced by the replacement of the classical monopolar phospholipids by bipolar amphiphiles (bolalipids), which can be inserted in a stretched manner in a phospholipid bilayer leading to a stabilization of this bilayer and, hence, the liposome. These bolalipids, originating in the membrane lipids of Archaea, are composed of two hydrophilic headgroups connected by one or two lipophilic alkyl chains. However, the use of natural archaeal membrane lipids is challenging since the cultivation of Archaea is elaborate and cost-intensive and the resulting lipids are diverse in their chemical structure. But also the total synthesis of these archaeal lipids is time-consuming and expensive.
Following up the last fact, the great vision of our group is to find an easy-to-synthesize bolalipid that can be used for the stabilization of orally administered liposomes. We therefore synthesized within the last years a couple of different single-chain and double-chain bolalipids with modifications in the long, headgroup-connecting alkyl chain. Some of these novel bolalipids show very promising results regarding the stabilization of liposomes (see below). Hence, the submitted project will firstly focus on an intensive physicochemical characterization of the mixing behavior of those artificial bolalipids with classical phospholipids either in pure manner (DPPC, etc.) or in natural mixture (EggPC, soy lecithin). The physicochemical characterization will be performed by means of calorimetrical (DSC, ITC), and IR-measurements, as well as by the use of electron microscopy (EM), dynamic light scattering, X-ray and neutron scattering, and fluorescence methods. Results from these investigations will allow conclusions on the structural prerequisites of bolalipids necessary for the insertion in a phospholipid bilayer in a stretched and, hence, stabilizing manner. The second part of this project will deal with formation and stability-tests of liposomes prepared from bolalipid-phospholipid-mixtures: the examination of various preparation techniques, dye-release-experiments in different artificial body fluids, and the long-term storage stability including the use of different lyoprotectors. An increased chemical and mechanical stability of liposomes, which result from the incorporation of bolalipids in a stretched transmembrane conformation, should improve the applicability of those liposomes as nano-dimensioned drug carrier systems for oral administration.
If we have identified one bolalipid or more that combines all properties and prerequisites and, hence, would be candidates for the use in oral drug delivery, we are aware of the fact that this/these new lipid(s) will be considered as New Chemical Entity, which will require an extensive preclinical safety testing. Therefore, the third part of this project is related to safety tests according to EMA and/or ICH guidelines. These tests-including toxicity tests in different cell models, hemolysis tests, and testing for mutagenicity-will be performed in cooperation with Dr. F. Erdmann (Dept. of Pharmacology and Toxicology, MLU).