Preparation and physicochemical characterization of novel mixed liposomes for medical applications

Liposomes are very useful aggregates due to the possibility to modulate their properties for the application area of concern. In this thesis the use of liposomes i) as delivery systems for natural substances with biological properties but low solubility in water such as (+)-usnic acid (UA) and curcuminoids (CUR) and ii) as sensors for a tumor biomarker, thymidine phosphorylase (TP) was investigated. The aim of this work is to evaluate how and to what extent changes in the chemical structure of liposomes components can affect their physicochemical properties. Liposomes as drug delivery systems Structurally related formulations were prepared and characterized to be used as UA or CUR delivery systems. In the case of UA, aggregates containing synthetic structurally related L-prolinol derivatives alone (micelles) or mixed with cholesterol in the presence (liposomes) or in the absence (quatsomes) of a natural phospholipid were investigated exploiting different methodologies of preparation and of removal of the unentrapped solute. The results of this investigation point out that the crucial role in affecting the aggregates and the solute properties is played by the synergistic effect between the chain length and the headgroup charge, besides the fluidity of the bilayer and the polarity of the solute. In particular, the z-potential of liposomes containing zwitterionic N-oxide synthetic surfactants that bear long chains was positive and not neutral as expected demonstrating that some parameters are not always easily predictable only on the basis of lipids molecular structure but also their organization in the bilayer is important. On the other hand, the preparation technique and the purification protocol affect mostly the stability of the aggregates. Analogue formulations containing L-prolinol derivatives were also prepared to investigate the correlation between the structure of the synthetic surfactant with the antibacterial activity of liposomal UA. The obtained results underline that a similar headgroup charge of the synthetic component is not enough to assure the same efficacy of the treatment. In the case of CUR, liposomes containing different saturated or unsaturated commercial phospholipids with or without cholesterol were investigated. All the formulations showed the ability to increase CUR solubility in water and to highly reduce their degradation to an extent strictly related to lipid molecular structure. Liposomes as sensors 5-Fluorouracil (5-FU) is a chemotherapeutic agent largely used for the treatment of many solid tumors but has a very narrow therapeutic window because the level of its three target enzymes can significantly vary among patients. The aim of this research was dosing the level of one of them (TP, chosen as model) with a simple colorimetric analysis using polydiacetilene liposomes. In fact, the polydiacetylene units present in the bilayer polymerize under UV light generating a very high conjugated skeleton that confers to the solution normally a blue colour. If an external stimulation (in this case the interaction of TP with 5-FU covalently linked to liposomes surface) occurs, the solution can turn to red to an extent proportional to the concentration of the target enzyme in solution. Different mixed polydiacetylene liposomes properly functionalized with 5-FU were prepared and tested. The obtained results demonstrate that the electrostatic interactions between the target enzyme and liposomes surface, the polarity of the headgroup region and the rigidity/lipid packing of the bilayer, besides the composition, are decisive for the response of the system. Also the steric hindrance of the polar headgroup, moving away the chains, can reduce or inhibit an extended polymerization. In fact, the self-organization of monomers influences the colour of the solution and precise angles and relative distances between chains must be respected to achieve the required conjugation of the polymer backbone.