VECTRIUM

Pathogenic bacteria are a permanent threat for Humanity even the discovery of antibiotics led to the fade-out of mass epidemies. Nevertheless humanity faces more and more antibiotic-resistant bacterial strains. Thus, the constant development of innovative antibacterial strategies is crucial, especially against Gram-negative bacteria found in many lethal infections. The current antibiotic arsenal is mainly composed of organic compounds when organometallic derivatives appear to be reserved principally to cancer. However, the toxicity profiles of current last resort antibiotics (colistin, polymyxin, oxazolidinones) make now organometallic derivatives competitive in the benefit/risk balance. Moreover some of these organometallic species proved to have the photophysical properties making them potential photosensitizers (PS) useable in antibacterial photodynamic therapy (PDT). In this context iridium(III) complexes are promising candidates. However, these chemical entities are not able to cross the bacterial envelope of Gram-negative bacteria to reach their intracellular biological targets. Bacterial iron uptake systems are gates through the envelope. Siderophores are small iron(III) chelating molecules secreted by bacteria to promote iron acquisition. Siderophores are able to use bacterial iron uptake systems to cross bacterial membrane reaching therefore bacterial inner space. Siderophores could be thus used as vectors to promote accumulation of PS based on iridium(III) into Gram-negative bacteria using a so called Trojan horse strategy. The recognition of the ferric-siderophore by specific outer membrane transporters will lead to the uptake of the conjugate inside the bacterium. This approach should, at one and at the same time, increase the penetration of organometallic species and reduce the peripheral toxicity of vectorized metals for host cells. Vectors to be used in our approach will be analogs of enterobactin, ferrichrome and pyochelin, three siderophores commonly used by Gram-negative pathogens considered to be critical (WHO list). These vectors will be synthesized by Partner 1 (Dr. Gaëtan MISLIN, UMR7242, Illkirch) and conjugated to Ir(III) complexes prepared by partner 2 (Dr. Sylvain GAILLARD, ENSICaen, Caen). Partner 1 will assess the antibacterial activities (MIC) of the siderophore-Ir(III) conjugates in the presence, and in the absence, of light, on Pseudomonas aeruginosa (planktonic and biofilms). This pathogenic bacterium, responsible for serious infections, is known to be poorly permeable to many xenobiotics. A comparison of results obtained in rich and iron-depleted media will give an overview of metal conjugates activities under various physiological conditions and will provide useful informations on involvement of iron uptake systems. Iron acquisition is completely different in humans and in bacteria: iron homeostasis does not require the presence of siderophores. Actually, eukaryotic membranes are poorly permeable to the ferric siderophores selected in our approach. Thus, metal conjugates are not expected to promote intracellular penetration, and toxicity, of iridium(III) in human cells. To evaluate this point more promising compounds will be tested on a human lung fibroblast infection model in order to evaluate both the cytotoxicity of the conjugates and the robustness of our antibacterial strategy in the presence of eukaryotic cells. Finally, the conception of a perennial therapeutic approach must include a study of the mechanisms potentially involved in the emergence of resistance. To this end, partner 3 (Pr Patrick PLESIAT, Besançon) will investigate the strategies developed by bacteria resistant to our approach. Furthermore this partner also depends on the National Reference Center on Resistances and could enlarge the screening to a wide collection of bacteria including antibiotic-resistant strains and clinical isolates.