Several polycations possessing substantial buffering capacity below physiological pH, such as lipopolyamines and polyamidoamine polymers, are efficient transfection agents per se-i.e., without the addition of cell targeting or membrane-disruption agents. This observation led us to test the cationic polymer polyethylenimine (PEI) for its genedelivery potential. Indeed, every third atom of PEI is a protonable amino nitrogen atom, which makes the polymeric network an effective "proton sponge" at virtually any pH. Luciferase reporter gene transfer with this polycation into a variety of cell lines and primary cells gave results comparable to, or even better than, lipopolyamines. Cytotoxicity was low and seen only at concentrations well above those required for optimal transfection. Delivery of oligonucleotides into embryonic neurons was followed by using a fluorescent probe.Virtually all neurons showed nuclear labeling, with no toxic effects. The optimal PEI cation/anion balance for in vitro transfection is only slightly on the cationic side, which is advantageous for in vivo delivery. Indeed, intracerebral luciferase gene transfer into newborn mice gave results comparable (for a given amount of DNA) to the in vitro transfection of primary rat brain endothelial cells or chicken embryonic neurons. Together, these properties make PEI a promising vector for gene therapy and an outstanding core for the design of more sophisticated devices. Our hypothesis is that its efficiency relies on extensive lysosome buffering that protects DNA from nuclease degradation, and consequent lysosomal swelling and rupture that provide an escape mechanism for the PEI/DNA particles. Nonviral gene-delivery techniques remain several orders of magnitude behind viral vectors when compared on the basis of the mean number of gene copies needed to transfect a cell. Despite this limitation, plasmid-mediated transfection has the major advantage that it raises none of the concerns of biological vectors for human therapy. Thus, much effort is presently devoted to improving nonviral techniques (1). Indeed, the advent of gene therapy has provided the impetus for improving, by appropriate chemical design, the efficiency of classical transfection agents such as cationic polymers (DEAE-dextran, Polybrene, polylysine) or inorganic aggregates (e.g., calcium phosphate). Various other polycationic cores have been developed, whether macromolecules (for review, see refs. 2-4), amphiphilic aggregates (for review, see ref. 5), or mixtures of both (6, 7), all of which ionically condense plasmid DNA and bind to the cell surface. Additional viral-like molecular properties have been added to these particles (2-9) to promote receptor-mediated endocytosis, fusogenicity, and karyophily. Among the cationic cores described so far, two are constitutively efficient gene-delivery agents without any extra virusderived function or lysosomotropic additive. Polyamidoamine cascade polymers (4) and lipopolyamines (5, 10-12), although quite different in chemical structures, ...

Optical imaging for biological applications requires more sensitive tools. Near-infrared persistent luminescence nanoparticles enable highly sensitive in vivo optical detection and complete avoidance of tissue autofluorescence. However, the actual generation of persistent luminescence nanoparticles necessitates ex vivo activation before systemic administration, which prevents long-term imaging in living animals. Here, we introduce a new generation of optical nanoprobes, based on chromium-doped zinc gallate, whose persistent luminescence can be activated in vivo through living tissues using highly penetrating low-energy red photons. Surface functionalization of this photonic probe can be adjusted to favour multiple biomedical applications such as tumour targeting. Notably, we show that cells can endocytose these nanoparticles in vitro and that, after intravenous injection, we can track labelled cells in vivo and follow their biodistribution by a simple whole animal optical detection, opening new perspectives for cell therapy research and for a variety of diagnosis applications.