Engineering a pH-Sensitive Liposomal MRI Agent by Modification of a Bacterial Channel.

MscL is a bacterial mechanosensitive channel that serves as a cellular emergency release valve, protecting the cell from lysis upon a drop in external osmolarity. The channel has an extremely large pore (30 Å) and can be purified and reconstituted into artificial membranes. Moreover, MscL is modified to open in response to alternative external stimuli including changes in pH. These properties suggest this channel's potential as a triggered "nanopore" for localized release of vesicular contents such as magnetic resonance imaging (MRI) contrast agents and drugs. Toward this end, several variants of pH-triggered MscL nanovalves are engineered. Stealth vesicles previously been shown to evade normal in vivo clearance and passively accumulate in inflamed and malignant tissues are reconstituted. These vesicles are loaded with 1,4,7,10-tetraazacyclododecane tetraacetic acid gadolinium complex (Gd-DOTA), an MRI contrast reagent, and the resulting nanodevices tested for their ability to release Gd-DOTA as evidenced by enhancement of the longitudinal relaxation rate (R1 ) of the bulk water proton spins. Nanovalves that are responsive to physiological pH changes are identified, but differ in sensitivity and efficacy, thus giving an array of nanovalves that could potentially be useful in different settings. These triggered nanodevices may be useful in delivering both diagnostic and therapeutic agents.

Redox- and hypoxia-responsive MRI contrast agents.

The development of responsive or "smart" magnetic resonance imaging (MRI) contrast agents that can report specific biomarker or biological events has been the focus of MRI contrast agent research over the past 20 years. Among various biological hallmarks of interest, tissue redox and hypoxia are particularly important owing to their roles in disease states and metabolic consequences. Herein we review the development of redox-/hypoxia-sensitive T1 shortening and paramagnetic chemical exchange saturation transfer (PARACEST) MRI contrast agents. Traditionally, the relaxivity of redox-sensitive Gd(3+) -based complexes is modulated through changes in the ligand structure or molecular rotation, while PARACEST sensors exploit the sensitivity of the metal-bound water exchange rate to electronic effects of the ligand-pendant arms and alterations in the coordination geometry. Newer designs involve complexes of redox-active metal ions in which the oxidation states have different magnetic properties. The challenges of translating redox- and hypoxia-sensitive agents in vivo are also addressed.