Synthesis, evaluation and metabolic studies of radiotracers containing a 4-(4-[18F]-fluorobenzyl)piperidin-1-yl moiety for the PET imaging of NR2B NMDA receptors.

In this study, novel specific PET radioligands containing the 4-(4-fluorobenzyl)piperidine moiety and selectively antagonistic for the NR2B subunit containing NMDA receptors were developed. Two antagonists, RGH-896 (1a) and 4-(4-fluorobenzyl)piperidinyl-1-methyl-2-benzimidazol-5-ol (2a), belonging to two different structural families, were radiolabeled by an aromatic nucleophilic radiofluorination followed by a reduction of the para-position carbonyl function. Radiotracers [18F]1a, [18F]2a or the pattern 4-(4-[18F]-fluorobenzyl)piperidine ([18F]6) demonstrated an identical in vivo behavior with high accumulation of radioactivity in bone and cartilage which would suggest a radiodefluorination of the radiotracers. The identification of metabolites from 6 by LC-MS-MS confirmed the significant degree of defluorination as a result of the in vivo hydroxylation in the benzyl ring. In conclusion, [18F]1a or [18F]2a are not suitable for imaging the NR2B NMDA receptors due to their poor brain penetration. We also argue for a cautious use of the radiolabeled pattern, 4-(4-[18F]-fluorobenzyl)piperidine, to develop PET radiotracers.

Nature as a source of inspiration for cationic lipid synthesis.

Synthetic gene delivery systems represent an attractive alternative to viral vectors for DNA transfection. Cationic lipids are one of the most widely used non-viral vectors for the delivery of DNA into cultured cells and are easily synthesized, leading to a large variety of well-characterized molecules. This review discusses strategies for the design of efficient cationic lipids that overcome the critical barriers of in vitro transfection. A particular focus is placed on natural hydrophilic headgroups and lipophilic tails that have been used to synthesize biocompatible and non-toxic cationic lipids. We also present chemical features that have been investigated to enhance the transfection efficiency of cationic lipids by promoting the escape of lipoplexes from the endosomal compartment and DNA release from DNA-liposome complexes. Transfection efficiency studies using these strategies are likely to improve the understanding of the mechanism of cationic lipid-mediated gene delivery and to help the rational design of novel cationic lipids.

Physicochemical parameters of non-viral vectors that govern transfection efficiency.

Gene therapy is based on the vectorization of nucleic acids to target cells and their subsequent expression. Cationic lipids and polymers are the most widely used vectors for the delivery of DNA into cultured cells. Nowadays, numerous reagents made of these cationic molecules are commercially available and used by researchers from the academic and industrial field. By contrast their evaluations in preclinical programs have revealed that their use for in vivo applications will be more problematic than their massive use in vitro. This is mostly due to the physicochemical properties of cationic vectors/DNA complexes, which are the result of their mode of interaction. Indeed, these cationic vectors interact through electrostatic forces with negatively charged DNA. This results in the formation of highly organized positively charged supramolecular structures where DNA molecules are condensed. Association of DNA with cationic lipids under a micellar or liposomal form leads to lamellar organization with DNA molecules sandwiched between lipid bilayers. Although the lamellar phase is the common described structure, as evidenced by small-angle X-ray scattering and electron microscopy, some cationic lipid combined with a hexagonal forming lipid could also result with DNA in an inverted hexagonal structure. Despite a lot of effort, the precise mechanism of gene transfer with cationic vector is still ill-defined. Here, our objective was to overview the main relationships between the physico chemical properties of cationic lipid/DNA complexes and their transfection efficiency. An overview of a new class of vectors consisting of amphiphilic block copolymers designed for in vivo delivery is also presented and discussed.