Monday May 27 2013 from 13:30 PM in room 107 - Towers 22-23 in Jussieu campus.

Rachidwill defend his Ph.D. entitled  : "Use of copolymer nanoparticles for the cellular targeting of photosensitizers: Study on model of membranes and correlation with their cellular internalization", performed under the supervision of Stéphanie Bonneau.

Abstract: 

Photodynamic therapy (PDT) is currently an alternative technique to traditional chemotherapy for cancer treatment. It is based on the ability of certain molecules called photosensitizers to generate reactive oxygen species after light irradiation. Most of these molecules are known for their preferential retention in tumor tissues, but a number of drawbacks limit their use. Particularly, their relative hydrophobicity limits their solubility in biological media. New forms of photosensitizers are being developed, such as the use of nanoparticles to convey to the targeted tissues, to reduce toxicity and increase their specificity. Among these, the amphiphilic polymer nanoparticles PEO-b-PCL are good candidates because of their biodegradability and ability to generate controlled nano-sized objects. In addition, these systems are used to control the release of the photosensitizer once transported into target cells. The aim of the work presented in this manuscript aims to understand the mechanisms involved in the cellular internalization of a photosensitizer, the pheophorbide-a (pheo-a), alone or via nanoparticles. The interaction of this photosensitizer with model of membranes mimicking the lipidic part of the biological ones has been studied, in particular, by focusing on the effects of cholesterol, which is a major membrane component, governing its physical properties. Our results show that involves, among other mechanisms, a significant reduction of the flip-flop rate of the phéo-a through the membrane. The ability of this photosensitizer to damage and permeabilize the membranes was then evaluated in comparison to that of an amphiphilic, tri-carboxylic chlorine, the chlorin e6. The interaction of the pheo-a with nanoparticles and its transfer from these compartments to the membranes were studied. No transfer of the pheo-a via the aqueous phase takes place, but the transfer requires a direct collision between the nanoparticles with the vesicles. At the cellular level, the nanoparticles showed a marked improvement in the amount of internalized pheo-a relative to the one internalized without nanoparticles. In addition, the mechanisms of internalization have been highlighted. With nanoparticles, the incorporation of pheo-a is biphasic. The first phase is assigned to the fast direct transfer of the photosensitizer from the nanoparticles to the membranes, in which it redistributes. It is followed by a second phase, assigned to slow rating endocytosis of the nanoparticles. 

Key words: PDT, photosensitizers, membranes, cells, cholesterol, nanoparticles, photo-permeabilization.