Design,Synthesis And Applications Of PolyDOTA Dendrimer-based Molecular Imaging Probes

Molecular imaging is an exponentially growing field that aims to image biological processes at cellular and/or molecular level in vivo.The development of molecular imaging will greatly benefit fundamental research and clinical diagnoses.The future molecular imaging requires probe,s with better sensitivity,stability,targeting and multi-ftunctionality.Dendrimer is considered an ideal nano-caier for molecular imaging probes.However,currently most dendrimer-based probes are constructed by a peripheral functionalization strategy,leaving the interior framework of dendrimer unexploited.Therefore,we developed a ligand-branching strategy that employs chelating hole as branching junction to const:ruct novel dendrimer.Such strategy not only combines the synthesis and functionalization procedure of functional dendrimer construction,but also exploits the interior framework of dendrimer.In this thesis,we constructed a series of polyDOTA framed dendrimers(PDOTA for short)by the ligand-branching strategy using DOTA(1,4,7,10-tetraazacyclodoaecane-1,4,7,10-tetraacetic acid),which is ubiquitous in molecular imaging field.The molecular imaging related properties of PDOTA were systematically studied.In Chapter 1,we present the research background of this thesis.The principles of magnetic resonance imaging(MRI),the SBM theory for MRI contrast agent,the research frontiers of MRI contrast agent:fluorescence imaging,investigated probes for MR-fluorescence bimodal imaging,dendrimer,and the research advances on dendrimer-based probes for molecular imaging are introduced and discussed.In Chapter 2,we present the design and synthesis of PDOTA,and the studies on the kinetic stabilities of PDOTA metal chelates(M-PDOTA).A series of PDOTA dendrimers with well-defined uniform nanostructure were efficiently synthesized.And our studies revealed that the kinetic stabilities of Gd-PDOIA and Mn-PDOTA are significantly higher than that of Gd-DOTA and Mn-DOTA,respectively.The high kinetic stabilities lay the foundation of the further bio-application studies of M-PDOTA.In Chapter 3,we systematically studied Gd-PDOTA as MRI contrast agent.The relaxivity values of Gd-PDOTA are significantly higher than that of Gd-DOTA at all magnetic fields studied.A synergistic enhancing effect on signal was also found in Gd-PDOTA injected mice.Moreover,Gd-G2-G4 are effective in cellular MRI contrast,which is beneflted from their positive zeta potentials.In Chapter 4,we systematically studied Mn-PDOTA as MRI contrast agent.The chelation of Mn2+ with PDOTA will produce less positive charge than Gd3+,reducing the cytotoxicity.The relaxivity values of Mn-PDOTA are closed to that of Gd-DOTA,making Mn-PDOTA a potential class of non-gadolinium-based MRI contrast agent.In Chapter 5,we studied Tb-PDOTA as fluorescence imaging probe.Tb-G2?G4 could efficiently enter cells,rendering it possible to do cellular fluorescence imaging.Interestingly,Tb-G2?G4 could localize in lysosome,and Tb-G2 had the highest co-localization efficiency among Tb-PDOTA.Since Tb-based fluorescence is highly photostable,Tb-G2 is an outstanding probe for long-term cell imaging and lysosome tracking.In Chapter 6,we expanded the PDOTA system by peripheral PEGylation and heterometallic chelation.Modifying PEG chains on Gd-PDOTA would significantly increase the kinetic stability and effectively change the in vivo behavior of the complexes.And through the construction of Tb/Gd core/shell PDOTA complexes we proved that PDOTA is a powerful platform for multi-modality imaging.In summary,for the first time,we constructed a series of PDOTA dendrimers by the ligand-branching strategy.These PDOTA dendrimers could efficiently chelate various kinds of metal ions.The resulting M-PDOTA complexes are much more kinetically stable than the corresponding M-DOTA complexes,indicating a high biosecurity of M-PDOTA complexes.Moreover,M-PDOTA also feature superior imaging properties,remarkable feasibility for functionalization,adaptable in vivo behavior,and enormous potential for multimodality imaging.Our studies illustrated that PDOTA is a powerful and versatile chelating platform for molecular imaging.We expect the ligand-branching strategy will encourage new ideas in construction of novel dendrimers with fancy structure,multi-function and high performance.